Cement Industry Process Technology - Holderbank Course (3 of 3)

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II

B05 - PT II

© Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:33:29 PM

Page 1

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems

C01 - Kiln Systems


6/23/2001 - 4:33:29 PM

Page 2

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview

Kiln Systems - Overview Urs Gasser PT 99/14501/E 1. PROCESS REQUIREMENTS FOR KILN SYSTEMS 2. PROCESS TYPES 2.1 General 3. WET PROCESS 3.1 General 3.2 Long Wet Process Kilns 3.3 Wet Process Kilns with Slurry Preheaters 4. SEMI WET PROCESS 4.1 General 4.2 Semi Wet Process Long Kilns 4.3 Semi Wet Grate Preheater Kilns 4.4 Semi-Wet Suspension Pre-heater Kiln 5. SEMI DRY PROCESS 5.1 Semi-Dry Process Long Kilns 5.2 Semi-Dry Process Grate Pre-heater Kilns 6. DRY PROCESS 6.1 Long Dry Kilns 6.2 Raw Meal Suspension Preheater Kilns 6.2.1

General

6.2.2

One and two Stage Cyclone Pre-heater Kilns

6.2.3

Four Stage Cyclone Pre-heater Kilns

6.2.4

Precalciner Kilns


6/23/2001 - 4:33:29 PM

Page 3

"Holderbank" - Cement Course 2000 SUMMARY Today’s kiln systems for burning cement clinker of major importance use a rotary kiln. Exceptions are vertical shaft kilns still used in certain geographical areas. With very rare exceptions, new plants use the dry process. However, there are still important markets where older wet process plants are predominant (USA, Russia). A first classification of the process can be made based on the water content of the kiln feed:

< 1% water

dry-process

10 ...

12% water

semi-dry-process

17 ...

21% water

semi-wet-process

25 ...

40% water

wet-process

♦ Dry-Process •

Precalciner kiln with 4 to 6 cyclone stages (contemporary technology): ∗ Separate tertiary air duct ∗ 50 - 60% fuel to the precalciner ∗ Large capacities possible > 10000 t/d ∗ Up to 4000 t/d in 1 string ∗ Heat consumption < 3000 kJ/kg possible (6 stages) ∗ Sensitive to circulation phenomena (-> kiln gas bypass!)

•

4-stage cyclone pre-heater kiln (standard technology 1970 to 1980): ∗ Cyclone stages (co-current flow) for raw meal preheating ∗ Large application world wide ∗ Capacities of up to 4500 t/d technically possible ∗ Heat consumption: 3150 to 3350 kJ/kg cli ∗ Sensitive to circulation phenomena (-> kiln gas bypass!)

•

2-stage cyclone pre-heater kiln: ∗ Less sensitive to circulation phenomena than 4-stage pre-heater ∗ Higher heat consumption than pre-heater with more stages

•

Shaft pre-heater kiln: ∗ Counter current heat exchange between hot gas and raw meal ∗ Practical efficiency inferior to cyclone pre-heater

•

Long-dry-kiln: ∗ Rather simple equipment ∗ High dust emission from kiln tube ∗ Without heat exchange internals: high heat consumption of up to 5100 kJ/kg cli ∗ With chains and/or crosses: 4200 kJ/kg cli achievable

♦ Semi-dry and semi wet process •

Grate pre-heater kiln (LEPOL, ACL): ∗ Raw meal must be suitable to be nodulised with water (semi-dry)


6/23/2001 - 4:33:29 PM

Page 4

"Holderbank" - Cement Course 2000 ∗ 3450 kJ/kg cli (no waste heat available for primary raw material drying) •

Long rotary kiln and suspension preheater: ∗ Filter cakes fed or slurry injection into vertical dryer; rather rare cases

♦ Wet-process •

Long wet kiln: ∗ Fed with raw meal slurry of approx. 32 - 42% water content ∗ Internal heat transfer improved by chains ∗ High heat consumption of 5300 to 6300 kJ/kg cli due to evaporation of water ∗ Heat consumption reduced by slurry thinners for a slurry with 25 - 30% H2O ∗ Slurry preheaters can reduce kiln size and improve heat exchange

Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 1. PROCESS REQUIREMENTS FOR KILN SYSTEMS

1.

PROCESS REQUIREMENTS FOR KILN SYSTEMS

The kiln system has to be designed to cope with the requirements of the chemical process during which the kiln feed material is converted into cement clinker. This process as a whole is endothermic and takes place at maximum material temperatures of 1450°C. Receiving its thermal energy from hot gases of up to 2000°C generated by combusting fuels, it is also referred to as pyroprocess. Type of reaction and temperature development are compiled in “sequence of reactions occurring in a rotary kiln” (table 1) and graphically as the “quasi-qualitative variation of minerals with temperature” (figure 1). The chemical process taking place in the kiln system where raw meal (input) is converted to cement clinker (output) can be subdivided into the following five steps: 1.

Drying

2.

Preheating

3.

Calcining

4.

Sintering

5.

Cooling

Process and equipment has been developed and improved with the aim at performing these steps forever improved economy, which means •

High availability

•

Low heat consumption

•

Low power consumption

•

Higher unit capacity

•

Stable kiln operation

•

Good, uniform clinker quality

Table 1

Sequence of Reactions occurring in a Rotary Kiln


6/23/2001 - 4:33:30 PM

Page 5

"Holderbank" - Cement Course 2000 Temperature range (°C)

Type of reaction

Heating Up 20 - 100

Evaporation of free H2O

100 - 300

Loss of physically absorbed water

400 - 900

Removal of structural H2O (H2O and OH groups) from clay minerals

> 500

Structural changes in silicate minerals

600 - 900

Dissociation of carbonates CO2 driven out)

> 800

Formation of belite, intermediate products, aluminate and ferrite

> 1250

Formation of liquid phase (aluminate and ferrite melt)

approx. 1450

Completion of reaction and re-crystallisation of alite and belite

Cooling 1300 - 1240

Crystallisation of liquid phase into mainly aluminate and ferrite

Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 2. PROCESS TYPES

2.

PROCESS TYPES

Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 2. PROCESS TYPES / 2.1 General

2.1

General

The criterion normally used to distinguish the process types is the moisture of the kiln feed material. Four basically different process types for clinker burning can be defined:

Process Type

Feed Material

Cons.

Dry process

Raw meal

Dry

Semi dry process

Nodules

Moist


Feed Moisture

Feed System

< 1% H2O

Mechanic, pneumatic

≈ 10 ... 12% H2O

Mechanic, pneumatic

6/23/2001 - 4:33:30 PM

Page 6

≈ 10 ... 12% H2O

"Holderbank" - Cement Course 2000 Semi wet process

Filter cake, nodules Moist

≈ 17 ... 21% H2O

Mechanic, pneumatic

Wet process

Slurry

≈ 25 ... 40% H2O

Hydraulic

Liquid

Table 1 gives a general survey of the various rotary kiln systems in operation for industrial clinker production. Shaft kilns, which are still used in China or experimental systems such as sintering grates or fluidised beds, are not considered in the scheme. We can distinguish two main groups of kiln systems: a)

Long kilns with or without internal heat exchanging installation

b)

Short or medium kilns with external preheaters (e.g. suspension preheaters, grates or external slurry preheaters)

The heat consumption of burning depends strongly on the water content of the kiln feed This can be illustrated by the typical specific heat consumption: The fuel consumption of wet kilns is nearly twice as high as for modern dry process suspension pre-heater kilns. The comparison of the heat economy within each process group (dry or wet) shows clearly: The more intensive the heat-exchange for drying and preheating, the lower the heat consumption. Other than based on the feed moisture, kiln systems can be grouped in different ways: Process Type

wet semi wet semi dry dry

>25% H2O in feed 17 - 21% H2O in feed 10 - 12% H2O in feed < 1% H2O in feed

Slurry nodules from slurry nodules from meal raw meal

Production Mode

batch+cont. continuous

< 200 t/d 300 t/d – 10’000 t/d

shaft kilns rotary kilns

2900 kJ/kg cli ( 700 kcal/kg cli)

state of the art system

> 6000 kJ/kg cli (> 1400 kcal/kg cli)

long wet or dry kilns, not optimum operation

20 to 65 kWh/t cli

kiln feed to clinker cooler

Heat Consumption

Power Consumption

OVERVIEW OF KILN AND PROCESS TYPES


6/23/2001 - 4:33:31 PM

Page 7

"Holderbank" - Cement Course 2000

When the concept for a new plant is developed, not only the present situation but also the possible future developments of all relevant factors must be taken into account. The following main parameters must be considered when selecting the kiln system: •

Raw material: ∗ moisture content ∗ grindability ∗ homogeneity of deposit ∗ number of components for raw mix ∗ chemical composition (sulphur, chlorides, alkalis, organic compounds etc) ∗ filtration properties of slurry (for semi-wet process only)

•

Plant installation and operating costs

•

Requirements for clinker quality (e.g. low alkali clinker)

•

Aspects of environmental protection (emission of dust, SOx, NOx, etc)

•

Technical standard of the country

Long wet (and dry) rotary kilns are the oldest and most simple type of installation to produce cement clinker. The pyroprocess takes place in a long rotating tube, which has usually internal equipment to improve heat transfer, and, in wet kilns, to reduce dust loss. Unit capacities of up to 2000 t/d are typically achieved, higher outputs are possible, however, they require kilns of gigantic dimensions. Today, economy requires plants for 3000 to 10’000 t/d. Therefore new plants are almost always based on the dry process with preheater, pre-calciner and reciprocating grate cooler. The semi wet process for a new plant could be preferred in special cases, e.g. where raw material with a high natural moisture must be used (e.g. quarry below water level). The three following graphs illustrate the development of the significance of the various processes within the Holderbank group, which can be considered representative of the global situation.


6/23/2001 - 4:33:32 PM

Page 8


Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 3. WET PROCESS

3.

WET PROCESS

Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 3. WET PROCESS / 3.1 General

3.1

General

The wet process was the most important process for clinker burning in the past and almost all plants were wet. Heterogeneous quarries and corrective addition were no problem; stirring of the liquid slurry in the slurry tanks provides very good batch-wise blending. Grinding was done in slurry mills, which consume 30%, less energy than dry ball mills, but at higher lining wear rates. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:33:32 PM

Page 9

"Holderbank" - Cement Course 2000 The disadvantage of the wet process is the high heat consumption. Compared to e.g. a suspension preheater kiln, the difference is more than 2000 kJ/kg clinker or 60 to 70%! Today, with efficient dry homogenising technology available, the wet process is no longer applied for new plants. Investments as well as operating costs of a wet system are higher than for dry systems of the same output. Technical development allows using more efficient kiln systems even where wet plants would have been built in earlier times. Another reason for preferring the wet process in the past was the production of low alkali cement (alkali content < 0,6%) and the fact that difficult circulation problems are easier to control in wet kilns. Today secondary firing or efficient bypass installations with precalciner are possibilities to keep these problems under control also in modern kiln systems. Because of the lower specific gas volume and the shorter rotary part, rotary kiln dimensions as well as gas handling, dedusting and fuel preparation can be designed accordingly smaller. Although new wet kilns are no longer considered for new plants, they still play an important role in the US as well as in many countries of Eastern Europe and Central Asia. Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 3. WET PROCESS / 3.2 Long Wet Process Kilns

3.2

Long Wet Process Kilns

Long wet kilns have been the most commonly used burning reactors for a very long time, but because of the high water content of the feed, their heat consumption is up to twice as high as for modern dry systems. The milled and homogenised raw material is a slurry with a water content of typically 32 to 42% and is pumped to the kiln inlet. In the first zone heat transfer for the evaporation of water is always increased by means of chain systems (extended surface, higher relative velocities, increase of turbulence). The chain systems should also reduce the dust losses and clean the kiln shell. These internal heat exchanger installations require very special know-how, based to a large degree on experience (see separate paper ‘chain systems’). In order to decrease fuel consumption the water content should be kept as low as possible. The limit is normally the pumpability of the slurry. It is basically possible to further reduce the slurry moisture by using slurry thinners. This technology has been successfully applied and will provide an economical advantage if adequate quantities are available at low cost, e.g. as industrial by-product. Example: Beauport (Canada): 28% feed moisture


6/23/2001 - 4:33:32 PM

Page 10

"Holderbank" - Cement Course 2000 Wet kilns are relatively insensitive to circulation problems because the critical temperature ranges are in the rotary part of the kiln (see also ‘circulation phenomena’). Low alkali clinker can be produced from high alkali raw material simply by selectively wasting of dust: The highest enriched kiln dust (e.g. from the last precipitator compartment) is removed from the process (i.e. dumped onto a dust pile) as necessary. The rest of the dust can be reintroduced to the kiln by dust scoops or insufflation into the burning zone. Today, discarding dust creates increasing problems because of restrictive permitting of dust piles. Note: Kiln dust cannot just be blended to the slurry because it would react and thicken the slurry. Typical technical data for long wet kilns with chains:

Heat consumption q

5’000 ... 6’300 kJ/kg cli (1’200 ... 1’500 kcal/kg cli)

Kiln exit gas temperature

150° ... 250°C

System pressure drop

0,5 ... 1,0 kPa

Dust emission in % of clinker production

5 ... 100%

Probably the largest wet process kiln in the world is installed at Holnam’s Clarksville plant (Michigan USA). This kiln has a diameter of 7,6 m and a length of 232 m with a daily capacity of about 3’600 t. Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 3. WET PROCESS / 3.3 Wet Process Kilns with Slurry Preheaters

3.3

Wet Process Kilns with Slurry Preheaters

External Slurry Preheaters In order to improve the heat exchange between gas and slurry and to reduce the kiln size, external slurry preheaters have been developed by MIAG (Kalzinator) and Krupp (Konzentrator). Both of them are revolving drums with special internal packing. These drums have about the same diameter as the kiln, its length being slightly smaller than the diameter. The capacity of these machines is limited to 800 -1000 t/d and frequently operating problems arise. Very often, external preheaters were large sources of false air. Internal Slurry Preheaters F.L. Smidth designed a slurry pre-heater system integrated into the kiln compartment, which should avoid the disadvantage of external slurry preheaters. In practice, this construction turned out to be very sensitive to clogging. A better system developed by Fives Cail Babcock is installed in the three kilns at Obourg. Lifting buckets and chain curtains produce a slurry curtain that keeps back a high amount of dust and improves heat exchange.


6/23/2001 - 4:33:34 PM

Page 11


Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 4. SEMI WET PROCESS

4.

SEMI WET PROCESS

Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 4. SEMI WET PROCESS / 4.1 General

4.1

General

A process is considered semi-wet if the kiln feed is produced from wet slurry. A mechanical water extraction process reduces the water content of the kiln feed to 17 to 21%. A number of filter presses operating batch-wise are commonly used, but also continuous filter band presses or similar equipment would be possible. Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 4. SEMI WET PROCESS / 4.2 Semi Wet Process Long Kilns

4.2

Semi Wet Process Long Kilns


6/23/2001 - 4:33:34 PM

Page 12

"Holderbank" - Cement Course 2000 Principally, long kilns with heat exchanger crosses can be fed with slurry, filter cakes or dry meal. Feeding filter cakes is a straightforward and simple solution and is used by Italcementi in some cases. Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 4. SEMI WET PROCESS / 4.3 Semi Wet Grate Preheater Kilns

4.3

Semi Wet Grate Preheater Kilns

Most of the semi-wet systems use a grate preheater kiln fed with filter cakes. A grate preheater system includes a short rotary kiln (similar to a four stage preheater kiln) where only calcining and sintering take place. For drying, preheating and partial calcining, a travelling grate is installed in front of the kiln, where heat of the kiln exhaust gases is used. For the semi-wet grate kiln, the slurry must be prepared in a special way so it can be fed to a travelling grate: The pumpable slurry as starting material is fed to filter presses where the moisture content is reduced to approx. 20% applying a filtration pressure of 15 to 20 bar. In a special type of extruder (Siebkneter), the filter cakes are converted into cylindrical nodules (diameter 15 ... 20 mm, length 30 ... 50 mm) and then fed to the preheater-grate. The economy of this way of preparation depends strongly on the filtration properties of the slurry. Operating and performance data are similar to the semi-dry grate preheater systems described under 5.2. Characteristic data of a semi-wet grate pre-heater system: Feed Nodules made from Moisture Content of the Feed

Slurry Filter Cake 10 ... 12%

Heat consumption q

3770 kJ/kg cli (≈ 900 kcal/kg cli)

Exit gas temperature after grate

100° ... 120°C


2,6 kPa

Example of a semi-wet LEPOL kiln: AB’s kiln 10 at the Lägerdorf plant (Germany) Maximum kiln capacity:

3’600 t/d

Kiln dimensions:

φ 6.0/5.6 m x 90 m

Grate dimensions:

5.6 x 61.7 m

Secondary firing with Fullers earth (special) (Shut down; replaced by semi wet precalciner kiln in 1996)


6/23/2001 - 4:33:36 PM

Page 13


Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 4. SEMI WET PROCESS / 4.4 Semi-Wet Suspension Pre-heater Kiln

4.4

Semi-Wet Suspension Pre-heater Kiln

The suspension preheater kiln is normally fed with dry meal (details see separate paper). However, there are some rare cases where suspension preheater kilns are fed with nodules prepared from slurry. These nodules should not be too strong because they must be cracked by thermal shock or abrasion before being fed to the kiln system via top stage of the pre-heater. A two-stage pre-heater kiln operated with semi-wet nodules was e.g. the Liesberg plant. There, the nodules were cracked in a vertical dryer before being fed to the preheater. The first modern kiln system using this principle has been built in the late 1980’s by FLS in Aalborg Cement’s RORDAL plant. It is a three stage two string kiln system with precalciner for a capacity of 4000 t/d. The high operating cost of the filter presses has been avoided by directly injecting the slurry into a drier-crusher followed by a vertical drier. The semi-wet process was selected because the raw material (chalk) is mined under water and has very high natural moisture. From the “Holderbank” group: Example of a semi-wet pre-heater/pre-calciner kiln: AB’s kiln 11 at the Lägerdorf plant (Germany) Maximum kiln capacity:

4’500 t/d at 3900 kJ/kg

Kiln dimensions:

φ 4.8 x 65 m; 2 supports, gearless friction drive

Preheater:

3 stages, 2 strings

Utilisation of various alternative fuels in both firings Supplied by Polysius; start-up: 1996 Filter cakes produced in already existing filter-presses


6/23/2001 - 4:33:36 PM

Page 14


Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 5. SEMI DRY PROCESS

5.

SEMI DRY PROCESS

The semi-dry process is characterised by the fact that kiln feed nodules are made from dry raw meal. Water is added in order to produce nodules with 10 - 12% moisture. Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 5. SEMI DRY PROCESS / 5.1 Semi-Dry Process Long Kilns

5.1

Semi-Dry Process Long Kilns

There are long kilns with heat exchanger crosses fed with nodules. This system was applied by Italcementi and looks very similar to an installation for semi-wet feed material. Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 5. SEMI DRY PROCESS / 5.2 Semi-Dry Process Grate Pre-heater Kilns

5.2

Semi-Dry Process Grate Pre-heater Kilns

The grate preheater kiln is by far the most popular semi-dry system. The principle of the grate preheater system for the semi-dry process is identical to the one used for the semi-wet process. What is different is the feed preparation: The dry raw material is mixed with water (10 ... 12%) and nodulised in a drum or preferably on a rotating plate (pan noduliser). This system can be used only for raw materials containing plastic components enabling the formation of nodules that are resistant against thermal shock and abrasion. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:33:38 PM

Page 15

"Holderbank" - Cement Course 2000 The main factor influencing plasticity is the mineralogical composition, especially the presence of montmorillonite. On the grate, heat exchange from the gas to the nodules forming a fixed bed layer of approx. 20 cm thickness is excellent. In some grate preheaters, precalcination is done successfully, often using even waste fuels (such as Fullers earth, acid sludge, waste lubricating oils etc.) utilising secondary firing. The only successfully working travelling grate pre-heater was available from Polysius and became known under the name LEPOL system (American licensee: Allis-Chalmers, ACL system).

This principle sketch shows a LEPOL kiln fed with nodules made out of dry raw meal. LEPOL kilns built after 1945 are equipped with two-pass grates; i.e. the exhaust gas is led twice through the nodule bed from top to bottom: The hot kiln gas passes first through a bed of dry and preheated nodules and subsequently, after an intermediary dedusting once again trough a layer of moist incoming nodules. It is believed that the nodules survive throughout the process resulting in a clinker with very uniform size. Furthermore, dust loads in the kiln atmosphere and dust emission out of the system are low. The nodules on the grate let only pass the fine dust while the coarse particles are retained. In cases of increased trace compound concentrations (especially alkali) in the raw material, the fine dust separated in the electrostatic precipitator is largely enriched with them. Only a small amount of dust has to be discarded to reduce the balance of these compounds in the kiln system. This effect makes the LEPOL kiln quite suitable to produce a low alkali clinker with rather low heat consumption. For this reason, it has been chosen in many cases, particularly in the USA. The following limits and disadvantages have to be considered: •

Only raw materials with good plastic properties can be used (semi-wet: filter cake nodules -> good filtration properties are required)

•

The grate chain is subject to wear.

•

Uneven temperature distribution across the grate can cause difficulties.

•

Additional theoretical heat consumption due to the water content of the feed (partially compensated by a low exit gas temperature).

•

Exhaust gases cannot be used in drying and grinding systems.

Characteristic data of a semi-dry grate pre-heater systems: Feed nodules made from © Holderbank Management & Consulting, 2000 Query:

dry raw meal 6/23/2001 - 4:33:41 PM

Page 16

"Holderbank" - Cement Feed nodules made from Course 2000 dry raw meal Moisture content of the feed nodules

10 ... 12%

Specific heat consumption q

3450 kJ/kg cli (= 820 kcal/kg cli)

Exit gas temperature after grate

100 ... 120°C


2.6 kPa

Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 6. DRY PROCESS

6.

DRY PROCESS

Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 6. DRY PROCESS / 6.1 Long Dry Kilns

6.1

Long Dry Kilns

Without internal heat exchange equipment The simplest kind of dry process installation is the long dry kiln without any internal heat exchange equipment (empty tube). With a heat consumption of 5100 kJ/kg cli (1200 kcal/kg cli) or about 90% of the wet process it must be considered very uneconomical. Advantages might be its simplicity and insensitivity to heavy circulation problems. This kiln type is suitable to be used in combination with waste heat recovery steam boilers for power © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:33:42 PM

Page 17

"Holderbank" - Cement Course 2000 generation. In that case, the waste heat contained in the hot kiln exhaust gases is further used to produce valuable energy. Characteristic kiln data: Heat consumption q

4500 ... 6000 kJ/kg cli

Kiln gas exit temperature

450° ... 500°C


0,5 ... 1,0 kPa

(1075 ... 1430 kcal/kg cli)

With internal heat exchange equipment Long dry kilns with internal heat exchange equipment (chains or crosses from steel or ceramic material) represent a more economical solution. Heat consumption of 4200 kJ/kg or even less can be achieved. Other typical operating figures are contained in annex 10. Characteristic kiln data: Heat consumption q

3800 ... 4500 kJ/kg cli


400° ... 450°C


1,0 ... 1,5 kPa

(910 ... 1075 kcal/kg cli)

Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 6. DRY PROCESS / 6.2 Raw Meal Suspension Preheater Kilns

6.2

Raw Meal Suspension Preheater Kilns

Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 6. DRY PROCESS / 6.2 Raw Meal Suspension Preheater Kilns / 6.2.1 General

6.2.1

General

During the last thirty years, the suspension preheater kiln became the dominant clinker manufacturing system. This system is fed by dry raw meal that is preferably prepared in a grinding and drying plant, using the kiln waste gases as a drying medium. This ground and dried raw meal is homogenised and then fed to the preheater where it is suspended in the kiln gas flow, where an extremely effective heat transfer takes place. More information is contained in the special section “Suspension Preheaters”. Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 6. DRY PROCESS / 6.2 Raw Meal Suspension Preheater Kilns / 6.2.2 One and two Stage Cyclone Pre-heater Kilns

6.2.2

One and two Stage Cyclone Pre-heater Kilns

Characteristic kiln data: one stage:

Heat consumption q

3750 ... 4000 kJ/kg cli


400° ... 500°C


1,5 ... 2,5 kPa


6/23/2001 - 4:33:46 PM

(900 ... 950 kcal/kg cli)

Page 18

"Holderbank" - Cement Course 2000 two stages:

Heat consumption q

3500 ... 3750 kJ/kg cli


400° ... 450°C


1,5 ... 2,5 kPa

(850 ... 900 kcal/kg cli)

Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 6. DRY PROCESS / 6.2 Raw Meal Suspension Preheater Kilns / 6.2.3 Four Stage Cyclone Pre-heater Kilns

6.2.3

Four Stage Cyclone Pre-heater Kilns

Until the mid 1980s, this arrangement belong to the systems with the lowest fuel consumption. It was offered in several configurations with capacities up to 4500 t/d, most of them being combinations of single or twin cyclone stages. The kiln exit gas includes still enough heat to dry raw material up to moisture content of 8% if the mill is running during all the kiln operation time. From this point of view, the remaining relatively high exit gas temperature cannot be considered fully as a loss, because it can substitute an auxiliary firing for raw material drying. The preheater system is installed in a steel or concrete tower with a height of about 60 to 120 m (6 stages) above the kiln inlet, depending on capacity and concept. The four to six stages preheater is most susceptible to circulation problems at presence of excessive concentration of circulation compounds causing clogging problems in the pre-heater system. The sketch shows a conventional four stage cyclone preheater system. In the 1970’s, production lines with more than approx. 2000 t/d had to be built with two parallel preheater strings. Today, one-string installations are possible for up to 4000 t/d. Characteristic operating figures of 4-stage pre-heater kilns: Heat consumption q small units

3350 ... 3550 kJ/kg cli (= 800 ... 850 kcal/kg cli)


6/23/2001 - 4:33:46 PM

Page 19

"Holderbank" - Cement Course 2000 (= 800 ... 850 kcal/kg cli) large units


kiln exit gas temperature

320° ... 350°C

kiln exit gas volume

approx. 1,5 Nm3/kg cli


4 ... 6 kPa

Dust loss relative to clinker

8 ... 15%

Transition chamber kiln gas temperature

approx. 1100°C

Material temperature

approx. 800°C

Process Technology / B05 - PT II / C01 - Kiln Systems / Kiln Systems - Overview / 6. DRY PROCESS / 6.2 Raw Meal Suspension Preheater Kilns / 6.2.4 Precalciner Kilns

6.2.4

Precalciner Kilns

For larger production capacities, a larger portion of the pyroprocess had to be relocated out of the rotary kiln in order to maintain reasonable kiln diameters without excessive thermal load of the burning © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:33:48 PM

Page 20

"Holderbank" - Cement Course 2000 zone. The process of dissociation of CO2 (calcination) is suitable to take place in a static reactor outside of the rotary kiln. Of the total heat consumption, 60 to 65% are required to achieve about 90% of calcination. 100% calcination must be avoided because clogging problems will seriously disturb kiln operation (beginning of clinker formation). The development of this reactor started with a secondary firing in the kiln riser duct sufficient for 35 to 40% calcination of the meal, combustion air still pulled through the kiln tube (=air through). It was therefore referred to as precalciner (PC) type AT. Only when hot cooler air (= tertiary air) for the PC fuel (= secondary fuel) was taken to the calciner in a separate duct, the so called tertiary air duct, the full benefit of this technology could be used. Today, only this type called PC-AS (=air separate) is considered a real precalciner. The elements of a precalciner kiln system are explained in the sketch. The strongest boost of calciner development was in the seventies in Japan, initiated by the demand for very large units exceeding the potential of conventional kilns with suspension preheaters. Only precalciner technology makes today’s largest units of 10’000 t/d possible. Two process alternatives of precalciner are used: •

in-line calciner (calciner installed in kiln gas flow)

•

separate-line calciner (calciner not passed by kiln gases)

More details on calciner technology are contained in a separate section. The operating data are very close to the ones of the corresponding preheater kiln system. In-line calciners have a tendency to higher gas exit temperature and system pressure drop; however, modern units are equipped with 5 or 6 preheater stages to compensate for this. Characteristic operating data of 4 to 6 stage precalciner kilns: Heat consumption q small units, 4 stage SP


large units, 5 stage SP


SP exit gas temp. 6 to 4 st. SP

290° ... 370°C

SP exit gas volume

approx. 1.3 to 1.5 Nm3/kg cli


4 ... 6 kPa

Dust loss relative to clinker

8 ... 15%

Transition chamber: kiln gas temperature

approx. 1100°C

Material temperature

approx. 800°C


6/23/2001 - 4:33:52 PM

Page 21

"Holderbank" - Cement Course 2000 More data of precalciner kiln systems are shown in the section “Precalciners”.

HEAT BALANCE WET / SEMI-DRY / 4-ST. PREHEATER / 5-ST. PREHEATER-PRECALCINER

WET PROCESS kJ/kg cli

Input Fuel kiln combustion

%

SEMI-DRY LEPOL kJ/kg cli

%

4-STAGE SP kJ/kg cli

%

6-STAGE SP-PC kJ/kg cli

%

5560

96.7%

3343

97.6%

3150

97.7%

1180

39.2%

25

0.4%

15

0.4%

13

0.4%

5

0.2%

0

0.0%

0

0.0%

0

0.0%

1775

58.9%

sensible heat

0

0.0%

0

0.0%

0

0.0%

8

0.3%

Kiln feed sensible heat

25

0.4%

30

0.9%

54

1.7%

45

1.5%

73

1.3%

17

0.5%

0

0.0%

0

0.0%

sensible heat Fuel PC combustion

sensible heat of water


6/23/2001 - 4:33:52 PM

Page 22

"Holderbank" water- Cement Course 2000 Insufflated air (PA, cooler)

Total inputs

Output

67

1.2%

20

0.6%

6

0.2%

0

0.0%

5750

100%

3425

100%

3223

100%

3013

100%

kJ/kg cli

%

kJ/kg cli

%

kJ/kg cli

%

kJ/kg cli

%

Heat of formation

1750

30.4%

1750

51.1%

1750

54.3%

1750

58.1%

Water evaporation

2370

41.2%

506

14.8%

13

0.4%

8

0.3%

Exhaust gas sens. heat

754

13.1%

314

9.2%

636

19.7%

553

18.4%

Exhaust gas dust sens. heat

25

0.4%

21

0.6%

18

0.6%

29

1.0%

Clinker

59

1.0%

50

1.5%

63

2.0%

83

2.8%

100

1.7%

276

8.1%

423

13.1%

288

9.6%

0

0.0%

160

4.7%

77

2.4%

60

2.0%

- Precalciner (or bottom stage)

0

0.0%

0

0.0%

20

0.6%

20

0.7%

- Kiln (+tertiary air duct)

530

9.2%

200

5.8%

200

6.2%

200

6.6%

10

0.2%

92

2.7%

10

0.3%

10

0.3%

Water cooling

0

0.0%

42

1.2%

0

0.0%

0

0.0%

Other outputs

0

0.0%

0

0.0%

0

0.0%

0

0.0%

152

2.6%

14

0.4%

13

0.4%

12

0.4%

5750

100%

3425

100%

3223

107%

3013

100%

Cooler waste air Radiation and convection : - Preheater

- Cooler

Rest

Total outputs


6/23/2001 - 4:33:56 PM

Page 23


HISTORICAL DEVELOPMENT

Annex 1

The word cement is more than 2000 years old, but impure lime has been used much longer as a building material. It is historically established, that the Phoenicians used a pozzolanic lime about 700 B.C. and also the Romans produced some sort of cement or hard burned lime. From the medieval ages, it is known that in Holland a type of hydraulic cement was formed out of lime and tuff in dome shaped kilns. Our cement, as we know it today, is now more than 200 years old, “invented” by the Englishman John Smeaton in 1756. It was burned in bottle kilns. The better known inventor of Portland cement was Joseph Aspdin, who patented his burning process in 1824. He also used dome kilns of approx. 36 ft height and 17 ft diameter with a production of 90 bbl (= 15 t) per charge, each of which took several days to produce. Fuel consumption was 50% of clinker weight in coal which corresponds to 15’500 kJ/kg cli (= 3’700 kcal/kg cli). In 1880 an important step forward was made with the development of the continuously working shaft kiln, which had a much better heat economy. An example of such a kiln was the “Dietzsche Etagenofen” which is shown in Annex 1. From 1877 experiments have been conducted with rotary kilns. In 1897 Hurry and Seaman developed the first successfully operating unit of this type in America. These first rotary kilns were wet process kilns with a daily capacity of 50 to 100 tons. Their heat consumption was again very high (about 30% of clinker in coal = 9’500 kJ/kg cli) and they had an incredible dust emission (usually more than one third of the whole production). In order to decrease heat consumption, chain systems were installed in wet kilns to improve heat transfer during drying. Behind long dry kilns, waste heat steam boilers were arranged for the same purpose. It took almost another 30 years, before a further substantial reduction of heat consumption could be achieved by reducing the water content of the feed and by a better heat exchange in the preheating a calcining zone. In 1930 an officer of the army of the tsar, Dr. Lellep, took an important step in this direction. He developed the travelling grate pre-heater, which is fed with moist nodules. This invention was taken over by Polysius and got the name LEPOL kiln. Some years later, there was a Czech patent of a cyclone raw meal pre-heater, and in 1953 Kloeckner-Humboldt-Deutz AG in Germany installed the first suspension pre-heater system for raw meal. This type of kiln now became dominant because of its heat economy and nowadays other systems are only chosen in special cases. In former years, the main reason for the selection of the wet process was, that effective homogenisation of © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:33:57 PM

Page 24

"Holderbank" - Cement Course 2000 ground raw material was not possible except in the form of slurry. In developing special techniques for dry material homogenisation such as mix beds, mixing chamber silos etc., this factor could be eliminated. Utilising a rather old idea, since about 1966 especially Japanese cement machine manufacturers have designed several successfully working precalcining kiln systems. Calcination is already done in a stationary calciner system, where secondary firing is installed. By this means, it is possible to design kiln systems with a comparatively small rotary part diameter but a very large capacity up to more than 10’000 t/d. Kiln systems built after 1990 include 6-stage preheaters with up to 4000 t/d per string, pure air calciners, designed for a variety of fuels and emission control. Using modern low primary air burners, low pressure drop cyclone designs and high recuperation efficiency coolers allow further reduction of heat and power consumption.


6/23/2001 - 4:33:58 PM

Page 25

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Rotary Kilns

Rotary Kilns U. Gasser PT 98/14362/E 1. General 2. Kiln Dimensioning 3. Mechanical Aspects of Rotary Kilns 3.1 Riding Ring Fixation, Kiln Shell Ovality 3.2 Kiln Seals 3.2.1

Kiln Inlet Seal

3.2.2

Kiln Outlet Seal

3.3 Kiln Drive


6/23/2001 - 4:33:58 PM

Page 26

"Holderbank" - Cement Course 2000 SUMMARY After over 100 years, the rotary kiln is used in all cement plants for clinker production. The following properties made it superior to other principles: ♦ suitable to cope with high temperatures ♦ easy to be lined with refractory bricks due to its shape ♦ material transport behaviour ♦ tight to ambient ♦ mechanically relatively simple ♦ large units possible The rotary kiln must be designed for process, combustion and mechanical requirements.

♦ Characteristic figures: ♦ Length L [m] , diameter D [m] and their ratio L/D [-]

♦

♦

♦ Slope [°], speed range [min-1] and drive [kWh]

♦

♦ Dimensioning criteria:

♦ Volume load

♦ [t/(d m3)]

♦

♦ Burning zone load

♦ [t/(d m2)]

♦

♦ Thermal burning zone load

♦ [MW/m2]

♦ ♦ Important mechanical features are: ♦ riding ring fixation ♦ roller station / alignment ♦ seals at inlet and outlet ♦ drive ♦ ♦ With modern precalciner technology, outputs exceeding 10’000 t/d per kiln are possible with diameters still below the 6.5 m of the largest wet kilns. ♦ There is a trend towards short L/D kilns with only two piers mainly because of lower investment. Process Technology / B05 - PT II / C01 - Kiln Systems / Rotary Kilns / 1. GENERAL

1.

GENERAL

Today, all clinker producing installations of industrial size use a rotary kiln. The rotary kiln is still the only feasible way to manage this high temperature process with process material of varying behaviour. One exception is the vertical shaft kiln still used in some parts of the world, e.g. China, however, for © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:33:59 PM

Page 27

"Holderbank" - Cement Course 2000 small unit capacities only. The other exceptions are few pilot installations based on sintering in a fluidized bed reactor. Like many other great ideas, the rotary kiln was invented towards the end of the 19th century and has found application in many different industries. In 1987, Hurry and Seaman in the USA developed the first successfully working rotary kiln to produce cement clinker. The first rotary cement kilns were using the wet process with one very long kiln tube, making it the dominating single piece of equipment of a plant. With technological progress, the kiln sections used for for drying, heating-up and calcining have gradually been replaced by other types of equipment, the rotary kiln remains to be the most suitable type of machine for the clinkerization process. The rotary kiln has to satisfy three types of requirements: Combustion:

as a combustion chamber for burning zone fuel

Process:

as a reactor for the clinker burning process

(→ retention time)

as a material conveyor

(→ slope, speed)

Mechanical:

stability of shape, carrying load, thermal flexibility, tightness

Remarks: ♦ Even though the rotary kiln is a relatively simple piece of equipment, nobody has developed a complete theoretical/mathematical model of its behaviour and process which would allow correct process simulation and equipment design. ♦ The rotary kiln is still the “heart” of the entire production line. Its OEE (overall equipment efficiency) depending mainly on hourly output and availability, is decisive for the success of a plant. ♦ The rotary kiln is designed to operate 24 hours a day, and the rest of the equipment upstream and downstream has to follow. ♦ Being a major cause for production cost (mechanical maintenance, refractories), a well managed kiln is vital for a successful plant. Figure 1:

Old and new kiln


6/23/2001 - 4:33:59 PM

Page 28

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Rotary Kilns / 2. KILN DIMENSIONING

2.

KILN DIMENSIONING

The kiln dimensions are defined with diameter D (for kilns with different diameter: burning zone D) and length L: L [m] and D [m]

resp.

L/D [m]

♦ For cement kilns, the actual L/D ratio range is: from 40 (for long wet kilns) to 11 (for modern short kilns with precalciner) ♦ The diameter D is the inner diameter Di of the kiln (steel-) shell. ♦ Process technological dimensioning of a kiln is based on empirical figures and experience from existing installations One limiting factor for the diameter is the mechanical stability of the ‘arch’ of the brick lining. Maximum diameters which can be safely realised with standard size bricks are about 6,5 m. The largest kiln in the “Holderbank” group is 232 m (wet process, 3750 t/d). The following process technological dimensioning criteria are mostly used:

Clinker Production Net Kiln Volume

[t/(d m3)]

Specific Zone Load

Clinker Production Net Burning Zone Cross Section

[t/(d m2)]

Thermal Burning Zone Load

Burning Zone Heat Input Net Bruning Zone Cross Section

[MW/m2)]

Specific Volume Load

Specific volume load and thermal burning zone (BZ) load have no physical significance. They are merely defined to make existing installations comparable. The specific load is indirectly a gas velocity, because generating a certain amount of thermal energy by fuel combustion results in a proportional gas flow which can be calculated. The thermal BZ load per cross section is considered the limiting factor for a modern kiln system. For a certain length/diameter ratio, which is typical for each kiln type, the thermal BZ load it is proportional to the heat load on the inside of the lining surface which is one of the main influencing factor on brick life. The limit usually respected is: © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:00 PM

Page 29


Max. Thermal BZ Load = 6 MW/m2 (=5.16 x 106 kcal/m2 h)

Other absolute limiting values of all the three factors are not known. Each supplier seems to have his own rules of kiln dimensioning. Since no theoretical formulas have been derived to calculate the kiln size on an analytical basis, it is possible, that the present limits of the dimensioning criteria may be surpassed even for the conventional processes. Figure 2:

Long and short L/D kilns

Process Technology / B05 - PT II / C01 - Kiln Systems / Rotary Kilns / 3. MECHANICAL ASPECTS OF ROTARY KILNS

3.

MECHANICAL ASPECTS OF ROTARY KILNS

The following aspects of kiln mechanical design are relevant for the process: ♦ Riding ring fixation, kiln shell ovality ♦ Kiln seals ♦ kiln drive ♦ refractory lining (separate paper) ♦ nose ring (covered in “refractory lining”) Process Technology / B05 - PT II / C01 - Kiln Systems / Rotary Kilns / 3. MECHANICAL ASPECTS OF ROTARY KILNS / 3.1 Riding Ring Fixation, Kiln Shell Ovality

3.1

Riding Ring Fixation, Kiln Shell Ovality

A rotary kiln should be designed as cheaply as possible, yet it must still be rigid to guarantee minimum wear of the lining. This requirement can be met, if the deformation of the kiln shell is reduced to a tolerable limit. The parameter expressing shell deformation at a certain point is the kiln shell ovality

ω

:


6/23/2001 - 4:34:00 PM

Page 30

"Holderbank" - Cement Course 2000 Definition of

ω

ω=2 (a - b)

with 2a and 2b as the main axis of an ellipse

:

Investigations have shown, that generally a maximum relative ovality 0,3% is allowed This ovality may be subdivided into two amounts:

ω

of

a) Ovality of the riding ring 3 cm due to external forces allowed value:

ω <= 0.2% dr a) Ovality of the kiln shell due to deformations by its own weight in loose riding rings and due to increased temperature. The following two requirements must be met to keep the kiln ovality within the tolerable limits: ♦ The riding rings must be rigid enough ♦ The clearance between the ring shoes and the riding ring should be minimum during operation. The following table shows some practical values:

Riding Ring No.

1

2

3

4

Clearance during operation [mm]

3-4

3-4

4-6

5-6

maximum [mm]

10-15

Riding rings with splined fixation provide much better support of the kiln shell. Because the kiln shell is laterally suspended in adequately designed carrying bars, ovality is much reduced resulting in noticeably better brick life.

Such systems are currently available from Polysius and FLS, the latter one is lso offered as retrofit. Splined tire fixations are integral part of gearless kiln drive systems. Figure 3:

Tire fixations


6/23/2001 - 4:34:01 PM

Page 31


Process Technology / B05 - PT II / C01 - Kiln Systems / Rotary Kilns / 3. MECHANICAL ASPECTS OF ROTARY KILNS / 3.2 Kiln Seals

3.2

Kiln Seals

In order to avoid the danger of hot gases and dust leaking into the atmosphere, the entire kiln system is operated at negative pressure. The pressure profile starts at ambient (grate cooler: above first grate, satellite and tube cooler: fresh air inlet) and becomes increasingly negative towards the kiln induced draft (ID) fan. Instead of leaking out from within the process, there is now a problem with ambient air being sucked into the system, called false air. Depending on the point of entry, false air has different undesired effects. That is why a lot of effort is made to keep process systems tight. Process Technology / B05 - PT II / C01 - Kiln Systems / Rotary Kilns / 3. MECHANICAL ASPECTS OF ROTARY KILNS / 3.2 Kiln Seals / 3.2.1 Kiln Inlet Seal

3.2.1

Kiln Inlet Seal

The kiln inlet seal (inlet: referring to material flow) is at point with negative pressure of less than 10 mmWG (modern 2-support kilns) up to 100 mmWG (long wet kilns with chains). Modern kilns with low suction have high temperatures (up to 1300°C) instead. False air entering the system causes ♦ Additional gas to be handled by kiln ID fan and dedusting system ♦ Unnecessary cooling of hot process gases reducing value of heat Kiln inlet seals: ♦ Sealing force by pneumatic cylinders (pneumatic); sealing-rings ♦ Sealing force by coil springs/levers or weights (mechanical); sealing-segments ♦ Sealing force by leaf springs and rope with weight; lamella (fish scale) Kiln inlet seals must be equipped with a dust return scoop ring to avoid spillage of kiln feed. Note: The inlet seal is designed to seal against cold fresh air from outside, but it can be damaged if it must seal hot gas from inside to ambient in case of system overpressure! (this happens sometimes during the heating-up phase)


6/23/2001 - 4:34:01 PM

Page 32

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Rotary Kilns / 3. MECHANICAL ASPECTS OF ROTARY KILNS / 3.2 Kiln Seals / 3.2.2 Kiln Outlet Seal

3.2.2

Kiln Outlet Seal

With grate and tube coolers, the kiln outlet seal is installed between kiln head and rotary kiln where pressure should be slightly negative. Kiln outlet seals used with grate coolers must be designed to cope with pressure pulsation with occasional positive pressure. Outlet seal and nosering (brick retainer) with cooling air fan can be considered one system. Here, the loss generated by false air reduces recuperation from the clinker cooler. Cold ambient air replaces hot secondary air from the cooler which has to be vented. Outlet seals designed specifically for this application of the following type are available: ♦ Pneumatic ♦ Mechanical ♦ Lamella (fish scale) ♦ Labyrinth (outdated) With planetary coolers, false air reduces the amount of cooling air resulting in higher clinker temperatures. The outlet seal is smaller, at lower temperature and negative pressure only. Figure 4:

Kiln seals

Process Technology / B05 - PT II / C01 - Kiln Systems / Rotary Kilns / 3. MECHANICAL ASPECTS OF ROTARY KILNS / 3.3 Kiln Drive

3.3

Kiln Drive

Kiln drives are designed for speeds between 1.0 and 4.0 min-1, depending on slope, process and kiln dimensions. Long wet kilns are typically operated at the low end of this speed range where some new high performance kilns (short L/D with precalciners) are running at the upper end. For over 10 years, rotary kilns have been driven by girth and pinion type drives. Decisive for their performance are: ♦ Correct dimensioning ♦ Correct alignment (even load distribution on the flanks of the teeth; no peaks) ♦ Adequate lubrication system and lubricant quality © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:02 PM

Page 33

"Holderbank" - Cement Course 2000 With the new two support short kilns (L/D < 13) with long overhangs, kiln shell deformation and burning zone much closer to the drive, it became more difficult to ascertain correct alignment. Because of the determined load distribution on two piers, it became possible to avoid the girth drive by using the kiln rollers to transfer the torque to the riding ring: the gearless drive (=friction drive) was introduced. It is currently available from Polysius (POLRO) and FLS-Fuller (ROTAX). The following elements are part of this system: •

Two supports

for defined load on the driven tire

•

Splined tire fixation

for safe torque transmission to the shell

•

Self-aligning roller station

for linear load pattern between roller and tire (friction)

Today, there are only few kilns with friction in operation; the first one was Lägerdorf 11 by Polysius. Detail optimization and long term experience are yet to be awaited. Most systems have hydraulic drives for two rollers. This provides smooth operation, but is expensive, rather complex (hydraulic unit) and has higher power consumption. Electric direct drive of only one roller has been installed in one case. Figure 5:

Kiln drives


6/23/2001 - 4:34:02 PM

Page 34

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater

Suspension Preheater U. Gasser PT 98/14363/E 1. General 1.1 History 1.2 Trend 2. Heat Exchange in a Suspension Preheater 2.1 Counter-Current Heat Exchange (Shaft Stage) 2.2 Co-Current Heat Exchange (Cyclone Stage) 2.3 Thermodynamic Limits 3. Preheater Types 3.1 Preheaters with Shaft Stages 3.1.1

Pure shaft preheaters:

3.1.2

Hybrid preheaters:

3.2 Preheaters with Cyclone Stages 3.3 Economical Number of Stages for Cyclone Preheaters 3.4 Minimum Gas Velocity 4. Design Features of Preheater-Cyclones 4.1 General 4.2 Dust Cycles 4.3 Features 4.3.1

Splash Box

4.3.2

Dip Tube (Immersion Tube, Vortex Finder, Thimble)

4.3.3

Meal Flap

4.3.4

Cyclone Shapes

5. Preheater Operation 5.1 Operating Problems of Suspension Preheaters 5.1.1

Circulation Phenomena.

6. New Developments 6.1 Horizontal Cyclone 6.2 TRS


6/23/2001 - 4:34:03 PM

Page 35

"Holderbank" - Cement Course 2000 SUMMARY Practically all modern kiln systems are equipped with a cyclone suspension preheater. New installations include a precalciner with tertiary air duct, so that the preheater and precalciner have become one unit. However, the preheater has a specific task and is not principally connected to the precalciner. Modern low pressure drop cyclones are the result of a development which started in 1932. ♦ Shaft Preheaters: •

Counter-current heat exchange

•

Limited production (around 1000 t/d)

•

Disappointing heat exchange mainly due to poor meal distribution

♦ Hybrid preheaters: •

Combination of shaft and cyclone stages

•

Bühler-Miag, Polysius, Prerov, Humboldt

•

Shaft stages often replaced by cyclone stages

♦ Cyclone preheaters: •

Co-current heat exchange

•

Successful concept, predominantly used

♦ Industrial installations of cyclone preheaters: •

•

•

Precalciner kiln with 4 to 6 cyclone stages (contemporary technology): ∗ Large capacities possible > 10000 t/d ∗ Up to 4000 t/d in 1 string ∗ Heat consumption < 3000 kJ/kg possible (6 stages) ∗ Sensitive to circulation phenomena (-> kiln gas bypass!) ∗ ∗ ∗ ∗ ∗

4-stage cyclone pre-heater kiln (standard technology 1970 to 1980): Cyclone for raw meal preheating Large application world wide Capacities of up to 4500 t/d technically possible Heat consumption: 3150 to 3350 kJ/kg cli Sensitive to circulation phenomena (-> kiln gas bypass!)

∗ ∗

2-stage cyclone pre-heater kiln: Less sensitive to circulating elements than 4-stage pre-heater Higher heat consumption than pre-heater with more stages

♦ Most recent innovations: •

Horizontal cyclone for “low profile” preheaters (Polysius)

•

Dip tube add-on RTS for 30% lower cyclone pressure drop

Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 1. GENERAL

1.

GENERAL

Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 1. GENERAL / 1.1 History


6/23/2001 - 4:34:03 PM

Page 36

"Holderbank" - Cement Course 2000 1.1

History

With dry process, the heat exchange for heating up and calcination takes place between hot kiln gas and dry powder. Since the high dust losses from long dry kilns made it almost impossible to achieve acceptable heat consumption: other heat exchange principles had to be applied. Since the temperature range to be covered is below 1000°C, where the meal behaves normally like dry powder, stationary reactors where the meal is in suspension with the hot gas can be used. The first patent for a suspension preheater using four co-current cyclone stages was applied for in 1932 and issued in 1934 by the patent office in Prague to a Danish engineer employed by FLS. Even though the concept was entirely described in the patent, it took another 20 years for industrial application in 1951 by the company Humboldt, now KHD. Other developments using shaft stages have been abandoned and today, a suspension preheater is actually a cyclone preheater. Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 1. GENERAL / 1.2 Trend

1.2

Trend

All new kiln systems and the majority of the ones with start-up date after 1970 are equipped with cyclone pre-heaters. Gradually, older plants with wet kilns or long dry kilns are shut down for good due to their age as well as their high specific production cost The portion of world’s cement produced with kilns using suspension pre-heaters is still growing, as can be seen by the development of the “Holderbank” plants. It looks as if it will exceed 95% one day because no feasible alternative solution changing this development is in sight. In combination with pre-calciners, units of 10’000 t/d have been built using up to four strings, five stages. Typically, 3500 t/d can be handled in one string, in a recent project even 4000 t/d have been proposed.

Figure 1:

Figure 2:


6/23/2001 - 4:34:04 PM

Page 37


Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 2. HEAT EXCHANGE IN A SUSPENSION PREHEATER

2.

HEAT EXCHANGE IN A SUSPENSION PREHEATER

Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 2. HEAT EXCHANGE IN A SUSPENSION PREHEATER / 2.1 Counter-Current Heat Exchange (Shaft Stage)

2.1

Counter-Current Heat Exchange (Shaft Stage)

The most efficient type of heat exchange is the counter-current principle. The flows of the heat releasing media and the heat absorbing media are in opposite directions. This provides optimum the temperature difference (=temperature gradient, in theory allowing almost complete heat exchange. In case of a suspension preheater, where powder is suspended in a gas, the heat exchange takes place in a “reactor” vessel where the hot gas enters from below and leaves at the top. The meal to be preheated is fed at the top. The meal retention time depends on distribution across the gas flow and the retention time, which is determined by the gas velocity. In industrial installations, the heat exchange proved to be far below expected, because even distribution of the meal was not achieved, particularly not with large units. Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 2. HEAT EXCHANGE IN A SUSPENSION PREHEATER / 2.2 Co-Current Heat Exchange (Cyclone Stage)

2.2

Co-Current Heat Exchange (Cyclone Stage)

Co-current heat exchange takes place if both heat exchanging media flow in the same direction. Because of the rapidly decreasing temperature difference, the meal can never reach gas inlet temperature. Good and reproducible results in industrial installations with this type lead to the predominance of this principle in the cement industry. The heat exchanger is a gas duct with velocities from 10 to 20 m/s, equipped with good meal dispersion devices. The purpose of the cyclone is primarily to separate meal from gas, and not to exchange heat! Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 2. HEAT EXCHANGE IN A SUSPENSION PREHEATER / 2.3 Thermodynamic Limits

2.3

Thermodynamic Limits

Regardless of the type of heat exchange, there is always a thermodynamic imbalance between hot © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:04 PM

Page 38

"Holderbank" - Cement Course 2000 gases from kiln and calciner and cold raw meal. The heat contained available in the hot gas leaving the rotary kiln exceeds the heat required for heating the meal to the temperature levels required for calcination. Another limit must be observed: Because the temperature gradient between gas and meal (T gas > T meal) must always be maintained, a higher calcination degree than 30% cannot be achieved without additional heat input. The following heat balance estimate shall illustrate this: Heat contained in the gas:

2300 kJ/kg cli

(1100°C; 1.3 Nm3/kg cli) Heat to preheat meal to 850°C:

1300 kJ/kg cli

(1.6 kg meal /kg cli) Heat required for 30% calcination:

650 kJ/kg cli

Rest (ideal heat exchange):

350 kJ/kg

(corresponding to 200 °C)

This shows that even if the heat of the gas above 850°C is used for partial calcination (about 30%), there is still excessive heat in the gas which would correspond to 200°C gas temperature. It is obvious that even with a very large number of stages (with accordingly small temperature gradients), there will always be excess heat! This waste heat is lost only for the kiln system, but not for the plant, since it can be used for raw material drying in the mill.

Figure 3:

Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 3. PREHEATER TYPES

3.

PREHEATER TYPES

Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 3. PREHEATER TYPES / 3.1 Preheaters with Shaft Stages

3.1

Preheaters with Shaft Stages


6/23/2001 - 4:34:04 PM

Page 39

"Holderbank" - Cement Course 2000 The rather disappointing performance of the shaft stage made it virtually disappear from the market. Many hybrid preheaters were equipped with one or two cyclone stages replacing the shaft stage. Shaft stages at the kiln inlet have the advantage to be less sensitive to build-ups. This could be an advantage in cases where elevated sulfur input in the kiln system must be expected. Several Suppliers built preheaters using shaft stages. Two groups can be distinguished: Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 3. PREHEATER TYPES / 3.1 Preheaters with Shaft Stages / 3.1.1 Pure shaft preheaters:

3.1.1

Pure shaft preheaters:

Polysius:

ZAB Dessau:

•

GEPOL

•

Self-supporting structure (no tower required)

•

Vertical tube with restrictions

•

For small capacities (up to ca. 1000 t/d)

•

Some applications in Eastern Europe

•

Similar to GEPOL, but not self-supporting

•

The Deuna plant had originally 4 ZAB shaft preheaters

Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 3. PREHEATER TYPES / 3.1 Preheaters with Shaft Stages / 3.1.2 Hybrid preheaters:

3.1.2

Hybrid preheaters:

Several suppliers used a combination of shaft and cyclone stages: Polysius:

Bühler-Miag:

Prerov:

•

DOPOL preheater (first generation)

•

The central swirl-pot (second lowest stage) was shaft stage

•

Replaced by DOPOL 90 from 1990

•

Gradually developed into a cyclone preheater

•

Up to ca. 3000 t/d

•

Lowest stage was shaft stage

•

Later often replaced by cyclone stage

•

One large shaft stage with dedusting cyclone

•

Shaft stage selfsupporting

•

Additional cyclone stage possible


6/23/2001 - 4:34:05 PM

Page 40

"Holderbank" - Cement Course 2000 • Additional cyclone stage possible

MBM:

•

Separate meal duct to kiln

•

As sensitive to circulation phenomena as a cyclone type

•

Bottom shaft stage with 4 cyclone stages

•

Only hybrid design still on the market

Figure 4:

Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 3. PREHEATER TYPES / 3.2 Preheaters with Cyclone Stages

3.2

Preheaters with Cyclone Stages

A quasi counter-current heat exchange can be achieved by serial installation of several co-current stages. The result is the multi-stage cyclone preheater as it is generally applied in modern cement plants. In the early years, one and two stage systems have been installed with long kilns, often to avoid problems caused by circulating phenomena. A large number of plants are equipped with four stages; the majority of them were built before 1990. Today, five stage preheaters represent the economical optimum. High raw material moisture leads occasionally to fewer stages, in combination with low temperature dedusting systems, or in areas with high fuel cost, six stages can be more economical. Number of stages depends thus on: ♦ Raw material moisture (i.e. drying heat requirement) ♦ Cost of thermal energy ♦ Cost of electrical energy ♦ Gas handling system (temperature limit, dew point) ♦ Soil conditions (foundations, earthquake zone -> height of structure) If raw material moisture shows significant seasonal variations, it can be economical to equip preheaters with “variable stages”. This is achieved by feeding all or part of the meal to the second highest stage or by skipping a stage. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:05 PM

Page 41


Note:

Numbering of stages is always from top to bottom:

top stage

=

stage 1.

Exception:

bottom stage

=

stage 1

Polysius:

Figure 5:

Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 3. PREHEATER TYPES / 3.3 Economical Number of Stages for Cyclone Preheaters

3.3

Economical Number of Stages for Cyclone Preheaters

For many years, the pressure drop across one preheater stage was up to 1,5 kPa (15 mbar). The reason for the 4-stage pre-heater being so widely used is, that it represented an optimum between investment cost (structure height, foundation), pressure drop and heat consumption. . The performance of comparable systems built in about the same period are within a relatively narrow range. About two third of the pressure drop of a stage occurs in the cyclone and depends on its shape/design and the size, the latter being the determining cost factor. New cyclone designs are now on the market with only 0,5 to 1,0 kPa (5-10 mbar) pressure drop per stage. Considering increasing energy cost, it is justified to install 5 or 6 pre-heater stages for new or modified kiln systems. The following table indicated the estimated effect of a 5th and a 6th cyclone stage: 4 to 5st

5 to 6st

Heat consumption

kJ/kg cli

- 80

- 50

Exhaust gas temperature

°C

- 40 to -50

- 20 to -30

Exhaust gas quantity

Nm3/kg cli

- 0,03

- 0,015

Drying capacity in RM

% H2O

from 8 - 6,5

From 6,5 5,5

Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 3. PREHEATER TYPES / 3.4 Minimum Gas Velocity © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:05 PM

Page 42


Minimum Gas Velocity

Dimensioning of a cyclone preheater is a careful consideration of the importance of separation efficiency, pressure drop, part load operation capability, size of the preheater and cost of the project. It must be mentioned that there is a lowest gas velocity in a cyclone preheater. If operation results in lower figures, the meal will not be lifted by the gas anymore, resulting in poor heat exchange and consequently high heat consumption, but also excessive temperatures . Large dimensions give lower velocities with low pressure drop, but also limit the lowest possible economical production. Figure 6:

Polysius

Figure 7:

FLS

Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 4. DESIGN FEATURES OF PREHEATER-CYCLONES

4.

DESIGN FEATURES OF PREHEATER-CYCLONES

Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 4. DESIGN FEATURES OF PREHEATER-CYCLONES / 4.1 General

4.1

General

Modern preheaters are designed for low pressure drop using the new cyclone design which must still provide good separation efficiency, particularly in the top and the bottom stage. Cyclone inlet velocities © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:06 PM

Page 43

"Holderbank" - Cement Course 2000 are designed in the range of 10 to 15 m/s. It has been found that the total pressure drop of one cyclone stage is caused by about 1/3 by the gas duct (i.e. lifting of the meal) and 2/3 by the cyclone. Since not much can be done regarding lifting of the meal, efforts have been made to improve the cyclone design in order to reduce total pressure drop: the low pressure (drop) cyclone was designed. Cyclone design means to optimize between high separation efficiency, low pressure drop and low cost (i.e. small size). Other than having the correct design parameters, all stages should be equipped with ♦ Dip Tubes (also called ‘immersion tubes’, ‘thimbles’ or ‘vortex finders’) ♦ Meal flaps ♦ Splash boxes (or splash plates). Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 4. DESIGN FEATURES OF PREHEATER-CYCLONES / 4.2 Dust Cycles

4.2

Dust Cycles

The entire kiln system is subject to dust cycles. Precondition is gas flow in opposite direction of pulverized process materials. This causes wear, unnecessary material transport and heat losses due to heat exchange in the wrong direction. In the preheater, internal dust cycles due to poor separation efficiency of the cyclones result in less than optimum preheating of meal. Unfortunately, it is almost impossible to measure dust loss from lower cyclones in normal operation. The only indicator is the temperature profiles of gas and meal, but even the meal temperature is not always easy tp measure. Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 4. DESIGN FEATURES OF PREHEATER-CYCLONES / 4.3 Features

4.3

Features

Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 4. DESIGN FEATURES OF PREHEATER-CYCLONES / 4.3 Features / 4.3.1 Splash Box

4.3.1

Splash Box

Early cyclone preheater designs had no splash boxes. Instead, the meal was fed into the gas at a higher point against the gas flow, creating some turbulence with a certain distribution effect. Modern cyclone preheaters must be equipped with correctly designed splash boxes for optimum meal distribution across the gas duct cross section. The principle is based on impact on a plate. In some installations, the bottom plate of the splash box can be adjusted. Note: No splash box must be installed at the kiln inlet! The hot meal from the bottom cyclone must enter the rotary kiln as smoothly as possible. Meal is easily picked up by the kiln gas and will create a dusty transition chamber. Figure 8:


6/23/2001 - 4:34:06 PM

Page 44


Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 4. DESIGN FEATURES OF PREHEATER-CYCLONES / 4.3 Features / 4.3.2 Dip Tube (Immersion Tube, Vortex Finder, Thimble)

4.3.2

Dip Tube (Immersion Tube, Vortex Finder, Thimble)

This integral element of the cyclone has a decisive influence on separation and pressure drop. It makes the gas to follow a 180 to 360° rotation thus creating the desired centrifugal force for the separation effect. In the colder upper stages (stage 1 to 3) it can be designed as simple extension of the outlet gas duct, made from steel plate. These upper stage dip tubes create usually no problems except when the preheater gets overheated, e.g. during start-up. Then, the dip tube can collapse, causing excessive pressure drop. In the hotter lower stages, mild steel ducts from one piece are not suitable. Several segmented designs made from heat resistant steel or ceramic material (Hasle) are available on the market. It is standard today that all stages are equipped with dip tubes. Note: It appears that some designs of segmented dip tubes have a tendency to unhook enabling individual elements to drop and to block the cyclone outlet! For older plants, installing a segmented dip tube in the lower stages is a optimization possibility which is often applied. Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 4. DESIGN FEATURES OF PREHEATER-CYCLONES / 4.3 Features / 4.3.3 Meal Flap

4.3.3

Meal Flap

In order to understand the purpose of the meal flap, the following two aspects must be mentioned: ♦ There is a pressure difference across a cyclone stage, i.e. between two subsequent cyclone gas outlets (maintained by the ID fan). ♦ Without meal, there are two ways the gas can flow from one stage to the next: through gas duct and through meal duct If there was an ideal kiln system, i.e. a system with 100% constant meal flow and never changing operation parameters, the meal duct diameter could be designed for just the meal. The meal would then fill the entire cross section, leaving no opening for the gas. In reality, there are fluctuations of meal and dropping build-ups, requiring oversized meal ducts. It is the purpose of the meal flap to close the free cross section not used by the meal, to avoid gas © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:06 PM

Page 45

"Holderbank" - Cement Course 2000 bypass. There are designs that open only when a certain weight pushes them open, thus creating meal fluctuations. Other designs (see figure) are adjustable so that they move only in case of meal peaks or lumps. Not operational meal flaps cause heat loss and allow build-up formation in meal ducts (circulating elements)! Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 4. DESIGN FEATURES OF PREHEATER-CYCLONES / 4.3 Features / 4.3.4 Cyclone Shapes

4.3.4

Cyclone Shapes

The separation efficiency of a cyclone gets better with longer dip tube and increasing distance between swirl (cylinder) and dust collecting cone, i.e. with high and slim shapes. The top stage of preheaters is designed for high separation efficiency. In order to save height, most suppliers install twin cyclones with the drawback that meal and gas have to be split. There are a few plants from FLS with only one top cyclone, avoiding this drawback. Figure 9:

Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 5. PREHEATER OPERATION

5.

PREHEATER OPERATION

The performance of a preheater is assessed based on the criteria: ♦ Temperature profile (first indicator: exit gas temperature) ♦ Pressure profile ♦ Oxygen profile Table

Typical Gas Temperature Profiles 4 stages

5 stages

6 stages

SP

PC

SP

PC

SP

PC

Stage 1

°C

350

360

300

310

270

280

Stage 2

°C

540

570

490

500

440

460

Stage 3

°C

710

740

630

650

580

600

Stage 4

°C

840

870

750

770

690

710


6/23/2001 - 4:34:07 PM

Page 46

"Holderbank" Stage 4 °C - Cement 840 Course 8702000

750

770

690

710

Stage 5

°C

-

-

840

870

770

800

Stage 6

°C

-

-

-

-

840

870

Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 5. PREHEATER OPERATION / 5.1 Operating Problems of Suspension Preheaters

5.1

Operating Problems of Suspension Preheaters

Some reasons for poor preheater performance frequently experienced: ♦ Worn out or non existing immersion tubes (often in bottom stage) ♦ Open inspection doors, leaky gaskets or holes in the pre-heater (cold false air leaks in, can be detected by hissing sound) ♦ Blocked or non existing meal flaps ♦ No splash boxes (specially older preheaters), combined with not optimum position of meal feed point (e.g. old DOPOL) ♦ Excessive dust circulation due to poor separation efficiency of cyclones Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 5. PREHEATER OPERATION / 5.1 Operating Problems of Suspension Preheaters / 5.1.1 Circulation Phenomena.

5.1.1

Circulation Phenomena.

Cyclone preheaters are sensitive to circulation phenomena. Cyclone blockages cause kiln stops resulting in production loss and dangerous cleaning actions. Possible causes are: ♦ Excessive input via feed or fuel (Cl, S, 1 Na, K) ♦ Chemical unbalance (sulphur, alkali ratio) ♦ Unfavourable kiln/burner operation ♦ Unfavourable design geometry of bottom stage and kiln gas riser duct area Countermeasures known today allow to solve the problems are: ♦ Change feed composition or fuel quality ♦ Improve burning conditions ♦ Install automatic cleaning (air cannon, big blasters) at critical locations ♦ Change temperature profile by installing a small secondary burner ♦ Install a kiln gas bypass* system *A bypass system is not desirable since it is expensive and causes loss of heat and material. It is therefore the last solution left and should be only considered if all other measures are not sufficient. The paper ‘circulating phenomena’ contains more details on this rather complex subject. Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 6. NEW DEVELOPMENTS

6.

NEW DEVELOPMENTS


6/23/2001 - 4:34:08 PM

Page 47

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 6. NEW DEVELOPMENTS / 6.1 Horizontal Cyclone

6.1

Horizontal Cyclone

Polysius have developed a “horizontal cyclone”. (not to be mixed up with earlier designs of Kawasaki!) This cyclone is a modified version of the conventional cyclone with the major difference that the gas outlet is also at the bottom, encircling the meal outlet. The heat exchanger duct is still from bottom to top, but the stages can be arranged next to each other instead on top of each other. This allows a significantly lower height of the preheater structure. It is expected that savings in civil cost can be achieved. Additional benefit is possible in cases where the maximum height is restricted (earthquake zones, scenery protection). Only top cyclones on conventional preheaters are in industrial operation, however. Any other experience is yet to be made. Process Technology / B05 - PT II / C01 - Kiln Systems / Suspension Preheater / 6. NEW DEVELOPMENTS / 6.2 TRS

6.2

TRS

The Austrian company Zyklontechnik have introduced a dip tube add-on device which will reduce pressure drop across the cyclone (not the entire stage!) by 30% at otherwise unchanged performance. The principle is to avoid the flow around the edge of the dip tube. Instead, the horizontal swirl from the gas inlet is maintained and can continue into the dip tube through an accurately shaped slot in the TRS. Prerequisite is aerodynamically correct cyclone design and very accurate manufacturing of the TRS, which cannot be made locally. The device can be mounted to the bottom of a shortened dip tube. If the inspection opening is large enough, the whole unit can be installed in one piece, otherwise it comes in pieces. Several TRS are in operation (not in preheaters, however) with performance equal to or exceeding the predicted figures. Figure 10:

Figure 11:


6/23/2001 - 4:34:08 PM

Page 48



6/23/2001 - 4:34:09 PM

Page 49

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems

Precalcining Systems U. Gasser VA 93/4055/E 1. INTRODUCTION 2. THEORETICAL ASPECTS OF PRECALCINING 2.1 2.1 Calcining of Raw Meal 2.2 Combustion in Precalciner 2.3 Specific Heat Consumption 2.4 True and Apparent Calcination Degree 3. BASIC ARRANGEMENTS OF PRECALCINING SYSTEMS 3.1 AS and AT Systems 3.2 In-Line, Off-Line and Separate Line Calciners 4. FEATURES OF PRECALCINERS 4.1 Main Benefits of Precalciner Technology 4.2 Limitations and Restrictions 4.3 Tertiary Air Damper and Kiln Riser Orifice 4.4 Circulation Problems and Bypass with PC Kilns 5. PRESENT STATE OF PRECALCINER DEVELOPMENT 5.1 Calciners from FCB 5.2 Calciners from FLS - FULLER 5.3 PYROCLON Calciners (KHD) 5.4 PREPOL® Calciners (Polysius) 5.5 Prerov-Calciner 5.6 Conclusion 6. SYNOPSIS OF PRECALCINERS 7. TEST QUESTIONS


6/23/2001 - 4:34:09 PM

Page 50

"Holderbank" - Cement Course 2000 SUMMARY When burning cement clinker in a suspension preheater kiln, about 2/3 of the total heat consumed or about 2000 kJ/kg are required for the dissociation of CaCO3 also known as calcination. The idea of precalcination is, to let this process take place before the meal enters the rotary kiln by introducing that part of the fuel, i.e. up to 65%, into a stationary reactor. Because the combustion air (tertiary air) is drawn through a separate duct parallel to the kiln directly from the cooler, the rotary kiln operates at significantly lower specific thermal load and gas flow. The main advantages of precalcination are: ♦ More stable kiln operation due to better kiln control via two separate fuel feed/control points ♦ More stable kiln operation due to controlled meal conditions at kiln inlet ♦ Reduced thermal load of burning zone ♦ Higher kiln availability ♦ Longer life of burning zone refractories ♦ Larger capacity with given kiln dimensions, resp. smaller kiln for given capacity ♦ Possibility of increasing capacity of existing kilns ♦ More favorable conditions regarding circulating element problems ♦ Allows shorter kilns (L/D <12, 2 supports) ♦ Lower NOx emissions The drawbacks of higher gas exit temperature after the bottom cyclone and the preheater higher pressure drop can be compensated by five preheater stages and modern low pressure drop cyclones. There are three basic precalciner arrangements available from several suppliers: in-line, off-line and separate line, all with separate tertiary air duct. Being the key for complete combustion, the main design criteria is gas retention time: 2 to 3.5 sec minimum, depending on fuel reactivity, 0.5 to 1 sec more for in-line calciners. Systems where 10 to 20% of the fuel is introduced to the riser duct are considered secondary firings (SF) and not precalciners. Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 1. INTRODUCTION

1.

INTRODUCTION

The idea of separating the calcining process from the burning process was already described in a patent as early as 1912. However, the first industrial precalciner was built by Humboldt-Wedag (KHD) only in 1966 (Fig. 1). It was the Polysius kiln in Dotternhausen (Germany) which was equipped with a special 5-stage suspension preheater with extended riser duct. This riser duct had a larger diameter and the shape of a gooseneck to provide more length thus more gas retention time enabling combustion of oilshale, a locally available fuel containing raw material. The combustion air (tertiary air) was still drawn through the rotary kiln. Additional burners were installed later at the bottom of the precalciner chamber. Tube type calciners using the gooseneck design are still being used by KHD (Pyroclon) and Polysius © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:10 PM

Page 51

"Holderbank" - Cement Course 2000 (Prepol). So it is obvious that the precalciner (PC) kiln was developed from a straight suspension preheater (SP) kiln. The process characteristics (heat balance etc.) of both SP and PC kiln systems are quite similar, the main difference being the fact that in case of the PC kiln, 50 to 60% of the fuel (heat) is introduced via a chamber between kiln inlet and bottom cyclone. This allows to match the process heat requirements more evenly leading to significant improvements. Since true precalciners with 50 to 60% PC fuel ratio require a separate tertiary air duct, almost all PC kilns feature a grate cooler. The demand for larger and larger capacities which started back in the 1970ies led to a rapid development of the new precalciner technology. The fastest growing market asking for the largest units was in Japan where most of the clinker is produced in PC kilns. During that period, 12 competing suppliers developed their own precalciners, 8 of them were Japanese (see para „synopsis of precalciners“). After the home market for cement plants started to stagnate, the Japanese suppliers exported their know-how via licenses as well as entire plants. During the late 1980ies, where only few new plants have been constructed world-wide, the Japanese activities came to a stop. The latest development of precalciner technology was aimed at ♦ Complete combustion, also for low reactivity fuels ♦ Suitability for a wide range of fuels ♦ Low emissions of NOx Since the Japanese competitors have virtually disappeared on the international market, the variety of precalciner systems is reduced. Five European suppliers (FCB, FLS-Fuller, KHD, Polysius and Prerov) offer precalciners, some even a choice of alternative solutions. Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 2. THEORETICAL ASPECTS OF PRECALCINING

2.

THEORETICAL ASPECTS OF PRECALCINING

Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 2. THEORETICAL ASPECTS OF PRECALCINING / 2.1 2.1 Calcining of Raw Meal

2.1

2.1

Calcining of Raw Meal

Among all reactions taking place when burning clinker, the calcining - also called decarbonisation requires the highest amount of energy: the dissociation of carbonates, primarily calciumcarbonate according to the reaction

CaCO3 + heat → CaO + CO2 in the raw meal requires approx. 1.3 MJ/kg raw meal corresponding to 2.0 MJ/kg cli. The DTA-curves (Fig. 2) illustrate very well the importance of calcining within the clinker burning process. Fig. 2 Differential Thermo-Analysis (DTA)-curves of a typical cement raw meal


6/23/2001 - 4:34:10 PM

Page 52


During the process of heating up a raw meal, the calcining does not happen suddenly at a well defined temperature, but starts at about 600 - 700°C and ends between 900 and 1000°C, following a so-called „S curve“ (Fig. 3). Exact shape and position of this curve vary from raw meal to raw meal. Fig. 3 General aspect of the calcining curve of a cement raw meal

Not only the temperature, but also the retention time of the raw meal is an important parameter of calcining. While the heat transfer from gas to suspended raw meal in a preheater stage is achieved a fraction of a second, the complete calcination at a temperature of about 900°C in suspension requires a reaction time in the range of 2 to 12 seconds. However, as only 90 to 95% of the calcining should take place in the precalciner in order to avoid clogging problems, a residence time of about 1 to 3 seconds has proven to be sufficient. To perform both above mentioned tasks, i.e. to keep raw meal in suspension for a few seconds at 850 to 900°C in a stationary vessel without clogging, is the common process target of all PC systems. Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 2. THEORETICAL ASPECTS OF PRECALCINING / 2.2 Combustion in Precalciner

2.2

Combustion in Precalciner

The combustion in the precalciner takes place under quite different conditions compared to the main firing because: ♦ The temperature of the combustion environment is in the order of 850 to 900°C (flame temperature of the main firing: around 2000°C). ♦ Some PC systems (in-line systems) use an air-gas mixture for combustion (main firing: pure © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:10 PM

Page 53

"Holderbank" - Cement Course 2000 primary and secondary air) while others use pure air (off-line and separate line systems). ♦ In all PC systems preheated raw meal is suspended in the combustion air or air-gas mixture respectively in order to absorb the heat released thereby maintaining the temperature at a comparatively low level. By all means must Sintering of material avoided, as this would lead to clogging in the precalciner stage. On the account of the less favorable combustion conditions complete combustion is not always readily obtained, it requires a certain experience to achieve optimum performance. Of the various parameters influencing the combustion performance, the following are perhaps the more important ones: ♦ Good mixing of the fuel with the available oxygen. (This is particularly difficult to achieve with in-line calciners.) Optimum fuel dispersion into the gas flow (liquid fuel: atomization) is essential. ♦ Retention time for combustion has to be sufficient. The combustion must be completed in the PC stage. Otherwise, it will continue in the next stage (post-combustion) where the temperature level is lower and therefore less favorable for the calcination (see S-curve). This results in not optimum utilization of the heat which leads eventually to higher fuel consumption. ♦ The flow pattern of the air/gas mixture (resp. tertiary air) has to be favorable for the combustion. ♦ The meal distribution in the combustion zone has to be optimum, i.e. causing minimum distortion of the combustion. (CaCO3 as well as CO2 can also react with C - carbon from the fuel - to produce CO!). It is known from experience that too high concentrations of raw meal can seriously impede the complete combustion. With the introduction of the separate air duct for the combustion air for the calciner, the new term of tertiary air had to be introduced: Primary air:

Air introduced via kiln burner

Secondary air:

Air from cooler to kiln burning zone

Tertiary air:

Air from cooler to PC for combustion

Introduction of fuel between kiln inlet and bottom cyclone - as secondary firing or via precalciner necessarily increases the temperature level. The gas exit temperatures from the lowest stage of a straight preheater kiln is only 790 to 820°C as compared to precalciner kilns where this temperature increases by some ten degrees to 840 to 870°C. Therefore, the preheater exit temperature is also somewhat higher entailing an increased heat loss, which is more pronounced with 4-stage preheaters. The performance of PC systems can primarily be judged on two characteristic values: ♦ The temperature difference between gas and material ex precalcining stage should be as low as possible, so as to minimize the heat losses of the exit gas. The reaction temperature in the precalciner depends of course on the raw meal and the required precalcining degree as well as tolerated NOx level. ♦ Complete combustion must be achieved as this directly influences the overall heat consumption of the system. It must be mentioned that this is strongly influenced by the excess of air. •

Note: Stating the amount of unburned matter in the gas is therefore only meaningful to assess a calciner system, if the amount of oxygen in the gas is indicated as well.


6/23/2001 - 4:34:10 PM

Page 54

"Holderbank" - Cement Course 2000 Solid, liquid and gaseous fuels are successfully fired in PC kiln systems. However, the location and position of the burners in the precalciners have to be adapted to the fuel particularities. This is specially important for gaseous fuels, which seem to be more difficult to burn in the PC chamber than other fuels. Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 2. THEORETICAL ASPECTS OF PRECALCINING / 2.3 Specific Heat Consumption

2.3

Specific Heat Consumption

From the above mentioned it can be concluded that PC systems have a tendency to slightly increased heat consumption, unless countermeasures are taken such as: ♦ Although equipping existing preheater kilns with precalcination usually results in a slight increase of the heat consumption, the average (annual) heat consumption may be equal or even lower on account of a more regular kiln performance. ♦ Also for new installations the heat consumption is about 50 - 100 kJ/kg cli higher than for conventional preheater kilns with 4 stages. Where the somewhat higher exit gas temperature cannot be fully used, say for raw material drying then it has become standard to install one or two additional preheater stages to reduce the heat consumption to a figure slightly, for 6 stages noticeably, below that of a conventional 4st SP kiln. The first PC kiln in Dotternhausen was in fact equipped with a 5-stage preheater. Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 2. THEORETICAL ASPECTS OF PRECALCINING / 2.4 True and Apparent Calcination Degree

2.4

True and Apparent Calcination Degree

An important parameter for controlling the precalciner operation is the calcination degree. It is important to know the meaning of the true and the apparent calcination degree. True calcination degree: Degree to which the calcination is completed, i.e. extent to which the CO2 is dissociated from the CaCO3. Extremes:

Raw meal Clinker

0% (LOI = 35%) 100% (LOI = 0%)

In reality, the calcination degree is determined using a hot meal sample taken from the meal duct of the bottom cyclone. Because of always present dust cycles between kiln / kiln inlet / kiln riser / bottom cyclone, this sample contains a certain amount of dust which was already in the kiln calcining zone and is higher or even fully calcined. This sample is therefore a mixture consisting of „fresh“ meal and dust circulated back and has a higher calcination degree than the pure „fresh“ hot meal. This means: The higher the dust concentration near the kiln inlet resp. the dust cycle, the higher the apparent calcination degree. Apparent calcination degree: The calcination degree determined from a hot meal sample taken from the meal duct of the bottom cyclone. Fig. 5

True and Apparent Calcination Degree, PC Fuel, Dust


6/23/2001 - 4:34:11 PM

Page 55


Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 3. BASIC ARRANGEMENTS OF PRECALCINING SYSTEMS

3.

BASIC ARRANGEMENTS OF PRECALCINING SYSTEMS

Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 3. BASIC ARRANGEMENTS OF PRECALCINING SYSTEMS / 3.1 AS and AT Systems

3.1

AS and AT Systems

The first precalciner in Dotternhausen used combustion air which was drawn through the kiln as excess air. This technology was maintained for several years and is known as AT System. However, in reality only up to 35% fuel could be introduced to the precalciner thus limiting its benefits. The AT type is thus no longer considered a precalciner; it is rather used for secondary firings where a high calcination degree at the kiln inlet is not the main target. Fig. 7 AS and AT Systems

Today, all precalciners are AS Systems using tertiary air which is extracted from the kiln hood or from the cooler roof and drawn via a separate tertiary air duct parallel to the kiln to the precalciner. This means that planetary coolers are not compatible with precalcination technology (i.e. AS systems). Table 1

Comparison of AS and AT System Item

AS

AT

Portion of fuel to the precalciner

up to 65%

max. 35%


6/23/2001 - 4:34:11 PM

Page 56

"Holderbank" - Cement Course 2000 precalciner Largest kiln in operation

8500 t/d, φ 6.2 x 105 m

4700 t/d, φ 5.2 x 80 m

Kiln φ for given capacity (st SP approx. 75-80% = 100%)

approx. 85-90%

Suitable type of cooler

only grate or rotary

all types

Suitable for extension of existing SP kiln

poor (cooler, tertiary air duct)

very good for low PC rates

Burning conditions in rotary kiln

normal flame temperature (normal excess air)

lower flame temperature and stable operation due to high excess air

Thermal load in burning zone (4st SP = 100%)

approx. 60-70% at 60% PC

approx. 85-90% at 30% PC

Behavior regarding circulating elements

like 4st SP kiln

due to the high O2-content of the kiln atmosphere, reduced volatility of sulfur and therefore decrease of encrustation in transition zone and riser pipe

Heat loss at 10% bypass (4st SP = 100%)

approx. 40% (bypass will be smaller than in 4st SP kiln)

approx. 90% (bypass will have same size as in 4st SP kiln)

Exhaust gas temperature (4st preheater)

higher than 4st SP

higher than 4st SP

Heat consumption

slightly higher than 4st SP


Pressure loss over preheater

higher than 4st SP


Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 3. BASIC ARRANGEMENTS OF PRECALCINING SYSTEMS / 3.2 In-Line, Off-Line and Separate Line Calciners

3.2

In-Line, Off-Line and Separate Line Calciners

This criteria refers to the position of the precalciner in the kiln system installation and is illustrated with Fig. 8 below. ♦ In-Line Calciners are installed in the kiln exhaust gas flow which means that the combustion takes place in an air/kiln gas mix. This precalciner can be considered an enlarged kiln riser duct. ♦ Off-Line Calciners are installed off the kiln exhaust gas flow. The combustion takes place in pure (tertiary) air which is also responsible for lifting up the meal. ♦ Separate Line Calciners are off-line calciners with a separate preheater string. Fig. 8 Precalciner Arrangements


6/23/2001 - 4:34:11 PM

Page 57


Table 2

Comparison of Calciner Arrangements In-Line

Off-Line

Separate Line

PC arrangement

Extended riser duct

Parallel to riser duct

Parallel to riser duct

Combustion atmosphere

Kiln gas and air mix

Pure air

Pure air

Preheater string

1 to 4 of same size

1 to 4 of same size

2 to 4, 2 different sizes

Advantages

Low NOx version (reducing kiln NOx)

Suitable for modification

Two independent combustions → Easy combustion control

Excess air used for combustion

Good combustion

Good combustion

Suitable for lump fuel

Suitable for modifications

Suitable for modifications

Mixing of air with gas

Higher peak temperature (NOx!)

Higher peak temperature (NOx!)

Larger volume required

PC drop-out can fill TAD

PC drop-out can fill TAD

Weak points

Incomplete combustion

Asymmetry regarding circulating elements

Height requirement (depending on type/design)

Requires 2 strings (not feasible for <3000 t/d) Strings of different sizes (problem >7000 t/d)


6/23/2001 - 4:34:12 PM

Page 58

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 4. FEATURES OF PRECALCINERS

4.

FEATURES OF PRECALCINERS

Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 4. FEATURES OF PRECALCINERS / 4.1 Main Benefits of Precalciner Technology

4.1

Main Benefits of Precalciner Technology

There are many advantages of precalciner technology which made it state of the art today. Some of them are listed here: 1) More stable kiln operation due to better kiln control via two separate fuel feed/control points. 2) More stable kiln operation due to controlled meal conditions at kiln inlet. 3) Reduced thermal load of burning zone. 4) Lower brick consumption as a result of 1. and 3. 5) More than double capacities possible with given kiln (10’000 t/d with 6 m x 95 m kiln). 6) Possibility of increasing capacity of existing kilns. 7) Reduced volatilization of circulating elements. 8) Reduction of cycles (S, Cl, Na20, K2O) with smaller bypass rate, i.e. lower losses. 9) Makes short kilns possible with 2 stations, L/D < 12 10) Possibilities of NOx reduction. 11) Lump fuel utilization in some cases. Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 4. FEATURES OF PRECALCINERS / 4.2 Limitations and Restrictions

4.2

Limitations and Restrictions

Even though the advantages of precalciner systems are doubtlessly convincing, not all types can be used in all cases. Limitations are: ♦ Additional installation (fuel dosing, calciner, tertiary air duct) as well as the relatively smaller rotary kiln sets a lower economical limit to PC systems for new plants at around 1200 t/d. ♦ Alternative fuels containing hazardous components can only be used in the main firing due to the high temperature level there. The potential to use such fuels is then lower for PC kilns. ♦ Higher exhaust gas temperature and higher pressure drop can be a drawback in specific cases. ♦ Separate line calciners for new installations are only feasible if a 2-string arrangement is required for the capacity, i.e. above 3500 t/d. Fig. 9 Comparison of wet, SP and PC Kilns (average curves)


6/23/2001 - 4:34:12 PM

Page 59


Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 4. FEATURES OF PRECALCINERS / 4.3 Tertiary Air Damper and Kiln Riser Orifice

4.3

Tertiary Air Damper and Kiln Riser Orifice

Off-line calciners as well as in-line calciners are usually equipped with one kiln ID fan. In order to allow control of the tertiary air/secondary air ratio, there must be a control device in at least one of the two gas paths (kiln resp. tertiary air duct). For efficient warming up of the preheater, a damper is usually installed in the tertiary air duct to avoid fresh air to bypass the main flame. Very often, this damper is used also, for controlling the tertiary air flow (Fig. 10a). However, experience shows that high temperature and clinker dust require a quite refined design of this tertiary air damper. In many cases, this damper operates only for a short period without problems. Another approach is to install the control device in the other path. Some suppliers (e.g. FLS and Kawasaki) have developed a kiln riser orifice which is successfully operating in several plants. This solution (Fig. 10b) is generally more expensive than the TA damper above, but performs well. Fig. 10 TA Damper and Kiln Orifice

Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 4. FEATURES OF PRECALCINERS / 4.4 Circulation Problems and Bypass with PC Kilns

4.4

Circulation Problems and Bypass with PC Kilns

Precalciner kiln systems have two major advantages regarding circulation problems.


6/23/2001 - 4:34:12 PM

Page 60

"Holderbank" - Cement Course 2000 ♦ Reduced volatilization in the rotary kiln because less than 50% of the heat is introduced in the burning zone. ♦ Less than 50% thermal and dust losses in case of a bypass compared to a straight SP kiln. The volatilization of circulating elements occurs primarily in the rotary kiln. The percentage of the volatilized elements which can be extracted with a bypass depends on ♦ volatilization rate in the kiln, and ♦ amount of kiln gas extracted via bypass (= bypass rate) which is expressed by the ratio: bypass gas gas at kiln inlet The highest possible reduction of circulating elements at a given volatilization rate would be if 100% of the gases at the kiln inlet could be extracted. this is only possible in the case of a precalciner but not with a straight preheater kiln. Accordingly are the heat losses approx. 50 to 60% lower at a given reduction because the concentration of volatilized circulating elements in the gas at the kiln inlet is much higher than for a SP kiln. Fig. 11 Bypass for PC Kilns

Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 5. PRESENT STATE OF PRECALCINER DEVELOPMENT

5.

PRESENT STATE OF PRECALCINER DEVELOPMENT

Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 5. PRESENT STATE OF PRECALCINER DEVELOPMENT / 5.1 Calciners from FCB

5.1

Calciners from FCB

FCB have been IHI licensees since the mid seventies for in-line calciners resulting in 8 operating installations and 6 under construction. The highest capacity is 3300 t/d (Tourah, Egypt). Together with Ciments Français FCB have designed a new type of calciner with low emissions suitable for low grade fuels called the FCB low NOx PC (Fig. 12). In combination with a low NOx kiln burner, FCB expect to achieve 150 - 350 ppm NOx at the stack with their new calciner. The first industrial prototype is scheduled for 1992. The FCB calciner looks like a vertical reactor with one three channel burner on the top. Tertiary air is introduced from the top as well as with the meal on two sides. Flow is vertical from top to bottom. Meal © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:13 PM

Page 61

"Holderbank" - Cement Course 2000 can be proportioned via three points on two levels. FCB claim to achieve: ♦ Hot spot ♦ Reducing atmosphere zone → NOx reduction ♦ Controlled flame ♦ No separation of coal and meal Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 5. PRESENT STATE OF PRECALCINER DEVELOPMENT / 5.2 Calciners from FLS - FULLER

5.2

Calciners from FLS - FULLER

The FLS range of calciners will be marketed by both FLS and FULLER. Three basic air separate calciner systems are available: ILC, SLC-S and SLC (Fig. 13). All these use a vessel type calciner which provides retention time by means of volume. Experiences with this system made no conceptual changes necessary. The only modification to be mentioned is the new tangential tertiary air inlet for the ILC system which allows larger calciner volume without requiring more height. Main features of the FLS calciner systems presently available are: ♦ Variable kiln orifice (Fig. 14) for the SLC-S calciner to control the ratio of secondary to tertiary air in place of the often troublesome damper in the tertiary air duct. ♦ Low NOx version by splitting the tertiary air creating a controlled area of reducing atmosphere in the lower part of the PC which is horizontally divided in two zones by an orifice. ♦ Variation of the calciner outlet temperature with the SLC-S system without changing the preheater temperature profile providing a „temperature window“ for NH3 injection. Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 5. PRESENT STATE OF PRECALCINER DEVELOPMENT / 5.3 PYROCLON Calciners (KHD)

5.3

PYROCLON Calciners (KHD)

The calciner systems by KHD (and Polysius) are based on the 1965 Dottenhausen „goose neck“ design, a tube type calciner. As PYROCLON-R, a whole range of versions has been developed (Fig. 15). A low NOx version of the RP version is not available. KHD tackle the problem of CO from incomplete combustion with coal firing by focusing on improved coal dosification. Incomplete mixture of waste gases from kiln and calciner is often found with tube type calciners. In order to achieve a good mixture, an essential prerequisite for low NOx systems using excess fuel zones in the precalciner, the 180°C elbow is substituted by a new reaction chamber, called PYROTOP (Fig. 16).A PYROCLON-R Low NOx with PYROTOP allows: ♦ Complete combustion of the calciner fuel ♦ Temperature controlled zones (NH3 injection) ♦ Improved mixing of gases ♦ Reduction of NOx


6/23/2001 - 4:34:13 PM

Page 62

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 5. PRESENT STATE OF PRECALCINER DEVELOPMENT / 5.4 PREPOL® Calciners (Polysius)

5.4

PREPOL® Calciners (Polysius)

Polysius calciners are all of the air separate (AS) h-line-type. It is generally accepted today that the calcination process takes place within a few seconds making the fuel reactivity the decisive design criteria for the calciner size. The „goose neck“-tube type calciner PREPOL by Polysius is presently available in three basic configurations (Fig. 17). Several Polysius calciners have been modified by the company CLE who added an RSP type pre-combustion chamber. The same principle is now incorporated in the PREPOL AS-CC calciner by Polysius. Polysius started in 1985 to develop their NOx reducing technology called MSC based on experience available from power stations with staged combustion. They have adapted this method to the requirements of the clinker burning process. Trial operation on cement plants have shown 35 - 45% reduction of NOx. The idea is to create a limited zone of reducing atmosphere near the transition chamber by adding a small amount of fuel to the rotary kiln exhaust gas via a small burner in the riser duct. For the NOx from the calciner fuel, the same principle is applied resulting in a second reducing zone. Such a system would have the following fuel inputs: ♦ < 50% main burner ♦ < 10% via primary DeNOx burner ♦ > 30% via precalciner ♦ < 10% via secondary DeNOx burner Experience on an industrial scale only will prove the capability of this system. One of the difficulties is how to control the kiln atmosphere without the gas analysis sampled near the kiln inlet. Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 5. PRESENT STATE OF PRECALCINER DEVELOPMENT / 5.5 Prerov-Calciner

5.5

Prerov-Calciner

The Czek company Prerov have developed a new precalciner (Fig. 18). It consists of a precombustion chamber (KKS) and a reaction chamber (KKN) with a vortex chamber and is comparable to Polysius’ PREPOL-AS CC. During 1992, the first installation will be commissioned in Southern Italy. Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 5. PRESENT STATE OF PRECALCINER DEVELOPMENT / 5.6 Conclusion

5.6

Conclusion

The development of tube type calciners and vessel type calciners has moved them closer to each other. The tube type calciners have received a swirl pot or a pre-combustion chamber for improved mixing and fuel burning and the vessel type calciners have become longer. The calciner without separate air duct also known as „air through“ actually operating only with 10 - 20% of the total fuel never fulfilled the expectations and has virtually disappeared, together with the planetary cooler. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:13 PM

Page 63

"Holderbank" - Cement Course 2000 Low NOx calciners have been developed based on the principle of locally reducing atmosphere by means of fuel excess zones. It can be expected that NOx from precalciner combustion can be reduced to around 700 - 800 ppm. Calciners can be designed to reduce NOx generated in the burning zone, or to keep NOx generated in the calciner low, or both. Since further NOx reduction to lower levels require methods such as NH3 injection, temperature control is very important. A modern calciner can be described as follows:

Type:

in-line with pre-combustion chamber

Fuel ratio:

50 - 60% (include. low NOx fuel in case of staged combustion

Fuel dosing:

low fluctuation

Fuel types:

various, including alternative fuels

Combustion environment:

pure air or air/kiln gas mix

Calciner size criteria:

fuel reactivity gas retention time (up to 4 - 5 sec.)

Feature:

enhanced turbulence

Tertiary air:

staged for reducing zone

Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 6. SYNOPSIS OF PRECALCINERS

6.

SYNOPSIS OF PRECALCINERS

The different PC systems as well as their developers and suppliers are summarized in Table 3. During the 1970ies the cement manufacturers greatly contributed to the development of the Japanese PC systems: Until 1985, ot 304 kilns with PC, 83 were located in Japan, totaling 35% of the capacity. This shows the explosive expansion of PC systems in Japan back than. Today, all new kilns have precalciner with tertiary air duct. Table 3

Synopsis of PC Systems

Trade Name

Signification

PASEC SLC

Separate Line Calciner

SLC-S

Separate Line Calciner Special

ILC

In-Line Calciner

ILC-D

In-Line Calciner Downdraft

ILC-E*

In-Line Calciner, Excess Air

Prepol AS

Air Separate

Prepol AS-CC

Controlled Combustion

Prepol AS-MSC

Multi Stage Combustion

Prepol AT*

Air Through

Pyroclon R

Regular = Air Separate

Pyroclon RP

Regular Parallel


Developer & Licenser

Plant Supplier & Licensee

Voert Alpine / SKET

ACT

F.L. Smidth

F.L. Smidth

Krupp-Polysius

Krupp-Polysius

KHD Humboldt Wedag

KHD Humboldt Wedag

6/23/2001 - 4:34:13 PM

Page 64

"Holderbank" Course Pyroclon RP - Cement Regular Parallel2000 Pyroclon R Low NOx Pyroclon R Low Nox with Pyrotop Pyroclon S*

Special = Air Through

EVS-PC (only fuel - oil)

Echangeur à voie sèche avec précalcination

Fives-Cail Babcock

Fives-Cail Babcock

KKS-KKN

n.a.

Prero

Prerov

SF

Suspension Flash Calciner

Ishikawajima-Harima Heavy Ind. Chichibu Cement

Ishikawajima-Harima Heavy Ind. Fuller Company / Fives-Cail Babcock

NSF

New SF

RSP

Reinforced Suspension Preheater

Onoda Cement

Onoda Engineering & Consulting Kawasaki Heavy Industries Allis-Chalmers CLE-Technip

KSV

Kawasaki Spouted Bed and Vortex Chamber

Kawasaki Heavy Industries


NKSV

New KSV

MFC

Mitsubishi Fluidized Calciner

Mitsubishi Mining & Cement

Mitsubishi Heavy Industries

GG

Reduction Gas Generator

Mitsubishi Heavy Industries

DDF

Dual Combustion and Denitration Nihon Cement Furnance

Kobe Steel

CSF (CFF)

Chichibu Suspension Flash Calciner

Chichibu Cement

Chichibu Cement (own plants)

SCS

Sumitomo Cross Suspension Preheater and Spouted Furnace Process

Sumitomo Cement

Kawasaki Heavy Industries Ishikawajima-Harima Heavy Industries

*Air through: secondary firing systems

Process Technology / B05 - PT II / C01 - Kiln Systems / Precalcining Systems / 7. TEST QUESTIONS

7.

TEST QUESTIONS

1) Which is the chemical reaction with the highest heat consumption within the clinker burning process? How much does it consume in absolute terms (kJ/kg clinker) and in percent of the total heat consumption of a modern kiln system? 2) Which are the three basic precalciner arrangements and what are their differences? 3) At what temperature does the calcination take place and how much CO2 is totally dissociated from the CaCO3? 4) Which are the benefits of precalciner technology? 5) Which is the most important design criteria for precalciner dimensioning? 6) Explain the term „apparent calcination degree“. How can it be determined and what is its significance? 7) How do the effects of a bypass compare in case of a straight preheater kiln and a precalciner kiln? Fig. 1 Sketch of Dotternhausen Kiln, the first Precalciner (KHD, 1966)


6/23/2001 - 4:34:14 PM

Page 65


Table 4:

Temperatures and Process Steps for Clinker Burning

Temperature [°C]

Process Step, Type of Reaction

Heat

20 - 100

Evaporation of free H2O

Endo

100 - 300

Loss of physically absorbed H2O

Endo

400 - 900

Removal of structural water

Endo

Structural changes in silicate minerals

Exo

Dissociation of CO2 from CaCO3

Endo

> 800

Formation of intermediate products Belite, Aluminate and Ferrite

Exo

> 1250

Formation of liquid phase (aluminate and ferrite melt)

Endo

Formation of alite

Exo

Crystallization of liquid phase into mainly aluminate and ferrite

Exo

> 500 600 - 900

1300 - 1240

For numerical calculations, an approximate quantity of CO2 from the raw material (dissociated from the calcites) can be used, regardless of the exact chemical composition. CO2 from raw mat = 0.28 Nm3/kg cli

Table 5:

Energy Balance of Process Steps for Clinker Burning

Endothermic Processes:

kJ/kg cli

kcal/kg cli

Dehydration of clays

165

40

Decarbonisation of calcite

1990

475

Heat of melting

105

25

Heating of raw materials (0 to 1450°C)

2050

490

Total endothermic

4310

1030


6/23/2001 - 4:34:14 PM

Page 66

"Holderbank" - Cement Course 2000 Total endothermic

4310

1030

Exothermic Processes:

kJ/kg cli

kcal/kg cli

Recrystallization of dehydrated clay

40

10

Heat of formation of clinker minerals

420

100

Crystallization of melt

105

25

Cooling of clinker

1400

335

Cooling of CO2 (ex calcite)

500

120

Cooling and condensation of H2O

85

20

Total exothermic

2550

610

Net Theor. Heat of Clinker Formation:

kJ/kg cli

kcal/kg cli

Endothermic - exothermic

1760

420

Heat consumption of Kiln System:

kJ/kg cli

kcal/kg cli

Average 4-stage SP system

3300

790

Modern 6-stage SP system

3000

720

Rel. Heat Requirement of Calcination: Average 4-stage SP system

60%

Modern 6-stage SP system

66%

Fig. 12 FCB Low-NOx Precalciner

Fig. 13 FLS


6/23/2001 - 4:34:15 PM

Page 67


Fig. 14 FLS Adjustable Kiln Orifice

Fig. 15 Pyroclon

Fig. 16 Pyrotop


6/23/2001 - 4:34:15 PM

Page 68


Fig. 17 Polysius

Fig. 18 Prerov

Fig. 19 EVS-PC


6/23/2001 - 4:34:15 PM

Page 69


Features of EVS-PC PC system Supplier:

Fives-Cail Babcock

Fig. 20

SF / NSF

Fig. 21

RSP

Features of RSP PC system Suppliers: Onoda Engineering & Consulting Kawasaki Heavy Industries Allis Chalmers Creusot - Loire Entreprises © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:15 PM

Page 70

"Holderbank" - Cement Course 2000 Fig. 22

KSV / NKSV

Features of KSV/NKSV PC system Supplier:


Fig. 23

MFC

Fig. 24

GG

Features of GG PC system Supplier:

Mitsubishi Heavy Industries System abandoned


6/23/2001 - 4:34:16 PM

Page 71


DD

Fig. 26

CSF

Features of CSF PC system Supplier: Fig. 27

Chichibu Cement in own plants Voest Alpine PASEC System

Fig. 28 FLS


6/23/2001 - 4:34:16 PM

Page 72



6/23/2001 - 4:34:16 PM

Page 73

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers

Clinker Coolers U. Gasser / D. Brassel PT 97/14232/E (Revision 1, February 1999) 1. INTRODUCTION 2. GENERAL CONSIDERATIONS 2.1 Heat Flow in a Kiln System 2.2 Definitions 2.3 Calculations 3. GRATE COOLERS 3.1 The Reciprocating Grate Cooler 3.1.1

Principle

3.1.2

History

3.1.3

Conventional Grate Coolers (1980’s)

3.1.4

Typical Grate Cooler Problems

3.1.5

Modern Grate Coolers (1990’s)

3.1.6

Design Highlights of Modern Grate Coolers

3.1.7

Clinker Crushers

3.1.8

Cooler control

3.1.9

Cooler Dedusting

3.1.10 Developments 3.2 The Cross Bar Cooler 3.2.1

Principle

3.2.2

Main features

3.2.3

Strengths and Weaknesses

3.3 The Travelling Grate Cooler 3.3.1

Principle

3.3.2


4. ROTATING COOLERS 4.1 The Rotary Cooler or Tube Cooler 4.1.1

Principle

4.1.2

Design Features

4.1.3

Cooling performance


6/23/2001 - 4:34:17 PM

Page 74

"Holderbank" - Cement Course 2000 4.1.4

Strengths / Weaknesses

4.2 The Planetary Cooler 4.2.1

Principle

4.2.2

Historical

4.2.3

Design features

4.2.4

Internal heat transfer equipment (see Fig. 26)

5. VERTICAL COOLERS 5.1 The Gravity Cooler (G - Cooler) 5.2 The Shaft Cooler

SUMMARY Clinker coolers have two tasks to fulfil: ♦ Recuperate as much heat as possible from the hot clinker by heating up the air used for combustion ♦ Cool the clinker from 1400°C to temperatures adequate for the subsequent process equipment, normally to 100 - 200°C. There are mainly two different types of clinker coolers in operation with the following features: Grate coolers ♦ Crossflow heat exchange through horizontal clinker bed with cold air from below. ♦ Cooling airflow exceeding combustion air requirement allows low clinker temperatures, but necessitates excess (waste) air dedusting. ♦ Modern cooler technology with sophisticated plates and forced aeration systems allow combustion air temperatures exceeding 1000°C. ♦ Trend to wider and fewer grates, less cooling air and fixed inlets ♦ Largest units: 10’000 t/d ♦ Travelling grate (Recupol): last unit built around 1980 Rotating coolers ♦ Rotary tube coolers with separate drive or planetary cooler attached to kiln shell ♦ Quasi counter-current flow heat exchange ♦ Cooling air determined by combustion air, no waste air ♦ Heat exchange (recuperation) determined by condition of internal heat transfer equipment ♦ Limited unit size, up to 3000 t/d ♦ Planetary cooler not suitable for precalciner technology ♦ Practically no new installation built anymore Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 1. INTRODUCTION


6/23/2001 - 4:34:17 PM

Page 75

"Holderbank" - Cement Course 2000 1.

INTRODUCTION

The clinker cooler is a vital part of the kiln system and has a decisive influence on the performance of the plant. Three key indicators characterize a good cooler: ♦ Maximum heat recuperation ♦ Minimum cooling air flow ♦ Unrestricted availability There have been periodic changes in trends during the past decades. Grate coolers were first introduced by Fuller Company (USA) around 1930. While its design was continuously being optimized, the grate cooler became the predominant type in the 1950's. In the late 1960's, the planetary cooler gained popularity which reached its peak in the 1970's, mainly due to its simplicity. Larger unit capacities with precalciner technology made the grate cooler the preferred solution again. A wave of grate cooler reengineering starting in the mid 1980's has generated a much improved grate cooler technology as well as a new situation on the suppliers' side. New problems were experienced and have been or are being solved. Since cement plants have life cycles of 40 years and more, numerous units of each cooler type, planetary, rotary or grate cooler of old or new designs, will remain in operation for many more years. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 2. GENERAL CONSIDERATIONS

2.

GENERAL CONSIDERATIONS

The clinker cooler has the following tasks to fulfil: ♦ Process internal heat recuperation by heat transfer from clinker to combustion air ♦ Reduce clinker temperature to facilitate clinker handling and storage ♦ Provide maximum cooling velocity to avoid unfavorable clinker phases and crystal size Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 2. GENERAL CONSIDERATIONS / 2.1 Heat Flow in a Kiln System

2.1

Heat Flow in a Kiln System

The importance of the cooler as a heat recuperator can be well demonstrated with a heat flow (Sanki) diagram. Figure 1

Clinker cooler and kiln system


6/23/2001 - 4:34:17 PM

Page 76


Figure 2

Energy turnover (Grate cooler)

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 2. GENERAL CONSIDERATIONS / 2.2 Definitions

2.2

Definitions

♦ As for other components of the kiln system, specific figures for clinker coolers refer to 1 kg of clinker. This eliminates the influence of plant size and allows direct comparison of clinker coolers of different types and sizes. ♦ Cooling air is the air which passes the clinker thus being heated up while cooling the clinker. It corresponds approximately to the combustion air requirement, only grate coolers allow additional air for better cooling. ♦ Primary air is the air which is required for proper functioning of the burner. Ambient air insufflated by a separate small fan plus the air from a pneumatic transport system, amounting from < 10% up to > 30% of the air required to combust that fuel. Some precalciner burners are equipped with primary air fans (for cooling) as well. ♦ Secondary air is the hot air entering the rotary kiln via clinker cooler. Its flow is determined by the combustion of the burning zone fuel. While cooling the clinker, it reaches temperatures of 600 to over 1000°C, depending on type and condition of the cooler. ♦ Tertiary air is that part of the combustion air which is required for combusting the precalciner fuel. It is extracted from kiln hood or cooler roof, and then taken along a duct (=tertiary air duct) parallel © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:17 PM

Page 77

"Holderbank" - Cement Course 2000 to the kiln to the precalciner. It reaches temperatures near or equal to the level of the secondary air. ♦ Middle air (grate cooler only) is extracted from the cooler roof if drying of process materials requires a temperature level which is higher than the waste air. If the quantity is small, up to 450°C can be expected at normal cooler operation. ♦ Waste air (grate cooler only) is also called cooler exit air or cooler excess air. The total cooling airflow from the fans is normally higher than the flow required for combustion (=tertiary + secondary air). The extra air, which has normally a temperature of 200 to 300°C, must be vented to ambient via a dedusting system. ♦ False air is cold air entering the system via kiln outlet seal, burner opening, casing or clinker discharge. It either dilutes secondary air thus reducing recuperated heat or adds load to the waste air system of grate coolers. ♦ Specific air volumes are airflows per kg of clinker (m3/kg cli, Nm3/kg cli). Independent of the kiln size, airflows of cooler systems can be directly compared. ♦ Specific loads express the relation of clinker production to a characteristic dimension of the cooler (t/d m, t/d m2, t/d m3). Exact definitions vary with cooler type. ♦ Radiation losses from the cooler casing/shell are particularly important for planetary coolers, where they actively support the cooling of the clinker. ♦ Efficiency expresses the quality of heat transfer from clinker to the air which is used for combustion in the burning zone and precalciner firing. Remark: Since the heat recuperated is proportional to hot air used for combustion and temperature, an efficiency figure is only meaningful if it is related to a heat consumption figure (resp. a combustion airflow). Figure 3

Clinker coolers - Definitions

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 2. GENERAL CONSIDERATIONS / 2.3 Calculations

2.3

Calculations

The calculations below are examples of heat balance investigations:


6/23/2001 - 4:34:18 PM

Page 78

"Holderbank" - Cement Course 2000 •

Heat in hot clinker Qcli :

Qcli = mcli* cpcli* (tcli - t ref)

Example with mcli =1 kg/h: tcli = 1400°C: Qcli = 1 kg/h * 1.090 kJ/kg°C * (1400°C-20°C) = 1504 kJ/h

•

Heat in hot air Qair :

Qair = Vair* cpair* (tair - t ref)

Example with V air = 1Nm3/h: tair = 1066°C: Qair = 1 Nm3/h * 1.421 kJ/Nm3°C * (1066°C-20°C) = 1486 kJ/h

•

Radiation loss Qrad :

Q rad =CR * ε * A {(t/100)4 4

(t0/100) }

Grate cooler Qrad = 20 kJ/kg cli (from experience)

Cooler efficiency ηcooler

ηcooler =

Q combustion air ∑ Qloss = 1− Q clinker from kiln Q clinker from kiln

The secondary (+ tertiary) air requirements are dictated by the amount of fuel fed to the burners. Per this definition, the efficiency of a cooler is getting better with increasing kiln heat consumption. It is thus obvious that a cooler efficiency figure is only meaningful if the corresponding heat consumption (or airflow) is indicated. Example:

production

5000 t/d

heat consumption

3000 kJ/kg cli

secondary and tertiary air temperatures

1066°C

Primary air main burner

10%

PC fuel ratio

60%

False air and excess air neglected (not realistic!) Q comb air: V Comb air

= 3000 MJ/kg cli * 0.26 Nm3/MJ * 5000/24*103 kg/h * (1-0.4*0.1) = 156'000 Nm3/h

t comb

air

Q comb air

= 1066°C → q combustion air = 1.421 kJ/Nm3° * (1066-20)° = 1486 kJ/Nm3 = V comb air * q comb air = 1486*156'000 kJ/h = 231'816 GJ/h

Q clinker: m clinker

= 5000 t/d /24 h/d *103 kg/t = 208'333 kg/h


6/23/2001 - 4:34:18 PM

Page 79

3 m "Holderbank" Cement Course 2000 =-5000 t/d /24 h/d *10 kg/t = 208'333 kg/h clinker

t clinker from

= 1400°C → q clinker from kiln = 1.09 kJ/kg° * (1400-20)° = 1504 kJ/kg

kiln

Q

clinker

Efficiency η

Figure 4

= 208'333 * 1504 kJ/kg = 313'333 GJ/h = 231'816 / 313'333 * 100% = 74.0%

Clinker cooler typical data (4-stage SP Kiln, 2’000 t/d)

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS

3.

GRATE COOLERS

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler

3.1

The Reciprocating Grate Cooler

The reciprocating grate cooler is the most widely applied type and is exclusively used for new plants. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.1 Principle

3.1.1

Principle

♦ The following major system components can be distinguished: •

Casing with kiln hood and connections for air at different temperature levels

•

Reciprocating grate with drive system

•

Aeration system with fans, undergrate compartments and direct air ducts

•

Riddling (= fall through) extraction system with hoppers, gates and transport

•

Clinker crusher

♦ Material transport The clinker is pushed by the vertical part of the front edge of the preceding plate. The entire grate consists of a combination of fixed and moving rows which results in a quasi-continuous motion of the clinker bed. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:19 PM

Page 80

"Holderbank" - Cement Course 2000 ♦ Heat exchange Heat exchange from clinker to air is according to the cross current principle. The cooling air penetrates the clinker bed which is laying on the grate from underneath and leaves it at the surface. While passing through the hot clinker, the air is accumulating heat which is transferred from the clinker. ♦ Cooling air Normally, ambient air is blown to underneath of the grate plates loaded with clinker by a number of cooling air fans. Delivery pressure must be sufficient to penetrate the clinker bed and to compensate for the expansion (increase of actual volume) of the air from heating it up Under ideal conditions, the required cooling air depends directly from the desired clinker temperature. One part of the cooling air is used for combustion in the kiln, the rest is cleaned and vented to ambient, unless it is further used, e.g. for drying. ♦ Cooling curve A simplified mathematical model for clinker cooling in a conventional, optimized grate cooler gives the relation between cooling air quantity and clinker temperature as follows:

T cli− Tamb = exp[− (Vair / 0.77)] Tcli in − Tamb with

= clinker temperature at cooler inlet

°C

T amb

= ambient temperature

°C

V air

= cooling air quantity

Nm3/kg cli

T cli in

The above approximation (curve Fig. 17: Tcli = 1400°C) has been found to give satisfactory results for conventional grate coolers from various suppliers. Figure 5

Reciprocating Grate Cooler: Design Features

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.2 History

3.1.2

History


6/23/2001 - 4:34:19 PM

Page 81

"Holderbank" - Cement Course 2000 It was the Fuller Company (USA) who introduced the first reciprocating grate cooler in the late 1930's with a grate slope of 15°. Fluidized material running down the grate leads to 10° grate inclination. The 10° cooler was predominantly used until the mid 1950's. Problems were encountered with those 10° coolers when the clinker was fine and started to fluidize. As an attempt to solve this problem, wedge grate plates were used. Another drawback of those 10° coolers was the building height required for larger units. In the mid 1950's, the first horizontal grate coolers were introduced. They were initially just 10° grates installed horizontally with accordingly reduced conveying capacity. Some of these coolers were severely damaged by overheating, due to fluidization and accumulation of hot fine clinker at the feed end. This drawback of the horizontal cooler lead to the development of the so-called combi cooler. Is has one (or formerly two) inclined grates with normally 3° slope, followed by one or two horizontal grates. Not all suppliers followed the same philosophies, so all three concepts (all horizontal, combi and all inclined) can be found all over the world. The planetary cooler boom period in the 1970's came to an end, when large production capacities were in demand. Precalciner technology required grate coolers which eventually needed to be reengineered again. Problems related to the clinker distribution, growing awareness of heat and power consumption as well as the demand for higher availability forced the suppliers to introduce new solutions. Initiated by the new company IKN, the grate cooler technology underwent significant changes since the mid 1980's. Modern grate plates, forced (direct) aeration and better gap design were introduced by all cooler makers helping to reduce cooling airflow and cooler size. The new approach lead to better recuperation in most cases. However, serious wear problems with the new systems forced most of the companies to modify their solutions once again. Today, in the mid 1990's, we are still gaining experience with latest designs. The ultimate solution would be the waste air free grate cooler with unlimited flexibility and availability. However, right now the cement industry would be happy with smooth operation, high recuperation, low cooling air and no cooler related kiln stops. Figure 6

Various configurations of reciprocating grate coolers

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.3 Conventional Grate Coolers (1980’s)

3.1.3

Conventional Grate Coolers (1980’s)


6/23/2001 - 4:34:19 PM

Page 82

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.3 Conventional Grate Coolers (1980’s) / 3.1.3.1 Typical Design Features

3.1.3.1

Typical Design Features

♦ Grate plates with round holes ♦ Two to three grates, depending on size ♦ Grate slope 0° or 3° or both, depending on supplier ♦ Mechanical excenter drives for reciprocating grate ♦ Chamber aeration ♦ Fan pressure 45 mbar (first) to 25 mbar (last) ♦ Smaller compartments at inlet, larger towards outlet ♦ Clinker riddling extraction with hoppers, gates and dragchain (some earlier designs: internal drag chain without hoppers) ♦ Hammer crusher at cooler discharge World’s largest kilns (10'000 t/d in Thailand) are equipped with conventional grate coolers from CPAG with 4 grates. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.3 Conventional Grate Coolers (1980’s) / 3.1.3.2 Strengths and Weaknesses of Conventional Grate Coolers

3.1.3.2

Strengths and Weaknesses of Conventional Grate Coolers

Strenghts

Weaknesses

•

Lower clinker end temperature due to • higher amount of cooling air

Waste air handling system (dedusting, fan) required

•

Possibility of adjusting cooling air and • grate speed provides higher flexibility

More complex cooler requires higher capital investment

•

Optimization possibilities during operation


•

Higher power consumption than planetary or tube cooler

•

Uneven clinker discharge / segregation leads to several problems

•

Red river

•

Snowmen

•

Air breakthrough (bubbling, geyser)

•

Reduced plate life

•

Excessive clinker fall through between gaps

6/23/2001 - 4:34:20 PM

Page 83


Causes and mechanism of those problems are further explained in the next paragraph. Figure 7

Conventional grate coolers: Design features

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.4 Typical Grate Cooler Problems

3.1.4

Typical Grate Cooler Problems

Most grate coolers show a tendency to one or more of the system inherent problems, and in many cases there is no real cure. Investigations of the causes lead to the development of the modern cooler technology. ♦ Segregation: Due to its physical properties, the clinker is lifted by the kiln rotation before it is discharged into the cooler. Installation of the grate axis offset from the cooler axis should compensate for this effect. However, since discharge behavior of finer and coarser clinker particles differ from each other, the clinker fractions are not evenly distributed across the grate. Fines are discharged later and are thus found predominantly on the rising side of the kiln shell (Fig. 8a). ♦ Thin clinker bed in recuperation zone: With a conventional grate cooler with chamber aeration, the clinker bed thickness is limited directly by the installed cooling fan pressure and indirectly by the quality of compartment seals and distribution of the clinker across the width. In order to avoid overheated plates, the operator will set the bed not higher than allowed to guarantee airflow through the plate carrying the clinker with the highest bed resistance. Thin bed operation leads to unfavorably high air to clinker ratio and poor heat exchange on the sides with consequently low recuperation efficiency. ♦ Red river: The infamous red river is one of the most feared problems with grate coolers. Due to segregation, fine clinker has always its preferred side (see above). Different bed resistance on either side and only one air chamber across the entire width often cause fluidization of the fine clinker laying on top. This fluidized clinker does no longer follow the speed of the grate, but shoots much faster towards the cooler discharge end. Because the residence time of that fine clinker is much reduced, it does not follow the general cooling curve and © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:20 PM

Page 84

"Holderbank" - Cement Course 2000 forms a red hot layer on top of the regularly cooled, already black clinker. Hence the term "red river". It is not the missed heat recuperation, but the red hot material being in touch with cooler walls, plates and side seals in the colder area where such temperatures should normally not occur. Premature destruction of those pieces results in poor availability, high maintenance and ultimately in loss of production and sales revenues. ♦ Snowman: The sticky consistence of the hot clinker leaving the kiln combined with the compaction at the drop point often leads to formation of solid clinker mountains on the grate. Not permeable for cooling air, they grow larger and disturb the flow pattern of the clinker in this anyway critical inlet area. ♦ Air breaking through: Due to the different resistance of the clinker bed and the fear of overheated plates, too much air is put on the first grate compared to the clinker bed. The result is air shooting through the bed, hardly taking any heat and thus not contributing to the heat exchange. In addition to that, the clinker is mixed which can be seen by the bubbling action, and the layered clinker bed (colder clinker below, hotter on top) is destroyed thus disturbing the cross flow heat exchange pattern. The results are low recuperation and too much heat going to the aftercooling zone. ♦ Figure 8a: Segregation at cooler inlet

Figure 8c

Red River


Figure 8b: Clinker bed depth effect on cooling

Figure 8d

6/23/2001 - 4:34:21 PM

Snowman

Page 85


Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.5 Modern Grate Coolers (1990’s)

3.1.5

Modern Grate Coolers (1990’s)

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.5 Modern Grate Coolers (1990’s) / 3.1.5.1 Design Features

3.1.5.1

Design Features

The successful clinker cooler has:

1) À Correct allocation of cooling air to clinker Á Sustainable gap widths in the entire cooler All new or redesigned clinker coolers are aiming at the above two goals: ♦ Modern grate plates, designed to cope with high temperature differences ♦ Inclined inlet section without moving rows ♦ Pattern of zones for individually adjustable aeration in recuperation zone ♦ Modern plates for a tight grate in the after cooling zone ♦ New, improved side seal plate design for tight gaps and low wear ♦ Careful undergrate compartment sealing ♦ Adequate seal air system with correct control ♦ Wider and shorter coolers; lower number of grates ♦ Improved and wear protected moving grate support and guidance ♦ Hydraulic grate drive with optimized control system ♦ Cooling air fans with inlet vane control and inlet nozzle for measuring flow ♦ Roller crusher Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.5 Modern Grate Coolers (1990’s) / 3.1.5.2 Strengths and Weaknesses of Modern Grate Coolers

3.1.5.2

Strengths and Weaknesses of Modern Grate Coolers

Strenghts


Weaknesses

6/23/2001 - 4:34:21 PM

Page 86

"Holderbank" - Cement Course 2000 Strenghts

Weaknesses

•

More constant heat recuperation → improved, smoother kiln operation

•

More complicated mechanical installation (varies with supplier)

•

Cooler inlet: improved clinker distribution across grate width

•

Higher secondary air temp. increases wear of nose ring and burner refractories

•

Higher actual (m3/h) tertiary air flow can increase dust entertainment at take off point

•

Teething problems with new designs -> design changes still in progress

•

Elimination / control of red river

•

Significantly reduced grate riddlings (clinker fall through)

•

Higher waste air temperature (valuable for drying)

•

Lower heat consumption due to higher heat recuperation (cooler efficiency)

•

Reduced power consumption due to less waste air

•

Lower civil cost due to more compact cooler

•

Lower investment due to smaller waste air system

•

Reduced cost for maintenance

Figure 9

Modern Grate Coolers: Design features

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.6 Design Highlights of Modern Grate Coolers

3.1.6

Design Highlights of Modern Grate Coolers

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.6 Design Highlights of Modern Grate Coolers / 3.1.6.1 Modern Grate Plates

3.1.6.1

Modern Grate Plates


6/23/2001 - 4:34:21 PM

Page 87

"Holderbank" - Cement Course 2000 In the mid 1980's, the first modern grate plates were installed in grate coolers by IKN and CPAG. They were designed for the following targets: ♦ Allow for lower air/clinker ratio in the recuperation zone for higher recuperation ♦ Improve clinker distribution across the grate width ♦ Assure that all grate plates are always sufficiently cooled by air The above targets were reached using the following ideas: •

Higher built-in pressure drop Similar to the effect of thick bed operation, a higher pressure drop across the plate reduces the relative influence of variations in permeability of the clinker bed.

•

No more fine clinker falling through Fine clinker falling through means loss of heat and thermal stress on the drag chain. For forced aeration (below) it is mandatory that no material can fall in the air ducts where it would cut off the air supply.

•

Forced (direct) aeration via air ducts In order to ensure that all plates get enough air, to allow individual allocation of air to different areas and to avoid that air escapes through gaps, groups of plates are supplied with air directly via a special duct system

•

Tight gaps between plates and plates/casing Not only through the grate surface, but also through gaps between plates within the same row as well as from one row to the next, fine clinker can fall through. Those gaps have to be sealed as well, e.g. by interlinked steps in the plate sides (Fuller, Polysius) or by bolting them together as packages (IKN).

The modern grate plates are the basis of modern cooler technology. Problems experienced with the first generation of modern grate plates lead to several detail modifications: ♦ Cracks in corners of air outlet openings → Solution: modified shape ♦ Plastic deformation caused premature failure with many designs → Solution: thermally flexible plates built from two or more pieces ♦ Preferred plate internal airflow left plates locally uncooled → Solution: plate internal guide vanes, optimized air channelling ♦ ♦ Modern grate cooler, as the IKN Pendulum Cooler, use also Pneumatic Hopper Drains (PHD) to withdraw the fine clinker fall through. Figure 10

Modern grate plates


6/23/2001 - 4:34:22 PM

Page 88


Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.6 Design Highlights of Modern Grate Coolers / 3.1.6.2 Air Ducts

3.1.6.2

Air Ducts

The concept of forced aeration, i.e. the idea to bring the air directly to the grate plates requires a flexible air connection between the (stationary) fan and the moving rows. Initially, the most obvious and simple approach was chosen: flexible hoses or bellows. IKN, CPAG, Polysius and Fuller used this solution at the beginning. However, experience showed that those hoses were sensitive to design (geometry), installation and material qualities. While many coolers operated without any problem, others showed frequent rupture of those hoses, very often causing severe plate damage and consequently kiln downtime. Meanwhile, all suppliers developed new solutions. Only KHD avoided these problems by using telescopic ducts from the beginning. The individual suppliers are now using the following standard solutions: ♦ Telescopic air connector (BMH-CPAG, KHD) ♦ Ball and socket type air connector (FLS, Fuller) ♦ Gate type air connector (Polysius) ♦ Open air beam (IKN) Figure 11

Forced (direct) aeration to moving rows: Flexible ducts


6/23/2001 - 4:34:22 PM

Page 89


Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.6 Design Highlights of Modern Grate Coolers / 3.1.6.3 Aeration Concept

3.1.6.3

Aeration Concept

It was soon recognized that only a few (6 to 8) rows of direct and individual aeration are not sufficient to improve clinker distribution or to eliminate/control red river formation. The number of rows with direct aeration was gradually increased and soon the suppliers started to equip the entire recuperation zone or even the entire cooler with direct aeration. Indeed, this improved the control possibilities, but created the following new drawbacks: ♦ Complicated and expensive equipment ♦ More parameters to control ♦ Difficult access underneath grate ♦ High number of potential problem areas (flexible hoses!) Ways had to be found to reduce the number of air ducts to the individually aerated cooler zones. There are two ways to achieve this: ♦ Reduce number of individually aerated zones ♦ Modify the air duct system Today, the following different solutions with varying degrees of experience are presently available from the suppliers: ♦ No moving rows requiring flexible air connectors in inlet section ♦ Longitudinal structural beams designed as air ducts ♦ Short air ducts from one moving row to the next (“Air bridge“) ♦ Direct aeration for fixed rows only (“hybrid aeration“) ♦ Full chamber aeration with modern grate plates Figure 12

Aeration patterns


6/23/2001 - 4:34:22 PM

Page 90

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.6 Design Highlights of Modern Grate Coolers / 3.1.6.4 Seal Air (Confining Air)

3.1.6.4

Seal Air (Confining Air)

When direct plate aeration was introduced, the significance of the seal air or confining air was not properly investigated. It was expected that direct individual aeration of the plates alone would be enough to get the desired improvement due to better air to clinker allocation. If the cooler grates were tight and had no or very narrow gaps between moving and fixed rows or between grate and cooler casing, this would indeed be true. However, real grates have large gaps, which is one of the reasons why direct aeration was introduced. The effect of insufficient seal air pressure for direct aerated grates can be explained as follows: ♦ High resistance in clinker bed (bed thickness, kiln upset, granulometry) ♦ Cooling air sneaks around plate edge to undergrate compartment instead ♦ Clinker dust carried in this air → abrasion / wear ♦ Gap becomes larger → seal air can escape → more “sneak“ air ♦ Stops for repair reduce availability and increase operating cost Today it is generally accepted that partition, sealing and pressurizing of the undergrate compartments is even more important than with chamber aerated coolers. Ideally, the partition of the undergrate compartments should repeat the pattern of the individually aerated grate zones of the grate itself. Since this would lead to very complicated and expensive designs with difficult access, simpler solutions had to be found. One of the most common countermeasures is, to install larger seal air fans. It was interesting to observe the installed cooling air to be gradually increased with each new project. This did not only lead to larger waste air systems but also to higher cooling fan motor power which partially offset the savings expected from modern coolers. The suppliers have proposed the following improvements: ♦ Larger seal air fans ♦ Seal air branched off from cooling air fans ♦ Seal air from booster fan using air from cooling air fans ♦ Undergrate pressure controlled by cooling air fan pressure ♦ Careful sealing of undergrate compartments ♦ No more moving rows in hot inlet zone Figure 13

Seal air systems


6/23/2001 - 4:34:23 PM

Page 91


Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.6 Design Highlights of Modern Grate Coolers / 3.1.6.5 Side Seal Systems

3.1.6.5

Side Seal Systems

Extremely serious wear problems occurred along the side seal plates on each side of the grate. Excessive fall through along the sides and shockingly short lifetime of the side seal plates, mainly in the recuperation zone, were the result. The main reasons for this problem can be listed as follows: ♦ The same seal element used for lateral and longitudinal movement ♦ Side seal plates fixed to cooler casing ♦ Entire thermal expansion to be compensated by (cold) gap on each side ♦ Side plates used for lateral guidance of the grate (older designs) ♦ More lateral thermal expansion of wider grates for large units The following new solutions have been developed and are now part of the contemporary standards: ♦ Entirely new side seal plate concepts ♦ Side seal plates bolted to cross beams of fixed rows (no longer to cooler casing) ♦ Joints for thermal lateral expansion and mechanical longitudinal movement between moving rows and casing separated ♦ Center grate guide for large coolers Figure 14

Side seal designs


6/23/2001 - 4:34:23 PM

Page 92


Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.7 Clinker Crushers

3.1.7

Clinker Crushers

All kiln systems produce larger than normal clinker lumps more or less frequently. Large balls of material enter the cooler when coating drops during kiln upsets. Such large clinker masses can only be cooled superficially and contain a lot of heat. Before being discharged to the clinker conveyor, they must at least be crushed to smaller particles. All clinker coolers, regardless of the type, are equipped with a clinker crusher. Traditionally, this is a hammer crusher which has proven to be reliable. In order to cool large clinker lumps, they must be crushed within the cooler. In reality, this means installing the crusher before the last grate. Early trials with hammer crushers were not successful, however. Based on the idea and experience with roller grate bottoms in shaft kilns (and shaft coolers), CPAG developed the roller crusher to be used as intermediate crusher in a step cooler. The advantages of the roller crusher make it also superior at the cooler outlet. Hydraulic or electric drives as well as different combinations of reversing rollers are available from various suppliers. Compared to the hammer crusher, the roller crusher is rated as follows:

Strengths

Weaknesses

•

low speed

•

higher initial investment

•

low wear

•

chokes easier

•

low dust generation

•

more difficult to design

•

equalization of material rushes

•

suitable for high temperatures

•

lower power consumption


6/23/2001 - 4:34:23 PM

Page 93


Figure 15a

Hammer crusher

Figure 15b

Roller crusher

Figure 16

Heat and air balance of a modern Grate cooler

Figure 17

Optimization


6/23/2001 - 4:34:23 PM

Page 94


Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.8 Cooler control

3.1.8

Cooler control

One of the advantages of the reciprocating grate cooler is its high flexibility, due to operating variables adjustable independently from kiln operation. Usually three main variables are controlled automatically. a) Grate speed In order to prevent the clinker bed resistance from exceeding the pressure capabilities of the cooling fans (which would mean too little cooling air and danger of heat damage), the bed resistance on the grate should be kept constant. To do this, each grate section drive is controlled by the undergrate pressure of the first or second compartment in each grate section. An increase in pressure indicates an increase in bed resistance (either more material in the cooler or finer material). The reaction is an increase of the grate speed, causing the bed to become thinner. If the undergrate pressure decreases, the drive slows down and the bed becomes thicker. Another possibility is to control only the first grate by the undergrate pressure, and to keep the speed of the following grates proportional to the speed of the first grate. More sophisticated control systems use the weighted average of several undergrate pressures to control first grate speed. In many cases, however, control systems amplify fluctuations from the kiln instead of smoothening them. Increasing the bandwidth of the control system has shown good results in several cases. b) Airflow This control is complementary to the grate speed control. It maintains a constant volume of cooling air entering the cooler independently from the grate underpressure. Each cooling fan is equipped with a piezometer sensor which will recognize an increase or decrease of the airflow and cause the cooling fan damper to close or open (in case of inlet vane damper control) or the fan motor speed to decrease or increase (in case of variable speed fan drives). During normal conditions the cooling fans operate at about 2/3 to 3/4 of their maximum performance so that enough spare capacity is left to cope with eventual kiln rushes. Together, grate speed and air flow control will on one hand ensure a sufficient cooling air supply to the cooler and, on the other hand, tend to provide more uniform combustion air temperature to the kiln. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:24 PM

Page 95

"Holderbank" - Cement Course 2000 c) Hood draft The third component of the cooler control system is the hood draft control. An automatically controlled grate cooler can improve the whole kiln operation and allows the operator to concentrate on other problems. The kiln hood pressure is used to regulate the cooler vent air fan speed to maintain a constant pre-set draft. As the draft tends to become positive, the cooler vent fan speed is increased. This takes more air from the cooler and maintains the draft setpoint. As with the other controls, reaction in the opposite direction is just as important. Coolers with radiation walls (IKN) allow hood draft control by one of the first cooling air fans. Figure 18

Cooler control

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.9 Cooler Dedusting

3.1.9

Cooler Dedusting

While dedusting of kiln exhaust gas can be commonly solved by using one type of dust collector only (electrostatic precipitator), the choice of the most adequate system for dedusting clinker cooler vent air raises quite often many discussions. This choice problem is basically a result of the special and fluctuating conditions of the vent air to be dedusted: normal operation

kiln upset

airflow (actual volume)

%

100

up to 150

air temperature

°C

200 - 250

up to 450

air dew point

°C

5 - 20

5 - 20

g/Nm3

5 - 15

25 - 35

dust load

The dust particle size distribution can vary in a wide range depending on the burning conditions in the kiln. Dimensioning of the dedusting equipment must take into account the worst conditions, in order to © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:24 PM

Page 96

"Holderbank" - Cement Course 2000 maintain the required clean gas dust content even at kiln upset condition. The types of dust collectors for this application are compared below. Today's trend is: ♦ multiclones will no longer be tolerated in new and many existing plants ♦ gravel bed filters have proved to be inefficient and expensive ♦ use of electrostatic precipitators is possible without restriction ♦ bag filters with cooling of the vent air in a heat exchanger are often used nowadays

Type of collector

Strengths

Weaknesses

multiclone

simple low investment cost low space requirement not sensitive to temperature peaks

poor efficiency for particles < 20 µm efficiency sensitive to gas flow fluctuation comparatively high pressure loss high operating cost

electrostatic precipitator

low pressure loss low operating cost low maintenance cost

big unit required or use of pulse generator -> high investment cost possibly water injection required

gravel bed filter

not sensitive to temperature peaks

highest investment cost highest pressure loss high operating cost

bag filter

high efficiency relatively low investment cost

no bags for temperatures up to 450°C Õ precooling required high pressure loss high operating cost high maintenance cost

Figure 19

Grate cooler dedusting


6/23/2001 - 4:34:24 PM

Page 97


Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.1 The Reciprocating Grate Cooler / 3.1.10 Developments

3.1.10 Developments Air recirculating (Duotherm) cooler A patent has been taken out in 1970 by the "Société des Ciments Français" concerning the recirculation of the vent air after sending it through a heat exchanger. The first application of the unconventional system has been realized in 1970 at the Beaucaire plant of the above mentioned company, on a 1500 t/d Fuller cooler. Initial experience gained with this installation was very satisfactory. Only few installations using this principle have been realized, e.g. in the Ulco plant. The main advantages and disadvantages of this system are:

Strengths

Weaknesses

•

no dust emission at all

•

•

simple

•

low investment cost

possible wear of fan blades (preventative measures necessary)

•

heat recovery possible (at various temperature levels)

•

•

maintenance and operating costs higher than conventional cooler dedusting system with EP

extension possible by adding further heat exchange units

Modern cooler technology and problems in some cases have pushed this idea in the background. However, it might be reactivated if it can be combined with modern cooler systems.

Dual pass cooler A completely new principle of cooling in a grate cooler has been introduced by Polysius in 1994: the dual pass cooler or REPOL-ZS. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:25 PM

Page 98

"Holderbank" - Cement Course 2000 This cooler can be considered a two-grate cooler with intermediate crusher where grate 1 and 2 are identical. The hot, 1400°C clinker from the kiln is fed on top of a layer of colder clinker already laying on the cooler grate. At the end of the grate, the now cold lower clinker layer is extracted via a special system consisting of reciprocating bars and a hopper. The upper layer which has reached about 500°C passes a roller crusher and is then returned to a intermediate hopper below the kiln from where it is fed onto the empty grate to pass the cooling air a second time, this time below the fresh hot clinker. One 1400 t/d unit is in operation in Germany using Jet-Ring technology. With less than 1.6 Nm3/kg cooling air, extremely low clinker temperatures have been reported. The crucial problems of this solution are intermediate transport and storage. In spite of the compact size, high cooling degree with low air flow and low plate temperatures, this cooler will only be successful if the intermediate temperature level can be increased and the heat losses reduced.

Figure 20a

Non venting cooler

Figure 20b

Dual pass cooler (Polysius)

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.2 The Cross Bar Cooler

3.2

The Cross Bar Cooler

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.2 The Cross Bar Cooler / 3.2.1 © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:25 PM

Page 99

"Holderbank" - Cement Course 2000 Principle

3.2.1

Principle

F.L.Smidth and Fuller developed together the new SF (Smidth - Fuller) Cross Bar Cooler representing a completely new concept. The basic idea was to develop a cooler in which conveying of clinker and air distribution systems are separated. The SF cooler has a clinker conveying device installed above an entirely fixed grate. In addition the cooler should be less complicated, more efficient and easier to operate than other grate coolers on the market. Sealing air is eliminated and the distribution of air is optimized for all modes of operation The thermal behavior of the SF cooler (e.g. heat balance, recuperation) is similar to the other grate coolers. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.2 The Cross Bar Cooler / 3.2.2 Main features

3.2.2

Main features

•

One inclined fixed grate.

•

Clinker conveying by cross bars, separate from air distribution.

•

No thermal stress of grate.

•

Minimum wear on grateplates due to a dead layer of clinker (50 mm) protecting the grate surface. The thickness is given by the space between the cross bars and the grate. (Anticipated service life time at least 5 years)

•

Dynamic flow control unit (mechanical flow regulator) for each grate plate. The mechanical flow regulator maintains a constant airflow through the grate and clinker bed, irrespective of the clinker bed height, particle size distribution, temperature, etc.

•

No fall through of clinker to the undergrate compartment. → Eliminating undergrate clinker transport resulting in low installation height for new plants.

•

Easy cooler operation by elimination of sealing air and automatic control of air distribution.

•

Modularized cooler concept → short delivery and installation time.

•

Different drive speeds across the cooler possible. → Additional control of clinker distribution.

•

Fewer and less expensive wear parts (easy to replace).

•

Easy visual inspection of undergrate compartment (clean undergrate, windows).

•

Sustainably high thermal cooler efficiency throughout the lifetime of the cooler. → Reduced system heat consumption.

Figure 21a:

SF Cross Bar Cooler


6/23/2001 - 4:34:25 PM

Page 100


Figure 21b:

SF cooler grate with cross bars

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.2 The Cross Bar Cooler / 3.2.3 Strengths and Weaknesses

3.2.3


Strengths


Weaknesses

6/23/2001 - 4:34:26 PM

Page 101

"Holderbank" Strengths - Cement Course 2000 •

No clinker fall through (no hoppers, no dragchain).

•

The grate is protected from overheating.

•

Very high availability is expected.

•

Wear and tear affects only the conveying system and not the air distribution system.

•

For each plate, the cooling air is individually controlled.

•

The amount of cooling air is about 1.6 to 1.8 Nm3/kg.

•

Reduced height and maintenance required since the undergrate clinker transport can be dropped.

•

Time for installation is short due to modular concept.

Weaknesses •

The clinker bed seems to be influenced by the conveying reciprocating cross bar, resulting in disturbed clinker layers.

•

In case of fine clinker and coating drops, air breakthroughs can occur.

•

The performance of the mechanical flow regulator (amount of cooling air) and its distribution is yet to be assessed.

•

Airflow through the fixed grate at the cooler inlet (CIS) can generate dust and dust cycle.

Remark: So far, no SF Cross Bar Cooler is in use within the “Holderbank” group and therefore no first hand experience is available. Worldwide, there are only three SF cross bar coolers installed. Two of a capacity of 450 t/d and one of 2000 t/d. (as of January 1999)

Figure 22a:

Cross Bars: Easy to replace wear parts

Figure 22b:

Mechanical flow regulator


6/23/2001 - 4:34:26 PM

Page 102


Figure 22c:

Modular concept: One module

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.3 The Travelling Grate Cooler

3.3

The Travelling Grate Cooler

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.3 The Travelling Grate Cooler / 3.3.1 Principle

3.3.1

Principle

The traveling grate cooler (Recupol) was originally developed by Polysius for use in combination with grate preheater (Lepol) kilns. Using the same principle and similar technology, it uses the same wear parts. The following main components can be distinguished: •

Casing with kiln hood and connections for air at different temperature levels

•

Inlet with water cooled chute (2nd generation) and pulsator

•

Traveling grate with return carrying idlers and drive system

•

Aeration system with fans, undergrate compartments

•

Riddling extraction system with chutes, flap gates, hoppers and transport


6/23/2001 - 4:34:27 PM

Page 103


Clinker crusher

♦ Material transport The clinker is carried by a horizontal traveling grate which works like a stationary caterpillar chain with perforated chain plates. In contrast to the reciprocating grate cooler, the clinker does not tumble over plate edges, but remains as undisturbed layered bed from inlet to discharge. ♦ Heat exchange Heat exchange takes place, like for the reciprocating grate according to the cross current principle. Because the layers remain, it should be even better, at least theoretically. ♦ Cooling air Ambient air is blown by a number of cooling air fans to underneath of the travelling grate plates carrying the clinker. Pressure and flow criteria of cooling air are basically as for the reciprocating grate cooler. ♦ Water cooled inlet chute In order to achieve rapid cooling in the inlet section, but also to protect the travelling grate from the highest clinker temperatures, Recupol coolers were equipped with a water cooled inlet chute. ♦

Key figures / KPI Specific grate loading: 25 - 30 t/d m2 (design) Largest units: 3000 t/d (Lägerdorf kiln 10)

♦ Figure 23

Travelling grate cooler

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 3. GRATE COOLERS / 3.3 The Travelling Grate Cooler / 3.3.2 Strengths and Weaknesses

3.3.2


Travelling grate cooler compared to reciprocating coolers: Strengths


Weaknesses

6/23/2001 - 4:34:27 PM

Page 104

"Holderbank" - Cement Course 2000 Strengths

Weaknesses

•

Possibility of replacing grate plates during operation (on the returning part)

•

•

Undisturbed, layered clinker bed is better for optimum heat exchange

Larger machine for the same grate area equipment requiring more space and higher civil cost

•

Lower specific grate loadings adding further to overall size

•

More expensive to build than a reciprocating grate cooler

•

The absence of clinker movement (see above) was often considered a disadvantage because of cases where a solid (fritted) layer on top of the clinker bed made it impermeable for air. For this reason, pulsators were installed for first cooling fans.

•

Much higher maintenance requirement with ageing equipment

•

Heat loss via cooling water for inlet chute

Due to the mentioned weaknesses, Polysius eventually decided to develop their own reciprocating grate cooler (Repol) around 1980: Figure 24

Travelling grate cooler: Design details

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS

4.

ROTATING COOLERS

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.1 The Rotary Cooler or Tube Cooler


6/23/2001 - 4:34:27 PM

Page 105


The Rotary Cooler or Tube Cooler

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.1 The Rotary Cooler or Tube Cooler / 4.1.1 Principle

4.1.1

Principle

The rotary cooler consists mainly of a rotating cylinder, similar to a rotary kiln. The clinker is fed through the inlet chute and is then cooled by air while being transported towards the outlet end. Cooling is performed in countercurrent flow. The tube is equipped with internal lifters which improve the heat transfer. About 2/3 (66%) of the cooler length is lined with refractory bricks. The rotary cooler is of simple design and is the oldest type of clinker coolers. It was seldom used for modern, large kiln systems. Therefore comparatively little design and operating experience is nowadays available for rotary coolers above 2000 t/d. However, the application of rotary coolers still offers certain advantages. Presently units up to 4500 t/d (dimensions dia 6.3/6.0 x 80 m) are in operation. It will be interesting to follow the future development of large rotary coolers. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.1 The Rotary Cooler or Tube Cooler / 4.1.2 Design Features

4.1.2

Design Features

♦ Arrangement of the rotary cooler is normally in the extension of the kiln axis; in many cases the reverse manner (underneath the kiln) has been applied. ♦ The diameter of the cooler is similar to that of a corresponding suspension preheater kiln. Likewise the rotating speed is in the same range as for the kiln (max. 3 rpm). Length/diameter ratio: L/D ~ 10. Many cooler tubes are designed with an extension in diameter in order to reduce air velocity. ♦ The inclination is comparatively high (in the order of 5%). ♦ Like for all rotating coolers, the internal heat transfer equipment is an important part of the rotary cooler. Its task is to generate additional area by scattering the clinker without generating too much dust. Basically a similar design may be applied as in a planetary cooler tube (see next chapter) however the following differences must be considered: •

The clinker falling heights are larger. Wear protection of shell and lining is essential.

•

At a comparative length position the clinker in a rotary cooler is hotter than in a planetary cooler.

Figure 25

Rotary cooler


6/23/2001 - 4:34:28 PM

Page 106


The following zones can typically be distinguished in a rotary cooler (simplified): A

Lined inlet zone

B

Lined crushing teeth zone (metallic teeth)

C

Lined cast lifter zone, lining protected by wearing plates (at least in the second half)

D

Cast lifter zone, shell protected by wearing plates (having air gap, giving also insulating effect)

E

Sheet metal zone with wearing plates

Construction materials have to be selected according to the high temperature and wear requirements. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.1 The Rotary Cooler or Tube Cooler / 4.1.3 Cooling performance

4.1.3

Cooling performance

Depending on the design and the shape of the lifters clinker outlet temperature usually tends to be high. In many cases it is necessary to enhance the cooling by injecting water into the tube (up to 60 g/kg clinker) in order to reach reasonably low clinker temperatures of 100° to 150°C. The cooling efficiency (heat recuperation) is equal or even slightly better than on a planetary cooler. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.1 The Rotary Cooler or Tube Cooler / 4.1.4 Strengths / Weaknesses

4.1.4

Strengths / Weaknesses


6/23/2001 - 4:34:28 PM

Page 107

"Holderbank" - Cement Course 2000 Strengths

Weaknesses

•

Simplicity of cooler design, robust piece of equipment.

•

Not recommended for large units (above 2000 t/d)

•

No special mechanical problems comparable to a rotary kiln.

•

•

No control loops.

•

Formation of build-ups ("snowmen") in the inlet chute. A water-cooled chute or a dislodging device is required in such case.

Easy commissioning.

•

•

No waste air and therefore no dedusting equipment required

Clinker outlet temperatures tend to be high and therefore water injection is usually required.

•

Electrical energy consumption up to 5 kWh/t lower compared to grate cooler.

•

•

Rotational speed can be adjusted and therefore upset kiln conditions can be handled easier than with a planetary cooler.

Due to large falling height wear protection in the tube must be reinforced (compared to a planetary cooler).

•

High kiln foundations are required.

•

Cooler inlet seal can contribute to additional false air inlet.

•

Suitable for AS type precalcining system tertiary (extraction of hot air is possible).

Figure 26

Internal transfer equipment for rotary and planetary coolers

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.2 The Planetary Cooler

4.2

The Planetary Cooler

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.2 The Planetary Cooler / 4.2.1 Principle

4.2.1

Principle

The planetary cooler is based on the same cooling principle as the rotary cooler in the preceding chapter. However, the essential difference of a planetary cooler is the number of individual cooling tubes. The flow of clinker is subdivided into 9 to 11 (usually 10) cooling tubes which are installed around the kiln circumference at the kiln outlet (see Fig. 15). Therefore the planetary tubes follow the © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:28 PM

Page 108

"Holderbank" - Cement Course 2000 kiln rotation. Because of their connection to the kiln rotation, planetary coolers do not need a separate drive. This fact already illustrates one main advantage of the planetary cooler: its simplicity in operation. Strictly speaking the cooling of clinker does not only start in the cooling tubes but already in the kiln. In the case of a planetary cooler the kiln burner pipe is always inserted into the rotary kiln so that a cooling zone behind the flame of 1.5 to 2.5 kiln diameters is created. This zone is called the "kiln internal cooling" zone and must be considered as an integral part of any planetary cooler. In this zone the temperature of the clinker drops from 1450° to 1200 - 1300°C. This temperature reduction is important for the protection of the inlet opening, the elbow and the first section of the cooling tubes. After this first cooling in the kiln internal cooling zone the clinker falls into the elbows when they reach their lowest point of kiln rotation. The hot clinker is then cooled by air in counterflow (the amount of air equals the amount of secondary air). The air is heated up to approx. 700°C. The clinker reaches final temperatures which are typically in the range of 140° to 240°C. A considerable amount of heat is also transferred to ambient by radiation and convection since approx. 75% of the cooler shell is not insulated. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.2 The Planetary Cooler / 4.2.2 Historical

4.2.2

Historical

Planetary coolers have been used since 1920. When large kiln units and grate coolers were developed planetary coolers were abandoned for many years. But about 1966 planetary coolers of large capacities were introduced. At that stage serious mechanical problems occurred on these first large planetary coolers. As a consequence a lot of work had to be done in order to improve the mechanical design of planetary coolers. As a result of extensive computer calculations and operating experience the planetary cooler became a mechanically reliable piece of equipment. In the late 1970's, the design had reached a high standard and a considerable level of perfection. Units of up to 5000 t/d were envisaged. With the demand for permanently larger units using precalciner technology with separate tertiary air dusts, the boom period of the planetary coolers came to an end. Figure 27

Planetary cooler

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.2 The Planetary Cooler / 4.2.3 Design features

4.2.3

Design features


6/23/2001 - 4:34:28 PM

Page 109

"Holderbank" - Cement Course 2000 Planetary coolers in the late 1970's had the following design features: ♦ Shell extension: The kiln shell is extended beyond the cooling tube outlets and is supported by an additional roller station. ♦ Fixation of cooling tubes: Fixed support of cooling tubes near inlet and loose support near outlet end. With larger coolers, the cooling tubes can consist of two separate sections requiring three supports. In that case two fixed supports are located near inlet and near outlet and a loose support is located at the interconnection point in the middle. ♦ Design of cooler supports: The kiln shell is reinforced (high thickness) where the cooler support structure for the cooler is welded on. The support structure (base and brackets) itself is of heavy design consisting of reinforcement ribs and box beams. ♦ Cooler length: Length/diameter ratio of tubes is approx. 10:1 ♦ Inlet openings: The inlet openings to the cooler elbows weaken the kiln shell and high mechanical and thermal stresses occur in that zone. The openings are made of oval shape and the kiln shell is considerably reinforced in its thickness (up to 140 mm in large kilns) in order to compensate for the weakening. In some cases a diagonal retaining bar (made of high heat resistant steel) is incorporated in the opening in order to avoid that large lumps can enter the cooler. ♦ Kiln-to-elbow joint: This joint is designed in a manner that no forces due to thermal expansion and deformation are transmitted from elbow to kiln. ♦ Elbow: In order to prevent that clinker is falling back into the kiln while the opening is on top position, the position of the cooling tube is displaced back against the direction of rotation. The elbow design must avoid excessive dust backspillage and wear. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 4. ROTATING COOLERS / 4.2 The Planetary Cooler / 4.2.4 Internal heat transfer equipment (see Fig. 26)

4.2.4

Internal heat transfer equipment (see Fig. 26)

Cooling performance depends strongly on efficient lifters of solid and durable design. Since high heat resistant metallic lifers are available on the market also the high temperature zones can be adequately equipped. Special high temperature alloys can be used for this purpose. They can withstand maximum temperatures of up to 1150°C. These alloys are usually characterized by a high chromium content of approx. 30% Cr. Other elements as Ni or Mo can occur in various proportions. Fig. 26 shows a typical arrangement of heat transfer internals. Breaking teeth are applied in the hottest zone. They are able to crush large lumps of clinker and create also a tumbling effect, which improves the heat transfer. They are of heavy design and mounted on separate supports. The first rows of lifters must be carefully selected regarding design and material. Their functioning is very important since they also protect the following lifters from overheating. Figure 28a

Temperature profile in planetary cooler


6/23/2001 - 4:34:29 PM

Page 110


Figure 28b

Water cooling for planetary coolers

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 5. VERTICAL COOLERS

5.

VERTICAL COOLERS

Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 5. VERTICAL COOLERS / 5.1 The Gravity Cooler (G Cooler)

5.1

The Gravity Cooler (G - Cooler)

The Claudius Peters Company have developed the “g-cooler”. The letter "g" stands for gravity since clinker movement is performed by gravity. This cooler is designed as an after cooler and can therefore only be used in connection with a primary cooler such as a short grate cooler or a planetary cooler. The installation together with a grate cooler is shown in Fig. 29. An intermediary crusher reduces the clinker size to 20 - 30 mm. The material of approx. 400°C is then filled by a drag chain into a vertical shaft. Cooling is performed by horizontal rows of tubes which are cooled by internal air flow. The heat is therefore exchanged indirectly and the air remains dust-free. The clinker slowly drops down (at a speed of 20 – 30 mm/s) and reaches final temperatures of approx. 100°C at the discharge. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:29 PM

Page 111

"Holderbank" - Cement Course 2000 There is no dedusting equipment required for the cooling air. However, the system according to Fig. 29 as a whole is usually not free from dusty waste air. In case of a suspension preheater kiln system there is still some waste air required on the grate cooler since the kiln cannot take all the hot air produced during the first cooling step. In addition, a marginal amount of dusty air is produced by the g-cooler itself (top and discharge). The application of this cooler type is often considered for kiln extension projects. If an existing grate cooler (or a planetary cooler) has to be operated at higher capacity the new clinker outlet temperature can become too high. In this case the clinker temperature can be reduced by a g-cooler used as an aftercooler. Process Technology / B05 - PT II / C01 - Kiln Systems / Clinker Coolers / 5. VERTICAL COOLERS / 5.2 The Shaft Cooler

5.2

The Shaft Cooler

A shaft cooler can be operated waste-air-free and theoretically offers an ideal countercurrent heat exchange and thus high recuperating efficiency. Based on the idea the first large shaft cooler was designed and constructed on a 3000 t/d kiln in 1973. The experience gained in the plant shows that it is possible to operate such equipment but some serious disadvantages have to be taken into account: ♦ All depends of the clinker granulometry! Theoretically, an extremely uniform clinker granulometry having no fines and no coarse material would be required. This is hardly achievable in a cement kiln. Therefore, fluctuations occur. ♦ High cooling air quantity (= secondary air) of 1.05 Nm3/kg cli is required but even so the clinker exit temperature of 350°C is very high. ♦ High power consumption (10 kWh/t) For the above reasons, the technical realization is not yet solved. The shaft cooler so far is not a reasonable alternative to the conventional clinker coolers. Figure 29

Gravity cooler (g-cooler, CPAG)

Figure 30

Shaft cooler


6/23/2001 - 4:34:29 PM

Page 112


Figure 31

Claudius Peters CPAG: Combi Cooler

Figure 32

FLS: Coolax Grate Cooler

Figure 33

Fuller: Controlled Flow Grate (CFG) Cooler


6/23/2001 - 4:34:30 PM

Page 113


Figure 34

IKN: Pendulum Cooler

Figure 35

KHD: Pyrostep Cooler

Figure 36

Polysius: Repol RS Cooler


6/23/2001 - 4:34:30 PM

Page 114



6/23/2001 - 4:34:31 PM

Page 115

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C02 - Internal Kiln Fittings

C02 - Internal Kiln Fittings


6/23/2001 - 4:34:32 PM

Page 116

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems

Kiln Chain Systems A. Obrist PT 96/ 14036 / E 1. INTRODUCTION 2. Functions of a Kiln Chain System 2.1 Heat Exchange 2.2 Cleaning of the Kiln Shell 2.3 Transport of Material 2.4 Reduce Dust Emissions 3. Individual Zones of a Chain System 3.1 Free Zone of the Kiln Inlet 3.2 Dust Curtain Zone 3.3 Plastic Zone 3.4 Granular Zone (Preheating Zone) 3.5 Heat Resistant Zone 1.6 Main Characteristic Data of the Individual Chain System Zones 4. Arrangement of Chains 4.1 Straight Curtain 4.2 Spiral Zone 1.3 Multiple Spiral Curtain 1.4 Triangular Curtain (Z-Curtain) 1.5 Garlands 1.6 Festoons 1.7 Spiral Garlands 1.8 Thermochains 5. Types of Chain Links 5.1 Round Links 5.2 Long Links 5.3 Oval Links 5.4 Other Types of Chain Links 6. Chain Material 6.1 Mild Steel Chains © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:32 PM

Page 117

"Holderbank" - Cement Course 2000 6.2 Heat Resistant Alloy Chains 7. Chain Hangers 8. Main Characteristic Data of Chain Systems 9. ANNEXES 10. LITERATURE 11. Test Questions


6/23/2001 - 4:34:33 PM

Page 118

"Holderbank" - Cement Course 2000 Summary: A kiln chain system has four main functions: ♦ It helps to increase the heat exchange between gas and raw meal ♦ It keeps the kiln shell (lining surface) clean ♦ It assists the transport of material through the kiln tube ♦ It helps to reduce the dust emission A properly designed chain system must respect the changing properties of material passing through the kiln tube. In a wet process kiln the material is fed as a liquid slurry and changes it properties subsequently in several steps inside the chain system to dry preheated granules. In accordance with the changing material properties different arrangement of chains (straight curtains, spirals, garlands, etc.) have to be used for individual parts of the system to satisfy the specific requirements. Also the chain densities and the height of the free tunnel below the chains have to be selected carefully in order to reach the maximum efficiency. The chain links can have different shapes (round, long, oval etc.), preferably round links. The chemical composition of the chains' alloy and its physical treatment (hardening) strongly influence the life time of the system. Different types of chain hangers can be used (single or multiple hangers, with or without shackles etc.). They have to guarantee a sufficient stability, to enable an easy installation and they should as far as possible assist the function of the chains. NOMENCLATURE Just a few symbols and names are to be explained before starting this lecture, the other ones will be explained in the respective chapters. Figure:

Dis

Diameter inside kiln steel shell

DIL

Diameter inside kiln lining

hfr

Theoretical free height under the chains (see attached sketch), expressed in mm or as % of DIL


6/23/2001 - 4:34:33 PM

Page 119

"Holderbank" - Cement Course 2000 expressed in mm or as % of DIL density of chains m2/m3

is calculated for individual parts (zones) of the system as the total surface area of chains in the respective zone divided by the volume inside lining of this zone

density of chains kg/m3

similar to the above mentioned density, but concerns the weight of chains instead of their surface

Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 1. INTRODUCTION

1.

INTRODUCTION

Wet process kilns cannot be successfully operated without internal kiln fittings, among which the kiln chains are the most typical and most frequently used ones. The number of existing wet process kilns is still high (~33% in "Holderbank" Group) and a conversion from wet to dry process is very expensive. By improving the existing chain systems or, where necessary, by installing a completely new chain system, the kiln operation can be upgraded considerably with relatively moderate investment costs. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 2. FUNCTIONS OF A KILN CHAIN SYSTEM

2.

FUNCTIONS OF A KILN CHAIN SYSTEM

The kiln chain system has 4 main functions: Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 2. FUNCTIONS OF A KILN CHAIN SYSTEM / 2.1 Heat Exchange

2.1

Heat Exchange

The heat exchange between hot gases and the raw material depends on the surface area exposed to the hot medium. In the parts of kiln where no chains are installed, this surface area consists of the surface of the material layer on the kiln bottom and of the surface of the remaining part of the kiln shell (resp. lining). By installing the chains a large additional surface area can be gained, exceeding that one mentioned above several times (up to 10 times and more) in the respective part of the kiln. By improving the heat exchange the specific heat demand is reduced and the kiln output is increased. In Fig. 1 different positions of a chain during one kiln rotation are shown. In position 1 the chain is exposed to the stream of hot gases and thus heated up. The cooling of the chain (which passes its heat to the layer of material) starts in position 2, continues in position 3 and ends in position 4. Figure 1:


6/23/2001 - 4:34:34 PM

Page 120


Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 2. FUNCTIONS OF A KILN CHAIN SYSTEM / 2.2 Cleaning of the Kiln Shell

2.2

Cleaning of the Kiln Shell

In the upper part of the kiln the characteristics of the wet, sticky raw material favors the formation of mud coating and mud rings. This would reduce the free kiln cross sectional area and thus obstruct the flow of material and gases. Growing mud rings make the kiln operation difficult. It is one of the main functions of the chain system to keep the internal kiln shell surface clean, free of coating or rings. Due to the kiln rotation the chains slide on the kiln shell (resp. lining) and destroy the rings and the coating. The sliding movement of a chain cleaning the kiln shell is shown in Fig. 1 (position 3). Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 2. FUNCTIONS OF A KILN CHAIN SYSTEM / 2.3 Transport of Material

2.3

Transport of Material

The properties of material in different parts of the kiln differ considerably. In some sections of the upper part of the kiln, where the material is sticky and plastic, its transporting is more difficult than in other sections. As a regular flow of material is of an eminent importance for a smooth kiln operation, it is necessary to install material flow assisting devices in some sections. Some special arrangements of chains can help to draw the material through the critical sections. This can be achieved by chains moving in the desired direction (garlands) or by a screw shaped arrangement of the chain fastening points. Other arrangements of chains can be an obstruction to the flow of material and should therefore never be used in the critical sections. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 2. FUNCTIONS OF A KILN CHAIN SYSTEM / 2.4 Reduce Dust Emissions

2.4

Reduce Dust Emissions

The gases leaving the kiln contain a certain amount of dust consisting mainly of partly calcined, hot raw material. The dust load of gases depends on the properties of the raw material and on the specific conditions of the kiln operation. Dust loss should be kept small, it means a loss of heat and material. The kiln chain system, mainly its upper part, can help to reduce the dust emission. Dust particles carried by the stream of gases stick to the wet surface of chains and later when these chains are emerged into the layer of material, this dust is passed over to the slurry. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 3. INDIVIDUAL ZONES OF A CHAIN SYSTEM © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:34 PM

Page 121


INDIVIDUAL ZONES OF A CHAIN SYSTEM

The material passing the chain system changes subsequently its properties - it loses water and is heated up. According to the different material properties the total chain system can be divided into several zones. These zones are: Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 3. INDIVIDUAL ZONES OF A CHAIN SYSTEM / 3.1 Free Zone of the Kiln Inlet

3.1

Free Zone of the Kiln Inlet

This short zone is considered to be a part of the chain system in spite of the fact that no chains are installed here. A sufficient amount of slurry should be accumulated in this zone in order to guarantee a constant and regular flow into the lower parts of the system. Good results have been obtained with the zone length of 1 to 1.5 kiln diameters. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 3. INDIVIDUAL ZONES OF A CHAIN SYSTEM / 3.2 Dust Curtain Zone

3.2

Dust Curtain Zone

The dust curtain zone is relatively short, its length does not exceed 0.5 DIL under normal conditions. The material entering this zone still has the relatively good flow properties of the kiln feed (slurry). When leaving this zone, the material has a lower water content and becomes more "plastic", essentially due to the inter-mixing of the dust previously retained by the chains in this zone. In order to achieve a good dust catching efficiency, the density of chains must be high (some 8 to 15 m2/m3) and the free height below the chains should be 18 - 27% of DIL. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 3. INDIVIDUAL ZONES OF A CHAIN SYSTEM / 3.3 Plastic Zone

3.3

Plastic Zone

The length of this zone depends on properties of raw material, slurry moisture, characteristics of the kiln operation etc. and can vary in a wide range (approx. between 1.5 and 4 DIL). The material in this zone is plastic and sticky, still relatively cold and wet and because of these properties it favors mud coating and mud ring formation. The transport of material through this zone is the most difficult one among all the zones of the chain system. Due to the material properties mentioned above the chains in this zone must have a good shell cleaning and material transporting efficiency. The density of chains should be relatively low, some 5 to 8 m2/m3. As to achieve a big free tunnel under the chains, the free height hfr should be approx. 30% or, if garland chains are installed in this zone, some 40%. Heavier (thick wire) chains should be installed. In order to be sure that the zone of plastic material will always stay inside the zone of chains which can treat it successfully, the respective arrangement of chains should be slightly extended in the downstream direction as to obtain a sufficient safety. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 3. INDIVIDUAL ZONES OF A CHAIN SYSTEM / 3.4 Granular Zone (Preheating Zone)


6/23/2001 - 4:34:34 PM

Page 122


Granular Zone (Preheating Zone)

The recommendable length of this zone depends on the desired material temperature and the rest water content at its discharge end. Good results have been achieved with a zone length between 2 and 4,5 DIL. The material entering this zone is not plastic any more, it forms granules which are easy to be transported and do not favor a mud ring formation. The granules should be dried and heated up in this zone. The chains should heat the material gently without unnecessary dust generation, they should enable a good heat exchange. Lighter (thin wire) chains should therefore be installed. A chain density of approx. 6 to 10 m2/m3 and a free height of approx. 25 to 30% can be recommended. This zone is sometimes divided into an upper and a lower part. Both parts have the same (or at least a similar) arrangement, but the lower part has a higher density of chains than the upper one. The damming effect of the lower part, caused by the thicker layer of chains on the kiln bottom, helps to increase the material retention time and improves the heat exchange. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 3. INDIVIDUAL ZONES OF A CHAIN SYSTEM / 3.5 Heat Resistant Zone

3.5

Heat Resistant Zone

This zone is relatively short, its length does not exceed 1,5 DIL. The material, dry and hot granules, can easily be transported. A very gentle treatment of the material is required in order to keep the dust creation as low as possible. The main function of the chains is to protect the upstream part of the system against heat radiation and too high a gas temperature. Chains made of heat resistant steel should be installed in this zone. Lighter (thin wire) chains should be preferred. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 3. INDIVIDUAL ZONES OF A CHAIN SYSTEM / 3.6 Main Characteristic Data of the Individual Chain System Zones

3.6

Main Characteristic Data of the Individual Chain System Zones

Table 1 Zone Free

Dust

Plastic

Curtain

Lower

Resistant

1 to 3

≤ 1,5

≤ 1,5

1.5 to 4

% DIL

18 - 27

40 / 30

25 to 30

Density

m2/m3

8 to 15

5 to 8

6 to 10

Moisture

%

DIL

hfr

Material temp. °C Chain temp.

°C

Gas temp.

°C

Heat

Upper

≤ 0.5

Length

≤ 1,5

Granular

30 to 40

15 - 25

20

100

<600

<400

<1'000

160 to 240

<1'100 Material flow


6/23/2001 - 4:34:35 PM

Page 123


Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 4. ARRANGEMENT OF CHAINS

4.

ARRANGEMENT OF CHAINS

Each individual zone of a chain system must have its own specific properties in order to satisfy the requirements mentioned previously. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 4. ARRANGEMENT OF CHAINS / 4.1 Straight Curtain

4.1

Straight Curtain

As shown in Fig. 2, this arrangement of chains is very simple. The chain fastening points form a ring. Several straight curtains are combined to a straight curtain zone. The distance between the individual straight curtains (rings or rows) should not be too long, this would lead - in order to achieve a sufficient density - to too great a number of chains per ring and thus to big heaps of chains on the kiln bottom obstructing the flow of material. Figure 2:

Straight Curtain

The main advantages of a straight curtain are its simple design and installation as well as an easy maintenance. Its main disadvantages are the poor shell cleaning efficiency and the fact that this arrangement does not assist the transport of material. The straight curtains should therefore not be used in the plastic zone or in the dust curtain, but they can be recommended for the granular zone. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 4. ARRANGEMENT OF CHAINS / 4.2 Spiral Zone

4.2

Spiral Zone

In a spiral curtain the chain fastening points follow the screw lines on the kiln shell. The spiral (screw) lines should have an inclination of approx. 30° (see Fig. 3). In order to assist the transport of material, the inclination must have the proper sense, i.e. the sense of rotation must be taken into consideration. Figure 3:

Spiral Curtain (4-start spiral)


6/23/2001 - 4:34:35 PM

Page 124


The arrangement with 4 screw lines per circumference, called a 4-start spiral, is most frequently used. This arrangement allows for a good shell cleaning effect and at the same time overlapping of chains can be avoided, as shown in Fig. 4a. The recommended chain length is namely approx. 0.7 D, and the circumferential distance between adjacent screw lines is π D/4 = 0.78 D. The benefits of this solution become clear if we compare it with other arrangements having a different number of screw lines. An arrangement with less than 4 screw lines per circumference (Fig. 4b) does not enable a full shell cleaning effect, because some parts of the shell have no contact with a chain. An arrangement with more than 4 screw lines leads necessarily to overlapping of chains (Fig. 4c) and thus to a higher layer of chains on the kiln bottom which is not favorable for the transport of material. Figure 4a-c:

The very good material transporting and the good shell cleaning efficiency are the main advantages of a spiral curtain. Its disadvantages are a slightly smaller heat exchanging efficiency, a more difficult brick lining work and a limited maximum chain density. For these reasons, the spiral curtain arrangement should not be used in the granular zone but it can be recommended for the plastic zone. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 4. ARRANGEMENT OF CHAINS / 4.3 Multiple Spiral Curtain

4.3

Multiple Spiral Curtain

A spiral curtain having 8 or more spirals per circumference is called a multiple spiral curtain. Such an arrangement enables to achieve high chain densities and the passages between chains are narrow, therefore, it can be used in the dust curtain zone. its material transporting efficiency is not as good as © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:35 PM

Page 125

"Holderbank" - Cement Course 2000 that one of a 4-start spiral, but it is still better than that one of any other arrangement enabling a similarly high chain density. A multiple spiral arrangement can therefore be recommended for dust curtain zones in such cases where the kiln feed flow properties are poor. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 4. ARRANGEMENT OF CHAINS / 4.4 Triangular Curtain (Z-Curtain)

4.4

Triangular Curtain (Z-Curtain)

In a triangular curtain, also called Z-curtain, the chain fastening points follow a zig-zag line (see Fig. 5). This arrangement enables to reach high chain densities without obstructing the flow of material and gases too much. Because of the equal distribution of chains the heap of chains on the kiln bottom is not too high in spite of the high chain density (see Fig. 6). Figure 5 & 6

The arrangement with angles of 60° and with 8 "triangles" per kiln circumference has proved to be the most successful one. In such an arrangement the layer of chains on the kiln bottom is only approx. 4 chains high (because 8 triangles have all together 16 fastening lines and the chain length of approx. 0.73 D corresponds to ≈ π D/4, therefore, each chain passes 16/4 = 4 lines). The dust catching efficiency of such an arrangement is very good, it was therefore used in the dust curtain zone in such cases where the kiln feed flow properties are relatively good. Frequently one single triangular curtain was installed, but two are also possible. Nowadays, for dust curtains a multiple start spiral arrangement is preferred to a triangular curtain because of its material transport efficiency. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 4. ARRANGEMENT OF CHAINS / 4.5 Garlands

4.5

Garlands

Chains having their both ends fastened to the kiln shell are called garlands (Fig. 7) The fastening points form straight rings in the kiln shell. The shape of a garland is characterized by the chain length, by the axial distance LAX and by the angle ∝ (see Fig. 7 and 8). Good results have been achieved with an angle ∝ ≈ 90 to 110° and a distance LAX ≈ 0.5 to 0.9 D. Figure 7 & 8:


6/23/2001 - 4:34:36 PM

Page 126


Due to the sliding movement of the garland chains on the kiln shell (resp. lining) surface, their shell cleaning and material transporting efficiency is very good and their heat exchange efficiency is approx. 1.5 times higher than that one of pending chains (see Fig. 9). Figure 9

As can be seen in Fig. 9 the garland chain should be hung in a "reverse" sense, i.e. in a sense differing from that one of a screw line, in order to assist the material transport. Due to the properties mentioned above, the garland chains have been used mainly in the plastic zone. In spite of the advantages mentioned above CS/PT does usually not recommend the installation of garlands because of their disadvantages, namely: ♦ complicated installation ♦ difficult maintenance ♦ relatively short life time ♦ at the ends of the garland zone the shall cleaning efficiency is very poor (no movement of chain) Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 4. ARRANGEMENT OF CHAINS / 4.6 Festoons

4.6

Festoons

Garlands without overlapping chains in the axial direction are called festoons (Fig. 10). The installation and maintenance of festoons are less difficult than that of garlands but their shell cleaning efficiency is poorer (the areas between the individual bays of festoons are not cleaned reliably). © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:36 PM

Page 127

"Holderbank" - Cement Course 2000 Figure 10

Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 4. ARRANGEMENT OF CHAINS / 4.7 Spiral Garlands

4.7

Spiral Garlands

An arrangement of garlands where the chain fastening points form screw lines ("spirals") is called spiral garlands. Its material transporting efficiency is even better than that one of normal straight garland zone, but the erection and maintenance are more complicated. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 4. ARRANGEMENT OF CHAINS / 4.8 Thermochains

4.8

Thermochains

Thermochains are a special type of festoons, meeting the following conditions: ♦ The axial distance LAX between the two fastening points is short (approx. 0.1 to 0.15 DIL). ♦ The angle ∝ between the two fastening points is approx. between 60° and 120°, preferably 90° to 120°. ♦ The free height under the chain hfr is approx. between 0.4 DIL (for ∝ = 120°) and 0.6 DIL (for ∝ = 90°). ♦ The chain (shackles included, if used) is approx. 1.1 times longer than the distance between the two fastening points, measured on the lining surface (see Fig. 11a and 11b). A good heat transfer and at the same time a gentle treatment of material preventing an unnecessary dust creation are the main advantages of thermochains. Their disadvantages are a low shell cleaning ability and a very limited material transporting efficiency. Thermochains only have a limited sliding movement on the kiln lining compared to garlands and for this reason the sense of hanging (reverse or non-reverse) does not make too much difference. Thermochains cannot be used in the upper and central part of the chain system where the shell cleaning efficiency is of an eminent importance. They should be used in the lowest (hot) part of the system in such cases when another type of chain arrangement enabling the same heat exchange would lead to an excessively high dust emission. Figure 11a & b:


6/23/2001 - 4:34:37 PM

Page 128


Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 5. TYPES OF CHAIN LINKS

5.

TYPES OF CHAIN LINKS

Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 5. TYPES OF CHAIN LINKS / 5.1 Round Links

5.1

Round Links

The round links (Fig. 12a) can rotate slowly when kiln is in operation which has two advantages: •

the wear is distributed equally over the whole link circumference, and therefore, in comparison with other types, round links have a longer life time.

•

due to the rotation of links their surface is kept clean which enables a good heat exchange (links are not surrounded by an insulating mud layer).

Because of the properties mentioned above round links should be preferred to other types mainly in those zones where the material is wet and sticky. For calculations of the chain surface area and chain weight, the following formula can be applied:

♦ surface area of 1 link ♦ volume of 1 link ♦ weigt of 1 link

O = π 2S(s + d ) V = 0•

s 4

G = V • γ (γ ≈ 7.8 t m 3 )

The above formulas are valid for round chain links with a round wire cross section. Some chain suppliers express the mentioned specific properties as chain surface area per 1 m of chain and chain weight per 1 m of chain. These values are formulated as follows:


6/23/2001 - 4:34:37 PM

Page 129

"Holderbank" - Cement Course 2000 01m =

01m link •

1000mmm d (mm )

G1m =

G1m link •

1000mmm d (mm )

Therefore, if the chain weight and surface area are to be calculated from the 1m specific data, the chain length LCH should be measured as shown in the following sketch: Figure

Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 5. TYPES OF CHAIN LINKS / 5.2 Long Links

5.2

Long Links

The long links (Fig. 12b) cannot rotate like the round ones, their life time is shorter and their self-cleaning ability poorer. For calculations of the chain surface area and chain weight, the following formulas can be applied: ♦ surface area of 1 link ♦ volume of 1 link ♦ weigt of 1 link

O = πs{2(l − d ) + π (s + d )} V = 0•

s 4

G = V • γ (γ ≈ 7.8 t m 3 )

The above formulas are valid for long chain links with a round wire cross section. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 5. TYPES OF CHAIN LINKS / 5.3 Oval Links

5.3

Oval Links

The oval links (Fig. 12c) are similar to long links, they have similar properties and their surface area and weight can be calculated (with a negligible mistake) by means of the formulas mentioned in the part 5.2. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:38 PM

Page 130

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 5. TYPES OF CHAIN LINKS / 5.4 Other Types of Chain Links

5.4

Other Types of Chain Links

Besides the links types mentioned above, the suppliers occasionally offer various other types of chain links such as asymmetric links, overlapping links (Fig. 12e), links with a non-round wire cross section (Fig. 12d) etc. These types are not to be recommended for cement kilns and are very rarely used, except the links with a non-round wire cross section. Figure 12: Type of Chain Links

Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 6. CHAIN MATERIAL

6.

CHAIN MATERIAL

Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 6. CHAIN MATERIAL / 6.1 Mild Steel Chains

6.1

Mild Steel Chains

A big majority of the kiln chains are made of mild steel. In order to withstand the friction between chains, between chain and raw material, between chain and hanger etc. they have to be made of a "through hardened" steel. This hardening (result of a thermal treatment) is one of the important chain properties and only experienced kiln chain suppliers are in possession of the necessary know-how for this procedure. For this reason, even the mild steel chains should be purchased from experienced suppliers. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 6. CHAIN MATERIAL / 6.2 Heat Resistant Alloy Chains

6.2

Heat Resistant Alloy Chains

Due to the thermal load of the kiln and the chain temperature at the hot end of the system, the portion of the heat resistant chains should be some 15% of the total weight of chains. Mild steel chains should not be installed in that part of the system where a chain temperature of 450°C or more is to be expected. The chain suppliers offer a lot of various heat resistant steel qualities. Besides the thermal treatment the chemical composition of the respective alloy is the most important criteria. The two main components are nickel and chromium. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:38 PM

Page 131

"Holderbank" - Cement Course 2000 Nickel increases the alloy resistivity against reducing kiln atmosphere, but a higher nickel content becomes dangerous if the raw material or kiln gases contain sulphur compounds which could react with it. Chromium increases the alloy resistivity against high temperatures, but a higher content of chromium makes the alloy sensible to sudden changes of temperature. Above 20% Cr and at operating temperatures between 600 - 900°C an intermetallic compound can occur (-phase) which makes the alloy very brittle and causes destruction. Whether this phenomenon occurs or not depends also on the Ni-content and on other elements. Alloys which are sensitive to -phase formation must be used at working temperatures above 900°C. Because of the properties mentioned above, it is necessary to find a compromise. Good results have been achieved with heat resistant alloys containing approx. 18 to 25% Cr and approx. 5 to 13% Ni. Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 7. CHAIN HANGERS

7.

CHAIN HANGERS

Chain hangers can be divided into 2 groups, namely single chain hangers and multiple chain hangers. The single hangers have only one chain fastening point, the multiple hangers have several fastening points. The single chain hangers should be preferred in cases, where larger distances between the chain hanging points are desired - this solution enables to keep the weight of hangers as low as possible. (Under favorable circumstances, the weight of hangers should not exceed some 15% of the weight of the chains). The chain can be fixed to the hanger by means of a shackle (Fig. 15). Specially designed hangers enable shackleless hanging of chains. In Fig. 13 and 14 some examples of the chain hangers are presented, namely: Fig. 13 a, b, c

single hangers with shackles

Fig. 13 d

single hanger, shackleless

Fig. 13 e, f Fig. 14 b, c

multiple hangers with shackles

Fig. 13 g Fig. 14 a

multiple hangers, shackleless

Figure13:


6/23/2001 - 4:34:38 PM

Page 132


Figure 14:

The gap between the plate of a multiple hanger and the steel kiln shell should not be too wide. Gaps exceeding some 20 mm enable, when the kiln lining is worn out, penetration of chains into the space between kiln shell and hanger. This leads to tangling of chains and finally to a destruction of chains and hangers. Figure 15:

Shackle

Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 8. MAIN CHARACTERISTIC DATA OF CHAIN SYSTEMS

8.

MAIN CHARACTERISTIC DATA OF CHAIN SYSTEMS


6/23/2001 - 4:34:39 PM

Page 133

"Holderbank" - Cement Course 2000 The following average data are to be understood only as a very rough guideline. Length of the total chain system

18 to 35% of total kiln length (some 6 to 10 D)

Total weight of chains: - smaller kilns (< 1000 t/d) - bigger kilns

9 to 12% of daily kiln output 11 to 20% of daily kiln output

Total surface area of chains: - smaller kilns - bigger kilns

Fe/Fii*) = 1.1 to 1.8 Fe/Fi = 1.5 to 2.6

* Fe = total surface area of chains Fi = total surface are of kiln shell (inside lining)

Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 9. ANNEXES

9.

ANNEXES

Annex 1:

Example of chain system record keeping (DA K5)

Annex 2:

Example of material sampling port arrangement on wet kiln (BP K1)

Annex 3:

Example material sampling and mass balance (BP K1)

Annex 4:

Example of material sampling port on wet kiln (BS K1)

Annex 1:

Example of chain system record keeping (DA K5)


6/23/2001 - 4:34:39 PM

Page 134


Annex 2:

Example of material sampling port arrangement on wet kiln (BP K1)

Annex 3:

Example material sampling and mass balance (BP K1)


6/23/2001 - 4:34:39 PM

Page 135


Annex 4:

Example of material sampling port on wet kiln (BS K1)

Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 10. LITERATURE

10.

LITERATURE

P. Weber

Wärmeübergang und Wasserverdampfung beim Nassdrehofen Zement-Kalk-Gips (1959), No. 5, p. 208 ff

Legrand:

Calcul des coeffients de perte de charge et de filtration d'un rideau de chaines Rew. Mater. Constr. (1961), No. 549, p. 327 - 332

De Beus,

Cement Technology: Design of Kiln Chain Systems Narzymski: Rock Products 69 (1966), No. 7, p. 77 ff

Bennet, C.S.:

Chain Experience in Wet Process Kilns Minerals Processing, Vol. 8 (1967), No. 10, p. 18 - 19

De Beus, A.J.:

Mind your Chain Dollar Minerals Processing, Vol. 8 (1967), No. 10, pa. 12 - 17

Feiser, C.F.:

Comments on Kiln Chain Developments in the Cement Industry Minerals Processing, Vol. 8 (1967), No. 9, p. 11 - 13


6/23/2001 - 4:34:40 PM

Page 136

"Holderbank" - Cement CourseProcessing, 2000 Minerals Vol. 8 (1967), No. 9, p. 11 - 13 Drayton, W.E.:

Know your Kiln's Chain System Rock Products (1972), H. 5, p. 88 ff

Numerous TC-VA Reports Numerous Reports of "Holderbank" Group Plants "Datenbank-Blatt" Collection of TC-VA

Process Technology / B05 - PT II / C02 - Internal Kiln Fittings / Kiln Chain Systems / 11. TEST QUESTIONS

11.

TEST QUESTIONS

1)

Name the main functions of a chain system and explain them!

2)

According to the changing properties of material passing along the kiln tube, which individual zones do we distinguish inside of a chain system? Describe them, define the main requirements which the chains have to meet in each individual zone!

3)

Name the different arrangements of chains and describe them!

4)

Each arrangement has advantages as well as disadvantages, please list them!

5)

Follow the stream of material in the kiln and define which chain arrangements can be used for the individual parts of the system! Explain why!

6)

Explain the reverse sense of hanging garlands! Compare it to the sense of hanging thermochains

7)

Would you recommend garlands for the downstream (hot) end of the system, thermochains for the upstream (cold) end? Please explain why!

8)

What are the specific advantages of round link chains?

9)

Describe the material and gas temperature profile along the system!


6/23/2001 - 4:34:40 PM

Page 137

"Holderbank" - Cement Course 2000 10)

What portion of heat resistant steel chains would you recommend for a system? Define the main criteria for estimating this amount!

11)

Which basic types of hangers do you know? In which case would you prefer single chain hangers?

12)

Name the main characteristic data of a chain system: Length, total weight and total surface area of chains?


6/23/2001 - 4:34:41 PM

Page 138

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C03 - Refractory Linings

C03 - Refractory Linings


6/23/2001 - 4:34:41 PM

Page 139

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems

Refractory Lining of Cement Kiln Systems

1. IMPORTANCE OF REFRACTORIES FOR CEMENT PRODUCTION 2. SUBDIVISION OF THE BURNING PROCESS AND SELECTION OF REFRACTORY QUALITIES 2.1 Drying Zone (applies only for wet and semi-wet process kilns) 2.2 Chain Zone 2.3 Preheating Zone 2.4 Calcining Zone 2.5 Transition Zone 2.6 Sintering Zone 2.7 Cooling Zone 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION 3.1 General 3.2 Brick Joints and Jointing Materials 3.3 Thermal Expansion Compensation 3.4 Lining Methods 3.5 Stripping and Removing of Old Lining 4. LINING OF PREHEATERS, KILN HOODS AND COOLERS 4.1 Suspension and Grate Preheaters 4.2 Kiln Hood 4.3 Clinker Coolers 5. OPERATIONAL ASPECTS REGARDING KILN LININGS 5.1 Heat-Up of Rotary Kiln Systems 5.2 Kiln Shell Deformation 5.3 Fluctuating Process Parameters 6. CONCLUSIONS 7. TEST QUESTIONS


6/23/2001 - 4:34:41 PM

Page 140

"Holderbank" - Cement Course 2000 SUMMARY Since the sole costs of refractory materials per ton of clinker produced play only a minor role in the overall manufacturing costs, the prior task of refractory lining optimization is to prolong the useful life of the installed materials which fact in turn increases the kiln availability for production. Selecting of the appropriate refractory qualities for the different kiln zones is of particular importance in various problem areas of the kiln system. As an example, alkali corrosion in calcining zones may be reduced by selecting acidic types of bricks, whereas eutectic reactions in the safety zone generally can only be countered by installing basic types of refractories. Apart from selecting the correct refractory materials, the work quality of lining installation is a key element in obtaining a long refractory life. Tight lining and adequate joint dimensions are important features. The operational influences on refractories materials, the work quality of lining installation is a key element in obtaining a long refractory life. Tight lining and adequate joint dimensions are important features. The operational influences on refractories performance are of thermal, chemical and mechanical nature. By considering an adequate heating-up procedure, thermal and mechanical damage can be avoided during start up. Measures to provide an optimum raw material composition on one hand and to avoid kiln feed fluctuations on the other hand may be necessary to decrease wear of chemical nature and to improve coating stability. In the area of kiln tyres, mechanical destruction of bricks can be caused by excessive kiln shell deformation. Continuous control of the mechanical condition of the kiln shell in the tyre areas allows to detect possible deterioration and to take appropriate measures in an early stage in order to prevent damage to the lining from this side. Refractory lifetime is generally not a matter of one single criterion. The influences described in this paper are nearly always jointly responsible for the results achieved and should be considered as an entirety. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 1. IMPORTANCE OF REFRACTORIES FOR CEMENT PRODUCTION

1.

IMPORTANCE OF REFRACTORIES FOR CEMENT PRODUCTION

Cement plans represent quite big amounts of invested capital which fact demands for a production as continuous as possible in order to guarantee a corresponding return. The sole cost of the refractory lining of a cement kiln, including the outlay for removal of the old and installation of the new materials amounts to barley 2 to 4% of the cement production costs. The losses caused by interruption of production, on the other hand, are already higher than the annual lining costs when the kiln has been idle for 10 to 15 days. Improvement or optimisation of the useful life of refractories with the object of impairing the availability of the kiln as little as possible by shut-down for relining, is therefore one of the major objectives for cement producers. The durability of refractory linings is mainly influenced by three factors: ♦ The choice of the quality of materials employed in the various zones. ♦ The installation of the lining with due consideration of the methods of placing, size of joints and jointing materials. ♦ Due attention to operational criteria which affect the durability of the lining, namely the correct procedure for heating-up and for cooling down of a kiln system on the one hand and the © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:41 PM

Page 141

"Holderbank" - Cement Course 2000 minimisation of process fluctuations in order to maintain the continuous operation of the kiln on the other hand. This paper contains accordingly remarks and information on how to consider the above criteria in order to improve refractory performance and kiln availability. For types and classification of refractory materials and other aspects related to materials technology, reference is made to the MA Cement Course Documents Vol. 2. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 2. SUBDIVISION OF THE BURNING PROCESS AND SELECTION OF REFRACTORY QUALITIES

2.

SUBDIVISION OF THE BURNING PROCESS AND SELECTION OF REFRACTORY QUALITIES

With regard to the requirements on refractory materials, the kiln system can be subdivided into various zones according to specific operating conditions (Fig. 1). The designation of the various zones refers to the respective reactions in the burning process which, however, are anything but clearly defined as they overlap in both directions. The point, at which the change from one refractory quality to another is made can usually be determined only by observing the behaviour of the lining in operation. Fig. 1

The main points of the following explanations are also summarised in Table 1. Table 1

Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 2. SUBDIVISION OF THE © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:41 PM

Page 142

"Holderbank" - Cement Course 2000 BURNING PROCESS AND SELECTION OF REFRACTORY QUALITIES / 2.1 Drying Zone (applies only for wet and semi-wet process kilns)

2.1

Drying Zone (applies only for wet and semi-wet process kilns)

In the drying zone, the water content of slurry or nodules is evaporated. This reaction is almost terminated at material temperatures of 100°C. In wet process kiln, the drying zone is generally a part of the chain zone. In grate pre-heater kilns, drying takes place in the first pre-heater chamber. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 2. SUBDIVISION OF THE BURNING PROCESS AND SELECTION OF REFRACTORY QUALITIES / 2.2 Chain Zone

2.2

Chain Zone

In wet process kilns, the chain zone can be subdivided into a drying and a preheating zone. The material temperature at the outlet of the chain zone depends on arrangement and qualities of chains as well as type of system, i.e. nodule or dust kiln, and is in the order of magnitude of 200 to 400°C. The length of the chain zone ranges from 7 to 9 kiln diameters in wet kilns and from 3 to 5 kiln diameters in long dry kilns. The lining of the chain zone should be of a dense, low alumina firebrick with high abrasion resistance to withstand the abrasive action of the chains. An air setting mortar should be used as there is insufficient temperature to vitrify a heat setting mortar. In wet kilns, furthermore, the mortar must be water resistant. Since the arrangement of the chain hangers requires extensive modification work, sometimes dense fireclay castables are installed, which, however, should be carefully compacted in order to obtain high abrasion resistance. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 2. SUBDIVISION OF THE BURNING PROCESS AND SELECTION OF REFRACTORY QUALITIES / 2.3 Preheating Zone

2.3

Preheating Zone

In the preheating zone, hydrate-water is driven off and the raw material is heated up to approx. 700°C. The length of the preheating zone in long kilns is approx. 4 to 8 kiln diameters (excluding chain zone). In short pre-heater kilns the preheating zone is a part of the pre-heater. Regarding the lining of preheaters, reference is made to paragraph 4.1: Lining of Suspension and Grate Preheaters. The preheating zone of long kilns is usually lined with low alumina firebricks or, for better thermal insulation, with lightweight firebricks. With respect of lining stability, two layer lining is generally not recommended in rotary kilns of more than 3.5 m diameter. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 2. SUBDIVISION OF THE BURNING PROCESS AND SELECTION OF REFRACTORY QUALITIES / 2.4 Calcining Zone

2.4

Calcining Zone

The calcining reaction starts already at material temperatures below 600°C and is completed at approx. 1200°C. Since, however, the most part of calcination takes place between raw material temperatures of 700 to 900°C, usually this range is designated as calcining zone. The length of the calcining zone in long kilns is in the order of magnitude of 6 to 9 kiln diameters. In © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:42 PM

Page 143

"Holderbank" - Cement Course 2000 short pre-heater kilns calcining takes partially place in the pre-heater. There, the length of the calcining zone in the kiln amounts to approx. 4 to 6, in kilns with precalciners 1 to 3 kiln diameters. From the refractory point of view, the calcining zone can still be lined with normal fireclay bricks or, for better insulation, with fireclay lightweight bricks. In case of alkali attack good operational results have been achieved with acidic light weight bricks with an SiO2 content above 65%. These bricks form with alkalis a vitreous layer of 2 to 3 mm thickness on the surface, which prevents the further alkali infiltration. Two layer linings are generally not recommended. There are, however, two layer bricks on the market, consisting of a dense working part and a porous insulating part. Such bricks are generally installed for heat saving purpose, if lightweight bricks show unsatisfactory operating results. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 2. SUBDIVISION OF THE BURNING PROCESS AND SELECTION OF REFRACTORY QUALITIES / 2.5 Transition Zone

2.5

Transition Zone

The transition zones are located on both sides of the sintering zone. Since the length of the sintering zone varies with process fluctuations, the transition zones are characterised in particular by unstable coating formation. Usually, the inlet-side transition zone is further subdivided into a colder section, so-called safety zone, and into a hot section, the proper transition zone. The safety zone has a length of approx. 2 kiln diameters and is usually lined with alumina rich bricks with Al2O3 content of 50 to 60%. The bricks must have good thermal shock resistance and low porosity in order to have good resistivity against chemical attack. The application limit of alumina rich or high alumina bricks in the transition zone is generally determined by eutectic reactions in the system Al2O3 CaO - SiO2 or by alkali-spalling. In such cases, lining lifetime can be improved only by installing basic bricks. The transition zones are often exposed to considerably higher loads than the sintering zone itself. Quite often no or only unstable coating is formed. Thus, the bricks are exposed directly to the flame radiation and considerable temperature variations. The lengths of the transition zones vary from 2 to 4 kiln diameters.They are usually lined with chrome free magnesia-spinell bricks produced of very pure, synthetic materials or with magnesia-chrome bricks containing approx. 60 to 70% MgO. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 2. SUBDIVISION OF THE BURNING PROCESS AND SELECTION OF REFRACTORY QUALITIES / 2.6 Sintering Zone

2.6

Sintering Zone

Although this zone is often referred to as burning zone preference is given to the expression sintering zone on account that this better describes the mechanism of reactions taking place. The sintering zone is generally covered with a more or less stable coating, formed with clinker and liquid phase. Liquid phase starts to form at raw material temperatures above approx. 1250°C. However, as the lining surface temperature is higher than the one of the raw material, coating formation starts already at raw material temperature above 1050 to 1150°C. The term ‘sintering zone’ can also be explained as zone of increased material diffusion with formation of C3S modifications, the latter starting at temperatures above approx. 1100°C. The maximum material temperature in the sintering zone is in the order of magnitude of 1400 to 1500°C at the beginning of the cooling zone. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:42 PM

Page 144

"Holderbank" - Cement Course 2000 The length of the sintering zone is generally between 3 to 5 kiln diameters and depends largely on the shape of the flame and type of fuel. Coal flames give generally short, oil flames medium and gas flames longer sintering zones. In kilns with precalciner, the sintering zone length amounts to 5 to 8 kiln diameters due to the higher specific material throughout. The bricks in the sintering zone are exposed to chemical attack by the liquid phase of the clinker and by alkali sulphates, high temperatures and, depending on coating stability, high thermal shocks. These conditions are best met by basic bricks due to their high refractoriness and good resistance against chemical attack. Thus, generally, chrome free magnesia-spinell bricks, magnesia-chrome or dolomite bricks are installed. When using chrome-free magnesia-spinell bricks, qualities particularly developed to improve coating adhesion should be chosen (qualities based on natural sinter). Dolomite bricks have generally good operating performance in zones with coating formation. The price for dolomite bricks is only approx. 60% of that of magnesite. A disadvantage of dolomite bricks is, however, its sensitivity to moisture. Thus, for longer storing times, these bricks are to be stored air-tight. During longer kiln stops the dolomite lining is to be protected against moisture by tightening the kiln tube and putting a hygroscopic agent (e.g. burned lime) in the kiln. Raw material analysis and tests can give some indications on selection of brick quality to be installed. The final decision, however, is often to be based on operating experience, i.e. by trial and error. Insulation of the burning zone with insulating back-lining is generally not recommended. By insulating, basically the hot face temperature of the bricks will be increased, resulting in reduced coating thickness and higher chemical and thermal load of the bricks. Furthermore, two layer lining is less stable and leads, particularly in big kilns, often to early failure due to relative movement and loosening of the lining. In cases, where a coating does not form, insulation may be helpful in reducing heat losses and protecting the kiln shell, particularly in the tyre area. In such specific cases basic bricks with back-linings of 40 mm hard fireclay slabs are sometimes installed. However, generally installation of two layer linings is not recommended due to reduced lining stability. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 2. SUBDIVISION OF THE BURNING PROCESS AND SELECTION OF REFRACTORY QUALITIES / 2.7 Cooling Zone

2.7

Cooling Zone

The cooling zone in the rotary kiln reaches approx. from the burner nozzle to the kiln outlet. In this zone, the clinker is cooled down from its maximum burning temperature of 1400 to 1500°C to approx. 1350°C in kilns with grate, rotary or shaft coolers and to approx. 1250°C in kilns with planetary coolers. The cooling zone in kilns with planetary coolers has a length of approx. 1.5 to 2 kiln diameters. It consists generally of a cam lining for better cooling efficiency and a dam ring for equalising of clinker distribution to the individual cooler inlet openings. For camlining and damring, dense high alumina bricks with approx. 80% Al2O3 and considerable thermal shock resistance are generally used. The backing of the damring can be made of hard fireclay bricks with adequate mechanical strength. The length of the cooling zone of kilns with grate, rotary or shaft coolers is generally 0.5 to 1 kiln diameter. It is usually lined with dense, abrasion resistant high alumina bricks containing 80% Al2O3. In case of heavy chemical attack it can be necessary to line this zone with basic bricks, which however, should have high thermal shock resistance (magnesia-spinell bricks). Since the outlet zone is often free of coating, kiln shell temperature will increase due to the higher thermal conductivity of basic bricks. Due to the high thermal and mechanical load of the bricks in this zone, insulating back-lining is © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:42 PM

Page 145

"Holderbank" - Cement Course 2000 not recommended. The end of the cooling zone, the nose ring, is one of the most critical points of cement kiln linings. Its lifetime is often lower than the one of high loaded sintering zones. Shape and quality of nose ring bricks requires therefore careful consideration. Basically, complicated special shapes should be avoided since special shaped bricks are often ‘hand-made’ and can have a much poorer quality than machine made bricks of the same composition. Nose ring design according to Fig. 2a and 2b would allow to use standard brick sizes with minor modifications and is to be preferred against the design according to Fig. 2c. Fig. 2a Nose Ring Design Using Bricks

Fig. 2b Nose Ring Design Using Bricks

Fig. 2c Nose Ring Design Using Bricks


6/23/2001 - 4:34:43 PM

Page 146


Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION

3.

IMPORTANT FEATURES OF REFRACTORIES INSTALLATION

Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION / 3.1 General

3.1

General

Depending on specific requirements and local conditions, refractory brick linings in rotary kilns are installed dry or with mortar. For specific applications, unshaped materials are used, which are installed by casting, gunning or ramming. Apart from selecting the correct refractory materials, the quality of the installation procedure is a key element in obtaining a durable refractory lining and long service life. High refractory lining quality requires: ♦ Tight installation of brick ♦ Correct compensation for thermal expansion with adequate joints ♦ Selection of suitable mortar for mortar joints ♦ Selection of the optimum lining method, so that kiln rotation in the cold state can be minimised Loose linings may result in spiral displacement of entire lining sections (see Fig. 3). The bricks jam in a twisted position and can no longer expand freely when hot. Result: spalling due to excessive mechanical stresses. Fig. 3

The lining ring must run true to the vertical kiln axis. Using a welding seam as reference, alignment lines can be drawn on the shell at regular intervals. Installation of closure bricks requires special care. Procedure (Fig. 4) 1) Tension the ring with a hydraulic spreader jack. 2) Insert the most tightly fitting combination of key bricks © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:43 PM

Page 147

"Holderbank" - Cement Course 2000 3) With basic lining: drive in key plates between axial joints of the ring until ring becomes completely stable. Use only key plate per joint. With linings containing Al2O3: closing with key plates should be avoided because iron reacts with Al2O3 at high temperature to form a compound with a low melting point. For this lining quality, the combination of key bricks must fit particularly well. Fig. 4

For closing lining rings, key bricks of various dimensions are available which can be combined to obtain optimum closures. If necessary, standard formats can be cut to the desired widths. The brick lengths of the last lining ring generally will have to be cut as well. Rotary kiln bricks may only be cut to 2/3 of their original length or width; if necessary two rows must be cut. Offset brick linings as shown in Fig. 5 may not be installed in rotary kilns. Each ring must be self-supporting in order to avoid shearing stress occurring as a result of relative lining motion. Fig. 5

Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION / 3.2 Brick Joints and Jointing Materials

3.2

Brick Joints and Jointing Materials

Bricks laid in mortar generally give better performance than dry placed bricks provided that the mortar used is of the correct quality. Mortar joints must be even and not thicker than 1.5 to 3 mm, preferably 2 mm. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:43 PM

Page 148

"Holderbank" - Cement Course 2000 To achieve the desired lining quality with mortar, qualified and experienced bricklayers are required, who regrettably are becoming a rarity in many countries. In contrast, dry bricking can be executed in less time by less specialised personnel. Dry lining is therefore the standard procedure for basic bricks. They are currently almost exclusively laid dry with or without steel plates in every radial joint. The steel plates react at high temperatures with the magnesia brick to form a highly refractory combination of magnesiaferrite which is claimed to give increased stability to the ring. Clench lining (dry lining without steel plates) is becoming more and more popular. Its main advantages are: ♦ tighter lining ♦ no handling of steel plates ♦ no separation of steel plates required when recycling old bricks Expansion allowances in axial and circumferential joints of the brick linings are sometimes necessary in order to compensate for the thermal expansion of the brick. With insufficient compensation of thermal expansion, the bricks can be mechanically overloaded at operating temperature. Excess compensation conversely leads to loose lining or even lining displacement and collapse of brick rings. Therefore the correct dimensioning of thermal expansion allowance is a most important feature of ensuring a stable lining. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION / 3.3 Thermal Expansion Compensation

3.3

Thermal Expansion Compensation

The thermal expansion of a refractory lining (re: Fig. 6 and 7) is partially compensated by the following factors: ♦ thermal expansion of kiln shell ♦ burning out of glue if sheet metal shims are glued to the brick surface ♦ lining ‘inaccuracy’ ♦ compressibility and pyroplastic behaviour of the bricks Fig. 6

Fig. 7


6/23/2001 - 4:34:44 PM

Page 149


The thermal expansion which cannot be absorbed by the above factors has to be compensated by: ♦ the elasticity and pyroplasticity of mortars ♦ the elasticity and softening of steel plates ♦ combustible materials (cardboard shims) The thermal expansion of refractory linings is strongly dependent on materials and operating conditions. For the correct dimensioning of the expansion allowance, the recommendations of the manufacturers of each brick type should be strictly followed. For dry lining consisting of basic bricks, most manufacturers recommend a longitudinal expansion allowance of 1%, i.e. installation of a 2 mm cardboard in each circumferential joint for 200 mm brick length (for designation of joints see Fig. 8). Most qualities of basic bricks are normally laid without cardboards in the axial joints in order to achieve the tightest possible lining at working temperature. Fig. 8

For some particularly dense, high fired qualities additional expansion allowances within the rings are recommended. In addition to the normal steel plate, a 1 mm thick, 50 mm wide cardboard is to be installed near the hot face of the axial joints after every 4th to 8th brick. To prevent the cardboards from slipping-in, they are bent at right angles to the brick face (see Fig. 9). Fig. 9


6/23/2001 - 4:34:44 PM

Page 150


If basic bricks are laid in mortar, additional expansion allowance in the axial joints is not necessary. In the circumferential joints an allowance of about 0.2% is made, i.e. a 2 mm cardboard is inserted between each fifth ring without mortar. Alumina and fireclay bricks as a rule are installed without expansion allowance in the interest of a stable lining. This is possible due to the lower thermal expansion of these bricks compared to the basic materials and due to good pyroplastic properties which permit accommodation of the stresses at high temperatures. Steel plates are not used with Alumina bricks, since steel and aluminium silicates form low melting compound at high temperature, which can lead to lining damage. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION / 3.4 Lining Methods

3.4

Lining Methods

The following factors have to be considered for kiln lining methods: ♦ safety ♦ tight installation of the bricks ♦ time required for lining The methods applied can be generally classified as: ♦ rotating methods, i.e. the kiln has to be turned during lining ♦ formwork methods: the bricks are installed on a curved formwork without turning of the kiln Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION / 3.4 Lining Methods / 3.4.1 Rotating Methods

3.4.1

Rotating Methods

With the rotating methods, kiln zones of 5 to 20 m length are lined at one time. Work is carried out only in the bottom of the kiln, which is rotated intermittently as the work of lining proceeds. The bricks must be fastened to the shell, e.g. by means of screw jacks or glues, as the ring of bricks is rotated into the overhead position before being closed. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION / 3.4 Lining Methods / 3.4.1 Rotating Methods / 3.4.1.1 Screw-Jack Method (Fig. 10)


6/23/2001 - 4:34:44 PM

Page 151

"Holderbank" - Cement Course 2000 3.4.1.1

Screw-Jack Method (Fig. 10)

The lining is fastened by means of screw-jacks, usually of steel. The bricks are clamped in position by means of wooden wedges. The screw-jacks should be furnished with pressure control facilities as e.g. discsprings or load cells. The screw-jack method should not be employed for kilns more than 4 m in diameter. Fig. 10

Lining Procedure 1) Lining of slightly more than the bottom half of the kiln. 2) Fastening of the lining by means of screw-jacks and wooden wedges. The distance of the screw-jacks is 0.5 - 1 m, depending on kiln diameter. 3) Turning the kiln by one quarter of circumference. 4) Lining of the third quarter. 5) Fastening according to point 2. 6) Turning of the kiln until the last section is on the bottom 7) Lining of the last section and keying with closure bricks. Disadvantages of the Screw-Jack Method ♦ Kiln has to be turned during lining. ♦ With large kilns there is a danger of the screw-jacks slipping and the shell being distorted. Average Lining Speed with the Screw-Jack Method ♦ 1 to 3.5 m2/h or 0.17 to 0.25 m2/man-hour Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION / 3.4 Lining Methods / 3.4.1 Rotating Methods / 3.4.1.2 Gluing Method

3.4.1.2

Gluing Method

With the gluing method the bricks are glued to the shell in an alternative manner, following a specific pattern according e.g. Fig. 11. The glues used are two component expoxy or polyurethane resins and for very low temperatures down to minus 25°C there are synthetic resins based on polyesters or polyacrylates. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:45 PM

Page 152


The glues decompose after heating up above 200°C and lose their effect. Lining procedure: 1) Cleaning of the shell down to the bright metal, if necessary with the aid of a grinder. The surface must be free from dust or grease. 2) Gluing of the first lining sector of approx. 5 brick rows. 3) Installation of the next sector without glue etc. The number of bricks between the glued sectors depend on brick weight, kiln diameter, glue properties, etc. Generally the brick and/or glue suppliers specify the gluing pattern. As a rule approx. 20% of the bricks laid are glued. Glue consumption amounts to approx. 0.4 to 0.8 kg/m2 of lined area or approx. 2 kg/m2 glued area. Using the gluing method, the following points require careful consideration: ♦ The start to be glued must be fitted together before the adhesives start to set. ♦ The kiln should not be turned before the glue has developed sufficient strength. ♦ The glue must be carefully prepared according to the suppliers specifications, particularly with respect to mixing ratio. Disadvantages of the gluing method: ♦ Kiln has to be turned during lining. ♦ Tightening of the brick rings before keying is not possible, but only the bricks between two glued sectors. Thus, lining mistakes are not easy to be localised. ♦ Safety risk if kiln is turned before glue has developed sufficient strength. With the gluing method, average lining speeds of 3 to 7 m2/h or 0.4 to 1.2 m2/man-hour can be achieved. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION / 3.4 Lining Methods / 3.4.2 Formwork Methods

3.4.2

Formwork Methods

The formwork methods can nowadays be considered as the standard lining method. They are safe in operation and permit very accurate installation of the bricks. With the curved formwork methods, first the lower half shell of the kiln is lined without any particular © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:45 PM

Page 153

"Holderbank" - Cement Course 2000 aid, and then, the upper half ring by ring with the aid of the curved formwork. There is a wide choice of equipment, form simple wooden forms to hydraulically operated mechanised forms. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION / 3.4 Lining Methods / 3.4.2 Formwork Methods / 3.4.2.1 Pogo Stick Method

3.4.2.1

Pogo Stick Method

The main equipment for the pogo stick method is: ♦ supporting construction ♦ working scaffold ♦ wooden curved drum ♦ spring loaded pogo sticks The working scaffold is installed upon the lined bottom half of the kiln. Then the bricks of the upper half section are installed and fixed by means of the pogo sticks. Generally two bricklayers and two helpers line from each side towards the top position. After keying, the pogo sticks are removed and the formwork is put into the new position. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION / 3.4 Lining Methods / 3.4.2 Formwork Methods / 3.4.2.2 Wooden Curved Form Method

3.4.2.2

Wooden Curved Form Method

The main equipment for the wooden curved form is: ♦ supporting framework ♦ working platform ♦ wooden curved formwork ♦ wooden wedges The supporting framework is so constructed to permit a fork lift vehicle to move beneath the working platform, lifting the brick pallets to the platform. It is installed on the lined bottom half of the kiln. The bricks of the upper section are then laid on the wooden arch and pressed against the shell by means of wooden wedges. Work is done from both sides towards the top by two men on each side. After keying of a ring the curved formwork is moved into the next position, thereby, normally the wooden wedges are loosening themselves. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION / 3.4 Lining Methods / 3.4.2 Formwork Methods / 3.4.2.3 Mechanised Curved Forms (Fig. 12)

3.4.2.3

Mechanised Curved Forms (Fig. 12)

Mechanised curved forms are mechanised variations of the simple arch forms. They basically consist of a travelling working platform on a supporting framework. The curved form is installed on the working platform and is equipped with mechanically or pneumatically operated pressure tools by which the bricks are pressed against the shell. The supporting frame is so constructed as to allow the passage of a fork lift vehicle. The most used mechanised lining machines are the Mult-O-Ring, the DAT-Appartus and the Reintjes lining machines. The Mult-O-Ring is equipped with two parallel rows of pressure tools which almost doubles the speed of work. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:45 PM

Page 154

"Holderbank" - Cement Course 2000 The lining procedure is similar to that with simple curved forms, except that the fastening of the bricks is mechanised. With mechanised lining machines very high lining speeds of up to 6.5 m2/h or 0.5 m2/man-hour for mortared linings and up to 9.5 m2/h or 0.75 m2/man-hour for dry lining can be achieved. Fig. 12

Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION / 3.4 Lining Methods / 3.4.3 Lining with Unshaped Materials

3.4.3

Lining with Unshaped Materials

Unshaped refractory materials can be installed by casting, gunning or ramming. The installed masses are fixed by metallic anchors which provide a good lining stability even in cases of local damages or spallings. Usually V-type anchors, generally delivered by the refractory supplier, are welded in a cross pattern on the shell. About 15 to 25 anchors per m2 are required, depending on lining thickness. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION / 3.4 Lining Methods / 3.4.3 Lining with Unshaped Materials / 3.4.3.1 Castables

3.4.3.1

Castables

Castables or refractory concrete contain a hydraulic setting agent (high alumina cement). They are installed generally in suitable formworks, e.g. by the rotocasting method (Fig. 13) and compacted by vibrating. In order to avoid spalling due to quick water evaporation during heating up, it can be advisable to perforate the surface by pushing steel wires of 5 mm thickness into the freshly placed castable at a distance of approx. 5 cm. The steel wires are to be removed after hardening of the castables. During the hardening period just after installation of the castable, its surface should be protected against too quick evaporation of water by means of wet cloths or water spray. Fig. 13


6/23/2001 - 4:34:45 PM

Page 155


Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION / 3.4 Lining Methods / 3.4.3 Lining with Unshaped Materials / 3.4.3.2 Guniting Materials

3.4.3.2

Guniting Materials

Guniting materials are supplied either with chemically or with hydraulically setting bonding agents. The materials are placed pneumatically, passing through a hose to a mixing nozzle where water is introduced (Fig. 14). Fig. 14

Guniting should always be done by specialised personnel, since quality of the lining as well as amount of losses in rebounding is highly depending on the ability of the operator. Depending on shape and position of the working surface losses in rebound amount from 10 to 50%. During the hardening period, the gunned surface must be protected against water evaporation similar to castables. Also perforation of the surface by means of steel wires can be advisable. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION / 3.4 Lining Methods / 3.4.3 Lining with Unshaped Materials / 3.4.3.3 Ramming Material

3.4.3.3

Ramming Material

Ramming materials are plastic or granular materials, generally chemically, seldom hydraulically bonded. Due to the low water content, rammed refractories generally have low porosity, high volume stability and strength and good resistance against chemical attack. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:46 PM

Page 156

"Holderbank" - Cement Course 2000 The masses are placed by compacting layer by layer of approx. 25 to 50 mm thickness each by means of a vibrating hammer fitted on a compressed airhammer. Appropriate and homogeneous compression is necessary for good operating performance of rammed materials, therefore, highly qualified personnel is required for placing. Chemically bonded materials should be installed only shortly before heating up. Spare parts lined with chemically bonded rammed materials must be tempered at approx. 250°C. After tempering, the lining surface must be protected against humidity by bituminous paint, in order that they are not damaged by absorbing water during storage. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION / 3.5 Stripping and Removing of Old Lining

3.5

Stripping and Removing of Old Lining

Stripping and removing of old lining and coating manually requires approx. the same time as the relining of the same zone with new bricks. By use of mechanised stripping machines, this dangerous and time consuming operation can be done much safer in considerably shorter time with less personnel. Manually, 6 to 7 men remove approx. 3 to 5 m2/h of old lining. By means of mechanised stripping machines, 2 to 3 men remove approx. 15 to 18 m2/h. The dismantling unit consists generally of a drilling machine chassis with an all-round pivoting arm on the end of which a compressed airhammer or drilling device is mounted (Fig. 15). For removal, small front-end-loaders are used. The shovel is shaped to fit the curve of the kiln shell. Fig. 15

The stripping procedure is generally done in the following steps: 1) If the coating is more than 25 cm thick it should be cleared away before the actual lining is to be opened up. 2) In small kilns of < 4 m diameter the removed coating must be transported away before the stripping of lining starts. 3) The process of opening up a slit should be done very careful in order not to damage the steel shell. 4) When the lining has been opened up, actual stripping can commence. Due to the opening already made in the lining, the stripping tool can more or less work parallel to the shell, thereby causing no damage.


6/23/2001 - 4:34:46 PM

Page 157

"Holderbank" - Cement Course 2000 5) After the upper half of the kiln is stripped, the kiln is rotated so that the remaining brickwork fall down. 6) The rubble can now be carted away by means of the front-end-loader. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 4. LINING OF PREHEATERS, KILN HOODS AND COOLERS

4.

LINING OF PREHEATERS, KILN HOODS AND COOLERS

Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 4. LINING OF PREHEATERS, KILN HOODS AND COOLERS / 4.1 Suspension and Grate Preheaters

4.1

Suspension and Grate Preheaters

The lining of preheaters should have good insulating properties, therefore, basically two or three layer linings are installed. The working lining generally consists of abrasion resistant fireclay bricks with low porosity. In case of alkali attack acid firebricks with SiO2-content above 65% are installed. Monolithic linings are used particularly for complicated shapes where the installation of bricks is difficult and expensive. For plane walls and gas ducts lining with bricks generally is easier and cheaper than lining with unshaped materials. A three layer insulating lining is generally composed of a backing of ceramic fibre board, an intermediate layer of insulting firebricks and the dense working lining. The bricks of the working lining are usually placed with chemically setting mortars in order to obtain gas tight linings. For compensation of thermal expansion, expansion joints are installed horizontally and vertically at distances of approx. 2 m, as well as in corners. The expansion joints have a width of approx. 15 to 20 mm and should be staggered and filled with ceramic fibre mats in order to avoid penetration of dust (Fig. 16). Fig. 16

For stabilising of the linings, anchor bricks and holding irons which are flexibly fitted with the steel shell are used. Roofs are either lined vaulted without use of anchors or flat with hanger bricks and holding irons. Suspended roofs are often lined combined with hanger bricks and unshaped materials. Pre-heater cyclones usually are lined with bricks and insulating back-lining, except the complicated shaped parts as e.g. cyclone inlet and control openings, which generally are lined with unshaped © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:46 PM

Page 158

"Holderbank" - Cement Course 2000 materials. Meal chutes should be lined with highly abrasion resistant materials. Usually dense castables with metallic anchors or prefabricated materials are installed. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 4. LINING OF PREHEATERS, KILN HOODS AND COOLERS / 4.2 Kiln Hood

4.2

Kiln Hood

For kilns equipped with planetary coolers, refractory lining in the hood section is limited to the hinged or otherwise movable kiln door with openings for burners, measuring instruments and observation. The most frequently used lining material is dense castable fireclay together with insulating backing. The lining of the kiln hood of kilns equipped with grate or rotary coolers generally consists of a dense working face and an insulating backing. The working lining must possess high thermal shock resistance and abrasion resistance. In the upper region, where the lining is exposed to direct flame radiation, alumina refractories with Al2O3-contents of up to 70% are used. The lower region is usually lined with dense fireclay. As insulating linings conventional insulating bricks as well as insulating gunning mixes or castables are used. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 4. LINING OF PREHEATERS, KILN HOODS AND COOLERS / 4.3 Clinker Coolers

4.3

Clinker Coolers

(Fig. 17) Approximately 15 to 35% of the length of planetary coolers is refractory lined. This lining should have a high resistivity against thermal shock and abrasion. For the cam lining section dense, high alumina bricks with high hardness and strength are used. The inlet bends are usually lined with dense high alumina, or even pure Corundum castables. For simpler geometric shapes conventional bricks of suitable quality can also be used. In rotary coolers, about 50% of the tube length is lined with refractory materials. In the inlet zones, similar as in planetary coolers, refractories of high strength fireclay are normally installed. The refractory lining of grate coolers, with the exception of the areas immediately above the grate (where direct contact with clinker occurs), is exposed to far a lower extent to thermal fluctuations and abrasion. Normally a two or three layer refractory wall and roof is installed, consisting of an insulating rear and a dense fireclay working lining. The wear zones immediately above the grate consist of particularly abrasion resistant bricks or castables. Fig. 17


6/23/2001 - 4:34:46 PM

Page 159


Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 5. OPERATIONAL ASPECTS REGARDING KILN LININGS

5.

OPERATIONAL ASPECTS REGARDING KILN LININGS

Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 5. OPERATIONAL ASPECTS REGARDING KILN LININGS / 5.1 Heat-Up of Rotary Kiln Systems

5.1

Heat-Up of Rotary Kiln Systems

The heating-up program, which mainly determines the amount of kiln temperature increase per unit of time and the timing of the raw material feed, must consider a number of factors such as type of refractory material, design of kiln system, mechanical systems etc.. The optimum heating program constitutes a compromise which best satisfies the various requirements. Optimum Heating Period: ♦ Refractory Materials •

Material quality

•

Type of bond

•

Installation method

♦ Riding Ring •

Riding ring clearance

•

Riding ring dimensions

•

Monitoring capabilities

♦ Exhaust Gas Flow •

Temperature profile in kiln

•

Permissible temperatures

•

Environmental regulations

Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 5. OPERATIONAL ASPECTS REGARDING KILN LININGS / 5.1 Heat-Up of Rotary Kiln Systems / 5.1.1 Requirements from a Refractory Materials Viewpoint

5.1.1

Requirements from a Refractory Materials Viewpoint

The minimum heating-up duration from the refractory materials viewpoint is given by the type of special © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:47 PM

Page 160

"Holderbank" - Cement Course 2000 bricks and castables installed. Generally, the heating-up specifications of the suppliers must be respected. The minimum heating-up time is indicated by that material which requires the longest heating-up period because the minimum heating-up time can generally be extended but not shortened without causing damage. Exception: basic brick linings tend to hydrate when kept at temperatures between 100 and 450°C for several days. Generally, the heating-up time is influenced by the following materials: ♦ Castable refractories with hydraulic bonding (Fig. 18) •

Approx. 24 hrs setting time at room temperature

•

Approx. 24 hrs drying time at 100 to 200°C

•

Heating-up at the rate of 25°C/h up to 500°C (expels crystal water)

•

In subsequent heating-up cycles, operational temperatures may be reached more rapidly. The speed of reaching operational temperatures might be limited by other factors as e.g. the tyre situation.

♦ Castable refractories with chemical bonding •

Approx. 8 to 16 hrs drying time at 100 to 200°C

•

Heating-up at the rate of 20 to 30°C/h up to 700°C (until chemical bonding is completed)

•

In subsequent heating-up cycles, operational temperatures may be reached more rapidly, as explained above.

♦ Untempered, phosphate bonded bricks •

Heating-up to 700°C (termination of chemical bonding) at the rate of 30°C/h

•

In subsequent heating-up cycles, operational temperature may be reached more rapidly, as explained above

♦ Jointing materials (mortars) •

Theoretically the same rules would apply as for casting mixes; however, the situation is not critical because of the limited joint thickness.

Fig. 18

Based on the above listed criteria, most suppliers of refractory materials recommend for the start-up of new kilns, and after major overhauls with monolithic linings, a maximum heating-up rate of approx. 25°C/h, in the sintering zone, up to a sintering zone temperature of 900°C. (Whereby a previous drying of heat exchanger systems and monolithically lined kiln zones is assumed.) After reaching 900°C, heating -up can continue at the rate of 50°C/h up to working temperature. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:47 PM

Page 161

"Holderbank" - Cement Course 2000 After minor repairs and after shut-downs not caused by defective lining, refractory materials suppliers generally recommend a heating-up rate in the sintering zone of 50°C/h. An example of heating-up scheme is shown in Fig. 19. Fig. 19

Faster heat-up times are sometimes reported. Some Japanese suppliers heat-up new kilns in 5 to 8 hours without any damage to lining and kiln system. Such procedures can, however, not be recommended in general, i.e. without considering the entire situation of lining quality and mechanical details of kiln design. Since expansion joints are dimensioned to produce optimum lining stability under standard working conditions, rapid heat build-up may cause spalling due to excessive mechanical stress of the brick surface because the lining face heats-up more rapidly than the kiln shell. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 5. OPERATIONAL ASPECTS REGARDING KILN LININGS / 5.1 Heat-Up of Rotary Kiln Systems / 5.1.2 Riding Rings (Floating Type)

5.1.2

Riding Rings (Floating Type)

From a riding ring point of view, the minimum heating-up period is determined by the time required to stabilise the temperature difference between the kiln shell and riding rings. The shell will heat-up more rapidly than the tyre. The critical temperature difference results in a constriction of the kiln shell, which will cause permanent deformation if the yield strength of the kiln shell material is exceeded. This in turn will cause excessive play in the riding ring after the normal working temperature is regained, as well as increased ovality of the kiln shell, a factor which may contribute to excessive refractory lining wear. As a general rule, the riding ring creep, i.e. the relative motion between riding ring and kiln shell should be monitored at regular intervals or, even better continuously, at least during the heat-up cycle. Various measures may be taken to protect the riding ring section from overheating. With imminent danger of seizure, the heating-up process should be slowed down or interrupted until a measurable amount of relative movement is again present. For this reason, the tyre creep may become the limiting factor in determining the heating-up rate. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 5. OPERATIONAL ASPECTS REGARDING KILN LININGS / 5.1 Heat-Up of Rotary Kiln Systems / 5.1.3 Exhaust Gas Control

5.1.3

Exhaust Gas Control

Temperature Gradient in Kiln System Already during the heating-up process a temperature gradient is sought which at the start of the raw © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:47 PM

Page 162

"Holderbank" - Cement Course 2000 meal feeding to the kiln is equivalent or similar to the temperature gradient prevailing under normal operating conditions. This condition can be approximately reached under ideal heat-up conditions and with correct flame patterns, because in a dynamic heating process, the cold kiln system is heated along the flow direction of the exit gas while the exit gases are simultaneously cooled. With optimum fuel addition, this procedure is ideally continued with material feed by preheating the raw meal in the counterflow. In practice, however, such a smooth transition is not possible because the amount of material fed can, for mechanical reasons, not be reduced to indefinitely small amounts and because the heat transfer to the meal is governed by endothermic and exothermic reactions. For correct temperature gradients, fuel combustion at lowest possible amount of excess air is a necessity. In this manner, the temperature gradient between gas and lining and the dwell time of the gases within the kiln system will result in optimum absorption of the available heat by the refractory lining. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 5. OPERATIONAL ASPECTS REGARDING KILN LININGS / 5.1 Heat-Up of Rotary Kiln Systems / 5.1.4 Shut-down of Kiln Systems

5.1.4

Shut-down of Kiln Systems

The procedure for shutting down a kiln depends mainly on whether it is a planned shut-down or an emergency stop due to a breakdown. To protect the lining, the cooling rate should, if possible, not exceed the maximum permissible heating rate, i.e. approx. 50°C/h measured in the sintering zone. Excessively rapid cooling may cause damage to the refractory lining due to thermal shock. After stopping raw meal and fuel feed, the exhaust gas damper is closed and the exhaust gas fan is stopped. Closing the exhaust gas path prevents gas circulation within the kiln and the temperature profile is more or less maintained. The cooling rate of 50°C/h will initially be reached by radiation alone. Only after a dull red heat is reached in the sintering zone cooling should continue with a light draft until the end of the cooling phase is reached. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 5. OPERATIONAL ASPECTS REGARDING KILN LININGS / 5.2 Kiln Shell Deformation

5.2

Kiln Shell Deformation

The refractory life and coating stability in the area of kiln tyres is significantly influenced by kiln deformation. As a result of changes in the kiln shell radius, there is a movement between the bricks which results in local surface pressure, leading to rapid wear and possibly total lining collapse. The measurement of kiln shell deformation can be made during kiln operation with the aid of the ‘Holderbank’ Shell-Test Gauge. The measuring principle is illustrated in Fig. 20. Fig. 20


6/23/2001 - 4:34:47 PM

Page 163


The yoke (A) of the Shell-test gauge covers a circular section with a chord of length of 1 m and height h. During kiln rotation the chord height h changes with the continuously changing curvature radius r of the kiln shell. This change is transferred via a plunger (D) and recorder (C) to produce a polar diagram (shell-test diagram). The closed line drawn on the diagram represents a geometrically similar figure to the deformed kiln shell cross section. The subsequent calculation is based on determining the difference between the longest and the shortest radius of the shell-test diagram after which the ovality is calculated according to the method of G. Rosenblad. The mechanical condition of kiln shell and kiln tyres with respect to allowable ovality should be regularly checked. For kiln with loose tyre design, an indication on the mechanical condition of the tyre section can be gained by continuous measurement of the difference in rotational speed of the kiln tyre and kiln shell. This method permits continuous indication/recording in the control room (Fig. 21). Fig. 21

Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 5. OPERATIONAL ASPECTS REGARDING KILN LININGS / 5.3 Fluctuating Process Parameters

5.3

Fluctuating Process Parameters


6/23/2001 - 4:34:48 PM

Page 164

"Holderbank" - Cement Course 2000 Unstable burning conditions of cement kilns belong still to the most frequent reasons of refractory lining failures. All types of used sintering zone refractories show structural changes between hot and cold face, infiltrated circulating elements of faded brick sections by overheating, do not immediately result in refractory failures. However, if kiln operating conditions change, followed by changed thermal profile or changed coating conditions, the bricks break off in large lumps since, due to its altered structure, their mechanical characteristics have deteriorated. Unstable burning conditions can be caused by ♦ Unstable kiln feed due to insufficient material flow control ♦ Fluctuating kiln feed composition due to insufficient homogenising of raw material ♦ Fluctuating flame length due to insufficient fuel flow control or fluctuating fuel quality ♦ Frequent kiln shut-down due to maintenance problems of auxiliary equipment, power failures or refractory failures. Without going into details it is evident, that these points need to be carefully watched and, if required, improved, in order to establish the conditions which make the improvements in the field of refractory lining selection, installation and operation effective. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 6. CONCLUSIONS

6.

CONCLUSIONS

Improvement of refractory lining lifetime is not a matter of one single criterion. There exist several fields of influence which often are jointly responsible for the results obtained. The following list contains suggestions concerning the most important and general consideration for achieving improvement in lining life. Since the major refractory problems occur in the sintering and transition zone, most of the considerations will concentrate on these kiln areas. Most important factors: 1) Optimisation of kiln feed (qualitatively and quantitatively) ∗ by selective quarrying procedures ∗ by selection of optimum raw material composition ∗ by appropriate homogenising of raw materials ∗ by improvement of kiln feeding equipment 2) Optimisation of flame shape and length ∗ by evaluation of the influence of adjustable flames ∗ by the most constant burning possible with the optimally established flame shape 3) Improvement of mechanical condition of kiln shell in the tyre area ∗ by adjustment to minimum possible tyre clearance ∗ by continuously monitoring tyre creep ∗ by cooling of kiln tyre areas when danger of seizure exists 4) Optimisation of brick quality in the sintering zone ∗ by methodical investigations (trials) with alternative brick qualities 5) Thermal load reduction ∗ by introduction of secondary firing (if applicable) © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:48 PM

Page 165

"Holderbank" - Cement Course 2000 6) Reduction of the number of kiln shut-downs ∗ by improving the mechanical and electrical reliability of the system ∗ by installation of emergency power supply 7) Optimisation of the bricking process ∗ by refining existing or selecting better methods ∗ by improvement of working conditions or methods ∗ by introducing clear instructions, control (bonus systems) ∗ by optimised bricking policy 8) Prevention of rapid heating or cooling ∗ by observing the important factors on kiln light-up ∗ by prevention of rapid cooling on shut-down This list is not claimed to be complete and some of the suggestions might not be realisable for a specific case on hand. As a check-list, however, it allows the identification of points of weakness and the establishment of further procedures in order to be able to define the actual problem and finally to solve it as effectively as possible. Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 7. TEST QUESTIONS

7.

TEST QUESTIONS

1) Please make a sketch of the various zones of a suspension pre-heater-, Lepol -and a long wet kiln and indicate what type of refractories are used in each zone. 2) Explain the different methods you know of refractory brick installation in a rotary kiln (sketches). 3) Where are the unshaped refractories mainly used in cement kiln system? 4) What are the main features of brick joints (jointing materials) for rotary kiln linings in the case of alumina and basic bricks, considering dry and mortar lining? 5) Establishment of a heating-up program for cement kilns is mainly based on considerations regarding three main factors. What are these considerations?


6/23/2001 - 4:34:48 PM

Page 166

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C04 - Firing Systems

C04 - Firing Systems


6/23/2001 - 4:34:50 PM

Page 167

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels

Firing Systems: Handling and Preparation of Noble Fuels D. Pauling PT 98/14353/E 1. Introduction 2. coal Firing Systems 2.1 Classification of Coal Firing Systems 2.2 Quality of Coal Preparation 2.3 Pulverized Coal Dosing 2.3.1

Feed Bins for Pulverized Coal

2.3.2

Weighing and dosing of pulverized coal

2.3.3

Most Common Pulverized Coal Dosing Systems

2.4 Pneumatic Transport of Pulverized Coal to the Burner 3. Oil firing systems 3.1 Fuel Oil Transfer from Delivery Point to the Storage Tank 3.2 Fuel Oil Storage 3.3 Fuel Oil Preparation 3.3.1

Heating with Steam

3.3.2

Heating with Thermal Oil:

3.3.3

Heating with Electricity

3.3.4

Heating with Flame Radiation

3.4 Quality of Fuel Oil Preparation 3.5 Control Loops in the Fuel Oil Circuit 4. Natural gas firing systems 4.1 Natural Gas Preparation 4.2 Safety Precautions 4.2.1

Flexible Hoses Bursting

4.2.2

Leak Tests

4.2.3

Explosions in the Kiln

5. list of references


6/23/2001 - 4:34:50 PM

Page 168

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 1. INTRODUCTION

1.

INTRODUCTION

Noble fuels are coal, fuel oil and natural gas. Handling and preparation of those fuels has to fulfill certain requirements in order to produce similar combustion conditions for those different fuels and avoid incomplete combustion, e.g. CO at kiln inlet or local reducing conditions due to combustion of fuel particles in the clinker bed. For coal firing the main types of firing systems are described (direct, semi-direct and indirect firing). For pulverized coal dosing and transport to the burner the important design criterias are outlined. The required coal dust qualities for a good combustion in the cement kiln are described. For fuel oil firing, preparation and heating systems are outlined. The required fuel oil qualities (pressure, viscosity and temperature) are given. For natural gas firing, preparation and safety precautions are described. Burners, injection characteristics and flames are not subject of this paper (see paper: "Burners and Flames"). Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 2. COAL FIRING SYSTEMS

2.

COAL FIRING SYSTEMS

Before the coal is fired, it has to be prepared according to the required fineness. The coal has to be dried to 0.5 - 1.5 % residual moisture content, since moisture in the coal means loss of calorific value, as the water has to be evaporated and heated up to flame temperature. Coal drying is done simultaneously with the grinding. Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 2. COAL FIRING SYSTEMS / 2.1 Classification of Coal Firing Systems

2.1

Classification of Coal Firing Systems

With reference to gas and material flow, the coal firing systems can be classified into four main groups which in total sum up to six individual systems (Fig. 1). Figure 1:

Classification of Coal Firing Systems


6/23/2001 - 4:34:50 PM

Page 169

"Holderbank" - Cement Course 2000 System 1 - Direct firing Represents the most simple case. The coal is ground in the mill, dried and blown into the kiln together with the drying gases. System 2 - Direct firing Basically describes the same solution with the exception of the mill working under positive pressure. This solution is generally applied to protect the fan when processing abrasive coal. System 3 - Direct firing In system 3, the fan is protected by separating the pulverized coal in a cyclone and feeding it after the fan into the primary air stream. System 3a - Direct firing with recirculation Same as system 3, but with recirculating drying air. This arrangement allows reduced primary air ratios. System 4 - Semi-direct firing Has little technical significance since the solution with intermediate storage of coal would generally be given preference. System 5a - Semi-indirect firing With system 5a, the kiln can be operated independently of short mill shut downs since the pulverized coal is stored in an intermediary storage bin. The exhaust air from the mill enters the kiln as primary air. System 5b - Semi-indirect firing with recirculation Same as system 5a, but with recirculating drying air. This arrangement allows reduced primary air ratios. System 6 - Indirect firing In system 6, the grinding installation is completely separated from the kiln. The pulverized coal is stored in an intermediary storage bin and the exhaust air from the mill is released through a filter into the atmosphere. By this way, the kiln operation is completely independent from the combined drying and grinding operation. Major Advantages / Disadvantages of the Different Coal Firing Systems:

Direct firing Conventional

modified

System 1 and 2 System 3a


Semi-indirect firing Conventional

Indirect firing

Modified

System 5a

6/23/2001 - 4:34:50 PM

System 5b

System 6

Page 170

"Holderbank" - Cement Course 2000 3a System 1 and 2 System Advantages

Disadvantages

•

Simple design

•

Low risk of explosions

•

Simple extinction of fire in the in the grinding system by • stopping coal feed. No spread of fire into silos

•

Combined operation with the kiln, therefore often not optimal operating conditions.

•

Mill shutdown = kiln shut-down

•

Number of kilns = number of mills thus reducuing the advantage of lower investment cost if several kilns are installed

•

•

•

Slow control loops, long dead time

•

Sampling for fineness control difficult

System 5b

Lower • primary air ratios and thus lower heat consumption compared to • conventional

No exhaust • gas, therefore no filter required, thus lower risk of explosions than with indirect firing

Independent • primary air fan

Only one mill required for several kilns

•

Easy sampling for fineness control

More complex installation requiring additionally: primary air fan, longer ducting

High primary air ratio up to 30%

•

System 5a

•

•

Short mill shut down not = kiln shutdown

More complex installation requiring additionally: cyclones , pulverized coal silos, pulverized coal feeders, measuring and control system

Investment cost higher than with a direct firing system (valid for one kiln only)

•

Risk of self-ignition of the pulverized coal in the storage silo

Lower • primary air ratios and thus lower • heat consumption • compared to conventional

• Independent primary air fan

•

High primary air ratio up to 30%, during start up and shut down of grinding plant disturbed kiln operation

•

System 6

More complex installation requiring additionally: primary air fan, longer ducting

Simple flame control Low primary air ratio Water vapour from coal drying is not introduced into the kiln

•

Short mill shut down not = kiln shutdown

•

Only one mill required for several kilns

•

Easy sampling for fine-ness control

•

More complex installation requiring additionally dedusting filter

•

Investment cost higher than with a direct firing (valid for one kiln only)

•

More vulnerable to fires and explosions in gas ducts and filter

•

Risk of self-ignition of the pulverized coal in storage silo

Impact of Firing System on Kiln Operation: •

Direct firing systems tend to enhance coal fluctuations and therefore disturb combustion.

•

If the mill vent air enters the kiln as primary air as it is the case with the direct- and semi-direct firing, the primary air ratio is higher than required for optimum combustion. At a given excess air factor, the primary air ratio has a direct influence on the heat recuperation efficiency of the cooler and finally on the overall kiln heat consumption. If the heat consumption can be reduced, the exhaust gas quantity is automatically decreasing, which offers potential for a capacity increase.

•

Another very important advantage of lower exhaust gas quantities is the effect of decreased gas velocities in the kiln. This on the other hand has the benefit of lower dust generation for wet kiln systems.

•

With a direct firing system the water vapor from coal drying enters the kiln with the primary air.


6/23/2001 - 4:34:51 PM

Page 171

"Holderbank" - Cement Course 2000 The water vapor has no direct influence on the combustion process, but increases the kiln exhaust gas quantity accordingly. A water content of 15% in the coal increases the exhaust gas quantity of a dry process kiln by approx. 1.5% and of a wet process kiln by 1.2 %. •

At the same time, the flame stability may decrease as a result of dilution of the primary air.

•

Flame shape is strongly influenced by the type of firing system. An indirect system will not only support a more stable flame, but also a shorter one, which results in smaller, more even distributed alite crystals with higher reactivity. Benefits are better quality of the clinker and a lower energy demand for the cement grinding.

•

For new projects direct firing will not be selected anymore because of the above mentioned disadvantages. Today indirect firing systems are "State of Technology". Conversion projects from direct to indirect firing for existing installations can not always be financially justified on the basis of reduced thermal energy consumption. In countries with low coal prices, pay back times of several years must be expected. However, what can make a conversion project interesting, are the positive effects on kiln operation and thus product quality.

Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 2. COAL FIRING SYSTEMS / 2.2 Quality of Coal Preparation

2.2

Quality of Coal Preparation

Inadequate coal preparation (fineness) can result in both burn-out problems (CO formation) and the presence of fuel in the material bed (increased volatility of sulfur). The combustion time of coal depends on the content of volatile elements. Fig. 2 shows the principal requirements for coal fineness in function of the volatile content. Figure 2:

The Grinding Fineness of Coal in Function of its Volatile Content

The aim is to comply with the following simple rule as an upper limit: •

Residue on the 90 µm sieve < ½ (% volatile components)

•

Residue on the 200 µm sieve < 2%

For low volatile and difficult to burn coal types such as petrol coke and anthracite, the above mentioned rule has to be tightened: •

Residue on 90 µm sieve for petrol coke and anthracite < 5 %

•

Residue on 200 µm sieve for petrol coke and anthracite < 1 %


6/23/2001 - 4:34:51 PM

Page 172

"Holderbank" - Cement Course 2000 It has to be pointed out, that both values, the residues on 90 µm and on 200 µm are important. The 90 µm values influence flame length and CO formation, excess residues on 200 µm create reducing conditions in the material bed and can be responsible for increased volatilization of sulfur. Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 2. COAL FIRING SYSTEMS / 2.3 Pulverized Coal Dosing

2.3

Pulverized Coal Dosing

For coal firing, in order to obtain perfect fuel feed, the entire feed system - from discharge from the coal dust silo, through weighing and dosing, to coal dust transport to the burner - must function as well as possible (Fig. 3). Figure 3:

Pulverized Coal Dosing

Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 2. COAL FIRING SYSTEMS / 2.3 Pulverized Coal Dosing / 2.3.1 Feed Bins for Pulverized Coal

2.3.1

Feed Bins for Pulverized Coal

The feed bin design has a decisive impact on feed rate control. A feed bin design ignoring a product's flow characteristics may result in inconsistent discharge rates due to problems such as arching, erratic flow and flushing, conditions that can not be corrected by any feeder system. Design of feed bin, activation and discharge: ♦ The capacity of the feed bin should be sufficient for at least 15 but not more than 60 minutes of kiln/precalciner operation ♦ The bin has to be designed for mass flow. ♦ The activated discharge opening section must be large enough to prevent bridging (at least 1200 mm in diameter for circular outlets and 600 x 1800 mm for slotted discharges). ♦ The discharge opening should be activated preferably by using mechanical discharge device such as paddle or agitator. ♦ Pulsed aeration systems for flow activation are only suitable for bins feeding loss-in-weight dosing systems. As a compromise aeration can help to solve discharge problems at existing bins, but should be avoided for new bins. Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 2. COAL FIRING SYSTEMS / 2.3 Pulverized Coal Dosing / 2.3.2 Weighing and dosing of pulverized coal © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:52 PM

Page 173


Weighing and dosing of pulverized coal

Proper weighing and dosing requires a uniform coal dust supply (feed bin discharge; see above). It is necessary to distinguish between accuracy of weighing and short term variations. The dosing system should meet the following requirements: ♦ Weighing accuracy: +/- 2% is normally accepted. ♦ Short term variations (referring to 10 sec. measurements): < +/- 1% (short term variations are responsible for CO peaks) ♦ Long term variations (approx. 10 min. to 1 hour): < +/- 0.5% ♦ Sensibility: < +/- 0.5% (Example: A dosing system with a maximum capacity of 5 t/h has to be capable to handle set point changes of +/- 25 kg/h). ♦ Adjustment range: 1:20 (of the maximum capacity). The best indicator for the accuracy of the dosing is the oxygen level at kiln exit. Poor dosing of coal dust leads to big fluctuation of the oxygen concentration. Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 2. COAL FIRING SYSTEMS / 2.3 Pulverized Coal Dosing / 2.3.3 Most Common Pulverized Coal Dosing Systems

2.3.3

Most Common Pulverized Coal Dosing Systems

At present two systems are on the market which offer the best solutions for dosing pulverized coal. ♦ Rotor Feed Scale (Pfister) Fig. 4 ♦ Coriolis Scale (Schenk)

Fig. 5

Only second choice are the following systems: ♦ Loss-In-Weight System (complex setup requiring skilled maintenance) ♦ Impact-Flow Meter (limited accuracy) Figure 4:

Rotor Feed Scale (Pfister)

Figure 5:

Coriolis Type Feed Scale (Schenk)


6/23/2001 - 4:34:52 PM

Page 174


Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 2. COAL FIRING SYSTEMS / 2.4 Pneumatic Transport of Pulverized Coal to the Burner

2.4

Pneumatic Transport of Pulverized Coal to the Burner

The highest accuracy of the dosing and feeding system is not useful for the kiln operation if the transport to the burner is not designed well. What is required is a high accuracy at the feed point to the process. This means that a careful design of the pneumatic transport of the pulverized coal to the burner is of utmost importance too. The coal dust transport should meet the following design criteria: ♦ Pneumatic transport velocity to burner is one of the most critical items for regular coal flow. To avoid pulsations caused by pocket formation in the pneumatic transport line, the transport velocity (from feeder to burner) should be in excess of 32 m/s. ♦ The fuel load carried by the air is not a critical value. Normal loads lie at about 5 kg/m3, but values of up to 12 kg/m3 are found without any operation problems. ♦ Fluctuations caused by the feeding device of coal to transport air (pneumatic pump, rotary air valve) have to be avoided by adequate design of the feeder (sizing, number, arrangement of rotary feeder cells, dedusting). ♦ Pressure fluctuations in the pneumatic transport: < +/- 5 mbar. ♦ Transport lines should run horizontally and vertically (no in-/declining sections). Long curves should be avoided because they lead to segregation of the coal dust through centrifugal forces and this in turn leads to plugging. Diversion pots have proved the best solution in three respects: 1. low wear and tear 2. low loss of pressure and 3. the coal dust is remixed with the transport air at every turn (Fig. 6). ♦ Maximum number of turns: 5 (preferably by diversion pots); first turn after the dosing no diversion pot ♦ Total length of the pneumatic transport line: < 120 m Figure 6:

Diverting Pots for Pulverized Coal Transport (Units: mm)


6/23/2001 - 4:34:52 PM

Page 175


Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 3. OIL FIRING SYSTEMS

3.

OIL FIRING SYSTEMS

The handling of fuel oil in a cement plant can be subdivided into the following steps: 1) Transfer to the storage tanks 2) Storage and extraction from storage tanks 3) Preparation, measuring, dosing 4) Atomization and combustion The last point will be delt with in the separate paper: "Burners and Flames". Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 3. OIL FIRING SYSTEMS / 3.1 Fuel Oil Transfer from Delivery Point to the Storage Tank

3.1

Fuel Oil Transfer from Delivery Point to the Storage Tank

For easy handling, fuel oil must have a temperature of about 50 to 60°C. If it is delivered at lower temperatures, which - due to the insulation of the wagons - is rather seldom, it has to be heated up. This can be done by circulating saturated steam (8 to 12 atm), thermal oil or electricity through the heating coils at the bottom of the railway wagons or trucks. Heating time depends on the boiler output, on the capacity of the wagon, on delivery temperature of the oil and on ambient temperature and lies between 2 and 6 (12, 24) hours (200 to 250 kg/h of steam is needed for a 20 tons capacity wagon). It is therefore common practice to do this - whenever required - in the afternoon, to heat up the oil during the night and to empty the wagons in the following morning. Via coarse strainers (for pump protection) the fuel oil is then pumped to the storage tanks (Fig. 7). Figure 7:

Fuel Oil Handling


6/23/2001 - 4:34:53 PM

Page 176


Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 3. OIL FIRING SYSTEMS / 3.2 Fuel Oil Storage

3.2

Fuel Oil Storage

The main storage requirements depend on the situation of the plant with respect of the fuel oil supply possibilities. A few plants are located sufficiently close to a refinery so that the oil is received by pipeline, directly from the refinery. Such cases require a minimum storage capacity. Where oil is delivered by truck or by rail, typical main storage capacities allow a kiln operation of 2 to 10 weeks. Tanks are usually designed as welded steel constructions. Due to the fuel oil forming an insulating layer on the walls, any particular insulation efforts are unnecessary. Suction heaters are used to maintain the fuel oil locally - i.e. in the area of the tank suction point - in a pumpable condition, i.e. at temperatures between 50 and 60°C. This is done in order to minimize the rate of deposit forming reactions, which doubles with each 10°C increase in fuel oil temperature. Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 3. OIL FIRING SYSTEMS / 3.3 Fuel Oil Preparation

3.3

Fuel Oil Preparation

Successful burning of oil requires that it is heated to approx. 140 - 170°C (see Chapter 3.4) in order to reduce its viscosity enough to allow it to be properly atomized by pressure atomization. Heating up of the fuel oil is usually accomplished through an assembly of equipment all contained on a common base. This minimizes expensive piping and valving and centralizes the equipment for ease of maintenance and control. Due to the foreign matter that all residual oils contain and the high rate of deposits that form at elevated temperatures, resulting in frequent maintenance, all equipment associated with and on the final heat and pump set is duplicated. Such a set would contain (see Fig. 7): ♦ 2 strainers with coarse meshes for pump protection ♦ 2 oil pumps (gear pumps or screw pumps) ♦ 2 heat exchangers for heating up the fuel oil to atomization temperature ♦ 2 strainers with fine meshes for control equipment and atomizer head protection The supply of heat mainly to the heat exchangers of the fuel oil preparation set, but also to the storage tank suction heater as well as to all oil carrying piping can be accomplished by: © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:53 PM

Page 177

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 3. OIL FIRING SYSTEMS / 3.3 Fuel Oil Preparation / 3.3.1 Heating with Steam

3.3.1

Heating with Steam

Steam has certainly been the most popular heat carrying medium for oil heating in the past (see Fig. 8). The principal problems associated with steam generation and its use are: ♦ feed water treatment ♦ steam trapping ♦ condensate handling ♦ high pressure operation ♦ freezing problems during plant stop Steam can be produced by: ♦ conventional oil fired steam generators ♦ electrical submersion heaters in a pressure vessel ♦ waste heat based steam generators (e.g. cooler exhaust air) Figure 8:

Fuel Oil Preparation System Based on Steam

Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 3. OIL FIRING SYSTEMS / 3.3 Fuel Oil Preparation / 3.3.2 Heating with Thermal Oil:

3.3.2

Heating with Thermal Oil:

The essential advantages of these inorganic, low flammability oils as a heat transfer medium are: ♦ operation in a constantly liquid phase ♦ low pressures even at operating temperatures of 250 to 300°C ♦ no freezing problems They might be treated up by: ♦ oil fired thermal oil heaters ♦ electrical submersion heaters ♦ waste heat based thermal oil heater (e.g. cooler exhaust air) © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:53 PM

Page 178

"Holderbank" - Cement Course 2000 Thermal oils are subjected to aging. Their quality has therefore to be checked in regular intervals of about one year. About every five years replacement by a new charge is required (see Fig. 9). Figure 9:

Fuel Oil Preparation System Based on Thermal Oil

Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 3. OIL FIRING SYSTEMS / 3.3 Fuel Oil Preparation / 3.3.3 Heating with Electricity

3.3.3

Heating with Electricity

Due to high operating costs, direct electrical heating of fuel oils is used for low capacities only. However, it is sometimes used as auxiliary heating for large systems to permit starting when the system is cold. Electrical power is also used in heating oil lines through "resistance heating". The oil line itself is used as the conductor for high current, low voltage power. Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 3. OIL FIRING SYSTEMS / 3.3 Fuel Oil Preparation / 3.3.4 Heating with Flame Radiation

3.3.4

Heating with Flame Radiation

The heating medium in this case is the flame itself. The thermal oil heater is an example of the direct fired heater. Replace the thermal oil with fuel oil and this, then, is the direct fired fuel oil heater. Since fuel oil cannot be heated to the same high temperature as the thermal oils, burner flame modulation (shape and length) within the heating chamber must be closely controlled to maintain a narrow oil temperature range, e.g. (120°C ± 2+C) over a wide range of oil flow. This close burner flame control must be maintained to prevent overheating and carbonization of the residual oil. Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 3. OIL FIRING SYSTEMS / 3.4 Quality of Fuel Oil Preparation

3.4

Quality of Fuel Oil Preparation

For heavy oil combustion, the kinematic viscosity at the burner nozzle must lie within the range of 12 to 20 cSt - preferably 12 - 15 cSt (upper limit 20 cSt) - this ensures that the droplet size needed for good combustion can be achieved. In today's heavy oil market, particularly in the South American OPEC countries, heavy oil is offered which has a significantly higher viscosity than the limit specified by DIN 51 603. It is therefore essential to keep track of the relationship viscosity - temperature and adjust the oil temperature as necessary.


6/23/2001 - 4:34:54 PM

Page 179

"Holderbank" - Cement Course 2000 Fig. 10 shows the kinematic viscosity of different fuel oil types in function of temperature. The upper limits for atomization and pumping are indicated. Fig. 11 shows a conversion table for the different viscosity units. Furthermore it is important to keep the oil temperature constant within a very narrow range to have a stable flame. Figure 10:

Kinematic Viscosity of Current Fuel Oils

Figure 11:

Conversion of Different Viscosity Scales

Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 3. OIL FIRING SYSTEMS / 3.5 Control Loops in the Fuel Oil Circuit

3.5

Control Loops in the Fuel Oil Circuit

Between storage tanks and fuel oil burners, there are generally four control loops installed, which have to keep constant the following values: 1) Fuel oil temperature at the storage tank suction point. 2) Pressure in the oil circuit line between storage tanks and preparation station (Bypass of a part of the flow back to the storage tank; see Fig 7). 3) Temperature of the fuel oil to be atomized (Preparation Station). 4) Atomizing pressure: Accomplished by means of a bypass valve which leads part of the flow back to the storage tank (see Fig. 7) or by means of variable speed high pressure pumps, which are directly controlled by the oil flow meter. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:54 PM

Page 180

"Holderbank" - Cement Course 2000 For burner nozzles with separate feed for axial and radial oil (Pillard, Unitherm), the oil pressure difference for optimum atomizing is set to 1,0 – 1,5 bar. However, as the accuracy of the reading on the oil manometer at the operating pressure of about 40 bar is unsatisfactory, it is recommended that both channels are equipped with flow meters. The pressure (flow characteristics given by the nozzle suppliers) can be taken into account in optimizing atomization. Furthermore, whenever a kiln stop occurs, the oil lance and the atomizer head have to be cleaned automatically by steam or compressed air in order to avoid overheating and coking of the oil. Continuation of burner cooling has to be assured by having the primary air fan connected to the auxiliary power generators. In cases of prolonged kiln stops removal of the oil lance is preferable, thus, also providing the opportunity to check the condition of the atomizer plate, which is very important for complete combustion. Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 4. NATURAL GAS FIRING SYSTEMS

4.

NATURAL GAS FIRING SYSTEMS

Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 4. NATURAL GAS FIRING SYSTEMS / 4.1 Natural Gas Preparation

4.1

Natural Gas Preparation

Gas distribution by means of pipelines is accomplished at pressures of 30 to 80 bars. At consumer's site the gas pressure is reduced to the required operational pressure, mostly by means of a two stage expansion process. The first stage takes place in the NG transfer station while the second runs off in the NG pressure reduction station. As a standard solution the NG transfer station is an independent, self-sustaining installation contained in a separate building (noise suppression). Similarly to the fuel oil preparation plant, all equipment is duplicated and provided with a number of bypass possibilities. The main equipment list is as follows (Fig. 12): ♦ Remote controlled main shut-off safety valve ♦ Transfer station inlet filters for protection of equipment from solid particles originating from the pipeline ♦ Thermal oil heated exchangers aiming to preheat the natural gas to such an extent that the following temperature drop due to expansion will not cause valve internal and external ice formation (Joule - Thompson effect: 0.3 to 0.5°C/bar) ♦ Safety shut off valves ♦ Pressure reduction valves (for reduction of the gas pressure to the pressure level of the plant internal distribution network of 3 to 10 bar) Figure 12:

Handling and Preparation of Natural Gas in the Cement Plant


6/23/2001 - 4:34:54 PM

Page 181


The heat value of the natural gas can be measured and recorded continuously by means of on-line calorimeters. Though this is often not done - plant people tend to rely on the heat values given by the gas suppliers - it would be worthwhile, since in some cases the heat values might vary in range of ±300 kJ/Nm3 from day to day. To enable leaks from the gas pipes to be detected surely and quickly, a powerful odorizer (e.g. mercaptan) is added to the gas just after the gas leaves the transfer station. The second stage of pressure reduction, taking place in the pressure reduction station, is located near the point of consumption (Fig. 13). With the exception of the NG heaters it contains about the same equipment as the transfer station. The aim of this installation is to completely even out supply network pressure fluctuations and to set the final pressure according to the requirements of the consumer i.e. the burner and kiln systems. Figure 13:

Secondary Pressure Reducing Unit

Immediately before the kiln, the gas stream is split up in order to supply the radial and the axial gas nozzle of the burner (Fig. 14). Figure: 14:

Kiln Ramp Unit


6/23/2001 - 4:34:55 PM

Page 182


Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 4. NATURAL GAS FIRING SYSTEMS / 4.2 Safety Precautions

4.2

Safety Precautions

Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 4. NATURAL GAS FIRING SYSTEMS / 4.2 Safety Precautions / 4.2.1 Flexible Hoses Bursting

4.2.1

Flexible Hoses Bursting

Since there is some risk of the flexible gas hoses between kiln burner and gas supply line bursting or of the proceeding valves etc. failing, pressure monitors for the maximum and minimum pressure are inserted immediately before the hoses concerned. In the event of an emergency stop, a safety stop valve, or two in series, are actuated to stop any further input of fuel at once. Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 4. NATURAL GAS FIRING SYSTEMS / 4.2 Safety Precautions / 4.2.2 Leak Tests

4.2.2

Leak Tests

To check the gas pipes and fittings inside the plant for leaks the following methods are used: ♦ Normally leaks can be detected naturally as a result of adding odorizer. ♦ When machines are switched off, the hissing sound of the escaping gas is easily discernible. ♦ A somewhat riskier method is to run a naked flame along the gas pipe. This results in a flaming torch being produced at the leak, which cannot be overlooked. There is no risk of this flame striking back into the supply pipe (quenching distance, lack of oxygen), but escaped gas could cause an explosion. When constructing buildings which contain gas pipes, it is essential to allow for sufficient ventilation. This point does not usually give rise to any difficulty in cement works. But to be quite sure, certain items of equipment can be fitted with guard flames from the start. Their task is to ignite any gas that escapes before a large quantity of explosive mixture has a chance to collect. A further possibility is to install gas detectors in critical places such as the gas preparation station or the burner tunnel. Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 4. NATURAL GAS FIRING SYSTEMS / 4.2 Safety Precautions / 4.2.3 Explosions in the Kiln

4.2.3

Explosions in the Kiln


6/23/2001 - 4:34:55 PM

Page 183

"Holderbank" - Cement Course 2000 The most important requirement is that the fuel should not be allowed to enter the kiln unintentionally or at an uncontrolled rate, as this is essential to prevent explosions occurring in the kiln itself or in the systems following it (e.g. preheater tower, EP). This means that the fuel input has to be stopped immediately in the event of the flame going out. In this respect it must be said that extinction of the flame in a hot kiln has never been observed so far, even during material rushes. Nevertheless during the start up of the cold kiln, lifting off and extinction of the flame can occur, for example caused by partly blocked burneroutlets which lead to increased injection speed of the gas. If the gas is injected with a too high speed, the flame can be blown out. Therefore careful observation of the flame during the whole start up period is of utmost importance. In the case of the flame going out, fuel supply has to be cut off immediately to prevent explosions. Excessive fuel input can also cause explosions because of CO accumulations. Therefore careful monitoring of CO concentrations is important. Process Technology / B05 - PT II / C04 - Firing Systems / Firing Systems: Handling and Preparation of Noble Fuels / 5. LIST OF REFERENCES

5.

LIST OF REFERENCES

1) "Firing Systems" VA 82/4898/E 2) "Flames and Burners" VA 93/4056/E 3) "State of Technology of Rotary Kiln Burners" F. Schneider, PT 96/14078/E 4) "Proportioning of Bulk Materials" F Bucher, PT 96/14071/E 5) "A Review of Coal Firing Systems and their Influence on Heat Consumption, Production and Kiln Operation" H. Meier, PT 96/14210/E


6/23/2001 - 4:34:55 PM

Page 184

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts

Safe Handling of Coal and other Combustible dusts F. Schneider (Original title: Basic safety theory of solid fuel preparation) 1. INTRODUCTION 2. BASIC PRINCIPLES FOR HANDLING PULVERIZED FUELS 2.1 Development of Dust Explosions and Fires 2.2 Possible Protective Measures against Dust Explosions and Fires 3. APPLICATION OF PROTECTIVE MEASURES IN THE INDUSTRIAL ENVIRONMENT 3.1 Preventive Safety Measures 3.2 Explosion Protection Techniques 4. LITERATURE 4.1 Approximate Values for Explosion Limits and Ignition Temperatures


6/23/2001 - 4:34:55 PM

Page 185


SUMMARY Dust explosions can only occur when - besides certain marginal conditions - the following factors simultaneously are present: ♦ Stirred-up, combustible dust ♦ Oxygen ♦ Source of ignition A basic distinction is made between active explosion protection techniques (prevention of the occurrence of explosions) and design related explosion protection (reduction of the effects of explosions). In practice, the following measures are applied: 1) Preventive Measures: Their aim is: ∗ to exclude possible ignition sources within the installation ∗ to prevent the building up of coal dust deposits, wherever possible ∗ to detect the source of a fire as soon as possible ∗ to extinguish the fire with a minimum of danger 2) Explosion Protection Measures: ∗ Either active protective measures involving inert gas operation. This is the case when the oxygen concentration within the pulverizing plant is kept below the critical limit - for solid fuels dust, as a rule, less than 12 to 14% - as long as combustible dust is present in the system. ∗ Or, alternatively, design related protective measures based on the use of explosion resistant construction in accordance with VDI Guidelines No. 3673. Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 1. INTRODUCTION

1.

INTRODUCTION

The operational safety of solid fuel plants is an important decision factor when the choice of the preparation system is being considered. For this reason, the three most important basic systems will be briefly reviewed here, differentiated according to their different methods of handling gas and coal dust (Fig. 1). Fig. 1 Firing Systems


6/23/2001 - 4:34:56 PM

Page 186


a) Direct firing In this system, the combustible dust is conveyed into the kiln together with the exhaust gases resulting from the drying-cum-grinding operation. This arrangement represents the simplest design and is easily controllable from the safety point of view. However, there are also considerable disadvantages involved with the use of this system in clinker manufacture with increasing ballast content of the fuel. b) Semi-direct firing In this system the combustible dust is separated in an intermediate silo, while the mill exhausts, possibly as recirculated air, are conveyed to the cement kiln as the primary air supply. This results in the technical disadvantages of direct firing being reduced to a certain extent - at the expense of a somewhat more complex installation - but all drying gases are still conveyed to the kiln. c) Indirect firing This solution is surely the best possible version when the operation of a rotary kiln is being considered. The pulverized fuel can be conveyed to the firing system from the silo independently from the pulverizing plant operation. The firing system can be operated with a primary air ratio designed for optimal flame generation, as the mill exhaust gases are filtered. Against this we have increased risks with respect to safety due to the operation of the filters and silos and higher control technique requirements. Further discussion of the decision criteria for the selection of an optimal preparation system is outside the scope of this lecture. However, it is certain that when factors such as ♦ the growing size of installation ♦ installations with several firing systems ♦ the use of fuels rich in ballast ♦ the use of fuels of widely differing quality characteristics are considered, the decision will be influenced in favor of the indirect firing system which needs far more advanced and sophisticated safety techniques than the simpler direct firing system does. For this reason the damage prevention possibilities discussed below refer basically to the indirect firing system and must be adjusted accordingly if they are applied to other systems. Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 2. BASIC PRINCIPLES FOR HANDLING PULVERIZED FUELS

2.

BASIC PRINCIPLES FOR HANDLING PULVERIZED FUELS


6/23/2001 - 4:34:56 PM

Page 187

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 2. BASIC PRINCIPLES FOR HANDLING PULVERIZED FUELS / 2.1 Development of Dust Explosions and Fires

2.1

Development of Dust Explosions and Fires

In order to effectively ensure the safety of a solid fuel preparation plant, we must first be aware of the sequence of the possible fuel reactions. Dust explosions can only occur if the following three conditions are simultaneously fulfilled (Fig. 2). a) Stirred-up, combustible dust present in explosive concentration. b) Air or oxygen above the critical concentration, for coal dust as a rule, above 14%, for lignite above 12%. c) An ignition source possessing energy above the minimal ignition energy (depending upon the type of dust). Fig. 2

Preliminary Conditions for an Explosion

After the ignition of an optimally explosive mixture in an enclosed space, the pressure increases more or less rapidly until it reaches the maximal explosion pressure Pmax, and then decreases more or less slowly to the original pressure, depending on the aerodynamic conditions (Fig. 3). While the maximum explosion pressure is almost independent of the container’s form and size, and in case of coal and lignite dusts, amounts to approximately seven to nine times the initial pressure the maximum rate of  dp    pressure rise  dt  max - which is a measure of the explosion violence - is dependent on the container volume in accordance with the cubic law:

 dp    × V 3 =cons tan t K st  dt  max 1

Kst is a material coefficient that depends on the type of dust, the degree of turbulence of the dust/air mixture at the moment of ignition, the grain size distribution, and the type of ignition source. The method for determining KSt is given in the VDI Guidelines No. 3673. Fig. 3 Pressure Development of an Explosion Over Time


6/23/2001 - 4:34:56 PM

Page 188


The degree of explosion violence of dusts is subdivided in industrial praxis into explosion classes, whereby the explosion class and KSt are related in the following manner: Table 1 Dust Explosion Class St0

KSt (bar . m . s-1) 0

St1

> 0 to 200

St2

> 200 to 300

St3

> 300

All types of mineral coals as well as the majority of lignites belong to explosions class St1. Table 2 illustrates arbitrarily selected comparative values for KSt characterizing different types of dust. Table 2 Dust Type

KSt (bar . m . s-1)

Hard coal

85

Lignite

150

Organic pigments

300

Aluminium

550

This comparison shows, that hard coal dust develops a less violent explosion than aluminium dust. It must be noted, that the value „KSt“ does not allow any conclusion as regards the risk involved with that particular dust. The main significance of KSt is for the dimensioning of design related protective measures. Smoldering fires, characterized by slowly smoldering combustion, can occur wherever combustible dust is stored for a longer period of time, whereby the ignition sources can be spontaneous combustion, initiated by external heat sources, mechanical sparks, or electrical sparks and arcs. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:56 PM

Page 189

"Holderbank" - Cement Course 2000 Combustion propagation in smoldering fires is quite possible in very low oxygen concentrations. Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 2. BASIC PRINCIPLES FOR HANDLING PULVERIZED FUELS / 2.2 Possible Protective Measures against Dust Explosions and Fires

2.2

Possible Protective Measures against Dust Explosions and Fires

In dust explosion protection techniques a distinction is made between active protective measures (prevention of the occurrence of explosions) and design related explosion protection (explosion resistant construction). Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 2. BASIC PRINCIPLES FOR HANDLING PULVERIZED FUELS / 2.2 Possible Protective Measures against Dust Explosions and Fires / 2.2.1 Active Explosion Protection

2.2.1

Active Explosion Protection

The active explosion protective techniques aim to exclude at least one of the three preliminary conditions necessary for an explosion, i.e.: ♦ Stirring-up of combustible dust ♦ Oxygen content above the critical concentration of generally 12% for lignite or 14% for hard coal ♦ Ignition source Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 2. BASIC PRINCIPLES FOR HANDLING PULVERIZED FUELS / 2.2 Possible Protective Measures against Dust Explosions and Fires / 2.2.1 Active Explosion Protection / 2.2.1.1 Ignition Source

2.2.1.1

Ignition Source

In a pulverizing plant, ignition sources cannot be excluded with absolute certainty. It is always possible that mechanical sparks will be generated by the action of foreign bodies or by friction between moving machine part or that the hot gas or coal feeding system will supply smoldering fuel particles. Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 2. BASIC PRINCIPLES FOR HANDLING PULVERIZED FUELS / 2.2 Possible Protective Measures against Dust Explosions and Fires / 2.2.1 Active Explosion Protection / 2.2.1.2 Combustible Dust

2.2.1.2

Combustible Dust

It is of course impossible to replace the combustible dust with a non-combustible material in the preparation of fuel. Thus, the only remaining possibility is the exclusion of air or oxygen respectively, or the reduction of the oxygen content in the fuel preparation plant. Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 2. BASIC PRINCIPLES FOR HANDLING PULVERIZED FUELS / 2.2 Possible Protective Measures against Dust Explosions and Fires / 2.2.1 Active Explosion Protection / 2.2.1.3 Air and Oxygen

2.2.1.3

Air and Oxygen

Dust explosions can be effectively prevented through inertization, i.e. the replacement of the oxygen in the air by a non-combustible gas, particularly CO2 or N2, if it can be ensured that the inert gas atmosphere will be maintained as long as combustible dust is present in the system. The maximal O2 concentration, below which no explosive propagation reactions of mineral coal dust © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:57 PM

Page 190

"Holderbank" - Cement Course 2000 are noted, is approx. 14%, the one for lignite approx. 12%. However, this concentration can vary in accordance with the type of fuel processed. As a safety margin of at least 2% O2 is required, the maximal permissible limit of 02 concentration for mineral coal dust is therefore as a rule 12%, for lignite 10%. Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 2. BASIC PRINCIPLES FOR HANDLING PULVERIZED FUELS / 2.2 Possible Protective Measures against Dust Explosions and Fires / 2.2.2 Design Related Explosion Protection

2.2.2

Design Related Explosion Protection

Reduction of the effects of already proceeding explosions, and therewith the protection of people and machines, can be achieved by: ♦ Explosion pressure resistant construction ♦ Explosion pressure venting measures ♦ Explosion suppression techniques (Fig. 4) Fig. 4 Passive Protection Measures

Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 2. BASIC PRINCIPLES FOR HANDLING PULVERIZED FUELS / 2.2 Possible Protective Measures against Dust Explosions and Fires / 2.2.2 Design Related Explosion Protection / 2.2.2.1 Explosion Pressure Resistant Construction

2.2.2.1

Explosion Pressure Resistant Construction

Explosion pressure resistant construction restrict any possible explosion to the dust conveying installation, whereby a certain amount of minor damage to the installation commensurate with the complexity of the facility is accepted. All dust conveying installation parts as well as the adjacent equipment and sealing elements must be designed to resist the maximal explosion pressure of 9 bar expected in the case of coal or lignite dust. If deformation of the container is accepted, the maximum permissible explosion pressure may be up to 50% above its design value (pressure shock resistant design). A design for 6 bar static overpressure is required for an expected maximum explosion pressure of 9 bar. Such construction methods are of course quite complex and expensive. However, in the event of an accident the installation is again operational within a short time. Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 2. BASIC PRINCIPLES FOR HANDLING PULVERIZED FUELS / 2.2 Possible Protective Measures against Dust Explosions and Fires / 2.2.2 Design Related Explosion Protection / 2.2.2.2 Explosion Pressure Venting Measures


6/23/2001 - 4:34:57 PM

Page 191


Explosion Pressure Venting Measures

In a broader sense explosion venting means all measures that serve to open temporarily or permanently the previously closed installation in a safe direction, at the beginning or after a certain spreading of an explosion. The purpose of this is to prevent any overstressing of the mechanical equipment beyond its pressure shock resistance. The strength of the equipment does not have to be designed for Pmax, but only for the reduced explosion pressure Pred (Fig. 5). A deformation of the container may again be acceptable, but it must not burst. Fig. 5 Pressure Response in Explosion-Pressure-Relief Techniques

The explosion pressure venting technique operates in the following manner: When the dynamic response pressure of the pressure venting installation is reached, predetermined breaking points, rip foils or doors open to vent the shock wave outdoors, mainly by means of amply dimensioned discharge channels. Immediately after the pressure venting system responds an increase in the temporal rate of pressure rise can often be observed which is due to the higher turbulence caused during the venting of the shock wave. The pressure rise then quickly stops at Pred. Guidelines concerning the design layout and dimensioning of the explosion pressure venting installations are contained in VDI Guidelines No. 3673. If the method of explosion venting is applied not only the inserts of the containers such as filter cloths etc. must be considered but the expected recoil forces as well. With a pressure venting area of 1 m2, a reduced explosion pressure of 2 bar, and under the assumption that the shock wave escapes with the velocity of sound, a thrust of approx. 15 t acts upon the housing to be protected. This must be properly supported or else the container may be torn from its foundations. Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 2. BASIC PRINCIPLES FOR HANDLING PULVERIZED FUELS / 2.2 Possible Protective Measures against Dust Explosions and Fires / 2.2.2 Design Related Explosion Protection / 2.2.2.3 Explosion Suppression

2.2.2.3

Explosion Suppression

In the explosion suppression techniques, the shock wave preceding the combustion front or the infrared radiation of the combustion area is detected by a device which quickly distributes extinguishing agents under a propellant pressure of 60 to 120 bar by means of detonator-operated valves. With a programmed dynamic response pressure threshold (Pdyn) of the detectors, the maximal explosion pressure is again lowered to a reduced level (Pred). Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 2. BASIC PRINCIPLES FOR HANDLING PULVERIZED FUELS / 2.2 Possible Protective Measures against Dust Explosions and Fires / 2.2.2 Design Related Explosion Protection / 2.2.2.4 Limitation: Explosions from Ducts into Containers


6/23/2001 - 4:34:57 PM

Page 192


Limitation: Explosions from Ducts into Containers

The described constructive protection techniques are effective under the condition that the reaction takes place as described in the paragraph 2.1. The description is applicable to most explosions that occur in pulverizing plants. However, if an explosion strikes from a duct into a container, and in doing so the residual dust deposited there is stirred up with great turbulence and ignited, the reaction within the duct and the adjacent container can develop into a detonation of such dimensions that the resulting pressures can amount to 50 times the original pressure, accompanied by a combustion front traveling at supersonic speed, so that any relief or suppression installation is too sluggish in action. However, such events are, fortunately, relatively rare in coal operations. As a limit for a spontaneous explosion propagation, an explosion characteristic of 100 bar.m.s-1 was observed under particular conditions in a 200 m long pipeline of 1800 mm diameter located at the experimental mining research station in Dortmund, while the usual values for coal are generally lower (approx. 85 bar.m.s-1). However, if the principles of design related explosion protection are to be consequently pursued, every duct conveying combustible dust in an explosive concentration and whose length exceeds five times its diameter must be safeguarded by an explosion vent placed ahead of its inlet into a container (such as a filter) (Fig. 6). Through this any explosion originating in the pipeline will be vented so that the protective measures taken with respect to the adjacent container can be designed in accordance with the criteria of an explosion starting in the container itself. Fig. 6 Venting of a Duct in Front of a Precipitator

Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 2. BASIC PRINCIPLES FOR HANDLING PULVERIZED FUELS / 2.2 Possible Protective Measures against Dust Explosions and Fires / 2.2.3 Prevention of Smoldering Fires

2.2.3

Prevention of Smoldering Fires

Smoldering fires in dust deposits are best prevented by preventing the possibility of greater quantities of dust accumulating. This is achieved through the appropriate design and slope of surfaces, pipelines and supports, as well as sufficiently high gas speeds within the conveyor systems. In silos where great quantities of combustible dust are stored for the plant’s own specific purposes, any combustion that may occur must be detected as early as possible by carefully monitoring of the dust temperature and the CO content of the silo atmosphere so that proper countermeasures can be taken. Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 3. APPLICATION OF PROTECTIVE MEASURES IN THE INDUSTRIAL ENVIRONMENT


6/23/2001 - 4:34:58 PM

Page 193


APPLICATION OF PROTECTIVE MEASURES IN THE INDUSTRIAL ENVIRONMENT

The fire and explosion protection measures described above result for practical applications on the one hand in a network of preventive safety measures that significantly reduce the risk of an accident in the operation of combustible dust installation, and on the other hand in actual explosion protection techniques that can prevent explosions, or at least shall hold the explosions within acceptable limits. Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 3. APPLICATION OF PROTECTIVE MEASURES IN THE INDUSTRIAL ENVIRONMENT / 3.1 Preventive Safety Measures

3.1

Preventive Safety Measures

The primary aim of preventive safety measures is to exclude possible ignition sources as causes of conflagration or explosion if at all possible. In addition, they are also intended to prevent secondary damage caused by the expulsion or stirring up of vast quantities of dust and their subsequent ignition. These essentially preventive safety measures can be listed as follows: ♦ Temperature measurement of •

mill exhausts

•

stored dust, preferably in silo entry and exit

♦ CO analysis of •

the silo atmosphere in silos

•

mill exhaust after the filter

♦ Prevention of local overheating caused by friction in conveyor belt systems, high speeds of screw conveyors, bucket elevators, rotary valves, and bearing, and/or the detection of increasing temperatures by measuring techniques. Relative velocities of moving parts < 1 m/s are considered safe, > 10 m/s are considered as potential ignition sources. ♦ Spark separators in air heaters ♦ Metal separator prior to the mill ♦ Prevention of electrostatic discharges by conductive connections and grounding of all installation parts ♦ Prevention of arcing in electrofilters by appropriate voltage control measures ♦ Prevention of dust accumulation possibilities: •

All surfaces to have a slope of at least 70° to the horizontal plane, especially in filter or silo cones

•

Regular disposal of dust deposits

•

Gas speeds in conduits of more than 22 m/s

♦ Protection of the stored dust from the effects of external heat, for instance by spraying the silo externally with cooling water ♦ Provision of inert gas supplies (e.g. CO2) for inertization of the silos in the case of smoldering fires ♦ Cleanliness of operating rooms •

Effective removal of the dust generated by means of proper dedusting installations

•

Safe elimination of dust deposits by means of suitable auxiliary material


6/23/2001 - 4:34:58 PM

Page 194

"Holderbank" - Cement Course 2000 From the point of view of safety a solid fuel pulverizing plant must be operated as continuously as possible, as critical situations often arise when the plant is not in operation. This fact must be considered when the capacity of the installation is being decided upon. Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 3. APPLICATION OF PROTECTIVE MEASURES IN THE INDUSTRIAL ENVIRONMENT / 3.2 Explosion Protection Techniques

3.2

Explosion Protection Techniques

Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 3. APPLICATION OF PROTECTIVE MEASURES IN THE INDUSTRIAL ENVIRONMENT / 3.2 Explosion Protection Techniques / 3.2.1 Inert Gas Operation

3.2.1

Inert Gas Operation

As described under 2.2.1, active explosion protection in solid fuel pulverizing is practically limited to inert gas operation, i.e. operation with a maximum of 10 to 12% oxygen in the pulverizing plant, depending on type of fuel, as ignition sources and the stirring-up of dust can never be excluded with absolute certainty. Active explosion protection can be applied if hot inert gases such as the kiln exhaust from cement kilns or hot gases from a combustion chamber, combined with a corresponding design for the mill’s recirculation gas are available. In the last case the dew point problem becomes significant, therefore this solution is rarely applicable for very moist fuels, or special measures will have to be taken for drying of the circulation gases. If the inert gas atmosphere can be maintained with absolute certainty through appropriate design and interlocking of the installation for as long as combustible dust is present in the system, design related protection measures become in principle redundant. In those cases where these conditions cannot be guaranteed, for example, because hot gases with higher oxygen content are being used such as clinker cooler exhausts, or because of dew point problems, design related explosion protection techniques must be rigorously applied. Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 3. APPLICATION OF PROTECTIVE MEASURES IN THE INDUSTRIAL ENVIRONMENT / 3.2 Explosion Protection Techniques / 3.2.2 Explosion Pressure Resistant Construction

3.2.2

Explosion Pressure Resistant Construction

Explosion pressure resistant construction, i.e. the dimensioning of the installation section to resist maximal explosion pressure, are mainly applicable where pressure venting methods cannot be used at all or only with difficulty, for geometrical reasons. This is mostly the case in mills, and definitely in all conduit pipe systems where the length of the system exceeds five times the tube diameter. As a rule such components are designed to withstand a static overpressure of 10 bar. Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 3. APPLICATION OF PROTECTIVE MEASURES IN THE INDUSTRIAL ENVIRONMENT / 3.2 Explosion Protection Techniques / 3.2.3 Explosion Pressure Venting Measures

3.2.3

Explosion Pressure Venting Measures

All combustible dust conveying components that are not in themselves designed to be explosion pressure resistant, such as cyclone, filters, pulverized fuel silos, etc. are to be provided with properly dimensioned devices for explosion pressure venting. Thereby containers and all interconnected aggregates such as bin vent filters, etc. must be dimensioned in pressure shock resistant design to © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:58 PM

Page 195

"Holderbank" - Cement Course 2000 withstand the reduced explosion pressure. Explosion venting openings within a particular building must be connected to properly dimensioned exhaust channels leading into the open. In order to prevent an explosion originating in the mill spreading into the filter via the conduit pipe, the conduit pipe must be equipped with an explosion vent in front of its connection to the filter. This measure is not required for pneumatic conveying systems as in this design the dust concentration is normally above the explosion limit. In addition, the minimal ignition energy is significantly higher under the operating conditions of pneumatic conveying than it is in the case of stirring-up combustible dust in containers. The area containing the vent opening for explosion pressure venting must not be accessible to anyone when the installation in operation. VDI Guidelines No. 3673 can serve as a basis for the design of such an explosion pressure venting system. Naturally, the system must be inspected regularly. Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 3. APPLICATION OF PROTECTIVE MEASURES IN THE INDUSTRIAL ENVIRONMENT / 3.2 Explosion Protection Techniques / 3.2.3 Explosion Pressure Venting Measures / 3.2.3.1 Underpressure Protection

3.2.3.1

Underpressure Protection

After venting an explosion in very large enclosures such as pulverized fuel silos through explosion flaps considerable underpressure can develop inside the silo due to dynamic effects and due to cooling down of the hot gases remaining in the silo after the explosion. Typical examples for the size of underpressure valves are given in Table 3. Guidelines for the individual design of underpressure valves can be taken from the relevant literature (10). Table 3 Volume

m3

100

1000

Diameter

m

3.4

.5

Cylindrical length

m

9.5

22.0

Plate thickness

mm

6

8

Max. negative pressure

mbar

100

25

Required aspiration area

m2

0.1

1.0

Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 3. APPLICATION OF PROTECTIVE MEASURES IN THE INDUSTRIAL ENVIRONMENT / 3.2 Explosion Protection Techniques / 3.2.4 Explosion Suppression

3.2.4

Explosion Suppression

Techniques of explosion suppression can basically replace all the previously mentioned methods. However, in practical experience it has been seen that in pulverizing plants, the costs involved in the consequent application of explosion suppression techniques are significantly higher than they are for explosion pressure venting techniques and explosion pressure resistant construction methods, both with respect to procurement and maintenance of the sensitive equipment. Thus applicability of explosion suppression may be primarily limited to existing, insufficiently protected pulverizing plants whose retrofitting in accordance with alternative protection techniques would be entirely uneconomical. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:58 PM

Page 196

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 3. APPLICATION OF PROTECTIVE MEASURES IN THE INDUSTRIAL ENVIRONMENT / 3.2 Explosion Protection Techniques / 3.2.5 Fire Extinguishing Measures

3.2.5

Fire Extinguishing Measures

If an accumulation of considerable quantities of combustible dust can be prevented inside the actual pulverizing plant (except in pulverized fuel silos), any fires that may arise following an explosion will not be able to grow to any significant size. The installation of a fire extinguishing system can nevertheless still be recommended for cloth filters and electofilters. In the case of smoldering fires in pulverized fuel silos, all further fuel supply must be stopped immediately. Following this, the silo exit must be made airtight and the silo atmosphere flooded with CO2. Sufficient time must now pass until the temperature conditions have normalized. An underpressure valve is required in order to avoid collapsing of the silo due to the vacuum produced during cooling down. The above procedures can take several days, depending on the size of the smolder location. An alternative technique is to deliver the fuel as quickly as possible to the burner system via the dosing and conveyor systems. Of course this method is possible only when the dosing and conveying systems are heat-resistant, dustproof and explosion resistant. In addition, under no circumstances is glowing fuel to be returned to the silo, as for instance via overflow feeders. Fig. 7 shows the practical preventive safety measures for pulverized fuel silos. In Fig. 8 the application of design related protective measures for solid fuel preparation is illustrated. Fig. 7 Preventive and Safety Measures for Coal Dust Silos

Fig. 8 Example to Show the Application of Design Related Explosion Protection


6/23/2001 - 4:34:59 PM

Page 197

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 4. LITERATURE

4.

LITERATURE

1) VDI Guidelines 2263 Verhütung von Staubbränden und Staubexplosionen 2) VDI Guidelines 3673 Druckentlastung von Staubexplosionen 3) VDI Report No. 304 Sichere Handhabung brennbarer Stäube 4) Arbeitskreis der chemischen Industrie, October 1, 1973 Sicherheitsmassnahmen gegen Staubbrände und Staubexplosionen Published by BASF, Bayer, Ciba-Geigy and Farbwerke Hoechst 5) Dr. W. Bartknecht Explosions, Course Prevention Protection Springer Verlag, Berlin, Heidelberg, New York, 1981 6) K.N. Palmer Dust Explosions and Fires London, Chapman and Hall, 1973 7) H. Wibbelhoff Der Umgang mit Kohlenstaub in der Zementindustrie Steine und Erden 2/1978 8) F. Schneider Kohlenaufbereitung und Kohlenfeuerung für Zementdrehöfen Zement, Kalk, Gips, No. 7/1976 9) E.W. Scholl, D. Reeh, W. Wiemann, M. Faber, G. Kühnen, H. Beck, N. Glienke Brenn- und Explosionskenngrössen von Stäuben STF-Report No. 2 - 79 (as well as BVS-Report) * * see paragraph 4.1 10)

W. Wiemann, R. Bauer, F. Möller Unterdruck-Sicherung von Silos nach Staubexplosionen bei Anwendung von Explosionsklappen VDI Report No. 701, 1988, Volume II

Process Technology / B05 - PT II / C04 - Firing Systems / Safe Handling of Coal and other Combustible dusts / 4. LITERATURE / 4.1 Approximate Values for Explosion Limits and Ignition Temperatures

4.1

Approximate Values for Explosion Limits and Ignition Temperatures

The numerical values of the following data are depending on the test procedure applied and can vary within certain limits according to the origin and geological age of the coals. The following values refer to the Literature (9). ♦ Explosion Limits 1) Dust concentration: ∗ lower explosion limits ∗ upper explosion limits

40 to 130 g/m3 2000 to 6000 g/m3

2) Oxygen concentration: © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:34:59 PM

Page 198

"Holderbank" - Cement Course 2000 ∗ hard coal 14% ∗ lignite 12% 3) Concentration of non-combustible parts (ash): ∗ hard coal (-medium volatile bituminous) 65% ♦ Ignition Temperature Cloud °C

Layer °C

Lignite

380 to 450

225 to 300

Hard coal

590 to 710

245 to 380

Petrol coke

690

280


6/23/2001 - 4:34:59 PM

Page 199

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C05 - Burners and Flames

C05 - Burners and Flames


6/23/2001 - 4:34:59 PM

Page 200

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames

Burners and Flames D. Pauling PT 98/14354/E 1. Terminology 2. Burners 2.1 Mono Channel - / Straight Burners 2.1.1

Burner Design

2.1.2

Burner Characterization

2.2 Multi Channel Burners 2.2.1

Burner Types

2.2.2

Burner Design Recommendations

2.3 Fuel Oil Atomizers 2.3.1

Mechanical Atomizers with Fixed Orifice and Variable Pressure

2.3.2

Mechanical Atomizers with Variable Orifice and Constant Pressure

2.3.3

Nozzles with Assisted Atomization through Steam or Compressed Air

2.4 Natural Gas Burners 3. flames 3.1 Prerequisites for the Ideal Flame 3.2 Flame Characteristics of the Different Burner Systems 3.2.1

Single Channel Burner

3.2.2

Multi Channel Burner

3.3 Factors Influencing the Flame 3.3.1

Primary Air Momentum

3.3.2

Position of the Burner in the Kiln

3.3.3

Alignment of the Burner in the Kiln

3.3.4

Secondary Air Temperature

3.3.5

Excess Air

3.3.6

Interaction Flame - Material Bed

3.3.7

Burner Dimensions

3.3.8

Pulverized Coal Characteristics

3.3.9

Fuel Oil Flame Adjustments

3.3.10 Natural Gas Flame Adjustments © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:00 PM

Page 201

"Holderbank" - Cement Course 2000 3.3.11 Combined Firing of Different Fuels 3.3.12 Oxygen Enrichment 3.4 Combustion Indicators 3.4.1

Clinker Quality

3.4.2

Sintering Zone Temperature

3.4.3

Coating Formation

3.4.4

Exhaust Gas Analysis

3.4.5

Kiln Inlet Temperature

3.4.6

Volatilization of Circulating Elements

3.5 NOx Formation 3.6 Flame Adjustment Procedure 4. secondary firing / precalciner 5. list of references 6. ANNEX 6.1 Formulas and Definitions for the Calculation of Burner Momentum and Swirl Number

Introduction The function of the burner is to introduce the fuel into the burning zone. The propagation of the combustion process depends on how fast the combustible comes into contact with oxygen. It is therefore the essential function of the burner to regulate this mixing process adequately in order to achieve a correct flame shape. This process must take place in such a fashion that the heat is released at exactly the right place without producing any damaging effects and without producing excessive pollutant elements such as NOx, SOx and CO. Consequently, any optimization of the burning process must start with the correct adjustment of the flame. This paper describes how the flame can be adjusted, what burner types are available and under what conditions they work best. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 1. TERMINOLOGY

1.

TERMINOLOGY

•

Primary air + secondary air + false air = combustion air

•

Stoichiometric combustion air + excess air = combustion air


6/23/2001 - 4:35:00 PM

Page 202

"Holderbank" - Cement Course 2000 Figure 1:

Terminology of Combustion Air

Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 2. BURNERS

2.

BURNERS

Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 2. BURNERS / 2.1 Mono Channel - / Straight Burners

2.1

Mono Channel - / Straight Burners

Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 2. BURNERS / 2.1 Mono Channel - / Straight Burners / 2.1.1 Burner Design

2.1.1

Burner Design

The mono channel burner is the most simple burner design. With this burner type, coal dust and all the primary air is injected together through a single tube. Usually this type is used for long kilns, equipped with direct firing. Mono channel / straight burners can also be used for fuel oil firing or for a combination of coal and oil firing (additional channel for the oil nozzle in the center). Conical burner tips can be used to increase the injection velocity (Fig. 2). Figure 2:

Straight Burner


6/23/2001 - 4:35:00 PM

Page 203

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 2. BURNERS / 2.1 Mono Channel - / Straight Burners / 2.1.2 Burner Characterization

2.1.2

Burner Characterization

A high axial impulsion (massflow of fuel and primary air multiplied with the injection velocity) leads to an intense mixing of the combustion air with the fuel. This intensive mixing has two effects: 1) strong and stable flame; good (complete) combustion 2) high NOx formation Recommended range of specific axial impulsion (Gax) for mono channel burners:

Gax =

M(transport air + fuel) ⋅ Vtransport air Q fuel

= 6−7

N MW

M:

Massflow Transport Air + Fuel (kg/s)

V:

Injection Velocity (m/s)

Q:

 MJ kg  ⋅  = [MW ]  Fuel Input (calorific value · fuel massflow)  kg s 

This corresponds with the old rule of thumb, which states that the kinetic energy of the primary air jet of a mono channel burner should be kept constant within certain limits:

(Velocity of Primary Air)2

(%Primary Air)

= 65'000

–

75'000

Even if this formula will not give optimal values in each case, it enables a rough estimate of the dimension of the burner if presupposed as a second condition that the primary air jet velocity should lie between 50 and 100 m/sec (valid for straight burners without swirl). Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 2. BURNERS / 2.2 Multi Channel Burners

2.2

Multi Channel Burners

Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 2. BURNERS / 2.2 Multi Channel Burners / 2.2.1 Burner Types

2.2.1

Burner Types

The most simple design of a burner is the mono channel burner (see Chap. 2.1). However, for optimum flame shaping when considering changing coal quality and different requirements from the point of view of raw mix burnability, burners with adjustable flame are to be preferred. In such burners, the © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:00 PM

Page 204

"Holderbank" - Cement Course 2000 primary air is usually divided into an axial and a radial component with the coal also introduced via a concentric ring tube. These burners are called multi channel burners, and are usually suitable for alternate or combined firing of coal, oil or gas. The axial air is injected in the direction of the kiln axis (similar to a mono channel burner where all the air is injected in axial direction). The radial air or swirl air is injected with a direction towards the kiln wall. The swirl component of the radial air creates a rotating air flow along the kiln axis (similar to the threat of a screw). This airflow is also pushing towards the outside, in direction of the kiln wall. Since the radial air channel is located inside the axial air channel (both are concentric ring channels), the radial air is opening up / widening the flow of the axial air. An increase of radial air versus axial air therefore creates a wider and shorter flame. An increase of axial air versus radial air create a longer flame. Besides flame shaping, the primary air also has to keep the burner pipe cool. A typical example of the first generation of multi channel burners is the Pillard 3-Channel Burner (Fig. 3). This burner has the coal channel in between the axial- and the radial air channel. A problem recognized with these burner types is that a shortening of the flame tends to produce a too wide flame (flame impeigements on the kiln wall). Furthermore coarse cool particles (residue on 200 µm sieve) can be thrown out of the primary air jet by the radial air. These particles can cause reducing condition on the clinker bed and high NOx formation. Figure 3:

Conventional 3-Channel Burner (Pillard)

A new generation of multi channel burners has therefore been designed. With special arrangements and constructions of the primary air channels the above mentioned negative effects can be avoided through the creation of a longer and more homogenious internal recirculation zone in the flame (see chapter 3.2.2). The leading burners of this generation are the Pillard Rotaflam and the KHD Pyrojet. The particular features of the Pillard Rotaflam Burner (Fig. 4) are the location of the coal channel inside the axial and radial air channels, as well as the flame holder / flame stabilizer (bluff-body-effect) in the enlarged center cross section. Figure 4:

Pillard Rotaflam Burner


6/23/2001 - 4:35:01 PM

Page 205


The particular feature of the KHD Pyrojet Burner (Fig. 5) is the jet air. The effect of this burner can be explained by the better and more uniform mixing of fuel and secondary air due to the jet air being introduced at nearly sonic velocity. For this reason the Pyrojet requires a compressor for the jet air. Figure 5:

KHD Pyrojet Burner

The FLS Swirlax Burner (Fig. 6) applies Pyrojet technology with a license from KHD. The experience in the Holderbank Group is limited. Figure. 6:

FLS Swirlax Burner

Unitherm offers an interesting solution with their M.A.S. Burner (Fig. 7), featuring only one primary air channel with adjustable swirl. However, so far with no application within the Holderbank Group.


6/23/2001 - 4:35:01 PM

Page 206


Unitherm M.A.S. Burner

Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 2. BURNERS / 2.2 Multi Channel Burners / 2.2.2 Burner Design Recommendations

2.2.2

Burner Design Recommendations

(Calculation of Gax and Sb : see Annex) • Specific Axial Impulsion:

Gax = 3 - 7

low volatile coal (10% volatiles):

Gax = 7

high volatile coal (35% volatiles):

Gax = 3

The axial impulsion affects the overall entrainment into the flame. In general a higher axial impulsion results in enhanced mixing and higher NOx emission levels.

• Swirl Number:

Normal Range:

Sb = 0.1 - 0.25

Maximum Range:

Sb = 0.4

In general higher tangential momentum (expressed through the swirl number) results in a more rapid heat release in the near burner zone and higher NOx emission levels. ♦ Primary Air Ratio: 10 - 12% ♦ Experience with these new generation (low primary air) burners has shown, that primary air ratios of 6 - 8% are on the technical limit below which it is no longer possible to guarantee stable combustion conditions. ♦ With primary air ratios of 6 - 8%, any disturbance of the burning process tends to shift combustion to the back kiln zone, producing high kiln inlet temperatures and poor clinker quality (underburning). Therefore in designing the primary air content for modern burners, a minimum of © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:02 PM

Page 207

"Holderbank" - Cement Course 2000 10% is recommended (including transport air in coal fired systems). ♦ Seen from a heat saving point of view the primary air ratio should be as low as possible in order to recuperate as much hot secondary air as possible. On the other hand, the kinetic energy of the fuel air mixture must be sufficiently high to provide a good mixture with the secondary air to ensure rapid burning. ♦ ♦ Axial air velocity (injection):

100 - 190 m/s (Pyrojet: 300 m/s)

♦ Radial air velocity (injection):

100 - 190 m/s

♦ Pressure of radial and axial air:

150 - 200 mbar (Pyrojet axial air: 0.5 - 1 bar)

♦ Transport air coal (injection):

20 - 30 m/s

Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 2. BURNERS / 2.3 Fuel Oil Atomizers

2.3

Fuel Oil Atomizers

Once properly prepared in terms of filtering, heating up and delivering to the burner with constant pressure and viscosity, the fuel oil must be atomized for effective mixing with the combustion air. Therefore fuel oil atomizing nozzles are used. These nozzles are located in the center of the burner, surrounded by the injection of the primary air. The oil nozzle is held in place by a jacked tube which is a fixed part of the burner. Thus the atomizing nozzle is retractable, which is necessary to change the orifice plate when increasing the throughput (only mechanical atomizers with fixed orifice - see below) or to take out the oil nozzle whenever it is not needed (e.g. switching to coal firing) to prevent overheating or coking of the unused atomizer. For fuel oil atomization different principles are employed: ♦ Mechanical atomization with fixed orifice and variable pressure ♦ Mechanical atomization with variable orifice and constant pressure ♦ Assisted atomization with steam or compressed air Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 2. BURNERS / 2.3 Fuel Oil Atomizers / 2.3.1 Mechanical Atomizers with Fixed Orifice and Variable Pressure

2.3.1

Mechanical Atomizers with Fixed Orifice and Variable Pressure

This type of atomization is the most common. Hereby the oil throughput is governed by the pressure of the fuel oil (within the range given by the selected discharge opening/orifice plate). With these atomizers the fuel oil flow in the atomizer head is often subdivided into an axial and a radial component. By adjusting the pressure and thus the ratio of these components, it is possible to alter the spray angle of the fuel jet. In general, an increase of the radial/tangential oil pressure leads to intensified swirling of radial and axial oil which has the tendency to shorten the flame. Typically the differential pressure is in the range of 1.5 bar (tangential minus axial oil pressure) with an overall pressure of approx. 40 bar. Since the reading accuracy of such small values, compared to the operating range of 40 bars, is unsatisfactory, it is suggested to equip both, radial and axial oil flow with oil flow measuring devices and optimize on flow basis using the flow-pressure curve of the nozzle © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:02 PM

Page 208

"Holderbank" - Cement Course 2000 supplier or to install a separate measurement of the pressure difference between radial and axial oil pressure. Flame shape control is, however, not only a result of atomizer adjustments, but also a function of primary air control. Fig. 8 and 9 show two current atomizers (Pillard and Unitherm) with radial-axial flow or alternatively return-flow for start-up operation. For return-flow, the axial oil flow is used to return a portion of the radial oil flow to the storage tank, in order to have a high flow velocity and oil pressure in the nozzle head (swirl chamber) despite the small amount of oil injected in the kiln (start up phase). Thus the turndown ratio can be increased, still with a good atomization. Atomizer turndown ratios of 10 to 1 are often given by the suppliers. Practical turn down ratios (without changing the orifice plate) however, are limited to values below 5 to 1 (even for return flow operation during start up). As an additional feature, the length of the swirl chamber in the Unitherm atomizer is adjustable. Fig. 10 (Coen Tri-Tip Nozzle) shows a mechanical atomizer with fixed orifice without radial-axial oil flow division. Figure 8:

Pillard MY Atomizer

Figure 9:

Unitherm Atomizer

Figure 10:

Coen Tri-Tip Nozzle


6/23/2001 - 4:35:02 PM

Page 209


Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 2. BURNERS / 2.3 Fuel Oil Atomizers / 2.3.2 Mechanical Atomizers with Variable Orifice and Constant Pressure

2.3.2

Mechanical Atomizers with Variable Orifice and Constant Pressure

This type of atomizer employs the adjustable needle valve principle for throughput control. By moving the needle position, contrary to the above described types, the orifice can be adjusted. Atomizing pressures are in the range of 20 bar. The turndown ratio are also limited. Needle value atomizers are mainly used by FLS for long wet kilns (see Fig. 11). Flame shaping is accomplished by adjusting the needle position, oil pressure and primary air. Figure 11:

FLS Atomizer (Needle Valve Principe)

1) tangential slots 2) swirl chamber Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 2. BURNERS / 2.3 Fuel Oil Atomizers / 2.3.3 Nozzles with Assisted Atomization through Steam or Compressed Air

2.3.3

Nozzles with Assisted Atomization through Steam or Compressed Air

This type of atomizer (Fig. 12) uses steam or compressed air instead of radial oil to create an intense swirl in front of the orifice plate. The advantage of these atomizers is the higher turndown ratio because even a small amount of oil can be atomized effectively with steam or compressed air. The disadvantage of these atomizers is the need for a significant amount of steam or compressed air, which cost money to produce.


6/23/2001 - 4:35:03 PM

Page 210


Pillard Atomizer with Assisted Atomization

Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 2. BURNERS / 2.4 Natural Gas Burners

2.4

Natural Gas Burners

In most modern gas burners the gas flow divided is into radial and axial gas. Primary air is not necessarily needed. However most burners use primary air for flame shaping and burner cooling. Pillard Rotagas Burner (Fig. 13) The Rotagas burner is the most recent development from Pillard. The burner is designed for 100% gas firing. Compared with the conventional Pillard Gas Rotaflam, the possibilities to adjust the flameshape have been ameliorated. Arrangement of the channels (from outside to the center): ♦ exterior, high pressure gas channel ♦ radial swirl air channel ♦ interior, low pressure gas channel ♦ central air channel ♦ center: jacket tube for ignition burner Figure 13:

Pillard Rotagas Burner

Pillard Rotaflam KGD Gas/Coal/Oil Burner (Fig. 14) © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:03 PM

Page 211

"Holderbank" - Cement Course 2000 The Rotaflam multipurpose burner is equipped for combined or separate firing of gas, coal and oil. Flame shaping is achieved with separate axial and radial primary air. Arrangement of the channels (from outside to the center): ♦ axial air ♦ radial swirl-air ♦ single gas channel ♦ pulverized coal channel ♦ central air / flame stabilizer ♦ center: jacket tube with oil atomizing nozzle Figure 14:

Pillard KGD Gas / Coal / Oil Burner

Gyro-Therm Gas Burner (Fig. 15) The Gyro-Therm burner applies a special flow phenomena to achieve the air/gas mixing. A "processing jet" is generated in a specifically designed nozzle. Experiences with this burner are limited. Figure 15:

Gyro-Therm Gas Burner

KHD Gas Burner (Fig. 16) This burner has been used in various kilns since a long time. Owing to the principle on which it operates, it requires a rather high supply pressure (3 - 5 bar) to allow the fuel throughput and the shape of the flame to be varied. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:03 PM

Page 212

"Holderbank" - Cement Course 2000 Primary air is not needed for this burner. Arrangement of the channels (from outside to center): ♦ axial gas channel ♦ center: radial gas Figure 16:

KHD Gas Burner

FLS Gas Burner (Fig. 17) Flame adjustment is done with this burner using the "needle valve method". Arrangement of the channels (from outside to center): ♦ primary air (10) ♦ secondary gas (15) ♦ primary gas (14) ♦ center: primary gas with regulating cone (13) Figure 17:

FLS Gas Burner

Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES

3.

FLAMES


6/23/2001 - 4:35:04 PM

Page 213

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.1 Prerequisites for the Ideal Flame

3.1

Prerequisites for the Ideal Flame

The optimization of the burning has to start with the correct adjustment of the flame. A look at the effects of poor burning makes this immediately clear: ♦ Unstable coating behavior, particularly in the transition zone, reduces the lining life. ♦ Incomplete burning and a local reducing atmosphere dramatically increase sulfur volatilization and build-up of coating in the preheater and in the kiln inlet area. Thus a significantly higher dust cycle is created which shifts the entire temperature profile toward the kiln inlet. ♦ With high CO-formation, secondary combustion forms at the back of the kiln which leads to ring formation. ♦ As a result, the kiln cannot operate at maximum output, the specific heat consumption increases and the efficiency of the unit drops. The "ideal" flame can prevent, or at least keep within limits, the operating problems described above. The flame is stable over the entire burn-out distance: ♦ By continually mixing hot secondary air into the burning zone. Therefore combustion can take place in a controlled manner over the entire flame length. ♦ No local temperature peaks are formed. ♦ No local reducing conditions develop over the clinker bed. ♦ Burn-out is complete at the end of the sinter zone. In addition this "ideal flame" has to be achieved with the lowest possible formation of NOx in the exhaust gas. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.2 Flame Characteristics of the Different Burner Systems

3.2

Flame Characteristics of the Different Burner Systems

Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.2 Flame Characteristics of the Different Burner Systems / 3.2.1 Single Channel Burner

3.2.1

Single Channel Burner

Straight burner pipes tend to produce an axial flame without internal recirculation. The heating up of the fuel jet to ignition temperatures is predominantly by external recirculation of the hot combustion gases (Fig. 18). Figure 18:

Flame Shape of Single Channel Burner


6/23/2001 - 4:35:04 PM

Page 214


Effects: ♦ Long sinter zone ♦ Long retention time of the kiln charge in the hot zone and thus high volatilization of alkalis and sulfur (very suitable for the production of low-alkali clinker) ♦ High NOx formation With a constant primary air ratio, the length of the flame reaches a minimum for a given primary air velocity. If the velocity is further increased, the primary air jet develops an excessive suction effect which results in a reverse flow of flue gases. The recirculating flue gas thins the secondary air so much that the flame becomes longer again. For the recommended range of the specific axial impulsion see chapter 2.1. However, for optimum flame shaping in response to changing production requirements, burners with adjustable flame (multi channel burners) are to be preferred. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.2 Flame Characteristics of the Different Burner Systems / 3.2.2 Multi Channel Burner

3.2.2

Multi Channel Burner

Multi channel burners can produce a divergent flame with internal and external recirculation zones. The ability to change the relationship between axial and radial air provides an important control mechanism for influencing the flame shape. A hollow cone flame shape is produced, which can be modified by adjusting the pressure and/or injection-orifice of the radial and axial air (Fig. 19). The first generation of three channel burners (e.g. Pillard 3-Channel) has some negative effects on the flame shape, if there is a high content of radial air used. Two different flame zones can appear: ♦ In the first zone with internal recirculation there is intense combustion. Depending on the arrangement of the swirling flow, in this zone coarse fuel particles are spun out of the flame and then burn quickly in the oxygen-rich atmosphere of the hot secondary air. ♦ In the second, long, instable zone, dominated by external recirculation, burn-out is completed. Effects: ♦ Peak temperatures in the internal recirculation zone. ♦ With very divergent flames, there are problems with the lining. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:04 PM

Page 215

"Holderbank" - Cement Course 2000 ♦ CO formation above the clinker bed caused by incomplete burn-out of the extended fuel jet. ♦ Unstable coating formation in a long transition zone, caused by an enlarged unsteady burn-out zone. ♦ Increased NOx formation because of the long retention time of the gasses at high temperatures. ♦ High sulfur volatilization because of the reducing zone above the clinker bed and the long retention time at relatively high temperatures. The new generation of three channel burners (e.g. Pillard Rotaflam and KHD Pyrojet - see also chapter 2.2) has been optimized so that these effects are largely avoided. The special arrangement and construction of the primary air channels make the internal recirculation zone (IRZ, Fig. 19) longer and more homogenous. This reduces the length of the burn-out zone with external recirculation. To reduce NOx formation, these burners have been optimized for very low primary air quantities. For a faster mixing of the primary air with the fuel, these burners have an enlarged flame stabilizer in form of a bluff body in the center. Figure 19:

Flame Shape of New Generation Multi Channel Burner

Effects: ♦ Homogeneous temperature distribution, no excessive temperature peaks. ♦ Low volatilization rate of alkalis and sulfur. ♦ Homogeneous recirculation zone, and therefore low NOx formation. ♦ In some cases the flame is too long. Therefore a rearrangement of the coal channel in between the axial and radial air (Pillard Rotaflam) is under discussion. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.3 Factors Influencing the Flame

3.3

Factors Influencing the Flame

In most cases the most favorable operation is achieved with a rather short and powerful flame, giving a high heat transfer rate to the material bed and a short and stable burning zone. The flame shape may be optimized during operation by adjusting the following parameters: Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.3 Factors Influencing the Flame / 3.3.1 Primary Air Momentum

3.3.1

Primary Air Momentum


6/23/2001 - 4:35:05 PM

Page 216

"Holderbank" - Cement Course 2000 A shortening of the flame is normally achieved by increasing the injection momentum of the primary air. With existing burners this can be achieved to a limited extent by increasing the radial air and decreasing axial air correspondingly. With jet burners (KHD) the flame can be optimized by varying number and diameter of jet nozzles and adjusting the jet air pressure. For burner design recommendations: see chapter 2.2.2. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.3 Factors Influencing the Flame / 3.3.2 Position of the Burner in the Kiln

3.3.2

Position of the Burner in the Kiln

One of the most pronounced influence on flame length is the position of the burner tip: Shifting the burner further in the kiln increases the flame length significantly and vice versa. This is because the turbulence field of the in-flowing secondary air significantly improves the mixing of the fuel jet with the air. In planetary cooler kilns this effect is less noticeable as the position of the burner tip is defined by the kiln's internal cooling zone. Recommendations for burner tip position (except planetary cooler):

♦ Dry kiln:

Distance from rotary kiln end to burner tip ≤ 1 m.

♦ Long wet kiln:

Distance rotary kiln end - burner tip approx. 1 m or a little more.

If the burner tip is too close to the rotary kiln end, overheating of the nose ring can occur. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.3 Factors Influencing the Flame / 3.3.3 Alignment of the Burner in the Kiln

3.3.3

Alignment of the Burner in the Kiln

Basically the burner should be aligned parallel to the kiln axis. In the cold kiln the burner should even be pointed slightly upwards, (specially long burners in kilns with planetary coolers), to compensate for the bending downwards in the hot kiln. If the burner is aligned horizontally (the kiln axis has an angle of approx. 3° to the horizontal) as is often seen, the flame tends to reach the material bed. A local reducing atmosphere is created resulting in high sulfur volatilization. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.3 Factors Influencing the Flame / 3.3.4 Secondary Air Temperature

3.3.4

Secondary Air Temperature

The secondary air temperature defines, firstly, the ignition behavior of the flame (black plume) and, secondly, the possible flame temperature. Insufficient secondary air temperature has to be compensated by fuel, and this means an increase in the combustion gas quantity and a lengthening of the temperature profile. In point of fact, the clinker cooler operation is one of the main factors influencing the flame. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.3 Factors Influencing the Flame / 3.3.5 Excess Air


6/23/2001 - 4:35:05 PM

Page 217


Excess Air

Some excess air is required for complete combustion. The optimum value for excess air to maintain the shortest possible sinter zone is about 10% (equal to 2% O2 at kiln inlet). Burning at a too low excess air factor increases the burning time and hence the flame length. This creates a reducing atmosphere which increases sulfur volatility thus leading eventually to clogging problems in the preheating zone. If the excess air is significantly higher than the optimum value, the temperature profile is extended again because of a too low flame. This results in an insufficient temperature gradient towards the material bed and a longer sinter zone. For this reason, for example, the secondary firing rate for Air-Through systems is restricted to about 25 ± 5%. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.3 Factors Influencing the Flame / 3.3.6 Interaction Flame - Material Bed

3.3.6

Interaction Flame - Material Bed

As the heat transfer from the flame to the material bed in the sinter zone is almost entirely through radiation, the key factors affecting heat transfer are the temperature and the emissivity of the flame. If radiation is reduced by a dusty kiln atmosphere, a long drawn-out temperature profile with long sinter zone is produced. In this situation, the clinker dust is overheated in the flame and often deposited in the transition zone or even further upstream the kiln in the form of a clinker ring. Ways to counter this effect include all those measures which serve to improve clinker granulation (short and hot flame, different raw mix design). Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.3 Factors Influencing the Flame / 3.3.7 Burner Dimensions

3.3.7

Burner Dimensions

Basically the burner must be of the right dimensions for nominal operation. This is observed particularly for burners in kilns with precalcination. Oversized burner nozzles have to be operated with unfavorable primary air settings (either too high primary air content or too low primary air speed) and should be adjusted for nominal operation. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.3 Factors Influencing the Flame / 3.3.8 Pulverized Coal Characteristics

3.3.8

Pulverized Coal Characteristics

♦ Volatile content: ♦ The combustion time of pulverized coal increases as the volatile content decreases, therefore low volatile coal has a longer burning time and ignition distance than bituminous coal. ♦ Grinding fineness: ♦ The burning time of a coal dust grain increases approx. with the square of its diameter. The combustion time of a grain of coal increases as its volatile content decreases. Thus, low volatile coal must be ground more finely in order to burn within the desired time, e.g. in order to produce the desired flame length. ♦ Recommendations for optimum grinding fineness: see paper "Firing Systems - Handling and Preparation of Noble Fuels". ♦ Ash content: © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:05 PM

Page 218

"Holderbank" - Cement Course 2000 ♦ A high content of ballast material (ash) has a retarding effect on the burning time caused by the reduced coal dust concentration and the lower flame temperatures as a result of the heat absorption of the ballast material. ♦ Rate of swelling: ♦ The higher the expansion of the coal grain during heating in the flame, the shorter the burning time. Coal types with high density expand / swell less. Therefore petrol coks has to be ground finer to reach the same combustion time as regular coal. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.3 Factors Influencing the Flame / 3.3.9 Fuel Oil Flame Adjustments

3.3.9

Fuel Oil Flame Adjustments

A faster burn out of the fuel oil can be achieved by lowering the oil viscosity / increasing the oil temperature (recommendations for optimum oil temperature: see paper "Firing Systems - Handling and Preparation of Noble Fuels") or by better atomization (see chapter 2.3). Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.3 Factors Influencing the Flame / 3.3.10 Natural Gas Flame Adjustments

3.3.10 Natural Gas Flame Adjustments The main requirement with natural gas burners is the possibility of producing a reverse flow zone in the center of the flame in order to achieve locally a reducing atmosphere where the hydrocarbon molecules agglomerate to larger chains. This is necessary to increase the emissivity of the gas flame, a prerequisite for heat transfer in the sintering zone. Adjusting the shape of the flame visually is almost impossible, because clearly defined flame contours are hardly recognizable. Optimization of the flame shape should be done following the combustion indicators (see chapter 3.5). Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.3 Factors Influencing the Flame / 3.3.11 Combined Firing of Different Fuels

3.3.11 Combined Firing of Different Fuels When firing two different fuels at the same time, the higher volatile fuel tends to burn more rapidly. this reduces the oxygen content so that the remaining fuel burns further to the back of the kiln. However, a small amount of high volatile fuel can also have a positive effect on the flame, because it accelerates the ignition and burning of the other fuel. In extreme cases, two separate burning zones are created. It is therefore important to improve the burning time of the less volatile fuel (e.g. by adjusting the fineness of grinding for coal). Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.3 Factors Influencing the Flame / 3.3.12 Oxygen Enrichment

3.3.12 Oxygen Enrichment By adding oxygen to the combustion air, the flame temperature can be increased significantly. At the same time the specific exhaust gas quantity is lowered. This decreases the energy losses of the exhaust gas and allows to increase the production capacity of the kiln. Practically feasible is the increase of O2 in the combustion air by 2 - 3% (from 21% to 24%). © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:06 PM

Page 219

"Holderbank" - Cement Course 2000 Disadvantages are the higher NOx formation, the lower cooler efficiency for planetary coolers and the oxygen costs. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.4 Combustion Indicators

3.4

Combustion Indicators

One of the main problems in the evaluation of the flame is that, in the rotary kiln, it is only possible to observe the flame visually to a limited extent. On closer consideration, however, there are a number of indicators which can provide much more information about the quality of the flame than can be obtained from simple visual observation. In the following, the most important operation indicators (combustion indicators) with direct relation to firing parameters are discussed: ♦ Clinker quality (free lime, liter weight) ♦ Burning zone temperature (pyrometer, NOx, amps) ♦ Coating formation (indicated by kiln shell temperature profile) ♦ Exhaust gas composition (CO, O2) ♦ Kiln inlet temperature ♦ Volatilization of circulating elements (hot meal analysis, encrustations in the preheater) Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.4 Combustion Indicators / 3.4.1 Clinker Quality

3.4.1

Clinker Quality

There exists a close interdependence between sintering zone temperature, granulometry, free lime and literweight of the clinker. The correlation of these parameters is to a high degree influenced by the flame shape. Fig. 20 shows an example, where by flame optimization, the literweight for the required free lime could be lowered. In other words, for the required clinker quality (free lime), burning could be done less hard (liter weight). Burning less hard leads to substantial savings of energy and refractories. Figure. 20: Correlation between Free Lime Content and Literweight with Two Different Operating Conditions

When modifying the burner settings, the correlation of the parameters shown above has to be closely © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:06 PM

Page 220

"Holderbank" - Cement Course 2000 recorded before and after any change to the burner in order to draw the relevant information for optimum burner settings. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.4 Combustion Indicators / 3.4.2 Sintering Zone Temperature

3.4.2

Sintering Zone Temperature

Information about the sintering zone temperature can be obtained by: ♦ Measuring the clinker bed temperature under the flame using a radiation pyrometer. ♦ Measuring the NOx concentration in the exhaust gas. ♦ Measuring the inclination of the kiln charge using a tallumeter. ♦ Measuring the kiln drive power consumption (Amps or kW) - (only reliable in some cases). It has to be noted, that all the above mentioned measurements do not supply absolute but rather relative temperature indications and that the NOx-level is also highly depending on the flame characteristics (see chapter 3.6). Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.4 Combustion Indicators / 3.4.3 Coating Formation

3.4.3

Coating Formation

The length of the sinter zone and transition zone gives a guide to the length and temperature profile of the flame. Ring formation can indicate poor combustion, incorrect burner setting, or insufficient fuel preparation (coal not fine enough or poor oil atomizing). Coating formation can be determined indirectly, by measuring the temperature profile of the kiln shell. The influence of burner adjustments on coating formation can be checked by recording the kiln shell temperature profile before and after any change to burner settings. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.4 Combustion Indicators / 3.4.4 Exhaust Gas Analysis

3.4.4

Exhaust Gas Analysis

Exhaust gas analysis at kiln inlet supplies valuable information on the completeness of the combustion. Due to factors such as fluctuations in fuel supply and quality, generally a too high O2 level would be required for 0% CO. Usually the kiln is set to an O2 level at kiln inlet, at which < 500 ppm CO is produced. A thus required O2 level in excess of 2.5% would indicate combustion problems. Too high CO levels do not only cause energy losses but do also increase Sulfur volatilization and may cause Sulfur rings and cloggings in the cyclones. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.4 Combustion Indicators / 3.4.5 Kiln Inlet Temperature

3.4.5

Kiln Inlet Temperature

With preheater kilns, the kiln inlet temperature (= back end temperature) supplies information on flame length and retarded combustion. The target is to have the kiln inlet temperature as low as possible. Kiln inlet temperatures in excess of 1100°C need improvement of the firing system. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.4 Combustion Indicators / 3.4.6 © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:06 PM

Page 221

"Holderbank" - Cement Course 2000 Volatilization of Circulating Elements

3.4.6

Volatilization of Circulating Elements

The flame has an important influence on the volatilization of the circulation elements, especifically on Sulfur. This is governed by factors such as retention time of the material in the hot zone (flame length) and local or general reducing atmosphere including the presence of oversize fuel particles in the material bed (Fig. 21). Figure 21:

Influence of Temperature and O2 Concentration on Sulfur Volatility

To assess the degree of volatilization of the circulating elements, the enrichment of SO3, CI and K2O in the hot meal has to be measured before and after any change to the firing system. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.5 NOx Formation

3.5

NOx Formation

NOx formation is dominated as well by peak temperatures as by the amount of air entrained into the primary fuel jet at ignition. NOx reduction measures are deduced essentially from these facts as follows: ♦ Low primary air ratio ♦ Flame front near the burner (short ignition distance) ♦ Flame shaping with the aim to avoid high peak temperatures with at the same time shorter flame ♦ Lower burning temperatures (free lime, raw mix) The minimum technically achievable NOx emission with measures related to the rotary kiln burner are in the order of magnitude of 800 to 1'000 mg/Nm3 (dry basis). Further reduction of NOx requires additional secondary measures such as staged combustion (air / fuel staging, reburning) at the precalciner or NH3 injection. For more details on NOx formations see paper "State of Technology of Rotary Kiln Burners". Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 3. FLAMES / 3.6 Flame Adjustment Procedure

3.6

Flame Adjustment Procedure

1) Follow the operating instruction of the supplier for a medium flame setting. 2) Wait until kiln is stable before undertaking any adjustment. 3) Progressively adjust parameters (axial/radial air, oil pressure, gas pressure) to get required flame. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:06 PM

Page 222

"Holderbank" - Cement Course 2000 ♦ Cautions: •

axial air outside channel also serves to cool the burner pipe; keep always min.8% of total primary air

•

watch continuously the corresponding combustion indicators

♦ The kiln reacts slowly to any change. It may take up to a few days to reach stable running conditions again. It is therefore useless to try to adjust a flame within one shift! 4) It is not recommended to operate the kiln with the shortest possible flame. A safety margin for adjustment in both directions should be maintained for control of burning zone disturbance. NOx Emission: In some countries with severe regulations, the NOx emission might be in a near future the most important parameter for flame adjustment. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 4. SECONDARY FIRING / PRECALCINER

4.

SECONDARY FIRING / PRECALCINER

The burning conditions for a secondary firing or precalciner burner are quite different from the kiln firing: ♦ In most cases the combustion takes place in a exhaust gas + air mixture instead of pure air. ♦ Combustion takes place in a very dusty atmosphere. ♦ Temperature range of 1000°C instead of 2000°C. Due to the poor burning conditions, incomplete combustion is quite common in precalciners. Beside CO, coal firing produces carbon skeletons and also CH4, which both cannot be traced by CO measuring equipment. Further signs for incomplete combustion are: ♦ Higher gas temperature at bottom cyclone outlet than at precalciner outlet. ♦ Only moderate drop of the gas temperature over the two lowermost cyclone stages. Both indication an after-burning within the preheater. This results in increased exhaust gas temperature and heat consumption. Improvement Measures: ♦ Avoiding fluctuations of the fuel feed. ♦ Grinding the coal to the required fineness. ♦ Providing enough gas retention time in the precalciner. As a rule of thumb for coal firing: •

gas retention time = 2 to 3 sec.

•

(kiln capacity [t/d]) / (precalciner volume inside lining [m3] ) = 7 ± 2 t/m3 d

Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 5. LIST OF REFERENCES

5.

LIST OF REFERENCES

1) "Firing Systems" VA 82/4898/E 2) "Flames and Burners" VA 93/4056/E © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:07 PM

Page 223

"Holderbank" - Cement Course 2000 3) "State of Technology of Rotary Kiln Burners" F. Schneider, PT 96/14078/E 4) W.L. van de Kamp / J.P. Smart IFRF Research Report CEMFLAM1 "The effect of burner design and operation and fuel type of cement kiln flames" Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 6. ANNEX

6.

ANNEX

Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 6. ANNEX / 6.1 Formulas and Definitions for the Calculation of Burner Momentum and Swirl Number

6.1

Formulas and Definitions for the Calculation of Burner Momentum and Swirl Number

Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 6. ANNEX / 6.1 Formulas and Definitions for the Calculation of Burner Momentum and Swirl Number / 6.1.1 Primary Air / Combustion Air:

6.1.1

Primary Air / Combustion Air:

Kiln heat consumption

Q

[MJ/kgcli]

Min. combustion air Amin.

0.26 x q

[Nm3/kgcli]

Good approximation for conventional fuels

Total combustion air A

n x Amin.

[Nm3/kgcli]

n = excess air factor, n>1

Excess combustion air

Amin. x (n-1)

[Nm3/kgcli]

Primary air ratio

Expressed in % Amin.

[%A min.]

Note: In order to get lower primary air ratio figures, burner suppliers usually relate primary air ratio to total combustion air. Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 6. ANNEX / 6.1 Formulas and Definitions for the Calculation of Burner Momentum and Swirl Number / 6.1.2 Burner Geometry:

6.1.2

Burner Geometry:

Following burner geometry calculations are based on the list of symbols and units stated below: ri

Burner channel radius of channel I

[m]

reg. i

Equivalent channel radius of channel I

[m]

Gx

Axial momentum

[N]

Gx.i

Axial momentum of channel I

[N]

Gax

Specific axial momentum

[N/MW]


6/23/2001 - 4:35:07 PM

Page 224

"Holderbank" Gax - Cement Course 2000 Gt

Tangential momentum

[N]

Mi

Mass flow through channel I

[kg/s]

Qfuel

Fuel heat input

[MW]

Sb

Burner swirl number

[-]

vi.ax

Axial velocity in channel I

[m/s]

vsw.tan

Tangential velocity on swirling channel

[m/s]

Figure A:

Typical Burner Geometry

Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 6. ANNEX / 6.1 Formulas and Definitions for the Calculation of Burner Momentum and Swirl Number / 6.1.3 Mono Channel Burner:

6.1.3

Mono Channel Burner:

Total specific axial momentum through burner [N/MW]:

Gax =

M

 tr 

+ c 

Q

×v

tr

fuel

 N   MW   

Process Technology / B05 - PT II / C05 - Burners and Flames / Burners and Flames / 6. ANNEX / 6.1 Formulas and Definitions for the Calculation of Burner Momentum and Swirl Number / 6.1.4 Multi Channel Burner:

6.1.4

Multi Channel Burner:

Total specific axial momentum through burner [N/MW]:

Gax =

(M

sw

× v sw ,ax + M (tr +c ) × v tr + M ax × v ax ,ax )  N   MW  Qfuel  

Burner Swirl Number: Swirl number =

[ ] [ ] [−] Σ(Axial Momentum [N ]× Characteristical Channel Radius [m ]) Tangential Momentum N × Characteristical Swirl Radius m


6/23/2001 - 4:35:07 PM

Page 225


Sb =

Gt [N ] × req .sw [m ]

Σ G xi [N ]× req.i [m]

[−]

A common method for the calculation of the characteristical or equivalent radius is to determine the radius of gyration for each individual channel cross-section as follows (Mathur and Maccallun - 1967):

req .i

( (

) )

2 × r2 − r1 [m] = 2 2 3 × r2 − r1 3

3

For a typical multi channel coal burner with axial-, transport- and swirl air, the burner swirl number can be calculated according to the following formula:

Sb =

Msw × v sw ,tan × req.sw Max × v ax × req .ax + M(tr + c ) × v tr × req.tr + Msw × v sw × req .sw


[−]

6/23/2001 - 4:35:08 PM

Page 226

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C06 - Combustion Engineering

C06 - Combustion Engineering


6/23/2001 - 4:35:08 PM

Page 227

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition

Combustion, Gasflows and Gas Composition A. Obrist VA 89/5584/E 1. INTRODUCTION 2. COMBUSTION 2.1 Basic Relations and Definitions 2.2 Calorific Value 2.3 Combustion Calculations 3. GAS FROM RAW MATERIAL 3.1 Gases from Dry Raw Meal 3.2 Water from Wet Raw Meal or from Raw Slurry 4. KILN GAS 4.1 Measurement of Exhaust Gas Composition 4.2 Immediate Calculations from Gas Composition 4.3 Calculation of Exhaust Gas Quantities 4.4 Calculation of the CO2 Content of Cement Kiln Exhaust Gas 5. FALSE AIR INVESTIGATIONS 5.1 Introduction 5.2 Evaluation 5.3 Example of an Investigation


6/23/2001 - 4:35:09 PM

Page 228

"Holderbank" - Cement Course 2000 SUMMARY In context with gas measurements on a cement kiln there are important numerical relations which must be understood. Such numerical relations involve the following subjects: ♦ Fuel properties, calorific value ♦ Raw meal properties ♦ Combustion calculations ♦ Gas composition ♦ Gas properties ♦ Gas quantities ♦ False air Calculations can be done with complete, exact formulas but sometimes also by using approximations. Approximations are never 100% precise but often sufficient for practical purposes. Important, basic approximations given in this chapter are e.g.: ♦ Min. combustion air

=

Amin ≈ 0.26 Nm3/MJ

♦ Min. combustion gas

=

Vmin ≈ 0.28 Nm3/MJ

The exhaust gas of a cement kiln consists of: ♦ Gas from raw meal ♦ Gas from combustion ♦ Excess air and false air ♦ Water from slurry or raw meal By considering the above contributions the exhaust gas quantity of a kiln can be calculated. This method of calculation and many other relations are given in this chapter. Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 1. INTRODUCTION

1.

INTRODUCTION

Once, in the Greenfield cement factory, they wanted to do a few measurements on their kiln system, which seemed to run at its capacity limit. The specialist who was in charge of these measurements wanted to know the actual gas flows in the kiln system. Unfortunately, a few years ago, the designers of the Greenfield kiln had to fight with some difficult space problems when they had to plan the layout of the gas ducts in the narrow space. They never had considered that somebody would have to take flow measurements in this ductwork. The shape of the gas ducts was far away from the „ideal, long straight tube“ for a precise flow measurement. After a hard job within the hot areas of the gas ducts the specialist realized that his results from the pitot tube measurement was still not as precise as he had imagined. He therefore started to think about his problem. Maybe, there was another method to come to a result? Obviously gas flow has something to do with the fuel combustion and also raw meal produces some gas. So, why not calculate the gas © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:09 PM

Page 229

"Holderbank" - Cement Course 2000 from other parameters such as fuel quantity and raw meal? And wouldn’t it be possible to calculate also the gas composition? But where do I find the necessary relations and formulas? With this kind of thoughts in mind the specialist in the Greenfield plant was about to use the paper on hand, and finally to do a more interesting and effective job. Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION

2.

COMBUSTION

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.1 Basic Relations and Definitions

2.1

Basic Relations and Definitions

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.1 Basic Relations and Definitions / 2.1.1 Combustion

2.1.1

Combustion

Combustion of fuels is a chemical reaction of fuel with oxygen (O2) according to the general scheme:

FUEL

+

OXYGEN

→

COMBUSTION PRODUCTS (CO2, H2O, SO2, Ash)

Combustion reactions usually go through intermediate steps where some intermediate products may occur. However after complete combustion (total oxidation) the resulting combustion products are of extremely simple nature because complete combustion always ends at only 3(!) simple gas molecules, namely CO2, H2O and SO2. The overall combustion reactions can therefore be characterized by three very simple combustion equations:

C + O2

→

CO2

2H + ½ O2

→

H2O

S + O2

→

SO2

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.1 Basic Relations and Definitions / 2.1.2 Air

2.1.2

Air

Oxygen for combustion of fuels is taken from the air. For the considerations within this chapter the composition of the dry air can be simplified (neglecting trace gases) as follows:


6/23/2001 - 4:35:09 PM

Page 230


OXYGEN (O2)

21.0 Vol%

NITROGEN (N2)

79.0 Vol%

AIR

100.0 Vol%

According to the local climate air contains some water vapor, e.g. 2 Vol% at 20°C and 80% relative humidity. Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.1 Basic Relations and Definitions / 2.1.3 Normal Conditions

2.1.3

Normal Conditions

„Normal conditions“ for gases are defined as: ♦ Pressure =

1.0133 bar

=

760 Torr

♦ Temperature

=

=

273.16°K

0°C

By the above conditions, the Nm3 (Normal-cubicmeter) is defined which is used as unit for gas quantities. Note: 1) The „normal conditions“ refer to the average atmospheric pressure at sea level (Fig. 1 at altitude = 0 m) and at the zero point of the Celsius temperature scale (0°C). 2) There is a similar definition of „standard conditions“ (USA) which refers to the 60°F point of the Fahrenheit temperature scale (30 inch Hg = 1.016 bar, 60°F = 15.6°C). Unfortunately these conditions are not equal to the „normal conditions“ in the metric system. Fig. 1 Average Barometric Pressure in Function of Altitude

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.1 Basic Relations and Definitions / 2.1.4 Kmol and Ideal Gas

2.1.4

Kmol and Ideal Gas

To characterize the quantity of substances in context with chemical reactions the unit kmol is used. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:09 PM

Page 231

"Holderbank" - Cement Course 2000 One kmol means a certain number of molecules (Avogadro’s constant = 6.02 x 1026/kmol). If the molecular weight of a compound is taken in kg it equals to 1 kmol of this substance. For example: The molecular weight of CO2 is 44. Therefore 44 kg of CO2 are equal to 1 kmol (which contains 6.02 x 1026 molecules) of CO2. One of the basic relations of chemistry and thermodynamics says that a certain number of 1 kmol of any gas molecules takes always the same volume (at constant pressure and temperature). The formula of the gas molecules does not have any influence on their volume in gaseous state. To say this more precisely it must be added that this applies only for ideal gases. Gases at low partial pressures and at temperatures above the critical point can be considered as ideal gases. All gases occurring in context with this chapter can be considered as ideal gases with sufficient accuracy (approx. 0.1%). This fact can be used by the following quantitative relationship: ♦ 1 kmol of an ideal gas takes a volume of 22.4 m3 at normal conditions (1.0133 bar, 0°C) Or in short form: ♦ 1 kmol of gas = 22.4 Nm3 Note that even H2O and CO2 behave nearly like ideal gases as long as they occur in gas mixtures at low partial pressures. Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.1 Basic Relations and Definitions / 2.1.5 Conversion of the Volume of Ideal Gases

2.1.5

Conversion of the Volume of Ideal Gases

Ideal gases or mixtures of ideal gases behave according to the fundamental equation: ♦ pv = RT (p = absolute pressure, V = volume, R = gas-constant, T = absolute temperature) Therefore, volume conversions from condition 1 to condition 2 can be done easily by means of the ratios of absolute temperature [K] and absolute pressure as follows:

 P  T  V2 =V1 ×  1  ×  2   P2   T1  Example: Convert V1 = 1 m3 at 350°C (623.16 K) and 0.9 bar to normal conditions:

 0.9 bar   273.16 K  3 V2 =1m 3  × =0.389Nm  1.0133 bar   623.16 K  This type of calculation is frequently used for practical gas flow calculations. However, for the considerations within this paper it will not be required in the following. Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.1 Basic Relations and Definitions / 2.1.6 Minimum Air and Air Factor

2.1.6

Minimum Air and Air Factor

To perform a complete combustion a theoretical minimum amount of oxygen O2 min) is required, © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:10 PM

Page 232

"Holderbank" - Cement Course 2000 depending on the type of fuel. The corresponding minimum quantity of air is called Amin:

O min Amin = 2 [Nm 3 ] 0.21 Practically a combustion requires always slightly more air than the theoretical minimum Amin in order to avoid local lack of O2 and unburnt products. The air factor „n“ is defined as the following ratio:

n=

A [ −] Amin

A is the effective air quantity, n must be always higher than 1 in order to maintain complete combustion. Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.2 Calorific Value

2.2

Calorific Value

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.2 Calorific Value / 2.2.1 Gross and Net Calorific Value (CV)

2.2.1

Gross and Net Calorific Value (CV)

The calorific value of a fuel sample is measured in a bomb calorimeter. The fuel sample and a surplus of oxygen are ignited in the bomb and after combustion the amount of heat is measured. The bomb is thereby cooled to room temperature level. By cooling the calorimeter the water vapor is condensed and therefore the heat of water condensation (2450 kJ/kg H2O at 20°C) is included in the resulting heat. The direct result (amount of heat) obtained from the calorimeter is therefore always the so called gross calorific value CVgross (in German: oberer Heizwert, Brennwert, Ho) of the fuel. The gross value, however, is not very significant for common technical applications because the effect of water condensation does usually not occur. Therefore the net calorific value (in German: unterer Heizwert, Hu) value is calculated by subtracting the heat of condensation, as follows: CVnet = CVgross - (water in combustion products) x 2450

[kJ/kg fuel]

The water in combustion products is calculated as follows:

H2O = H2Ofuel + 9 x Hfuel

[kg/kg fuel]

↑hydrogen in fuel [kg/kg fuel]

Important Note: Within this chapter and also within the cement course chapter on Heat Balances the net calorific value is used as reference. Unless otherwise noted, fuel energy or fuel heat always refers to net calorific value. Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.2 Calorific Value / 2.2.2 Calculation of CV © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:10 PM

Page 233


Calculation of CV

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.2 Calorific Value / 2.2.2 Calculation of CV / 2.2.2.1 Gaseous Fuels

2.2.2.1

Gaseous Fuels

If the gas components are known by analysis the calorific value can be calculated exactly by adding the contributions of the pure gas components. The gas analysis is given as volume % (which is the same as mole %) and the calorific value is usually referred to one Nm3 (not kg of gas).

Gas Component

Formula

CV net [MJ/Nm3]

Methane

CH4

35.8

Ethane

C2H6

63.8

Propane

C3H8

91.3

Butane (gas)

C4H10

118.8

Pentane (gas)

C5H12

146.1

Ethylene

C2H4

59.1

Propylene

C3H6

86.1

Acetylene

C2H2

56.1

Carbon monoxide

CO

12.6

Hydrogen

H2

10.8

Hydrogen sulfide

H2S

23.2

Example: Natural gas:

CH4

=

90.5%

C2H6

=

2.0%

C3H8

=

0.5%

CO2

=

0.4%

N2

=

6.6% 100.0%

CV = 0.905 x 35.8 + 0.02 x 63.8 + 0.005 x 91.3 = 34.1 MJ/Nm3 Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.2 Calorific Value / 2.2.2 Calculation of CV / 2.2.2.2 Liquid and Solid Fuels


6/23/2001 - 4:35:10 PM

Page 234


Liquid and Solid Fuels

Usual liquid and solid fuels (fossil fuel oil and coal) consist of complex organic compounds. Usually the ultimate analysis or the elemental analysis (content of C, H, S, N, O) is available. Strictly speaking it is impossible to calculate the exact calorific value of a fuel only from its elemental composition without knowing the kind of its organic compounds. However, as long as fossil fuel oils and coals are considered, the following approximation produces fairly accurate results: CVnet ≈

34.8 x C + 93.9 x H + 10.5 x S 6.2 x N - 10.8 x 0 - 2.5 x W

[MJ/kg]

C, H, S, N, O, W are the weight fractions [kg/kg fuel] of carbon, hydrogen, sulfur, nitrogen, oxygen and water. The above approximation produces also good results for other organic materials such as wood, paper and peat. But is should not be used for extreme cases such as e.g. pure carbon or pure sulfur. Note that for exact determination of the CV of fossil fuel oils and coals only the calorimeter method and not the above approximate calculation - can give the correct result. Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.2 Calorific Value / 2.2.3 Examples of Calorific Values

2.2.3

Examples of Calorific Values

(Including also alternative fuels)

CV [MJ/kg] (net) Pure polyethylene

46

Light oil

42

Heavy oil

40

Pure polystyrene

40

Pure rubber (without inert material)

36

Anthracite

34

Waste oils, various refinery wastes

30 to 40

Petcoke

33

Waste tires (with steel and inert material)

28 to 32

Bituminous coal (low ash)

29

Bituminous (high ash)

24

Acid sludge, acid tar (from oil refining)

16 to 22

Lignite (10% moisture)

16 to 21

Pot liners (from aluminium smelter)

20

PVC

19


High grade

6/23/2001 - 4:35:11 PM

Page 235

"Holderbank" - Cement Course 2000 PVC

19

Palm nut shells (10% moisture)

19

Pressed olive cake

18

Dried peat (10% moisture)

18

Fuller earth (from oil refining)

Medium grade

13 to 18

Dried wood, bark, saw dust (10% moisture)

16

Rice husks (10% moisture)

16

Shredder wastes

15

RDF (from domestic refuse, 10% moisture)

15

Cardboard, paper (air dry)

15

Dried sewage sludge (10% moisture)

10

Domestic refuse (30% moisture)

8.5

Pure iron (heat of oxidation!, occurs e.g. in waste tires)

7.5

Low grade

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.3 Combustion Calculations

2.3

Combustion Calculations

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.3 Combustion Calculations / 2.3.1 Exact Calculations

2.3.1

Exact Calculations

The following calculations are based on the combustion equations (see 2.1.1) and some basic relations. To demonstrate the calculation method, the simple example of combustion of pure carbon is given as example. Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.3 Combustion Calculations / 2.3.1 Exact Calculations / 2.3.1.1 Combustion of Pure Carbon (C)

2.3.1.1

Combustion of Pure Carbon (C)

The combustion of 1 kg of pure carbon (C) without any excess air (n = 1) is considered. The combustion equation and the corresponding weights and volumes are as follows:

C = 12 kg/kmol

+

02 = 32 kg/kmol = 22.4 Nm3/kmol


→

C02 = 44 kg/kmol = 22.4 Nm3/kmol

6/23/2001 - 4:35:11 PM

Page 236

"Holderbank" - Cement Course 2000 The above equation refers to 1 kmol or 12 kg of C. The final results are wanted per 1 kg of C i.e. per 1 kg of fuel. ♦ The minimum oxygen is: O2min

=

22.4 Nm3/12 kg

=

1.87 Nm3/kg C

=

8.89 Nm3/kg C

♦ Air contains 21% O2, therefore: Amin

=

1.87 Nm3/kg / 0.21

The combustion products are only CO2 and N2 (coming from Amin): CO2

=

22.4 Nm3/12 kg

=

1.87 Nm3/kg C

N2

=

Amin x 0.79

=

7.02 Nm3/kg C

Total combustion gas

=

Vmin

=

8.89 Nm3/kg C

Calorific value of pure C

=

CV (from handbook)

=

32.8 MJ/kg C

If Amin and Vmin are referred to the CV the following results are obtained: Amin/CV

=

0.271 Nm3/MJ

Vmin/CV

=

0.271 Nm3/MJ

From this simple example it would appear that Amin = Vmin, but in general the Vmin will be a few percent higher than Amin. Nevertheless the example of pure C gives already a fairly representative impression of a typical combustion calculation. Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.3 Combustion Calculations / 2.3.1 Exact Calculations / 2.3.1.2 General Case

2.3.1.2

General Case

In general a fuel is given either by its elemental composition or by its volume composition in case of gases. Therefore two types of formulas are required (2.3.1.2.1 and 2.3.1.2.2) which are based either on weight composition or on volume composition of the fuel. Amin = Minimum air Vmin = Minimum combustion air Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.3 Combustion Calculations / 2.3.1 Exact Calculations / 2.3.1.2 General Case / 2.3.1.2.1 Calculation based on Elemental Analysis / Weight Composition

2.3.1.2.1 Calculation based on Elemental Analysis / Weight Composition C, H, S, N, O, H2O are the weight fractions (kg/kg fuel) of carbon, hydrogen, sulfur, nitrogen, oxygen and water in the fuel. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:11 PM

Page 237

"Holderbank" - Cement Course 2000 Amin = 8.89 x C + 26.5 x H + 3.3 x S - 3.3 x 0

Vmin =

[Nm3/kg fuel]

0.79 x Amin + 0.8 x N + 1.87 x C + 0.7 x S + 11.2 x H + 1.24 x H2O [Nm3/kg fuel]

+ Amin x H2O air

The terms in the Vmin-formula mean: ♦ 0.79 x Amin + 0.8 x N

→

N2



♦ 1.87 x C

→

CO2



♦ 0.7 x S

→

SO2



♦ 11.2 x H + 1.24 x H2O+

→

H2O

•

dry gas

wet

+ Amin x H20 air

Example: Coal:

C

=

71.0% Moisture of air = O

H

=

4.0%

O

=

6.0%

N

=

1.5%

S

=

1.0%

H2O

=

0.5%

Ash

=

16.0% 100.0%

= 7.207 Nm3/kg coal

Amin

=

8.89 x 0.71 + 26.5 x 0.04 + 3.3 x 0.01- 3.3 x 0.06

Vmin

=

0.79 x Amin + 0.8 x 0.015 + 1.87 x 0.71+ 0.7 x 0.01 = 7.494 Nm3/kg coal + 11.2 x 0.04 + 1.24 X 0.005


6/23/2001 - 4:35:12 PM

Page 238


Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.3 Combustion Calculations / 2.3.1 Exact Calculations / 2.3.1.2 General Case / 2.3.1.2.2 Calculation based on Volume (or Mole) Composition (only for gaseous fuels)

2.3.1.2.2 Calculation based on Volume (or Mole) Composition (only for gaseous fuels) The general formula of any compound (also inert gases) in the fuel gas is defined as: Cc Hh Ss Oo Nn

Examples:

CH4

→

c=1

h=4

C2H6

→

c=2

h=6

CO

→

c=1

o=1

H2S

→

h=2

s=1

N2

→

n=2

A fuel gas is a mixture of various compounds, each having a certain volume fraction of vf (Nm3/Nm3). The combustion calculations need a summation of all compounds in the mixture, therefore the Σ (sigma) sign appears in the equations.

Amin =

1 h o  vf ×  c + + s −  ∑ 0.21 4 2 

Vmin =0.79 × Amin

[Nm 3 / Nm 3 fuel ]

h n + ∑ vf ×  + c + s +  + Amin × H 2Oair 2 2

[Nm 3 / Nm 3 fuel ]

The above two formulas apply for any type of gaseous compounds (combustible or even inert) and their mixtures. The terms in the Vmin-formula mean: 0.79 x Amin + Σ vf x (n/2)

→

N2



Σ vf x (c)

→

CO2



Σ vf x (s)

→

SO2




dry gas

6/23/2001 - 4:35:12 PM

Page 239

"Holderbank" - Cement Course → 2000 Σ vf x (s)

SO2



Σ vf x (h/2) + Amin x H2Oair

→

H2O

}

Example:

wet

90%

CH4

→

vf = 0.9

c=1

h=4

4%

C2H6

→

vf = 0.04

c=2

h =6

3%

CO

→

vf = 0.03

c=1

o=1

2%

N2

→

vf = 0.02

n=2

1%

O2

→

vf = 0.01

o=2

1  6  2   1   4 0.9 × 1 +  + 0.04 ×  2 +  + 0.03 × 1 −  + 0.01×  −  =9.262Nm 3 / Nm 3  0.21  4  2   2   4  6 2  4  Vmin =0.79 × Amin + 0.9 × 1 +  + 0.04 ×  2 +  + 0.03 × (1) + 0.02 ×  =10.267Nm 3 / Nm 3 2 2  2   Amin =

Composition of Vmin The composition of Vmin (N2, CO2, SO2, H2O) can be easily calculated by identification of the terms of the formula of Vmin. The best way to show this is on the previous example of coal where a Vmin of 7.494 Nm3/kg coal was obtained.

Comp.

Calculation Term

Nm3/kg coal

% in Vmin

N2

0.79 x 7.207 + 0.8 x 0.015 (Amin)

5.706

76.1

CO2

1.87 x 0.71

1.327

17.7

SO2

0.7 x 0.01

0.007

0.1

H2O

11.2 x 0.04 + 1.24 x 0.005

0.454

6.1

7.494

100%

Total

Note: Due to the absorption of SO2 with cement raw meal the above calculation of SO2 will not produce the true amount of SO2 in the gas of a cement kiln. Fig. 2 shows typical compositions of Vmin for 6 common types of fuels. Fig. 2 Typical Compositions of Vmin (for 6 common fuels)


6/23/2001 - 4:35:13 PM

Page 240


Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 2. COMBUSTION / 2.3 Combustion Calculations / 2.3.2 Approximate Calculations

2.3.2

Approximate Calculations

In the previous paragraph 2.3.1 the exact calculation procedures for Amin and Vmin are shown. In many practical cases it is, however, not necessary to do exact calculations if approximations of sufficient accuracy can be given. Such approximates have been found by investigating a large number of different types of fuels (see HMC report VA 81/4849/D). For all fuels of practical use in the cement industry such as coal, fuel oil, natural gas and most of the alternative fuels the following approximations for Amin and Vmin can be used:

Minimum air:

Amin ≈ 0.26 x CV

Minimum combustion gas (wet):

Vmin ≈ 0.28 x CV

where Amin resp. Vmin = [Nm3] and CV = [MJ] Or expressed in words: Each MJ (Megajoule) of combustion energy requires 0.26 Nm3 minimum air and produces 0.28 Nm3 minimum combustion gas. By using the above approximations, a very quick determination of gas quantities is possible. Note that no analysis of the fuel is required. Furthermore, the above approximations can also be used if the CV of the fuel is not given expressively. For example, if the specific heat consumption of a cement kiln is 3.5 MJ/kg clinker the specific Amin and Vmin per kg clinker can be calculated directly: ♦ Amin = 3.5 x 0.26 = 0.91 Nm3/kg cli ♦ Vmin = 3.5 x 0.28 = 0.98 Nm3/kg cli If the type of fuel is known the actual factors can be taken from the following table in order to obtain © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:14 PM

Page 241

"Holderbank" - Cement Course 2000 somewhat more accurate results.

Type of Fuel

Amin-Factor (Nm3/MJ)

Vmin-Factor (Nm3/MJ)

Anthracite, coke

at 1% moisture

0.26

0.27

Bituminous coal (low to medium volatile)

at 1...2% moisture

0.26

0.28

Lignite (high volatile)

at 10% moisture

0.26

0.29

Wood / Peat

at 10...15% moisture

0.26

0.30

Light oil

0.26

0.29

Heavy oil (bunker oil)

0.26

0.28

Tar

0.26

0.28

Waste tires (rubber)

0.26

0.27

Natural gas (high CH4 content)

0.27

0.30

Natural gas (approx. 24% C2H6)

0.26

0.30

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 3. GAS FROM RAW MATERIAL

3.

GAS FROM RAW MATERIAL

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 3. GAS FROM RAW MATERIAL / 3.1 Gases from Dry Raw Meal

3.1

Gases from Dry Raw Meal

If cement raw meal is heated up, hydrate water and CO2 are released. The true quantity of dry raw meal (not kiln feed) required to produce 1 kg of clinker is called R. Normally R equals to approx. 1.55 kg/kg cli. R must be calculated from the loss on ignition:

R=

1 1 − L.O.I

[kg/kg cli]

↑ loss on ignition of raw meal in [kg/kg meal]

The above formula applies if the kiln dust is completely returned into the kiln. If some dust is discarded (without return to kiln) the factor R increases accordingly. The quantity of hydrate water released from dry raw meal is calculated as follows: © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:14 PM

Page 242


H2Ohyd = R x hydrate content x 1.24

[Nm3/kg cli]

↑ (kg hydrate water/kg raw meal)

The quantity of CO2 released from dry raw meal is calculated as follows: [Nm3/kg cli]

C02 = [R x (1 - hydrate content) - 1] x 0.509

↑ (kg hydrate water /kg raw meal)

Normal dry cement raw meals release approximately the following gas quantities per one kg of clinker:

H2Ohyd

≈

0.01 Nm3/kg cli

CO2

≈

0.27 Nm3/kg cli

Total

≈

0.28 Nm3/kg cli

In most cases it is sufficient to use the above approximation. If dust is discarded the above value must be increased accordingly. Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 3. GAS FROM RAW MATERIAL / 3.2 Water from Wet Raw Meal or from Raw Slurry

3.2

Water from Wet Raw Meal or from Raw Slurry

If w (kg/kg) means the moisture or slurry water content of the kiln feed, the amount of water vapor is calculated as:

w H 2O= × R × 1.24 1− w

[Nm3/kg cli]

In case of a dry kiln the above quantity is usually negligible, but it is essential on a wet kiln. At a slurry water content of e.g. 35% it amounts to 1.03 Nm3/kg clinker. Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS

4.

KILN GAS

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.1 © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:14 PM

Page 243

"Holderbank" - Cement Course 2000 Measurement of Exhaust Gas Composition

4.1

Measurement of Exhaust Gas Composition

Within this section O2, CO2, CO and N2 are considered. Trace gases such as NOx and SO2 and hydrocarbons belong to the scope of emission measurements and are usually in the order of less than 0.1% and are therefore not significant with regard to the main components (O2, CO2, CO, N2). Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.1 Measurement of Exhaust Gas Composition / 4.1.1 Gas Sampling

4.1.1

Gas Sampling

Although gas sampling may appear as one of the easiest things, it is in fact the source of more than 50% of all measuring errors! Sampling can be done either continuously or by extracting a gas sample into a rubber bladder. The aim of sampling is to obtain a representative gas sample from a gas duct into the analyzer. The possible problems and solutions in general are as follows:

Problem

Solution

Gas is not homogeneous within the cross section of the gas duct, because gases do not mix well (sample not representative)

Sample the average of the total cross section (instead of one sampling point). Avoid sampling points where poor upstream mixing seems obvious. Prefer sampling points after fans (mixing effect of fan blades).

False air is entering into gas sampling system(too much O2 in gas sample)

Check tightness of sampling system, especially when sample is extracted from high underpressure. Avoid small sampling tubing which may become clogged by dust (and produce high suction pressure at low sample gas flow). In case of spot sampling with rubber bladder: rinse bladder at least once with sample gas.

O2 may react with steel tube at > 400°C (too little O2 in sample)

Use hot extraction tubes made of sicromal, ceramics or quartz.

CO2 may be absorbed by dust and condensate (too little CO2 in sample)

Use a filter directly at the hot extraction point. Keep sampling system free of dust and condensate(e.g. rubber bladder must be clean inside). To a certain extent reliable sampling for CO2 will always be a problem as long as gas cooling is applied.


6/23/2001 - 4:35:15 PM

Page 244

"Holderbank" - Cement Course 2000 as long as gas cooling is applied. a problem Long storage time in rubber bladder (too little CO2 and too much O2 due to diffusion)

Analyze rubber bladder after 30 min at the latest.

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.1 Measurement of Exhaust Gas Composition / 4.1.2 The Orsat Apparatus

4.1.2

The Orsat Apparatus

An Orsat apparatus is shown in Fig. 3. A gas sample is filled into a burette volume of 100%. Step by step, CO2, O2 and CO are absorbed by contacting the gas sample several times with the absorbing agent. The absorbed gas volume is measured after each step in the burette. Fig. 3 The Orsat Apparatus

It is important to maintain the following, correct sequence of absorption: 1) CO2

→

1) KOH solution

2) O2

1) →

2) pyrogallic acid / KOH

3) CO

1) →

2) Cu2 Cl2 solution

In order to avoid measuring errors the following hints are useful: 1) Check tightness of Orsat apparatus first. 2) Use fresh absorption liquids. 3) While taking a reading always hold liquid level in burette and expansion tank at same height (constant pressure). 4) Check O2 absorption by measuring ambient air (21% O2). ♦ Advantages of the Orsat: •

Measuring principle is very clear and simple

•

The Orsat is available (or at least known) in every cement factory


6/23/2001 - 4:35:15 PM

Page 245

"Holderbank" - Cement Course 2000 ♦ Disadvantages of the Orsat: •

Due to is principle a continuous measurement is impossible

•

Analyzing is time consuming

•

Although the Orsat can measure CO it is impossible to detect small traces of CO ≤ 0.1%. Therefore the CO result is only useful in cases of „bad“ combustion producing about 0.3 to 1% CO.

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.1 Measurement of Exhaust Gas Composition / 4.1.3 Other Gas Analyzers

4.1.3

Other Gas Analyzers

In most cases continuous gas analyzers which are permanently installed are used for process control in a cement kiln (see chapter Exhaust Gas Analysis). Such permanent analyzers measure only at one location and are not flexible enough for an investigation of the complete kiln system (involving a large number of measuring points). More and more portable gas analyzers are available on the market. They can usually measure O2, CO and combustibles continuously. At fairly low costs (less than $4’000) a quite handy and useful equipment can be purchased. For an investigation on a cement kiln this is virtually the ideal tool to perform extensive and quick measurements at various locations in the plant. In the following a few important measuring principles are compared. Frequent Measuring Principles for O2 ♦Paramagnetic effect of O2 •

Either thermomagnetical or magneto-mechanical principle is used.

•

Thermomagnetical principle is cross-sensitive to gases having other heat conductivities than O2 e.g. CO2.

•

Magneto-mechanical principle with cell containing movable „dumb-bell“ is not cross-sensitive to CO2 (see Fig. 4A).

•

Analyzers based on the paramagnetic effect are the most common type of permanent installation in the cement industry.

Fig. 4A Frequent Measuring Principles for Gases Paramagnetic O2 Sensor

♦ Electrochemical voltage effect of hot zirconium oxide (Fig. 4B) © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:15 PM

Page 246


E=

•

Working temperatures above 600°C.

•

Measuring the voltage of an electrochemical cell, according to the equation:

 PO2 RT × ln nF  PO2Re f

  

[V]

(R = Gas constant, T = [K], n = 4, F = Faraday constant, PO2 = partial pressure of oxygen) •

Logarithmic output signal, depending on PO2.

•

Principle is seldom used.

Fig. 4B Frequent Measuring Principles of Gases Zirconiumoxide Cell

♦ „Self consuming“ electromechanical sensors (Fig. 4C) •

Basic design of such a measuring cell is similar to a normal battery. Cell consists of anode, electrolyte and air cathode (see Fig. 4C).

•

O2 enters via diffusion barrier to cathode.

•

At the air-cathode O2 is reduced to OH. Thereby an electrical current is produced which is proportional to the O2 content and is used as output signal.

•

Above reaction causes an irreversible consumption of the anode (lead). Therefore lifetime of the cell is limited.

•

Practical lifetime of such a cell is about 1 year (even if cell is not in use a certain aging occurs).

•

Reliable measurements without major cross-sensitivities.

•

Calibration only with air (zero is self adjusting).

•

Application: Portable low cost analyzers.

•

Similar applications: Other electrochemical sensors have been developed for CO, SO2 and NOx working on similar principles. By special design and compensation the effect of „self-consumption“ could be avoided. These analyzers are widely used for portable applications but not for permanent emission control.

Fig. 4C

Frequent Measuring Principles for Gases -


6/23/2001 - 4:35:16 PM

Page 247

"Holderbank" - Cement Course 2000 Electrochemical Cell

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.1 Measurement of Exhaust Gas Composition / 4.1.4 Reference to Dry or Wet

4.1.4

Reference to Dry or Wet

The most frequent type of gas sampling includes a cooling of the gas to approx. 0°C (or at least to room temperature). By this cooling the water vapor is condensed and the measurement in the gas analyzer refers to dry gas. The reference to dry gas will be considered as „normal case“ in this paper (unless otherwise noted). The „classic analyzer“ - the Orsat apparatus - refers always to dry gas. For this particular analyzer even some residual water content in the gas sample will not affect the final result, the reading means exactly the dry composition (in this case it would even be wrong to make any correction for residual water vapor in the gas sample!). Note that there are exceptional cases e.g. where gas samples are not condensed but introduced directly in a hot zirconium oxide cell. In such a case the result will refer to wet gas. Furthermore sampling systems without gas coolers but with dilution of the hot gas instead (thus avoiding any water condensation) exist, but they are hardly used for O2, CO, CO2. Summarizing, sampling with gas cooling and reference to dry gas is considered as normal, i.e. is used for about 99% of all practical measurements. Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.1 Measurement of Exhaust Gas Composition / 4.1.5 Determination of Water Content

4.1.5

Determination of Water Content

Since all measuring systems described above are not able to measure water, other methods have to be used if water really has to be measured. Important methods are: ♦ Two temperatures method: •

Measuring of wet bulb and dry bulb temperature.

•

If the gas temperature is < 150°C wet and dry bulb temperature can be measured directly in the gas duct. If the gas temperature is > 150°C an extraction of the gas and cooling down to < 150°C is required.

•

Evaluation of gas moisture according to calculation sheet 32599e (see measuring techniques part of cement course).

•

Method is preferred due to its simplicity, however accuracy at high moisture contents may not


6/23/2001 - 4:35:16 PM

Page 248

"Holderbank" - Cement Course 2000 be always sufficient. ♦ Condensation method: •

A gas sample is extracted for a certain time and cooled by ice water, thus water is condensed.

•

The remaining dry gas extracted is measured by a volume counter.

•

By measuring the weight of the condensate the moisture content of the gas can be calculated.

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.2 Immediate Calculations from Gas Composition

4.2

Immediate Calculations from Gas Composition

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.2 Immediate Calculations from Gas Composition / 4.2.1 Dew Point

4.2.1

Dew Point

From the water content the dew point can be calculated. ♦ Vf [Nm3/Nm3w] ♦ Ptot [bar]

is the volume fraction of water vapor in the wet gas

is the total pressure (usually assumed as 1.0133 bar)

By expressing the vapor pressure in form of a numerical equation the following dew point formula was developed for precise calculations:

τ=

336.48 − 179 5.3362 − 17.045 + ln(Vf × Ptot )

[C]

Example: Vf = 0.10 Nm3/Nm3w Ptot = 1.0133 bar

τ=

336.48

5.3362 − 17.045 + ln(Vf × Ptot )

− 179

τ =46.09 o C The corresponding inverse function giving the saturation pressure of water (PW) is as follows:

 3591 113216   [bar] PW =EXP 11.4297 − + 2  t + 179 ( ) t + 179  

t = 0...120°C, max. deviation ± 0.03%. A section of the above function is shown in Fig. 5 which can be used for graphical evaluation of the © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:16 PM

Page 249

"Holderbank" - Cement Course 2000 dew point. Fig. 5 Determination of Dew Point

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.2 Immediate Calculations from Gas Composition / 4.2.2 Air Factor n

4.2.2

Air Factor n

The air factor n can be calculated precisely from the gas composition by considering the N2, according to:

n=

N2eff N2 eff = N2min N2 eff − N2 excess

By using the O2 and N2 content of the gas the following important formula can be derived:

n=

N2 1 = 0.79 O N2 − × O2 1 − 3.76 2 0.21 N2

If CO is present the following, general formula applies:

1 n= O − 0.5CO 1 − 3.76 2 N2 Note that e.g. the CO2 from raw meal will not affect the n although it does affect the gas composition (the ratio O2/N2 is constant when CO2 is added to the gas). Therefore this formula is very useful and applicable for any case. On the other hand O2, CO and N2 are required as inputs which needs a complete Orsat analysis (N2 is the rest composition after absorption of O2, CO and CO2). Example: O2

= 4%

N2 = 68%

(CO = O)

1 n= =1.28 4 1 − 3.76 × 68 © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:17 PM

Page 250

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.2 Immediate Calculations from Gas Composition / 4.2.3 Combustion Efficiency

4.2.3

Combustion Efficiency

Unburned gases are a sign of incomplete combustion. The most important unburned gas is CO, whereas hydrocarbons are usually < 0.1% on a cement kiln and can be neglected with regard to energy losses. If only CO is known the loss due to incomplete combustion is: Q=

(dry gas quantity) x CO x 12.6 ↑[Nm3]

[MJ]

↑[Nm3/Nm3]

Example: On a cement kiln the fuel combustion is 3.3 MJ/kg clinker, the dry exhaust gas quantity is 1.3 Nm3/kg clinker and the CO content = 0.2%. q = 1.3 x 0.002 x 12.6

=

0.033 MJ/kg cli

0.033MJ =0.01=1% Relative loss = 3.3MJ Some gas analyzers have also a combustible sensor based on a catalytic combustion of the sample gas. Such a sensor responses to all types of combustible gases (CO, H2, CH4, C2H6 etc.). Although the main combustible component is not CH4 (methane) but rather CO it is common to express the result as „CH4 equivalent“. If the total of combustibles is given as their „CH4 equivalent“ the loss is:

Q=

(dry gas quantity) x CH4 x 35.8 ↑[Nm3]

[MJ]

↑[Nm3/Nm3]

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.2 Immediate Calculations from Gas Composition / 4.2.4 Gas Density

4.2.4

Gas Density

The gas density at normal conditions can be calculated by a simple mix calculation by using the following densities:

Gas

M [kg/kmol]

Density* [kg/Nm3]

O2

32.0

1.429

CO2

44.0

1.964


6/23/2001 - 4:35:17 PM

Page 251

CO2 "Holderbank" - Cement Course 2000 CO

28.0

1.250

„N2“**

28.15

1.257

H2O

18.0

0.804

*

Density in a gas mixture (considered) as ideal gas

**

Not as pure N2 but as „rest“ in the air (including also Ar and trace gases)

Note:

For the calculation of the true gas density all concentrations (O2, CO2, CO, N2, H2O) are to be referred to wet gas here.

Example: Exhaust gas density of a suspension preheater kiln with coal firing: Gas component

Composition of dry gas [Vol%]

Composition of wet gas [Vol%]

O2 CO2 CO N2

4% 31% 0% 65%

0.9 x 4 = 3.6% 0.9 x 31 = 27.9%

H2O

--

10 %

Total

100%

100 %

Density

=

0.9 x 65 = 58.5%

   

100 - 10 = 90%

0.036 x 1.429 + 0.279 x 1.964 + 0.585 x 1.257 + 0.10 x 0.804

=

1.415 kg/Nm3

(at normal conditions)

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.3 Calculation of Exhaust Gas Quantities

4.3

Calculation of Exhaust Gas Quantities

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.3 Calculation of Exhaust Gas Quantities / 4.3.1 Firing / Hot Gas Generator

4.3.1

Firing / Hot Gas Generator

The following calculation applies for any type of firing where gases are coming only from fuel combustion and excess air (no gases from raw meal). Problem:

The fuel consumption Q = [MJ/h] of the firing is known and the complete exhaust gas


6/23/2001 - 4:35:18 PM

Page 252

"Holderbank" - Cement Course 2000 analysis (CO2, O2, CO, N2) is given. By means of this information the gas flow [Nm3/h] shall be calculated. Solution:

Calculate first the n according to 4.2.2:

1 n= O − 0.5CO 1 − 3.76 2 N2 The gas flow V [Nm3/h] is then: Q [Vmin + (n - 1) Amin] [Nm3/h]

V=

Vmin and Amin are in [Nm3/MJ] and can either be calculated exactly according to 2.3.1 or just simply by introducing the approximations according to 2.3.2, namely: ♦ Vmin ≈ 0.28 Nm3/MJ ♦ Amin ≈ 0.26 Nm3/MJ If the complete gas analysis is not known but only the O2 concentration (dry) a further approximation for the dry amount of Vmin, namely 0.25 Nm3/MJ, has to be introduced which results in the following approximation formula:

 %O2  V =Q × 0.28 + 0.25  21 − %O2   ↑ [MJ/h]

[Nm3/h]

O2 ref. to dry

Note: If the O2 was based on wet gas the above formula would become even more simple, namely:

V ≈Q × 0.28 ×

21 21 − %O2

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.3 Calculation of Exhaust Gas Quantities / 4.3.2 Cement Kiln

4.3.2

Cement Kiln

On a cement kiln it is common practice to work with specific quantities referred to 1 kg of clinker, therefore the fuel consumption is expressed in q [MJ/kg cli] and V means here [Nm3/kg cli]. The general calculation formula is:

V=

q (Vmin + (n -1) x Amin) + (gas from RM) ↑ [MJ/kg cli]


[Nm3/kg cli]

↑ [Nm3/kg cli]

6/23/2001 - 4:35:18 PM

Page 253


n is calculated from the complete gas composition according to 4.2.2. If only O2 concentrations are available the following approximations can be used for dry kilns:

- from combustion

0.28 * q

[Nm3/kg cli]

- from raw meal

0.28

[Nm3/kg cli]

- excess air plus false air

(0.28 + 0.25 × q ) ×

%O2 21 − %O2

%O2 Total = V = 0.28 (q+1) + (0.28+0.25xq) x 21 − %O2

[Nm3/kg cli]

[Nm3/kg cli]

↑ [MJ/kg cli]

In case of wet raw meal or for wet kilns additional water has to be added (see 3.2) to the above result. The above approximation can be used in form of diagram (Fig. 6) Fig. 6 Quick Determination of Kiln Exhaust Gas Quantity

Example: Dry kiln, q = 3.3 MJ/kg cli, O2 = 3.5%

3 .5 Total gas = 0.28 (3.3+1) + (0.28+0.25x3.3) x 21 − 3.5

= 1.425 Nm3/kg cli

Three typical results for cement kilns are shown in Fig. 7. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:19 PM

Page 254

"Holderbank" - Cement Course 2000 Fig. 7 Typical Exhaust Gas Quantities for Three Cement Kiln Systems

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.4 Calculation of the CO2 Content of Cement Kiln Exhaust Gas

4.4

Calculation of the CO2 Content of Cement Kiln Exhaust Gas

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.4 Calculation of the CO2 Content of Cement Kiln Exhaust Gas / 4.4.1 Introduction

4.4.1

Introduction

If no measured CO2 concentration is available it is possible to complete the gas analysis with a calculated CO2 concentration. This can be useful e.g. for calculation of density or specific heat. It is possible to calculate the CO2 content at any point in the exhaust gas system (suspension preheater, cooling tower, raw mill, filter) of a cement kiln if the corresponding O2 concentration is known. The calculation is based on two main facts: 1) The CO2 balance applies (CO2 comes from combustion and from raw meal) 2) Starting from the O2 concentration the corresponding dilution with air can be quantified. In addition a few other preconditions are to be observed here: ♦ Reference is made to dry gas composition. ♦ The assumption is made that all CO2 from raw meal is in the kiln gas (i.e. after complete calcination). ♦ No gas is lost or extracted from the gas stream under consideration (e.g.: an upstream extraction of a bypass gas is excluded here). ♦ For numerical calculations the following approximations are used: •

CO2 from raw meal

≈

•

N2 from combustion at n = 1 N2 = 0.79 x Amin + N2 from fuel = 0.79 x 0.26 + 0.0015 ≈ 0.207 Nm3/MJ (this approximation is very accurate for all types of fuels)


0.27 Nm3/kg cli

6/23/2001 - 4:35:19 PM

Page 255

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.4 Calculation of the CO2 Content of Cement Kiln Exhaust Gas / 4.4.2 Maximum CO2 Content

4.4.2

Maximum CO2 Content

In a first step the influence of dilution (or excess air) is assumed as zero (0% O2). Therefore a theoretical, maximum CO2 content can be calculated from the CO2 balance. The dry gas contains only CO2 and N2 which can be calculated as follows:

CO2

= 0.27 + cf x q

[Nm3/kg cli]

N2

= 0.207 x q

[Nm3/kg cli]

q [MJ/kg cli]

=

cf [Nm3/MJ]

specific heat consumption =

CO2 from fuel, referred to CV

cf depends on the type of fuel and can be calculated according to paragraph 2.3.1.2 (by considering only the CO2 term in the Vmin formula). Typical values for cf are:

Coal

=

0.050 Nm3 CO2/MJ

Fuel oil

=

0.039 Nm3 CO2/MJ

Natural gas

=

0.028 Nm3 CO2/MJ

The concentration CO2max is calculated from the above quantities of CO2 and N2:

CO2 CO2 max = CO2 + N2

[Nm3/Nm3]

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 4. KILN GAS / 4.4 Calculation of the CO2 Content of Cement Kiln Exhaust Gas / 4.4.3 True CO2 Content

4.4.3

True CO2 Content

Due to dilution of the gas with air and due to possible formation of some CO the true CO2 content is:


6/23/2001 - 4:35:19 PM

Page 256


O CO 0.79   CO2 =CO2 max 1 − 2 +  − CO  0.21 2 0.21 

[Nm3/Nm3]

CO2, O2, CO = [Nm3/Nm3] Fig. 8 shows a graphical mode of evaluation of a gas composition based on the formulas given in this paragraph. Fig. 8 Relation Heat Consumption/Exhaust Gas Composition for Cement Kilns (CO2 = 0.27 Nm3/kg cli)

Notes: 1) Although it is theoretically possible to calculate also the specific heat consumption from a given gas composition (backward calculation) this procedure is not recommended. In most of the practical cases this will be quite inaccurate because of the lack of a precise gas composition (CO2 readings are often too low due to systematic sampling error). 2) It is not recommended to introduce calculated CO2 values in the n-formula according to 4.2.2, because „calculation with calculated values“ may finally end in a somewhat doubtful result. The air factor n should therefore only be calculated from a measured gas composition (or alternative calculations based on O2 only should be used). Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 5. FALSE AIR INVESTIGATIONS

5.

FALSE AIR INVESTIGATIONS

Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 5. FALSE AIR INVESTIGATIONS / 5.1 Introduction

5.1

Introduction

Underpressures at various locations in the gas system may vary from 0 to -100 mbar. Leakages of the equipment can therefore cause considerable amounts of false air which increase the gas flow. In addition to the obvious increase of gas flow, false air can also be detrimental for the heat © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:20 PM

Page 257

"Holderbank" - Cement Course 2000 consumption. If the exhaust gas fan (or the mill fan) is running at full capacity any additional false air causes a reduction of the kiln production capacity. Shortage of production may become an essential financial loss for the cement factory. Therefore measures against false air must be taken. The first step is always to locate the major leakage points. For this purpose it is necessary to measure a complete O2 profile of the kiln system. This may e.g. include: ♦ suspension preheater ♦ cooling tower ♦ kiln fan ♦ raw mill ♦ E.P. ♦ mill fan As already mentioned in 4.1.3 such a profile can be measured easily with a portable O2 analyzer. A large number of sampling points and repeated checks can be realized. Proper sampling is essential for such an investigation! During such a measurement the operating conditions of the system should be constant in order to obtain a consistent O2 profile. Proceed quickly from one sampling point to the next and finally re-check all O2 concentrations if they are really stable. Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 5. FALSE AIR INVESTIGATIONS / 5.2 Evaluation

5.2

Evaluation

The measured O2 profile may already give a qualitative impression of the tightness of the gas system. For a quantitative information in form of real gas flows (Nm3/h) the approximation formula from 4.3.2 is used here (because it is based only on O2). If m denotes the clinker production in [kg/h] the complete formula is:

%O2 V = m [0.28 x (q + 1) + (0.25 X q + 0.28) 21 − %O2 ] ↑ [kg/h]

[Nm3/h]

↑ [MJ/kg cli]

V is the gas flow at the measuring point. The amount of false air is easily calculated as the increase of V between two measuring points. Note: V does not include water evaporation e.g. from raw slurry or from a cooling tower. It is also obvious from the formula that V will not be influenced by any water evaporation because O2 is based on dry gas composition. For the purpose of false air calculation it is not required to add these additional water quantities. It © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:20 PM

Page 258

"Holderbank" - Cement Course 2000 would be even wrong to add such quantities to V before calculating the false air as difference between two V-valves. Process Technology / B05 - PT II / C06 - Combustion Engineering / Combustion, Gasflows and Gas Composition / 5. FALSE AIR INVESTIGATIONS / 5.3 Example of an Investigation

5.3

Example of an Investigation

On a dry suspension preheater kiln with cooling tower and roller mill in combined operation the O2 profile was checked by means of 5 sampling points. The clinker production is 100 t/h with a specific consumption of 3400 kJ/kg cli. Fig. 9 shows the evaluation of the results by calculating the V [Nm3/h] for every sampling point. Fig. 9 False Air Investigation on a Kiln System

V equals to the true wet exhaust gas quantity at the sampling points 1 and 2. For the points 3, 4 and 5 the amount of water vapor from cooling tower and the amount of water evaporated in the raw mill is not included in V. The false air flows are simply calculated as the increase of V. The final result gives the amounts of false air inleak (Nm3/h) for each section. These values are needed in order to evaluate the possible potential for optimization. Conclusion: For the example according to Fig. 9 the exhaust gas quantity after preheater is 145’800 Nm3/h which is finally mixed with up to 101’700 Nm3/h or 70% of false air when it reaches the chimney. Therefore clear optimization potentials exist in the area of the cooling tower, the raw mill and the electrofilter.


6/23/2001 - 4:35:20 PM

Page 259

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C07 - Alternative Fuels

C07 - Alternative Fuels


6/23/2001 - 4:35:21 PM

Page 260

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels

Use of Alternative Fuels A. Obrist PT 96/14024/E (Modification 2000) 1. INTRODUCTION 2. TYPES OF ALTERNATIVE FUELS 3. UTILIZATION IN CEMENT KILNS 3.1 List of Applications 3.2 Feedpoints for Alternative Fuels 3.3 Substitution effect and potential capacity loss 3.4 Supply and Inlet Control 4. EMISSIONS IN CONTEXT WITH ALTERNATIVE FUELS 4.1 Introduction 4.2 General Features of Cement Kiln Systems 4.3 Special rules regarding emission behaviour on cement kilns 5. ADVANTAGES / DISADVANTAGES 6. PRACTICAL APPLICATIONS 6.1 Waste Tires 6.2 Domestic Refuse / RDF 6.3 Burning of Contaminated Waste Oil 6.4 Burning pure waste oil 6.5 Burning of Waste Wood at Rekingen 6.6 Mixed examples

SUMMARY The use of alternative fuels (AF) in cement kilns can save costs and contribute to the solution of environmental problems. The paper on hand concentrates on technical and environmental aspects. Rules on how to use alternative fuels and possible impacts are given. Practical examples are attached (flowsheets). Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 1. INTRODUCTION

1.

INTRODUCTION

♦ Burning of alternative fuels (AF) in cement kilns offers unique advantages from an environmental © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:21 PM

Page 261

"Holderbank" - Cement Course 2000 point of view (high temperatures, long retention, no solid residues, no increase of emission, overall reduction of CO2 emission). ♦ Using alternative fuels saves costs. Two main factors contribute to this 1) Thermal substitution rate (there are technical limits) 2) Low or even negative energy price (USD per GJ) for AF’s ♦ Logically only fuels that are significantly cheaper than conventional fuels (USD per GJ) can create cost savings. However, even if AF’s are cheaper all the additional costs involved have to be considered to make it profitable (preparation, additional production costs, maintenance, reduction of OEE, etc.) ♦ Within the Holderbank, Group 52 plants are using significant amounts of AF. The average thermal substitution rate of all 105 plants is 12.3% (1998). The fuel cost substitution rate (which is not the same as thermal substitution rate) is not yet being reported and the difficulty is to get an objective and fair consideration of all additional costs involved. From the basic principle cost substitution rates of over 100% are possible at negative fuel prices, but so far very exceptional. Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 2. TYPES OF ALTERNATIVE FUELS

2.

TYPES OF ALTERNATIVE FUELS

By definition, fuels, which are not traded in the normal fuel market, are considered as "alternative fuels". Petcoke e.g. is not classified as “alternative fuel” and is listed in a separate application list (not treated here). Alternative fuels can be roughly divided into solid and liquid fuels (gaseous is negligible). Whether it is simple or difficult to use an alternative fuel depends much on its physical properties. E.g. it may be very simple to use waste oil which has been purified by the supplier. On the other hand it is impossible to use e.g. raw domestic refuse directly as solid fuel, because it is of poor quality and very inhomogeneous. The only practical way to burn it in a cement kiln is a sophisticated pre-treatment to produce RDF (refuse derived fuel). Table (1) shows a list of alternative fuels in the order of their CV compared to conventional fuels. The calorific value alone does not directly indicate the potential to save costs. E.g. waste tires are as good as coal from the viewpoint of CV but require expensive handling and tend to cause negative impacts on the kiln process, so an adequate compensation must be included in the price (disposal fee). Table 1

Various Alternative and Conventional Fuels, grouped according to their CV (*= conventional fuel)

Material

CV [MJ/kg] net

Pure polyethylene 3)

46

* Light oil

42

* Heavy oil

40

Tar (by-product) © Holderbank Management & Consulting, 2000 Query:

38 6/23/2001 - 4:35:21 PM

Page 262

"Holderbank" - Cement Course 2000 Tar (by-product)

38

Pure rubber (without inert material)

36

* Anthracite

34

Aluminium metal 1)

31

Waste oils, various refinery wastes

30 to 40

* Petcoke

33

Waste tires

28 to 32

* Bituminous coal (low ash)

29

* Bituminous coal (high ash)

24

Liquid mix (CSS from SCORIBEL or SYNFUEL from Safety Kleen)

20 to 30 16 to 20 (MJ/Nm3)

Landfill gas Acid sludge, acid tar (from oil reprocessing)

16 to 22

* Lignite (10% moisture)

16 to 21

Pot liners (from aluminium smelter) PVC

20

3)

19

Palm nut shells (10% moisture)

19

Pressed olive cake

18

Dried wood, bark, saw dust (10% moisture)

16

Rice husks (10% moisture)

16

Car shredder wastes

15

RDF (from domestic refuse, 10% moisture)

15

Animal meal

15

Cardboard, paper (air dry)

15

Impregnated saw dust (25% moisture)

10 to 12


10

Fuller’s earth (from oil purification, LD actual)

10

Domestic refuse (30% moisture)

8.5


7.5

Pure iron 2)

7.5

1)

Al metal may occur e.g. in composite packaging wastes and is oxidised to Al2O3

2)

Fe metal occurs e.g. in waste tires and is oxidised to Fe2O3

3)

Usually not in pure form, but contained in mixed plastics

Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 3. UTILIZATION IN CEMENT KILNS


6/23/2001 - 4:35:21 PM

Page 263


UTILIZATION IN CEMENT KILNS

Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 3. UTILIZATION IN CEMENT KILNS / 3.1 List of Applications

3.1

List of Applications

Practical experience and practical applications are the key items in the field of alternative fuels. It is important to know where practical applications or tests have been realized and obtaining the experience from such cases. At HMC/TPT a database on practical applications or test or projects in context with alternative fuels is used and updated regularly. It includes more than 200 plants inside and outside of the Holderbank Group. A typical printout for the first few examples looks as follows:


6/23/2001 - 4:35:22 PM

Page 264


Fig. 1 „Holderbank“ Alternative Fuels

Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 3. UTILIZATION IN CEMENT KILNS / 3.2 Feedpoints for Alternative Fuels

3.2

Feedpoints for Alternative Fuels

Fig. 2 Feedpoints for AF © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:22 PM

Page 265


Regarding the selection of feedpoints the following comments apply: ♦ Solid fuels of large size tend to produce more combustion problems especially when pushed to high substitution rates. So the practical substitution rates are often below the above optimum figures. Remedy is possible by better preparation (size reduction), if economically feasible. ♦ In exceptional cases solid fuels can be transferred into a combustible gas by means of a process integrated gasifier. The gasifier is then the “ultimate preparation” which allows a comparatively easy burning. Since such solutions are expensive they are reserved to special applications (the example of tire gasification is mentioned in this paper). ♦ The feed point via kiln feed is forbidden because of the emission problems during preheating (VOC, CO). This feed point is reserved for alternative materials with no organics. The only exception would be kiln systems where the kiln feed enters the combustion zone without preheating (one stage precalciner kiln at FC) or kilns with VOC removal system (carbonfilter SG, oxidiser at DU).

Fig. 3 Feed Points for Alternative Fuels to Cement Kilns

Regarding the different kiln systems the following rules apply: A Circulation Phenomena © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:23 PM

Page 266

"Holderbank" - Cement Course 2000 ♦

Kiln systems with cyclon suspension preheater and without bypass are most sensitive to circulating phenomena. If the following criteria are not respected, the AF use will cause severe problems or will fail - Keep total chlorine input below 200 – 300 mg/kg clinker (from all fuels and raw materials). If this limit is exceeded a bypass is required. The cyclone preheater without bypass is not forgiving excessive Cl input, it will just plug. - Keep sulfur cycles under control! Unlike Cl the most critical factor is not the possible sulfur input by AF but the impact of poor AF combustion on sulfur volatilisation. This will promote a high sulfur cycle and sulfur pluggings. Remedies: improve combustion, higher O2 at kiln inlet, enhanced preheater cleaning.

♦

Kiln systems with grate preheater (Lepol) are of similar sensitivity to circulating phenomena as cyclone preheater kilns. Condensation of volatile elements in the nodule bed on the preheater can disturb its permeability and thus the kiln operation.

♦

Long dry kilns or long wet kilns are more forgiving in terms of circulating phenomena as they have no critical narrow cross sections. However, rings and build-ups in the rotary kiln also occur but it takes longer until they grow to a critical size. If the kiln system works with 100% dust reintroduction the sustainable chlorine limit is the same as on a cyclone preheater kiln (200 – 300 mg/kg clinker). The difference to the cyclone preheater kiln is that it is easy to realize a valve for chlorine on a wet kiln if the kiln is equipped with an EP dedusting. This allows to extract a highly enriched fine dust selectively that removes chlorine effectively when being discarded. Like that up to 5000 mg/kg cli chlorine input can be handled. In this case the discarded dust causes an additional disposal problem, because it cannot be added to the cement due to the chlorine limit for cement (0.1% Cl).

♦ Chlorine limit Regarding the chlorine input the following diagram helps to get a quick overview of what can be accepted as total input if then chlorine would come only from AFR (whether it’s a Fuel or a Raw material does not matter here). The following limits apply: Limit A:

Normal SP kiln with completely closed dust loop

Limit B:

SP kiln with some 20% bypass or wet kiln discarding medium dust quantities

Limit C: bypass

Maximum possible for wet kiln discarding high dust or SP kiln with 100%

Example: An AFR with 1% Cl at a relative input of 10 g AFR per kg clinker creates an input of 100 mg Cl/kg cli, which is not critical (assuming no other inputs of Cl).

Fig. 4 Chlorine Input by AFR


6/23/2001 - 4:35:24 PM

Page 267


B Temperature and gas residence line If stable toxic organic compounds in AF’s are an issue the main kiln features for their destruction have to be known. ♦ -

Main firing Flame temperature 1800 – 2000°C Total gas residence time in rotary kiln depends on kiln system as follows short kiln (2 support) approx. 3 sec. normal SP/PC kiln more than 5 sec. long wet or dry kiln more than 10 sec.

So typically a gas residence time of approx. 5 sec. above 1200°C can be expected. ♦

Secondary firing (no PC), with unextended riser duct

♦

Precalciner with tertiary air

1 sec. above 820°C

2-4 sec. above 860°C (in case of hot spot design peaks up to 1200°C)

For optimum combustion and safe destruction of stable organics only the main firing shall be used. Example: waste oil that is contaminated with traces of PCB. Other feedpoints are reserved for less critical AF’s or if they are used for critical substances tests may become necessary. To avoid extensive tests it is often easier to select the main firing. Finally the above temperatures are not valid for start up or upset conditions so critical AF’s should only be used under normal operating conditions. Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 3. UTILIZATION IN CEMENT KILNS / 3.3 Substitution effect and potential capacity loss

3.3

Substitution effect and potential capacity loss

Introduction If low grade fuels are used to substitute high grade conventional fuels (coal, oil, gas) the kiln will react with certain effects that will increase the thermal consumption and decrease the maximum kiln © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:24 PM

Page 268

"Holderbank" - Cement Course 2000 capacity. Both phenomena are related to each other. If the energy costs for AF’s are low or even negative one may think the increase in heat consumption is not that negative because the additional consumption can be covered with low cost AF. This is only half the truth. If maximum production is required at the same time every ton of clinker that cannot be produced means a financial loss. Roughly every one % increase in heat consumption also means one % loss in potential kiln capacity. And if kiln availability is lower due to AF’s the OEE can decrease even further.

Fig. 5 Conventional Fuel versus AF

Factors for increased heat consumption Why can AF’s increase the thermal losses on a cement kiln system and thus create “induced losses”? There is a defined number of reasons that contribute to such effects as follows: 1)

High water content in AF A high water content increases both exhaust gas quantity and exhaust gas temperature. Consequence: increased heat loss in the exhaust gas that needs to be compensated by more fuel.

2)

High ash content in AF A high ash content reduces the amount of kiln feed that passes through the preheating zone and decreases the heat recovery by kiln feed. The exhaust gas temperature increases. Consequence: increased exhaust gas loss that needs to be compensated by more fuel. Note: the same effect happens if cold raw material is added directly into the precalciner.

3)

Reduced combustion properties Certain AF’s have poor combustion properties because of too coarse granulometry. Depending on the control strategy this can mean increased CO losses or increased O2 level to compensate this. Example: whole tires at kiln inlet Consequence: No matter what the strategy is, the final result is always a higher energy loss in the exhaust gas, which has to be compensated by more fuel.

4)

Fluctuating AF feed (at good combustion properties)


6/23/2001 - 4:35:24 PM

Page 269

"Holderbank" - Cement Course 2000 Fluctuations in AF energy input can result from -

Inhomogeneous AF properties (CV)

-

Fluctuations of the dosing rate due to more difficult handling properties

Both items have the same impact as for item 3). What happens if a temporary excess of energy input occurs? Either a CO loss is generated or the O2 has to be set so high that no CO is generated. Consequence: same as in case 3). (The difference to case 3) is that this can happen even with fuels that have good combustion properties.) Example: poorly homogenized liquids or poor performance of dosing system. 5)

Cold air introduction Solid coarse AF usually need a high amount of air for pneumatic injection or an air leakage can occur at a poorly sealed feed chute for AF. This has the same effect as an increase of the primary air on a burner. Consequence: Inleak of additional cold air (due to AF) causes higher fuel consumption.

6)

More circulation phenomena due to AF Circulation phenomena do not only occur because AF can introduce circulating elements but also be a result of poor combustion (local CO formation). Although the evaporation and condensation of circulating elements induces a heat transport from the hot zone to the colder zones, the direct effect on balance heat consumption is not that critical. What really disturbs is the unstable kiln operation, which results in reduced availability and higher average fuel consumption. Consequence: reduced availability due to unstable operation/stoppages and increased average fuel consumption.

The majority of the above factors (1, 2, 4 and 5) can be quantified of predicted and the other factors are based on experience. (One of the services that can be provided by HMC/TPT.) Practical substitution effects (examples): -

Liquids with < 10% H2O and good homogeneity

95 – 100%

-

Very low grade waste as raw domestic refuse

70% or lower

So the substitution effect would be typically between 70 – 100% as long as the applications are approached in a engaged and professional way and no unusual difficulties occur. For low grade AF (high ash, high water, coarse, inhomogeneous) the lower limit of 70 % would be typical.

Conclusion 1.

The potential fuel substitution value of an AF (USD/GJ) cannot be calculated by just using its net CV. A reduced effect of typically 70 – 100% can occur. This is only on basis of immediate additional thermal losses (not yet taking into account other costs that emerge when using AF).

2.

When reporting the true heat consumption of a kiln, we have to accept higher consumption when


6/23/2001 - 4:35:26 PM

Page 270

"Holderbank" - Cement Course 2000 using low grade AF's. Manipulating CV's for AF to get the same consumption (on paper) is physically incorrect and not a good reporting practise. 3.

The potential capacity loss when using AF’s is directly linked to the % increase in heat consumption. Increased heat consumption and possibly also reduced kiln availability can have an important impact on OEE.

Fig. 6 Factors for Increased Heat Consumption

Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 3. UTILIZATION IN CEMENT KILNS / 3.4 Supply and Inlet Control

3.4

Supply and Inlet Control

Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 3. UTILIZATION IN CEMENT KILNS / 3.4 Supply and Inlet Control / 3.4.1 Organisation of supply

3.4.1

Organisation of supply

To get AF to the cement plant can be basically done in two ways: 1)

Get AF directly from a waste source.

2)

Get AF through a specialised company that prepares an adequate waste blend for the cement.

Both ways are being used. The possibility 1) is adequate for certain wastes that can be used with minimum or no pre-treatment before shipped to the cement plant. A standard example would be tires. The possibility 2) is a more professional approach, which involves also better integration in the whole waste market. Specialized companies emerged in the previous years and their number is still growing. Classical examples were -

SCORI

(France)

-

SCORIBEL

(Belgium/Obourg)

-

SYSTECH

(USA/Lafarge)

-

SAFETY KLEEN

(USA/partly active for Holnam)

With the exception of Safety Kleen all these organisations were controlled by the cement industry. The new companies that have emerged within Holderbank recently are mostly orientated on the © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:26 PM

Page 271

"Holderbank" - Cement Course 2000 SCORIBEL/Obourg model. From a technical viewpoint of a cement plant these companies provide the following functions: -

Allocating adequate waste categories to the cement plant

-

Control of properties

-

Preparation/pretreatment (in particular blending/homogenising)

The preparation of waste into a useful cement kiln AF is done externally. This seems to become the preferred approach. The supply companies within Holderbank have a common platform: the VESTA Forum. Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 3. UTILIZATION IN CEMENT KILNS / 3.4 Supply and Inlet Control / 3.4.2 Delivery control

3.4.2

Delivery control

A delivery control at the cement plant is essential. In case of hazardeous wastes this is anyway a must (given by the permit) and this does not need to be explained further. What is less obvious is that even harmless or non-hazardeous AF’s need to be checked when delivered to the cement plant. The problem is that AF’s can be contaminated with undesirable inpurities. Whether this happens intentionally or not, it needs to be excluded. Examples: ♦

Waste oil

♦ The original motor oil would not be critical from its properties but the waste product that is finally delivered to the cement plant may be contaminated, e.g. with

♦

-

Solvents (a small quantity of solvents decreases the flash point drastically and thus the safety can become a problem)

-

PCB (PCB contaminated oils have a high disposal fee and the cement kiln could be abused to get rid of them cheaply, PCB is not detectable by simple test methods)

Waste tires

♦ When accepting waste tires it is mandatory to have a visual delivery control and the receiving area must not allow uncontrolled access for various suppliers. Some plants that believed they could do it without any control finally paid a high price because they were abused as a dumping area for non usable sizes, rims and other materials (for which they had to pay for the disposal). ♦ ♦

Chlorine

♦ It happened from time to time that suppliers came up with new solid waste mixes (RDF) or mixed plastics where they claimed very low chlorine contents. This is not always true, but difficult to disprove. There is in fact a problem that sampling of solid RDF is hardly representative and the chlorine analysis are often lower than the average bulk. So do not believe, but check what you get for chlorine, it could hurt your kiln operation.

Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 3. UTILIZATION IN CEMENT KILNS / 3.4 Supply © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:27 PM

Page 272

"Holderbank" - Cement Course 2000 and Inlet Control / 3.4.3 Check-list for Properties of Waste Fuels

3.4.3

Check-list for Properties of Waste Fuels

Table 2

Checklist for Properties of Waste Fuels

•

Type of waste

Name, trade name origin

•

Physical state: solid liquid gaseous solid/liquid

Size, form, grindability viscosity at ...°C, impurities mixing proportions

•

Density

kg/m3

•

Calorific value (net)

MJ/kg

•

Proximate analysis

Moisture, ash, volatiles, Cfix

•

Ultimate analysis

C, H, O, N, S

•

Halogens

Cl, Br, F

•

Ash composition

CaO, SiO2, Al2O3, Fe2O3, K2O, Na2O, P2O5, etc

•

Heavy metals

Hg, Cd, Tl, Be, As, Co, Cr, Pb, Zu, V, etc.

•

Flashpoint

°C

•

Explosivity

non-explosive

•

Toxicity

toxic /non toxic, safety precautions, warnings

•

Legal restrictions containing transport and storage

•

Storage

Chemical or natural degradation, putrefaction phenomena, segregations, precipitations,

•

Corrosivity

Construction materials required

•

Mixing possibilities

Mixing with oil, water, solvents

•

Quantities to be used

min, max, average (now, in future)

•

Fluctuations in quality

Quality specification

Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 4. EMISSIONS IN CONTEXT WITH ALTERNATIVE FUELS

4.

EMISSIONS IN CONTEXT WITH ALTERNATIVE FUELS


6/23/2001 - 4:35:27 PM

Page 273

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 4. EMISSIONS IN CONTEXT WITH ALTERNATIVE FUELS / 4.1 Introduction

4.1

Introduction

If alternative fuels are used to substitute conventional fuels the cement kiln emissions are often not increased and may even drop.

Fig. 6 Emission influence AF

Emissions do occur but they are hardly caused by alternative fuels. Emission results mainly from the raw material and from the high temperature process (NOx) and the fuels have only a limited influence. E.g. the SO2 emission on a suspension preheater kiln does not depend on the sulfur in the fuel. The difficult part can be how to handle the normal emissions if burning AF’s attracts public interest and implies more stringent emission rules.

Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 4. EMISSIONS IN CONTEXT WITH ALTERNATIVE FUELS / 4.2 General Features of Cement Kiln Systems

4.2

General Features of Cement Kiln Systems

"Barriers" which prevent toxic substances from being emitted or becoming harmful to the environment (see figure 7)

Fig. 7 Environmental Aspects: „Emission Barriers“


6/23/2001 - 4:35:27 PM

Page 274


# 1 High incineration temperature In the sintering zone flame temperatures of some 2000°C are required for process reasons. Even very stable organic compounds (e.g. PCB) are completely destroyed. This argument does not apply for secondary firings.

# 2 Contact with Fine, Dispersed Raw Meal Intensive contact of gas and raw meal is required for process reasons (heat transfer). This produces gas purification through absorption of toxic compounds while contact occurs in counter-current pattern. Excellent retention of acid gases (e.g. HCl, SO2) and also of most of the heavy metals is achieved in SP-preheaters and raw mills. The key is the contact of gas with fine suspended particles. This does not apply for the bypass gas extraction, which must be considered e.g. in an emission estimate.

# 3 Low Final Gas Temperature (favourable equilibrium) Condensation or absorption on surface active raw meal reduces the concentrations of toxic elements according to the physical/chemical equilibrium. This effect strongly depends on the gas temperature. The lower the stack temperature the lower will be the equilibrium concentrations of the vapours of toxic compounds. Examples for very low stack temperatures are: ♦ Kiln gas after passing the raw mill (during combined operation)~ 100°C ♦ It is therefore possible to keep emission levels low, while the gases are passing the raw mill (argument #2 and #3 are equally important). ♦ # 4 Efficient Dedusting Equipment The high absorption capacity of the kiln system avoids emissions but on the other hand can cause enrichments of the filter dust for certain elements that can reach the outer dust cycle (e.g. Tl). An efficient dedusting prevents enriched dust from getting into the atmosphere. Also no EP shut offs are acceptable. Moreover, excessive dust emissions have an over proportional negative psychological impact as all neighbours can see it.


6/23/2001 - 4:35:29 PM

Page 275

"Holderbank" - Cement Course 2000 # 5 Safe Disposal of Trace Elements Trace elements or heavy metals cannot be destroyed nor can they disappear. If they are fed into the cement clinkering process and are not emitted they must have an outlet. Unlike other incineration systems, which produce concentrated and often toxic by-products, a cement kiln with complete dust reintroduction offers the unique possibility to incorporate trace elements in the clinker production in diluted and immobile form. These trace elements occur in concentrations which are usually not different from clinker, which is produced without alternative fuels and they are not leachable (exception: hexavalent Cr during make up with water). If the dust is not completely reintroduced into the kiln or if a bypass is required the above argument - in its simple form - is no longer true.

Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 4. EMISSIONS IN CONTEXT WITH ALTERNATIVE FUELS / 4.3 Special rules regarding emission behaviour on cement kilns

4.3

Special rules regarding emission behaviour on cement kilns

♦ The reality with emission is usually too complex for a safe and accurate prediction. However, from an engineering point of view it is better to have some ideas or rules about the emission behaviour to roughly identify what could be critical or not. ♦ Low emission of Cl, F and Br, < 0.1% of balance input. ♦ Low volatile heavy metals are not critical. Emission usually < 0.1% of balance inputs. ♦ Medium or high volatile heavy metals can reach the outer dust cycle (Cd, Tl) or even escape in form of vaporous compounds (Hg). Whereas Cd and Tl are still below < 1% emission Hg can be emitted almost totally (again this depends on process conditions). ♦ SO2 emissions are not depending on fuel sulfur in case of a SP kiln. Wet kilns however show a moderate influence by S on emissions. ♦ AF burning in the secondary firing usually decreases the NOx (0 – 30%). ♦ CO as discussed previously is often increased when using the secondary firing. Fluctuating energy input may also cause CO peaks. ♦ Virtually no organics result from AF burning (even in cases of CO formation it does not necessarily correlate with organics). The improper use of AF via kiln feed is of course excluded from this consideration. ♦ Dioxine/furan emissions on SP kilns are not critical in view of a limit of 0.1 ng TE/m3. Again, there is no correlation with AF burning.

Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 5. ADVANTAGES / DISADVANTAGES

5.

ADVANTAGES / DISADVANTAGES

Nowadays the destruction of wastes in special incineration plants is being improved e.g. by addition of more effective gas cleaning. Under this aspect the question may arise whether it still makes sense to incinerate wastes in cement kilns instead of in special incinerators. To answer this question the advantages and disadvantages of a cement kiln must be compared as follows: © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:29 PM

Page 276


Advantages

Disadvantages

Possibility of high temperature incineration (up to 2000°C) which destroys toxic organic compound completely The majority of heavy metals can be captured at > 99.9% in the kiln system (absorption by raw meal)

Some limitations have to be considered, e.g. Hg, Tl, Cr

Acid gases are retained efficiently (e.g. no HCl emission)

Because of kiln internal circulation phenomena, cement kilns and especially SP-kilns cannot accept high inputs of chlorine

No solid residues occur because the ash is incorporated in the clinker. No landfill is required

In the case of wet kilns or bypass installations solid residues in the form of dust may need disposal

If the necessary rules are observed there will be no influence on emissions and clinker quality

For psychological reasons some customers may not accept cement which is produced by using „waste“

No necessity for a new incinerator since the cement kiln is already existing High environmental awareness helps to allocate certain wastes to cement plants

Realization of a project may be difficult and time consuming at the level of public discussion and obtaining of permission

Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 6. PRACTICAL APPLICATIONS

6.

PRACTICAL APPLICATIONS

Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 6. PRACTICAL APPLICATIONS / 6.1 Waste Tires

6.1

Waste Tires

Energy potential ♦ Calorific value (depends on quantity of steel included) 28 to 32 MJ/kg

Comparison for an industrialized country (per capita): a) 500 kg cement/cap. a

Energy required to burn clinker at (at 3.6 MJ/kg) = 1800 MJ/a cap.

b) Energy from waste tires at © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:29 PM

Page 277

"Holderbank" - Cement Course 2000 6 kg tires/cap. a (at 30 MJ/kg)

=

Theoretical overall fuel replacement (if tires were fully available for cement industry)

180 MJ/a cap. 10%

Conclusion: Tires are an important energy source and so far the most frequent application of AF. The practical attractivity depends on the disposal fee that is available. Current values (in different areas) range from 0 – 60 USD/t. Typical Composition of Tires

Constituents Rubber

36.0%

Filler (soot, SiO2)

37.0%

ZnO

1.2%

Softeners

3.0%

Sulfur

1.3%

Steel, textiles

18.0%

Rest

3.5%

Total

100%

Chemical analysis C

70%

H

7%

S

1 ...3%

Cl

0.2...0.6%

Fe

15%

ZnO

2%

SiO2 + rest

5%

Cr

97 ppm

Ni

77 ppm

Pb

60 to 760 ppm

Cd

5 to 10 ppm

Tl

0.2 to 0.3 ppm


6/23/2001 - 4:35:29 PM

Page 278


Incineration in Cement Kilns Incineration of waste tires in cement kilns has nowadays become a frequent method. At least 40 cement plants are known to do so. They can usually substitute 10 - 20% of their fuel requirements. From an environmental point of view this method is considered as proven and advantageous (energetical recycling, low emission, no solid residues). It is often well accepted by the authorities. Application of feeding methods: 1) Whole tires This is the most frequent application, based on the secondary firing principle. Originally, this was first developed and used on dry SP-kilns but then also extended to long dry or wet kilns. ♦ a) Kiln inlet of suspension preheater kilns A feed system according to the figure 8 involves an investment of some USD 2 Mio. for a fully automatic installation. ♦

b) Mid kiln introduction device on long dry or wet kilns The principle is similar to a) but the introduction chute is rotating with the kiln shell, i.e. the tire feed is coupled with the kiln revolution. Figure 9 shows the introduction chute of the "Cadence" system as used at the Joliette plant.

2) Shredded tires / TDF TDF = tire derived fuel ♦

Shredded tires allow for a more regular fuel input into the kiln and have a higher density (advantages for transport and storage).

♦

Shredding costs are some 30 - 60 UDS/t. Sometimes this is already paid by disposal Shredding is normally not operated by the cement producer.

♦

The use of shredded tires < 300 mm on suspension preheater kilns as in figure 10 is rare because it would cost less to use complete tires at the kiln inlet.

♦

The use of shredded tires < 50 mm has some applications on long kilns in North America where still many long dry and wet kilns are in operation. The tire chips are injected into the burning zone. Figure 11 shows the example of the Seattle plant with 15% substitution. Shredded tires or TDF < 50 mm are successfully used on precalciners according to figure 12 in the plants Midlothian, Theodore, Ramos Arizpe and Lägerdorf.

3)

Ground tires (< 5 mm): Theoretically, ground tires would be the ideal fuel for any primary firing (without compromise). However, the costs for grinding are usually prohibitive. Ground rubber as granulate is normally too expensive as fuel. Nevertheless, a Group Plant in Germany and HCB have tried this and gone through a learning process.

fees.

4) Pyrolisis/Gazification of tires: The Japanese have realised gasifications for cement kilns and reported more than a decade ago. It was based on a reactor (shaft) with understochiometric air addition at 700°C. The hot gas produced was directly sent to the cement kiln. ♦ 1999 a new gasifier for whole tires (industrial scale) was commissioned by Polysius at Jura Cement in Wildegg (Switzerland). The hot combustible gases are used in the precalciner. Investment for a 3 t/h installation is in the order of 3 Mio. USD. (Fig. 13) © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:30 PM

Page 279

"Holderbank" - Cement Course 2000 ♦ The gasification can potentially help to optimize the use of tires, which still needs to be demonstrated on a long-term basis. The costs are significantly higher than for burning the tires directly. Direct burning - not gasification - should always be the first option to be investigated. ♦ Fig. 8 Whole Tire Handling and Lump Fuel Kiln Feed (HCB-Eclépens)

Fig. 9 “Cadence valve” on Joliette kiln

Fig. 10 Handling of Tyre Chips and Lump Alternative Fuels at Altkirch


6/23/2001 - 4:35:30 PM

Page 280


Burning of TDF ( Tire Derived Fuel) at the Seattle plant

Fig. 12

Generic tire chip feeding system

Fig. 13 Integrated Gasifier (Polysius) for whole tires

Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 6. PRACTICAL APPLICATIONS / 6.2 Domestic Refuse / RDF

6.2

Domestic Refuse / RDF

♦ Example Germany:


6/23/2001 - 4:35:32 PM

Page 281

"Holderbank" - Cement Course 2000 Domestic refuse

Cement consumption

Quantity

400 kg/cap a

450 kg/cap a

Energy content

3.4 GJ/cap a

1.7 GJ/cap a

(heat energy)

♦ The energy content contained in the domestic refuse is twice the energy consumption of the cement! ♦ However, a complete use of the energy from raw domestic refuse in the cement industry is by no means feasible. Reasons: •

poor homogeneity, inadequate size, difficult handling

•

Cl-content of 0.5...1% Cl which can cause clogging problems in the kiln

•

low calorific value (8 to 10 MJ/kg)

•

low density and high transport costs per heat unit

•

competition to existing incinerators

Conclusion: Domestic refuse needs intensive processing in order to eliminate undesired fractions and to obtain a reasonable burnable fraction. Such a fraction may represent 30...50% of the original refuse, the rest needs further disposal. The burnable fraction is called RDF (refuse derived fuel) and offers somewhat better properties, e.g. a CV of 12...16 MJ/kg. Experience: The first application was in the early 80’s at BCI/Westbury, now stopped. The most important application today is the Wittekind plant in Erwitte (Germany) according to figure 14 with 50% substitution and a chlorine bypass. Otherwise very few plants have realized major applications. Fig. 14 Processing of Domestic Refuse and Incineration of RDF in a SP Kiln

Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 6. PRACTICAL APPLICATIONS / 6.3 Burning of Contaminated Waste Oil


6/23/2001 - 4:35:38 PM

Page 282


Burning of Contaminated Waste Oil

The burning of waste oil in cement kilns has a long tradition. In the late 80’s and early 90’s new efforts have been made to investigate the influence of contaminants. Extensive measurement programs have been performed to demonstrate all possible impacts on the environment. Important examples come from Germany and Austria. One of the first plants to publish the results of its measurements in 1988 was Phoenix in Beckum/Germany. They burnt waste oil, which was contaminated with PCB (0 to 1000 ppm). Emissions of dioxins were also measured. This project was 50% sponsored by the German Umweltbundesamt (UBA). The Lägerdorf plant and the Gmunden plant (Austria) have also followed this at an even higher degree of perfection. The design of the original Lägerdorf installation is shown in the figure 15. A large program of measurements has been done and published. This program also includes emission measurements of SO2, NOx, heavy metals, F, chlorinated organics, PCB, Dioxins/Furans. It could be demonstrated that a limit for dioxins of 0.1 ng TE/m3 (toxic equivalent) could be easily met and that these emissions are not influenced by burning contaminated waste oils. A similar result was obtained at Gmunden. Special efforts were also made with regard to the delivery control, which meant a considerable extension to the existing laboratory. Limiting values for the waste oil in Gmunden:

Pb

<

5000 ppm

Hg

<

2 ppm

Tl

<

10 ppm

Cd

<

60 ppm

PCB

<

100 ppm

F

<

600 ppm

S

<

5%

Cl

<

1%

N

<

5%

H2O

<

15%

Sediments

<

5%

CV net

>

25’000 kJ/kg

The original installation Lägerdorf according to figure 15 is designed for low flashpoints (< 21°C), therefore, use of solvents is also possible. Fig. 15 Burning of Waste Oil at Lägerdorf Plant


6/23/2001 - 4:35:40 PM

Page 283


Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 6. PRACTICAL APPLICATIONS / 6.4 Burning pure waste oil

6.4

Burning pure waste oil

The installation from Untervaz according to figure 16 results from a former heavy oil system and is adequate for high quality waste oil with high flashpoint (> 55°C). Fig. 16 Handling of Waste Oil at BCU Untervaz

Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 6. PRACTICAL APPLICATIONS / 6.5 Burning of Waste Wood at Rekingen

6.5

Burning of Waste Wood at Rekingen

This installation to process waste wood to a dry saw dust and burn it in a cement kiln went into operation in 1994. It was stopped in 1997 because the Rekingen plant was closed (market reasons). The final stage of this project would have been 70’000 t/a of processed waste wood or 50% fuel substitution. The treatment of the incoming wood consisted of delivery control, primary crushing, sorting fine grinding and drying. The projected costs were 25 Mio. Swiss Francs and therefore among the highest ever realized for a single project for an AF. During the project phase the fees for waste wood were overestimated. When it came into operation, the actual market prices for waste wood were much lower and the installation could not be amortized.


6/23/2001 - 4:35:40 PM

Page 284

"Holderbank" - Cement Course 2000 Fig. 17 Use of Waste Wood as Fuel at the Rekingen Plant

Process Technology / B05 - PT II / C07 - Alternative Fuels / Use of Alternative Fuels / 6. PRACTICAL APPLICATIONS / 6.6 Mixed examples

6.6

Mixed examples

The following illustrations originate from our reports describing practical AFR applications in our Group plants. The illustrations are self-explaining. Content: -

Liquid AF at Altkirch

-

Liquid AF at Obourg

-

Distillation Residues and Animal Fat at Untervaz

-

Mid kiln firing for bales at Obourg

-

Dried Sewage Sludge and Animal Meal at Untervaz

-

Solid AF (Impregnated saw dust) at Eclépens

-

Tire Chips and Ferrocarbon at Lägerdorf

-

Fly ash at Lägerdorf

Fig. 18 Handling of Liquid Alternative Fuels (CSL) at Altkirch (Solvents, Oil-Emulsions, Pasty Liquids)


6/23/2001 - 4:35:44 PM

Page 285


Fig. 19

Handling of Solvents (CSL: Combustible de Substitution Liquide) at Ciments d’Obourg

Fig. 20 Handling of Destillation Residue and Animal Fat at BCU Untervaz

Fig. 21 Mid Kiln Installation at Ciments d’Obourg


6/23/2001 - 4:35:46 PM

Page 286


Fig. 22 Handling of Dried Sewage Sludge and Animal Meal at Untervaz

Fig. 23 Handling and injecting of solid fuel at “HCB Eclépens (impregnated saw dust, shredded plastic, animal meal)

Fig. 24 Handling of Tire Chips and Ferrocarbon at Lägerdorf


6/23/2001 - 4:35:48 PM

Page 287


Fig. 25 Handling of fly ash at Lägerdorf


6/23/2001 - 4:35:52 PM

Page 288

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels

Preparation of Alternative Fuels A. Flacher PT 00/14667/E 1. INTRODUCTION 2. FROM WASTE TO ALTERNATIVE FUEL 2.1 Waste Properties 2.2 Processing of Waste 3. LIQUID ALTERNATIVE FUEL PREPARATION 3.1 Pollution of Liquid Waste 3.2 Homogenizing 3.3 Safety 4. PREPARATION OF ALTERNATIVE FUEL FROM PASTY WASTE 4.1 High Viscosity Liquid Facility 4.2 Impregnation Facility 5. SOLID ALTERNATIVE FUEL PREPARATION 5.1 Shredding and Granulation of Solid Waste 6. EXAMPLES 6.1 Typical Liquid Waste Handling 6.2 SCORIBEL Impregnated Sawdust Production 6.3 Industrial Plastic Waste Shredding at PLASTREC AG

SUMMARY Alternative fuels used in the cement manufacturing process originate from waste, which often doesn’t have suitable physical properties for the kiln process. This is where alternative fuel preparation is needed. This paper focuses on common waste treatment techniques for alternative fuel production. For a complete understanding of the subject, the lecture of the Cement Course paper “Use of Alternative Fuels“ is recommended.

Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 1. INTRODUCTION

1.

INTRODUCTION

Waste is a residual product of some sort of process. Its physical properties are characterized by this © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:54 PM

Page 289

"Holderbank" - Cement Course 2000 process. Alternative fuel, on the other hand, is a combustible for clinker manufacture. It must have physical properties suited for incineration in a cement kiln. In many cases the properties of waste and alternative fuel do not correspond. This is where alternative fuel preparation is required.

Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 2. FROM WASTE TO ALTERNATIVE FUEL

2.

FROM WASTE TO ALTERNATIVE FUEL

Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 2. FROM WASTE TO ALTERNATIVE FUEL / 2.1 Waste Properties

2.1

Waste Properties

When waste material is considered for alternative fuel use, first of all, its chemical composition (circulating elements, ash composition) must be suited for the kiln process. Then there are other properties and aspects, which need to be looked at: ♦ The viscosity of liquid waste determines the design of the storage and handling installation. ♦ Corresponding to the granulometry of solid waste, a size reduction process might have to be considered and the suitable kiln feeding point is chosen. ♦ Water contained in liquids requires mixing for better homogeneity of the alternative fuel. Water contained in solid waste has an impact on the handling as it might change the flowability of the material. ♦ Foreign bodies and impurities need to be separated in order to reduce wear and the risk of blockages. ♦ Safety is very important when dealing with waste, e.g. toxicity and flashpoint need to be considered. ♦ Additional aspects refer to the waste supply, such as annual tonnage as well as form and schedule of delivery.

Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 2. FROM WASTE TO ALTERNATIVE FUEL / 2.2 Processing of Waste

2.2

Processing of Waste

As for any other process, for the preparation of alternative fuel the most simple and inexpensive but also safe process is suited best. A mechanical waste treatment is therefore chosen in almost any case. Similar to the mechanical processes in cement manufacture the preparation of alternative fuel comprises of the following main processes: ♦ Screening to separate foreign and oversized material which might cause handling or wear problems in fuel preparation and firing installations. ♦ Size reduction to fit the fuel to the foreseen firing point (e.g. preheater, main burner), as well as to allow a proper dosing and to ensure a good combustion. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:54 PM

Page 290

"Holderbank" - Cement Course 2000 ♦ Homogenizing and mixing to feed to the kiln process a fuel of constant quality. Picture 1 gives and overview of the different processes from waste to alternative fuel. Explanation is given in the following chapters.

Picture 1:

Processes from waste to alternative fuel

Thermal waste processing (gasification/pyrolysis) is applied in a few cement plants. It is operated in conjunction with the kiln. Gasification allows separating the combustible fraction of a waste in a gaseous form from the non-combustible part. The investment cost for such installation is comparably high, which makes this process worthwhile only when considerable disposal fees are received.

Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 3. LIQUID ALTERNATIVE FUEL PREPARATION

3.

LIQUID ALTERNATIVE FUEL PREPARATION

In many cement plants, liquid waste has been the first waste material to be accepted and used as alternative fuel because little preparation is necessary and, in certain cases, existing installations may be used. However, there are special aspects that need to be considered when handling liquid waste such as waste oil and solvents.

Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 3. LIQUID ALTERNATIVE FUEL PREPARATION / 3.1 Pollution of Liquid Waste

3.1

Pollution of Liquid Waste

Liquid waste is likely to be polluted with foreign bodies (metal pieces, sand, plastics etc.). A coarse filtering upon reception is therefore necessary. With in-line shredders and grinders remaining particles can be crushed to avoid blockages. Even after a second filter in the kiln firing line, there are still solid particles present in the liquid. The pumping and kiln injection system must therefore be designed accordingly. Picture 2:

Screw pole pump, suited for waste oil containing solid particles


6/23/2001 - 4:35:54 PM

Page 291


Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 3. LIQUID ALTERNATIVE FUEL PREPARATION / 3.2 Homogenizing

3.2

Homogenizing

Due to the presence of solids and often also mixtures of liquids with different densities, a mixing tank is required. A recirculation circuit or mechanical mixer ensures a good blending.

Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 3. LIQUID ALTERNATIVE FUEL PREPARATION / 3.3 Safety

3.3

Safety

The handling of solvents or liquid waste containing solvents represents a safety risk since there is a high potential for explosion. The decisive parameter for the safe design of a liquid installation is the liquid’s flashpoint (the flashpoint is the temperature at which the evaporations of a combustible liquid form an inflammable gas). Pure motor oil has a flashpoint of up to 200°C whereas other liquids, such as solvents, can have one below 0°C. Another safety risk to workers and the environment represent toxic substances. Special protection and safety procedures are required. The guideline for this should always be the safety standards applied in the chemical industry. Picture 3:

Protected worker sampling liquid waste


6/23/2001 - 4:35:55 PM

Page 292


Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 4. PREPARATION OF ALTERNATIVE FUEL FROM PASTY WASTE

4.

PREPARATION OF ALTERNATIVE FUEL FROM PASTY WASTE

In industrial areas there are often considerable amounts of pasty or sludgy waste materials available, such as ♦ resin, paint, varnish ♦ oil sludges from tank cleaning ♦ destillation residues ♦ grease, soap. Many of these materials are hazardous. With regard to the variable clinker production cost, they can therefore be of particular interest as considerable disposal fees may be received. From the point of view of incineration, it is important that such waste is burnt at high temperatures with sufficient residence time (The combustion criteria for destruction of halogenated waste for example is 1‘200 °C with 2 seconds residence time). This is only ensured at the main burner of a cement kiln. Therefore, the hazardous, pasty waste must be pre-treated for incineration in the primary firing. There are two common ways of sludge pre-treatment for incineration in the primary firing, see below. For mainly non-hazardous sludges there is the possibility of kiln inlet feeding. With such application, the sludge is screened to take out coarse foreign bodies before being pumped to the kiln by means of a concrete pump. Sludge feeding at the kiln inlet is limited due to the high water input. Picture 4:

Sludge feeding installation to kiln inlet (Apaxco plant)

Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 4. PREPARATION OF ALTERNATIVE FUEL FROM PASTY WASTE / 4.1 High Viscosity Liquid Facility

4.1

High Viscosity Liquid Facility

A so-called high viscosity liquid facility aims to liquefy the sludgy waste. This is achieved by mixing the sludge with liquid fuel. The important aspect of this process is the compatibility of the substances involved. To avoid chemical reaction, sludge and liquid need to fulfill certain requirements. This © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:55 PM

Page 293

"Holderbank" - Cement Course 2000 however, limits the use of such pre-treatment system to certain substances, which is a drawback with regard to flexibility. Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 4. PREPARATION OF ALTERNATIVE FUEL FROM PASTY WASTE / 4.2 Impregnation Facility

4.2

Impregnation Facility

The other common sludge pre-treatment process aims to prepare a pourable, fine, solid alternative fuel. This is achieved by mixing the sludges with an impregnation support. Saw dust is commonly used for this purpose. It has a good absorbency and the impregnated product shows little stickiness. Alternatively, filter cakes and also animal meal have been used as impregnation aid. In what follows the different steps of an impregnation facility are explained. Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 4. PREPARATION OF ALTERNATIVE FUEL FROM PASTY WASTE / 4.2 Impregnation Facility / 4.2.1 Reception and Preparation of Primary Materials

4.2.1

Reception and Preparation of Primary Materials

The sludgy waste is delivered either in bulk or small containers (typically 200 l steel drums) and emptied into pits. There are drums, which cannot be emptied by gravity or reasonable manual effort. For such containers a robust shredding facility is needed. Picture 5:

Drum emptying

According to chemical analysis of the incoming waste and the product requirements, the unloaded materials are mixed and then shredded. The prepared mix is conveyed to a mixing station by means of a high pressure pump. For an optimum product quality and minimum use of impregnation aid the viscosity of the batch mix is adjusted. If necessary, liquid phases are extracted from the drums or added to the mixing pit. Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 4. PREPARATION OF ALTERNATIVE FUEL FROM PASTY WASTE / 4.2 Impregnation Facility / 4.2.2 Production of Impregnated Alternative Fuel and Dispatch

4.2.2

Production of Impregnated Alternative Fuel and Dispatch

Once there is a batch of sludge prepared it needs to be mixed with sawdust. The mixing ratio is about one third of saw dust and two thirds of sludge. For a simple installation, it is possible to mix the components by means of a wheel loader. The more sophisticated process is to use a continuously operated intensive mixer. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:55 PM

Page 294

"Holderbank" - Cement Course 2000 Legend: 1: mixing container 2: mixing tool 3: material guiding arm 4: discharge opening 5: housing

Picture 6:

Intensive mixer (type Eirich)

Before storage, metals are separated and the product is screened in a drum screen. The product is fine (mostly – 10 mm) and well suited for incineration at the main firing. For storage moving floor type silos are best suited.

Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 4. PREPARATION OF ALTERNATIVE FUEL FROM PASTY WASTE / 4.2 Impregnation Facility / 4.2.3 Organic Emission

4.2.3

Organic Emission

Due to the handling of unconfined chemicals there is a lot of organic emission. This requires an appropriate protection of the workforce on the one hand. On the other hand, an aspiration and thermal treatment of the emission might be necessary. Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 5. SOLID ALTERNATIVE FUEL PREPARATION

5.

SOLID ALTERNATIVE FUEL PREPARATION

Solid waste can be fed in a coarse form to the kiln (e.g. whole tyres) but often needs to have a finer granulometry. A size reduction is necessary and determined mainly by the following process factors: ♦ transportation cost ♦ handling properties ♦ kiln feeding point ♦ thermal substitution rate © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:35:56 PM

Page 295

"Holderbank" - Cement Course 2000 ♦ kiln behavior. The mechanical processes used for the size reduction of solid waste are shredding and granulation. They are typically applied for materials such as wood, rubber, plastics or waste tyres. Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 5. SOLID ALTERNATIVE FUEL PREPARATION / 5.1 Shredding and Granulation of Solid Waste

5.1

Shredding and Granulation of Solid Waste

Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 5. SOLID ALTERNATIVE FUEL PREPARATION / 5.1 Shredding and Granulation of Solid Waste / 5.1.1 Shredders

5.1.1

Shredders

A shredder is a machine for primary size reduction of solid materials. Its operating principle is based on a tearing and shearing action of slowly rotating tools (10 – 40 rpm) agitated by a high torque drive. Picture 7:

Typical shredder design with two counterrotating shafts

Typically, a shredder consists of two counterrotating shafts. On these shafts engaging disks with hook type knives are mounted to grab and tear the material fed. Such shredder is preferably used for the size reduction of high-density waste (e.g. wood and tyres). Waste materials with a low bulk density such as plastic films require a shredder with large feed opening and a mechanism to push the material towards the cutting rotor. For such application single rotor shredders are used. Depending on the knife size and operation (multiple stage, closed circuit) a minimum chip size of 20 – 50 mm can be achieved. Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 5. SOLID ALTERNATIVE FUEL PREPARATION / 5.1 Shredding and Granulation of Solid Waste / 5.1.2 Granulators

5.1.2

Granulators

Granulators (also called grinders) are generally used for secondary size reduction. The typical design consists of a single rotor with a fixed counter knife and an outlet sieve. There is a mechanism to push waste material towards the rotor where knives grab it. After being cut at the fixed counter knife the waste chips are further milled before they pass the outlet sieve. A granulator operates at higher rpm than a shredder, above about 100 rpm. Picture 8: Typical granulator design with one shaft and outlet sieve


6/23/2001 - 4:35:56 PM

Page 296


Due to their high rotor speed, granulators are used for low abrasive material. Waste containing considerable amounts of metals and/or mineral material should not be granulated because of excessive wear. The spacing between the rotor knife and the fixed counter knife needs to be small (about 0.5 mm) to efficiently cut fine material like plastic films.

Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 5. SOLID ALTERNATIVE FUEL PREPARATION / 5.1 Shredding and Granulation of Solid Waste / 5.1.3 Multi-Stage Size Reduction

5.1.3

Multi-Stage Size Reduction

For the firing of waste materials at the preheater/precalciner, a single stage size reduction is sufficient in most cases. If bulky waste material is to be fired at the main burner a multiple stage shredding and granulation is required for the efficient production of an alternative fuel of a few millimeter size. Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 6. EXAMPLES

6.

EXAMPLES

Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 6. EXAMPLES / 6.1 Typical Liquid Waste Handling

6.1

Typical Liquid Waste Handling

A typical handling facility for the reception of liquid waste and the production of liquid alternative fuel is shown in picture 9. The installation consists of the following main elements: ♦ Liquid waste decanting tank with primary filtration ♦ Shredder to crush oversized particles for protection of the pump and to avoid blockages. ♦ Discharge pump, typically of centrifugal type ♦ Liquid storage tank with mechanical mixer ♦ Loading pump, typically of centrifugal type ♦ In case of low flashpoint liquid, a nitrogen inertisation system


6/23/2001 - 4:35:57 PM

Page 297

"Holderbank" - Cement Course 2000 ♦ Sprinkling system for fire suppression Picture 9:

Typical liquid waste handling installation

Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 6. EXAMPLES / 6.2 SCORIBEL Impregnated Sawdust Production

6.2

SCORIBEL Impregnated Sawdust Production

SCORIBEL is a Belgium based subsidiary of Ciments d’Obourg. At the impregnation workshop of its Seneffe plant solid, pasty and liquid waste is mixed with an impregnation aid to form a fine, solid alternative fuel for incineration at the main firing of cement kilns. The installation consists of three main parts: ♦ A reception section for industrial waste in bulk or containers as well as the impregnation aids sawdust and filter cake. In a closed building the liquid, pasty and solid waste is premixed and shredded. ♦ By means of a continuously operated intensive mixer the waste sludge is mixed with the impregnation aid before being screened. ♦ In two moving floor type silos the product is stored and homogenized before being loaded to trucks. For reduction of organic emission air is aspirated at the main sources of emission and sent to a thermal treatment unit. The plant’s design production capacity is 15 t/h, actually it is operated at more than 20 t/h. Investment cost for the impregnation facility (excl. air treatment): BEF 250 Mio.

Picture 10:

flowsheet of SCORIBEL’s impregnation workshop


6/23/2001 - 4:35:59 PM

Page 298


Process Technology / B05 - PT II / C07 - Alternative Fuels / Preparation of Alternative Fuels / 6. EXAMPLES / 6.3 Industrial Plastic Waste Shredding at PLASTREC AG

6.3

Industrial Plastic Waste Shredding at PLASTREC AG

PLASTREC AG, a subsidiary of HCB, operates a facility for the shredding of industrial plastic and rubber waste to a product for use at the primary firing of cement kilns. The waste materials received are cables, textile reinforced plastic, residues from manufacture of hard plastic items, plastic films and rubber. The facility is designed to mechanically process the incoming waste to a product of 10 mm size. This is achieved by pre-shredding and subsequent grinding of the material. Two shredders and two granulators, each operated in parallel, are used for the two-stage process. Material transport is done by belt conveyors for raw and pre-shredded material, and pneumatically for the finished product. The plant has an hourly production rate of 1.5 – 2.0 t. The annual production is about 5‘000 t. The consumption of electrical energy is 100 kWh/t for the whole facility. Investment cost for whole installation: CHF 1.2 Mio.

Picture 11:

flowsheet of PLASTREC plastic shredding plant


6/23/2001 - 4:36:01 PM

Page 299

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C08 - Circulation Phenomena

C08 - Circulation Phenomena


6/23/2001 - 4:36:04 PM

Page 300

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process

Circulation Phenomena in the Clinkerization Process René Hasler, Daniel Brassel PT 99/14503/E 1. INTRODUCTION 2. MECHANISM OF THE CIRCULATION PHENOMENA 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM 3.1 Input of Circulating Elements 3.2 Enrichment of Circulating Elements / Endangered Zones for Encrustation Formation 3.3 Output of Circulating Elements 3.4 Volatility of Circulating Elements 3.5 Condensation of Circulating Elements 4. TYPICAL APPEARANCE OF BUILD-UPS 5. KILN OPERATION PROBLEMS DUE TO CIRCULATING ELEMENTS 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS 6.1 Material Balance 6.2 Criteria and Indicators to Assess the Build-up Problem 6.3 Example of a Circulation Phenomena Problem 7. MEASURES AGAINST BUILD-UP FORMATION 7.1 General Measures 7.2 “Intelligent“ Cleaning 7.3 Measures against Chloride Problems 7.4 Measures against Sulfur Problems 7.5 Measures against Alkali Problems 8. MATHEMATICAL MODEL TO SIMULATE THE CYCLES OF THE CIRCULATING ELEMENTS


6/23/2001 - 4:36:04 PM

Page 301

"Holderbank" - Cement Course 2000 SUMMARY This chapter describes the circulation of volatile elements in the kiln system. It indicates the tolerable inputs of circulating elements so that no excessive build-up and clogging problems arise. In particular it shall serve as guideline how an encrustation problem, caused by volatile elements, is systematically solved. Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 1. INTRODUCTION

1.

INTRODUCTION

Alkali, sulfur and chlorine compounds (hereafter called circulating or volatile elements, see below) in raw materials and fuels utilized for the cement manufacture, when present in high concentrations often given rise to difficulties in kiln operation with build-up formation, mainly in the preheater and the kiln inlet section. Volatile Elements (VE): Sulfur

SO3

Potassium

K2O

Sodium

Na2O

Chlorine

Cl

The build-up formations cause cyclone blockages or restrictions of the kiln inlet area so that the kiln has to be stopped for cleaning. In extreme cases more than 200 annual kiln stops due to blockages may occur, so that the impact on kiln availability and productivity can become a severe issue. Suspension preheaters armed with dozens of shock blowers (Fig. 1a, 1b) to prevent build-up formation illustrate the importance of this problem. Fig. 1a

Fig. 1b


6/23/2001 - 4:36:05 PM

Page 302


Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 2. MECHANISM OF THE CIRCULATION PHENOMENA

2.

MECHANISM OF THE CIRCULATION PHENOMENA

Depending on the degree of volatility, the circulating elements evaporate in the sintering zone of the cement kiln and are carried by the gases to colder zones, where they mainly condense on the raw meal and partly also on the surrounding walls. Afterwards they return with the raw meal into the sintering zone where they partly reevaporate depending on the degree of volatility. This repeated process through the kiln leads to the establishment of internal cycles (see Fig. 5). Finally the cycles reach equilibrium so that the output of circulating elements is equal to their input by the raw materials and the fuels (cp. Fig. 2). Fig. 2 Circulation of Volatile Elements within the Kiln System

Almost all the circulating elements finally leave the system with the clinker. However, this is only the case when beforehand sufficiently high internal cycles of the volatile elements have been formed. The extents of these cycles depend on the degree of volatility of the circulating elements. As the latter recondense on the colder raw meal and the surrounding walls, the formed sticky molten salts are able to reduce the fluidability of the raw meal and, if present in sufficient quantities, to glue it finally on the walls. From time to time, especially during a change of the temperature profile, pieces of build-ups fall down and mainly block the cyclone outlets (Fig. 3). Fig. 3 Build-ups due to Circulating Elements in the Various Kiln Systems © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:05 PM

Page 303


If the amount of molten salts becomes too high, either because of an excessive input of volatile elements or due to a high degree of volatility, the installation of a kiln gas bypass becomes necessary in order to extract part of the circulating elements from the kiln system (Fig. 4a, 4b, 4c). Fig. 4a Conventional Bypass with Water Injection

Fig. 4b Hot Gas Bypass without Water Injection

Fig. 4c Bypass with Gas Feedback


6/23/2001 - 4:36:05 PM

Page 304


A small part of the circulating elements leave the kiln system with the main exhaust gas dust. The latter is efficiently precipitated in the dedusting unit and is normally reintroduced into the kiln system. This is called the external cycle of the circulating elements (Fig. 5). Fig. 5 Circulation Phenomena: Internal and External Cycle

In normal cases a negligible amount of circulating elements is emitted by the stack into the atmosphere. However, the emission of SO2 is not negligible anymore, when the sulfur in the raw material is present in form of sulfide (FeS2, PbS, ZnS) or organic compounds. In this latter case the sulfides are volatilized in the temperature range of 400 to 600°C and leave the kiln system partly as gaseous SO2 emission (Fig. 6 and section 3.3.4). Fig. 6 SO2 Emission in case of Sulfides


6/23/2001 - 4:36:06 PM

Page 305


Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM

3.

CIRCULATING ELEMENTS IN THE KILN SYSTEM

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.1 Input of Circulating Elements

3.1

Input of Circulating Elements

In the following the typical inputs of circulating elements by the raw materials and the fuel(s) are indicated. It is differentiated between input ranges where usually no encrustation and build-up problems arise and input ranges that usually lead to severe clogging problems. Important: All figures may serve as rough guidelines only. The real limits for the build-up and clogging formation depend on a lot of individual parameters like the degree of volatilization, the temperature profile, the completeness of combustion and the excess air factor as well as the kiln system itself. The indicated values are guidelines for suspension preheater kiln only. Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.1 Input of Circulating Elements / 3.1.1 Input by the Raw Materials (loss free basis)

3.1.1

Input by the Raw Materials (loss free basis)

♦ Alkalis (K, Na): Generally appear as interlayer cations in the clay minerals and the feldspars. ♦ Sulfur: The sulfur is introduced in several mineralogical forms: •

as sulfate:

•

as sulfide: pyrite FeS2, organic compounds In the following only the sulfates are considered.

gypsum anhydrate

CaSO4 ⋅ 2H2O CaSO4

♦ Chlorine: The chlorides are mainly introduced as NaCl (from seawater) or KCl. Note: The„kiln feed“ normally includes already external cycle (see Fig. 2) so that its concentration on circulating elements is higher than the one of the pure raw mix. Typical limits for 4 to 5 stage SP kilns are listed in section 6.2.2. Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.1 Input of Circulating Elements / 3.1.2 Input by the Fuel(s)

3.1.2

Input by the Fuel(s)

Mainly sulfur is introduced by the fuels such as Coal, Coke, fuel oil and so on. In Addition alternative fuels often contain a reasonable amount of sulfur. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:06 PM

Page 306

"Holderbank" - Cement Course 2000 Typical limits for 4 to 5 stage SP kilns are shown in section 6.2.2. Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.2 Enrichment of Circulating Elements / Endangered Zones for Encrustation Formation

3.2

Enrichment of Circulating Elements / Endangered Zones for Encrustation Formation

Depending on the individual circulating element and its compounds, the condensing point lies in the temperature range of 650 to 1000°C. Condensation: 650 – 800°C : 800 – 1000°C :

Chlorides and its compounds Sulfates

Therefore the endangered zones for the formation of build-ups by the condensed circulating elements depend on one hand on the circulating element and its compounds and on the other hand on the kiln system itself (see also Fig. 3). In the following the build-up zones for the various kiln systems are indicated. At the colder end usually the encrustations by the chlorides are found whereas at the hotter end the ones of the sulfates are met. Kiln System

Zones of Build-ups

Precalciner kiln / 4 or 5 stage SP kiln

The two lowest cyclones stage and their riser ducts, kiln inlet area (first section of rotary part)

Lepol kiln

Second preheating chamber, kiln inlet (first section of rotary part)

3 stage SP kiln

Lowest cyclone and its riser duct, kiln inlet, first section of rotary part

2 stage SP kiln

Lowest riser duct, kiln inlet, first section of rotary part

Long dry kiln / wet

Calcining zone of rotary part (transition zone)

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.3 Output of Circulating Elements

3.3

Output of Circulating Elements

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.3 Output of Circulating Elements / 3.3.1 Clinker

3.3.1

Clinker

♦ The chlorides are so volatile that they hardly leave the kiln via clinker. In exceptional cases, when the sintering zone has largely cooled down or when embedded in big material lumps, the chlorides may leave the kiln in major quantities. However, normally the chlorides form a large cycle within the kiln system and they need to be extracted by a kiln gas bypass. ♦ The sulfur and the alkalis leave the kiln system normally via clinker either as definite compounds: © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:07 PM

Page 307

"Holderbank" - Cement Course 2000 ♦

K2SO4, K3Na(SO4), Na2SO4 Ca2K2(SO4)3 CaSO4 (rare!)

♦

or in solid solution in clinker minerals: K with the Belite, Aluminate Na with the Aluminate SO3 with the Belite

♦ ♦ The calciumanhydrate CaSO4 is rather volatile (decomposes at temperatures > 1000°C) and forms therefore a large sulfur cycle in the kiln system. Therefore it is very important that there is sufficient alkalis to combine with the sulfur and to leave the kiln system as alkali/sulfur compound. Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.3 Output of Circulating Elements / 3.3.2 Kiln Gas Bypass

3.3.2

Kiln Gas Bypass

Kiln gas bypasses (see Fig. 4) are mainly used to extract the very volatile chlorides from SP preheater kilns. Chloride bypasses withdraw typically 5 to 15% of the kiln gases. The hot gases from the kiln inlet are quenched down by fresh air, sometimes supported by injection of water into the quench chamber, to a temperature below 600°C. The gaseous chlorides condense onto the withdrawn dust particles and are separated finally in an electrostatic precipitator or a bagfilter (see Fig. 5a, 5b, 5c). In rare cases kiln gas bypasses are also applied in case of too much CaSO4 and not sufficient alkalis. Also for the production of low alkali clinker large kiln gas bypasses are used (20 to 50% of the kiln gases) in order to withdraw the alkalis from the kiln charge and to produce a clinker with an alkali content of < 0.6% (i.e. Na2Oeq < 0.6%). Thereby the alkalis are volatilized by hard burning and by injection of chlorides into the kiln (see also section 0). The bypass dust must be discarded e.g. into the quarry or is partly reutilized as additive to the cement, or in rare cases may be sold as filler material. A ballpark figure for the amount of bypass dust: 15 g/kg cli per 10% bypass Due to the extraction of the bypass gases additional heat loss arise. The specific value per % bypass depends on the kiln system: •

Precalciner kilns: 8 – 11 kJ/kg cli

•

Preheater kilns: 20 kJ/kg cli

Typical chemical concentrations of bypass dusts are found in Fig. 7 and 8. Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.3 Output of Circulating Elements / 3.3.3 Kiln Dust

3.3.3

Kiln Dust

Another possibility to withdraw circulating elements from the kiln system is via external cycle (see Fig. 5) which is interrupted and partly discarded. The enrichment of this kiln dust by circulating elements depends on the kiln system. Whereas kiln dusts from long wet kilns are highly enriched, the ones from SP kilns show nearly the same chemical composition than the raw meal. Typical chemical © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:07 PM

Page 308

"Holderbank" - Cement Course 2000 concentrations of dusts from the different kiln systems are shown on Fig. 7 and 8. Thereby the following legend applies: WL

=

long wet kiln

DL

=

long dry kiln

DG

=

dry kiln with grate preheater (LEPOL)

DS

=

SP kilns:

Fig. 7

F

Filter dust from kiln exhaust gas

BP

Bypass dust

Kiln Dust Analyses

Fig. 8 Kiln Dust Analyses

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.3 Output of Circulating Elements / 3.3.4 Emission by Exhaust Gas

3.3.4

Emission by Exhaust Gas

Normally there is no emission of circulating elements by the exhaust gas. The only gaseous component, the SO2 which does not condense at low temperatures is effectively absorbed by the free lime at the lowest cyclone stage. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:08 PM

Page 309

"Holderbank" - Cement Course 2000 However, a source of SO2 emission is the raw material sulfur in form of sulfide (e.g. pyrite). The sulfides decompose at temperatures between 400 to 600°C, forming SO2. Approximately 70% of the SO2 immediately reacts with the CaCO3 of the kiln feed to CaSO3 and finally at higher temperatures to CaSO4. The residual 30% of the SO2 are partly absorbed on their way with the flue gases to the stack, mainly in the raw mill and in the conditioning tower. However, a certain portion of the SO2 is finally emitted by the stack, depending on the kiln and raw mill system (see Fig. 6). Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.4 Volatility of Circulating Elements

3.4

Volatility of Circulating Elements

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.4 Volatility of Circulating Elements / 3.4.1 Definition of Total Volatility

3.4.1

Definition of Total Volatility

The total volatility of a circulating element or a compound of it is defined as shown in Fig. 9. Fig. 9 The Volatility of a Circulating Element

The total volatility of a circulating element or of a compound of it indicates which portion of it is volatilized in the kiln and does not leave the kiln straight with the clinker. The total volatility is:

ϕ = 1−

ccli cHM

whereas ϕ = factor for total volatility cHM = concentration of the circulating element at the kiln inlet ccli = concentration of the circulating element in the clinker Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.4 Volatility of Circulating Elements / 3.4.2 Affinity of the Circulating Elements

3.4.2

Affinity of the Circulating Elements

Circulating elements have a strong affinity to other elements and form chemical compounds. The © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:08 PM

Page 310

"Holderbank" - Cement Course 2000 following affinity order has been observed (see also Fig. 10): 1) The chlorine combines first with the alkalis, forming KCI, NaCl. The residual chlorine, if any, combines with the calcium, forming CaCl2. 2) The residual alkalis combine with the sulfur, forming K2SO4, Na2SO4. They form also double salts Ca2K2(SO4) 2, K3Na(SO4) 2. The residual alkalis, if any, combine with CO, forming K2CO3, Na2CO3. 3) The residual sulfur (SO3 or SO2) combines with CaO, forming CaSO4. Fig. 10

Affinity

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.4 Volatility of Circulating Elements / 3.4.3 Volatility of the Circulating Element Compounds

3.4.3

Volatility of the Circulating Element Compounds

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.4 Volatility of Circulating Elements / 3.4.3 Volatility of the Circulating Element Compounds / 3.4.3.1 General

3.4.3.1

General

♦ In the kiln the alkalis are liberated from the clay mineral lattice. They partly dissociate into the gas phase and recombine with other elements according to the affinity order as described in paragraph 3.4.2. The rest recombines directly in the charge material with chlorine or sulfur or is integrated into the lattice of the clinker minerals (belite and aluminate, see paragraph 3.3.1). Especially the sodium is little volatile and goes preferably direct into the clinker (Fig. 11). Fig. 11 Circulation of Alkalis


6/23/2001 - 4:36:09 PM

Page 311


The chlorides liberated during heating of the material and combustion of the fuel react with the alkalis to form alkali chloride. This reaction takes place either in the charge material or, after vaporization, in the kiln gas (Fig. 12). Fig. 12 Circulation of Chlorine

♦ Upon volatilization of sulfur at burning zone temperatures in the cement kiln, sulfur dioxide is the predominant component. The gaseous SO2 derives either from the dissociated sulfur of the combustibles or the decomposed CaSO4 and the partly volatilized Alk2SO4 from the charge material. Subsequently if there is an excess of alkalis, the internal sulfur cycle is caused primarily by the reaction with these, which takes place almost entirely in the rotary kiln. The alkali sulfates formed in this reaction are, in so far as they are present in vapor form, precipitated on the material. This occurs mainly in the rotary kiln itself, but partly also in the preheater. The precipitated alkali sulfates thus travel through the kiln again, pass through the sintering zone, and are partly discharged from the kiln with the clinker, while some of the alkali sulfates remain in the cycle. The calcium sulfate (CaSO4) is formed as a result of the reaction between SO2 and CaO. It is partly present already in the raw materials or is formed preferably in the temperature range of 800 to 900°C as illustrated in Fig. 13. Above a temperature level of 1000°C the calcium sulfate starts decomposing, first rather slowly and beyond a temperature of 1300°C rapidly. Sulfur dioxide again is formed. If the alkali input is not high enough to combine the whole incoming sulfur as alkali sulfate, high sulfur dioxide concentrations in the kiln gas arise. In such circumstances calcium sulfate may pass un-decomposed through the sintering zone leaving the kiln embedded in the belite mineral or even as © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:09 PM

Page 312

"Holderbank" - Cement Course 2000 CaSO4. The sulfur cycle is illustrated in Fig. 14. Fig. 13 SO2 Absorption by CaO resp. CaCO3

Fig. 14 Circulation of Sulfur

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.4 Volatility of Circulating Elements / 3.4.3 Volatility of the Circulating Element Compounds / 3.4.3.2 Volatility of the Compounds of Circulating Elements

3.4.3.2

Volatility of the Compounds of Circulating Elements

Fig. 15 indicates the relation between vapor pressure of various alkali compounds at different temperatures. This demonstrates that the volatilization mechanism is dependent upon the form of combination of the particular volatile element. Fig. 15 Vapor Pressure


6/23/2001 - 4:36:10 PM

Page 313


Chlorides:

KCl, NaCl, CaCl2

At a temperature level of 1200 to 1300°C the chlorides are already volatilized to a great extent. At sintering zone temperatures they are almost entirely volatilized so that the total volatility factor ϕ is approx. 0.97 to 1. Sulfates:

Alk2SO4, CaSO4

Basically, the Alk2SO4 are little volatile, whereas the CaSO4 is highly volatile. Therefore the most important criteria for the total sulfur volatility is the molar ratio between the alkalis and the sulfur itself, corrected by the chlorine: Molar alkali / sulfur ratio:

K 2O Na2O Cl + − Alk 62 71 = 94 SO3 SO3 80 Alk ≈ 1 .2 SO Desirable value: 3 If there are sufficient alkalis available to combine with the total sulfur input (from raw materials as well as from the fuel), the total volatility for the sulfur is ϕ = 0.3 to 0.5. However, if there is a surplus of sulfur over the alkalis, the highly volatile CaSO4 is formed. Its volatility is approximately ϕ = 0.9, but can increase up to ϕ = 1, depending on the operating conditions of the burning process. In general the sulfur volatility is very much depending on the operating conditions of the burning process, such as ♦ the maximum temperature in the sintering zone ♦ the retention time of the kiln charge material at high sinter zone temperatures ♦ the granulometry of the kiln charge material (diffusion to the surface of the granules) ♦ the partial pressure of O2 in the kiln atmosphere ♦ the partial pressure of SO2 in the kiln atmosphere


6/23/2001 - 4:36:10 PM

Page 314

"Holderbank" - Cement Course 2000 For more details see next paragraph 3.4.4. Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.4 Volatility of Circulating Elements / 3.4.4 Parameters Influencing the Volatility of the Sulfur Compounds

3.4.4

Parameters Influencing the Volatility of the Sulfur Compounds

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.4 Volatility of Circulating Elements / 3.4.4 Parameters Influencing the Volatility of the Sulfur Compounds / 3.4.4.1 Maximum Temperature in the Sintering Zone

3.4.4.1

Maximum Temperature in the Sintering Zone

The essential feature of the volatility - temperature curve is that above a certain critical temperature the volatility increases first exponentially and then slowly reaches a maximum (Fig. 16). Fig. 16 Volatility-Temperature Curve

For sulfur this critical temperature is in the order of the burning temperature. Consequently hard burnable raw mixes or overheating of the kiln charge material (low free limes in the clinker) have a very important bearing on sulfur volatility. Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.4 Volatility of Circulating Elements / 3.4.4 Parameters Influencing the Volatility of the Sulfur Compounds / 3.4.4.2 Retention Time of the kiln Charge Material at High Burning Temperatures

3.4.4.2

Retention Time of the kiln Charge Material at High Burning Temperatures

Volatilization of the sulfur compounds is a dynamic process. The longer the kiln charge material is exposed to the high burning temperatures, the higher the total amount of volatilized sulfur. Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.4 Volatility of Circulating Elements / 3.4.4 Parameters Influencing the Volatility of the Sulfur Compounds / 3.4.4.3 Granulometry of the Kiln Charge Material

3.4.4.3

Granulometry of the Kiln Charge Material

If the kiln charge material is well granulated, the sulfur takes more time to diffuse to the surface of the granule. Consequently less sulfur volatilizes. On the other hand poor granulometry conditions result in higher dust formation which negatively influences the temperature profile in the kiln and shifts it towards the kiln inlet.


6/23/2001 - 4:36:10 PM

Page 315

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.4 Volatility of Circulating Elements / 3.4.4 Parameters Influencing the Volatility of the Sulfur Compounds / 3.4.4.4 Kiln Atmosphere

3.4.4.4

Kiln Atmosphere

The composition of the kiln atmosphere is an essential factor for the sulfur volatility:

Men (SO4 )m ⇔n ⋅ MeO + m ⋅ SO2 +

m O2 2

The equilibrium of the dissociation is shifted with increasing O2 partial pressure and SO2 partial pressure in favor of the sulfates. Too little excess air or also local reducing kiln atmosphere increases the volatility of the sulfur. This behavior is illustrated for the K2SO4:

K 2 SO 4 ↔ K 2 O + SO 2 +½O 2

Ù O2 excess in burning zone:

K 2 SO 4 ← K 2 O + SO 2 + ½O 2 ← Ù O2 deficiency in burning zone: ← K 2 SO 4 → K 2 O + SO 2 + ½O 2

Laboratory tests have been executed for the primary volatility ∈1, i.e. the volatility of the sulfur when exposed the first time to high temperatures. It is remarkable that already at a temperature level of 1000°C, the sulfur volatility goes up to 1 (100% volatility) if there is no oxygen in the kiln atmosphere. This is the case if the combustible produces a local reducing atmosphere in the kiln charge material e.g. when burning whole tires at the kiln inlet. From these tests it can be derived that the main influence of the oxygen content on an increase of the sulfur volatility is produced between 0 to 2% O2. Beyond 2% O2 the influence is negligible (see Fig 17). Fig. 17 Sulfur Volatility depending on Oxygen content

The partial pressure of the sulfur dioxide (SO2) also decreases the sulfur volatility. That means that if the sulfur cycle in the kiln is large enough, the dissociation of the sulfates decreases and the sulfates can gradually leave the kiln with the clinker. However, this is not much of a help, as the required high sulfur cycles already cause encrustation problems! Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM / 3.5 Condensation of Circulating Elements © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:11 PM

Page 316


Condensation of Circulating Elements

As previously stated, the circulating elements volatilize in the hot zones of the kiln and condense at the colder areas of the kiln system (internal cycle). Melting point and boiling point (at 1 bar) of some chlorides, sulfates and carbonates are listed below. Generally speaking it can be stated that the chlorides condense at lower temperatures than the sulfates.

Chlorides

Sulfates

Carbonates

Melting Point [°C]

Boiling Point [°C] at 1 bar

NaCl

801

1413

KCl

776

1500 (sublim.)

CaCl2

772

> 1600

Na2SO4

884

?

K2SO4

1069

1689

CaSO4

1280 (d)

-

Na2CO3

851

(decomp.)

K2CO3

891

(decomp.)

In the melting phase secondary compounds are formed which are more complex than the original volatile elements: Secondary Compounds: Chlorides

Sulfates

Carbonates

Spurrites

KCl

K2SO4

K2CO3,

2 C2S ⋅ CaCO3

(NaCl)

K3Na (SO4)2

(Na2CO3)

2 C2S ⋅ CaSO4

Na2SO4 Ca2K2 (SO4)3 CaSO4

Furthermore, investigations have revealed that calcium sulfate, calcium oxide and alkali sulfate form eutectic melts which can have much lower melting temperatures than the single compounds (Fig. 18). Under presence of chlorides these melting temperatures are even more lowered to temperatures below 700°C. Very low melting temperatures show also the alkali carbonates. However, in such a case the alkalis can be sulfatizated by the addition of gypsum so that it presents normally not a major problem. Fig. 18 Melt Intervals in the System © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:11 PM

Page 317

"Holderbank" - Cement Course 2000 CaSO4 - K2SO4 - Na2SO4

All these melts, when present in sufficient quantities, lead to severe encrustations and build-up formation in the preheater and kiln inlet area. A further problem is given by the fact that the volatilization process goes along with an endothermic reaction whereas the condensation is exothermic. By this way an important amount of heat is drawn from the sintering zone to the kiln inlet so that the area of condensation is shifted even more towards the cold end of the kiln system. Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 4. TYPICAL APPEARANCE OF BUILD-UPS

4.

TYPICAL APPEARANCE OF BUILD-UPS

Typical appearance of build-ups and deposits are shown in Fig. 19, 20 and 21. Fig. 19 Macrographs of Deposit Types

Fig. 20 a) .. d) S.E.M. Micrographs of Deposits Fig. 20 a) REM 84/506 HD - 1st cyclone: KCl blocks embedded in fine matrix


6/23/2001 - 4:36:12 PM

Page 318


Fig. 20 b) REM 84/525 HD - 2nd cyclone: CaSO4 and C2S

Fig. 20 c) REM 84/502 HV - 4th cyclone: Binding nature of glassy KCl matrix

Fig. 20 d) REM 84/550 GM - 4th cyclone: KCl crystal structure filling pore


6/23/2001 - 4:36:12 PM

Page 319


Fig. 21 a) .. d) S.E.M. Micrographs of Deposits Fig. 21 a) REM 84/113 TU - Riser pipe: CaSO4 and C2S

Fig. 21 b) REM 84/117 TU - Riser pipe: K2SO4 and Ca SO4 border

Fig. 21 c) REM 84/72 WU - Kiln inlet: Fly ash balls


6/23/2001 - 4:36:13 PM

Page 320


Fig. 21 d) REM 84/69 WU - Kiln inlet: Primary spurrite

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 5. KILN OPERATION PROBLEMS DUE TO CIRCULATING ELEMENTS

5.

KILN OPERATION PROBLEMS DUE TO CIRCULATING ELEMENTS

The consequences due to high internal cycles of the circulating elements are rather severe. First it starts by an increased encrustation and build-up formation at the kiln inlet area and the lower part of the preheater. As a consequence the pressure loss across the system increases and at the same time also the inbleeding false air quantity increases. This reduces the maximum kiln draft and thus the maximum clinker production. When the kiln operator tries to compensate the lower available kiln draft by a lower excess air rate, the situation becomes even worse as the sulfur cycle further increases. Studies have revealed that a high chloride cycle impedes also complete combustion. This in turn further increases the sulfur cycle. High sulfur cycles lead to a poorly granulated clinker and therefore to dust formation. The dust entrains the heat from the burning zone to the kiln inlet, so that the cycles of the volatile elements further increase due to a longer residence time at high temperatures. Furthermore, the high cycles of volatile elements transport the heat of the sintering zone to the area of © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:13 PM

Page 321

"Holderbank" - Cement Course 2000 condensation by the endothermic - exothermic reactions, causing the same effect as the dust cycles described above. The result of these mechanisms is always the same: ♦

Frequent kiln stops due to encrustation and clogging problems, i.e. reduced kiln utilization factor (Õ reduction of OEE)

♦

Reduction of the maximum kiln production

♦

Higher heat consumption

♦

Formation of unstable coating at the transition zone and thus high refractory consumption

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS

6.

IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS

Chlorides, sulfates, alkalis or any combination of them can cause encrustation and build-up problems. Therefore a detailed analysis is a prerequisite to find an appropriate solution. The analysis can consist of eight steps as follows:

À

Systematic analysis of the hot meal (cp section 6.1) (every shift: LOI, SO3, Cl, K2O, Na2O)

Á

Comparison with the defined limits of circulating elements in the hot meal! Ù Chlorine problem, sulfur problem, alkali problem of combination?

Â

Performing of a material balance with the inputs and outputs of the circulating elements (cp section 6.1). Ù Where do the circulating elements come from?

Ã

Comparison with typical tolerable inputs of circulating elements by raw materials and fuels. Ù Which amount is critical?

Ä

Calculation of the molar alkali / sulfur ratio and comparison with standard. Ù Are there enough alkali present to withdraw the sulfur within the clinker?

Å

Calculation of the sulfur volatility and comparison with the standard. Ù Why sulfur problem?: Õ too high inputs? Õ unfavorable alkali / sulfur ratio? Õ because of kiln operation?

Æ

Detailed recording where the build-ups occur and possible a chemical analysis of a typical build-up peace.

Ç

Specific measures against build-up formation (see section 7).

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS / 6.1 Material Balance

6.1

Material Balance

♦ Definition of the balance boundary (Fig. 22): © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:14 PM

Page 322


Shall the external cycle be included or excluded?

•

Please note that the kiln feed includes the external cycle so that the concentration on volatile elements is higher than the real inputs by the raw material. In this case the kiln dust must be taken into account as further output!

♦ Taking material samples over a sufficiently large period •

The sampling period should be at least 8 hours of representative kiln operation.

•

From every input and output an hourly spot sample shall be taken. The spot samples of the individual inputs and outputs are finally combined to one integrated sample.

•

The following quantities for the individual spot samples shall be taken: ∗ raw mix (before grinding): ∼ 20.0 kg meal or dust: ∼ 0.5 kg clinker: ∼ 5.0 kg fuel: ∼ 0.5 kg

♦ Besides the samples for the material balance also samples of the hot meal at kiln inlet (outlet of the lower most cyclone stage) shall be taken, observing the same frequency and sample quantity as stated above. Fig. 22 Material Balance Boundaries

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS / 6.2 Criteria and Indicators to Assess the Build-up Problem

6.2

Criteria and Indicators to Assess the Build-up Problem

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS / 6.2 Criteria and Indicators to Assess the Build-up Problem / 6.2.1 Process Variables

6.2.1

Process Variables

♦ Position of the build-ups •

If the build-ups are found up to the second lowest cyclone stage or even to the third lowest cyclone stage the problems could origin from excessive chloride cycles or in rare cases from too high alkali cycles.

•

If the build-ups are found mainly at the lowest cyclone stage, the lowest riser dust and the kiln inlet, the problems normally are caused by excessive sulfur cycles.


6/23/2001 - 4:36:14 PM

Page 323

"Holderbank" - Cement Course 2000 ♦ Completeness of combustion •

CO at kiln inlet > 0.1% and/or O2 < 1.5 to 2% cause high sulfur cycles.

•

If no secondary firing or precalcination is applied also the CO and O2 concentrations after preheater can be taken to judge the completeness of combustion (CO must be < 0.1% and O2 usually 3 to 4%, depending on the inbleeding amount of false air).

♦ Temperature profile •

Excessive temperatures within and after the preheater indicate a disturbed temperature profile in the rotary kiln and thus a too high sulfur volatilization.

♦ Pressure profile •

The pressure profile over the preheater indicates where the build-ups are located.

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS / 6.2 Criteria and Indicators to Assess the Build-up Problem / 6.2.2 Material Balance

6.2.2

Material Balance

First a material balance must be performed as described in paragraph 6.1. Afterwards, the following evaluation shall be done: ♦ Input of circulating elements •

It has to be determined in which form the sulfur is present in the raw materials, as sulfate or sulfide. When present as sulfide, part of it will leave the kiln system as emission via exhaust gas stack! Therefore the sulfur emission by the stack has to be deducted from the overall sulfur when judging a potential sulfur problem in the kiln.

•

The inputs of circulating elements (without emitted sulfur through the stack!) should be compared to the ones given below in order to check whether they are in a normal range where usually no build-ups are formed or whether they are too high. This, of course, is only a rough guideline.

Typical Input Limits for Circulating Elements (for 4 to 5 stage SP kiln only) ♦ Raw Material (loss free basis) Cl

SO3 1)

K2O

< 0.02%

Normal case, no problems

> 0.05%

Heavy clogging problems, depending on the sulfur cycle

< 0.5%


> 1.25%

Heavy clogging problems

< 1.0%


> 1.5%

Problems with encrustations, depending on degree of sulfatization (molar alkali/sulfur ratio)


6/23/2001 - 4:36:15 PM

Page 324

"Holderbank" - Cement Course 2000 Na2O

1)

Little volatile, thus no problems due to recirculation of Na2O

Sulfates: Sulfur in form of sulfides behaves in a different way, see section 3.3.4 Emission by Exhaust Gas

♦ Fuel(s) Coal:

S < 1.5%

No clogging problems, (corresponding to depending on sulfur and alkalis input by approx. the raw materials and the completeness < 5 g SO3/kg cli) of combustion

Coke:

S < 2%

Fuel oil:

S < 2.5%

Coal:

S > 3%

Coke:

S > 4%

Fuel oil:

S > 5%

Gas:

Sulfur content is normally zero!

Severe clogging problems, depending on the alkalis and sulfur input by the raw materials and the completeness of combustion

(corresponding to approx. > 10 g SO3/kg cli)

♦ Alkali / sulfur ratio (A/S) •

The A/S ratio should be preferably at 1.2 or in a range between 0.8 to 1.5. Compare it to the A/S ratio of the investigated case, deducting first the emitted sulfur from the total sulfur input by the raw mix and the fuel.

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS / 6.2 Criteria and Indicators to Assess the Build-up Problem / 6.2.3 Enrichment of the Circulating Elements in the Hot Meal

6.2.3

Enrichment of the Circulating Elements in the Hot Meal

♦ Fig. 23 illustrates the tolerable concentrations of circulating elements in the „hot meal“, i.e. in the meal ex lowest cyclone stage. Fig. 23 Concentrations in the Hot Meal © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:15 PM

Page 325


♦ Fig. 24 considers the fact that normally both chlorides and sulfates are present in the hot meal and that both together determine the tolerable limits of concentrations. The actual case shall also be compared with this diagram. Fig. 24 Maximum Tolerable Concentrations in the Hot Meal

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS / 6.2 Criteria and Indicators to Assess the Build-up Problem / 6.2.4 Total Sulfur Volatility

6.2.4

Total Sulfur Volatility

♦ With the formula for the total volatility (see section 3.4.1) the individual total volatilities of the sulfates shall be calculated. •

If ϕ of the sulfur is > 0.7 and if at the same time the A/S ratio is within the range of 0.8 to 1.2, the sulfur cycle is definitely too high and needs improvements by measures as stated in section 7.4.

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS / 6.3 Example of a Circulation Phenomena Problem

6.3

Example of a Circulation Phenomena Problem

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS / 6.3 Example of a Circulation Phenomena Problem / 6.3.1 Material Balance and Hot Meal Analysis

6.3.1

Material Balance and Hot Meal Analysis


6/23/2001 - 4:36:16 PM

Page 326


Material Balance Inputs Raw mix:

SO3

=

0.5% cli

(loss free basis)

K2O

=

0.5% cli

Na2O

=

0.2% cli

Cl

=

0.05% cli

S

=

2%, SO3 = 5%

Hu

=

26’800 kJ/kg, 6400 kcal/kg

q

=

3350 kJ/kg, 800 kcal/kg

SO3

=

0.85%

K2O

=

0.5%

Na2O

=

0.2%

Cl

=

0.03%

SO2

=

400 mg/Nm3 (= 0.1% SO3)

SO3

=

4.5% cli

K2O

=

4% cli

Na2O

=

0.3% cli

Cl

=

2% cli

Coal: Specific heat consumption: Outputs Clinker:

SO2 emission: Hot Meal Analysis (loss free basis)

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS / 6.3 Example of a Circulation Phenomena Problem / 6.3.2 Analysis of the Example

6.3.2

Analysis of the Example

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS / 6.3 Example of a Circulation Phenomena Problem / 6.3.2 Analysis of the Example / 6.3.2.1 Material Balance

6.3.2.1

Material Balance

Material Balance Input

Raw Mix


SO3 [% cli]

K2O [% cli]

Na2O [% cli]

Cl [% cli]

0.5

0.5

0.2

0.05

6/23/2001 - 4:36:22 PM

Page 327

"Holderbank" - Cement 2000 Raw Course Mix Input

Output

0.5

0.5

0.2

0.05

Coal

0.6

Total

1.1

0.5

0.2

0.05

Clinker

0.85

0.5

0.2

0.03

SO2 emission

0.1

Total

0.95

0.5

0.2

0.03

0.15

0.0

0.0

0.02

Balance Error

♦ The total sulfur output is smaller than the total input. This often is the case when performing a material balance. There are two possible explanations: •

The samples are not representative.

•

The process was not in a steady state. Sulfur was kept back in the system, forming encrustations. During cleaning, the sulfur containing deposits left the kiln with the clinker but were not catched representatively by the sampling procedure.

♦ The total chloride output is smaller than the total input. Here the same problems arise as described above. Normally the clinker does not contain more than 0.01% Cl due to the high chloride volatility. However, if the chloride cycle becomes very large, values up to 0.05% Cl in the clinker have already been measured. Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS / 6.3 Example of a Circulation Phenomena Problem / 6.3.2 Analysis of the Example / 6.3.2.2 Form of Sulfur in the Raw Materials

6.3.2.2

Form of Sulfur in the Raw Materials

Note that the raw material contains sulfidic sulfur (FeS2) due to the SO2 emission. The real sulfate input to the kiln is therefore: from raw mix to kiln

0.5 to 0.1 =

coal

0.4% SO3 + 0.6% SO3

total input to the kiln

1.0% SO3

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS / 6.3 Example of a Circulation Phenomena Problem / 6.3.2 Analysis of the Example / 6.3.2.3 Comparison with Typical Inputs of Circulating Elements (see section 6.2.2)

6.3.2.3

Comparison with Typical Inputs of Circulating Elements (see section 6.2.2)

Inputs by the raw materials: SO3 = 0.4% (sulfates only!)

Ù o.k.

K2O and Na2O

Ù o.k.

Cl = 0.05%

Ù very high, bypass required


6/23/2001 - 4:36:22 PM

Page 328

"Holderbank" - Cement Course 2000 Sulfur inputs by the coal: Ù high, but within the normal range

S = 2%

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS / 6.3 Example of a Circulation Phenomena Problem / 6.3.2 Analysis of the Example / 6.3.2.4 Alkali / Sulfur Ratio

6.3.2.4

Alkali / Sulfur Ratio

%K 2O %Na2O %Cl 0.5 0.2 0.05 + − + + 94 62 71 = 0.63 94 62 71 A/S = = *) %SO3 1 .0 80 80 *)

total input to the kiln (see section 6.3.2.2)

þ The A/S ratio is outside of the desirable range of 0.8 to 1.5. Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS / 6.3 Example of a Circulation Phenomena Problem / 6.3.2 Analysis of the Example / 6.3.2.5 Enrichment of Circulating Elements in the Hot Meal

6.3.2.5

Enrichment of Circulating Elements in the Hot Meal

♦ Comparison with tolerable concentrations of circulating elements (see section 6.2.2) SO3 = 4.5%

Ù High, far beyond normal limit, problems with encrustation

Alkalis: 0.66 ⋅ 4 + 0.3 =2.9%

Ù Slightly above normal, but highly desirable due to the large sulfur input!

Cl = 2%

Ù High, far beyond normal limit, problems with encrustations

♦ Comparison with diagram for maximum concentrations (see section 6.2.2) ♦

SO3 = 4.5%

♦ Ù frequent blockages to be expected

♦

Cl = 2%

♦

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS / 6.3 Example of a Circulation Phenomena Problem / 6.3.2 Analysis of the Example / 6.3.2.6 Total Sulfur Volatility (SO3)


6/23/2001 - 4:36:23 PM

Page 329


Total Sulfur Volatility (SO3)

ϕ = 1−

ccli 0.85 = 1− = 0.81 cHM 4 .5

ϕ > 0.7: Ù Because of the low A/S ratio, a substantial portion of the sulfur is in form of CaSO4 which leads to the high volatility of > 0.7. Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 7. MEASURES AGAINST BUILD-UP FORMATION

7.

MEASURES AGAINST BUILD-UP FORMATION

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 7. MEASURES AGAINST BUILD-UP FORMATION / 7.1 General Measures

7.1

General Measures

♦ Reduction of the Inputs of Circulating Elements •

The most obvious measure against build-up formation is to reduce the input of circulating elements. It is normally not possible to change the main raw materials. However, sometimes a minor component that contains a substantial amount of circulating elements can be replaced. In most cases the sulfur content of the fuel is more easy to change. Please note that the fuel may also contain considerable amounts of chlorides.

•

The circulating elements of the raw materials in the quarry deposits are often not homogeneously distributed. In such cases prehomogenization of the raw materials combined with selective quarrying helps to reduce peak inputs of circulating elements into the kiln system.

♦ Smooth Kiln Operation •

In many cases the cycles of circulating elements are frequently increased by an inadequate kiln operation applying to drastic changes in fuel feed and draft.

•

Also the best kiln operator cannot apply during his daily work of 8 h such a super constant kiln operation. Therefore the author is of the strong opinion that only a fully automatic kiln control will provide optimal results (cp. LINKman).

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 7. MEASURES AGAINST BUILD-UP FORMATION / 7.2 “Intelligent“ Cleaning

7.2

“Intelligent“ Cleaning

Important: The more circulating elements are introduced into the kiln system the better and more efficient cleaning methods have to be applied! ♦ Preheaters have to be controlled and cleaned at least once a shift to remove immediately possible build-ups! Therefore a experienced tower guard is needed. ♦ On each platform enough cleaning slots are required to manually clean the riser duct and the inlet chamber whenever it is necessary. At particularly critical spots additional slots should be installed. ♦ Critical locations where always material deposits are built shall be armed with air blasters. Please note that mostly several blasters for one location must be installed since their cleaning radius is rather small (< 0.5 m). The blasters should always shoot in the direction of material flow (see Fig. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:23 PM

Page 330

"Holderbank" - Cement Course 2000 25). Fig. 25 Application of Air Cannons

♦ The riser ducts shall be protected by introducing the raw meal from the upper stage as low as possible (Fig. 26). That way the circulating elements are able to condense at the cooler meal. Fig. 26

Protection of the Riser Duct Walls against Condensation of Circulating Elements

The meal ducts should be equipped with pendulum flaps in order to avoid a hot gas bypass through the ducts into the cyclones (Fig. 27). Fig. 27 Example of a Pendulum Flape for the hot meal duct of the lowest cyclone in a suspension preheater (Polysius)


6/23/2001 - 4:36:24 PM

Page 331

"Holderbank" - Cement Course 2000 ♦ Meal ducts have to have a sufficient inclination (> 55°), a sufficient large diameter and must not have any sharp bends. ♦ The cyclone outlets for the hot meal should be sufficiently large. If frequent bridging of the outflowing raw meal occurs, the outlet section shall be first equipped with air blasters and, if this measure does not help, it must be enlarged. ♦ The installation of coating neutral refractory at the transition chamber and the lowest riser duct has produced good results in many applications and is state of the art. ♦ Any false air entrance at the kiln inlet area and the lower preheater part must be avoided (continuous supervision and maintenance). Otherwise false air is likely to increase the formation of build-ups. ♦ For efficient cleaning special high-pressure water pumps (e.g. type WOMA) shall be applied. The application of this pump is dangerous and needs special training of the operators. There is also a danger of destroying the refractory lining! ♦ Adequate meal distribution boxes shall be installed, which distribute the raw meal over the whole cross section. An example is given in Fig. 28. Fig. 28 Meal Distribution Boxes by Polysius

♦ Dead corners in riser ducts, cyclones and the transition chamber shall be avoided. ♦ Cleaning should only be done if the pressure profile or a quick inspection indicate a need for. Opening of the big access doors disturbs kiln operation and let enter cold air, which favors build-up formation. Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 7. MEASURES AGAINST BUILD-UP FORMATION / 7.3 Measures against Chloride Problems

7.3

Measures against Chloride Problems

Important: With the clinker chlorine can only be withdrawn to a very limited amount (0.01 to maximum 0.02% Cl). Therefore the measures against chlorine build-ups are limited. ♦ Reduction of the Volatility •

There is little chance to decrease the chlorine and chlorine volatility as evaporation takes place at a low temperature range (800 to 1000°C).

♦ Discarding of Dust (external cycle) © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:24 PM

Page 332


For long dry and wet kilns as well as for Lepol kilns the chlorides are effectively withdrawn from the kiln by discarding the finest fraction of the total of the kiln dust.

•

For preheater kiln dust discarding is normally not very efficient as the accumulation of chlorides in the dust is small. However, in case of a large internal chloride cycle this measure could help to keep it at a lower level. The effect of the measure can be calculated by a material balance.

♦ Reduction of the Sulfur Cycle •

Build-up formation is provoked by all circulating elements together. As the chloride cycle cannot be effectively influenced, all possible measures should be taken to reduce the sulfur cycle as it increases the tolerance threshold for chlorine.

♦ Installation of a Kiln Gas Bypass •

If the chloride input of a SP kiln is larger than 0.02 to 0.03% (loss free basis) a chloride bypass must be installed. Whether the higher limit of 0.03% can be tolerated depends on the expected sulfur cycle.

•

A rule of thumb to calculate the quantity of required bypass gases is: •

% Bypass = % Cl input by the raw material (loss free) x 100 •

•

Example: % Cl input by the raw material = 0.05% Õ Required bypass size = 0.05 100 = 5%

The Bypass has to be installed at point of highest concentration in the gas of chlorine and alkalis.

Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 7. MEASURES AGAINST BUILD-UP FORMATION / 7.4 Measures against Sulfur Problems

7.4

Measures against Sulfur Problems

Important: In contrast to chlorine sulfur can be withdrawn in high quantities with the clinker. All measure aim to reduce the volatility in order to introduce the sulfur into the clinker. ♦ Reduction of the sulfur input (raw material and fuel) ♦ Adjusting of the molar alkali / sulfur ratio between 0.8 and 1.2; best to a value beyond 1. K2SO4 is little volatile, CaSO4 is highly volatile and can be withdrawn only in form of double-salts or within the belite. ♦ Keep the lime saturation factor as low as possible so that the sulfur can dissolve in the belite. ♦ Possibly modification of the raw mix in order to improve the burnability and the granulation of the clinker (decrease of the silica ratio). These measures help to reduce the required maximum temperature in the burning zone and to reduce the sulfur volatilization from the clinker granules. ♦ Minimal fluctuations in the chemistry and the quantity of the kiln feed so that constant burning conditions can be maintained. ♦ Avoid overburning! High sulfur content means a porous clinker. In this case not a high litre weight is needed to produce low free lime! For quality control drop litre weight or adjust regularly the rated value for the litre weight to the free lime values. ♦ High sulfur cycles produce a dusty kiln atmosphere. Do not burn down the dust at all costs in order to avoid overheating. ♦

Sufficient excess air at the kiln inlet to avoid reducing conditions (2% O2). Please note


6/23/2001 - 4:36:25 PM

Page 333

"Holderbank" - Cement Course 2000 that the measurement of O2 at kiln inlet is a spot sample and does not necessarily represent the whole inlet cross section. The 2% O2 can therefore be a guide value only. Attention: Too much excess air produces high kiln inlet temperatures, which again increase the sulfur volatility. Use carefully excess air! Do not tolerate any CO at the kiln inlet, i.e. CO < 0.05%. ♦

Introduce automatic kiln control (LINKman) in order to stabalize the kiln operation.

♦

The burner should be directed parallel to the kiln axis and should not point to the kiln charge in order to avoid local reducing burning conditions.

♦

The burner itself should be of the latest design, which produces a short and stable flame in order to have the shortest possible retention time of the kiln charge in the high temperature zone and low kiln inlet temperatures are achieved. As a result the sulfur volatility is minimized.

♦

Apply secondary firing or precalcination to reduce the thermal load in the sintering zone. This possibility is very limited when the kiln inlet temperatures are too high and reducing conditions occur. In this case heavy sulfur build-ups or even build-ups from the first clinker mineralization result.

♦

Very good dispersion of all liquid fuels in order to achieve a short flame (enough viscosity and atomizing pressure).

♦

Solid fuels (also alternative fuels) should be enough prepared. The main fuel at the main burner should have the fineness of coal! Use coarser fractions only limited in the main burner (10 to 20% heat). Coarse particles make the flame longer and so increase the sulfur volatility. Introduce additional coarse fuel fractions separate above the flame and not directly into the flame.

♦

If solid fuels (e.g. used tires) are burnt at the kiln inlet, lifters shall be installed in order to keep the fuel pieces at the surface of the kiln charge and to avoid local reducing burning conditions.

♦

Permanent control of pressure fluctuations in the pneumatic pipes at the burner head. Pulsation < ± 5 mbar! Avoid long transport pipe and keep the velocity > 30 m/s.

♦

In case of extremely high sulfur inputs and a alkali deficiency at the same time kiln gas bypasses are applied. But these are by far not as efficient as for chlorine problems. •

♦

Remark: Especially for long dry and wet kilns discarding of dust is a very effective measure to withdraw the sulfur from the kiln. This does not hold true for SP kilns as the accumulation of sulfur in the external cycle is small. For SP kilns the installation of a kiln gas bypass may become necessary if there are almost no alkalis to extract the sulfur from the kiln.

Shifting of the condensation area of the sulfur towards the kiln inlet and the first zone of the rotary part. Õ This effect is achieved by increasing the A/S ratio and by avoiding high kiln inlet temperatures.

♦ Avoid overburning of the clinker, Fig. 29 demonstrates the strong influence of burning zone temperature (indicated by the concentration of NOx on the accumulation of SO3 and especially CaSO4 in the hot meal. Furthermore also the influence of the excess air is shown. Fig. 29 Enrichment of SO3 in the Hot Meal of the Lowest Cyclone Stage


6/23/2001 - 4:36:25 PM

Page 334


Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 7. MEASURES AGAINST BUILD-UP FORMATION / 7.5 Measures against Alkali Problems

7.5

Measures against Alkali Problems

♦ Alkali Volatility •

In absence of sulfur the alkali volatility is very high and creates problems in the preheater. In such a case the raw mix can be sulfatizated by addition of gypsum.

♦ Low Alkali Clinker •

If low alkali clinker must be produced, all measures must be taken to increase the alkali volatility, such as ∗ reducing the sulfur input ∗ producing a long and stable flame ∗ applying hard burning (CaOfree < 1), if possible reducing the burnability by increasing the silica ration ∗ applying a minimum of excess air ∗ chlorination of the raw mix either by burning chloride ∗ containing solvents or adding CaCl2

♦ In case of long dry or wet kilns the alkalis are withdrawn by discarding a fraction or the total dust of the external cycle. In case of a SP kiln a (large) kiln gas bypass is required. Process Technology / B05 - PT II / C08 - Circulation Phenomena / Circulation Phenomena in the Clinkerization Process / 8. 8. MATHEMATICAL MODEL TO SIMULATE THE CYCLES OF THE CIRCULATING ELEMENTS

8.

8. MATHEMATICAL MODEL TO SIMULATE THE CYCLES OF THE CIRCULATING ELEMENTS

The cycles of the circulating elements in the kiln system can be simulated by a mathematical model (Fig. 30). This model is used for bypass calculations and for simulating special effects on the behavior of the volatile cycles. Fig. 30 Mathematical Model to Simulate the Cycles of the Circulating Elements in the Kiln System (by Weber)


6/23/2001 - 4:36:26 PM

Page 335



6/23/2001 - 4:36:26 PM

Page 336

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C09 - Heat Balance

C09 - Heat Balance


6/23/2001 - 4:36:27 PM

Page 337

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics

Heat Balances of Kilns and Coolers and Related Topics Th. Richner / A. Obrist PT 99/14496/E (update of VA 8/5180/E) 1. INTRODUCTION 2. OVERVIEW OF COMPLETE HEAT BALANCE PROCEDURE 2.1 Why to do a Heat Balance? 2.2 How to Proceed 3. BASIS OF BALANCE AND REMARKS REGARDING EXECUTION 3.1 Balance Limits 3.2 Guidelines for Test Duration 3.3 Kiln Operation 3.4 Kiln Data 4. HEAT BALANCE CALCULATIONS 4.1 General Remarks 4.2 Determination of Clinker Production 4.3 Heat from Fuel 4.4 Burnable Components in Raw Material 4.5 Loss due to Incomplete Combustion 4.6 Heat of Formation 4.7 Heat due to Partly Decarbonized Material 4.8 Heat of Evaporation 4.9 Sensible Heat 4.10

Heat Loss due to Radiation and Convection

5. HEAT BALANCES 5.1 General 5.2 Examples of Heat Balances of Various Kiln Systems 6. SPECIAL PART 6.1 Influence of Reference Temperature 6.2 Heat of Formation 6.3 Radiation Heat Transfer


6/23/2001 - 4:36:27 PM

Page 338

"Holderbank" - Cement Course 2000 6.4 Convective Heat Transfer 6.5 Effect of Thermal Improvements 6.6 Heat Transfer in Preheaters and Coolers and Improvement Potential 7. TEST QUESTIONS 8. LITERATURE 9. SYMBOLS AND UNITS


6/23/2001 - 4:36:27 PM

Page 339

"Holderbank" - Cement Course 2000 SUMMARY A heat balance is an efficient tool to measure the actual state of a kiln system. It allows a better assessment of the heat consumption and reveals the potentials for improvements (regarding the thermal aspects). The principle of a heat balance is rather simple: we select our system limits for the balance and measure all inputs and outputs. After the necessary measuring campaign we need to calculate the various heat items, e.g.: ♦ Heat of fuel combustion ♦ Combustibles in raw meal or exhaust gas ♦ Heat of formation ♦ Heat of evaporation ♦ Sensible heats of all gas and mass flows ♦ Radiation and convection heat The heat balance shows clearly how the heat is spent among the individual items. This information is therefore most suitable to detect abnormal operating conditions or potential for improvements. As a next step we can decide which practical measures should be taken in order to achieve improvements in the thermal energy consumption. When improvements are realized we can often take into account that the saving of fuel is by a factor of 1.3 to 1.5 higher than the primary improvements on the balance item (multiplication factor), as long as we consider the high temperature zone. Considerable improvements can be realized by reducing shell losses, false air inleaks, heat exchange in certain preheater types and in clinker coolers. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 1. INTRODUCTION

1.

INTRODUCTION

Heat balances on a kiln system can offer extremely useful information on the thermal performance of the system. Heat balances show where or how the fuel heat is consumed, based on the simple principle of: input = output Unnecessary energy losses can be easily detected. The principle aim of this chapter is to serve a practical guide for doing heat balances on cement kilns. It can be used as working paper and does neither require special thermodynamic knowledge nor supplementary literature or tables. For the more interested reader some special aspects are treated in a separate chapter. The principle of heat balance may be easily transferred to other systems such as preheaters, coolers and drying systems. Therefore the use of this chapter can be extended to other systems than cement kilns. In this chapter, only SI units are used, which means that heat is always given in kJ (kilo Joule). Conversion calculations within the chapter will no more be required. Incidentally this may also © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:28 PM

Page 340

"Holderbank" - Cement Course 2000 contribute to the consequent use of SI units. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 2. OVERVIEW OF COMPLETE HEAT BALANCE PROCEDURE

2.

OVERVIEW OF COMPLETE HEAT BALANCE PROCEDURE

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 2. OVERVIEW OF COMPLETE HEAT BALANCE PROCEDURE / 2.1 Why to do a Heat Balance?

2.1

Why to do a Heat Balance?

Various reasons or circumstances may cause a need for a heat balance measurement. The following situations may justify a heat balance: ♦ Performance test ♦ Recording of kiln performance before/after a modification ♦ Unusually high heat consumption or abnormal kiln operational data ♦ Kiln optimization campaign It may be self explaining that an extensive heat balance also costs money, especially if a large number of people are involved. Therefore the costs may be put in relation to the obtainable benefits. A potential improvement of say 100 kJ/kg of heat consumption on a 3000 t/d kiln means a savings in the order of US$ 200’000 per year (at current fuel prices). In such a case it may be worthwhile to invest some money for a detailed investigation including a complete heat balance. Although the specific heat consumption proper could also be determined by measuring nothing but fuel heat and clinker production, a complete heat balance does offer considerably more information and security. The consistency of the measured data is proved much better, and the balance shows clearly where the heat is consumed. A heat balance is obviously a very efficient tool for assessment of thermal efficiency. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 2. OVERVIEW OF COMPLETE HEAT BALANCE PROCEDURE / 2.2 How to Proceed

2.2

How to Proceed

A heat balance does not only mean calculation of heat balance items. The complete procedure usually includes the following steps: Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 2. OVERVIEW OF COMPLETE HEAT BALANCE PROCEDURE / 2.2 How to Proceed / 2.2.1 1st Step: Preparation

2.2.1

1st Step: Preparation

The extent of works to be done depends on the completeness and reliability which is desired. A careful planning and preparation is recommendable. The following basic items must be clarified: ♦ What has to be measured (kind and location of measuring / sampling points)? ♦ Duration of test? ♦ Frequency of measurements (continuous recording, spot measurements, time intervals, etc.)? Under above preconditions the number of people required and the necessary measuring equipment can be determined. Temporary equipment may usually become necessary, whereas the existing © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:28 PM

Page 341

"Holderbank" - Cement Course 2000 permanent instrumentation should be carefully checked and calibrated. The following list may be used as checklist for a test preparation: ♦ People available for test period? ♦ Does everybody know what to do at what time? ♦ Necessary logsheets for manual recordings ready? ♦ Data recording system (electronic, pen recorder, etc.) available? ♦ Flow of information among test team? ♦ Calibration or checks of instruments and scales done (flowmeters, orifice plates, venturis, dp-cells, thermocouples, etc.)? ♦ Temporary measuring equipment available? Complete? Correct span? Functionning properly? For example: •

pitot tube

•

U-tube manometer / electronic manometer

•

mobile thermometer

•

radiation pyrometer

•

gas analyzer / Orsat

•

sampling equipment for gas

♦ Sampling procedure for solids (e.g. meal, dust, clinker) clarified? ♦ Analyzing facilities? Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 2. OVERVIEW OF COMPLETE HEAT BALANCE PROCEDURE / 2.2 How to Proceed / 2.2.2 2nd Step: Execution

2.2.2

2nd Step: Execution

An important precondition for a good test is a steady kiln operation. The test should only be started if the system has reached a constant equilibrium state. During the test, variations of operating parameters should be avoided. For the measuring techniques reference is made to the corresponding chapter. It is recommended to check completeness and reliability of measurements already during the test, afterwards missing or uncertain information may create problems at the final evaluation. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 2. OVERVIEW OF COMPLETE HEAT BALANCE PROCEDURE / 2.2 How to Proceed / 2.2.3 3rd Step: Evaluation

2.2.3

3rd Step: Evaluation

This step is the main scope of this chapter. Evaluation of data means to establish a heat balance calculation according to the principle “input = output“. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 2. OVERVIEW OF COMPLETE HEAT BALANCE PROCEDURE / 2.2 How to Proceed / 2.2.4 4th Step: Discussion

2.2.4

4th Step: Discussion


6/23/2001 - 4:36:28 PM

Page 342

"Holderbank" - Cement Course 2000 A heat balance as such must bring some practical conclusions otherwise it would be only of academical interest. The following items may be considered: ♦ Acceptable (normal) heat consumption? ♦ Are heat balance items normal for given kiln systems? ♦ Measures in order to improve heat economy of the system? The last item can become quite an extensive work and the economical feasibility must be considered as well. Such subjects, however, are beyond the scope of this chapter.

Note: Summarizing, it is obvious that the 3rd step „Evaluation“ is only a limited part in the whole context. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 3. BASIS OF BALANCE AND REMARKS REGARDING EXECUTION

3.

BASIS OF BALANCE AND REMARKS REGARDING EXECUTION

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 3. BASIS OF BALANCE AND REMARKS REGARDING EXECUTION / 3.1 Balance Limits

3.1

Balance Limits

In many balance reports the boundary for the balance is not shown expressively. As long as „standard cases“ are considered it may sometimes seem evident how the balance limits were selected. Nevertheless it is advantageous to indicate always clearly where the boundary for the balance is. This will avoid misunderstandings and reduce error possibilities. For that purpose a (simplified) flowsheet is required. In this scheme the boundary must be marked by a line which envelopes the system under consideration. When a measuring campaign in a plant is planned such definitions should be made already in the planning phase, i.e before the test. Fig. 1 Examples of Balance Boundaries

The boundary generates various cutpoints with ingoing or outgoing solid- and gas streams. Likewise radiation and convection heat crosses the boundary. Each cutpoint means a certain item in the heat balance because it represents a heat flow either into or © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:29 PM

Page 343

"Holderbank" - Cement Course 2000 out of the system. By this measure the system is clearly defined and nothing can be forgotten. Basically, any shape of the boundary could theoretically be chosen. To give an extreme example: The boundary could even cut a rotary kiln at half length! However, the boundary must be selected according to practical considerations. This means that the cutpoints which are generated must be: ♦ easily accessible for reliable measurements ♦ of practical interest in the whole context. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 3. BASIS OF BALANCE AND REMARKS REGARDING EXECUTION / 3.2 Guidelines for Test Duration

3.2

Guidelines for Test Duration

A long test duration would allow for a good accuracy but the available time is limited by practical considerations. As a rough guideline the minimum test duration should be about ten times the material retention time in the complete kiln system: ♦ test duration = 10 x retention time Other factors like regularity of kiln operation also influence the test duration. If the process is very unsteady longer times should be envisaged. On a normal suspension preheater kiln the retention time of the material is in the order of one hour. In contrast, on a lime shaft kiln retention times above 24 hours may occur. Therefore the necessary test durations for these two cases must be completely different. Although test durations must be set individually the following list may be used as rough guideline:

Type of Kiln

Test Duration (hours) for Heat Balance

Suspension preheater kiln with precalciner

12

Suspension preheater kiln without precalciner

12 to 24

Long dry / wet kiln

24

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 3. BASIS OF BALANCE AND REMARKS REGARDING EXECUTION / 3.3 Kiln Operation

3.3

Kiln Operation

During the test, the kiln must run at constant and steady conditions. Changing of setpoints should be avoided whenever possible. Interruptions have to be logged. If serious problems occur, the test has to be extended or even postponed. Therefore it is often worthwhile to plan a certain time reserve. From a theoretical point of view a proper balance can only be made if the system runs at steady conditions. E.g. during heating-up heat is stored in the system and there is no balance between input © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:29 PM

Page 344

"Holderbank" - Cement Course 2000 and output (input > output). Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 3. BASIS OF BALANCE AND REMARKS REGARDING EXECUTION / 3.4 Kiln Data

3.4

Kiln Data

In order to facilitate the final discussion it is usually necessary to collect the main data of the system such as: a) System ∗ process ∗ type of kiln ∗ nominal capacity ∗ type of preheater / precalciner ∗ type of cooler ∗ supplier ∗ year of commissioning ∗ fuel and firing system ∗ type of burner nozzle ∗ dust reintroduction system ∗ dimensions of main equipment (sizes, inclinations, etc.) ∗ data on fans, derives, etc. b) Operation ∗ various operating data (rpm, kW, temperature and pressure profiles along kiln system, grate speed, undergrate pressures, etc.) ∗ electric power readings (before / after test) ∗ chemical analysis of raw meal, dust(s) and clinker, LSF, SR, AR, etc. Above data are not necessarily required for heat balance calculations proper, but they should be included in a complete balance report in order to describe the system and to give more information on its performance. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS

4.

HEAT BALANCE CALCULATIONS

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.1 General Remarks

4.1

General Remarks

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.1 General Remarks / 4.1.1 Symbols and Units

4.1.1

Symbols and Units

A

m2

area

CR

W/m2K4

radiation constant

cP

kJ/kg C

specific heat (at const. pressure),


6/23/2001 - 4:36:29 PM

Page 345

cP "Holderbank" - Cement Course 2000 or kJ/Nm3 C

specific heat capacity

CV

kJ/kg

calorific value

D

m

diameter 2

g

m/s

h

kJ/kg or kJ/Nm

gravity constant heat content (specific) 3

or kJ/kg cli L

m

length

m

kg

mass

or kg/kg

specific mass

mf

kg/h

mass flow

Qf

kW

heat flow (1 kW = 1 kJ/s)

t

C

temperature (Centigrade)

T

K

temperature (Kelvin)

v

m/s

velocity

w

kg/kg

water content

∝

W/m2K

heat transfer coefficient

ε

-

emissivity (for radiation)

λ

W/m C

heat conductivity

ρ

kg/m3

density

Greek Letters

Dimensionless Numbers Nu

Nusselt number (for heat transfer)

Pr

Prandtl number

R

kiln feed (raw meal) / clinker-ratio

Re

Reynolds number

Indices conv

convection

rad

radiation

tot

total

o

ambient condition or zero condition

Conversion Factors Length Area

1 inch

0.0254 m

1 ft

0.3048 m

1 sq. ft

0.092903 m2


6/23/2001 - 4:36:30 PM

Page 346


0.092903 m2

Volume, Volume Flow

1 cu.ft

0.028316 m3

1 cu.ft/min

1.699 m3/h (actual m3)

1 lb.

0.45359 kg

Mass

Pressure

Energy

Temperature Conversion Heat Flow

Specific Heat Heat Transfer Coeffic.

Standard Conditions for Gases

Nm3 =act.m3 ×

1 short ton (USA)

907.185 kg

1 bar

105 N/m2

1 mm H2O-Col.

9.806 N/m2

1 atm.

1.013 bar

1 kJ

1000 J

1 MJ

1000 kJ

1 kWh

3600 kJ

1 kcal

4.187 kJ

1 BTU

1.055 kJ

C=

5/9(F - 32)

K=

273.15 + C

1 kW

1000 W = 1 kJ/s

1 kcal/h

1.163 W

1 BTU/h

0.29307 W

1 kcal/kg C

4.187 kJ/kg C

1 BTU/lb F

1 kcal/kg C

=

=

4187 J/kg C

4.187 kJ/kg C

2

1.163 W/m C

2

1 BTU/ft h F

5.678 W/m2C

Standard Conditions

0°C and 1 atm (1.013 bar)

1 kcal/m h C

2

273.15 p(bar) × 273.16 + t(C) 1.013bar

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.1 General Remarks / 4.1.2 Reference Temperature

4.1.2

Reference Temperature

We will set the usual reference temperature to 20°C i.e. the sensible heat of mass flows at 20°C becomes zero. All „heats of transformation“ such as combustion, evaporation, formation are also based on 20°C reference. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.1 General Remarks / 4.1.3 Input / Output © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:30 PM

Page 347


Input / Output

Whether a heat item represents an input or output is determined by the direction of mass flow, according to: ♦ into boundary

=

input

♦ out of boundary

=

output

This is a simple and useful convention (not a natural law). If „heats of transformation“ occur within the boundary the net heat effect is normally used as criterion: ♦ heat producing process

=

♦ heat consuming process =

input output

A heat producing process is e.g. the fuel combustion (exothermic). Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.1 General Remarks / 4.1.4 Reference Quantity

4.1.4

Reference Quantity

The heat balance is referred to 1 kg of clinker produced. This requires a general calculation step (division by clinker production) which is not shown in the following formulas in order to maintain a more simple presentation.

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.2 Determination of Clinker Production

4.2

Determination of Clinker Production

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.2 Determination of Clinker Production / 4.2.1 General

4.2.1

General

The clinker production during a test is a key figure for all the following calculations. We will refer all flows and heats to 1 kg of clinker produced. Determination of clinker production can be made by direct weighing of the production which is by far the best method. If this can be done the following chapter 4.2.2 can be ignored. But in some cases no direct measurement is possible. Therefore, an indirect determination may be necessary as shown in the following. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.2 Determination of Clinker Production / 4.2.2 Indirect Determination

4.2.2

Indirect Determination

It is well known, that from approx. 1.56 kg raw material (not kiln feed!) 1 kg clinker can be produced. Starting from this fact it seems to be easily possible to calculate the clinker production. However, the above factor of 1.56 if of limited practical help because it can be superimposed by dust return and depends on some other influences. The best method in such a case is to establish a mass balance for the system. Similar to a heat © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:31 PM

Page 348

"Holderbank" - Cement Course 2000 balance a boundary for a mass balance can be defined. Thus the clinker production becomes:

Clinker =

    

+ kiln feed + coal ash + dust insufflated - dust losses

    

all calculated on L.o.I. free basis!

This principle is quite simple. The following additional remarks can be given: ♦ The boundary for the mass balance does not necessarily have to coincide with the heat balance boundary. ♦ The mass balance boundary must cut the kiln feed measuring point (because the mass flow is known at this point). ♦ The balance is always made on a loss-on-ignition-free (L.o.I.-free) base (no balance for the entire quantity including L.o.I. can be made since part of it is converted to gas, which is not included here). Regarding only the dust loss from the exhaust gas there are two basic possibilities:

a) Dust Flow crosses mass balance boundary

•

mass balance is influenced by dust flow

•

note that is not significant whether or how the dust is returned (outside the boundary)!


6/23/2001 - 4:36:31 PM

Page 349

"Holderbank" - Cement Course 2000 • b) Dust flow does not leave mass balance boundary (internal dust return)

•

mass balance is not influenced by dust flow

•

internal dust return must be steady, i.e. no storage being built up (silos!)

• Two practical examples illustrating above two cases are shown in the following. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.2 Determination of Clinker Production / 4.2.3 Examples

4.2.3

Examples

a) External Dust Recirculation (outside of boundary) Fig. 2 Mass Balance / External Dust Recirculation

mf (dry) [t/h]

L.o.I. [-]

Kiln Feed (KF)

204.0

Coal Ash (Ash)

1.8

Dust in Exhaust (D)

11.0

0.315

- 7.54

Bypass Dust (BpD)

2.1

0.070

- 1.95


0.357

mf (1-L.o.I.) [t/h]

∼0

131.17 + 1.80

6/23/2001 - 4:36:32 PM

Page 350

"Holderbank" - Cement Course2.1 2000 Bypass Dust (BpD)

0.070 ∼0

Clinker (Cli)

- 1.95 123.48 t/h

Note that the kiln feed data (L.o.I.) may slightly change when going from combined operation to direct operation or vice versa!

Kiln Feed / Clinker = R = 204.0 / 123.48 = 1.652

(all included)

b1) Internal Dust Recirculation (inside of boundary) Fig. 3 Mass Balance / Internal Dust Recirculation

During direct operation the following calculation method applies:

mf (dry) [t/h] Kiln Feed (KF)

204.0

Coal Ash (Ash)

1.8

Bypass Dust (BpD)

2.1

Clinker (Cli)

L.o.I. [-] 0.357 ∼0 0.070 ∼0

mf (1-L.o.I.) [t/h] 131.17 + 1.80 - 1.95 131.02 t/h

Kiln Feed/Clinker = R = 204.0/131.02 = 1.557 Note the difference to example a)! Incidentally the example b1) is shown for comparison to example a) and does by no means represent a superior solution of the dust return! The following example b2) shows a very similar way of calculation to example b1). b2) Internal Dust Return (wet kiln) © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:32 PM

Page 351

"Holderbank" - Cement Course 2000 Fig. 4 Mass Balance / Internal Dust Recirculation

mf (wet) [t/h]

mf (dry) [t/h]

L.o.I. [-]

mf (1-L.o.I.) [t/h]

Kiln Feed (KF)

42

26.46

0.360

16.93

Coal Ash (Ash)

-

0.8

∼0

0.8

∼0

17.73 t/h

Clinker

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.3 Heat from Fuel

4.3

Heat from Fuel

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.3 Heat from Fuel / 4.3.1 Fuel Firing

4.3.1

Fuel Firing

Fuel can be introduced at the following locations: ♦ Kiln firing ♦ Secondary firing or precalciner ♦ Burnable components in raw meal Generally the heat from fuel is calculated: h = m · CV

(kJ/kg cli)

h

=

heat (kJ/kg cli)

m

=

specific fuel consumption (kg/kg cli or Nm3/kg cli)

CV

=

calorific value (kJ/kg fuel or kJ/Nm3 fuel)

For the calorific value CV only the low (or net) value is used within the „Holderbank“ Group (for © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:33 PM

Page 352

"Holderbank" - Cement Course 2000 conversion see chapter „Combustion Engineering“). Heat balance referring to high (or gross) heat value are sometimes encountered. But here an important note must be made: the use of high heat values is based on a quite different method of considering heat losses. Therefore those balances cannot be compared directly to our type of balance. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.3 Heat from Fuel / 4.3.2 Coal Firing

4.3.2

Coal Firing

For coal firing it is essential that the CV and the consumption (m) refer both to the same conditions. The normal convention is to refer to the state as fed to the kiln burner. The state as fed to burner may sometimes be different from the state as analyzed. Differences can occur because of dust addition in coal mill, loss of volatile matter or just simply by changed moisture conditions. If the actual CV at the given coal moisture w is not known it can be calculated from the value CVo at the moisture wo:

CV = [(1-w) / (1-w0)] · (CV0 + w0 ·r) - w ·r where:

r

w, wo

=

2450 kJ/kg

=

heat of water evaporation at reference temperature 20°C

=

weight fractions of water

Examples: a) Coal, CV at 5% moisture = 26’500 kJ/kg CV at 1% moisture (as fed to burner) = ? CV = [(1-0.01) / (1-0.05)] · (26500 + 0.05·2450) - 0.01·2.450 CV =

27’464 kJ/kg

If above coal (1% moisture) is fired at a specific rate of 0.1230 kg/kg cli: h = 0.1230 · 27’464 =

3’378 kJ/kg cli

b) If the CV for dry matter is known (CVdry) the general formula can be simplified (for wo = 0): CV = (1 - w) CVdry - w · r CVdry = 28 MJ/kg, moisture 2% CV = 28 · (1 - 0.02) - 0.02 · 2.45 =

27.39 MJ/kg

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.4 Burnable Components in Raw Material

4.4

Burnable Components in Raw Material


6/23/2001 - 4:36:33 PM

Page 353

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.4 Burnable Components in Raw Material / 4.4.1 Organic Matter

4.4.1

Organic Matter

Organic matter can be present in the raw meal e.g. in form of oil shale or exceptionally even in form of free crude oil. Such material is partly volatilized in the preheating zone and leaves the system partly as unburnt light hydrocarbons. If the latter are actually measured and considered in the total heat balance it is recommended to determine the calorific value of the raw meal (not directly, but by extracting its organic matter). Thus the heat input becomes: h = CVlow · R

(kJ/kg cli)

where: CVlow =

calorific value due to organic matter, referred to 1 kg raw meal

R

kiln feed / clinker - factor

=

If light hydrocarbons in the exhaust gas are not measured nor considered in the total heat balance anyway the following approach will produce better results: Determine the organic content only (org. C, measured by burning in pure O2 to CO2). Then consider exclusively the organic C (the rest can be neglected). The approximate heat input becomes: h = C · R · 33’000 kJ/kg

(kJ/kg cli)

where C

=

organic carbon content in raw meal

Above mentioned is only an approximation but often used due to its simplicity. Example: R

=

1.6 kg/kg cli, C = 0.2% carbon

q

=

0.002 · 1.6 · 33’000 = 106 kJ/kg cli

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.4 Burnable Components in Raw Material / 4.4.2 Inorganic Matter

4.4.2

Inorganic Matter

In certain cases residues of non-oxidized pyrite (FeS2) can occur in the raw meal. The heat input becomes: h = S · R · 12’930 kJ/kg

(kJ/kg cli)

where S

=

weight fraction of sulfur (expressed as S!) from pyrite in raw meal

Example: R

=

1.6 kg/kg cli, 0.05% S (from pyrite) in raw meal

h

=

0.0005 · 1.6 · 12’930 =


10 kJ/kg cli

6/23/2001 - 4:36:34 PM

Page 354

"Holderbank" - Cement Course 2000 The practical heat effect in the above case is only marginal. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.5 Loss due to Incomplete Combustion

4.5

Loss due to Incomplete Combustion

If unburnt gases such as CO, H2, CH4 occur in the exhaust gas an additional heat output occurs. The loss can be calculated to: h

=

m · (CO · 12’640 + H2 · 10’800 + CH4 · 35’840)

m

=

specific gas quantity (Nm3/kg cli)

(kJ/kg cli)

CO, H2, CH4 = volume fractions in exhaust (referred to wet gas) In most cases only CO is measured and then the calculation becomes: h = m · CO · 12’640

(kJ/kg cli)

Example m

=

1.50 Nm3/kg cli (SP kiln)

CO

=

0.2% (ref to wet)

q

=

1.50 · 0.002 · 12’640 = 38 kJ/kg cli

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.6 Heat of Formation

4.6

Heat of Formation

The heat of formation takes into account all main reactions which occur when raw meal is transformed to clinker, as follows: RAW MEAL ♦

Heat of dehydration of clay (endothermic)

♦

Heat of decarbonation of CaCO3 + MgCO3 (endothermic)

♦

Heat of formation of clinker (exothermic!)

CLINKER In most of the practical cases it is sufficient to assume a constant value of h = 1750 kJ/kg cli The value mentioned before represents a heat output since the overall reaction is endothermic (heat consuming). The main contribution to the overall value comes from the decarbonation of CaCO3 (approx. 2100 kJ/kg cli). The heat of formation may naturally have some variations from one raw meal to the other. But due to the narrow range which is specified for the cement clinker composition no major deviations (of say more than +/- 50 kJ/kg cli) have to be expected in normal cases. If the CaO in clinker does not originate from CaCO3, but from raw material sources containing less CO2 than the CaO balance (such as coal ash or partly decarbonized raw mix) the heat of formation would © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:34 PM

Page 355

"Holderbank" - Cement Course 2000 theoretically change. However, this problem can be easily avoided by the following definition: ♦ All mass flows (as kiln feed, dust losses, coal ash) which carry a certain amount of CaO not coming from CaCO3 are considered as „heat flows“ according to their „non-carbonatic“ CaO content (see chapter 4.7). By following the above definition it is not necessary to make alterations at the heat of formation. An „abnormal“ situation can simply be treated by creating additional inputs or outputs, but not by changing the „standard“ heat of formation. The procedure is based on the idea of an „ideal“ clinker burning from pure carbonates. If those ideal conditions do not apply we set corrections in the corresponding heat inputs or outputs. Such principles are well known from calculation of bypass CaO-losses. Additional information on the heat of formation is given in chapter 6.2. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.7 Heat due to Partly Decarbonized Material

4.7

Heat due to Partly Decarbonized Material

Decomposition of carbonates in an essential heat effect in a cement kiln. Usually one thinks of the two extreme cases of either complete presence of carbonates (CaCO3, MgCO3) or complete absence of carbonates (e.g. clinker). But there can also be cases in between which influence the heat balance. Material streams can transport non-sensible heat due to partly decarbonized material (CaO) or non-carbonatic CaO. The following possibilities exist: 1) raw meal (containing e.g. CaO free or non carbonatic, CaO-bearing minerals) 2) exhaust gas dust (partly decarbonized) 3) bypass dust (largely decarbonized) 4) coal ash The heat can either be negative or positive according to the direction of flow (ingoing or outgoing). If item 2) (exhaust gas) is considered in the balance it is essential to consider also item 1) (raw meal)! In many cases kiln dust (which contains some free CaO) is returned to the kiln but not necessarily at exactly the same rate as the outcoming dust (e.g. on a system combined with raw mill, alternating between combined and direct operation). Even if dust is returned the net heat effect of item 1) minus item 2) may not automatically be zero! If the CaO and CO2 is known from chemical analysis the non carbonatic part CaOnc can be calculated: CaOnc = CaO - (56 / 44) CO2 Strictly speaking, above formula is only correct if no other carbonates than CaCO3 occur. If MgCO3 is present in form of carbonates the „carbonatic MgO“ has to be taken into account: CaOnc = CaO - 56 [(CO2/44) - (MgO/40)] Thus the heat is calculated to h = m · CaOnc · 3150 m

=

CaOnc =

(kJ/kg cli)

spec. mass (kg/kg cli) „non-carbonatic“ CaO, expresses as weight fraction


6/23/2001 - 4:36:35 PM

Page 356

"Holderbank" - Cement Course 2000 Above formula does not consider the small possible influence of MgCO3 decarbonation heat. If the assumption can be made that all MgO occurs in non-carbonatic form (e.g. after a heat treatment in the 700°C range) the following improved formula can be applied: h = m (CaOnc . 3150 + MgO . 2710)

(kJ/kg cli)

Examples 1) Raw Meal and Kiln Dust (dust returned, measurements refer to balance limits, normal raw mix)

Raw Meal

Kiln Dust

R

= 1.65 kg/kg cli

m

= 0.09 kg/kg cli

CaO

= 42.3%

CaO

= 43.5%

MgO

= 0.9%

MgO

= 1.1%

CO2

= 34.0%

CO2

= 30.5%

CaOnc = 42.3-56 [(34/44) - (0.9/40) = 0.3% h

= 1.65 · 0.003 · 3150 = 16 kJ/kg cli (input)

CaOnc = 43.5-56 [(30.5/44) - (1.1/40) = 6.2% = 0.09 · 0.062 · 3150 = 18 kJ/kg cli (output)

h

In the above example the net heat effect is virtually zero and may be completely neglected. But this may not be used as a general rule as shown by the next example.

2) Raw Meal and Kiln Dust (similar to example 1), but raw mix containing a major proportion of non-carbonatic CaO)

Raw Meal

Kiln Dust

R

= 1.60 kg/kg cli

m

= 0.09 kg/kg cli

CaO

= 43.2%

CaO

= 43.5%

MgO

= 0.9%

MgO

= 1.1%

CO2

= 30.0%

CO2

= 30.5%

CaOnc = 43.2-56 [(30/44) - (0.9/40) = 6.28% h

= 1.60 · 0.0628 · 3150 = 317 kJ/kg cli (input)


CaOnc = 43.5-56 [(30.5/44) - (1.1/40) = 6.2% h

= 0.09 · 0.062 · 3150 = 18 kJ/kg cli (output) 6/23/2001 - 4:36:35 PM

Page 357


Although non-carbonatic CaO occurs seldom, it may largely contribute to the heat balance (in the latter example a net effect of 299 kJ/kg cli!). The practical problem, however, is to determine the CaOnc with sufficient precision. 3) Bypass Dust

m

=

0.04 kg/kg cli (quantity of bypass dust)

CaO

=

56.2%

MgO

=

1.0%

CO2

=

1.8%

CaOnc

=

56.2 - (56/44) * 1.8

=

53.9%

q

=

0.04 * 0.539 * 3150 + 0.01 * 2710

=

95 kJ/kg cli (output)

4) Coal Ash

m

=

0.02 kg/kg cli (quantity of ash)

CaO

=

21%

MgO

=

2%

q

=

0.02 (0.21 * 3150 + 0.02 * 2710)

=

14 kJ/kg cli (input)

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.8 Heat of Evaporation

4.8

Heat of Evaporation

If water is evaporated within the balance limits the heat of evaporation becomes h = m · 2450

(kJ/kg cli)

m = evaporated water (kg/kg cli) Above items means an output. A large source of water evaporation is usually the slurry feed to a wet kiln. Although the calculation of heat of evaporation is obviously simple a few notes are given: ♦

Only the free water of the kiln feed is considered (the hydrate water is already included in the heat of formation!).


6/23/2001 - 4:36:36 PM

Page 358

"Holderbank" - Cement Course 2000 ♦ Water evaporation can also be caused by water injection into preheater, kiln, cooler, etc. ♦ The water evaporated from the fuel must not be included (this effect is already included in the net heat value, provided it has been determined as described in paragraph 4.3). Examples a) Wet Kiln: slurry water content = 35% R

=

1.56 kg/kg cli (dry)

m

=

0.35 / (1-0.35) · 1.56 =

0.84 kg/kg cli

h

=

0.84 · 2450

2058 kJ/kg cli

=

b) Water Spray into Planetary Cooler m

=

0.05 kg/kg cli

h

=

0.05 * 2450 = 123 kJ/kg cli

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.9 Sensible Heat

4.9

Sensible Heat

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.9 Sensible Heat / 4.9.1 General

4.9.1

General

Generally the sensible heats are calculated as follows: h = m · cp · (t - 20°C)

(kJ/kg cli)

m

=

specific mass (kg/kg cli or Nm3/kg cli)

cp

=

average specific heat (kJ/kg C or kJ/Nm3 C)

t

=

temperature of m (C)

Above formula uses a reference temperature of 20°C, i.e. sensible heats of material and gas flows at 20°C are zero. The value h (kJ/kg cli) can either be positive (if t > 20°C) or negative (if t < 20°C). Whether h represents an input or an output (see 4.1) depends only on the direction of the flow „m“: ♦ if m = entering boundary → h = input ♦ if m = leaving boundary

→ h = output

Basically the value m can be expressed as kg or Nm3. It is then logical that the cp values used must also refer to same unit (per kg or per Nm3). For convenience we use the following convention: ♦ for solid flows

→ unit = kg

♦ for gaseous flows → unit = Nm3 1) © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:36 PM

Page 359

"Holderbank" - Cement Course 2000 1)

The application of the unit „kg“ for gases would also be thinkable and would even facilitate the mass balance calculations. But since Nm3 is well established in the cement industry and since the cp-diagrams refer to Nm3 only, Nm3 is used in this chapter. The necessary cp values are to be found in the diagrams Fig. 5 and 6 (for solids and fuels) and Fig. 7 (for gases).

The same basic mode of calculation of sensible heat can be adapted for all material and gas flows crossing the boundary such as: ♦ Raw meal ♦ Clinker ♦ Dust(s) ♦ Exhaust gas, bypass gas, waste gas, etc. ♦ Cooling air, false air, etc. Fig. 5 Cp of Solids

Fig. 6 cp of Liquids and Fuels Temp.

cp of Liquid Water

0°C

4.22 kJ/kg C

50°C

4.18 kJ/kg C

100°C

4.22 kJ/kg C


6/23/2001 - 4:36:37 PM

Page 360


Fig. 7 cp of Gases

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.9 Sensible Heat / 4.9.2 Calculation of cp of Mixtures

4.9.2

Calculation of cp of Mixtures

The cp values are usually tabulated for pure substances. If a mixture is present it may become necessary to calculate the cp starting from the given composition. This is better shown in a practical example (instead of a general formula):

Example: Exhaust gas of suspension preheater kiln at 360 °C ♦ Measured by Orsat (dry): •

CO2

=

27.2% (CO = O)

•

O2

=

4.3%

•

N2

=

100 - 27.2 - 4.3 = 68.5%

♦ From moisture determination: •

H2O

=

0.08 Nm3/Nm3 wet


6/23/2001 - 4:36:37 PM

Page 361

"Holderbank" - Cement Course 2000 What is the cp value of the mixture (wet gas)?

Dry Gas (Vol %)

Wet Gas (Vol %)

cp (pure (kJ/Nm3C)

cp x Vol. Frac. (kJ/Nm3C)

CO2

27.2

25.0

1.92

0.480

O2

4.3

4.0

1.37

0.055

N2

68.5

63.0

1.31

0.825

H2O

0

8.0

1.55

0.124

Total

100%

100%

-

1.484 ≅ 1.48

Result: cp of mixture = 1.48 kJ/Nm3C (at 360°C) The above value is typical for a normal SP kiln. We will always find values in the 1.5 kJ/Nm3C range.

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.10 Heat Loss due to Radiation and Convection

4.10

Heat Loss due to Radiation and Convection

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.10 Heat Loss due to Radiation and Convection / 4.10.1 General

4.10.1 General In practice it is quite convenient to treat both, radiation and convection heat transfer together. Although the physical laws of these two heat transfer phenomenas are different they are usually given as a total. The determination of total heat transfer coefficients by using simplified working diagrams will be accurate enough. Therefore the theory how to calculate the values is treated in separate chapters (6.3 and 6.4). The user of this chapter does usually not need to go into details of radiation and convection heat transfer theories. In addition, the calculation from the complete theory would hardly increase the practical accuracy, since all these theories contain a certain inaccuracy. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.10 Heat Loss due to Radiation and Convection / 4.10.2 Radiation Heat Transfer

4.10.2 Radiation Heat Transfer Radiation heat transfer depends on the surface temperature and the emissivity ε (0...1). Typical values for ε: Type of Surface

Temperature °C

ε

Rough oxidized steel

100

1

Rough oxidized steel

400

0.9


6/23/2001 - 4:36:38 PM

Page 362

"Holderbank" Cement Course 2000 Rough oxidized- steel 400

0.9

White lime paint (on steel)

400

0.8

Aluminium paint

100

0.2 to 0.4

Aluminium, rolled sheet metal

100 to 500

0.08

Aluminium, polished or bright foil

100 to 500

0.04

In most cases an ε value has to be set at the pyrometer used for the measurement (depending on the type). For pyrometers giving readings for a constant emissivity ε = 1. Fig. 8 can be used for correction. The pure radiation heat transfer coefficient ∝rad (W/m2C) is shown in Fig. 9. Note that the temperature axis in Fig. 9 means temperature difference to ambient (same convention as for the following Fig. 10 and 11 - see paragraph 4.10.3 and 4.10.4). Radiation heat transfer calculation does usually not create special problems as long as the radiating surfaces face freely towards ambient. A particular situation occurs on a planetary cooler where certain directions of radiation are shielded. That means not all areas are able for radiation towards the ambient.

Here, it would be obviously wrong to consider the total of the tube surfaces as radiation area. As a first approach the outer enveloping cylinder area (enveloping all tubes, see sketch) can be taken as reference area. Fig. 8 Relation between Emissivity ε and True and Apparent Surface Temperature


6/23/2001 - 4:36:38 PM

Page 363

"Holderbank" - Cement Course 2000 Fig. 9 Radiation Heat Transfer Coefficient

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.10 Heat Loss due to Radiation and Convection / 4.10.3 Convection Heat Transfer

4.10.3 Convection Heat Transfer Two different types of convection can be distinguished: ♦ Free convection (occurs by natural thermal draft, at low wind velocities) ♦ Forced convection (occurs at high wind velocities) In practice an intermediate region between these two extreme cases can be encountered, depending on the wind velocity v (m/s). The resulting ∝conv can be taken from Fig. 10. Fig. 10 Convection Heat Transfer Coefficient

The diagram Fig. 10 refers to a normal kiln diameter range from 3 m to 6 m. Note that the temperature axis in Fig. 10 means temperature difference to ambient, not temperature itself. ♦ Influence of diameter •

Free convection (v = 0 m/s): ∗ The free convection coefficient is not depending on the diameter (or height) of equipment. If


6/23/2001 - 4:36:39 PM

Page 364

"Holderbank" - Cement Course 2000 there is no wind Fig. 10 can be applied for any dimension. •

∗ ∗

Forced convection: In contrast to the free convection the forced convection does depend on the diameter. For this reason the application range of Fig. 10 is limited to 3 m...6 m, but only if high wind velocities occur. At low wind velocities no diameter limits exist. For very small tubes (or gas ducts) the following corrections of ∝conv (increases) can be made: For ∅ 2 m + 15%  For ∅ 1 m + 30%  only at high wind velocities (min. 2 m/s) For ∅ 0.5 m + 50% 

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.10 Heat Loss due to Radiation and Convection / 4.10.4 Total Heat Transfer Coefficient (Radiation and Convection)

4.10.4 Total Heat Transfer Coefficient (Radiation and Convection) For the determination of the total heat transfer coefficient ∝tot (W/m2C) two cases are possible: a) Standard Case ε = 0.9 (rough oxidized steel surface) This particular case is rather simple. Direct use of Fig. 11 is possible. (Go into diagram with temperature difference and read out the total heat transfer coefficient.)

b) Non-standard Case ε differs clearly from 0.9 and/or correction of convective heat transfer coefficient is necessary. Make use of Fig. 9 and 10. ∝tot = ∝rad + ∝conv (Caution: never add free convection + forced convection, Fig. 10 does already include the overlapping of both effects.)

Fig. 11 Radiation and Convection Heat Transfer Coefficient (total)


6/23/2001 - 4:36:40 PM

Page 365


Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 4. HEAT BALANCE CALCULATIONS / 4.10 Heat Loss due to Radiation and Convection / 4.10.5 Total Heat Flow

4.10.5 Total Heat Flow The total heat flow from radiation and convection heat transfer is calculated according to: Qf

=

αtot · A · (t - t0)

Qf

=

heat flow (W); 1kW = 1000W

αtot

=

αrad + αconv = total heat transfer coefficient (W/m2C)

A

=

shell area (m2)

t

=

shell temperature (C)

t0

=

ambient temperature (C)

(W)

From the heat flow Qf the specific loss can be calculated: h

=

Qf (kW) / mf (t/h) · 3.6

(kJ/kg cli)

mf

=

clinker production (t/h)

3.6

=

conversion factor, because of unit (t/h) for mf

If the temperature of a shell area is not approximately constant it is necessary to subdivide the area into individual sections. On a rotary kiln we may e.g. select about 10 (or more) individual cylindrical areas, each of them having the surface: Ai = Π · Di · Li (m2) Example: ♦ Rotary kiln ♦ Clinker production = 1900 t/d = 79.17 t/h ♦ ∅ 4.4 m x 67 m ♦ Average emissivity: ε = 0.9 ♦ Ambient temperature = 20°C ♦ Average wind velocity v = 1 m/s (nearly free convection!) ♦ Shell temperature profile as indicated in the following table: Length Position (m)

Element Length (m)

Dia-meter Element Area 1) (m) (m2)

Surface Temp. (°C)

ε (-)

∝ total2) (W/m2C)

0 - 5

5

4.40

69.1

230

0.9

22.7

331’000

5 - 10

5

4.40

69.1

360

0.9

32.9

774’000


6/23/2001 - 4:36:40 PM

Qf (heat flow) (W)

Page 366

"Holderbank" Course 2000 5 - 10 5 - Cement 4.40 69.1

360

0.9

32.9

774’000

10 - 15

5

4.40

69.1

310

0.9

28.6

574’000

15 - 25

10

4.40

138.2

220

0.9

22.1

611’000

25 - 35

10

4.40

138.2

330

0.9

30.3

1’299’000

35 - 45

10

4.40

138.2

260

0.9

24.9

825’000

45 - 55

10

4.40

138.2

290

0.9

27.1

1’011’000

55 - 60

5

4.40

69.1

270

0.9

25.6

442’000

60 - 67

7

4.40

96.8

220

0.9

22.1

428’000

Total

67 m

926.1 m

Total heat loss

6'295 ⋅ 3.6 Specific heat loss = 79.17t / h

=

=

6’295’000 W

6’295 kW

286 kJ/kg cli

A = ∏ . D . L (cylinder)

1) 2)

2

radiation plus convection (see Fig. 11)

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 5. HEAT BALANCES

5.

HEAT BALANCES

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 5. HEAT BALANCES / 5.1 General

5.1

General

The results for heat inputs and outputs as calculated according to the preceding paragraph 4 are summarized in a balance sheet. Table 1 (complete kiln system) and Table 2 (cooler only) are given as examples. They may also be used as checklist for completeness of own measurements and calculations. ♦ „Rest“-item: •

A real balance (from complete date) will always include an item „rest“, where all measuring errors (no measurement is 100% accurate) and non-considered items are included. The amount of the „rest“ item should not exceed 3% of the total heat input.

♦ Heat consumption: •

The actual heat consumption is the total of fuel input(s) which is not exactly the same as the total of heat inputs. If there are more than one fuel input, it is advantageous to indicate the actual (true) heat consumption separately.

♦ Cooler balance: •

On a cooler, reliable measurement of secondary air heat is virtually impossible. Therefore, this value is determined by balance calculation and no rest item is given. The thermal efficiency of the cooler is usually defined as: η = heat of secondary (and tertiary) air (%) heat of hot clinker


6/23/2001 - 4:36:41 PM

Page 367


The % values in the balance sheet, however, are based on the total of inputs. This can cause a slight difference from the percentage of above η.

Table 1

Heat Balance of Cement Kiln System

(General case) - All referred to 1 kg clinker - Reference temperature

= 20°C

- Ambient temperature

= ...... °C

- Production

= ...... t/h

- Specific heat cons.

= ...... kJ/kg cli

Specifications

Temp.

(kg/kg cli), (Nm3/kg cli) (kW) etc.

(°C)

Heat (kJ/kg cli)

(%)

INPUT Fuel combustion - primary firing - secondary firing

-

Burnable matter in kiln feed

-

Raw meal: sensible heat Fuel: sensible heat Primary air: sensible heat Cooler air: sensible heat CaO (non-carbonatic) in kiln feed

-

Total of inputs

-

100%

OUTPUT Heat of formation

-

-

Water evaporation: - kiln feed - water spray(s)

-

Exhaust gas: - sensible heat - dust CaO-loss - unburnt gases (CO, etc.)

-

Cooler: - waste air sensible heat - middle air sensible heat - clinker exit sensible heat Bypass losses: - sensible heat - dust sensible heat - dust CaO-loss - unburnt gases (CO, etc.)


-

6/23/2001 - 4:36:41 PM

Page 368

"Holderbank" - Cement - unburnt gases (CO, etc.)Course 2000 Radiation and Convection: - preheater - rotary kiln - cooler - tert. Air duct

-

kW kW kW kW kW

Rest Total of outputs

Table 2

-

100%

Heat Balance of Clinker Grate Cooler

All referred to 1 kg clinker Reference temperature

= 20°C

Ambient temperature

= ... °C

Specifications

Temp. (°C)

Heat (kJ/kg cli)

(%)

INPUT Hot clinker Cooling air Total input

100%

OUTPUT Secondary air (incl. dust) Tertiary air Middle air Waste air Clinker outlet Water spray Radiation and convection Total output

100%

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 5. HEAT BALANCES / 5.2 Examples of Heat Balances of Various Kiln Systems

5.2

Examples of Heat Balances of Various Kiln Systems

Heat balances are given in Table 3 of various kiln systems. While comparing these it has to kept in mind, however, that kiln systems cannot only be judged based on these heat balances. The heat consumption of kiln system may depend on many items, those of major influence are: © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:42 PM

Page 369

"Holderbank" - Cement Course 2000 ♦ Kiln size, i.e. production rate ♦ Heat loss due to radiation and convection ♦ Secondary air quantity and temperature ♦ Operating conditions of kiln Furthermore related systems have also to be taken into account. Consider a grate preheater kiln (Lepol kiln): Its specific heat consumption may almost be as low as that of a 4-stage preheater kiln. However the grate preheater kiln, the semi-dry type, requires that additional heat is spent for material drying, whereas the exhaust gas of the 4-stage preheater kiln may be utilized to dry up to 8% water content of the raw material. The table 3 gives heat balances of three different kiln systems. ♦ Wet kiln: •

The wet kiln has a production capacity of approximately 3000 t/d. The slurry water content is 38%.

♦ Lepol kiln: •

The heat balance of the Lepol kiln is of a comparatively small unit. The nodule moisture content is 12%. In general grate preheater kilns of the semi-dry type have a heat consumption only slightly higher than the 4-stage preheater kilns.

♦ Dry suspension preheater: ♦ •

The heat balance of the 4-stage preheater kiln is typical for a unit in the 2000 to 3000 t/d range.

Table 3

Heat Balances of Wet, Grate Preheater and Preheater Kilns WET PROCESS

SEMI-DRY (LEPOL)

DRY PRE-HEATER (4-STAGE)

kJ/kg cli

%

kJ/kg cli

%

kJ/kg cli

%

from sensible heat

25

0.4

15

0.4

13

0.4

from combustion

5560

96.7

3343

97.6

3150

97.7

from sensible heat

25

0.4

30

0.9

54

1.7

from sensible heat of water

71

0.2

17

0.5

-

-

from sensible heat of all the air supplied (prim. sec.)

67

1.2

20

0.6

6

0.2

TOTAL INPUT

5750

100

3425

100

3223

100

Heat of formation

1750

30.4

1750

51.1

1750

54.3

Evaporation of water from raw meal

2370

41.2

506

14.8

13

0.4

Exhaust gas sensible heat

754

13.1

314

9.2

636

19.7

1. INPUT FUEL

RAW MEAL

COMBUSTION AIR

2. OUTPUT


6/23/2001 - 4:36:43 PM

Page 370

"Holderbank" - Cement Course Exhaust gas sensible heat 754 2000 13.1

314

9.2

636

19.7

Dust sensible heat

25

0.4

21

0.6

18

0.6

Incomplete combustion (CO)

-

-

-

-

-

-

Clinker exit temperature

59

1.0

50

1.5

63

2.0

Cooler exhaust gases

100

1.7

276

8.1

423

13.1

Losses due to radiation and convection

540

9.4

452

13.2

297

9.2

Water cooling (Recupol inlet chute)

-

-

42

1.2

-

-

Rest

152

2.6

14

0.4

23

0.7

TOTAL OUTPUT

5750

100

3425

100

3223

100

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART

6.

SPECIAL PART

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.1 Influence of Reference Temperature

6.1

Influence of Reference Temperature

The reference temperature for a heat balance is usually set to 20°C. At this temperature all sensible heats become zero. This is quite practical because the normal ambient temperature is always near 20°C. In addition the choice of the reference temperature has various other consequences which are normally not mentioned expressively. As long as the calculation procedure according to this chapter is applied we will not run into any practical problems. But if we want to go a bit deeper inside the matter we are soon confronted with some special questions as will be shown in the following. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.1 Influence of Reference Temperature / 6.1.1 „Heat of Transformation“

6.1.1

„Heat of Transformation“

By the term „heat of transformation“ we summarize here all heat effects by chemical reactions and changing of the physical state (combustion, heat of formation, evaporation). In order to understand the meaning of the reference temperature on these heat effects the following diagram is shown. It gives an example of an (ideal) isothermal process, including upheating and cooling. Fig.12


6/23/2001 - 4:36:43 PM

Page 371


We note from the diagram that the „heat of transformation“ at reference temperature and at true reaction temperature are not identical. The reason is the different specific heat content of the starting material and the reaction products. It is also obvious that any other level for the reference temperature would basically change the „heat of transformation“. The „heat of transformation“ at a reference temperature 20°C should therefore be seen in its proper sense: It is the heat effect under the convention, that we start at 20°C and end exactly at those 20°C. The real process (heating up, transformation, cooling) has no influence on the result, regardless of the true temperatures. This is a direct consequence of the principle of energy conservation. Therefore, we may take into account that all „heats of transformation“ which we use in a heat balance are based on reference temperature 20°C. Such values should not be confused with the heat effect at the true temperature of transformation or any other temperature. The items of interest are: ♦ Calorific value (combustion heat): •

The CV must also be based on 20°C reference. The error which occurs when choosing a reference of e.g. 0°C is fortunately so small that it is within the normal accuracy of a CV determination.

♦ Heat of formation: •

Regarding the heat of decarbonation only we may sometimes find values for the true reaction temperature (decarbonation, in the 800°C range). Such values shall not be used for a heat balance at 20°C reference, because this would produce an essential error!

♦ Heat of evaporation of water: •

Note that we must use the evaporation heat at 20°C (2450 kJ/kg) in the balances and not the value at say 100°C (2260 kJ/kg). The effects of upheating of water or vapor are automatically considered by the normal balance calculation procedure (items for sensible heats).

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.1 Influence of Reference Temperature / 6.1.2 Specific Heats

6.1.2

Specific Heats

The specific heats used in this chapter are average values which are consequently based on 20°C reference temperature. We may illustrate this fact by an upheating process starting at 20°C as shown in the following graph: © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:44 PM

Page 372

"Holderbank" - Cement Course 2000 Fig.13

The average specific heat can be graphically explained as the inclination of the straight line from Po to P. It is obvious that this inclination depends from the reference point Po at 20°C. At the point P the tangent to the heat curve is also shown by a dotted line. The inclination of this line is the actual specific heat at the temperature t. This value is different from the average specific heat. Moreover, it does not depend from any reference point. As a practical consequence we should never use specific heats from any source in the literature without checking what they really mean. In most cases actual values are given and not averages. Therefore, an integration or averaging would be required before we can use them for calculation of heat contents. Special caution should be made in the high temperature range where actual value and average value may be considerably different! In the 20° to 200°C range the practical differences, however, are rather small. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.2 Heat of Formation

6.2

Heat of Formation

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.2 Heat of Formation / 6.2.1 General

6.2.1

General

The heat of formation expresses the theoretical heat required for producing 1 kg of clinker. The following steps are taken into account:

Approx. Temperature

Heat Effect

Step 1

Expelling of hydrate water (+ transformation of clay minerals)

∼ 500°C

negative (heat consumed)

Step 2

Decarbonation of MgCO3 (first) and CaCO3 (CaCO3 → CaO + CO2)

∼ 850°C

negative (heat consumed)

Step 3

Formation of clinker minerals (C2S, C3S, C3A, C4AF)

∼ 1400°C

positive (heat produced)


6/23/2001 - 4:36:44 PM

Page 373


The net heat produced by the overall reaction is negative i.e. heat is consumed (listed as output). The heat formation is defined at 20°C. In other words it expresses the theoretical amount of heat to transform raw meal at 20°C to 1 kg clinker at 20°C (if no heat losses would occur). It is therefore also considered as the theoretical minimum heat required for burning clinker. The heat of formation is not at all depending on the way of the actual reaction (i.e. temperature versus time). The only precondition of course is that the reactions (step 1, 2, 3) have really been completed. Although the basic principle of the heat of formation seems quite simple a few conventions (or definitions) are necessary here: 1) The hydrate water is expelled is considered as vapour (therefore no additional heat of evaporation has to be introduced into the balance!). 2) As a „standard case“ the assumption is made that the CaO is present as CaCO3 in the raw meal (in non carbonatic CaO occurs we do not change the heat of formation, but add heat inputs according to paragraph 4.7). 3) Burnable components in the raw mix are considered separately as inputs (see paragraph 4.4.2). Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.2 Heat of Formation / 6.2.2 Calculation of Heat of Formation

6.2.2

Calculation of Heat of Formation

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.2 Heat of Formation / 6.2.2 Calculation of Heat of Formation / 6.2.2.1 Introduction

6.2.2.1

Introduction

It is difficult to determine the heat of formation directly be calorimetric measurements. Therefore calculation methods have been developed in order to take into account the varying influences of raw mix properties. Although formulas are given the following they should be applied with care. As long as the raw mix does not clearly deviate from the „standard“ value of ∼ 1750 kJ/kg the calculated figures must not necessarily give more accurate results than the 1750 kJ/kg. Even extensive formulas for heat of formation contain a certain incertitude due to the partial lack of accurate thermodynamic standard values (for the minerals which are involved). Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.2 Heat of Formation / 6.2.2 Calculation of Heat of Formation / 6.2.2.2 Calculation Formulas

6.2.2.2

Calculation Formulas

The following formulas are partly based on the work of H. zur Strassen (Lit. 1) however a few recent adaptations have been made: We will use the following notation: CaO, MgO, SiO2, Fe2O3, Al2O3 = (kg/kg cli)

Clinker analysis, expressed as weight fractions


6/23/2001 - 4:36:45 PM

Page 374

"Holderbank" - Cement Course 2000 H2O (kg/kg cli)=

Hydrate water in raw material, referred to kg clinker (not to raw meal)

The following general formula for the heat of formation applies: h = CaO · 3200 + MgO · 2710 - SiO2 · 2140 - Fe2O3 · 250 + hRes

(kJ/kg cli)

The last contribution „hRes“ depends on the Al2O3 and H2O (hydrate) content. The calculation of „hRes“ depends on the information which is available on hydrate water and/or type of clay. Usually, the more information is known the better the result will be.

Case No.

Hydrate Water (H2O)

Type of Clay

Formula for hRes (kJ/kg cli)

1

not known

not known

hRes

= Al2O3 . 1720

2

known

not known

hRes

= Al2O3 . 120 + H2O . 5520

3

not known

known

hRes

= (Al2O3)K . 2220 + (Al2O3)M . 1310 + (Al2O3)I . 1640

4

known

known

hRes

= (Al2O3)K . 1400 + (Al2O3)M . 620 + (Al2O3)I . 760 + H2O . 2450

In case 3 and 4 a distinction has to be made from which type of clay the Al2O3 originates. The indices K, M, I denote: K

=

Kaolinite

M

=

Montmorillonite

I

=

Illite

Note on the formulas for hRes: No. 1: For general purposes, if no information on hydrate water available. No. 2: Takes into account the actual hydrate water content. It is also a very good approach if the hydrate water does not only originate from clays but for example from Ca(OH)2. Note that the coefficient for H2O (hydrate) is more than twice the normal heat of evaporation 1) Nos. 3 + 4: Take into account the type of clay. (Hydrate water must originate from clay only, otherwise take No. 2). When the CaO is introduced into the formula it is not required to know whether the CaO results from CaCO3 or not. If any non carbonatic CaO is entering into the system we will take into account a balance heat input by definition (see paragraph 4.7) and therefore the calculated heat of formation is © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:45 PM

Page 375

"Holderbank" - Cement Course 2000 not affected. 1)

The older formula from H. zur Strassen did allocate only 2450 kJ to H2O, but instead 930 kJ to the Al2O3, which made it difficult to extrapolate it for other hydrates than from clay. It is certainly more logical to refer the heat to the H2O, which is actually responsible for the heat consumption. Examples 1)

-

Clinker composition:

CaO

=

64.8 %

MgO

=

1.2 %

SiO2

=

22.6 %

Fe2O3

=

3.3 %

Al2O3

=

5.2 %

SO3

=

0.52%

TiO2

=

0.29%

Na2O

=

0.08%

Mn2O3 =

0.03%

K2O

=

1.20%

L.o.I

0.54%

=

-

No information on hydrate water in raw meal available

h=

0.648 · 3200 + 0.012 · 2710 - 0.226 · 2140 - 0.033 · 250 + 0.052 · 1720 = 1704 kJ/kg cli

2)

-Clinker composition:

CaO

=

65.2 %

MgO

=

1.2 %

SiO2

=

22.9 %

Fe2O3

=

3.0 %

Al2O3

=

5.0 %

h=

Raw material = 1.1% hydrate water R = 1.57 kg/kg → 0.017 kg hydrate water/kg clinker 0.652 · 3200 + 0.012 · 2710 - 0.229 · 2140 - 0.03 · 250 + 0.050 · 120 + 0.017 · 5520 = 1721 kJ/kg cli

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.2 Heat of Formation / 6.2.2 Calculation of Heat of Formation / 6.2.2.3 Sulfatization Effects

6.2.2.3

Sulfatization Effects

The SO2 can react with the alkalis K2O or Na2O (but also with CaO). For the heat effect the following © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:46 PM

Page 376

"Holderbank" - Cement Course 2000 (simplified) reaction is considered as typical: Na2O + SO2 + ½ O2 → Na2SO4 + heat (molar heat effects of K2O and Na2O are of similar size!) The overall degree of sulfatization reactions is usually not known. For the heat effect the following lower and upper boundaries can be calculated: lower boundary upper boundary

= =

- 8370 · [(SO3)cli + (SO3)D - (SO3)R] - 10800 · [(Na2O)R - (Na2O)cli] - 7120 · [(K2O)R - (K2O)cli] + 8370 · [(SO3)R - (SO3)cli] (kJ/kg cli)

(kJ/kg cli)

where SO3, Na2O, K2O are expressed as kg/kg cli (referred to clinker!). The indices denote:

cli D R

= = =

in clinker in dust in raw meal

An average value can be introduced into the heat balance. Preferably this item is just listed under the item heat of formation (under output). Its sign is then negative, i.e. the outputs are reduced. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.2 Heat of Formation / 6.2.2 Calculation of Heat of Formation / 6.2.2.4 Lime Kilns

6.2.2.4

Lime Kilns

The formula for heat of formation of burnt lime is quite similar to that of cement clinker. The following main reactions are taken into account: CaCO3 MgCO3 CaO + 2 SiO2

→ → →

CaO + CO2 MgO + CO2 C2S (dicalciumilicate)

Unlike on cement clinker the CaO in the lime product does not exclusively consist of „non carbonatic“ CaO, but some residual CaCO3 is also present. Therefore the non carbonatic proportion CaOnc has to be calculated first: CaOnc = CaOtot - CO2 · (56/44) The heat of formation becomes: h = CaOnc · 3150 + MgO · 2710 - SiO2 . 2100

(kJ/kg lime)

CaOnc, MgO, SiO2 denote the weight fractions in lime product. If the lime is present in lump form and the SiO2 does originate e.g. from coal ash it is unlikely that C2S mineral is formed. In such cases only the SiO2 which comes from the limestone feed has to be considered in the formula. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.3 Radiation Heat Transfer

6.3

Radiation Heat Transfer

The basic radiation formula is © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:46 PM

Page 377

"Holderbank" - Cement Course 2000 Qf

=

CR · ε · A [ (T/100)4 - (T0/100)4 ]

Qf CR ε A T To

= = = = = =

heat flow [W] radiation constant = 5.67 W/m2K4 emissivity (0...1) radiating area (m2) absolute temperature of radiating surface [K] absolute temperature of ambient [K]

[W]

(T = 273+t)

Strictly speaking above formula is only valid for cylinders of infinite length which radiate towards an ambient which is far away from the cylinder. No obstacles may shield off the radiation. Though its simplifications this formula may be taken as a good approach for most cases. By dividing Qf by T - To and A, the ∝-value (heat transfer coefficient) can be obtained:

C ⋅ε α= R T − T0

 T  4  T0  4  2  −   (W / m C )   100   100  

Or alternatively the following formula, which is identical to the latter comes out:

4  T    α =CR ⋅ ε ⋅ 100  100 

3

 1  ∆T  2  2   (W / m C ) 1 +   4  T  

T + T0 T= 2 = average temperature (K)

∆T =T − T0 = temperature difference (K or C) From the second formula it becomes evident that (at small temperature differences) ∝ increases with the third (not fourth!) power of the average temperature. In addition, the ∝-value does converge towards a finite value at zero temperature difference. For practical evaluation of the ∝ (radiation) Fig. 9 (see paragraph 4.10.2) is available. It is based on a constant ambient temperature of To = 293 K (20°). If the true ambient temperature is actually not exactly 20°C one commits a slight error. Usually this can be neglected. In some extreme or exceptional cases the above formulas can be used for calculations. Since the emissivity ε is often not known precisely and does also depend on the temperature it is usually not worthwhile to do small corrections on the ∝-values obtained from Fig. 9. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.4 Convective Heat Transfer

6.4

Convective Heat Transfer

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.4 Convective Heat Transfer / 6.4.1 Free Convection

6.4.1

Free Convection

Free convection occurs due to density differences between hot air (at the shell surface) and ambient © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:47 PM

Page 378

"Holderbank" - Cement Course 2000 air. A natural updraft causes a convective air movement. For vertical plates or horizontal cylinders the following relationship applies: Nu = 0.13 (Pr . Gr) 1/3 where: Nu = Pr

(-)

=

Gr =

αD / λ =

Nusselt number (-)

cpη / λ =

Prandtl number (-)

D 3 ⋅ g ⋅ ρ 2 ⋅ ∆T η 2 ⋅ T0

=

Grashof number (-)

∝

(W/m2K)


D

(m)

characteristical dimension *

λ

(W/mK)

heat conductivity

cp

(J/kg K)

specific heat

g

(m/s2)

gravity constant = 9.81 m/s2

ρ

(kg/m3)

gas density

∆T

(K)

temperature difference (T - To)

η

(kg/ms)

dynamic viscosity

To

(K)

absolute ambient temperature

* for a cylinder with diameter d → D = (Π/2) · d Pr · Gr > 109

Application range:

As long as there is a minimum temperature difference of a few degrees and the diameter range is over 1 m, above limitation does not affect the calculation. A mathematical transformation of the basic equation reveals that the free convection heat transfer does not at all depend on the characteristical dimension! It follows: 1

1

 c ⋅ λ2 ⋅ g ⋅ δ 2  3  ∆T  3    (W / m 2K ) α =0.13 p   T  η    0  At ∆T = 0 the free convection becomes zero (which is different from the behavior of the radiation heat transfer!). The numerical values for cp, λ, ρ, η must be taken from tables for air at the average temperature between surface and ambient (use SI-units only). Note that the density ρ depends also on the barometric pressure and therefore the result will depend on the altitude above sea level (∝ ∼ p2/3). As a general guideline the convective heat transfer drops by about 8% per 1000 m of altitude. By using numerical approximations for the properties of air the following relationship has been developed (dimensional equation): α ≅ 1.4 · (ρ0 · ρ · ∆T)1/3 © Holderbank Management & Consulting, 2000 Query:

(W/m2K) 6/23/2001 - 4:36:48 PM

Page 379

"Holderbank" - Cement Course 2000 ρo

(kg/m3)=

density at ambient temperature

ρ

(kg/m3)=

density at average temperature

Though its simplicity the latter formula covers the temperature range from 0...500°C with an accuracy of better than 1%! This is more than enough for practical purposes. At sea level and at 20°C ambient it follows

  ∆T α ≅1.6  ∆T  1+ 2 ⋅T 0 

     

1 3

(W / m 2K )

Above relationship is also an excellent numerical approximation of the curve for v = O in Fig. 10 (see paragraph 4.10.3), which is actually based on complete computer calculations out of properties for air. Table Temp.ϑ

Properties of Air at Pressure = 1 bar ρ

°C

kg/m

-180 -160

cp 3

β

λ 3

η 3

ν 6

α 6

2

Pr 6

2

kJ/kg K

10 /K

10 W/mK

10 kg/ms

10 m /s

10 m /s

1

3.8515

1.071

11.701

9.0

6.44

1.67

2.18

0.77

3.1258

1.036

9.320

10.9

7.85

2.51

3.37

0.75

-140

2.6391

1.021

7.758

12.7

9.20

3.48

4.71

0.74

-120

2.2867

1.014

6.659

14.6

10.49

4.587

6.30

0.73

-100

2.0186

1.011

5.846

16.4

11.72

5.806

8.04

0.72

-80

1.8073

1.009

5.219

18.16

12.89

7.132

9.96

0.72

-60

1.6364

1.007

4.719

19.83

14.02

8.567

12.0

0.71

-40

1.4952

1.006

4.304

21.45

15.09

10.09

14.3

0.71

-20

1.3765

1.006

3.962

23.01

16.15

11.73

16.6

0.71

0

1.2754

1.006

3.671

24.54

17.10

13.41

19.1

0.70

20

1.1881

1.007

3.419

26.03

17.98

15.13

21.8

0.70

40

1.1120

1.008

3.200

27.49

18.81

16.92

24.5

0.69

60

1.0452

1.009

3.007

28.94

19.73

18.88

27.4

0.69

80

0.9859

1.010

2.836

30.38

20.73

21.02

30.5

0.69

100

0.9329

1.012

2.684

31.81

21.60

23.15

33.7

0.69

120

0.8854

1.014

2.547

33.23

22.43

25.33

37.0

0.68

140

0.8425

1.017

2.423

34.66

23.19

27.53

40.5

0.68

160

0.8036

1.020

2.311

36.07

24.01

29.88

44.0

0.68

180

0.7681

1.023

2.209

37.49

24.91

32.43

47.7

0.68

200

0.7356

1.026

2.115

38.91

25.70

34.94

51.6

0.68

250

0.6653

1.035

1.912

42.43

27.40

41.18

61.6

0.67

300

0.6072

1.046

1.745

45.91

29.20

48.09

72.3

0.67

350

0.5585

1.057

1.605

49.31

30.90

55.33

83.5

0.66

400

0.5170

1.069

1.485

52.57

32.55

62.95

95.1

0.66

450

0.4813

1.081

1.383

55.64

34.00

70.64

107

0.66

500

0.4502

1.093

1.293

58.48

35.50

78.86

119

0.66

600

0.3986

1.116

1.145

63.5

38.30

96.08

143

0.67

700

0.3577

1.137

1.027

67.8

40.87

114.3

166

0.69

800

0.3243

1.155

0.932

71.3

43.32

133.6

190

0.70


6/23/2001 - 4:36:48 PM

Page 380

"Holderbank" Course 2000 800 0.3243 - Cement 1.155 0.932 71.3

43.32

133.6

190

0.70

900

0.2967

1.171

0.852

74.3

45.65

153.9

214

0.72

1000

0.2734

1.185

0.786

76.8

47.88

175.1

237

0.7

↓ (actual cp, not average)

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.4 Convective Heat Transfer / 6.4.2 Forced Convection

6.4.2

Forced Convection

Forced convection occurs at comparatively high wind velocity and dominates the convective heat transfer, i.e. the free convection is suppressed. The calculation of forced convection is depending on many factors, such as: ♦ Wind velocity ♦ Direction of the wind ♦ Velocity distribution and flow obstacles ♦ Uniformity of wind ♦ Reynolds number (depends on kiln diameter). There are a few more influence factors than in case of the free convection. Generally speaking the calculation of forced convection heat transfer contains more possible sources of error than free convection. As a simplification, we will assume a cylinder in a non disturbed flow of a constant velocity v (at 90° against kiln axis). Fig. 14

For air the following formula apply: Nu

=

0.0239 · Re 0.805

for Re =

40’000...400’000

Nu

=

0.00672 · Re 0.905

for Re >

400’000


6/23/2001 - 4:36:50 PM

Page 381


vD v ⋅ D ⋅ ρ = =Re ynoldsNumber ν η αD Nu= =Nusselt Number λ

Re=

The properties η, λ, ϑ have to be taken at average air temperature. There are other formulas in use which can give different results, the above formulas are preferred due to their simplicity. In any case there is always a incertitude from the mode of calculation itself. Two main factors determine the ∝-value: ♦ Velocity v ♦ Average temperature (between surface and ambient) In addition the ∝ does also depend on the diameter D. If the equation for high Reynolds number (Re > 400’000) is solved for ∝, the following relationship is obtained: ∝

∼ D-0.095 ↓ proportional

This means that the ∝ value does not much depend on D! Therefore it is possible to work with constant values within a certain diameter range. This actually the base of Fig. 10 (see paragraph 4.10.3) which is calculated for a common range from 3...4...6 m diameter. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.4 Convective Heat Transfer / 6.4.3 Free Convection Plus Forced Convection

6.4.3

Free Convection Plus Forced Convection

If the convection is clearly dominated either by free or forced convection the final determination of the representative ∝ does not give any problem, since the higher value has to be taken. If the two values are of the same order they must be combined with an appropriate method. It would be certainly wrong to add the two values. A better approach is the square addition:

α tot = α 2 free + α 2 forced It is also valid if either the free convection or the forced convection dominates. Fig. 10 is based on the above method. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.5 Effect of Thermal Improvements

6.5

Effect of Thermal Improvements

According to the actual condition of an existing kiln system (heat consumption, heat balance, other operating data) we can envisage an optimization campaign. From the thermal point of view we can take certain measures in order to reduce the specific heat consumption. For example: ♦ Better insulation of rotary kiln or preheater/precalciner in order to reduce radiation losses (except the sintering zone). © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:51 PM

Page 382

"Holderbank" - Cement Course 2000 ♦ Improvement of the cooler efficiency (optimization of grate cooler operation or installation of highly efficient internal equipment in planetary or rotary cooler). ♦ Reducing of false air inleaks at kiln seals or at preheater. ♦ Reducing of internal dust circulations in cooler, kiln or preheater (improves the internal counter-current heat transfer). ♦ Modification of raw mix in order to decrease the sintering temperature which in turn will also reduce heat losses. A further effect of such a measure can be the reduction of internal dust circulations due to better clinker granulometry (see above). ♦ Modification of raw mix in order to decrease the heat required for decarbonation, e.g. by making use of non-carbonatic CaO raw material sources. This possibility, however, is very rare and often not feasible. (Note the basic difference to the measures which tend to decrease the sintering temperature or increase the proportion of melting phase!) The above measures are just a few typical examples. When one goes into such items, an important phenomenon will appear soon: The so called „loss multiplication“ factor for thermal losses (or savings). What does this mean? If a saving (or loss) in the high temperature zone in a kiln of say 100 kJ occurs the possible gain in fuel heat consumption will not be 100 kJ but rather 130 to 150 kJ. That means that the primary saving (in terms of heat balance item) will be multiplied by a factor of up to 1.5. At the first glance the above principle seems to be contradicious because it would violate the principle of heat balance or the energy law. However, what really happens is a differential change of more than only one heat balance item. To illustrate this fact we start from an example where we have reduced the shell radiation losses by 100 kJ/kg cli. The following differential balance situation occurs: Fig. 15

The corresponding multiplication factor for the above case is: multiplication factor

=

- 140 kJ/kg - 100 kJ/kg

=

1.40

The above fact does result from the thermal behavior of the system and can be verified by simulation models (not by a simple balance only).


6/23/2001 - 4:36:51 PM

Page 383

"Holderbank" - Cement Course 2000 A factor in the 1.4-range is quite typical for the situation in the high temperature zone (above 800°C) of a cement kiln. The main effect of a change in this zone will be a corresponding change at the exhaust gas, but also other minor effects will occur (e.g. at cooler losses). A “differential balance“ of heat can still be applied according to:

Input Change

Output Change

fuel

radiation

=

- 100 kJ/kg

exhaust

=

- 40 kJ/kg

total

=

- 140 kJ/kg

total

=

=

- 140 kJ/kg

- 140 kJ/kg

The principle of loss multiplication does not only apply for radiation in the high temperature zone but also for the heat which is recuperated in the clinker cooler. Regarding the false air inleaks the corresponding deterioration of heat consumption is often under-estimated. If false air inleaks into the high temperature zone it does not only cause a heat loss because this air must be heated up to the exhaust gas temperature of the kiln system! It actually causes much more losses than what would result from such a simplified calculation approach. As a rule of thumb we may consider the following two main effects in order to come to a realistic result: ♦ Heating up of false air at the temperature of the high temperature zone which can be set approximately to 800°C (end of HT-zone). ♦ Multiplication of the above heat requirement by a loss multiplication factor. The above calculation is a rough approach. By more sophisticated simulation models we find e.g. that a false air inleak into the high temperature zone of 0.08 Nm3/kg cli can cause an additional heat consumption of 100 kJ/kg cli. In contrast, the inleak in the low temperature zone (e.g. air lift on SP-kiln) is much less critical. Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 6. SPECIAL PART / 6.6 Heat Transfer in Preheaters and Coolers and Improvement Potential

6.6

Heat Transfer in Preheaters and Coolers and Improvement Potential

Normally we are considering a cement kiln as an equipment for burning cement clinker. As an essential feature we must be able to generate a high sintering temperature in the order of 1450°C (material temperature). But a kiln system is of course much more than a generator of sintering temperature. It is also a system of heat exchangers which allows for burning at low heat consumption. Generally speaking we will find two essential heat-exchanging systems on any cement kiln: a) raw meal preheater b) clinker cooler Low heat consumption is only possible if the above two „heat exchangers“ perform optimally. How can we get optimum heat exchange performance? Form the basic theory it is known that even in © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:52 PM

Page 384

"Holderbank" - Cement Course 2000 case of an exchanger which is perfectly insulated against ambient temperature influence, three important conditions are required for optimum heat exchange: 1) Optimum heat transfer rate (here: from gas to solid) → high specific contact or surface area, high ∝ (W/m2C). 2) The two heat exchanging streams must flow in counter-current manner, or at least in an arrangement which has similar characteristics to a counter-current system (e.g. overall arrangement of a 4-stage cyclone preheater). 3) If we want to recover heat from a „flow 1“ completely into a „flow 2“ the „flow 2“ must have at least the same heat equivalence as „flow 1“: (flow 2) x (cp2) ≥ (flow 1) x (cp1)

[kW/C] or [kJ/kg cli C]

flow: [kg/s] or [kg/kg cli] cp: [kJ/kg C] In the above equation the cp values are considered as constant (approximation). Graphically this means that the heat characteristic curves of the two heat flows must be in a certain relationship as shown here:

What are the practical consequences for a cement kiln? We consider: A) Raw meal preheater B) Clinker cooler A)

Raw meal preheater

1)

Heat transfer rate: Optimum heat transfer rate and optimum specific surface (small particles) and distribution has been almost achieved in the cyclone suspension preheater. After every cyclone stage material and gas come to almost complete temperature approach and there is usually little to improve on that. Improvement are rather to be done where the heat exchange takes place in the rotary part itself. Especially on wet and long dry kilns the kiln internal fittings are essential for good heat exchange.

∗

∗

2) Counter-current principle: ∗ Counter-current flow in its proper sense does not exist in case of a cyclone suspension preheater. Instead, we have usually four co-current heat exchanging units, but the overall © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:52 PM

Page 385

"Holderbank" - Cement Course 2000 arrangement acts as counter-current system. To reach an ideal state one would have to apply an infinite number of cyclone stages. Practically the common arrangement of 4 or 5 stages can be considered as sufficient. ∗ True counter-current preheaters are shaft preheaters or preheating in long rotary kilns. Such preheaters would theoretically be ideal. Practically they are less efficient because of distribution problems and backmixing effect (internal material circulations) and comparatively high losses to outside (in case of a rotary kiln). 3) Equivalence of heat flow characteristics: ∗ A general feature of any raw meal preheater is the surplus of heat input by the hot gases. After the calcining step the hot gas has a temperature of approx. 850°C and the specific quantity is always above ∼ 1.3 Nm3/kg cli even in case of an optimum kiln system (4-stage SP kiln). This amount of heat is too much, it cannot be used completely for preheating of raw meal (∼ 1.56 kg RM/kg cli) up to calcining temperature (∼ 800°C). Therefore a certain amount of waste heat will occur even in an ideal case. Theoretically we should not exceed ∼ 1 Nm3/kg cli for ideal recuperation. Practically this cannot be realized, not even on modern kiln systems which produce comparatively little exhaust gas. ∗ Graphically we have the following heat situation in a raw meal preheater: Fig. 16

Above diagram is simplified but typical for any preheater. Because of the „heat surplus“ of the exhaust gas it is not possible to achieve an ideal recuperation even at perfect counter-current heat transfer (e.g. infinite number of cyclone stages). The exhaust gas will always give a certain residual heat content. Practically this means that all kind of improvements on the preheater have a limited potential. In the example of a 4-stage preheater kiln we can add a fifth stage which causes a reduction of about 100 kJ/kg cli in fuel heat consumption. More than 5 stages will bring only marginal economical point of view. *) Even at 5 stages we may check if the necessary investment and the (possible) increase of pressure drop can be justified by the local cost structure. *)

Instead of constructing more than 5 conventional cyclone stages one would prefer here „non-conventional“ preheaters, such as the cross-suspension-preheater (two strings with cross flow of raw meals). B)

Clinker Coolers

1) ∗

Heat transfer rate: On a planetary or a rotary cooler we have the possibility to increase the heat transfer rate by


6/23/2001 - 4:36:52 PM

Page 386

"Holderbank" - Cement Course 2000 installation of efficient internal equipment (tumblers, lifters) which increase the active heat transfer area by better moving and scattering of the clinker. If a cooler has worn out internal equipment or equipment of inadequate design we may realize a considerable potential for improvements. Improvements may also result from a more uniform clinker granulometry (less internal dust circulations). ∗ On a grate cooler we find quite a different situation. The real problem is not the heat transfer rate between a piece of clinker and the cooling air but rather the uniform air distribution through the clinker bed. Also here we may realize a considerable improvement (thick bed operation, mechanical modifications at inlet zone etc.). 2) Counter-current principle: ∗ There is an obvious difference between planetary/rotary coolers and the grate cooler: - planetary / rotary → counter-current flow - grate cooler → almost cross-current flow ∗ The grate cooler has a nearly cross-current performance and has therefore, from its principle, a limited heat recovery potential. Improvements are possible when air recirculation (of hot waste air) to the first grate section is applied in order to increase the heat content of the secondary air. Another quite different measure is the increasing of the bed thickness in order to come more towards a counter-current-like exchange (similar to a shaft cooler). The disadvantage is an increase of the cooling air pressure. 3) Equivalence of heat characteristic ∗ On a clinker cooler we would theoretically require approx. 0.77 Nm3/kg cli of cooling air in order to transfer the clinker heat completely to the secondary air (under perfect counter-current heat exchange). ∗ The practical figures are usually above 0.8 Nm3/kg cli. Compared to a raw meal preheater we have an inverse situation: The heat from the hot clinker could theoretically be completely recovered (under ideal conditions)! Of course, we know that the common, practical figures are often below 70%. The latter fact illustrates that from the basic principle there is still a considerable heat potential which is not used for reducing the kiln heat consumption. ∗ In this context it is also important to note that the practical efficiency of any type of clinker cooler increases with higher cooling air quantity. As a consequence we should draw as much cooling air as possible through the cooler and therefore avoid or reduce excessive primary air quantities or false air inleaks at the kiln hood or kiln seals. The improvement does not come from the heat transfer proper but rather from the improved „heat characteristic“ (air/clinker ratio). Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 7. TEST QUESTIONS

7.

TEST QUESTIONS

1) Give an example where it can be worthwhile to execute a complete heat balance on a kiln. 2) Summarize all important measurement points which are needed for doing a complete heat balance on a suspension preheater kiln. 3) What is the usual value (or range) for the specific heat (kJ/Nm3 C) of exit gas of an SP-kiln at 350°C? 4) What is the sensible heat content of 1 kg clinker at 1450°C, expressed as kJ/kg cli? 5) What is the standard value (or range) for heat of formation (kJ/kg cli) for clinker burning? Which heat effects are included in above value? © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:53 PM

Page 387

"Holderbank" - Cement Course 2000 6) Determine the heat transfer coefficient (W/m2C) for the total heat transfer by radiation plus free convection. The temperature of the kiln shell section is 200°C (ambient = 20°C, ε = 0.9). 7) When has the forced convection heat transfer (instead of free convection) to be considered? How is the above value affected in case of smaller dimensions (say 1 m instead of 5 m diameter)? 8) If the shell losses in the calcining zone can be reduced by 50 kJ/kg cli, what will be the approximate saving of fuel heat (kJ/kg cli)? 9) What is the approximate fuel heat which can be saved through the installation of a fifth cyclone stage on a suspension preheater kiln? What would be the approximate amount of false air reduction (Nm3/kg cli into the high temperature zone) in order to achieve a similar fuel heat saving? Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 8. LITERATURE

8.

LITERATURE

1) Zur Strassen, H. Der theoretische Wärmebedarf des Zementbrandes ZKG 10 (1957), Vol. 1, p. 1-12 1) Jakob, M. Heat transfer, Vol. I (1949), p. 529 2) Hilpert, R. Wärmeabgabe von geheizten Drähten und Rohren im Luftstrom Forsch.-Ing.-Wes., Vol. 4 (1939), p. 215-224 3) Gygi, H. Thermodynamics of the cement kiln, third industrial symposium on the chemistry of cement 4) Eigen, H. Beitrag zur Thermodynamik des Drehofens Tonindustrie-Zeitung 82 (1958), No. 16, p. 337-341 5) Frankenberger, R. Beitrag zur Berechnung des Wärmeübergangs in Zementdrehöfen Dissertation, Technische Universität Clausthal (1969) 6) Kühle, W. Untersuchung über die äussere Wärmeabgabe von Drehöfen durch Strahlung und Konvektion Zement-Kalk-Gips, Vol. 6, 1970, p. 263 7) VDZ Unterlagen „Wärmetechnische Berechnungen“ Verein Deutscher Zementwerke E.V., Düsseldorf 8) VDI-Wärematlas Berechnungsblätter für den Wäremübergang VDI-Verlag GmbH, Düsseldorf 9) Barin, I. and Knacke, O. Thermochemical properties of inorganic substances Springer Verlag, Berlin, Heidelberg, New York 10)

Elkajer, P. (FLS) Die Bestimmung des Wärmeverbrauches mit vierstufigem Zyklonvorwärmer durch Aufstellung


6/23/2001 - 4:36:53 PM

Page 388

"Holderbank" - Cement Course 2000 eines mathematischen Modelles Zement-Kalk-Gips, Vol. 2, 1980 11)

Gardeik, H.O. Berechnung des Wandwärmeverlustes von Drehöfen und Mühlen Zement-Kalk-Gips, Vol. 2, 1980

12)

Rother, W. Ausführung von Rohmehl-Wärmetauschern unter Berücksichtigung heutiger Kostenfaktoren Zement-Kalk-Gips, Vol. 2, 1982, p. 66 ff.

Process Technology / B05 - PT II / C09 - Heat Balance / Heat Balances of Kilns and Coolers and Related Topics / 9. SYMBOLS AND UNITS

9.

SYMBOLS AND UNITS

A

m2

area

CR

W/m2K4

radiation constant

cP

kJ/kg C

specific heat (at const. pressure),

or kJ/Nm3 C

specific heat capacity

CV

kJ/kg

calorific value

D

m

diameter

g

m/s2

gravity constant

h

kJ/kg

heat content (specific)

or kJ/Nm3 or kJ/kg cli L

m

length

m

kg

mass

or kg/kg

specific mass

mf

kg/h

mass flow

Qf

kW

heat flow (1 kW = 1 kJ/s)

t

C

temperature (Centigrade)

T

K

temperature (Kelvin)

v

m/s

velocity

w

kg/kg

water content

∝

W/m2K


ε

-

emissivity (for radiation)

λ

W/m C

heat conductivity

ρ

kg/m3

density

Greek Letters

Dimensionless Numbers Nu © Holderbank Management & Consulting, 2000 Query:

Nusselt number (for heat transfer) 6/23/2001 - 4:36:54 PM

Page 389

"Holderbank" - Cement Course 2000 Nu

Nusselt number (for heat transfer)

Pr

Prandtl number

R

kiln feed (raw meal) / clinker-ratio

Re

Reynolds number

Indices conv

convection

rad

radiation

tot

total

o

ambient condition or zero condition

Conversion Factors Length

1 inch

0.0254 m

1 ft

0.3048 m

Area

1 sq. ft

0.092903 m2

Volume, Volume Flow

1 cu.ft

0.028316 m3

1 cu.ft/min

1.699 m3/h (actual m3)

1 lb.

0.45359 kg

1 short ton (USA)

907.185 kg

Mass

Pressure Energy

Temperature Conversion Heat Flow

Specific Heat Heat Transfer Coeffic.

Standard Conditions for Gases

Nm 3 =act .m 3 ×

1 bar

105 N/m2

1 atm.

1.013 bar

1 kJ

1000 J

1 MJ

1000 kJ

1 kWh

3600 kJ

1 kcal

4.187 kJ

1 BTU

1.055 kJ

C=

5/9(F - 32)

K=

273.15 + C

1 kW

1000 W = 1 kJ/s

1 kcal/h

1.163 W

1 BTU/h

0.29307 W

1 kcal/kg C

4.187 kJ/kg C =

4187 J/kg C

1 BTU/lb F

1 kcal/kg C

4.187 kJ/kg C

1 kcal/m2h C

1.163 W/m2 C

1 BTU/ft2h F

5.678 W/m2C

Standard Conditions

0°C and 1 atm. (1.013 bar)

=

2.73.15 p(bar ) × 273.16 + t (c ) 1.013bar


6/23/2001 - 4:36:54 PM

Page 390



6/23/2001 - 4:36:55 PM

Page 391

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C10 - Main Fans

C10 - Main Fans


6/23/2001 - 4:36:55 PM

Page 392

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C10 - Main Fans / Main Fans

Main Fans Authors: W. Zeller, Th. Richner, D. Brassel PT 99/14500/E 1. Design and efficiency of fan impellers 1.1 Fan impeller types 1.2 Selection criteria 1.3 Fan applications in the cement industry 2. Fan performance curves 2.1 System Resistance Curve 2.2 Fan curves 2.2.1

Fan equations

2.2.2

Adjusting fan performance curves

3. Flow control 3.1 Damper control 3.2 Radial inlet vane 3.3 Speed control 3.3.1

Hydraulic transmission with fixed speed motor

3.3.2

Speed-controlled electric motors

4. Possible problems with fans 4.1 Vibrations 4.1.1

Variable speed operation

4.1.2

Thermal effects

4.1.3

Hot shutdowns

4.2 Material build-up 4.2.1

Kiln exhaust fan build-up

4.2.2

Recommendations against build-up

4.3 Erosion 4.3.1

Erosion types

4.3.2

Improper duct connection

4.3.3

Effects of impeller speed and wheel inlet velocity

4.4 Wear protection 4.4.1

Protection of parts subjected to abrasion


6/23/2001 - 4:36:55 PM

Page 393


Deflection of abrasive particles

4.4.3

Liner materials

4.5 Bearings 5. Fan Capacity Adjustment 5.1 Fan capacity too low 5.2 Fan capacity too high 6. Troubleshooting 7. Start-up of fans 8. Fan impeller arrangement and connections 8.1 Assembly 8.1.1

Overhung assembly (Fig. 8.1)

8.1.2

Center hung assembly (Fig. 8.2)

8.2 Foundations 8.3 Connections 8.3.1

Inlet connections

8.3.2

Outlet connections

9. Information Sources

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 1. DESIGN AND EFFICIENCY OF FAN IMPELLERS

1.

DESIGN AND EFFICIENCY OF FAN IMPELLERS

Fans are essential components of the cement manufacturing process and merit therefore particular considerations with respect to •

design and efficiency of the impeller

•

fan size and operating point

•

flow control

•

reliability

Main fans in a Cement Plant are found as •

Kiln ID Fan

•

Raw Mill Fan

•

Cooler Exhaust Gas Fan

•

Kiln Dedusting Fan

Altogether these fans consume between 30 and 50% of the plants total electrical energy. Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 1. DESIGN AND EFFICIENCY OF FAN IMPELLERS / 1.1 Fan impeller types

1.1

Fan impeller types


6/23/2001 - 4:36:55 PM

Page 394

"Holderbank" - Cement Course 2000 There are four basic blade forms used in industrial induced draft service: •

backward airfoil blades

•

backward curved blades

•

backward inclined blades

•

radial straight blades

Fig. 1.1 shows design and efficiency of these impellers.

Fig. 1.1:

Design and efficiency of impellers

TYPE

backward airfoil blades

backward curved blades


EFFICIENCY η

> 84 %

up to 82 %

6/23/2001 - 4:36:56 PM

APPLICATION

for clean gas applications (dust < 50 g/m3)

for gas with a dust concentration < 100 g/m3

Page 395


Backward inclined blades

up to 80 %

for gas with a dustload up to 100 g/m3

radial straight blades

60 - 75 %

for gas with a high dustload (dust > 100 g/m3)

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 1. DESIGN AND EFFICIENCY OF FAN IMPELLERS / 1.2 Selection criteria

1.2

Selection criteria

It is of great importance that critical process equipment, such as fans, is selected on the basis of proven ability in order to provide maximum reliability rather than on an efficiency rating. In situations where more than one blade form will meet a performance requirement, it then becomes necessary to select the one form that will be most overall cost-effective. For the selection process the supplier should provide the operating and service manual for the equipment type being considered. The operating and service report should include all occurrences that require fan maintenance. To assist in fan type selection, there are at least four important points to be considered. ♦ Fan efficiency: Because many of the higher efficiency fans can only achieve their stated efficiency within a narrow operating range, a true energy evaluation must consider the actual operating point and alternate © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:56 PM

Page 396

"Holderbank" - Cement Course 2000 operating points on a time basis. Many systems include a built-in safety factor, which results in reduced efficiency when operated at constant speed with damper regulation. ♦ Continuous operation: Any type of equipment will require maintenance. In blade form selection, blade build-up (cp. section 4.2) and erosion (cp. section 4.3) have the greatest effect on fan operation. Build-up on the wheel results in reduced performance. There is an increased tendency for material to build up on blades as the blade angle is tilted back from radial. This build-up can accumulate to the degree that it restricts and alters air passages, reducing both efficiency and performance. ♦ Mechanical design: All fan rotors are subject to centrifugal force. Depending on blade form and angle, different types of stress occur in a blade. The radial Blade is in tension, while bending and tensile stresses act on the backward inclined/Airfoil design. Bending stresses are more subject to fatigue stresses. ♦ Equipment costs Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 1. DESIGN AND EFFICIENCY OF FAN IMPELLERS / 1.3 Fan applications in the cement industry

1.3

Fan applications in the cement industry

The following table shows an overview of fan applications in the cement industry. Location

Dust load [g/Nm3]

Coal Mill < 0.15 Filter exhaust Separator / cyclone < 100 exhaust

Blades mainly used

Max. Temp [°C]

max. speed [rpm]

Flow regulation

Rotor protection

Stator protection

Efficiency

F/C/A

150

1800

VC/ILD

(WP)

-

70 - 85

R/F/C

150

1200

ILD

HSWP

WP( HSWP)

55 - 75

[%]

Raw Mill Filter exhaust

< 0.15

F/C/A*

300

1800

VC/ILD/VS

-

-

70 - 85

Separator / cyclone exhaust

< 100

F/C

300

1200

ILD/VS

(HW/HWSP)

WP

65 - 75

Filter exhaust

< 0.15

F/C/A

200

1800

VC/ILD

-

-

75 - 85

Separator / cyclone exhaust

< 100

F/C

200

1200

ILD

(WP/HSWP)

WP

65 - 75

Preheater exhaust

< 100

R/F/C

450

1200

ILD/VS

(WP)

-

55 - 75

Kiln line filter exhaust

< 0.15

F/C/A*

350

1200

VC/ILD/VS

-

-

70 - 85

Recirculation fan

< 20

R/F

450

750

ILD/VS

HSWP

WP

60 - 70

Kiln line filter exhaust

<0.15

F/C/A*

450

1800

VC/ILD/VS

-

-

70 - 85

Filter exhaust

<0.15

F/C

450

1200

VC/ILD/VS

(WP)

-

70 - 88

Filter exhaust

0.5

F/C

450

1200

ILD/VS

WP

(WP)

70 - 80

Cyclone exhaust

<5

R/F

450

1200

ILD/VS

HSWP

WP

65 - 75

Recirculating

< 15

R

450

750

ILD

HSWP

WP(HSWP)

60 - 70

Cooler F.D. fans

Traces

F/C/A

50

2200

VC/VS

-

-

75 - 85

Cement Mill

Dry Process

Semi-dry process

Cooler

* in case of bag filter


6/23/2001 - 4:36:57 PM

Page 397

"Holderbank" - Cement Course 2000 Abbreviations: a.

Blade Type

b.

R:

radial blade

F:

backward inclined flat plate

C:

backward curved plate

A:

backward airfoil blade

Flow regulation (cp. section 3)

c.

VC:

Vane control set at the fan inlet

ILD:

Inlet louver damper set on the inlet box

VS:

Variable speed

Rotor and stator protection against abrasion (cp. section 0) WP:

bolted or welded wear plates - mild steel - quenched steel - wear-resistant steel

HSWP:

bolted mild steel wear plates with hard surfacing - chromium carbide - tungsten carbide

(WP/HSWP): optional

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 2. FAN PERFORMANCE CURVES

2.

FAN PERFORMANCE CURVES

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 2. FAN PERFORMANCE CURVES / 2.1 System Resistance Curve

2.1

System Resistance Curve

♦ The System Resistance Curve is the relation between pressure and volume flow for the given system. For a system in which only air is moved and the geometry of the ductwork is constant, the pressure drop is proportional to the square of the flow rate. ♦ In most of the applications around the cement plant, this curve is constantly changing because of the changing system conditions. For instance, the cooler under-grate fan system resistance curve depends not only on the geometry of the ductwork and cooler, but also on size distribution and thickness of the clinker bed on the grate above the compartment. ♦ The system resistance curve of a roller mill system is dependent on the geometry of the ducts, the amount of material being transported by the gas, the composition of the gases and the speed of the classifier. Because of these variables, it is essential to understand that a system resistance curve can vary significantly from the slope of the curve illustrated in e.g. Fig. 2.3. ♦ Principally the curve is derived as described next. Fig 2.1 schematically shows a system. Fig. 2.1:

System with fan


6/23/2001 - 4:36:58 PM

Page 398


♦ Total pressure (dynamic and static part) at S:

psuc = ps − ∆ps with ∆ps = pressure loss due to friction of the suction side &2 = fV

( )

♦ Total pressure (dynamic and static part) at P:

ppress = pp − ∆pp with ∆pp = pressure loss due to friction of the pressure side f V& 2 =

♦

( )

Pressure difference ∆pfan, which has to be produced by the fan, consists of a static and dynamic part and is a function of the gas velocity or gas flow.

( )

2 ∆p = ppress - psuc = = ∆pstat + ∆pdyn = f(w2) = f V&

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 2. FAN PERFORMANCE CURVES / 2.2 Fan curves

2.2

Fan curves

The fan performance curve is derived from laboratory data when the flow conditions to and from the fan are ideal. Since these conditions seldom exist in cement plant fan locations. The fan curve data cannot be considered reliable when applied to field conditions. Because of this uncertainty of the prevailing conditions, we find that engineers specify oversized fans during the plant design stage. For example, typical factors that can effect the performance of an under-grate cooler fan include a dirty inlet screen; a structural column too close to the inlet; a silencer at fan inlet or the influence of an adjacent fan. Likewise, similar factors influencing an induced-draft fan include asymmetrical inlet duct which effects streamline distribution to the fan or heavier than normal blade wear pads that restrict blade passage geometry. A typical fan curve (see Fig. 2.2) shows the quantity of air on the horizontal axis and the fan static pressure and fan power plotted on the vertical axis. The conditions of density and flow are at the inlet of the fan. The actual operating point of the fan will be the intersection of the fan curve with the system resistance curve (cp. section 2.1). ♦

The efficiency of a fan is best close to its design point of operation. The farther off these “ideal” conditions, the lower the efficiency. The operating points of equal fan


6/23/2001 - 4:36:58 PM

Page 399

"Holderbank" - Cement Course 2000 efficiency are located on oval shaped curves around the maximum point. Fig. 2.2:

Fan characteristics and efficiencies by different fan speed

The fan curve depends on the impeller speed and the physical properties of the gas (cp. section 2.2.2) The pressure which is produced by the fan equals the pressure difference between the total pressure at the fan outlet and the fan inlet: ∆p = ppress - psuc = = ∆pstat + ∆pdyn For most fans in cement plants, it is satisfactory to assume that the fan's pressure is the difference between the static pressure at the inlet and outlet of the fans. The fan power is the power at the fan shaft. For most plant applications where the fan has constant speed and is direct coupled, this can be assumed to be 96 % of the power consumed by the drive motor when the motor is fully loaded. Since power factor and motor efficiency vary with motor load, it is difficult to measure fan shaft power input without a kilowatt meter. However, near full motor load conditions, the amp reading is a good indicator of shaft power. For fans with variable speed drives, especially hydraulic or magnetic clutch drives, the efficiency of the drive can be very poor so the power at the fan shaft cannot be assumed to be a linear function of the power consumed by the motor. Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 2. FAN PERFORMANCE CURVES / 2.2 Fan curves / 2.2.1 Fan equations

2.2.1

Fan equations

Power requirement:

N=

V& ∆p 10 −3 η [kW]

N

: Required power

[kW]

V&

: Gas flow

[m3/s] (not [Nm3/s])

∆p : Total increase of pressure in fan

[Pa]

η

[-]

: Fan efficiency

Depending on the blade shape of the impeller, the power requirement to draw the gas through a given system (and therefore for determined pressure losses) can vary in a relatively wide range. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:36:59 PM

Page 400

"Holderbank" - Cement Course 2000 As the required fan power is given by the equation above, it is obvious that considerable savings can be achieved with the most efficient impeller. Total pressure increase: Formula for a rough calculation of the total pressure ∆p = ppress - psuc = ∆pstat + ∆pdyn:

0 .6 ρ n 2 d 2 π 2 ∆p ≈ [Pa] 3600 ρ

: Density

[kg/m3]

n

: Fan speed

[rpm]

d

: Impeller diameter

[m]

Remark: ∆p is just a rough guideline and depends very much on the blades shape and the rotor efficiency. Example: Effect of fan efficiency Plant: 4-stage SP kiln, 3000 t/d clinker Requirement for kiln ID-fan: V& = 115 m3/s at 350°C ∆p = 6000 Pa Ù two efficiencies available: η1 = 0.75 η2 = 0.85

V& ∆p 10 −3 115 ⋅ 6000 ⋅ 10 −3 = = 920 kW η1 0.75 V& ∆p 10 −3 115 ⋅ 6000 ⋅ 10 −3 N2 = = = 812 kW η2 0.85 N1 =

Power saving

= 108 kW = 0.86 kWh/t cli

Assuming an operation time of 7500 h/year and an energy price of 0.05 US$/kWh the yearly saving will amount to 40’500 US$!

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 2. FAN PERFORMANCE CURVES / 2.2 Fan curves / 2.2.2 Adjusting fan performance curves

2.2.2

Adjusting fan performance curves

It was mentioned above that the fan performance curve is given at specific conditions of density and fan speed. Most often, it is necessary to correct the fan curve for density and speed other than the predicted conditions. Fig. 2.3 shows the influence of density changes on the fan performance. Indices: 1: reference conditions (or original fan curve characteristic) 2: actual conditions Gas density correction:


6/23/2001 - 4:37:00 PM

Page 401

"Holderbank" - Cement Course 2000 A correction of the density is necessary if: ♦ temperature changes ♦ the chemical composition of gas changes ♦ the altitude changes (height above sea level).

pstat 1 ρ T = 1 = 2 pstat 2 ρ2 T1

Volume flow and efficiency of the fan are unaffected by gas density changes. Remark:

À Calculation of actual density ρ=

M p 273 ⋅ ⋅ 22.4 1013 T + 273

ρ

density

[kg/m3]

M

molecular weight of gas

[kg/kmol]

p

actual pressure

[mbar]

T

actual temperature

[°C]

Á Ambient pressure, depending on the altitudes can be calculated by: p = 1013 ⋅ exp[− 0.001255 ⋅ h]

h

altitude above sea level

[m]

Gas

Density [kg/Nm3]

Molecular weight [kg/kmol]

O2

1.429

32

CO2

1.964

44

N2

1.250

28

Air

1.292

29

H2O

0.804

18

Fan speed correction:

Volume flow:

n  V&2 = V&1 ⋅  2   n1 


6/23/2001 - 4:37:00 PM

Page 402


pstat 2 Static pressure:

Power requirement:

n  = pstat 1 ⋅  2   n1 

n  N2 = N1 ⋅  2   n1 

2

3

(for η1 = η2) Fan wheel dimensions correction:

d  V&2 = V&1 ⋅  2   d1 

Volume flow:


Power requirement:

3

b  ⋅  2   b1 

d  = pstat 1 ⋅  2   d1 

d  N2 = N1 ⋅  2   d1 

5

2

b  ⋅  2   b1 

whereas

d b

impeller diameter impeller width

[m] [m]

Fig. 2.3:

Influence of density changes on system resistance and fan performance curves

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 3. FLOW CONTROL

3.

FLOW CONTROL

Whether a fan ever will operate at a high efficiency is to a large extent already decided when the fan size is selected. Since the efficiency of the fan has a maximum at a specific operating point the fan ought to be sized so that it can operate at such conditions most of the time. However, since the fan performance needs to © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:37:01 PM

Page 403

"Holderbank" - Cement Course 2000 be flexible to meet variable requirements, the fan size will, to some extent at least, be designed for the maximum requirement. Reduced requirements have to be met by fan control dampers or variable speed control. Besides the efficiency of the fan itself, the type of flow control has the strongest influence on the overall power consumption. Damper control generally results in higher power consumption and so more if the damper is installed at the fan outlet. Therefore the damper should always be installed at fan inlet. Inlet vane control may be satisfactory from 100 % down to about 70 % of maximum flow, but the power demand becomes high when the flow is reduced further. Speed control is virtually ideal. Fig 3.1 shows different arrangements for inlet and outlet dampers. Fig. 3.1:

Design possibilities for fan dampers

Figure 3.2 shows how fan power consumption is affected by the different flow regulation methods. Fig. 3.2:

Comparison: Types of Flow Regulation

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 3. FLOW CONTROL / 3.1 Damper control


6/23/2001 - 4:37:02 PM

Page 404


Damper control

A parallel blade inlet damper is preferred over either outlet damper or an opposed-blade inlet damper. The parallel-blade inlet damper pre-spins the incoming air in the direction of wheel rotation, resulting in lower energy consumption in the regulation range of 100 - 80% of maximum flow. Fig. 3.3 shows the parallel inlet damper arrangement. Fig. 3.3:

Parallel inlet box damper

The inlet box damper influences the fan curve (see Fig. 3.4). Fig. 3.4:

Flow regulation by inlet box damper

Fig. 3.5 shows the operating point at the intersection of the system resistance curve and the fan curve. By reducing the airflow, the fan theoretically produces a pressure above 80 mbar. Since the system behaves like the system resistance curve, the damper induces a pressure loss of about 40 mbar. Fig. 3.5:

Flow regulation by outlet damper


6/23/2001 - 4:37:04 PM

Page 405


Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 3. FLOW CONTROL / 3.2 Radial inlet vane

3.2

Radial inlet vane

The radial inlet vane mounted direct at the fan inlet pre-spins the incoming air still better in the direction of wheel rotation, resulting in a wider range of stable regulation (100 - 70%) and less energy consumption. Fig. 3.6 shows the parallel inlet damper arrangement. Fig 3.6:

Radial inlet vane

Their use is mainly recommended in connection with over hung arrangement fans with low rates of dust, thus limited to applications after filters or in clean air, e.g. for cooler under-grate fans. This arrangement is normally more costly. Fig. 3.7:

Flow regulation by inlet vane damper


6/23/2001 - 4:37:04 PM

Page 406


Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 3. FLOW CONTROL / 3.3 Speed control

3.3

Speed control

Variable speed controlled fans have more fan characteristic curves but only one characteristic curve of the duct system (see Fig. 3.8). It is obvious that the variable speed drive is the most efficient type of fan control since no additional resistance for e.g. damper is built into the system. On the other hand a variable speed control is more capital intensive than a damper control. Fig. 3.8:

Flow regulation by speed control

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 3. FLOW CONTROL / 3.3 Speed control / 3.3.1 Hydraulic transmission with fixed speed motor

3.3.1

Hydraulic transmission with fixed speed motor

Hydraulic transmission in connection with a fixed speed motor can be an option for speed ranges from 100% down to 85% of maximum speed, but the energy efficiency becomes low when the flow is further reduced. Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 3. FLOW CONTROL / 3.3 Speed control / 3.3.2 Speed-controlled electric motors


6/23/2001 - 4:37:05 PM

Page 407


Speed-controlled electric motors

Flow control by variation of the fan speed is most efficient with regard to energy savings and permits also the reduction of wear on the fan wheel. ♦ DC motors have limitations in high speed and power (roughly 1000 rpm for 1500 kW, 600 to 800 rpm for 2000 kW motor) and require a lot of maintenance work (motor ventilation circuits, carbon brushes etc.), so their use is not recommended any longer. ♦ AC motors with slip recovery. These motors have a limited speed range down from 100 - 30%. Its cost increases with the width of the range. ♦ Synchronous motors with AC variable frequency control Both types of AC motors are well suited for high power (500 to 5000 kW). Fan equations at variable speed:

n  V&2 = V&1 ⋅  2   n1 

Volume flow:


Power requirement:

n  = pstat 1 ⋅  2   n1 

n  N2 = N1 ⋅  2   n1 

2

3

(for η1 = η2) Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS

4.

POSSIBLE PROBLEMS WITH FANS

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.1 Vibrations

4.1

Vibrations

Main reasons for fan vibrations are build-ups of process material (refer to section 4.2) and wear/erosion on the fan impeller (refer to section 4.3/0). A moderate level of vibration can be tolerated from a mechanical design point of view, although it certainly reduces the bearing lifetime to a certain extent. Therefore it should be tolerated only to avoid additional main equipment downtime. If vibrations are excessive, fan impeller balancing, cleaning or replacement is required. For balancing special vibration detectors are used on site or the wheel has to be shop balanced. General Machinery vibration severity as per ANSI S2.41 (Fig. 4.1 a) for use as a guide in judging vibration as a warning of impending trouble) 1.

Rigid support The fundamental natural frequency of the machine/support system is higher than the operating speed


6/23/2001 - 4:37:05 PM

Page 408

"Holderbank" - Cement Coursespeed 2000 than the operating

2.

excellent

0.

to

2.54

mm/s vibration velocity (Peak)

good

2.55

to

6.35

mm/s

alarm

6.36

to

12.7

mm/s

shutdown

>

12.7

mm/s

Flexible support The fundamental natural frequency of the machine/support system is lower than the operating speed excellent

0.

to

3.81


good

3.81

to

10.16

mm/s

alarm

10.17

to

19.1

mm/s

shutdown

>

19.1

mm/s

Vibration severity criteria (10 Hz to 1 kHz) per ISO 2372 (Fig. 4.1 b) 1.

Large machines with rigid foundations whose natural frequency exceeds machine speed good

0.0

to

2.54


allowable

2.55

to

6.35

mm/s

just tolerable

6.36

to

15.84

mm/s

>

15.84

mm/s

not permissible 2.

Large machines operating at speeds above foundation natural frequency good

0

to

4


allowable

4

to

10

mm/s

just tolerable

10

to

25.4

mm/s

>

25.4

mm/s

not permissible

If the tolerable vibration levels are exceeded, the fan must be shut down, which usually results in costly production losses. To extend the periods between shutdowns due to vibration, the use of automatic balancing devices, mounted on the fan shaft may be considered. Depending on their size, they are capable of automatic compensation of a certain rotor unbalance. For manufacturers of such devices, refer to the information source at the end of this report. Fig. 4.1 a:

Vibration severity chart (ANSI S2.41)


6/23/2001 - 4:37:06 PM

Page 409


Fig. 4.1 b:

Vibration severity chart (ISO 2372)

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.1 Vibrations / 4.1.1 Variable speed operation

4.1.1

Variable speed operation

All electrical variable speed drive systems can generate harmful harmonics that result in torque pulsation. Such harmonics can be predicted and filtered, but often at high cost. For fans with variable speed control it must be verified by the motor supplier that the pulsating torque do not cause excessive vibrations. Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.1 Vibrations / 4.1.2 Thermal effects

4.1.2

Thermal effects

Some typical problem areas are: ♦ Due to expansion joint problems, forces due to thermal expansion of ducts are transmitted to the fan housing, resulting in damage of the housing or interference between wheel and housing. It can also cause excessive force on foundation bolts that sometimes can result in foundation cracks. ♦ Rapid temperature changes in a system require proper design of the wheel-to-shaft fit. This will assure that looseness and resulting vibration sensitivity will be avoided.


6/23/2001 - 4:37:06 PM

Page 410

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.1 Vibrations / 4.1.3 Hot shutdowns

4.1.3

Hot shutdowns

Thermal shaft set is a usual concern on centre-hung fans above 120°C when shut down in the hot condition. It is generally agreed that this thermal bowing of the shaft occurs due to uneven thermal gradients across the wheel and shaft assembly at hot shutdown. This results in small asymmetrical distortions, often sufficient to cause excessive unbalance forces during start-up. In some cases the resulting vibrations are within acceptable limits. If so, it is usually found that the unbalance forces disappear after 12 to 36 hours of operation. The preferred, but expensive solution is to have an auxiliary drive to slowly rotate the wheel and shaft assembly, which should be engaged immediately after hot shut-down to avoid the undesirable thermal distortion. Auxiliary drives are typically designed to maintain a minimum speed (40 to 60 rpm) as the fan slows down. They are not intended for use in starting the fan rotor from a dead stop. Most hot gas fans work well without auxiliary drive, since hot shutdowns are infrequent and of short duration and the natural draft of the chimney keeps the rotor in slow motion for some time. The tendency is to install hot gas fans without auxiliary drives. Often it is also the case that variable speed drives have a turndown ratio of 10:1, which corresponds normally to less than 70 rpm. Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.2 Material build-up

4.2

Material build-up

A well maintained fan is generally a reliable piece of equipment. However, in case of material build-ups within the fan, serious operational limitations could occur, such as vibrations. Possible reasons for material build-ups are: •

Mineralogical composition of the material

•

Burning conditions

•

Duct/fan arrangement

•

High temperatures

•

Inappropriate blade shape and angles

•

High fan speed

•

Dew point problems

After balancing, the rotor of a fan will still have a certain residual imbalance, the value of which depends on the balance quality grade. The unbalance force can be calculated with the formula:

F = m ⋅ ω 2 ⋅ eper [N] m ω n eper

rotor mass angular velocity 2πn/60 rotor speed permissible residual specific unbalance

[kg] [s-1] [rpm] [m]

Industrial fans often handle dust-laden gases and thus dust deposition on the impeller may occur, increasing the initial unbalance. Asymmetrical wear on the impeller has the same effect. These © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:37:07 PM

Page 411

"Holderbank" - Cement Course 2000 additional unbalances can be considerable compared to the permissible unbalance according to the balance quality grade. If the resulting vibration exceeds the tolerable limits, the fan has to be shut down to clean the wheel. See Fig. 4.1 c for permissible unbalance. Fig. 4.1 c:

Balance quality grades

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.2 Material build-up / 4.2.1 Kiln exhaust fan build-up

4.2.1

Kiln exhaust fan build-up

One idea about the causes of build-up is that some particles are "sticky" at temperatures above 300°C and begin to build up on the rotor surfaces. The impact energy of the particles striking the rotor surface (especially at an angle of 90°) is also converted to heat and results in additional softening. Other particles with a higher softening point are caught in the sticky material and increase the coating layer thickness. The originally soft build-up gets harder under the influence of heat and pressure (from centrifugal force and the impact of other particles). Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.2 Material build-up / 4.2.2 Recommendations against build-up

4.2.2

Recommendations against build-up

♦ The fan rotor should be designed for the smoothest possible flow lines to reduce the impact energy of dust particles. Backward curved and airfoil are the best blade forms. Airfoil blades must be designed carefully to prevent material from getting inside and regular inspection of the fan blades is mandatory. Backward curved blades must be inclined enough to prevent the "hard" build-up on the front surface and radial enough to prevent the "soft" build-up on the back-surface. ♦ Fans should be designed for low gas and particle velocity at the fan inlet. This reduces the impact energy of particles against the rotor and can be achieved by 1) Double inlet instead of single inlet fans 2) Larger diameter / lower speed fans 3) If possible, the peripheral speed at the rotor inlet opening should be limited to about 76 m/s, and wheel inlet velocity should not exceed 38 m/s 4) The fan shaft should be oversized to reduce unbalance effects. The design critical speed (considering a bearing oil film thickness and a build-up thickness of 25 mm on all leading surfaces of the rotor blades) should be at least 1.25 times the operating speed of the fan. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:37:07 PM

Page 412

"Holderbank" - Cement Course 2000 Suppliers generally do not recommend to spray water directly onto a hot fan wheel, but in practice it has been done over years with success. The rapid cooling effect removes build-up effectively, however, the strength properties of the wheel material may be affected adversely if the injection rate is too high. Several different techniques are used to clean the fan, none with guarantied success however: ∗ “Sand blasting” by addition of sand to the gas flow for short periods (Fig. 4.2) ∗ Acoustic resonance (Fig. 4.3) ∗ Compressed air injection (Fig. 4.4) ∗ Steam injection (Fig. 4.4) The following measures are performed below operational fan speed or even require a fan stop. ∗ “Knocking” with pneumatic hammer ∗ High pressure water spray (up to 300 bar) ∗ H2O injection (Fig. 4.4) (rather delicate; it has to be carefully dealt with) Fig. 4.2:

Sand cleaning device

Fig. 4.3:

Acoustic cleaning device

Fig. 4.4:

Compressed air, H2O or steam Cleaning Devices


6/23/2001 - 4:37:07 PM

Page 413


Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.3 Erosion

4.3

Erosion

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.3 Erosion / 4.3.1 Erosion types

4.3.1

Erosion types

It has been established that maximum erosion occurs when the angle of incidence between particle and surface is between 20 and 40 degrees. This erosion, referred to as "ductile", is thought to be the consequence of microscopic melting, which occurs when sharp-edged, hard particles scratch the surface. Particles striking at an angle of about 90 degrees to the surface erode according to another mechanism. The kinetic energy of the particle creates a stress at the contact surface that can exceed the elastic limit of the material, thus forming a surface crack. This type of erosion is called "fragile". Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.3 Erosion / 4.3.2 Improper duct connection

4.3.2

Improper duct connection

Figure 4.5a shows a duct arrangement encountered on a raw mill system. Due to the uneven material distribution one side of the double-inlet impeller wore out much faster. Figure 4.5b shows the recommended modification. Fig. 4.5a, Fig. 4.5b


6/23/2001 - 4:37:08 PM

Page 414


Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.3 Erosion / 4.3.3 Effects of impeller speed and wheel inlet velocity

4.3.3

Effects of impeller speed and wheel inlet velocity

As mentioned earlier, erosion is proportional to the square of wheel inlet velocity and to the second to third power of the relative gas velocity w1 at the rotor inlet. With a given rotor size this velocity is proportional to rotor speed [rpm]. As a rough guideline we can expect that the erosion will vary with the speed to the power of 3. A large fan will therefore resist erosion better then a smaller one, operating in the same conditions, due to lower fan speed, larger surfaces.

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.4 Wear protection

4.4

Wear protection

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.4 Wear protection / 4.4.1 Protection of parts subjected to abrasion

4.4.1

Protection of parts subjected to abrasion

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.4 Wear protection / 4.4.1 Protection of parts subjected to abrasion / 4.4.1.1 Direct protection

4.4.1.1

Direct protection

♦ Increased thickness of wear parts This should only be done when abrasion is very light (ID behind electrostatic precipitators) ♦ Direct hard surfacing on impeller by deposits by electrode or fusion projection This process is very efficient if the deposits are well chosen and properly applied. However, the thickness of the deposit is limited. The base structure of the impeller can be affected (dilution of hard surfacing and significant addition of energy) and the mechanical characteristics of the steel can deteriorate, especially after several maintenance operations. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:37:08 PM

Page 415

"Holderbank" - Cement Course 2000 ♦ Glued-on ceramics Their hardness is very high and they are very resistant to ductile abrasion Temperature must be limited and the ceramics tend to come off at the blade end (high centrifugal forces). Application on static parts is easier and more reliable. Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.4 Wear protection / 4.4.1 Protection of parts subjected to abrasion / 4.4.1.2 Protection by wear plates

4.4.1.2

Protection by wear plates

♦ Corners or mild steel plates (or steel of the same grade as the base structure) added on to the blade and on the center plate. This protection is applicable only where abrasion is low. If the protection area is not wide enough, the base plate can also be attacked. ♦ Wear-plates in abrasion resistant steel. This process is applied particularly on flat-bladed impellers. The wear-plates are fitted on with countersunk screws. Replacement is easy, but the solution can be insufficient if abrasion is very high. ♦ Mild steel wear plates with hard surfacing by electrode or hardsurface spray coating. This solution has the double advantage of easy replacement and high resistance to abrasion when the type of deposit is well chosen. Moreover, hard surfacing on site is easily carried out with no risk for the base structure. Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.4 Wear protection / 4.4.2 Deflection of abrasive particles

4.4.2

Deflection of abrasive particles

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.4 Wear protection / 4.4.2 Deflection of abrasive particles / 4.4.2.1 Deflector plates on impeller (Fig. 4.6)

4.4.2.1

Deflector plates on impeller (Fig. 4.6)

Fig. 4.6a shows the fan inlet and the rotating impeller. A, B and C are flow lines for the gas and lines 1, 2 and 3 represent the trajectories for particles of varying size. Line 1 refers to a very small particle that closely follows the gas flow line. With increasing particle size the trajectories 2 and 3 deviate from the gas flow lines. The particles hit the back-plate of the impeller and erode it. Fig. 4.6b depicts the passage of two different particle sizes between two blades. Line 1 is the trajectory of a small particle, line 2 of a coarser one. Most particles will hit the blade during their passage through the impeller. With suitably located deflector plates (Fig. 4.6c), the particle trajectories can be altered so that no particles will hit the blades. The location and direction of the deflectors depends on the particle size distribution, density of gas and particle, and fan speed and size. Fig. 4.6:

Wear protection by particle deflection


6/23/2001 - 4:37:09 PM

Page 416


Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.4 Wear protection / 4.4.3 Liner materials

4.4.3

Liner materials

Due to the need for ductility in the structural members of the wheel, the wheel itself is usually not capable of high resistance to erosion. It is therefore necessary to install liners with a higher hardness classification to provide sufficient protection against wear. Material hardness is an indication of its resistance to erosion. A very good liner material is chromium carbide with an average hardness of 600 Brinell. Figure 4.7 shows erosion test results of nine different materials. Fig. 4.7:

Erosion Test on Fan (Solyvent-Ventec)

1) Alloy of tungsten and nickel-chromium. Flame spray coating. 2) Alloy of nickel, chromium and cobalt. Flame spray coating followed by fusion. 3) Alloy of tungsten, cobalt carbides and nickel-chromium. Flame spray coating followed by fusion. 4) Special chromium cast iron. Special electric arc welding. 5) Chromium cast iron with chromium carbides. Electric arc welding. 6) Chromium cast iron. Semi-automatic electric arc welding. 7) Ceramic powder containing basically aluminium oxide. Flame spray coating. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:37:09 PM

Page 417

"Holderbank" - Cement Course 2000 8) Same as 7, but of different hardness. 9) Ceramic tiles containing basically aluminium oxide. Glued to the blades. Carbon steel sheets with a protective layer of chromium carbide are normally available in standard sizes and various thicknesses. Cutting discs or cutting plasma can cut them to the necessary form and size. The liner fixation can be done by bolting or by welding the carbon steel base plate to the existing structure of the wheel or casing. Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 4. POSSIBLE PROBLEMS WITH FANS / 4.5 Bearings

4.5

Bearings

Bearing problems are usually caused by operating conditions and not by deficiencies in the bearings. By following regular operating and maintenance procedures, many bearing problems will be avoided. Most problems are due to hot bearings. For hints see section 5. Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 5. FAN CAPACITY ADJUSTMENT

5.

FAN CAPACITY ADJUSTMENT

Fan capacity adjustments in the field are practically limited to the speed adjustment and this within a limited range: ~ 0 - 15 % for flow increase and ~ 0 - 30 % for flow reduction. Other capacity adjustments require mainly dimensional modifications of the fan wheel, which have to be carried out in the workshop. Those are more costly and critical to execute. Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 5. FAN CAPACITY ADJUSTMENT / 5.1 Fan capacity too low

5.1

Fan capacity too low

Important variables that are hampered by insufficient gas flow are e.g. ♦ kiln production ♦ separator efficiency ♦ mill performance (throughput, drying capacity, mill venting) Possible remedies: ♦ Design changes in the system (reduce false air, reduce pressure drop) the fan consumes no additional energy! ♦ reduce/rectify false air inleaks ♦ reduce unnecessary pressure drops caused by e.g. •

not fully open dampers

•

improper design of inlet/outlet connections

•

no turning vanes in bends (refer to section 8.3)

•

dust deposits in ducts and bends

•

too narrow ducts (appropriate air speed must however be maintained to avoid settling of dust)

♦ Changes in fan design, speed © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:37:09 PM

Page 418


Speed increase ∗ volume flow is directly proportional to fan speed ∗ fan absorbed power rises with the third power of fan speed ∗ fan works with lower than design efficiency ∗ normally only possible with V-belt drives ∗ sound emission rises with fifth power of speed ∗ higher wear, if fan handles dust-laden gas ∗ increased sensitivity to rotor imbalance caused by dust deposits on blades ∗ speed increase is limited by mechanical strength of rotor ∗ speed increase is limited by fan critical and resonant speed

•

Fan wheel diameter increase ∗ practically limited to wheels where the blades do not extend to the full diameter of the rotor sideplates (plates can then be welded-in to increase the effective fan wheel diameter) ∗ volume flow rises with the third power of wheel diameter ∗ fan absorbed power rises with the fifth power of wheel diameter

•

Replacement of rotor with inherent low efficiency by one designed for high efficiency ∗ e.g. replacement of radial-blade wheel by backward-inclined or backward-curved blade wheel. ∗ limitations with regard to diameter, width (and speed, if wheel is directly coupled to motor via flexible coupling) of the new wheel exist if fan housing and motor are to be kept.

♦ New fan Can be designed for optimum performance at the new operating point Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 5. FAN CAPACITY ADJUSTMENT / 5.2 Fan capacity too high

5.2

Fan capacity too high

May be due to e.g. oversized fan, decline in production, process changes Possible remedies: ♦ Flow reduction by damper Widely used solution, but inefficient and expensive (fan energy consumption) ♦ Flow reduction by inlet vane damper More energy-efficient than damper, but can be recommended only for flow regulation purposes, not for permanent use at lower capacity (expensive, may be difficult to fit into existing equipment) ♦ Changes in fan design, speed •

Lower speed ∗ volume flow decreases proportional to fan speed ∗ absorbed power decreases with third power of fan speed ∗ lower sound emission ∗ lower wear, if handling dust-laden gas ∗ lower sensitivity to rotor imbalance caused by e.g. dust deposits on blades

•

Rotor width reduction ∗ applicable if fan must deliver the same pressure as before, but at lower volume flow

•

Rotor diameter / width reduction ∗ volume flow decreases with the third power of rotor diameter ∗ volume flow decreases proportional to rotor width


6/23/2001 - 4:37:10 PM

Page 419

"Holderbank" - Cement Course 2000 ∗ fan delivery pressure decreases with the square of rotor diameter ∗ absorbed power decreases with the fifth power of rotor diameter ∗ absorbed power decreases proportional to width ∗ lower sound emission •

To maintain a good efficiency, it is not sufficient to decrease the diameter only, but also to adapt the rotor width. For a permanent capacity reduction this is a good solution

If the old transmission is kept, speed reduction is normally reversible, i.e. fan capacity can be increased again if necessary, whereas a reduction in rotor diameter / width are not. Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 6. TROUBLESHOOTING

6.

TROUBLESHOOTING

Problem

Check for

Noise

1. 2. 3. 4. 5. 6. 7. 8. 9.

Squealing V-belts, due to misalignment or improper tensioning Defective bearings, or bearing seal rubbing Misalignment of bearing seal Misaligned housing-shaft seal Foreign matter in fan housing Rubbing of shaft seal, wheel to inlet piece, or wheel to housing Heat flinger is contacting guard Coupling failure Untreated expansion joints

Poor performance

1. 2. 3. 4.

Incorrect fan rotation Wheel is off-center, poor inlet piece fit-up allows recirculation of air Fan speed too low/high Poor duct design, installation of elbow or turning vanes could remedy problem Inlet damper installed backwards (counter-rotation) System resistance is excessive compared to design requirements (partially closed damper may be the cause) Density may be different from design density

5. 6. 7.


6/23/2001 - 4:37:10 PM

Page 420

"Holderbank" - Cement Course 2000 Problem High bearing temperature

Check for 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11 12. 13. 14.

Excessive starting time

1. 2. 3.

4. 5. 6.

Defective bearings caused by electrical arc due to improper grounding of nearby welding Over-lubrication Improper lubrication or contaminated lubricant Lack of lubrication, cooling fluid, or circulation High ambient temperatures or direct exposure to sunlight Misalignment Excessive thrust loading High vibration Inadvertently exchanged bearing caps (mismatched) Bearing race turning inside housing Moisture in bearing V-belts too tight Improper location ; not enough room for free axial movement of floating bearing in its housing at elevated temperatures) Heat flinger missing Motor improperly sized for fan wheel WR2 Inlet dampers not closed during start-up Properly selected time-delay starter/fusing required (many industrial fans take up to 20 - 25 seconds to reach operating speed) Temperature at inlet is excessively low (high density) Low voltage at motor terminals Inadequate system resistance


6/23/2001 - 4:37:11 PM

Page 421

"Holderbank" - Cement Course 2000 Vibrations

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Loose bolts in bearings and pedestals, or improper mounting Defective bearings Improper alignment of bearings or couplings Out-of-balance fan wheel Loose set-screws holding wheel to shaft Weld cracking Improper fan wheel clearance to inlet piece(s) Material build-up and/or wear on wheel Ensure expansion joints in ductwork are not fully compressed Misalignment or loose V-belts Improper wheel rotation Operation near system critical speed Shaft bent or distorted during high-temperature shutdown Defective motor Resonant frequencies of structural steel mounting Beat frequency with other fans on common base Loose hub-to-shaft fit

Duct pulsation 1. 2. 3.

Control volume with a radial inlet damper Install speed variation Change to a special "surgeless" blower design

High motor temperature

Improper ventilation of cooling air to motor (may be blocked by dirt) Input power problems (especially low voltage) High amperage High ambient temperature

1. 2. 3. 4.

Component

Problem

Probable cause, remedies

Bearings

Noise

1. 2. 3.

Imperfection in bearing elements Improper clearance Internal wear of bearing parts

Freezing water jacket

1.

When stopping water flow in freezing weather, blow out lower portion of bearing housing water cavity

Erosion

1. 2. 3. 4. 5. 6. 7.

Reduce dust loading Reduce rpm Redesign inlet ductwork Damper setting Damper design Better liner material Alternate blade design

Fan Wheel


6/23/2001 - 4:37:11 PM

Page 422

"Holderbank" - Cement Course 2000 Component

Problem

Probable cause, remedies

Buildup

1. 2. 3. 4.

Vibration

1. 2. 3. 4. 5. 6. 7. 8.

Shaft

Hubs

Cracks at section change

-

Reduce dust loading May be affected by system temperature change Blade form Alternate wheel material, apply "slippery" material Rectify build-up Rectify erosion Tighten foundation bolts Correct misalignments Improve supporting structure Check effects of ductwork thermal expansion Shaft bow due to "thermal set" Special considerations, refer to factory Get new shaft that is machined to eliminate stress raisers

Natural frequency too close to running value

Redesign shaft

Out of round at bearing -

Replace shaft

Bowing and torsion problems

Refer to factory

-

Shaft dropped or damaged during transit or installation

Get new shaft

Loose fit on shaft

-

Tighten interference fit

Insufficient stiffness

-

Redesign

Cracks in casting

-

Change to weldment

Erosion

-

Build up or replace

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 7. START-UP OF FANS

7.

START-UP OF FANS

Before starting the fan for the first time, complete the following list: 1) Uncouple motor from fan and check motor (fan) for proper rotation. 2) Shut off power by disconnecting motor main breaker. 3) Check and tighten hold-down bolts. 4) Check and tighten rotor set-screws. © Holderbank Management & Consulting, 2000 Query:

6/23/2001 - 4:37:11 PM

Page 423

"Holderbank" - Cement Course 2000 5) Check couplings and bearing for proper alignment. 6) Move rotor to see if it is rotating freely and maintains proper inlet piece/rotor clearance. 7) Check that fan wheel is balanced. 8) Check fan and ducts for any foreign material or dirt build-up. 9) Check that physical position of damper corresponds to indication at actuator and control panel. 10) Secure all access doors. 11) Check lubrication of bearings, couplings, drive unit etc. 12) Couple the motor again to fan and secure and check safety guards for clearance. 13) Close dampers for adequate system resistance to prevent drive unit from overloading. 14) Supply water to water-cooled bearings and start lubrication pump. 15) Make sure that all persons are away from fan and out of any other equipment of the system to which the fan is connected. 16) Connect electric motor by closing main circuit breaker of the motor. Start equipment according to recommendations of drive unit and starting equipment supplier. 17) Allow fan to reach full speed, then shut down. Make immediate corrections if any vibrations or unusual sounds have been detected. 18) During a run-in period make observations of bearings at least once an hour. Higher bearing temperatures may occur if bearings are over-lubricated. 19) Refer to trouble-shooting guide for any unusual occurrences encountered during the run-in period. Only after any vibrations, misalignments, etc. have been corrected, may the fan be restarted. Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 8. FAN IMPELLER ARRANGEMENT AND CONNECTIONS

8.

FAN IMPELLER ARRANGEMENT AND CONNECTIONS

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 8. FAN IMPELLER ARRANGEMENT AND CONNECTIONS / 8.1 Assembly

8.1

Assembly

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 8. FAN IMPELLER ARRANGEMENT AND CONNECTIONS / 8.1 Assembly / 8.1.1 Overhung assembly (Fig. 8.1)

8.1.1

Overhung assembly (Fig. 8.1)

This is possible if the fan operates in an environment with little risk of clogging and associated imbalance and the impeller diameter is not too large (less than approx. 2.7 m). Advantages: ♦ lower purchase and installation costs than centre hung assembly ♦ connection of upstream duct directly to fan inlet is possible (no inlet box) Fig. 8.1:

Fan arrangement - overhung arrangement


6/23/2001 - 4:37:12 PM

Page 424


Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 8. FAN IMPELLER ARRANGEMENT AND CONNECTIONS / 8.1 Assembly / 8.1.2 Center hung assembly (Fig. 8.2)

8.1.2

Center hung assembly (Fig. 8.2)

This assembly is more robust and absorbs the dynamic effects of rotor imbalance better than the above solution. Its use is recommended if ♦ the dust load is high with a risk of clogging or wear of the impeller, causing rotor imbalance ♦ when the size and weight of the impeller makes overhung assembly delicate Fig. 8.2:

Fan arrangement - arrangement between bearings

Figure 8.3 indicates the application range for the two arrangements. Fig. 8.3


6/23/2001 - 4:37:12 PM

Page 425


Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 8. FAN IMPELLER ARRANGEMENT AND CONNECTIONS / 8.2 Foundations

8.2

Foundations

Fig. 8.4

1) Directly onto concrete with separate motor-bearing base-plate (this is the most economical solution and most widely used). 2) Onto concrete with common base plate with centerline axis support maintaining the rotor-bearing-motor alignment with the stator (more expensive solution, but easy to install onto the foundations). Used for hot gas fans where thermal expansion must be taken into account. 3) Common base-plate with centerline axis support resting on anti-vibration mounts (this chassis must be perfectly rigid and is thus very expensive). Used only in very special cases (fans mounted on steel structures). 4) With spring-supported concrete block resting on anti-vibration mounts and supporting the fan (a spring supported block is less expensive than a common base-plate with centerline support and allows for remarkable vibration absorption. Its weight, between 10 - 60 tons must be included in design calculations). Used only in very special cases.


6/23/2001 - 4:37:13 PM

Page 426

"Holderbank" - Cement Course 2000 Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 8. FAN IMPELLER ARRANGEMENT AND CONNECTIONS / 8.3 Connections

8.3

Connections

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 8. FAN IMPELLER ARRANGEMENT AND CONNECTIONS / 8.3 Connections / 8.3.1 Inlet connections

8.3.1

Inlet connections

Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 8. FAN IMPELLER ARRANGEMENT AND CONNECTIONS / 8.3 Connections / 8.3.2 Outlet connections

8.3.2

Outlet connections


6/23/2001 - 4:37:13 PM

Page 427


Process Technology / B05 - PT II / C10 - Main Fans / Main Fans / 9. INFORMATION SOURCES

9.

INFORMATION SOURCES

♦ Robinson Industries, Inc., Zelienople, PA, USA ♦ TLT-Babcock, Inc., Akron, Ohio, USA ♦ Venti Oelde, Oelde, Germany ♦ Solyvent-Ventec, Chalon-Sur-Saône, Cedex, France ♦ Balance Dynamics Corporation, Ann Arbor, Michigan, USA, Fax # 313 994 3690


6/23/2001 - 4:37:13 PM

Page 428

Cement Industry Process Technology - Holderbank Course (3 of 3)

Recommend Documents