Improvement in Performance of SCR Catalyst Using 

Combustion Additives: Technology Status Update 

Paper # Control 2 

Mandar Gadgil, Larry Pierson, Elizabeth Hansen 

Babcock & Wilcox Power Generation Group, Inc., 20 S Van Buren Ave., Barberton, Ohio 44203 

Derek Silbaugh 

Black Hills Corporation, 20 Wyodak Road, Gillette Wyoming 82718 

John Meier 

Nalco Company, 1601 W Diehl Road, Naperville, IL 60563 

ABSTRACT

Significant progress has been made in combustion additives technology for improvement of 

selective catalytic reduction (SCR) catalyst performance in the last two years. During that time,

Babcock & Wilcox Power Generation Group, Inc. (B&W PGG) has successfully completed a

long-term injection demonstration at Black Hills Corporation’s Wygen 2 power plant where 

a substantial improvement in SCR catalyst life was realized due to the control of gas-phase 

phosphorus poisoning of the catalyst. As a result of this successful demonstration, a permanent 

injection system is in the planning stages for commercial installation at a plant burning Powder 

River Basin (PRB) coal. As a further improvement to this technology, B&W PGG has developed 

a blended additive to simultaneously achieve gas-phase phosphorus control and enhanced 

mercury oxidation. Results of a field test of this blended additive at a midwestern power plant 

burning PRB coal with staged combustion technology will be presented. The dual benefits of the 

blended additive provide significant cost savings both in terms of capital and operating cost due 

to improved catalyst life and reduction in the cost of mercury compliance. 

INTRODUCTION 

Selective catalytic reduction (SCR) is a widely used technology for post-combustion control of 

nitrogen oxides (NOx) [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 catalysts typically contain vanadium (V) as the active 

catalytic element supported on a titanium oxide (TiO2) substrate. SCR catalysts are used for NOx

control in power plants firing a wide variety of fuels, including both bituminous and sub 

bituminous PRB coals. Low NOx burners with air staging, commonly referred to as staged 

combustion, is another technology used to reduce NOx emissions by lowering NOx formation in 

the furnace. 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 

1
temperature zone, in which air necessary to complete combustion is then reintroduced. 

Utilization of staged combustion and low NOx burners, coupled with an SCR, is a common 

practice used 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 outlet emission level. For example, if we consider two identical 

plants in which one unit is staged and the other unit is non staged, the non-staged plant 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/or chemical poisoning. Plugging 

and masking are classified as physical deactivation. In chemical deactivation, one or more 

catalyst poisons react with the vanadium (V) present in the catalyst, thus reducing the catalyst's 

activity for NOx reduction. The rate of chemical deactivation depends on the type of fuel burned 

and the combustion environment. Combustion of different coals results in the generation of a 

variety of different catalyst poisons. In plants burning bituminous coals, arsenic (As) in the coal 

is the main factor causing the chemical deactivation through poisoning of the catalyst. In PRB 

fired plants, the main catalyst poison is phosphorus (P). In lignite fired plants, the main catalyst 

poisons are believed to be potassium (K) and sodium (Na) in addition to P. 

Power plants burning PRB coal and operating with non-staged combustion do not appear to 

suffer from catalyst deactivation due to phosphorus as rapidly as power plants using staged 

combustion. Furthermore, many of the staged power plants burning PRB coal experience 

unacceptable SCR catalyst deactivation levels early in the catalyst life cycle. The analysis of 

catalyst samples having low catalytic activity from a number of different plants had two features 

in common. All of the analyzed samples had high amounts of P accumulated on the catalyst, and 

all were taken from plants that utilize staged combustion with low NOx burners. Fuel analysis of 

PRB coal indicated the presence of about 0.5 to 1.0% by weight percentage P in coal ash. At a 

recent SCR conference [2] the link between P deactivation of an SCR catalyst and staged 

combustion was highlighted. It was stated that following a low NOx burner retrofit, the SCR 

catalyst began experiencing faster 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 phosphorus poisoning, in 

particular gas-phase phosphorus poisoning, and the development of a solution to control the rate 

of SCR catalyst deactivation by B&W PGG’s patent-pending technology. 

FIELD RESEARCH ON PHOSPHORUS-DEACTIVATION OF SCR 

CATALYST 

B&W PGG initiated an investigation of catalyst deactivation by phosphorus. There are two 

sources by which phosphorus can poison 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 an SCR. Flyash samples were collected from plants experiencing rapid catalyst 

2
deactivation, and separated into particle size fractions. These samples were analyzed by using 

leaching techniques to determine whether or not flyash-bound phosphorus was reacting with the 

SCR catalyst thereby resulting in chemical deactivation of the catalyst. Two pH levels for this 

leaching study were chosen, one being slightly acidic (pH = 5) and the other being slightly 

alkaline (pH = 9), since these two are the most likely pH conditions that occur during startup and 

shutdown of an SCR. It was observed that under these two pH conditions, low amounts of 

phosphorus leach out of the flyash, leading to the conclusion that flyash-bound phosphorus is 

unlikely the main source of SCR catalyst deactivation. Following this analysis, an effort began 

to determine if gas-phase P was the main deactivation agent similar to gas-phase arsenic 

poisoning of SCR catalysts used in plants burning eastern bituminous coal [3]. 

To support this next effort, B&W PGG developed a modified procedure to reliably measure gasphase 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. This variability was attributed to particulate-bound 

phosphorus breaking through into the impingers and reporting as gas-phase phosphorus. 

Following investigations, it was decided to modify the standard EPA Method 29 (M29) for gasphase phosphorus measurement. The modifications included installing an in-stack 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 deionized (DI) water as it was 

observed that flyash-bound 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. 

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 gasphase and flyash) a mass balance was conducted for the total phosphorus entering the furnace, 

entering 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 

entering the SCR reactor was calculated. 

3
0

10

20

30

40

50

60

70

80

1 2 3 4 5 6 7 8 9 10 11 12

Gas‐Phase

P in ppb

Run id

Gas phase P at 100% PRB staged Units

gas phase P at plant A inlet

NOx 0.22 lb/Mbtu

Gas pahse P at Wyegn II

inlet NOx 0.15 lb/Mbtu

Figure 1: Gas-phase phosphorus measurement by B&W PGG-modified method at SCR inlet location of 

different PRB-fired plants. 

LABORATORY RESEARCH ON PHOSPHORUS-DEACTIVATED 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 

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 way 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 points) during non-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 

4
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 a 

phosphorus-bearing inorganic compound resulting in the release of gas-phase phosphorus. This 

mechanism could potentially explain increased deactivation of the catalyst by phosphorus under 

staged combustion conditions. In staged combustion as well as in poor combustion there is much 

more carbon monoxide (CO) in the lower furnace which accelerates the release of gas-phase 

phosphorus as compared to non-staged combustion. 

ADDITIVE DEVELOPMENT FOR CONTROL OF GAS-PHASE 

PHOSPHORUS 

B&W PGG embarked on a developmental program to identify additives and/or methods that are 

capable of controlling and/or reducing gas-phase phosphorus compounds. A novel material and 

method was developed which was unexpectedly determined to bind gas-phase phosphorus 

generated during combustion and yield non gas-phase phosphorus. The iron carbonate additive 

material and method lowers the concentration of gas-phase phosphorus or its aerosols entering 

the SCR. Detailed studies were performed to investigate the effects of various materials on 

boiler operation and to determine whether or not various additives created additional concerns 

relative to slagging and corrosion. The addition rate of such materials relative to the amount of 

coal being burned was also analyzed. Finally, leaching studies were performed to observe 

stability of this new particulate-bound phosphorus under standard SCR operating conditions. 

The Wygen 2 power plant located in Gillette, Wyoming, is a 100 MW B&W PGG subcritical 

PRB coal-fired power plant. This plant uses staged combustion and an SCR for NOx control. 

This plant previously experienced rapid SCR catalyst deactivation due to phosphorus poisoning. 

A proposed material and methodology 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 a pneumatic supply process. Gas-phase phosphorus was measured before, during and 

after the injection test. It was observed that the gas-phase phosphorus concentration decreased 

by between 40 to 50% during testing as compared to the concentration of gas-phase phosphorus 

prior to testing. The result of this test is shown in Figure 2. A memory effect was observed after 

injection of the material was stopped. The memory effect highlights two points the first point is 

the fact that more material may have been injected than needed and it may be possible to further 

optimize the injection rate upon future studies. The second point is that the injected material 

may have the ability to scavenge gas-phase phosphorus after the material is entrained in the flue 

gas upstream of the SCR catalyst. 

5
Figure 2: Demonstration of reduction in gas-phase phosphorus at SCR inlet as a result of additive 

injection at Wygen II. 

A second test was performed to demonstrate repeatability of the data. As a part of the 

repeatability test baseline testing was performed for two days (10/12 through 10/14). The 

variation in the measured values in ppb can be explained by small changes in coal phosphorus 

contents. Continuous injection of iron carbonate was performed after baseline testing but gas 

phase phosphorus sampling was done only on last day of injection (10/20). This result is shown 

in Figure 3. A similar observation in the percentage reduction of gas-phase phosphorus was 

observed in this test under similar injection rates. This demonstrates the ability of the material to 

lower a gas-phase phosphorus concentration prior to entry of the flue gas into the SCR. 

6
Figure 3: Demonstration of reduction in gas-phase phosphorus at SCR inlet as a result of additive 

injection with good repeatability (the second test campaign). 

To demonstrate the improvement in catalyst life performance, a long-term injection test was 

performed at Wygen 2. The injection test lasted for around 14,000 hours. The inlet NOx at the 

SCR inlet is around 0.15 lb/MBtu. Previously, the catalyst rapidly deactivated due to 

phosphorus poisoning as evidenced by the deactivation curve in Figure 4. 

7
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 2000 4000 6000 8000 10000 12000 14000 16000

Actual deactivation for Wy

2 without additive

Expected Wy 2  deactivation

Catalyst hours in service

Catalyts activity K/Ko

Figure 4: Expected and actual catalyst deactivation trend from Wygen 2. 

Iron carbonate injection started in March 2010. The catalyst activity is measured approximately 

every one thousand hours using an in situ catalyst activity measuring procedure developed by 

Fossil Energy Research Corporation (FERCO). The deactivation trend at Wygen 2 as a result of 

the material injection for the last 14,000 hours is shown in Figure 5. 

8
Figure 5: Actual catalyst deactivation trend for Wygen 2 with continuous material injection. 

s observed from Figure 5 it is evident that as a result of the additive injection, the deactivation 

e 

t 

ased on the result of the Wygen 2 testing, B&W PGG developed various projections for 

tion. 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 2000 4000 6000 8000 10000 12000 14000 16000

Actual deactivation for Wy

2 without additive

Actual  Wy 2  deactivation

Expected Wy 2  deactivation

Catalyst hours in service

Catalyts activity K/Ko

A

of catalyst has been very gradual, closely following the expected/theoretical deactivation trend. 

This improvement in catalyst performance can be attributed to the injected material converting 

reducing and/or controlling the concentration of the gas-phase phosphorus in the flue gas. Whil

not wishing to be bound to any one theory, it is believed that the material reacts with gas-phase 

phosphorus to produce a less detrimental particulate bound phosphorus. This reduces the amoun

of gas-phase phosphorus that can cause poisoning of the catalyst which results in deactivation of 

the catalyst. 

B

theoretical catalyst life at Wygen 2 as a result of a lower rate of catalyst poisoning/deactiva

The projections are as shown in Table 1. 

9
Table 1: Theoretical improvement in life of catalyst at Wygen 2 as a result of additive injection 

Unit Number of Catalyst 

Layers 

End of Life Activity 

k/Ko 

Theoretical Life 

Hours 

Wygen 3 no additive 

injection 

2 0.43 16,000 

Wygen 2 with additive 1 0.55 16,000 

Wygen 2 with additive 2 0.55 24,000 

Wygen 2 with additive 3 0.55 32,000 

Due to B&W PGG's gas-phase phosphorus control material, the rate of catalyst deactivation 

and/or poisoning can be favorably controlled thereby resulting in an improvement in the usable 

life of an SCR catalyst. This results in the potential to realize fewer catalyst changes throughout 

a 20-year operating period, resulting in significant cost savings for the utility. When the material 

is provided to the furnace, it binds with, controls and/or reduces the gas-phase phosphorus in the 

flue gas prior to entry of the flue gas into the SCR. Since a reduced amount of gas-phase 

phosphorus is prevented from reaching the SCR catalyst, catalytic life is improved, regardless of 

whether the SCR catalyst is new catalyst or regenerated catalyst. 

 

BLENDED ADDITIVES FOR GAS-PHASE PHOSPHORUS CONTROL 

AND ENHANCED MERCURY OXIDATION 

A blended additive was also investigated. The blended additive contains two parts: one part is 

the gas-phase phosphorus control material, and the other part is a halogen source selected for its 

ability to control mercury emissions. These two parts are mixed in a certain relative weight 

percentage to achieve a desired level of mercury oxidation while simultaneously achieving gasphase phosphorus control, thereby achieving two objectives using one injection system. The 

benefit of having higher fraction of total mercury in oxidized state is that, the oxidized mercury 

can be easily removed by flue gas desulfurization (FGD) equipment, therefore lowering the cost 

of mercury emissions compliance. The blended additive was tested at a 300 MW unit of a 

midwestern utility burning PRB coal. This plant’s air quality control system (AQCS) includes of 

an SCR, a circulating dry scrubber (CDS) and a fabric filter. Nalco Company’s MerControl 5897 

additive was selected to be the halogen source. The blended additive was a mixture of iron 

carbonate and MerControl 5897 powder mixed in a desired relative weight percentage. 

The objective of the blended additive injection program was to demonstrate the simultaneous 

reduction in gas-phase phosphorus at the SCR inlet and an increased fraction of oxidized 

mercury at the air heater outlet. The extent of the oxidized fraction of mercury at the air heater 

outlet should be sufficient to achieve stack mercury emissions at or below limits set by the 

Mercury Air Toxics Standards (MATS). This limit for power plants burning bituminous or 

subbituminous coal is 1.2 lb/TBtu. 

A similar reduction in gas-phase phosphorus at the SCR inlet as that of Wygen 2 was observed 

during the injection testing at this midwestern utility. The results are shown in Figure 6. The 

average value of gas-phase phosphorus before blended additive injection was around 34 ppb. The 

average value of gas-phase phosphorus at the SCR inlet after the blended additive injection was 

10
reduced to 21 ppb as a result of the injection. This demonstrated about a 40% reduction in gasphase phosphorus at the SCR inlet as a result of additive injection. This percentage reduction 

value is very similar to the 40 to 50% reduction observed at Wygen 2 during the short-term 

injection testing as shown in Figure 2. 

0

5

10

15

20

25

30

35

40

45

50

10‐Jan 11‐Jan 11‐Jan 12‐Jan 12‐Jan 13‐Jan 13‐Jan 14‐Jan 14‐Jan 15‐Jan 15‐Jan Gas‐Phase P in ppb

Without

Injection

With  Injection

Figure 6: Gas-phase phosphorus reduction as a result of blended additive injection at a 300 MW 

utility boiler. 

Halogen injection in both liquid and in powder form as a blended additive was performed. The 

main reason for testing liquid and powder halogen injection was to demonstrate that the same 

extent of mercury oxidation could be achieved irrespective of the form of halogen. The injection 

testing was performed over a five-day period. The results of the injection testing are shown in 

Table 2. 

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Table 2: Blended additive injection results 

Day Additive Plant Load 

MW 

Coal Mercury 

in lb/TBtu 

Average 

Percentage 

Oxidized 

Mercury @ Air 

Heater Outlet 

8 Hours 

Average Stack 

Hg in lb/TBtu 

1 Baseline 250 7 30 4.8 

2 Liquid chlorine 250 8.1 51 1.8 

3 Liquid bromine 275 13 76 1.1 

4 Powdered bromine 250 8.5 76 1.2 

5 Powdered chlorine 150 13 71 1.8 

As seen in the Table 2, on the day of baseline testing the average percentage of total mercury 

measured as oxidized at the air heater outlet sampling location was 30%. This low oxidized 

mercury fraction was expected since PRB coal typically has low concentrations of halogen 

components. The stack emission of mercury was 4.8 lb/TBtu, significantly higher than the 

MATS limit of 1.2 lb/TBtu. On the second day of testing, a calcium chloride solution was added 

to the coal. The equivalent liquid chlorine concentration was between 100 to 300 ppm, consistent 

with typical industry practice. The liquid chlorine additive improved the average oxidized 

percentage of mercury at the air heater outlet from 30 to 51%. In turn, the stack average value of 

mercury emission decreased to 1.8 lb/TBtu. Though this value was significantly lower than the 

baseline value of 4.8 lb/TBtu, it was still above the MATS limit. On the third day of testing, 

Nalco’s MerControl 5897 in liquid form was added to the coal as a source of bromine. The 

equivalent concentration of liquid bromine provided by the MerControl 5897 additive was 

consistent with typical industry practice of 10 to 30 ppm. The MerControl 5897 additive greatly 

improved the oxidized percentage of mercury at the air heater outlet. The average mercury 

oxidation measured was 76%, with an average stack mercury emission of 1.1 lb/TBtu, which was 

below the MATS compliance limit of 1.2 lb/TBtu. 

On the fourth day of testing, a blend of FeCO3 and MerControl 5897 in powder form was added 

to the pulvarizer. The equivalent concentration of liquid bromine provided by the MerControl 

5897 additive was 10 to 30 ppm, the same concentration used during the third day of testing. For 

the fourth day of testing, the average percentage oxidized mercury was 76%, close to the 71% 

value measured on the third day of testing with the liquid MerControl 5897 injection. The 

average stack mercury emission on the fourth day of testing met the 1.2 lb/TBtu MATS limit. 

These two days of testing demonstrated that it was possible to achieve the same percentage of 

mercury oxidation regardless of the form (liquid or powder) of halogen injected. The 

observations from the blended additive testing were as follows 

1) The results obtained at the 300 MW Midwestern utility substantiate earlier results 

obtained at Wygen 2 during the short-term demonstration. This indicates that if additive 

injection is used on a long-term basis, a significant improvement in the catalyst 

performance can be achieved. 

2) This was the first time a blended additive of iron carbonate and halogen was used to 

achieve a simultaneous reduction in gas-phase phosphorus and an increase in oxidized 

12
fraction of mercury. As a result it may be possible to achieve an improvement in the 

performance of SCR catalyst by controlling and/or reducing catalyst poisoning due to 

gas-phase phosphorus and obtain higher mercury oxidization. The enhanced mercury 

oxidation also demonstrated demonstrates that the MATS mercury emissions limit of 1.2 

lb/TBtu can be met using an SCR and flue gas desulfurization (FGD) only without using 

activated carbon injection. 

 

 

CONCLUSION 

It has been determined through various testing by B&W PGG that gas-phase phosphorus 

negatively impacts the catalytic lifespan of an SCR catalyst. In view of this, B&W PGG has 

successfully developed a gas-phase phosphorus control material and method to control and/or 

reduce the concentration of gas-phase phosphorus in a flue gas. Additionally, by combining 

B&W PGG's gas-phase phosphorus control material with a suitable mercury control technology, 

it has been demonstrated that simultaneous control of gas-phase phosphorus and mercury 

emissions is possible. 

ACKNOWLEDGEMENT 

B&W PGG wishes to acknowledge and sincerely appreciates the cooperation obtained from 

Black Hills Power Corporation’s Wygen 2 generation facility in support of this long-term 

additive testing. Additionally, special thanks are extended to Mr. John Meier and Nalco 

Company for participating in the testing of B&W PGG's additive material and method for 

controlling gas-phase phosphorus. 

REFERENCES 

1) Stultz, S.C. and Kitto, J.B., Steam/its generation and use, 40th edition, The Babcock & 

Wilcox Company, 1992. 

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. 

Copyright © 2012 Babcock & Wilcox Power Generation Group, Inc. 

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All rights reserved. 

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copyright holder. Permission requests should be addressed to: Marketing Communications, 

Babcock & Wilcox Power Generation Group, Inc., P.O. Box 351, Barberton, Ohio, U.S.A. 

44203-0351. 

Disclaimer 

Although the information presented in this work is believed to be reliable, this work is published 

with the understanding that Babcock & Wilcox Power Generation Group, Inc. (B&W PGG) and 

the authors and contributors to this work are supplying general information and are not 

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