Heating Value of Air Fuel Mixtures

Once the heating value of the specific fuel being burned and the amount of air required for complete combustion of the flammables is determined, the heating value of the air-fuel mixture, or air-fuel charge as it is sometimes called, can be determined as follows:

Heating value of _ air-fuel mixture

Heating value of fuel (1 cf fuel + cf air required)

HHV of air-fuel mixture

LHVof air-fuel mixture

96.16 Btu/cf

For the natural gas described as C1045H4034O0010N0036, the (A/F)S is 16.39 (283.242/17.282), which can be inte6-preted as 16.39 lbm dry air requirement for each lbm of gas burned (16.39 lbm-air/lbm-gas, or any other equivalent mass units).

To determine the actual air-fuel ratio for lean-burn conditions, an excess air factor X is defined as:

The hypothetical pipeline gas fuel mixture (neglecting compressibility) yields the following:

Adjustments of the HHV and LHV of the air-fuel mixture are made by recalculating the values with more or less air. Excess air (lean) will result in decreased Btu/cf (kJ/m3) values and excess fuel (rich) will result in increased Btu/cf (kJ/m3) values.

All of these computations are based on complete stoichiometric combustion. Actual combustion may deviate from this for several reasons. In boiler applications, for example, it is common to use a leaner mixture. In the event of imperfect mixing of fuel and air, or other such imperfections, this provides a margin of error on the side of excess air rather than excess fuel. While both result in decreased efficiency, air is less costly than fuel. Another consideration is exhaust emissions. Rich burn may result in increased emission of excess carbon monoxide (CO) as well as the accumulation of soot on heat exchange surfaces, while lean burn may result in higher NOX emissions.

Although a rich mixture is often used to improve performance in internal combustion engine applications, emissions requirements have led to a movement toward lean-burn rather than rich-burn engines. For both boilers and engines, however, increasingly stringent emissions requirements have produced a carefully planned richer or leaner operation.

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