766 NITROGEN OXIDES REDUCTION
where
m molar ratio of ammino to NO x
h NO x removal efficiency
S limit of ammonia slip, ppmdv
NO x,in inlet concentration of NO x, ppmdvWith the values of m and h know, one can then calculate the
space velocity. The volume of catalyst is then determined by
dividing the volumetric gas flow by the space velocity.
The last parameter that affects SCR performance is dust
loading. This parameter is particularly important when con-
sidering coal-fired plants. The dust from the coal pulveriza-
tion process can accumulate on the SCR catalyst, creating
plugging and fouling problems and decreasing performance.
Thus, in pulverized coal-fired applications, soot blowers are
sometimes installed upstream of the catalyst layers to “dust
off” the catalyst surface.
Selective noncatalytic reduction (SNCR) The other
major type of post combustion control is selective non-
catalytic reduction. SNCR reduces NO x through the same
reactions as SCR. However, SNCR differs from selective
catalytic reduction in a number of ways, most notably the
lack of catalyst for reaction. Because of the lack of cata-
lyst, NO x reduction in a SNCR must occur at higher tem-
peratures. Typically, the temperature of the process is in the
range of 1,600–2,000F, with an optimum temperature of
approximately 1,750F.^26 In a SNCR system, a urea based
compound can also be used besides ammonia as a reagent.
Urea based compounds break down to ammonia and carbon
dioxide after injection into the flue gas stream. SNCR does
not typically provide the same reductions as SCR, but some
studies and tests have shown NO x reductions using SNCR to
be in the range of 60–80%.
As noted, one of the most important operating param-
eters for SNCR performance is the temperature. Typically,
for ammonia injection, the temperatures should be main-
tained from 1,600F to 1,750F. For urea based compounds,
1,000F to 1,900F is the optimum temperature range. If the
temperature increases above the optimal range, then the
reagent will react more with oxygen than with the NO x , lead-
ing to increased NO x formation. On the other hand, if the
temperature falls below the optimal range, then less ammo-
nia will react, creating more ammonia slip.
The disadvantages of using SNCR are the same as that of
SCR. Both ammonium sulfate and ammonium bisulfate can
form in a SNCR system, creating the fouling and corrosion
problems. Ammonium chloride, NH 4 Cl can also be formed,
resulting in plumes that are more visible. Furthermore, SNCR
requires strict control of the temperature and does not pro-
vide the same reductions in NO x as SCR.
The advantages of SNCR over SCR include lower
capital and operating costs and safer handling of chemi-
cals when urea based reagents are used. A comparative
study of the two processes on a 200 ton/hr oil and gas fired
boiler revealed that the SNCR would require a capital cost of
$0.4 million as compared to a capital cost of $2 million for
SCR. The NO x reductions in this study for the SNCR were
predicted to be 65–70%.^31Fluidized bed combustors or boilers For new power
or cogeneration plants, one method of meeting the increas-
ingly stringent air pollutant emission standards is by selecting
fluidized bed combustion as the technology for combusting
the fuel. There are a number of different fluidized bed tech-
nologies available: (1) atmospheric fluidized bed combustion
(AFBC); (2) circulating fluidized bed combustion (CFBC);
and (3) pressurized fluidized bed combustion (PFBC). Based
on the technology selected and on process variations, the FBC
technology has an efficiency range of 37–45%. Figure 10^32 is
a schematic of a typical PFBC.
The basic principle behind all FBC’s is that air is injected
upwards through a bed of inert particles (i.e. sand) at a suffi-
cient rate so as to give the bed fluid-like characteristics. The
particle bed is normally heated to just above the ignition
temperature of the fuel, normally 850C. Most conventional
boilers are operated at 1600C. Recall that NO x emissions
increase with increasing flame temperature. Thus, one can
see that NO x emissions will be reduced substantially in a
FBC because of the reduced combustion temperature. As
a matter of fact, almost all NO x emissions from an FBC
can be attributed to fuel NOx. Combined with other NO x
control techniques, such as staged combustion or SNCR,
FBC provides a reasonable alternative to conventional fired
boilers for reducing pollutant emissions without sacrificing
efficiency.Mobile Source ControlSpark ignition engines There are a number of methods used to
reduce pollutant emissions from spark ignition engines. These
methods are based on two different approaches: (1) combus-
tion based; (2) exhaust gas treatment.
Combustion based emission controls for NO x include
spark retard and exhaust gas recirculation (EGR).^18 In spark
retard, combustion initiation is delayed to reduce the peak
pressure and peak temperature. As noted earlier, reducing
the peak temperature of combustion results in lower NO x
emissions. The other type of method, EGR, involves the
recirculation of cool combustion products for mixing with
the pre-combusted air and fuel mixture. This method reduces
the peak temperature through dilution and by increasing the
specific heat of the gas due to the addition of water carbon
dioxide. Although effective in reducing NO x emissions,
spark retard and EGR must be somewhat limited due to the
engine power loss, increased fuel consumption and increased
carbon monoxide emissions associated with these control
measures.
There are two basic types of exhaust gas treatment:
(1) noncatalytic thermal reactors; and (2) catalytic convert-
ers^18. Noncatalytic thermal reactors can reduce hydrocarbon
and CO emissions substantially, but is not as efficient in alle-
viating the NO x problem. In such a system, NOx is limited by
ammonia reduction. However, the ammonia-NO x reaction,
without catalyst, requires a high and narrow range of tem-
peratures, thereby limiting the application of this technol-
ogy. On the other hand, catalytic converters have become the
standard on most automobiles in the United States. The basicC014_002_r03.indd 766C014_002_r03.indd 766 11/18/2005 1:26:56 PM11/18/2005 1:26:56 PM