BR-1841 Deactivation of SCR Catalyst by Phosphorous ...

Technical Paper

BR-1841

Deactivation of Selective Catalytic Reduction
(SCR) Catalyst by Phosphorous:
Proposed Mechanism and Possible Solution

Authors:
M. Gadgil
B.S. Ghorishi
K.A. Larson
X. Guo
Babcock & Wilcox
Power Generation Group, Inc.
Barberton, Ohio, U.S.A.
D. Silbaugh
Black Hills Power Corporation
Gillette, Wyoming

Presented to:
EPRI Power Plant Air Pollutant
Control Mega Symposium
Date:
August 30 - Sept. 2, 2010
Location:
Baltimore, Maryland, U.S.A.

Deactivation of Selective Catalytic Reduction (SCR) Catalyst by
Phosphorous: Proposed Mechanism and Possible Solution

M. Gadgil D. Silbaugh
B.S. Ghorishi Black Hills Power Corporation
K.A. Larson
Gillette, Wyoing, U.S.A.
X. Guo
Babcock & Wilcox BR-1841
Power Generation Group, Inc.
Barberton, Ohio, U.S.A.

Presented to:
EPRI Mega Symposium 2010
August 30 - Sept. 2, 2010
Baltimore, Maryland, U.S.A.

Abstract or what is commonly referred to as a staged combustion

A number of power plants burning western Powder design. A staged combustion design diverts secondary air
River Basin (PRB) coal have experienced an unanticipated
rapid deactivation of catalyst during the Selective Catalytic away from the combustion zone and then introduces it into
Reduction (SCR) process. This rapid deactivation is not
observed on all PRB units. One key potential contributor the upper furnace through overfire air (OFA) ports. Lower
to this deactivation process is believed to be gas-phase
phosphorous (P). This deactivation phenomenon has been NOx levels are obtained as a result of the oxygen deficient/
primarily observed at SCR installations firing PRB coal with fuel rich reducing atmosphere created in the lower furnace
staged combustion. The effects of air staging on the genera-
tion of gas-phase P and an interpretation of the mechanism in combination with OFA ports located physically above
of P poisoning of the catalyst are discussed. As a possible
solution to reduce or eliminate rapid catalyst deactivation, the peak furnace temperature zone, in which air necessary
full-scale field test results applying Babcock & Wilcox
Power Generation Group’s (B&W PGG) patent-pending to complete combustion is then reintroduced. Utilization
additive technology at Black Hills Power’s 100 MWe Wygen
II unit are presented. Results obtained so far have indicated of staged combustion and low NOx burners coupled with
the effectiveness of this technology on reducing gas-phase an SCR are common practice to achieve the desired NOx
phosphorous concentration entering the reactor. control at an economical cost. As such, there are many power

Introduction plants burning PRB coal with low NOx burners and staged
combustion. Since staged combustion lowers the furnace
Selective catalytic reduction (SCR) is a widely used
technology for post combustion nitrogen oxides (NOx) outlet NOx levels, the construction and operating costs of the
reduction [1]. In the SCR DeNOx process, NOx is reduced SCR are lower for a given compliance level. For example,
to nitrogen (N2) and water vapor (H2O) by reaction with am-
monia (NH3). This reaction takes place in the presence of a if we consider two identical plants, where one unit is staged
catalyst. Commercial catalyst typically contains vanadium
(V) as the active catalytic element supported on a titanium and the other unit is unstaged, the plant without OFA will
oxide (TiO2) substrate. SCR catalyst is used for NOx control
in power plants firing both eastern bituminous and western have a NOx concentration leaving the boiler approximately
PRB coals. Another technology for lowering NOx formation 25-30% more than the plant with staged combustion. Staged
in the furnace is the use of low NOx burners with air staging,
combustion will result in lower operating cost due to less

ammonia consumption.

Exposure to the flue gas over a period of time results in

the loss of catalyst reactivity. The catalyst deactivation can

be due to ash plugging, masking, and chemical poisoning.

Plugging and masking are classified as physical deactiva-

tion. In chemical deactivation the catalyst poisons react

with V, thus reducing the extent of DeNOx reaction. The
chemical deactivation depends on the type of fuel burned

and the combustion environment. Combustion of different

coals can result in different SCR poisons. In plants burning

eastern bituminous coals, arsenic (As) in the coal is the main

factor causing the chemical deactivation. In PRB coal-fired

Babcock & Wilcox Power Generation Group 1

plants, the main chemical deactivation agent is believed in-stack thimble and a backup filter to collect the ash inside
to be P. In lignite coal-fired plants, the main deactivation the duct. The impinger solutions used to collect the gas
agents are believed to be potassium (K) and sodium (Na) were also modified. The first impinger solution was filled
in addition to P. with deionized (DI) water as it was observed that P (flyash
or gas-phase) was insoluble in DI water and slightly acidic
Power plants burning PRB coal and operating unstaged do solutions. The other two impingers were filled with a mixture
not appear to suffer from rapid catalyst deactivation. Many of of hydrogen peroxide and nitric acid. The first impinger
the staged power plants burning PRB coal experienced rapid served as an additional filter to catch any particulate break-
SCR catalyst deactivation early in the life of the catalyst. through. This technique was tested at different power plants
The analysis of many different deactivated catalyst samples burning PRB coal and was found to be a reliable method to
from different locations where early deactivation occurred measure gas-phase P.
had two features in common. All of the analyzed samples
had higher amounts of P accumulated on the catalyst, and Figure 1 summarizes the results obtained at the inlet of
the plants were utilizing staged combustion with low NOx various SCRs. Using B&W PGG’s measurement techniques
burners. Fuel analysis of PRB coal indicated the presence of (both gas-phase and flyash) a mass balance was conducted
about 0.5-1.0% P in coal ash. In a recent SCR conference [3] for the total P entering the furnace, entering the SCR, and
the link between P deactivation of SCR catalyst and staged then leaving the SCR. This mass balance was successfully
combustion was highlighted. It was stated that following closed within acceptable error limits (+ or -20%). Simul-
low NOx burner retrofit, the catalyst started experiencing taneous gas-phase P sampling at the SCR inlet and outlet
higher deactivation. This trend was observed in plants with showed higher gas-phase P at SCR inlet as compared to
different combustion configurations such as wall-, tangen- SCR outlet. This further confirmed the accumulation of
tial- and cyclone-fired. gas-phase P inside the SCR catalyst. Based on the mass
balance calculations, a rate of gas-phase P emission enter-
Field research on P-deactivation of SCR ing and leaving the SCR reactor was calculated. Based
on this accumulation rate over the period of exposure and
B&W PGG initiated an investigation of catalyst deacti- an analysis of SCR catalyst for accumulated P, the role of
vation by P in 2007. There are two sources by which P can gas-phase P in chemical deactivation of SCR catalyst was
enter the SCR catalyst. One is in the form of gas-phase P; further confirmed.
the other is P associated with the flyash. The latter source
was investigated first as there was no reliable technique at Laboratory research of P-deactivated
that time to measure gas-phase P concentration entering the SCRs and the role of staged combustion
SCR. Size-separated flyash samples were collected from
the plants experiencing rapid catalyst deactivation. These Advanced Raman spectroscopy, scanning electron
samples were analyzed by using leaching techniques to de- microscopy/Energy dispersive spectroscopy, and intrinsic
termine the possibility of flyash P transferring into the SCR kinetic studies were performed by the Babcock &Wilcox
catalyst causing a chemical deactivation. Two pHs for this Research Center (BWRC) to understand P deactivation
leaching study were chosen with one being slightly acidic of SCR catalyst. It was observed that mechanism of gas-
(pH = 5) and the other an alkaline (pH = 9) since these are phase P deactivation of SCR catalyst involved chemical
the two most likely pH conditions that can be found under deactivation by means of possible reaction between V and
startup and shutdown of the SCR. It was observed that under P forming V-P-O compounds. This mechanism appeared
the two pH conditions negligible amounts of P can leach out, to be very similar to the deactivation of SCR catalyst by
leading to the conclusion that flyash P is an unlikely source of As where it was found that As also reacted with V to form
SCR catalyst deactivation. Following this analysis, an effort
began to determine if gas-phase P was the main deactivation Fig. 1 Gas-phase phosphorous measurement by B&W PGG-modi-
agent similar to gas-phase As, as observed in many plants fied method at SCR inlet location of different PRB-fired plants.
burning eastern bituminous coal [4].

To support this next effort, B&W PGG developed a modi-
fied procedure to reliably measure gas-phase P at high dust
locations such as the SCR inlet. The main problem at high
dust locations is the interference of the particulate-bound
phosphorous with the impinger solution. As a result of this
interference, the prior gas-phase P measurements determined
by other methods produced highly variable results. B&W
PGG believes that this variability was due to particulate P
breaking through into the impingers and reporting as gas-
phase P. Following numerous investigations, it was decided
to modify the standard EPA Method 29 (M29) for gas-phase
P measurement. The modifications included installing an

2 Babcock & Wilcox Power Generation Group

V-As compounds, thus deactivating the catalyst. The link Fig. 2 Demonstration of reduction in gas-phase at SCR inlet as a
between catalyst deactivation and staged combustion was result of additive injection at Wygen II.
still unclear at the start of this research.
conditions before the injection testing and also during and
Mineral analysis of the PRB coal revealed that the P was after the injection test. It was observed that the gas-phase
mainly associated with calcium (Ca) in the form of calcium concentration decreased by about 50-60% during additive
phosphate, which is a very stable compound (high melting injection testing as compared to the baseline conditions. The
and boiling point) under unstaged combustion conditions. result of this test is shown in Figure 2. Memory effect was
A coal sequential extraction technique also revealed that a observed after the injection of additive was stopped. The
negligible amount of P was organically associated within the memory effect highlights two points. One is the fact that
coal matrix. It is believed that organic P is readily released more material was injected and it will be possible to optimize
into the gas phase. Further review of the staged combustion the injection rate in the future. The second point highlighted
process highlighted the presence of reducing atmosphere in by memory effect is the injected material may have scav-
the furnace as previously discussed. Based on the observed enged the gas-phase phosphorous after it got deposited on
phenomenon of the formation of vapor phase silicon mono- walls or economizer tubes, etc., in the upper furnace.
oxide (SiO) under reducing conditions as mentioned by
Quann [2] due to carbothermic reduction of SiO2, a theory A second test was performed to check for repeatability of
was proposed involving carbothermic reduction of phospho- the data. This result is shown in Figure 3. Similar observation
rous-bearing inorganic compound resulting in the release of in the percentage reduction of gas-phase P was observed in
gas-phase phosphorous. The reaction involved consists of this test under similar injection rates. This demonstrated the
P-bearing inorganic compound + CO (g)- P (g) + CO2 ability of the additive to lower the gas-phase P concentration
(g). This mechanism could potentially explain increased entering the SCR.
deactivation of the catalyst by P under staged combustion
conditions. In staged combustion there is much more carbon Fig. 3 Demonstration of reduction in gas-phase at SCR inlet as a
monoxide (CO) in the lower furnace which accelerates the result of additive injection with good repeatability (the second test
release of gas-phase P as compared to unstaged combustion. campaign).

Additive development for control of
gas-phase P

In this study a similarity between As and P poisoning of
the SCR catalyst was established. A commercially avail-
able technique for As poisoning mitigation is the injection
of limestone with coal [4]. Ca compounds will react with
gas-phase As, sequestering it in a solid form, thus prevent-
ing gas-phase As attack on the SCR catalyst. Based on
this technique B&W PGG embarked on an investigation
for potential additives that can sequester gas-phase P com-
pounds. A suitable, cost-effective and naturally occurring
material was identified which could tie up gas-phase P under
combustion conditions as particulate-bound P. This lowers
the concentration of gas-phase P or its aerosols entering the
SCR. Detailed studies were performed to investigate the ef-
fects of this material on boiler operation and to confirm no
additional concerns, particularly in the areas of slagging and
corrosion. The required coal addition rate of this material
was also analyzed. Finally, leaching studies were performed
to observe stability of this new particulate-bound P under
standard SCR operating conditions.

The Wygen II power plant located in Gillette, Wyoming,
is a 100 MWe B&W PGG subcritical PRB coal-fired power
plant. This plant uses deeply staged combustion and an SCR
for NOx control. This plant previously experienced rapid
SCR catalyst deactivation due to P poisoning. The B&W
PGG additive was first tested at Wygen II in two short-term
(4-5 days) injection tests in 2009. The additive was injected
at rates less than 0.5% of the total plant coal flow rate using
pneumatic conveying. Gas-phase P was measured at baseline

Babcock & Wilcox Power Generation Group 3

To demonstrate that lower gas-phase P levels entering the Acknowledgement
SCR will result in decreased deactivation rates for staged
combustion designs a long-term test is currently being con- B&W PGG wishes to acknowledge and sincerely ap-
ducted at Wygen II. This long-term injection will continue preciates the cooperation obtained from Black Hills Power
until the end of this year (2010) resulting in approximately Corporation’s Wygen II generation facility in support of this
6900 hours of operation. A catalyst sample will be obtained long-term additive testing.
at the end of the injection period to test its activity and P
accumulation. Although not conclusive, a sample has been References
obtained from approximately 2000 hours of operation.
1. Stultz, S.C. and Kitto, J.B., Steam/its generation and
Conclusion use, 40th edition, The Babcock & Wilcox Company,
1992.
Gas-phase P causes rapid deactivation of SCR catalyst.
Higher gas-phase P concentrations are produced as a result 2. Quann, R.J., “Ash vaporization under simulated pul-
of staged combustion. The greater the extent of combustion verized coal combustion conditions,” PhD thesis.
staging, the higher the rate of deactivation as a result of
increased release of gas-phase P. B&W PGG has developed 3. 2009 EPRI SCR workshop presentation by Ceram
a coal additive that has the ability to significantly reduce Environmental.
the gas-phase phosphorous concentration entering the SCR
catalyst. Long-term field testing is currently underway at 4. Ake, Terrence, et al, “Limestone injection for protec-
Black Hills’ Wygen II power plant to demonstrate the ef- tion of SCR catalyst,” Presented to EPRI-DOE-EPA-
fectiveness of the B&W PGG additive in lowering gas-phase AWMA Combined Power Plant Air Pollutant Control
P accumulation in the catalyst, and thus improvement in the Mega Symposium, Washington, DC, 2003.
catalyst life.

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a Babcock & Wilcox company
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