424 POWER PLANT ENGINEERING13.11.4 Soot
Soot is composed of particles of a few hundred angstroms in size. Generally, soot is a fluffy
carbon material, though it is generally not pure carbon. Soot forms under hot, fuel-rich conditions, and,
once formed, oxidizes relatively slowly. (Soot requires on order of 100 milliseconds to oxidize at typical
combustion temperatures.) Soot is controlled by adding air to the flame zone, thus eliminating the hot,
rich pockets of gas, which produce it, or by providing a secondary combustion zone of sufficient O 2 ,
temperature, and time to oxidize the soot.
A low emission combustor would have CO and NOx exhaust concentrations of less than about
100 ppm (parts per million by volume) each, and an UHC emission of under about 10 ppm. A very low
emission system would have CO and NO of about 10 ppm each, and UHC less than a few ppm's. High
emission burners have CO and NOx emissions in the several hundred to a few thousand-ppm ranges.
New combustors and emission control methods are being developed ‘every day,’ and for some
situations the goal of ‘zero emissions’ is being approached. Of course, the term ‘zero emissions’ does
not mean absolute zero emission. It depends somewhat on the ‘reference point.’ Generally, it means CO
and NOx emissions in the few ppm range. For example, Honda recently announced a special new ex-
haust catalyst system, which gives very low emissions.
13.12 POLUTION DUE TO LIQUID FUEL
When a liquid fuel, such as oil, is burned, there are additional aspects of the combustion process
and the pollutant formation. Specifically:13.12.1 Atomization
It is common practice to inject the fuel into the combustor (or premixer) through a nozzle, which
atomizes the fuel. That is, the continuous stream of fuel is broken up into a mist of tiny droplets. There
are many types of nozzles, some of which rely on very high feed pressures to atomize the fuel, and some
of which rely on assistance from steam and air to effect good atomization. Generally, the finer the spray
produced by the nozzle the better the combustion process.13.12.2 Vaporization
The fuel droplets vaporize as they receive heat by mixing with the hot gases in the combustion
chamber. Heat can also be received by radiation from any hot refractory wall of the combustion cham-
ber.13.12.3 Modes of Combustion
If the vaporization process is fast compared to the reaction chemistry, the combustion of the
liquid fuel occurs mainly as clouds of vapor. Thus, the sequence of processes is atomization, vaporiza-
tion, mixing, and chemical reaction, and the 4-step chemical mechanism given above under Alkanes is
valid. On the other hand, if the vaporization process is slow, droplet burning can occur. That is, indi-
vidual flames may encircle individual droplets, thereby effecting oxidation of the oil.
Generally, though not necessarily, the UHC, CO, and NOx emissions increase with oil burning
compared to gaseous fuel burning. There are several reasons for this, including: