# Commercial Application of Urea SNCR for NOx RACT Compliance on a 112 MWe Pulverized Coal Boiler

### COMMERCIAL APPLICATION OF UREA SNCR FOR NOx RACT COMPLIANCE ON A 112**~We** 

### PULVERIZED COAL BOILER1



### James E. Staudt, Ph.D. Robert P. Casill, 

### P.E. Research-Cottrell



## ABSTRACT Montaup Electric Company, a subsidiary of Eastern Utilities Associates, operates a tangentially-fired pulverized coal bdiler at its Somerset, MA generating station. NOx emissions from the 112 MWe Montaup boiler #8 must be reduced from their uncontrolled levels in order to comply with the Reasonably Available Control Technology (RACT) requirements promulgated by the Commonwealth of Massachusetts. According to RACT, NOx emissions from the unit must not exceed 0.38 lb/MMBtu when firing coal or 0.25 lb/MMBtu when firing oil.
For reduction of NOx emissions from Montaup boiler #8, combustion controls and flue gas treatment were considered. Montaup Electric chose Selective NonCatalytic Reduction (SNCR) technology as its primary means of reducing NOx from baseline levels to the levels required for RACT compliance. The SNCR technology operates by injection ofcontrolled amounts of aqueous urea into the furnace to reduce the NOx to nitrogen, water and carbon 

### by

Thomas S. Tsai Leonard J. Ariagno Eastern Utilities

dioxide.Total projectscope includeddesign engineering, equipmentsupply, installation, and system startup. Optimization and startup of the system were completed in early March. Startup testing demonstrated that the system can achieve compliance levels across the load range 35% to 100% MCR while maintaining ammonia slip at about 10 ppm or less. This paper will describe the system provided to Montaup Electric and will discuss the results of startup and optimization testing. 

## BACKGROUND Montaup Electric Company operates a 11 2 MWe boiler at its Somerset, MA Generating Station. Boiler #8 began operation in 1959. It is a natural circulation, reheat, singledrum radiant boiler with a dry bottom that is rated at 800,000 lbs/hr steam flow at the superheater outlet at 1,925 psig and 1000

## °F. The furnace has four corners and four levels of tilting, tangentially fired coal buckets. Between the four coal buckets in each corner are three oil guns. The fuel for the boiler was changed from coal to oil in the early 1970's, and in 1984 the boiler

1Submitted to EPRI/EPA 1995 Joint Symposium on Stationary Combustion NOx Control, Kansas City, MO, May 16-19,1995 

### 1was retrofit to operate primarily on coal. A Continuous Emission Monitoring System (CEMS) was recently installed on the boiler, and a new Distributed Control System (DCS) is planned for installation in April and May of this year. Plans are to maintain boiler #8 in operation well beyond the year 2000.
Boiler #8 operates on dispatch typically in a load following mode. During the day and during other high demand periods the boiler operates atfull capacity.lnthe evening and on weekends the boiler load is frequently reduced to 35% to 55% MCR. Maintaining boiler #8 within environmental compliance under this wide range of operating conditions is, therefore, of critical importance.
In the Commonwealth of Massachusetts, Reasonably Available Control Technology (RACT) for control of NOx from tangentially-fired utility boilers is defined as compliance with the following emissions performance levels:

Coal Oil Coal +Oil 0.381b/MMBtu 0.25 lb/MMBtu Weighted average of above two values

Under some conditions, particularly at part loads, boiler #8 cofires oil and coal. On an annual basis about 90% of the thermal input is from coal. Uncontrolled NOx levels measured from boiler #8 during temperature mapping and more recently during SNCR system initial optimization testing are shown in Figure 1 as a function of load. As shown in Figure 1, at part load conditions NOx levels can be much greater than those atfull load conditions due tothe increased excess air levels that are necessaryto maintainreheat and superheat temperatures at their optimum level. Manipulation of furnace air distribution enables some reduction of uncontrolled NOx. Simulated Over Fire Air (SOFA), performed during temperature testing in December 1993 by feeding coal through only the lower three coal buckets, produced the lowest NOx levels. However, the NOx reduction possible in this manner alone is not adequate to achieve the NOx emissions levels required in Massachusetts for compliance with RACT. Additional NOx control measures are necessary. To comply with RACT, Montaup Electric evaluated several technologies, including low NOx burners, Selective Non-Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR). Capital and operating cost, and performance guarantees overthe entire load range were the key issues in selecting a technology· for RACT compliance. SNCR using aqueous urea solution was selected by Eastern Utilities for RACTcompliance on Montaup Electric's boiler #8. Research-Cottrell supplied all engineering, equipment, installation, and startupservices needed for implementation of the SNCR system, including initial furnace temperature mapping.
SYSTEM DESCRIPTION The SNCR system provided to Montaup Electric included reagent storage, all pumping systems, and twenty eight wallmounted injectors arranged at four elevations in the furnace as in Figure 2. The four zones of injection are necessary in order to achieve NOx reduction over the operating range of 35% to 100% MCR when firing coal and/or No. 6 fuel oil. Of



### 2

## Figure 1,MontaupElectric BaselineNOx vs. Load

1.10

-------------------------------------------------------- 1.00

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -[j

- - - - - - - - - - - - - - - - - - - - - - - 0.90 

### c 0

## ~ 0.80 

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# I**~** 8 

## =**~** 

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- - - - - - - - - 0.40

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- - - - - - -

### ..±S..QF.A

- - - - - 0.30

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 0.20

------------------------------- -·------------------------ QlO

-------------------------------------------------------- 0 30 40 

## so 60 70 80 90 100 110 120 Load(MWe) 

# I+ 12193 

# I Figure2 Boiler#8 

## andUreaInjectorLocations

3thetwentyeightinjectors, nineteenutilized existing furnace penetrations. Nine new penetrations were installed to accommodatetheinjectors. Major pieces of equipment are schematically displayed in Figure 3 and include:

• a 20,000gallon storage tankfor outdoor storage of 50% by weight aqueous urea solution

• a circulation loop with circulation pumps and in-line heater to maintain the temperature ofthe concentrated urea solution above the urea crystallization temperature

• a chemical metering and mixing system that provides a measured amount of reagent to the injection levels at a prescribed pressure

• distribution modules that modulate liquid flowrate and atomizing air pressure to the individual injectors

• two-fluid, air-atomized injectors that automatically retract on zones 1 and 2 whenthese zones are not in operation

• a control system composed of a Programmable Logic Controller, computer interface, local control panels that enable local manual control of equipment, and associated valves and instrumentation Total project duration, from receipt of order to completion of startup and optimization, was about eleven months. Installation of the boiler penetrations was achieved during an outage of about two weeks. Control of the system is feed-forward based upon inputs of steam flow (or oil flow) and mill configuration. From these signals the PLC will determine which injectors are operated and the maximum and minimum reagent that can be pumped to each injection level. A NOx feedback signal from the CEMS to the PLC enables control of the reagent flowrate within this range to m1n1m1ze overcontrol or undercontrol. The operator can adjust the setpoints in the control system as he sees fit. 

### Results Of Initial Start Up and Optimization Testing
Initial startup and optimization testing was performed to confirm the operability of the SNCR system and to establishthesetpoints for all system components, including injector atomization parameters and the control system look-up table set points. During this period the system performance was characterized by:

• NOx reduction • ammonia slip • reagent consumption NOx was measured by the plant's CEMS.
Ammonia slip was determined by wet chemical means. Flue gas samples from downstream ofthe economizerweredrawn through a known volume of sulfuric acid solution. The solution was analyzed for concentration of ammonia ion using an ammonia ion selective electrode. By this method ammonia concentration in the flue gas could be determined. Samples were taken by Research-Cottrell and an independent company contracted by MontaupElectric. Correspondencebetween

4

## Figure3 

## Process Flow Diagram ofSNCR System

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Dletrtbullon

MaduiH



### 5independent measurements was generally very good. Reagent treatment rate in this paper is expressed in terms of gallons per hour of 50% aqueous urea solution and in terms of NormalizedStoichiometricRatio (NSR). NSR isthe ratio ofthe actual stoichiometric ratio (of urea to uncontrolled NOx> to the stoichiometric ratio for theoretically 100% NOx reduction and 100% chemical utilization. For urea, NSR is equal to:



### NSR

## = molesofurea X 2 

### molesofuncontrolledNOx

FullLoad Testing At 100% MCR conditions, NOx reduction was achieved through injection of reagent into the two uppermost injection zones (zones #3 and #4). Full load optimization testing results, shown in Figure 4 as ammonia slip versus NOx reduction, indicate that at full load conditions NOx reductions in excess of 50% may result in ammonia slip in excess of 10 ppm. Figure 5 shows the results of testing at full load with variation of treatment rate and a constant 1 to 2 bias in reagent flow between zones 3 and 4, respectively. The testing demonstrated that NOx compliance (at or below 0.38 lb/MMBtu) could be achieved with less than 10 ppm of ammonia slip.
Mid-Load Testing Boiler #8 operates periodically between 85 and 105 MWe and very little between 60 and 85 MWe. Normally the boiler passes through these load levels during early morning load ramp up to full load or during evening load ramp down to about 35-60 MWe (net). Although the boiler operates less often in these middle**lof:~ds,** the high baseline NOx levels under these load conditions determined some of the design limitations ofthe SNCR system provided to Montaup Electric.
At 95 MWe, uncontrolled NOx levels can exceed 0.90 lb/MMBtu. In terms of total NOx emissions, this is the worst case condition for the boiler. However, baseline NOx levels can be reduced by adjustment of furnace air distribution. Figure 6 shows the results of testing at 95 MWe with an initial NOx of 0.63 lb/MMBtu. Testing demonstrated that NOx can be reduced to compliance levels with roughly 10 ppm or less of ammonia slip.
In order to test the limits of the SNCR system and to demonstrate the economic benefits of reducing baseline NOx through combustion air adjustment, testing of the SNCR system was performed at high NOx levels (0.94 lb/MMBtu) and 95 MWe. Testingshowedthatunderthese conditions NOx could not be reduced by the SNCR system to below 0.38 due to the high chemical flowrates required, which exceeded pump capacity. These results were expected since the system provided to Montaup Electric was not designed to accommodate this extreme case. In any event, operation in this manner is uneconomical due to the very high chemical consumption. Reagent chemical costs ranged about $0.80-$1.00per gallon during this testing. However, as shown in Figure 7, atthe lower baseline NOx level of 0.63 lb/MMBtu, chemical consumptions can be greatly reduced and compliance more economically achieved.
At 85 MWe, baseline NOx emissions during testing ranged from about 0.90-0.98 lb/MMBtu.This isthe lowestload condition on Boiler #8 where only coal is fired. Below 85 MWe, Montaup Electric usually co-fires



### 60.90 0.80 0.70



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## z 0.20 0.10 11 10 9 8 

## Figure 4, Montaup Electric FullLoad NH3

### slip vs NOx Reduction



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## Montaup Electric Boiler#8 

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### NSR

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### 7

# Figure6, MontaupElectric Boiler#8, 95 MWe NOx NH3 

## slip vs NSR
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### 0.30 0.20 0.10

### 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

I 

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# ab1MMB1U)J 

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# Figure7,MontaupElectric NOx vs. ReagentFlowrate 95 MWe, NOx Baseline0.93, 0.63**---,------~------------------------------------------------------** 1 I**---~-------------~-----------------------------------------------**



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# ·Isome oil with the coal. The results of testing at 85 MWe are shown in Figure 8. Testing showed that NOx emissions below 0.381b/MMBtucould be achieved with less than 5 ppm ammonia slip by injection through zone 3 and 4 injectors. Injection of reagent through zone 2 and 3 injectors could not achieve adequate NOx reduction because of the higher temperatures in these zones at this load condition.
LowLoad Testing
The results of SNCR testing at40MWe are shown in Figure 9. NOx emissions were reduced to nearly 0.30 lb/MMBtu while maintaining below 10 ppm ammonia slip with reagent injection through zones 1 and 2. Reduction to below 0.30 lb/MMBtu was possible; however, ammonia slip increased above 10 ppm. Had NOx levels below 0.30 lb/MMBtu been necessary under this load condition, it could have been provided for by design of a different injector configuration. Figure 8 also demonstrates the benefit of independent zone control with injection into two zones. Injection through zone 2 injectors alone produced high ammonia slip (over 20 ppm - the highest ammonia slip measured during the test period) with no advantage in NOx reduction. Injection into zones 1 and 2 produced lower ammonia slip with good NOx reduction.
LoadFollowing
Because Montaup Electric boiler #8 operates over a wide load range, the SNCR system was designed to follow load and maintainthe unitincompliance overthefull load range. The boiler, in fact, spends very little time at intermediate loads. Typically daytime operation is ator near 100% MCR. At timesthatthe unit is not operating at or near full load, it is operating in the range of about 35%-55% MCR. Since the unit normally passesthrough intermediate loads and these are the loads where NOx is most difficult to control on this unit, this is a challenging application for a control system. The control system must very quickly respond to a situation that initially requires increasing reagent injection rates while changing injection levels, and then requires decreasing injection rates while injection levels continue to change.
In the short time that Montaup Electric has operated the SNCR system, the SNCR system's response to load changes has been observed. From this limited experience, the system seems to take about 30 minutes to fully adjust to a major load transient, as shown in Figure 10.This, however, is expected to be improved through adjustment of control system time constants and could be further improved, if needed, by use of in-duct NOx analyzers for feedback. Several minutes pass before the stack CEMS will indicate a change in NOx withinthe furnace, which results in a lag in the SNCR system response. In-duct NOx analyzers would reduce that lag time to seconds. Nonetheless, although NOx may pass above the NOx setpoint for a short time during the transient, the system consistently stabilizes at or below compliance levels. Figure 10 also shows a transient in the SNCR system while the CEMS was taken out of service, which exacerbated the NOx excursion during the load transient. The SNCR control system provided to Montaup Electric has a feature to properly accommodate for such situations; however, it will not be in operation until after Montaup Electric completes its DCS installation this spring. With this feature enabled and with additional operating experience, Montaup Electric operators can adjust the SNCR system controls to minimize any NOx excursions during transients.



### 9

# Figure 8, MontaupElectric Boiler#8, 85 MWe NOx and NH3

## slipvs NSR, vary injection zones

1.00

- - - I

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 7

I 0.90 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 6 

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### 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 

### NSR 

# I• NO, (IJMMB1U)

# I J

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# I Figure9, MontaupElectric Boiler#8, 40MWe NOx and NH3

## vs NSR, zones 1&2or2only

0.80

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11Carbon Monoxide Emissions Carbon monoxide (CO) emissions can potentially increase as a result of SNCR withurea. However, noticeable increases in CO emissions were notobserved during the start up and optimization of the SNCR system.
Economics The fully installed capital cost ofthis SNCR system is in the range of $1 5-$1 6 per KWe. This cost includes a technology license fee which is a significant portion of this total cost. Were Montaup Electric a member of E.P.R.I., the license fee would have been substantially less. Under full load conditions it was demonstrated that compliance could be achieved with roughly 150gallons per hour of reagent. Using a price of $0.80-$1.00 per gallon, this equatesto a reagent cost of $700-$880 per ton of NOx removed, or about 1.1 to 1.3 mils per KW-hr. 

### SUMMARY Start up and optimization of the SNCR system at Eastern Utilities 112 MWe Montaup Electric Boiler #8 demonstrated thatthe system is capable of reducing NOx to below Massachusetts RACT compliance levels overthe load range of 35% to 100% MCR. Over this load range ammonia slip remained at or below 10 ppm. Operation of the system also demonstrated the economic benefits of minimizing baseline NOx to minimize reagent consumption. Montaup Electric plans to incorporate automated combustion controls that will reduce their baseline NOx and enable operation of the SNCR system under the most economical conditions. While experience with SNCR operation during load changes is limited at this time, the control system provided has demonstrated that it can follow a load change and adjust SNCR system operation to bring the NOx emissions in compliance at the new load. Although a transient in NOx emissions may occur for a few minutes while the SNCR system adjusts tothe new condition, the system consistently brings NOx emissions into compliance levels. System response of the SNCR system will be improved through operator experience and adjustment of system control parameters, thereby minimizing these transients.



### 12