# Deactivation of Selective Catalytic Reduction (SCR) Catalyst by Phosphorus: Proposed Mechanism and Solution

Authors: M. Gadgil

Babcock & Wilcox Power Generation Group, Inc. Barberton, Ohio, U.S.A.

Presented to: International Pittsburgh Coal Conference

Date: September 12-15, 2011

Location: Pittsburgh, Pennsylvania, U.S.A. 

# Technical Paper

BR-1856Babcock & Wilcox Power Generation Group 1 

# Deactivation of Selective Catalytic Reduction (SCR) Cata

lyst by Phosphorus: Proposed Mechanism and Solution

M. Gadgil Babcock & Wilcox Power Generation Group, Inc. Barberton, Ohio, U.S.A.

Presented at: International Pittsburgh Coal Conference September 12-15, 2011 Pittsburgh, Pennsylvania, U.S.A. BR-1856



## Abstract

Selective catalytic reduction (SCR) is a commonly used technology for post-combustion nitrogen oxides (NO x ) emission compliance. One of the important co-benefits of the SCR catalyst is simultaneous mercury oxidation along
with NO x reduction. The oxidized mercury can then be captured in downstream wet or dry scrubbers. Exposure of SCR catalyst to coal combustion flue gas results in the deactivation of catalyst. Air staged combustion is another very common technique used to lower NO x formation during coal combustion. Utilities firing Powder River Basin (PRB) coal with air staged combustion have been experiencing rapid deactivation of SCR catalyst. Utilities firing PRB coal with un-staged combustion have not experienced rapid deactivation. The rapid catalyst deactivation is believed to be primarily caused by gas-phase phosphorus (P). The investigation of this problem by Babcock & Wilcox Power Generation Group, Inc. (B&W PGG) began with field sampling of fly ash at utilities experiencing rapid catalyst de

activation. Size-separated fly ash samples were subjected to leaching tests to determine leachability of particulate-bound phosphorus. The leaching tests were conducted at pH level of 5, 7 and 9. Leaching studies ruled out particulate-bound phosphorus as the source of rapid deactivation. B&W PGG modified the existing gas-phase phosphorus sampling method to reliably measure gas-phase phos

phorus at high dust locations such as the SCR inlet. This modified method confirmed the presence of high gas-phase phosphorus concentrations at the SCR inlet of staged PRB units. Analysis of deactivated SCR catalyst logs showed the presence of vanadium-phosphorus bonds indicating achemical deactivation occurs due to the interaction of gasphase phosphorus with vanadium that is present in the SCR catalyst. It was also observed that the staged combustion process was contributing to release of gas-phase phospho

rus due to carbothermic reduction of phosphorus-bearing
minerals. A novel combustion additive and method was developed to tie up the gas-phase phosphorus before the SCR catalyst to reduce the rate of catalyst deactivation. A short-term proof of concept test was conducted at Wygen II power plant located in Gillette, Wyoming, to demonstrate reduction in gas-phase phosphorus due to the injection of combustion additive. Long-term additive testing has been proceeding at Wygen II since March 2010. In this paper, we will present the results of long-term com

bustion additive injection testing to demonstrate reduction in the rate of catalyst deactivation and thus, improvement in SCR catalyst life using B&W’s patent-pending combustion additive as compared to earlier experience. Results will also be presented for simultaneous phosphorus mitigation and mercury oxidation using a blended combustion additive.



## Introduction

Selective catalytic reduction (SCR) is a widely used technology for post-combustion nitrogen oxides (NO x ) reduction [1]. In the SCR NO x reduction process, NO x is reduced to nitrogen (N 2 ) and water vapor (H 2 O) by reaction with ammonia (NH 3 ). This reaction takes place in the pres

ence of a catalyst. Commercial catalyst typically contains2 Babcock & Wilcox Power Generation Group vanadium (V) as the active catalytic element supported on a titanium oxide (TiO 2 ) substrate. SCR catalyst is used for

NO x control in power plants firing both bituminous and PRB coals. Another technology for lowering NO x formation in
the furnace is the use of low NO x burners with air staging, or what is commonly referred to as staged combustion. A staged combustion design diverts secondary air away from the combustion zone and then introduces it into the up

per furnace through overfire air (OFA) ports. Lower NO x levels are obtained as a result of the oxygen deficient/fuel rich reducing atmosphere created in the lower furnace in combination with OFA ports located physically above the peak furnace temperature zone, in which air necessary to complete combustion is then reintroduced. Utilization of staged combustion and low NO x burners coupled with an SCR are common practice to achieve the desired NO x control at an economical cost. As such, there are many power plants burning PRB coal with low NO x burners and staged com

bustion. Since staged combustion lowers the furnace outlet

NO x levels, the construction and operating costs of the SCR are lower for a given compliance level. For example, if we consider two identical plants in which one unit is staged and the other unit is un-staged, the plant without OFA will have

a NO x concentration leaving the boiler approximately 25 to 30% more than the plant with staged combustion. Staged 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 vanadium (V), thus reducing the extent of NO x 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 bituminous coals, arsenic (As) in the coal is the main factor causing the chemical deactivation. In PRB coal-fired plants,the main chemical deactivation agent is P. In lignite coalfired plants, the main deactivation agents are believed to be potassium (K) and sodium (Na) in addition to phosphorus. Power plants burning PRB coal and operating un-staged do not appear to suffer from rapid catalyst deactivation. Many of the staged power plants burning PRB coal experi

enced rapid SCR catalyst deactivation early in the life of the catalyst. The analysis of many different deactivated catalyst samples from different locations where early deactivation occurred had two features in common. All of the analyzed samples had higher amounts of phosphorus accumulated on the catalyst, and the plants were utilizing staged combustion
with low NO x burners. Fuel analysis of PRB coal indicated the presence of about 0.5 to 1.0% P in coal ash. In a recent SCR conference [2] the link between P deactivation of SCR catalyst and staged combustion was highlighted. It was stated that following a low NO x burner retrofit, the catalyst began experiencing higher deactivation. This trend was observed in plants with different combustion configurations such as wall-, tangential- and Cyclone-fired. This paper describes the efforts at B&W PGG in understanding SCR catalyst deactivation by gas-phase phosphorus poisoning, and the de

velopment of a solution to minimize such fast SCR catalyst deactivation by B&W PGG’s patent-pending technology.



## Field Research on Phosphorus-Deac

- tivation of SCR

Recognizing a potential problem, B&W PGG initiated an investigation of catalyst deactivation by phosphorus. There are two sources by which phosphorus can enter the SCR catalyst. One is in the form of gas-phase phosphorus; the other is associated with the fly ash. The latter source was investigated first as there was no reliable technique at that time to measure gas-phase phosphorus concentration enter

ing the SCR. Size-separated fly ash samples were collected from the plants experiencing rapid catalyst deactivation. These samples were analyzed by using leaching techniques to determine the possibility of fly ash-bound phosphorus transferring into the SCR catalyst causing a chemical deac

tivation. Two pHs for this leaching study were chosen with one being slightly acidic (pH = 5) and the other an alkaline (pH = 9) since these are the two most likely pH conditions that can be found under startup and shutdown of the SCR. It was observed that under the two pH conditions, negligible amounts of phosphorus can leach out, leading to the conclu

sion that fly ash-bound phosphorus is an unlikely source of SCR catalyst deactivation. Following this analysis, an effort began to determine if gas-phase phosphorus was the main deactivation agent similar to gas-phase arsenic, as observed in many plants burning eastern bituminous coal [3].To sup

port this next effort, B&W PGG developed a modified pro

cedure to reliably measure gas-phase phosphorus at high dust locations such as the SCR inlet. The main problem at high dust locations is the interference of the particulate-bound phosphorus with the impinger solution. As a result of this interference, the prior gas-phase phosphorus measurements determined by other methods produced highly variable results. B&W PGG believes that this variability was due to particulate-bound phosphorus breaking through into the impingers and reporting as gas-phase phosphorus. Follow

ing numerous investigations, it was decided to modify the standard EPA Method 29 (M29) for gas-phase phosphorusmeasurement. The modifications included installing an instack thimble and a backup filter to collect the ash inside the duct. The impinger solutions used to collect the gas were also modified. The first impinger solution was filledwith de-ionized (DI) water as it was observed that fly ashbound phosphorus or gas-phase phosphorus was insoluble in DI water and slightly acidic solutions. The other two impingers were filled with a mixture of hydrogen peroxide and nitric acid. The first impinger served as an additional filter to catch any particulate breakthrough. This technique was tested at different power plants burning PRB coal with staged combustion and was found to be a reliable method to measure gas-phase phosphorus.Babcock & Wilcox Power Generation Group 3 Figure 1 summarizes the results of gas-phase phosphorus sampling using B&W PGG’s modified method at the inlet of various SCRs. Using B&W PGG’s measurement techniques (both gas-phase and fly ash) a mass balance was conducted for the total phosphorus entering the furnace, entering the SCR, and then leaving the SCR. This mass balance was successfully closed within acceptable error limits (±20%). Based on the mass balance calculations, a mass flow rate of gas-phase phosphorus emission entering and leaving the SCR reactor was calculated. Simultaneous gas-phase phosphorus sampling at the SCR inlet and outlet showed higher gas-phase phosphorus mass flow rate at the SCR inlet as compared to the SCR outlet as shown in Figure 2. This confirmed the accumulation of gas-phase phosphorus inside the SCR catalyst. Based on this accumulation rate over the period of exposure and an analysis of SCR catalyst for accumulated phosphorus, the role of gas-phase phosphorus in chemical deactivation of SCR catalyst was confirmed.



## Laboratory Research of PhosphorusDeactivated SCRs and the Role of Staged Combustion

Advanced Raman spectroscopy, temperature-pro

grammed adsorption/desorption, scanning electron micros

copy/Energy dispersive spectroscopy, and intrinsic kinetic studies were performed by B&W PGG at the Babcock &Wilcox Research Center (BWRC) to understand phospho

rus deactivation of SCR catalyst. It was observed that the mechanism of gas-phase phosphorus deactivation of SCR catalyst involved chemical deactivation by means of possible reaction between vanadium and phosphorus forming V-P-O compounds. This mechanism appeared to be very similar to the deactivation of SCR catalyst by arsenic where it was
found that arsenic also reacted with vanadium to form V-As compounds, thus deactivating the catalyst. The link between catalyst deactivation and staged combustion was still unclear
at the start of this research. Mineral analysis of the PRB coal revealed that the phos

phorus was mainly associated with calcium (Ca) in the form of calcium phosphate, which is a very stable compound (high melting and boiling point) under un-staged combustion con

ditions. A coal sequential extraction technique also revealed that a negligible amount of phosphorus was organically as

sociated within the coal matrix. It is believed that organic phosphorus is readily released into the gas phase. Further review of the staged combustion process highlighted the presence of a reducing atmosphere in the furnace. Based on the observed phenomenon of the formation of vapor phase silicon mono-oxide (SiO) under reducing conditions as mentioned by Quann [3] due to carbothermic reduction of
SiO 2 , a theory was proposed involving carbothermic reduc

tion of phosphorus-bearing inorganic compound resulting in the release of gas-phase phosphorus. The reaction involved consists of phosphorus-bearing inorganic compound + CO (g)- → P (g) + CO 2 (g). This mechanism could potentially explain increased deactivation of the catalyst by phosphorus under staged combustion conditions. In staged combustion there is much more carbon monoxide (CO) in the lower fur

nace which accelerates the release of gas-phase phosphorus as compared to un-staged combustion.



## Additive Development for Control of Gas-Phase Phosphorus

In this study a similarity between arsenic and phospho

rus poisoning of the SCR catalyst was presumed. Based on this, B&W PGG embarked on a developmental program to identify additives and methods that are capable of mitigat

ing and/or reducing gas-phase phosphorus compounds. A
novel and cost-effective material was identified which was found to tie up gas-phase phosphorus under combustion conditions and yield particulate-bound phosphorus. This additive and method lowers the concentration of gas-phase phosphorus or its aerosols entering the SCR. Detailed studies were performed to investigate the effects of this material on boiler operation and to confirm that there were no additional concerns, particularly in the areas of slagging and corro

sion. The required coal addition rate of this material was also analyzed. Finally, leaching studies were performed to Fig. 1 Gas-phase phosphorus measurement by B&W
PGG-modified method at SCR inlet location of different PRB-fired plants. Fig. 2 Gas-phase phosphorus mass flow rate measure

ment as determined by simultaneous SCR inlet and outlet gas-phase sampling by B&W PGG-modified method.4 Babcock & Wilcox Power Generation Group observe stability of this new particulate-bound phosphorus 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 NO x control. This plant previously experienced rapid SCR catalyst deactivation due to phosphorus poisoning. The B&W PGG technology was first tested at Wygen II in two short-term (4 to 5 days) injection tests. The additive was injected at rates less than 0.5% of the total plant coal flow rate using pneumatic conveying. Gas-phase phosphorus was measured at baseline conditions before the injection testing and also during and after the injection test. It was observed that the gas-phase concentration decreased by about 50 to about 60% during additive injection testing as compared to
the baseline conditions. The result of this test is shown in Figure 3. A memory effect was observed after the injection of additive was stopped. The memory effect highlights two points. One is the fact that more material was injected than needed and it will be possible to optimize the injection rate in the future. The second point highlighted by memory effect is the injected material may have scavenged the gas-phase phosphorus after it was deposited on walls or economizer tubes, etc., in the upper furnace. A second test was performed to check for repeatability of the data. This result is shown in Figure 4. Similar observa

tion in the percentage reduction of gas-phase phosphorus was observed in this test under similar injection rates. Thisdemonstrated the ability of the additive to lower the gasphase phosphorus concentration entering the SCR. To demonstrate the improvement in catalyst life perfor

mance, a long-term injection test is ongoing at Wygen II. This is a 100 MW unit burning PRB coal. The inlet NO x at the SCR inlet is around 0.15 lb/MBtu. This catalyst has been previously rapidly deactivated due to phosphorus poisoning as shown by the deactivation curve in Figure 5.
A similar deactivation trend is also observed at the ad

jacent Wygen III unit. Both Wygen II and III use the same coal from the same Wyodak mine as they are mine mouth plants. They both use the same air staged combustion burners supplied by B&W PGG with inlet NO x at Wygen III at 0.16 lb/MBtu. As a result of this similarity in combustion and coal, it was expected that Wygen III should also deactivate as rapidly as Wygen II. See Figure 6. The additive injection has been ongoing since March 2010. The catalyst activity is measured approximately every one thousand hours using the in situ catalyst activity measuring procedure developed by Fossil Energy Research Fig. 5 Expected and actual catalyst deactivation trend from Wygen II.

Fig. 6 Catalyst deactivation trend for Wygen II and Wygen III. Fig. 3 Demonstration of reduction in gas-phase phospho

rus at SCR inlet as a result of additive injection at Wygen II.

Fig. 4 Demonstration of reduction in gas-phase phospho

rus at SCR inlet as a result of additive injection with good repeatability (the second test campaign).Babcock & Wilcox Power Generation Group 5 Corporation (FERCO). The deactivation trend at Wygen II as a result of the additive injection for the last 12,000 hours is shown in Figure 7. As observed from Figure 7 it is evident that as a result of the additive injection, the deactivation of catalyst has been very gradual and following closely with the expected deac

tivation trend. This improvement in the catalyst deactivation performance is a result of the additive injection process that converts the gas-phase phosphorus into particulate-bound phosphorus. This eliminates the phosphorus deactivation component for the catalyst resulting in slower deactivation. As a part of the additive development process a blended version of the additive was also developed. The blended additive consists of two parts. One part is the phosphorus mitigation additive and the other part is a halogen source. These two parts are mixed in a certain weight percentage to achieve desired mercury oxidation and at the same time achieve phosphorus mitigation, thus satisfying two objec

tives by one injection. Figure 8 shows the results of mercury oxidation from Wygen II as a result of blended additive injection. As seen from Figure 8 it was possible to achieve about 80% mercury oxidation as a result of blended additive injec

tion at Wygen II, even with one layer of regenerated catalyst. 

## Conclusion

Gas-phase phosphorus causes rapid deactivation of SCR catalyst. Higher gas-phase phosphorus concentrations are produced as a result of staged combustion. The greater the extent of combustion staging, the higher the rate of deactiva

tion as a result of increased release of gas-phase phospho

rus. B&W PGG has developed a coal additive and method that has the ability to reduce and/or mitigate the gas-phase phosphorus concentration entering the SCR catalyst and thus improve the life of the SCR catalyst as observed from Wygen II injection testing.



## Acknowledgement

B&W PGG wishes to acknowledge and sincerely ap

preciates the cooperation obtained from Black Hills Power Corporation’s Wygen II generation facility in support of this long-term additive testing.



## References

1. Stultz, S.C. and Kitto, J.B., Steam/its generation and

use , 41st edition, The Babcock & Wilcox Company,

2005.

2. 2009 EPRI SCR workshop presentation by Ceram
Environmental.

3. Quann, R.J., “Ash vaporization under simulated pulverized coal combustion conditions,” PhD thesis, January 1982, Massachusetts Institute of Technology. Fig. 7 Actual catalyst deactivation trend for Wygen II with continuous additive injection.

Fig. 8 Mercury oxidation as a result of blended additive injection.Copyright© 2011 by Babcock & Wilcox Power Generation Group, Inc. a Babcock & Wilcox company All rights reserved.

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