Page 234 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

210 7 Combustion Process and Air Emission Formation

SO 2 þ 1=2O 2 $ SO 3 ð7:52Þ

For coal combustion, sulfur in fuels is eventually oxidized to SO 2 and/or SO 3 ,
with a small amount being bound to ashes. For combustion of liquid fuels in engine,
SO 2 concentration in the engine exhaust depends on the type of fuel burned. Marine
engine exhaust usually has the highest SO 2 emission, and it can be in a level that is
comparable with coal fired power plants, (about 1 %) because marine diesel fuels
are of very low quality and high sulfur content.
The chemical equilibrium constant in Eq. (7.36) indicates that the concentration
of SO 3 increases with the decreasing combustion temperature. When the temper-
ature is below 900 K, SO 3 would have been the dominant SO x air pollutant at
equilibrium. However, in a typical flue gas or engine exhaust, SO 2 is still domi-
nating over SO 3 . One of the reasons is that the conversion from SO 2 to SO 3 is too
slow to reach an equilibrium state. In a typical engineering practice, about 3 % of
SO 2 is converted into SO 3 . Typical SO 2 concentration in a coal fired power plant
flue gas stream is in the order of 1,000 ppm (0.1 %). It shall be much lower in
modern gasoline or natural gas fired stationary combustion facilities because sulfur
has been removed from the fuels.
The oxidation of SO 2 to SO 3 is usually slow unless the combustion temperature
is above 1,100 °C or there are catalysts present to expedite the reaction. For
example, sulphur in the liquid fuels supplied to automobiles is oxidized to SO 2 in
the engine. But in the exhaust SO 3 is present due to the presence of various catalytic
metals in the fuel, engine housing, and/or the filtration systems. For stationary
combustion sources like a coal-fired power plant, there are also various metals in
the fuel, and some of them may serve as catalysts for the oxidation of SO 2 .

7.7 NO x

Nitrogen oxides (NO x ) represent the following seven oxides of nitrogen [45].
• nitric oxide (NO)
• nitrogen dioxide (NO 2 )
• nitrous oxide (N 2 O)
• dinitrogen dioxide (N 2 O 2 )
• dinitrogen trioxide (N 2 O 3 )
• dinitrogen tetroxide (N 2 O 4 )
• dinitrogen pentoxide (N 2 O 5 )
However, NO x are often referred only to NO and NO 2 by the environmental
protection agencies in most jurisdictions. It is because these two gases are the major
contributors to air pollution. Therefore, only NO and NO 2 are introduced in the
following section.