Power Plant Engineering

13.9.4 ACID FOG
A recently noted major acid pollutant is acid fog. Its origin is the same as acid rain or snow, i.e.,
sulfuric and nitric oxides from power plants and, to a lesser extent, motor vehicles. It forms by the
mixing of these pollutants with water vapor near the ground. The acid vapors then begin to condense
around very tiny particles of fog or smog, pick up more water vapor from the humid air, and turn into
acid fog. When the water in the fog burns off (evaporates) due to the sun or other causes, drops of nearly
pure sulfuric acid are left behind. It is these drops that make acid fog so acidic. In Los Angeles and
Bakersfield in southern California, the mists have a pH of 3.0 compared with 4 or 4.5 for acid rain. Acid
fog 100 times as acidic as acid rain has been detected. Cases have been reported where people had

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POLLUTION AND ITS CONTROL
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trouble breathing when it was foggy. The problems of fog are now believed by some to be more serious
than those of smog in these areas.
Many researchers consider the effects of acid precipitation, especially the changing of soil chem-
istry, to be irreversible and fear its long-range effects. Monitoring programs of air, soil, and water are
being instituted to ascertain these long-range effects. How-ever, the uncertainty about the real extent of
the problem adds to the prevailing disquiet regarding it.
13.10 POLUTION DUE TO COMBUSTION OF FUEL
13.10.1 Gas Fuel
When a gaseous fuel, such as natural gas, burns it undergoes a series of processes. These are as
follows:
•
Either in a premixer or in the combustion chamber, the fuel must mix with air.
•
The fuel (and the air) must mix with hot, burning and burnt gases present in the chamber. The
burning and burnt gases provide heat and active species (O, H, and OH), which upon mixing
with the fresh fuel and air cause the fresh fuel to ignite and react with the air.
•
The fuel then undergoes a series of chemical reactions.