Environmental Engineering Reference
In-Depth Information
downstream
of the scrubber, in contrast to the wet scrubber, where the particle removal system,
usually an ESP, is
upstream
of the scrubber. Thus, the dry scrubber does not create a gelatinous
sludge that is difficult to transport and dispose of, but it does impose a larger load on the particle
removal system.
The dry scrubber SO
2
removal efficiency is not as high as that of the wet scrubber, amounting
to 70-90%. Its capital and operating costs are somewhat lower than those of a wet scrubber.
In the United States, all new coal-fired power plants must install a scrubber, either a wet or a
dry scrubber, depending on the sulfur content of the coal. At present, in the United States about
25% of coal-fired power plants have installed a scrubber. In Germany and Japan practically all
coal-fired power plants have installed a scrubber.
5.2.9.4 Nitrogen Oxide Control
The other major category of pollutants that emanates from fossil fuel combustion is nitrogen oxides,
called NO
x
, which includes nitric oxide NO, nitrogen dioxide NO
2
(and its dimer N
2
O
4
), nitrogen
trioxide NO
3
, pentoxide N
2
O
5
, and nitrous oxide N
2
O. Other than NO and NO
2
, the other oxides
are emitted in minuscule quantities, so that NO
x
usually implies the sum of NO and NO
2
.
NO
x
is a pernicious pollutant because it is a respiratory tract irritant and it is a precursor to
photo-oxidants, including ozone, and acid deposition (see Section 9.2.6). At this time, the U.S. EPA
plans to introduce a stricter ambient standard for ozone, which necessarily implies a more strin-
gent control of NO
x
emissions from both stationary and mobile sources, including electric power
plants.
Coal and oil contain organic nitrogen in their molecular structure. When burnt, these fuels
produce the so-called
fuel
NO
x
. In addition, all fossil fuels produce
thermal
NO
x
. This results from
the recombination of atmospheric nitrogen and oxygen under conditions of the high temperatures
prevailing in the flame of fossil fuel combustion:
N
2
+
O
2
↔
2NO
(5.7)
The recombination involves intermediate radicals, such as atomic oxygen and nitrogen, and organic
radicals, which are formed at the high temperatures. As the combustion gases cool, the formed NO
does not revert to N
2
and O
2
, as it would if thermodynamic equilibrium prevailed at the gas stack
temperatures. As the flue gas traverses the stack, a part of the NO oxidizes into NO
2
and other
nitrogen oxides.
Coal and oil combustion produce both fuel and thermal NO
x
, whereas natural gas produces
only thermal NO
x
. As a rule of thumb, coal and oil produce about equal amounts of fuel and thermal
NO
x
. The flue gas of uncontrolled coal and oil combustion contains thousands of parts per million
by volume of NO
x
, whereas that of natural gas contains half as much.
Because organic nitrogen cannot be removed prior to combustion of the fuel, NO
x
emission
control can only be achieved during and after combustion.
During Combustion
Low-NO
x
Burner.
A low-NO
x
burner (LNB) employs a process called
staged combustion
. The
fact that NO
x
formation is a function of air-to-fuel ratio (by weight) in the flame is exploited in
LNB. This ratio affects the flame temperature and the availability of free radicals that participate
in the NO
x
formation process. A plot of NO
x
concentration in the flame versus air-to-fuel ratio
