COMBUSTION CALCULATIONS

Solid & Liquid Fuels:

Astoi = 11.53*C + 34.34*(H2 - (O2/8)) + 4.29*S

where

Astoi = Dry stoichiometric air required for combustion, lb/lb of fuel

C = weight fraction of Carbon in Solid/Liquid fuel

H2 = weight fraction of Hydrogen in Solid/Liquid fuel

O2 = weight fraction of Oxygen in Solid/Liquid fuel

S = weight fraction of sulfur in Solid/Liquid Fuel

Gaseous Fuels:

Astoi = 2.47*CO + 34.34*H2 + 17.265*CH4 + 13.3*C2H2 + 14.81*C2H4 + 16.12*C2H6 + 15.69*C3H8 + 15.44*C4H10 - 4.32*O2

where

Astoi = Dry stoichiometric air required for combustion, lb/lb of fuel

CO = weight fraction of Carbon Monoxide Gaseous fuel

H2 = weight fraction of Hydrogen in Gaseous fuel

CH4 = weight fraction of Methane in Gaseous Fuel

C2H2 = weight fraction of acetylene in Gaseous Fuel

C2H4 = weight fraction of ethylene in Gaseous Fuel

C2H6 = weight fraction of ethane in Gaseous Fuel

C3H8 = weight fraction of propane in Gaseous Fuel

O2 = weight fraction of Oxygen in Gaseous fuel

HHV's (Higher Heating Value) for API fuel oils:

HHV = 17887 + 57.5*°API - 102.2*S

where

°API = Degree API of Fuel Oil (measure of specific gravity)

S = weight percent of sulfur in fuel oil

MM Btu method of combustion:

Examples for calculation of thoretical/stoichiometric Air:

Example 1:

A Fuel oil has the following characteristics:

Degree API 28 °API

weight percent C = 87.5 %

weight percent H = 12.5 %

weight percent S = 0 %

weight percent O = 0 %

Calculate the amount of theoretical air required per million Btu fired for above fuel oil

This method for combustion is based on the concept that the weight of air required in the combustion of a unit

weight of any commercial fuel is more nearly proportional to the unit heat value than to the unit weight of the fuel.

Hence the quantity of air required is expressed in lb per million btu (lb/MMBtu) fired.

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COMBUSTION CALCULATIONS

Calculations:

HHV = 19497 Btu/lb

Astoi = 14.38 lb/lb fuel

1 MM Btu fired requires = 51.29 lb fuel

Hence air required per 1 MM Btu fuel = 737.6 lb

Example 2:

Natural gas is used as fired fuel for boiler. It has the following volumetric analysis:

CH4 = 83.4 %

C2H6 = 15.8 %

N2 = 0.8 %

Calculate the amount of theoretical air required per million Btu fired for natural gas

Calculations:

Component volume % Molecular wt weight % wt fraction Combustion

Constants

or HHV,

Btu/lb

CH4 83.4 16 72.89 0.7289 23876 yellow colored cell are inputs

C2H6 15.8 30 25.89 0.2589 22320

N2 0.8 28 1.22 0.0122 -

Astoi = 16.76 lb/lb fuel

HHV = 23181.0 Btu/lb

1 MM Btu fired requires = 43.14 lb fuel

Hence air required per 1 MM Btu fuel = 722.9 lb

Total Atmospheric Air required for Combustion in fired Heaters:

Atot = Astoi*(1000000/HHV of fuel)*(1+ H)*(1 + Aexc)

where

Atot = total atmospheric air, lb/MM Btu

Astoi = Dry stoichiometric or theoretical air required, lb/lb fuel

HHV = Higher Heating Value, Btu/lb

H= absolute humidity of atmospheric air expressed as a fraction - lb of water vapor per 1 lb of dry air

Aexc = excess air expressed as a fraction (10% excess air will be entered as 0.1, 15% = 0.15 etc.)

(to be calculated from % Relative Humidity (RH) & Dry Bulb Temperature using psychrometric chart,

example: at 105°F & 100% RH, H = 0.0507 from Humid Air chart)

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COMBUSTION CALCULATIONS

Common Combustion Reactions & the Heats of Combustion:

Heating Value (Btu/lb)

HHV LHV

H2 + 1/2O2 = H2O 61100 51600

C + O2 = CO2 14093 14093

C + 1/2O2 = CO 4440 4440

CO + 1/2O2 = CO2 4345 4345

S + O2 = SO2 10160 10160

CH4 + 2O2 = CO2 + 2H2O 23885 21500

C2H6 + 3.5O2 = 2CO2 + 3H2O 22263 20370

C3H8 + 5O2 = 3CO2 + 4H2O 21646 19929

C4H10 + 6.5O2 = 4CO2 + 5H2O 21293 19665

Note that where no water is formed during combustion reaction the HHV & LHV are the same.

Heating Value (LHV(net) & HHV(gross)) @ 60°F, 14.696 psia (Source: Fig 23-2, GPSA Engg. Data Book)

Btu/ft3,

ideal gas,

14.696

psia

Btu/lbm

Liquid

Btu/ft3,

ideal gas,

14.696

psia

Btu/lbm

Liquid

Btu/gal

Liquid

Methane 909.4 - 1010 - -

Ethane 1618.7 20277 1769.6 22181 65869

Propane 2314.9 19757 2516.1 21489 90830

Isobutane 3000.4 19437 3251.9 21079 98917

n-Butane 3010.8 19494 3262.3 21136 102911

Isopentane 3699 19303 4000.9 20891 108805

n-Pentane 3706.9 19335 4008.9 20923 110091

Neopentane 3682.9 19235 3984.7 20822 103577

n-Hexane 4403.8 19232 4755.9 20783 115021

Hydrogen 273.8 - 324.2 - -

Carbon

Monoxide320.5 - 320.5 - -

Prepared by: Ankur Srivastava

Chemical Engineer

e-mail: ankur2061@yahoo.co.in

Disclaimer : The information and methods included within this spreadsheet are presented for combustion air

calculations. It is intended to be used by technically skilled persons at their own discretion. I do not warrant the

suitability or accuracy of these methods.

Heating Value, 60°F

Net Gross

Compound

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COMBUSTION CALCULATIONS

2.47*CO + 34.34*H2 + 17.265*CH4 + 13.3*C2H2 + 14.81*C2H4 + 16.12*C2H6 + 15.69*C3H8 + 15.44*C4H10 - 4.32*O2

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COMBUSTION CALCULATIONS

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COMBUSTION CALCULATIONS

Disclaimer : The information and methods included within this spreadsheet are presented for combustion air

calculations. It is intended to be used by technically skilled persons at their own discretion. I do not warrant the

suitability or accuracy of these methods.

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