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-1856
Babcock & Wilcox Power Generation Group 1

Deactivation of Selective Catalytic Reduction (SCR) Catalyst 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 (NOx) 

emission compliance. One of the important co-benefits of 

the SCR catalyst is simultaneous mercury oxidation along 

with NOx 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 NOx 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 deactivation. 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 phosphorus 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 a 

chemical 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 phosphorus 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 combustion 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 (NOx) 

reduction [1]. In the SCR NOx reduction process, NOx is 

reduced to nitrogen (N2) and water vapor (H2O) by reaction 

with ammonia (NH3). This reaction takes place in the presence of a catalyst. Commercial catalyst typically contains 
2 Babcock & Wilcox Power Generation Group

vanadium (V) as the active catalytic element supported on 

a titanium oxide (TiO2) substrate. SCR catalyst is used for 

NOx control in power plants firing both bituminous and PRB 

coals. Another technology for lowering NOx formation in 

the furnace is the use of low NOx 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 upper furnace through overfire air (OFA) ports. Lower NOx 

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 NOx burners coupled with an 

SCR are common practice to achieve the desired NOx control 

at an economical cost. As such, there are many power plants 

burning PRB coal with low NOx burners and staged combustion. Since staged combustion lowers the furnace outlet 

NOx 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 NOx 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 deactivation. In chemical deactivation the catalyst poisons react with 

vanadium (V), thus reducing the extent of NOx 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 experienced 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 NOx 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 NOx 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 development 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 entering 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 deactivation. 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 conclusion 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 support this next effort, B&W PGG developed a modified procedure 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. Following numerous investigations, it was decided to modify the 

standard EPA Method 29 (M29) for gas-phase phosphorus 

measurement. 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 filled 

with 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-programmed adsorption/desorption, scanning electron microscopy/Energy dispersive spectroscopy, and intrinsic kinetic 

studies were performed by B&W PGG at the Babcock 

&Wilcox Research Center (BWRC) to understand phosphorus 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 phosphorus 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 conditions. A coal sequential extraction technique also revealed 

that a negligible amount of phosphorus was organically associated 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 

SiO2, a theory was proposed involving carbothermic reduction 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) + CO2 (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 furnace 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 phosphorus 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 mitigating 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 corrosion. 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 measurement 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 NOx 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 observation in the percentage reduction of gas-phase phosphorus 

was observed in this test under similar injection rates. This 

demonstrated the ability of the additive to lower the gasphase phosphorus concentration entering the SCR.

To demonstrate the improvement in catalyst life performance, a long-term injection test is ongoing at Wygen II. 

This is a 100 MW unit burning PRB coal. The inlet NOx 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 adjacent 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 NOx 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 phosphorus at SCR inlet as a result of additive injection at Wygen II.

Fig. 4 Demonstration of reduction in gas-phase phosphorus 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 deactivation 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 objectives 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 injection 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 deactivation as a result of increased release of gas-phase phosphorus. 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 appreciates 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

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