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Combustion Fundamentals
To understand the fonnation of pollutants in combustion systems, we must first under-
stand the nature of the fuels being burned, the thennodynamics of the combustion pro-
cess, and some aspects of flame structure. In this chapter we discuss fundamental aspects
of hydrocarbon fuel combustion that relate directly to the fonnation of pollutants or to
the control of emissions. Questions of flame stability, detonations, and several other
important aspects of combustion science are beyond the scope of the present discussion
and will not be treated. Specific pollution control problems will be addressed in detail
in later chapters.
FUELS
Of the spectrum of fuels currently in widespread use, the simplest in composition is
natural gas, which consists primarily of methane but includes a number of other con-
stituents as well. The compositions of other gaseous fuels are generally more complex,
but they are, at least, readily detennined. Table 2.1 illustrates the range of compositions
encountered in gaseous fuels, both natural and synthetic.
Infonnation on the composition of liquid or solid fuels is generally much more
limited than that for gaseous fuels. Rarely is the molecular composition known since
liquid fuels are usually complex mixtures of a large number of hydrocarbon species. The
most commonly reported composition data are derived from the ultimate analysis, which
consists of measurements of the elemental composition of the fuel, generally presented
as mass fractions of carbon, hydrogen,
Adiabatic Flame Temperature
Combustion reactions generally occur very fast, on the order of 1 ms and little heat or
work transfer takes place on the time scale of combustion. For this reason the maximum
temperature achieved in the combustion process is often near that for adiabatic combus-
tion. This so-called adiabatic flame temperature may readily be calculated by applying
the first law of thermodynamics to an adiabatic combustor. Consider a steady-flow com-
bustor, illustrated in Figure 2.5, burning a fuel with composition CH m