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This document shows how Thermo Utilities, MS Excel Add-ins can be
used for calculation of combustion.
A fossil fuel with the following composition by mass:
C 70%; H 18.5%; O 3%; N 4%; S 1.5%; ash 3%
has been burned in a boiler, when 100% excess air is supplied.
Combustion efficiency is 0.75 Calculate:
1- the stoichiometric air-to-fuel (A/F) ratio
2- the A/F ratio
3- analysis of combustion products (dry and wet)
4- temperature of exhaust gases
Air is supplied at atmospheric pressure and 18 C with 0.008 specific
humidity. The fuel has an average temperature of 35 C when enters
the boiler. Use Dulong formula to estimate the net calorific value of the fuel.The
specific heat capacity of fuel is 3.2 kJ/kg,K.
Combustion Equations
Combustion equation for coal:
C + O2 => CO2 (12 kg C)+(32 kg O) => (34 kg CO2)
Combustion equation for hydrogen:
2 H2 + O2 => 2 H2O (4 kg H)+(32 kg O) => (36 kg H2O)
Combustion equation for sulphur:
S + O2 => SO2 (32 kg S) + (32 kg O) => (64 kg SO2)
Fuel Analysis
Constituent Mass fraction Required oxygen Product mass
kg/kg fuel kg/kg fuel
Carbon 0.700 1.867 2.567
Hydrogen 0.185 1.480 1.665
Oxygen 0.030 -0.020 0.010
Nitrogen 0.040 0.000 0.040
Sulphur 0.015 0.015 0.030
Ash 0.030 0.000 0.030
1.000 3.342 4.342
Analysis of Supplied Air
Specific Humidity 0.008
Composition by mass
Constituent Dry Air Humid Air
N2 0.76280 0.75670
O2 0.23290 0.23104
CO2 0.00300 0.00298
Ar 0.00130 0.00129
H2O 0.00000 0.00800
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SO2 0.00000 0.00000
1.00000 1.00000
Air required per kg of fuel 14.46 Stoichiometric A/F ratio kg/kg
Excess Air 1
Actual A/F ratio kg/kg 28.92757
Exhaust Gases Wet Mass Dry Mass
Constituent Mass Composition Composition
N2 21.92942 0.73373 0.78344
O2 3.34167 0.11181 0.11938
CO2 2.65276 0.08876 0.09477
Ar 0.03730 0.00125 0.00133
H2O 1.89642 0.06345 0.00000
SO2 0.03000 0.00100 0.00107
29.88757 1.00000 1.00000
Exhaust Gases Volume
Constituent Kg/kmol Mole Fraction Composition
N2 28 0.02620 0.74260
O2 32 0.00349 0.09901
CO2 44 0.00202 0.05716
Ar 40 0.00003 0.00088
H2O 18 0.00353 0.09990
SO2 64 0.00002 0.00044
0.03529 1.00000
Mass balance
Fuel 1.00000
Supplied Air 28.92757
29.92757
Exhaust Gases 29.88757
Ash 0.03000
29.91757
Dulong suggests the following formula for gross calorific value (GCV) of fossil fuels when oxygen
content is less than 10%
GCV = 337 C +1442 (H - O/8) + 93 S
GCV is in (kJ/kg). C, H, O, S are percentages on weight basis for carbon, hydrogen, oxygen and
sulphur. The net calorific value for a constant pressure combustion is:
NCV = GCV - mc * hfg
mc is the mass of condensate per unit quantity of fuel and hfg is the latent heat of steam at 25
degree Celsius which is 2442 kJ/kg.
Supplied Air Temp. 18
Fuel Cp 3.2 Err:511
Gross Calorific Value, GCV 49866 kJ/kg
Net Calorific Value, NCV 49711 kJ/kg
Combustion efficiency 0.75
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Enthalpy Mass Flow m*h
Err:511 kg/s kJ/s
Supplied Air Err:511 28.93 Err:511
Fuel 64.00 1.00 64.00
Fuel Energy Supplied 49710.80 1.00 37283.10
Err:511
Exhaust Gases Err:511 29.93 Err:511
Exhaust Gases Temp Err:511 Err:511
Taftan Data
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roduct mass
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n oxygen
gen and
at 25
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