Page 110 - Fundamentals of Air Pollution

80 6. Sources of Air Pollution

instant, when the temperature is high enough to cause the reaction to
begin.
The physical state of the fuel for a combustion process dictates the type
of system to be used for burning. A fuel may be composed of volatile
material, fixed carbon, or both. The volatile material burns as a gas and
exhibits a visible flame, whereas the fixed carbon burns without a visible
flame in a solid form. If a fuel is in the gaseous state, such as natural gas,
it is very reactive and can be fired with a simple burner.
If a fuel is in the liquid state, such as fuel oil, most of it must be vaporized
to the gaseous state before combustion occurs. This vaporization can be
accomplished by supplying heat from an outside source, but usually the
liquid fuel is first atomized and then the finely divided fuel particles are
sprayed into a hot combustion chamber to accomplish the gasification.
With a solid fuel, such as coal or wood, a series of steps are involved in
combustion. These steps occur in a definite order, and the combustion
device must be designed with these steps in mind. Figure 6-6 shows what
happens to a typical solid fuel during the combustion process.
The cycle of operation of the combustion source is very important as far
as emissions are concerned. A steady process, such as a large steam boiler,
operates with a fairly uniform load and a continuous fuel flow. The effluent
gases, along with any air pollutants, are discharged steadily and continually
from the stack. An automobile engine, on the other hand, is a series of
intermittent sources. The emissions from the automotive engine will be
vastly different from those from the boiler in terms of both quantity and
quality. A four-cylinder automotive engine operating at 2500 rpm has 5000
separate combustion processes started and completed each minute of its

Fig. 6-6. Solid fuel combustion schematic.