POWER December 2013 - 47

EMISSIONS
surrounding the SCR. From an SCR perspective,
de-NOx
performance is well defined
and controlled within the SCR reactor
by catalyst selection, cross-sectional area,
number of modules, and so on. However,
total Hg removal must carefully consider
the mercury removal performance of equipment
downstream of the SCR, such as the
air heater, particulate control device(s), and
the FGD. A number of system-level factors
relative to characterizing and understanding
catalyst performance must also be considered
(Table 1).
Cormetech's early product development
work to quantify the co-benefits SCR + FGD
mercury removal used a mercury activity test
reactor system in conjunction with a multilayer
catalyst system to perform multiple
parametric tests. For example, one set of
tests characterized layer position and halogen
content for a given catalyst. Tests were also
conducted on new as well as aged catalysts
taken from operating plants. This critical
data enables accurate prediction of catalyst
Hg oxidation performance for unique applications.
Typical results from those tests show
that NOx
reduction is a strong function of the
operating temperature of the reactor and the
Table 1. Key factors that influence
SCR de-NOx performance and Hg
oxidation performance. Of particular
importance are the concentrations of halogens
and reducing agents, such as CO and
NH3, present in the stack gas. Halogens,
whether present in the fuel or as an additive,
favorably influence SCR catalyst performance.
The ammonia concentration is a function of
the desired de-NOx performance, layer position,
and chemistry. Source: Cormetech Inc.
De-NOx
NOx inleta
Efficiencya
Slipa
Temperature
SO2 conversionb
Mercury
NOx inlet
NH3 efficiency
NH3 slip
Hg oxidationa
Temperature
Fuel->contaminants->K/Ko SO2 conversionb
Reactor condition
O2, H2O, SO2 (lower
impact)
Fuel->contaminates
->K/Ko
Reactor condition
Halogen (fuel or additive)
Layer
position (NH3)
CO
O2, H2O, SO2 (can be
larger impact)
Notes: a. Performance threshold; b. Catalyst
formulation
December 2013 | POWER
HCl present in the gas. In addition, the Hg0
oxidation to Hg2+
is a strong function of the
halogen content (HBr) and operating temperature
(Figure 1).
Catalyst Design and Selection
Traditional catalyst management techniques
provide very accurate predictions of de-NOx
performance for various fuels and fuel additives.
At the heart of the catalyst management
process for de-NOx
Equation 4: K de-NOx
molar ratio = 1
managed simultaneously. Thus it is important
to develop equations and tools to accurately
model mercury oxidation performance as has
been done with de-NOx
. Again, a simplified
approach to describe catalyst oxidation potential
is shown in Figure 1 and represented
by Equation 5.
Equation 5: K HgOx
is a simplified method to
describe catalyst oxidation potential, shown
in Figure 1 and represented by Equation 4.
NOx efficiency), where K de-NOx
lyst de-NOx
NH3:NOx
/AV = ln (1- de=
cataactivity,
AV = area velocity, α =
The addition of mercury oxidation as a
performance feature of the SCR adds complexity
to the management process where
both de-NOx
and Hg oxidation needs must be
oxidation) at α = variable, where K HgOx
/AV = ln (1- Hg0
=
catalyst Hg oxidation activity and AV = area
velocity
Although the equations are similar, the
usefulness of the equation for Hg oxidation is
influenced to a greater extent by the ammonia
concentration. Therefore, instead of simplifying
the assessment to one molar ratio,
multiple molar ratios must be considered to
properly assess the performance capability of
a given catalyst layer in a particular position
within the reactor. Plant-specific conditions
related to fuel parameters, such as halogen
1. MATRS test data. The SCR Hg0 oxidation rate is a function of halogen content and
temperature (right). The SCR performance of the same catalyst is shown for comparison (left).
The parametric studies used fresh and field-aged catalyst. Source: Cormetech Inc.
400C
340C
100
90
80
70
60
50
40
30
20
10
20
40 60 80 100 120
HCl (ppmvda)
100
90
80
70
60
50
40
30
20
10
0.0
0.5
1.0
1.5
HBr (ppmvda)
2. COMET catalysts perform. Test results of COMET versus a standard de-NOx catalyst
illustrate a significant improvement in Hg0 oxidation rates. The ability of the catalyst to oxidize
mercury is represented by the K HgOx, the catalyst oxidation activity (Equation 5). The data was
taken at 403C; 107 ppm of NOx; 3.5% O2; 14% H2O; 245 ppm SO2; and 8 ppm of HCl. Source:
Cormetech Inc.
Standard
COMET
2
1.5
1
0.5
1
NH3
ppm
CO ppm
HBr ppm
2
3
100
www.powermag.com
4
0.1
5
100
0.1
1
6
21
7
21
0.1
8
21
100
0.1
9
21
1
10
86
47
2.0
2.5
Relative KHgOx
deNOx (%)
Hg oxidation (%)

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POWER December 2013

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