accompanied by light, it is called combustion. Three ingredients have to be present for combustion to occur: a source of ignition (heat), fuel, and oxygen. Remove any one of these from this triangle and combustion will not take place.

The source of oxygen in most combustion processes is atmospheric air. Air comprises oxygen (20.946 mol%), nitrogen (78.102 mol%), argon (0.916 mol%), carbon dioxide (0.033 mol%), and a variety of other gases in smaller amounts. In rough calculations, the air composition is frequently taken to be 21 vol.% oxygen and 79 vol.% inert gases lumped under the name of atmospheric nitrogen. Based on National Bureau of Standards 1978 data, a more exact approach indicates that each mole of oxygen involved in combustion will carry with it:

(1 - 0.20946) _ 3 J74 moles of 0.20946 atmospheric nitrogen

The molar mass of air, in English system units, is 28.9625, i.e., each mole of air has a mass of 28.9625 lbm. However, because atmospheric nitrogen contains traces of other species, its molar mass is slightly different from the 28.0134 lbm value of pure nitrogen. Its mass is 28.1580 lbm/mol.

Table 5-8 summarizes the molecular and weight relationships between fuel and oxygen for constituents commonly involved in combustion. The heat of combustion for each constituent is also tabulated.

There are many ways to compute the amount of air

Wy. Substance

Molecular Formula Weight1

DcnaiLy," lb per ftv

Specific Volumeh ft"^ [KT ]Ej

Specific f. ravi t^^ l«tr=lj

] ItkiL uf Combuarion'' Bt.U por f C: Rctl p<3r IL.i

ft" per ft1-' i>f CotubusLlble Requiiud for Combustion Flue Hroduc.ts


lb per lb of Comlyu&¿jl)le tíaquired f/i>nibt3tMjri Flue Gas Producta

Theor air Jh: 10.909 Htu





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