96

= 164

7,097

Table 5-8 Molecular and Weight Relationships Between Fuel and Oxygen for Common Combustion Constituents. Source: Babcock & Wilcox

Table 5-8 Molecular and Weight Relationships Between Fuel and Oxygen for Common Combustion Constituents. Source: Babcock & Wilcox needed for complete combustion of various fuels. As a general rule, roughly 10 cf of air is required for every 1,000 Btu (or 1 m3 of air for every 3.77 MJ) of fuel. However, there is usually a need to perform more precise, fuel-specific computations.

Since the highest efficiency is achieved in most applications by minimizing the amount of hot flue products leaving the gas-fired device at temperatures above the ambient, the idea of using the bare minimum of air to achieve complete combustion is attractive. When the flue products contain no oxygen or fuel, with all reactants having been adjusted to chemically correct proportions to produce water and carbon dioxide only, the process is called stoichiometric combustion. Thus, there is a specific air-fuel ratio that results in complete combustion for every combustible hydrocarbon.

One common method computes the amount of oxygen required to achieve stoichiometry by determining the amount of oxygen required for complete oxidation of each combustible in the fuel. It sums these amounts, and then determines the amount of air, in moles, by dividing this sum by 0.20946 (the molar fraction of oxygen in the air). Referring to methane combustion illustrated by Equation 5-2, each carbon atom will combine with two atoms of

Table 5-9 Composition of Sample Pipeline Gas

Pipeline Gas

Mole

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