R E V I E W N O. 1 3 9
Petcoke Combustion Characteristics Reprint of article published in World World Cement, April 2002. By Gary Gary R. Roy Roy,, F.L.Sm .L.Smidth idth Inc. Inc.,, USA.
Gary R Roy, F.L.Smidth, USA, examines the way in which the combustion of petcoke has had an effect on modern calciner and burner designs.
Introduction Cement producers all over the world are striving to lower their production costs, one on e ef effe fect ctiv ive e me meth thod od of wh whic ich h is th the e substitution of traditional fuels such as coal, oil and natural gas with petcoke. However, as many producers have experienced, the use of petcoke as a fuel presents sen ts sev severa erall cha challe llenge ngess that mus mustt be addres add ressed. sed. Its hig high h sul sulfur fur con conten tentt can present pres ent ope operati rationa onall dif diffic ficult ulties ies if not properly addressed, and the fact that it burns at a much slower rate than normal coals means that for many existing kiln systems it is not possible to fire 100% petcoke in the kiln and calciner without special design considerations. Many Ma ny of th the e cl clas assi sica call FL FLS S ca calc lcin iner erss have hav e bee been n suc succes cessfu sfully lly ret retrof rofitt itted ed to enable ena ble pet petcok coke e fir firing ing,, and new pla plants nts specifi spec ificall cally y desi designe gned d to ena enable ble 100 100% % petcoke firing have recently been commissio mis sioned ned.. Thi Thiss art articl icle e hig highli hlight ghtss the equipment and the experience from several plants that have been designed or modified by F.L.Smidth to enable 100% petcoke firing. Petcoke as fuel Petroleum coke is the solid residue that remains after extraction of all valuable liquid liq uid and gas gaseou eouss com compon ponent entss fro from m crude oil. The volatile content range is typically 5 - 15%, depending on the coking process. The main difficulty in burning petcoke is its low reactivity due to this low volatile content. This low reactivity can be compensated for in a number of ways, such as finer grinding, high moment mom entum um rot rotary ary kil kiln n bur burner ner des design ign and calciner design. One tra tradit dition ional al solu solutio tion n is to gri grind nd the coke to a much finer residue than stan st anda dard rd co coal al.. Fi Figu gure re 1 sh show owss th the e
Figure 1. Recommended fineness for pulverised fuels.
Figure 2. Duoflex burner.
Reprinted from WORLD CEMENT April 2002
%O2
ppm CO
ppm NOx
Stage I
3.8
292
631
Stage IV
2.5
44
560
Loop duct outlet
3.2
150
560
Calciner outlet
3.8
765
530
Calciner ‘hot zone’
4.3
1352
540
Riser duct
0.6
32 000
615
Kiln exit
2.5
7
955
Figure 3. Operation of ILC with 100% petcoke firing.
relationship between the volati relationship volatile le content of the fuel and the required fineness to enable good combustion in the kiln and calciner. Most petcokes have a high sulfur content. High sulfur petcoke is sold at a low price but may require extra ext ra pre precau cautio tions ns in the cem cement ent kil kiln. n. Ope Operat ration ional al problems, due to the increased internal sulfur circulation ti on in th the e ki kiln ln sy syst stem em th that at re resu sult lts, s, ma may y of ofte ten n be solved by burner design and operational mode, or by returning dust to the upper end of the rotary kiln.
Ignition, combustion and burnout characteristics The initi initial al de-v de-volat olatilisa ilisation tion and combustion of the volatile matter in the coke particles is very fast, whereas the combustio combu stion n of the residual coke may then the n req requir uire e sev severa erall sec second ondss to com com-plete. The burning rate of an individual char particle depends primarily on its size (dp), the amount of oxygen present in the local atmosphere and the local temperature (T). In the very hig high h tem temper peratu atures res of the flame in the rotary kiln, the combustion busti on rate is determined determined by the diffusi fu sion on of th the e ox oxyg ygen en mo mole lecu cule less towards the surface of the shrinking char particle against the wind of CO and CO2. At these temperatures, the main mai n inf influe luence nce of the bur burnou noutt tim time e will be the size of the particle and the amou am ount nt of ox oxyg ygen en pr pres esen ent. t. A hi high gh mome mo ment ntum um bu burn rner er,, su such ch as th the e Duoflex kiln burner depicted in Figure 2, which provides a vigorous mixing of the particles with the local atmosphere, increases the probability that the char particle will encounter oxygen to facilitate the quick burnout. In the ca calci lciner ner,, th the e te tempe mpera ratur ture e cannot can not exc exceed eed the equ equili ilibri brium um tem tem-perature for calcination (850 - 900 ˚C), as long as a reasonable amount of carbonate bon ate is pre presen sent. t. At this moderate moderate temper tem peratu ature re lev level, el, eve even n a rel relati ativel vely y fine fi ne gr grou ound nd pe petc tcok oke e me meal al bu burn rnss so slowly that the heat produced is used immedi imm ediate ately ly for cal calcin cining ing.. The rea reacction rate is controlled by the speed in which whi ch oxy oxygen gen com combin bines es with car carbon bon on the par partic ticle le sur surfac face. e. The rea reacti ction on between the oxygen and the char particl ti cle e is th the e li limi miti ting ng fa fact ctor or,, an and d th the e burnout time will be roughly proportional to dp. It will therefore require a calciner with a considerable retention time ti me to co comp mple lete te th the e co comb mbus usti tion on process before the exit of the calciner. As the com combus busti tion on rat rate e dou double bless every time T is raised by 70 ˚C, it can be a great advantage to have a localised ‘hot zone’ in the calciner to promote faster combustion.
Petcoke firing firing in the in-line in-line calciner (ILC) (ILC) In an IL ILC C ca calc lcin iner er,, th the e ki kiln ln exh exhau aust st ga gase sess pa pass ss up through the riser duct and into the calciner vessel. Hot air from the cooler is brought to the calciner via an air duct to provide oxygen for combustion of the calciner fuel. As the initial average percentage of O2 in the calciner is considerably lower than 21%, and the average temperature is typically less than 900 ˚C,
Reprinted Rep rinted from W ORLD CEMENT April 2002
it is ve very ry di difffi ficu cult lt to bu burn rn a lo low w vo vola lati tile le fu fuel el.. Furthermore, Furthe rmore, most older ILC calciners on existi existing ng systems, designed for coal firing, have a gas retention time significantly less than 3 sec. Under these conditions, it is not possible to burn 100% petcoke unless ground to an extreme fineness, and then only if other measures are taken. FLS has use used d exp experi erienc ence e fro from m sev severa erall ILC pla plants nts that have been converted to petcoke firing. In these cases, a portion of the petcoke is introduced directly into the tertiary air duct before it has mixed with the kiln gas. As such, it is an advantage to have a high efficiency cooler so that the tertiary air temperature will be as hi high gh as po poss ssib ible le to pr prom omot ote e ig igni niti tion on.. Th The e remaining petcoke is distributed into the lower cone section of the calciner. The raw meal is added above the cone section and distributed along the walls of the calciner by means of the double deflector in order to provide a relatively raw meal free ‘hot zone’. A small portion of the raw meal can be diverted to the riser duct to limit build-ups. In these cases, the petcoke must still be ground to a fineness of approximately 1% retained on 90 microns. FLS recently commissioned a new ILC kiln specifically designed to fire 100% petcoke. In this case, the retention ti on ti time me of th the e ca calc lcin iner er wa wass de desi sig gne ned d to be 7 sec by means of a long loop duct between the main calciner vessel and the bottom stage. All of the fuel was injec inj ected ted in into to th the e kil kiln n ri riser ser du duct ct.. Th The e pe petco tcoke ke had a volat vol atile ile co conte ntent nt of 18% 18%,, and wa wass gr groun ound d to 7% re reta taine ined d on 90 mi micr cron ons. s. Th The e me meal al fr from om th the e se seco cond nd lo lowe west st cy cycl clon one e is split between the riser duct and the top of the calciner lower cone to create the ‘hot zone’ in order to facilitate the combustion of the petcoke. The notch in the centre of the calciner and the sharp bend in the top of the loop duct promote mixing of the calciner gases to aid in the complete burnout of the fuel. Gas measurements reveal that although the CO level is very high in the bottom of the calciner, the CO exiting the bottom stage was less than 50 ppm, and the residual carbon (% C) in th the e bo bott ttom om st stage age mat mater erial ial str stream eam wa wass 0. 0.06% 06%,, which indicates almost complete combustion (Figure 3).
from th from the e sec secon ond d lo lowe west st cy cycl clon one e of th the e CC-st stri ring ng betwee bet ween n the low lower er and upp upper er cal calcin ciner er sec sectio tions. ns. F.L. .L.Smi Smidth dth has a ref refere erence nce of sev severa erall SLC SLC-S -S 100 100% % petcok pet coke e fir fired ed pla plants nts and a 100 100% % ant anthra hracit cite-f e-fire ired d plant, which is a fuel that is even more difficult to burn than petcoke petcoke.. An SLC plant with two calciners was recently converted ver ted to 100% pet petcok coke e fir firing ing.. The mod modifi ificat cation ion included the addition of diverter gates to split the meal between the lower and upper portions of the calciner and the riser duct, restrictions in the centre of the calciner and lowering the solid fuel injection points in the calciners (Figure 4). In the kilns used today, the calciners are 100% fire fi red d wi with th pe petc tcok oke e co cont ntai aini ning ng 5% vo vola lati tile less an and d 6.74 6. 74% % S, wh whic ich h is gr grou ound nd to a fi fine nene ness ss of 0. 0.5% 5% residue on the 90 micron sieve. The hot zones of the calciners are operated at 1050 ˚C, and the gas retention time is 3 sec. The main burner fuel is a mixtur mixture e of petcoke, anthracite and lignite, adjusted so that the SO3 of the clinker is close to 1.6%. The clinker production rate is 10 200 tpd at a fuel consumption of 725 kcal/kg clinker. Two 5600 tpd SLC kilns are also firing 100% petcoke. In these kilns, the meal is again divided so that the ‘hot zone’ is created, and they are able to have stable operation even with the coke, which contains only 11% volatiles and is ground to a fineness of only 9 - 12% retained on the 90 micron sieve.
Calciners tailor-made for low volatile fuels The SLC-D system features a down draught calciner (DDC), in which there is a downward flow of gas, raw meal and fuel. The combustion air containing 21% O2 carries the raw meal in suspension, where it is introduced tangentially at the top of the calciner. This tangential action causes the raw meal to travel in a spiral movement close to the cylinder wall, protecting the walls from the high temperature zone. The fuel is fired from the top in a Duoflex burner, which crea cr eate tess a we well ll-m -mix ixed ed fl flam ame e in th the e ce cent ntre re of th the e
Petcoke firing in separate line calciners (SLC) FLS has been able to burn petcoke in a large number of existing in g SL SLC C ca calc lcin iner ers. s. Ag Agai ain, n, a ‘h ‘hot ot zone zo ne’’ is cr crea eate ted; d; th this is ti time me by dividi div iding ng the cal calcin ciner er vess vessel el int into o two sections, with only a portion of the raw meal flow directed to the lower section. This results in a lower raw meal concentration in the bottom section, which facilitates a high temperature so that the petcoke will burn at a sufficient rate. This ‘double calciner’ solution is fair fairly ly eas easily ily imp implem lement ented ed in most SLC plants, whenever there is ample height for the splitter gate ga te to di divi vide de th the e ra raw w me meal al
Figure 4. Modification of SLC for 100% petcoke firing. Reprinte Rep rinted d from W ORLD CEMENT April 2002
Figure 5. Operating data from 100% petcoke fired SLC-D kiln.
calciner vessel. The very high combustion rate is due to the high flame temperature that is only possible in the raw meal free atmosphere (Figure 5). The outlet from the down draught calciner is connected with the kiln riser pipe. The rotational motion of the cal calcin ciner er out output put pro provid vides es ef effic ficien ientt mix mixing ing between the hot combustion gases, raw meal and the kiln gas flow. The calcined raw meal separated to the kiln obtains a high degree of calcination, for example
in one plant 95% calcination was achieved, with only 845 ˚C in the lowermost cyclone. The DDC arr arrang angemen ementt is als also o eas easily ily ret retrof rofitt itted ed into an existing SP or ILC system to help boost production or adapt for the firing of low volatile fuels. The first 100% petcoke fired DDC was inserted in a kiln line producing white clinker. The high degree of combustion 99.8% is attained, although the combustio bus tion n air air/te /terti rtiary ary air is onl only y 280 ˚C. Unlike Unlike gre grey y cement plants, white pyroprocessing systems cannot retrieve 800 - 900 ˚C hot air from the clinker cooler. The calciner must make do with warm air generated by a heat exchanger using preheater exit gas. The preheated raw meal from the second lowest cyclone is split between the kiln riser pipe and the tertiary airr ri ai rise serr le lead adin ing g up to th the e ca calc lcin iner er.. Wi With th th this is arrangement, the raw meal is actually preheating the combustion air prior to its entry into the DDC (Figure 5). This is the ultimate case that shows that the final design of the calciner, and the ability to create and maintain a hot zone, are the critical components in enabling a calciner to burn 100% petcoke efficiently efficiently..
Bibliography
HUNDEBØL S., ‘Recent HUNDEBØL ‘Recent advancement advancement in petroleum petroleum coke use for cement manufacturing’, Proceedings of International Exhibition and Seminar on Energy and Environment in Cement, Construction and Allied Sectors, Sectors , January 2002, New Delhi, India. KAPLAN E. & NEDDER N., ‘Petroleum ‘Petroleum Coke Utilisation for Cement Kiln Firing’, Proceedings Firing’, Proceedings of 2001 IEEE meeting in Vancouver , Vancouver , 251. KEEFE, KEEF E, B.P., B.P., ‘Plant Modernization Modernization Projects Employ New Calciner Calciner Designs’, International Designs’, International Cement Journal , January 1997, 40-46.
Reprinte Rep rinted d from W ORLD CEMENT April 2002
Data in this brochure is intended for preliminary project planing only. Manufacturer reserves the right to modify equipment details and/or specifications without notice.
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