Combustion of biomass  fuel, Using Thermo Utilities, MS Excel Add-ins

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:

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 Vondracek 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	0,540	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.