## 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 W yodak 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 (S CR) 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 su ccessful demonstration, a permanent injection system is in the planning stages for co mmercial installation at a plant burning Powder River Basin (PRB) coal. As a further improveme nt 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 technol ogy will be presented. The dual benefits of the blended additive provide significant cost savings both in term s of capital and operating cost due to improved catalyst lif e and reduction in the cost of mercury compliance.



### INTRODUCTION

Selective catalytic reduction (SCR) is a widely used technology for pos t-combustion control of nitrogen oxides (NO

x ) [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 presence of a catalyst. Comm ercial catalysts typically contain vanadium (V) as the active catalytic element supported on a titanium oxide (TiO 2 ) substrate. SCR catalysts are used for NO

x control in power plants firing a wide variet y of fuels, including both bituminous and sub

bituminous PRB coals. Low NO

x burners with air staging, co mmonly referred to as staged combustion, is another technology used to reduce NO

x emissions by lowering NO

x 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 NO

x levels are obtained as a result of the oxygen deficient/fu el rich reducing atmosphere created in the lower furnace in combination with OFA ports located physically a bove the peak furnace

1temperature zone, in which air necessary to complete combustion is then reintroduced. Utilization of staged combustion and low NO

x burners, coupled with an SCR, is a common practice used 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 combustion. Since staged

combustion lowers the furnace outlet NO

x 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 othe r unit is non staged, the non-staged plant will have

a NO

x concentration leaving the boile r 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 chemi cal poisoning. Plugging and masking are classified as physical deactiva tion. In chemical deactivation, one or more catalyst poisons react with the va nadium (V) present in the cataly st, thus reducing the catalyst's activity for NO

x reduction. The rate of chemical deactivat ion 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 co als, arsenic (As) in the coal is the main factor causing the chemical deactiv ation through poisoning of the catalyst. In PRB fired plants, the main catalyst po ison 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 w ith 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 pl ants 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 NO

x 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 de activation of an SCR catalyst and staged combustion was highlighted. It wa s stated that following a low NO

x burner retrofit, the SCR catalyst began experiencing faster deactivation. This trend was obser ved in plants with different combustion configurations such as wall-, tangentia l- and Cyclone-fired. This paper describes the

efforts at B&W PGG in unders tanding SCR catalyst deactiva tion 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 PHOSPH ORUS-DEACTIVATION OF SCR

CATALYST

B&W PGG initiated an investigation of catal yst deactivation by phosphor us. There are two sources by which phosphorus can poison the SCR catal yst. One is in the form of gas-phase phosphorus; the other is associated with the fly as h. 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 collect ed from plants experiencing rapid catalyst

2deactivation, and separated into particle size fr actions. 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 deactiva tion 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 cond itions, 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 d eactivation 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 pro cedure 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 m easurements 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 sligh tly acidic solutions. The other two impingers were filled with a mixt ure 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 m easure gas-phase phosphorus.

Figure 1 summarizes the results of gas-phase phosphorus sampling usi ng B&W PGG’s modified

method at the inlet of various SCRs. Usi ng 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 successf ully closed within ac ceptable error limits (±20%). Based on the mass balance calculations, a mass flow rate of gas-phase phosphorus entering the SCR reactor was calculated.

30 10 20 30 40 50 60 70 80

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



## 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 PHOS PHORUS-DEACTIVATED SCRs

AND THE ROLE OF STAGED COMBUSTION

Advanced Raman spectroscopy, temperature-pr ogrammed adsorption/desorption, scanning electron microscopy/energy dispersive spectroscopy, and intrinsic kinetic studies were performed at the Babcock &Wilcox Research Center (BWR C) to understand phosphorus deactivation of SCR catalyst. It was observed that the mech anism of gas-phase phosphorus deactivation of SCR catalyst involved chemical deactivation by wa y of possible reaction between vanadium and phosphorus forming V-P-O compounds. This mechan ism 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 deactiv ating the catalyst. Th e link between catalyst deactivation and staged combustion was still unclear at the start of this research.

Mineral analysis of the PRB co al 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 phos phorus is readily released into the gas phase. Further review of the staged combustion pr ocess highlighted the presence of a reducing atmosphere in the furnace. Based on the observed phenomenon of the formation of vapor phase

4silicon mono-oxide (SiO) under reducing conditions as mentioned by Quann [3] due to

carbothermic reduction of SiO 2 , 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 combusti on 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 co mpounds. A novel material and method was developed which was unexpectedly determined to bind gas-phase phosphorus generated during combustion and yield non gas-phas e 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 create d additional concerns relative to slagging and corrosion. The addition rate of such materials relative to the amount of coal being burned was also analyzed. Finall y, leaching studies were performed to observe stability of this new particulate-bound phos phorus 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 NO

x control. This plant previously experienced rapid SCR cat alyst deactivation due to phosphorus poisoning. A proposed material and methodolog y 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 wa s observed that the gas-phase ph osphorus concentration decreased by between 40 to 50% during testing as compared to the concentration of gas-phase phosphorus prior to testing. The result of th is test is shown in Figure 2. A memory effect was observed after injection of the material was stopped. The memory effect highlights two poi nts the first point is the fact that more material may have been inject ed than needed and it may be possible to further optimize the injection rate upon future studies. Th e 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.

5Figure 2: Demonstration of reduction in gas-phase phos phorus at SCR inlet as a result of additive
injection at Wygen II.

A second test was performed to demonstrate re peatability of the data. As a part of the repeatability test ba seline 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 ba seline 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 pe rcentage 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 prio r to entry of the flue gas into the SCR.

6Figure 3: Demonstration of reduction in gas-phase phos phorus 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 NO

x at the SCR inlet is around 0.15 lb/MBtu. Previously , the catalyst rapidly deactivated due to phosphorus poisoning as evidenced by th e deactivation curve in Figure 4.

70 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 serviceCatalytsactivityK/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 measur ing procedure developed by Fossil Energy Research Corporation (FERCO). Th e deactivation trend at Wygen 2 as a result of the material injection for the last 14,000 hours is shown in Figure 5.

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

s observed from Figure 5 it is evident that as a result of the additive in jection, 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 serviceCatalytsactivityK/Ko

A of catalyst has been very gradua l, closely following the expected/t heoretical deactivation trend. This improvement in catalyst performance can be attributed to the inject ed material converting reducing and/or controlling the conc entration 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 detrim ental particulate bound phosphorus . This reduces the amoun of gas-phase phosphorus that can cau se poisoning of the catalyst wh ich results in deactivation of the catalyst.

B theoretical catalyst life at Wygen 2 as a result of a lower rate of catal yst poisoning/deactiva The projections are as shown in Table 1.

9Table 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 real ize fewer catalyst changes throughout a 20-year operating period, resul ting 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 catalys t, catalytic life is improved, regardless of whether the SCR catalyst is new ca talyst or regenerated catalyst.



### BLENDED ADDITIVES FOR GAS-P HASE 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 mate rial, and the other part is a halogen source selected for its ability to control mercury emissi ons. These two parts are mixed in a certain relative weight percentage to achieve a desire d level of mercury oxidation wh ile simultaneously achieving gasphase phosphorus control, ther eby achieving two objectives us ing 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 ga s 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 co al. This plant’s air quality c ontrol 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 sour ce. 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 addi tive 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 fracti on of mercury at the air heater

outlet should be sufficient to achieve stack merc ury 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. Th e results are shown in Figure 6. The average value of gas-phase phosphorus before blended additive inje ction was around 34 ppb. The average value of gas-phase phosphor us at the SCR inlet after the blended additive injection was

10reduced to 21 ppb as a result of the injection. This demonstrat ed 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% reducti on 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

‐ JanGas‐PhasePinppb 

### 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 haloge n 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 testi ng are shown in

Table 2.

11Table 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 te sting the average percentage of total mercury

measured as oxidized at the air heater outle t 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 wa s 4.8 lb/TBtu, significantly higher than the MATS limit of 1.2 lb/TBtu. On the second day of te sting, a calcium chloride solution was added to the coal. The equivalent liqui d chlorine concentration was be tween 100 to 300 ppm, consistent with typical industry practice. The liquid chlo rine 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 th is 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 bromin e 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 st ack 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 FeCO 3 and MerControl 5897 in powder form was added to the pulvarizer. The equivalent concentra tion of liquid bromine provided by the MerControl 5897 additive was 10 to 30 ppm, the same concentra tion used during the third day of testing. For the fourth day of testing, the av erage percentage oxidized merc ury was 76%, close to the 71% value measured on the third day of testing with the liquid MerCont rol 5897 injection. The average stack mercury emission on the fourth da y 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 ha logen injected. The observations from the blended additive testing were as follows

1) The results obtained at the 300 MW Midweste rn utility substantia te earlier results
obtained at Wygen 2 during the short-term demonstration. This indicates that if additive
injection is used on a long-te rm 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

12fraction of mercury. As a result it may be possible to achieve an improvement in the
performance of SCR catalyst by controlling a nd/or reducing catalyst poisoning due to
gas-phase phosphorus and obtain higher me rcury oxidization. The enhanced mercury

oxidation also demonstrated demonstrates th at the MATS mercury emissions limit of 1.2
lb/TBtu can be met using an SCR and flue gas desulfurization (F GD) only without using
activated carbon injection.



### CONCLUSION

It has been determined through various te sting 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 c ontrol 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 c ontrol 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 sincerel y appreciates the coope ration obtained from Black Hills Power Corporation’ s Wygen 2 generation facility in support of this long-term additive testing. Additionally, special tha nks 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 presenta tion by Ceram Environmental. 3) Quann, R.J., “Ash vaporization under simula ted pulverized coal combustion conditions,” PhD thesis, January 1982, Massach usetts Institute of Technology.

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1314 All rights reserved.

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