POLLUTION AND ITS CONTROL 421trouble 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.
13.10.2 Methane
If the fuel is methane (the primary constituent of natural gas), the following reaction steps occur.
Due to the heat and attack by the active species, the methane reacts to a methyl radical (CH 3 ),
which reacts to formaldehyde (HCHO). The formaldehyde reacts to a formal radical (HCO), which then
forms carbon monoxide (CO). Through these steps, the active species are used up and H 2 and H 2 O are
formed in addition to the CO. Overall, the process is as follows:
CH 4 + O 2 → CO + H 2 + H 2 O
Thus, the original fuel is converted into two new fuels (CO and H 2 O) and into one product
(H 2 O). The process occurs very quickly, within a fraction of a millisecond to a few milliseconds, de-
pending on the flame temperature, pressure, and fuel-air ratio. (Actually, methane is one of the slower
burning hydrocarbon gases.) The process is called Oxidative Pyrolysis.
Following oxidative pyrolysis, the H 2 oxidizes, forming H 2 O, replenishing the active species,
and releasing heat. This occurs very quickly, usually in less than a millisecond.
H 2 + (1/2) O 2 → H 2 O
Finally, the CO oxidizes, forming CO 2 and releasing more heat. This process is generally slower
than the other chemical steps, and typically requires a few to several milliseconds to occur.
CO + (1/2) O 2 → CO 2
The combustor may be thought of as having flame zones, in which the free radical activity is
high, the fuel undergoes oxidative pyrolysis, the hydrogen oxidizes, and the CO begins to oxidize, and
a post-flame zone, in which the CO continues to oxidize and reaches its final exhaust concentration.