TMTS Associates Inc.

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

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	--	--

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