Page 474 - Air Pollution Control Engineering

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446 Lawerence K. Wang et al.

total pollution control system. Because the implementation of a single control technology
may not in itself be adequate and may even directly cause other emission problems, it
is often desirable to consider combinations of several different control techniques so
that the final system selected is optimized from both economic and environmental
considerations.

2. PROCESS MODIFICATION
Process modification should be utilized as the first and last steps when planning to
control air pollution emissions. In most processes, there are many ways to obtain the
desired end product. One or more of these alternatives may eliminate or, at least, reduce
the emission of pollutants. Combustion operations are perhaps the best known in this
regard. Boilers have been redesigned to reduce nitrogen oxide (NO ) emissions by
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permitting the use of recirculated air, reducing hot zones, and eliminating flue gas
quenching. Automobile engines are good examples in which redesign to eliminate cold
spots and to recirculate vapors has reduced hydrocarbon (HC) emissions, and changes
in timing, electronic charge distribution, and air-to-fuel ratios have improved fuel econ-
omy while reducing emissions. Furthermore, an automobile engine using the advanced
fuel-cell technology is being developed in the United States for emission reduction.
High-efficiency control devices may need to be installed to meet regulations if pro-
cess alterations do not result in an adequate reduction of pollution quantities. However,
process improvements can change the character of the emissions to make their control
easier. An ideal control device would close the process loop and return valuable prod-
uct to the system. In these situations, it may be necessary to modify the process system
so that it can successfully accept the returned material.
Combustion processes are examples of systems that can be modified to produce
fewer pollutants (e.g., NO and HC), to accept return of recovered pollutants (e.g., HC),
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and to eliminate formation of pollutants. Elimination of fly ash and SO can be accom-
2
plished, for example, by conversion from solid fuel (coal) to gaseous fuel (natural gas,
which is methane, or compressed natural gas [CNG], which is propane). This, however,
simply relocates the pollution control facilities because adequate gaseous fuels currently
are not available, although they could be in the future if produced from coal, solid waste,
or some other abundant raw material. Air pollution control would then be required at
these conversion facilities. Liquefaction of these raw materials can also produce a low-
pollution fuel, but the same constraint applies (i.e., air pollution control facilities will
be required at the conversion site).

3. VEHICLE AIR POLLUTION AND ITS CONTROL

3.1. Background
Transportation vehicles in the United States have been the single largest source of air
pollution emissions. In 1972, it was estimated that nearly 104 million tons of trans-
portation source pollutants were released, which amounted to 48.7 weight percent
(wt%) of total air pollution emissions. For the same year, the emission concentration of
the various transportation-source pollutants by weight percent was 74.5% carbon
monoxide (CO), 15.6% hydrocarbons (HCs), 8.4% nitrogen oxides (NO ), 0.8% partic-
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