A biomass fuel with the following composition by mass:
C 54%; H 22%; O 12%; N 4%; S 1.3%; ash 6.7%
C1, percentage carbon on the dry ash-free basis 17%
has been burned in a boiler, when 150% excess air is supplied.
Combustion efficiency is 0.75 Calculate:
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 Vondracek formula to estimate the net calorific value of the fuel. The specific heat capacity of fuel is 3.2 kJ/kg,K.
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 |
0,540 |
1,440 |
1,980 |
Hydrogen |
0,220 |
1,760 |
1,980 |
Oxygen |
0,120 |
-0,020 |
0,100 |
Nitrogen |
0,040 |
0,000 |
0,040 |
Sulphur |
0,013 |
0,013 |
0,026 |
Ash |
0,067 |
0,000 |
0,067 |
. |
1,000 |
3,193 |
4,193 |
Carbon on dry ash-free basis |
0,170 |
. |
. |
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 |
. |
SO2 |
0,00000 |
0,00000 |
. |
. |
1,00000 |
1,00000 |
. |
Air required per kg of fuel |
13,82 |
Stoichiometric A/F ratio |
kg/kg |
Excess Air |
1,5 |
. |
. |
Actual A/F ratio kg/kg |
34,55077 |
. |
. |
Exhaust Gases |
. |
Wet Mass |
Dry Mass |
Constituent |
Mass |
Composition |
Composition |
N2 |
26,18449 |
0,74001 |
0,79042 |
O2 |
4,78950 |
0,13536 |
0,14458 |
CO2 |
2,08282 |
0,05886 |
0,06287 |
Ar |
0,04456 |
0,00126 |
0,00135 |
H2O |
2,25641 |
0,06377 |
0,00000 |
SO2 |
0,02600 |
0,00073 |
0,00078 |
. |
35,38377 |
1,00000 |
1,00000 |
Exhaust Gases |
. |
. |
Volume |
Constituent |
Kg/kmol |
Mole Fraction |
Composition |
N2 |
28 |
0,02643 |
0,74275 |
O2 |
32 |
0,00423 |
0,11888 |
CO2 |
44 |
0,00134 |
0,03760 |
Ar |
40 |
0,00003 |
0,00088 |
H2O |
18 |
0,00354 |
0,09956 |
SO2 |
64 |
0,00001 |
0,00032 |
. |
. |
0,03558 |
1,00000 |
Mass balance |
. |
. |
. |
Fuel |
1,00000 |
. |
. |
Supplied Air |
34,55077 |
. |
. |
. |
35,55077 |
. |
. |
. |
. |
. |
. |
Exhaust Gases |
35,38377 |
, |
. |
Ash |
0,06700 |
. |
. |
. |
35,45077 |
. |
. |
Vondracek suggests the following formula for gross calorific value (GCV) of fossil fuels when oxygen content exceeds 10%
GCV = (337 -0.261 C1) C +1130 (H - O/10) + 105 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 |
kJ/(kg.K) |
. |
Gross Calorific Value, GCV |
41836 |
kJ/kg |
. |
Net Calorific Value, NCV |
41680 |
kJ/kg |
. |
Combustion efficiency |
0,75 |
. |
. |
. |
. |
. |
. |
. |
Enthalpy |
Mass Flow |
m*h |
. |
kJ/kg |
kg/s |
kJ/s |
Supplied Air |
38,31 |
34,55 |
1323,51 |
Fuel |
64,00 |
1,00 |
64,00 |
Fuel Energy Supplied |
41680,38 |
1,00 |
31260,28 |
. |
. |
. |
32647,79 |
Exhaust Gases |
918,34 |
35,55 |
32647,79 |
Exhaust Gases Temp |
671, |
C |
. |