PRINCIPLES OF COMBUSTION
INTRODUCTION Combustion is a chemical reaction • “Rapid oxygenation/oxidation” • energy state by releasing some energy • Usually produces visible radiance
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FUEL PROPERTIES he primary combustion elements of liquid • T heating fuel are: • Carbon (84% - 89%) • Hydrogen (7% - 16%) • The other constituents that combust, such as sulphur, generally contribute less than 1% of the total energy released • Water, contrary to popular belief, does not combust but in fact takes up energy 2009 March
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COMBUSTION CHEMISTRY The generalised combustion reaction for hydrocarbon fuels can be shown as:
CxHy + O2 = CO2 + H2O + Energy • C H is the hydrocarbon fuel, O is oxygen y (Thex actual conversion of hydrocarbon 2molecules in the combustion process is more complex as there are many intermediate conversions that take place.) 2009 March
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OXYGEN ISSUES • Oxygen is found in air • Oxygen constitutes ~20,9% of the , nitrogen (and a few trace gases) • Thus for any given amount of oxygen required, we need times the volume of air (1 divided by4,8 0.209) 2009 March
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FUEL CHEMISTRY • Carbon has a molecular mass of 12 • Oxygen has a mol mass of 16, but as a gas (O2) has a mol mass of 32 • hus carbon dioxide (CO 2) has a mol mass of 44 • Water (H 2O) has a mol mass of 18 • Above is useful for emissions modelling:
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EMISSION MODELLING MasosFfueBl urnt Input Carbon Hydrogen Sulphur Oxygen
Output CO2 H2O SO2
March 2009
1000 k g /h r Mol Mass Content in fuel (g/mol) (mass%) 12.01 88% 1.01 12% 31.97 3% 8 0% Totals 1 Mol Mass (g/mol) 28.01 10.016 47.97 Totals
Content in flue gas (mass%) 76% 22% 2%
Kmol/hr 73.272 119.048 0.938 207.945 401
Mass burnt (kg/hr) 880 120 30 1,664 2,694
Kmol/hr 73.272 59.524 0.938 Total
Mass in flue gas (kg/hr) 2,052 596 45 2,694 7
COMBUSTION EFFICIENCY • combustion efficiency is affected by the manner in which the combustion occurs • That is, the • air:fuel ratio • degree of atomising (liquid fuels) • fuel-air mixing • flame temperature • flame shape • fuel residence time in the combustion zone
• And the amount of heat lost out of the system 2009 March
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AIR:FUEL RATIO • The theoretical air:fuel ratio for complete combustion is known as the “STOICHIOMETRIC” ratio • In practice this ratio does not achieve complete combustion as the degree of mixing is never sufficient to allow every oxygen • Thus a certain amount of excess oxygen (air) is required to achieve full combustion • The range of excess oxygen required to achieve complete combustion in practical applications is between 1% and 5% depending on the combustion appliance • This implies that an excess air requirement of 5% - 25% is necessary, as there is only ~21% oxygen in air.
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AIR:FUEL RATIO FLUE GAS ANALYSIS 17 16 15
CARB ON
M U L O V
14
MONOXIDE
CARBON DIOXIDE
13 12
B S A G E U L F F O T N E C R E P
11 10 9 8 7 6 5 4
OXYGEN 3 2 1 0 -60
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
% EXCESS AIR
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AIR:FUEL RATIO • An excess air is necessary for amount completeofcombustion • oo much excess air is undesirable as it reduces efficiency by absorbing and carrying away heat • Typically the energy loss due to excess air is in the order of 1,2% for every 10% of excess air by volume 2009 March
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ATOMISING • Applies to liquid fuels only • Is required to generate an even spray of droplets sufficiently small to allow good mixing with the oxygen to ac ieve comp ete com ustion usua y <50 microns in diameter) • Atomisation is dependent on fuel pressure and viscosity, atomising air or steam pressure, nozzle and burner design • The viscosity can be regulated by controlling the fuel oil temperature 2009 March
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ATOMISING Primary causes of poor atomisation are: • • • • • • •
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Worn nozzles Insufficient fuel-oil pressure Excessive fuel-oil viscosity Insufficient atomising air or steam pressure Incorrect nozzle size – excessive turndown Poor nozzle design Excessive fuel viscosity (>20 cSt)
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FUEL:AIR MIXING • The effectiveness of the burner in achieving adequate mixing of the fuel and air is crucial to efficient combustion • he burner must provide a stable spray of atomised fuel particles expanding into the combustion air in a manner that will sustain good combustion • The quarl helps sustain the shape of the flame necessary for good combustion 2009 March
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FUEL:AIR MIXING Causes of poor mixing: • Imbalanced air:fuel pressures •I n rr l r n r • Worn burner parts • Misaligned burners • Damaged or badly made burner tile (quarl) • Dirty or blocked swirl plates 2009 March
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FLAME CHARACTERISTICS • Heavy require time than light oils and gasfuel to oils achieve full more combustion (respectively 0,1s – 0,01s – 0,001s). Thus the length of the • Flame shapes are important as short flames may not provide sufficient residence time for full combustion and “woolly” flames allow unburnt fuel mixture to escape from the side of the flame.
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HEAT LOSSES Useful energy (heat) is lost in the following manner: • Poor combustion (0% - 20%) • Insufficient radiance (5% - 15%) • Lost out of appliance from poor insulation (2,5% - 15%) • Up the stack (2% - 10%) • Heating up excess air (0,5% - 3%) 2009 March
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SANKEY DIAGRAM
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STACK LOSSES • The load energy in the combustion gases is a loss heat of useful • Therefore the stack temperature should be • The volume of gas should be minimised (excess air) • Stack temperature in a boiler application goes up when the heat transfer surfaces become dirty 2009 March
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STACK LOSSES 16 FUEL OIL
COMPOSITION Carbon - 86% Hydrogen - 12% Sulphur - 1,4%
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G E U L F IN 12 E M U L O V Y B 10 E ID X IO D N 8 O B R A C %
60 -
STACK TEMPERATURE500 C %
40 L
400 C
CARBON DIOXIDE - WET
300 C
200 C
20
6
100 C
4 0
25
50
75
100
125
150
175
200
% EXCESS COMBUSTION AIR
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MEASUREMENT • It is virtually to set a burner’s air:fuel ratio byimpossible eye to ensure complete combustion (minimum CO) and minimum excess • The only reliable way is to measure the Oxygen (O2) and Carbon Monoxide (CO< 10 ppm) content in the stack • The burner should be set for minimum O in the 2 stack gas without producing more than 10ppm of CO over a range of turn-down 2009 March
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CONTROLS • The only effective way is to install combustion analysers and control the f l: irmix r m i ll • There is a range of such instruments and systems on the market
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CONCLUSION • Good combustion requires constant attention to detail, keeping all parts in w rkin r r • Significant savings can be made by controlling and measuring the combustion process
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