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Combustion Data and Basic Calculations
For more detail and expanded information on multiple fuels, see North American Combustion Handbook, Vol. 1, B&W Steam book, and Perry’s Chemical Engineers’ Handbook.
FINDING EXCESS AIR BASED ON STACK OXYGEN
Shortcut to find/check approximate level of excess air %:
Excess air % = 90 (O2%/(21-O2%)) where O2% is dry basis
Example: Excess air at 11.1% O2 = 90(11.1/(21-11.1)) = 100%
CALCULATE ADIABATIC FLAME TEMPERATURE (AFT)
Q = M x Cp x temperature difference
Temperature difference = Q/(M x Cp)
Example: 1 mole CH4, 16 lb X LHV 21,500 Btu/lb = 344,000 Btu
At 100% excess air, M = 568 lb for air plus fuel
Temp diff = 344,000/(568 lb x 0.282) = 2,150 F
With 60 F base temperature at standard conditions:
Adiabatic Flame Temp = 60 + temp diff = 2,210 F
Caution: Cp varies with gas temp and gas; Use integrated heat capacities of gases in place of specific heat via equations or tables
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SELECTED COMBUSTION CONSTANTS |
COMPONENT |
Mol
Weight |
HHV Btu
per lb fuel |
lb air reg'd
per lb fuel |
Methane |
16 |
23,896 |
17.3 |
Ethane |
30 |
22,282 |
16.2 |
Propane |
44 |
21,523 |
15.7 |
Hydrogen |
2 |
60,991 |
34.3 |
Carbon Monoxide |
28 |
4,323 |
2.42 |
Carbon Dioxide |
44 |
-- |
-- |
Oxygen |
32 |
-- |
-- |
Nitrogen |
28 |
-- |
-- |
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RULES OF THUMB FOR COMBUSTION AIR ESTIMATION
For air volume:
Btu/hr input/6,000 = combustion air flow, scfm
For air mass:
765 lb/air required to burn 1 MM Btu of fuel
Example:
A 10 MM Btu/hr burner running at 0% excess air requires:
10,000,000/6,000 = 1,670 scfm air or 10 x 765 = 7,650 lb/hr air
HEAT EXCHANGER EFFECTIVENESS
E = Ch * (Th in - Th out)/Cmin * (Th in - Tc in)
Where Cmin is the smaller of mhcph or mccpc, the hourly heat capacity
The rate of heat exchange is
Q = E Cmin * (Th in - Tc in)
Reference: F. Kreith, 3rd ed. Principles of Heat Transfer, Intext Education Publisher, NY, 1973
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TMTS Associates, Inc. 399 Pavillion Street, Atlanta, GA 30315. 404-627-4722
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