386 ENVIRONMENTK ENGINEERING
Figure 20-1. Points of possible air pollution control.(exhaust) is so difficult to trap and treat. If automobile exhaust could be channeled
to a central treatment facility, treatment could be more efficient in controlling each
individual car.
Recycling of exhaust gases is one means of control. Although automobiles could
not meet 1990 exhaust emission standards only by recycling exhaust and blow-by gases,
this method proved a valuable start to automobile emission controls. Many stationary
industries recycle exhaust gases, usually CO and volatile organic compounds, as fuel
for the process, since even CO releases heat when burned to COz.
Process exhaust gases channeled through one or more stacks are relatively easy
to collect, but fugitive emissions from windows and doors, cracks in the walls, and
dust raised during on-site transportation of partially processed materials pose a difficult
collection problem. Some industries must overhaul the entire plant air flow system to
provide adequate control.
COOLING
The exhaust gases to be treated are sometimes too hot for the control equipment, and
must first be cooled. Cooling may also drop the temperature below the condensation
point of some pollutants, so that they may be collected as liquids. Dilution, quenching,