NITROGEN OXIDES REDUCTION 765
- amount of ammonia
- ammonia slip
- catalyst surface area
- dust loading.
Ammonia slip and sulfur content of the fuel are two param-
eters that are interrelated and must be closely monitored.
Theoretically, according to the balanced chemical reac-
tions, only one mole of ammonia is needed to break down
one mole of NO x. Because mixing of the ammonia-air mix-
ture and distribution of the mixture with the flue gas is not
ideal, ammonia is normally injected at higher quantities
than is theoretically required. This overinjection can lead to
residual ammonia escaping into the flue gas, which is called
ammonia slip. Although higher values of ammonia slip lead
to increased NO x reductions (to an asymptotic limit), it is
environmentally detrimental to allow excess ammonia to the
atmosphere. Furthermore, because the sulfur in the fuel leads
to SO 2 and SO 3 production, excess ammonia can result in the
formation of ammonium sulfate and ammonium bisulfate by
the following reactions:^29SO 3 2NH 3 H 2 O → (NH 4 ) 2 SO 4
SO 3 NH 3 H 2 O → NH 4 HSO 4.The formation of sulfates can lead to higher particulate
concentrations in the flue gas and can lead to catalyst poi-
soning and equipment deterioration. The formation of sul-
fates can be mitigated by keeping the temperature of the
gas relatively high ( 600 F) and by limiting ammonia slip
to 10 ppm for low sulfur fuels and to 3–5 ppm for high
sulfur fuels.
Oxygen and water vapor concentrations have opposing
effects on NO x reduction. Typically, higher oxygen con-
centrations result in better catalyst performance leading to
improved NO x reductions, to a certain limit. On the other
hand, water vapor decreases the performance of the catalyst
as its concentration increases.
Obviously, the type of catalyst is extremely important to
the performance of the SCR. As noted earlier, catalysts are
available in a variety of materials. In addition, the catalyst
shape can vary, including rings, pellets, honeycomb plates and
parallel-ridged plates. Although it depends on the particular
catalyst, flue gas temperature is critical to the performance of
the catalyst. Typically 600F to 700F is the optimum range of
operation. Below these temperatures, formation of ammonium
sulfates becomes more substantial leading to further complica-
tions. Figures 8 and 9^30 show the temperature dependence of
ammonia slip and NO x reductions on two catalysts, VNX and
ZNX. The VNX catalyst is a low temperature catalyst and is
comprised of V 2 O 5 and TiO 2. The ZNX catalyst is a high tem-
perature zeolite catalyst. In addition to temperature, the age of
the catalyst affects the catalyst performance. Usually, the rate
of deterioration is large at the onset of operation, but levels
out after the initial settlement. Another important parameter
is the specific surface area of the catalyst. The larger the area
of the catalyst exposed to the flue gas, the better the SCR
performance.In addition to the surface area of the catalyst, the residence
time of the flue gas in the reactor is critical to good perfor-
mance. Related to the residence time is the space velocity,
which is a parameter that is normally used for SCR reactor
design. The efficiency of the SCR can be expressed in terms
of the space velocity through the following equation:^29h m(1 e k/SV )
where
h NO x removal efficiency
m molar ratio of ammonia to NO x
SV space velocity
k an activity constant that takes into account most of
the operating parameters listed earlier.The molar ratio of ammonia to NO x is determined by:^29m h S/NO x, (^) in
100
90
80
70
60
50
250 300 350 400 450
0
10
20
30
40
50
Temperature(°C)
NOx Conversion (%) NH 3 Slip (% Unconverted NH 3 )
NH 3 /NOx =1.0
NH 3 /NOx =0.9
FIGURE 8 NOx Conversion and NH 3 Slip for VNXTM SCR
Catalyst.^30
100
90
80
70
60
50
250
0
10
20
30
40
50
Temperature(°C)
NOx Conversion (%) NH 3 Slip (% Unconverted NH 3 )
NH 3 /NOx =1.0
NH 3 /NOx =0.9
300 350 400 450 500 550 600 650
FIGURE 9 NOx Conversion and NH 3 Slip for ZNXTH SCR
Catalyst.^30
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