Power Plant Engineering

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 NO
x
emissions increase with oil burning
compared to gaseous fuel burning. There are several reasons for this, including:

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POLLUTION AND ITS CONTROL
425
•
The additional time required for vaporization,
•
The more complex hydrocarbons involved in the fuel,
•
Poorer mixing of the fuel vapor with the air,
•
Higher localized temperatures (leading to higher NO
x
),
•
And the formation of deposits in the combustor which can adversely affect combustion ‘good-
ness’ by affecting flame shape and burner aerodynamics.
Also, some liquid fuels contain sulfur, organic nitrogen, and mineral elements, which lead to
additional pollution. Generally, distillate fuel oil and diesel fuel are low in these compounds, and gaso-
line and aviation fuel are even lower yet in these compounds. Other (heavier) fuel oils are relatively high
in these compounds. Sulfur in the fuel reacts essentially completely to sulfur dioxide (SO
2
) upon com-
bustion. Also, a small fraction of the sulfur oxidizes to sulfur trioxide (SO
3
), which is a problem because
it will readily form sulfur acid mists if the exhaust temperature is too low. Thus, in order to prevent
corrosion of the energy system equipment, the exhaust temperature may be maintained higher than it
would be without the sulfur. Thus, thermodynamic efficiency is degraded, and the ratio of CO
2
to unit of
electrical energy produced increases. Sulfate particulate emission can also result from the sulfur. (Within
the 3-way catalyst of an automotive engine, the small amount of SO
2
present can be reduced to hydrogen
sulfide (H
2
S). Ever smell this?) Organic nitrogen in the fuel, also called fuel-bound nitrogen, will end up