Info

Mass per fuel mole NG =

17.282 lbm HC

= 16.557 lbm

Noting that any free oxygen present in the fuel will react, reducing the need for combustion air, the stoichio-metric combustion equation of any mixture of sulfur-free hydrocarbons represented by CaHbOcNd, including natural gas, may be written as:

CaHbOcNd + [(4a + b - 2c) /4]x (O2 + 3.774N2) ^ CO2 + (b/2) H2O + 0.9435 x [(4a + b - 2c)]N2 + (d/2)N2

Based on the given example of pipeline quality gas, the above equation becomes:

C1.045H4.034O0.010N0.036 + 2.0485 O2 + 7.731 N2 ^ 1.045 CO2 + 2.017H2O + 7.731 N2 + 0.018 N2

In many applications, combustion air requirements may be estimated with reasonable accuracy. For each volume of the fuel above, for example, roughly 9.78 volumes of air (2.049 volumes of oxygen and 7.731 volumes of nitrogen) have to be added to achieve theoretical complete combustion.

Table 5-11 shows theoretical air requirements for various fuels. In the first column, theoretical air has been tabulated on a mass per mass of fuel basis. The wide variation of values, however, is of little significance when comparing the various fuels. But, when the theoretical air is converted to a mass per unit heat input from fuel basis, as shown in Columns 3 and 4, the theoretical air varies little among fuels. Note that values are listed in lbm per 10,000 Btu and that carbon and hydrogen, the principal combustible fuel elements, are shown for reference. Table 5-12 provides a fuel analysis and shows the theoretical air, fuel, and moisture content for representative samples of heavy oil and natural gas fuels. Note that constituents are shown by weight for the oil sample and by volume for the natural gas sample.

Since perfect mixing of fuel and air is never achieved in practice and a portion of the combustion air volume may be occupied by water vapor, the fuel has to be burned with slight excess air. Humid air will contain varying percentages of moisture, depending on temperature and degree of saturation — typically the proportion will be

Fuel

Theoretical Air, lb/lb HHV Fuel Btu/lb

Theoretical Air Typical Range lb/104 Btu lb/104 Btu

Fuel

Theoretical Air, lb/lb HHV Fuel Btu/lb

Theoretical Air Typical Range lb/104 Btu lb/104 Btu

Bituminous coal

(VM* >30%) Subbituminous coal (VM* >30%)

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