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

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

.

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

kJ/(kg.K)

.

Gross Calorific Value, GCV

49866

kJ/kg

.

Net Calorific Value, NCV

49711

kJ/kg

.

Combustion efficiency

0,75

.

.

.

.

.

.

.

Enthalpy

Mass Flow

m*h

.

kJ/kg

kg/s

kJ/s

Supplied Air

38,31

28,93

1108,10

Fuel

64,00

1,00

64,00

Fuel Energy Supplied

49710,80

1,00

37283,10

.

.

.

38455,21

Exhaust Gases

1284,94

29,93

38455,21

Exhaust Gases Temp

964,

C

.


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