## Optimizing Selective Non-Catalytic Reduction Systems for CostEffective Operation on Coal-Fired Electric Utility Boilers

by

Jam e s E. Staudt, Ph.D. Andover Technology Partners Keith Kehrer John Poczynec Delmarva Pow e r

Roland Cote Robert Pierce Montaup Electric Rui Afonso Allen Sload David Miles Ne w E n g l a n d P o we r



## Abstract

Selective Non-Catalytic Reduction (SNCR) system s have been used to reduce NOx

em issions f r om coal f i red electric utility boilers. Experience has shown that the prim ary cost of reducing NOx with SNCR technology m o st often is the cost of reagent. It is therefore desirable to reduce the reagent cost to the lowest possible am ount while not adversely im pacting other aspects of plant operation. Adverse effects that need to be considered in any com p rehensive evaluation include im pacts on fuel cost as well as im pacts of such things as am m onia slip. And, while the param e ters that im pact perform a nce of an SNCR system are com m on am ong boilers, the behavior of these
param e ters will vary widely between units. Hence, the operators of plants using SNCR f o r reduction of NOx will f r equently f i nd dif f e rent operating strategies m o st ef f e ctive in

m i nim i zing the total operating cost associated with reducing NOx.

This paper will discuss the operating experience and results of optim ization program s at three different coal fired electric utility bo ilers, each one having different circum stances and each one using a different approach to m i nim i zing the cost of NOx reduction while using SNCR. New England Power utilized advanced instrum e ntation and controls with changes to soot blower operation to reduce consum ption of reagent by over 50% while

m i nim i zing adverse im pact to heat rate and reducing am m onia slip. Montaup Electric carefully m odified its furnace operations to reduce its reagent consum ption by over 50% while m a intaining design heat rate. Delm arva Power reduced its reagent consum ption through m odification of furnace operating settings and cofiring of natural gas under som e conditions, with a m i nim a l im pact to heat rate. This paper will discuss the approaches used at these f acilities and will present operating data f r om the program s.

Presented at ICAC Forum ’98, Raleigh-Durham , NC, March 18-20, 1998
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## Introduction

Selective Non-Catalytic Reduction is a technology that is being used at sixteen electric utility boilers to reduce NOx em issions. It is expected to play a signif i cant role in f u ture U.S. electric utility NOx reduction program s. As with all control technologies, deploym ent and operation of SNCR will incur a cost. Capital cost is generally low

com p ared to other techO$gies; however, the cost of reducing NOx through SNCR is driven prim arily by variable operating costs - generally dom inated by the cost of reagent. Hence, it is norm a lly in the operator’s best interest to m i nim i ze the reagent consum ption and any other costs associated with operating the SNCR system

. Suppliers of SNCR technology generally provide system s designed to m eet the guaranteed perform a nce at the conditions where the operator historically runs the unit. Due to a shortage of tim e and inform ation, the SNCR vendor generally cannot anticipate operating changes to the f acility that m a y benef it the operator when using SNCR technology. Frequently, after the operators have gained som e experience with the SNCR system they will discover im proved m e thods of operating the unit.

This paper will highlight successful SNCR optim ization program s at three facilities equipped with urea-based SNCR, com m only ref e rred to as NOxOUT. All three f acilities are equipped with NOxOUT system s that were m eeting or exceeding the expected system perform a nce prior to the optim ization. In each case, after having som e experience with the NOxOUT system on their unit, the facility operators identified m e thods of reducing the overall cost of reducing NOx while operating the NOxOUT system . In m o st cases, the operating adjustm e nts discussed here would not have been considered were it not for the existence of the NOxOUT system at their f acilities. The three f acilities have three uniquely different situations that cause different approaches to be appropriate.


## Operating Cost - Discussion

W h en reducing NOx through com bustion cont rols, reburning, fuel switching, SNCR or another NOx control m e thod, the facility' s operating costs m a y be increased in several ways. Efforts to reduce any single operating cost com ponent could favorably or unfavorably im pact the other operating cost param e ters. A detailed cost evaluation of NOx reduction technologies should include consid eration of all of the effects so that trade-offs can be assessed and the system operation potentially optim ized for lower overall cost operation.

SNCR reagent consum ption is driven prim arily by the baseline NOx level, the controlled NOx level, the gas flowrate (boiler size), the furnace tem p erature and the effectiveness in distributing the reagent. The last two param e ters - tem p erature and distribution - strongly influence chem ical utilization, a m easure of the efficiency of the urea in reducing NOx. The outlet NOx and the boiler size are either im posed requirem e nts or fixed param e ters. The operator does have the ability, however, to im pact the baseline NOx, the tem p erature
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of the SNCR reaction and the effectiveness in di stributing the reagent. To the extent that the operator can m a nipulate these variables to reduce the am ount of SNCR reagent that is

consum ed, the im pact that these variables have on the plant operating costs need to be included when considering the cost of reducing NOx.

Hence, when adjusting plant param e ters in an effort to reduce overall NOx reduction costs, it is essential to capture all of the cost im pacts so that the trade of f

s can be f u lly understood. The cost com ponents that should be considered include the following:

• The direct cost of operating the urea SNCR system , estim ated to be equal to roughly $1.00 per gallon of NOxOUT reagent consum ed. NOxOUT reagent, an SNCR reagent that is a 50% (by weight) aqueous urea solution with other additives, usually costs a little under $1.00 per gallon. However, $1.00/gallon was used in this analysis to generate the inform ation presented in this report in order to m a ke the m a th easy and cover the costs associated with other requirem e nts for operating the system such as plant service air, electric power, m a intenance, etc.

• If, for the purpose of enhancing NOx reduction system perform a nce, the furnace is operated at conditions where steam tem p eratures are not optim al, Heat Rate will be poorer. The additional cost of operating the unit at a particular load will be referred to as the Heat Rate Penalty. The operator can alter steam tem p erature by adjusting burner tilts, soot blowing or other m eans. The Heat Rate Penalty is specif i c to the boiler and is typically a function of superheat and reheat steam tem p eratures.

• W h en a prem ium f

u el, such as natural gas, is substituted f o r the prim ary f u el in order to reduce com bustion NOx and urea consum ption, the additional cost of this prem ium fuel should be considered and balanced against the savings from reduced urea

consum ption. The Fuel Prem ium is determ ined by m u ltiplying the cost difference between gas and coal in ($/MMBTU) tim es the heat input of the gas.

• If the boiler is capacity lim ited as a result of operation of the NOx reduction system
, the value of lost MW -hr incurred should be included in the costs. In m o st cases, a reduction in capacity is not necessary. But, a reduction m a y be necessary for som e facilities. The value of lost m e gawatt hours is determ ined by m u ltiplying the lost MW by the value of a MW hr for that day and tim e. It is im portant to recognize that during the sum m e r m onths there are often high-dem a nd days when the value of lost capacity could be very costly. For the purpose this analysis, capacity reduction was not considered as a m eans to optim ize NOx reduction system operation.

The aforem entioned costs do not include the heat rate penalty that is associated with evaporation of water carrier or incom p lete com bustion as indicated by LOI. These are costs that have been considered by previous investigators and are acknowledged but not investigated in this paper.

Costs that should be evaluated in a m o re com p rehensive analysis include additional

m a intenance that m i ght be associated with som e of the operating conditions evaluated in these program s.

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## Effects of Process Variables

Baseline NOx level To reduce chem ical consum ption it is generally m o st effective to first pursue a strategy of

m i nim i zing the NOx produced by the com bustion of fuel, thereby reducing the am ount of NOx that m u st be reduced. This can be done through som e m odification of the prim ary

com bustion process, changing of fuel to natural gas, or though the use of reburning prior to the SNCR system . However, once baseline NOx has been reduced, it is possible to

m a ke other process adjustm e nts that can significantly im prove the SNCR process

econom ics. The adjustm e nts generally focus on im proving reagent utilization.

Reagent Utilization Reagent utilization is a m easure of the ef f ectiveness of the reagent in reducing NOx. It is the objective of the SNCR user to m a xim i ze utilization to m i nim i ze the cost of reagent. Utilization is m a them atically equal to the percent reduction divided by the Norm alized Stoichiom e tric Ratio (NSR, the stoichiom e tric ratio of reducing reagent to NOx,

norm a lized with the balanced chem ical equation). If the utilization is 50%, f o r exam ple, then that m eans twice as m u ch urea is being used as that predicted by a balanced chem ical equation of the SNCR reaction for urea. Utilization is very strongly affected by initial NOx, controlled NOx, furnace tem p erature, ability to distribute the reagent, and the concentration of other species, such as CO and O 2 . In com m e rcial SNCR system s, utilization is typically between 20% and 60%.

For exam ple, Figure 1 shows % NOx reduction and % utilization plotted against NSR f o r an SNCR system . NOx reduction will increase with NSR. However, the slope of the curve becom e s shallow with increasing treatm e nt rates, indicating that further NOx reductions are only possible with m u ch higher treatm e nt rates. This is also reflected in the utilization, which drops rapidly at first and then drops less rapidly to near 20%.

Effects of Temperature on Utilization - It is well established that SNCR has a reasonably
narrow tem p erature window in which to properly react the reagent. If the tem p erature is too high, poor NOx reduction and poor chem ical utilization will result. If the
tem p erature is too low, high am m onia slip will result. In the region where operation is best, operators will balance im proved reduction or utilization against the risk of high

am m onia slip (see Figure 2). Since a high furnace tem p erature generally m eans low utilization and low utilization m eans high chem ical consum ption, it is desirable to inject urea at the lowest possible tem p erature without incurring high am m onia slip levels. Figure 3 shows the relationship between utilization and Furnace Exit Gas Tem p erature (FEGT) at one facility tested under conditions where the process was being pressed for very high reductions. Chem ical utilization was near 20% at the tim e and FEGT was adjusted to determ ine sensitivity of utilization to tem p erature. The relationship is very clear. In this case, a 20 ° F drop in FEGT resulted in nearly a 4% im provem e nt in utilization. This relationship m a y be m o re or less pronounced for any particular facility. In som e boilers, it m a y be desirable to operate the boiler under conditions where steam

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tem p eratures m a y be less than optim al for good heat rate because the value of the im proved reagent consum ption will m o re than com p ensate f o r the additional f u el cost.

Injector Optimization - Distribution of urea plays a critical role in chem ical utilization. Injector type and location are critical desi gn decisions when a vendor engineers an SNCR system . SNCR technology vendors norm a lly perform com puter sim u lation with

com putational fluid dynam i c m odeling to select injector locations and type. The

m odeling is norm a lly perform e d in conjunction with a test program where furnace
tem p erature and gas species m easurem ents are taken. Once installed, these injectors

m

u st be adjusted to ensure that atom ization and distribution are optim al. Annual adjustm e nt of the injectors with m o re frequent checking of injector set points is

recom m e nded for best perform a nce. Figure 4 shows the results of injector optim ization of a NOxOUT system that had been installed and operated interm ittently for about 18

m onths. The system had been optim ized 12 m onths earlier. The NOx and flowrate values on Figure 4 are five-m inute rolling averages. W h ile m a intaining the boiler conditions and SNCR injection steady, the liquid flowrate and atom ization air setpoints were adjusted on the distribution m odules. The objective during injector optim ization is to im prove NOx reduction and am m onia slip solely by im proving reagent distribution through adjustm e nt of atom ization, injector orientation, or nozzle tip changes. Am m onia slip m easurem ents

were m onitored continuously during these tests and there were no indications of high

am m onia slip. As can be seen from Figure 4, an im provem e nt in NOx of about 0.04 lb/MMBTU was achieved sim p ly through adjustm e nts to nozzle atom ization param e ters. And, if we were to control at the original outlet NOx level, chem ical consum ption could be reduced. As a result of this work, new injector atom ization set points were established at this f acility.

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F i g. 1. R e d ' n an d U t il ization v s . N S R

0% 10% 20% 30% 40% 50% 60% 70%

0 0 .5 1 1

. 5 2 2 .5
NS RRed'n/Util. % Red' n U t iliz a t io n

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Figure 2. Effect of Temperature on NOx Reduction and Ammonia Slip

1800F N Ox REDUCTIO N

N H3 SLIP HIGH NH3 SLIP
LOW NOx REDUCTION

TEMPERATURE

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Fig. 4. Ure a Nozzle Optimiza t i on

0. 0 0 0. 1 0 0. 2 0 0. 3 0 0. 4 0 0. 5 0 0. 6 020-Jun-9708:20:0020-Jun-9708:30:0020-Jun-9708:40:0020-Jun-9708:50:0020-Jun-9709:00:0020-Jun-9709:10:0020-Jun-9709:20:0020-Jun-9709:30:00 NOx

ur ea f l ow /100 inj e ctor adjus tm ent 5- m i nute r o lling aver ages of NOx and urea f l ow Fig. 4. Ure a Nozzle Optimiza t i on

0. 0 0 0. 1 0 0. 2 0 0. 3 0 0. 4 0 0. 5 0 0. 6 020-Jun-9708:20:0020-Jun-9708:30:0020-Jun-9708:40:0020-Jun-9708:50:0020-Jun-9709:00:0020-Jun-9709:10:0020-Jun-9709:20:0020-Jun-9709:30:00 NOx

ur ea f l ow /100 inj e ctor adjus tm ent 5- m i nute r o lling aver ages of NOx and urea f l ow

Andover Technology Partners 112 Tucker Farm Road, North Andover, MA 01845 phone: (978) 683-9599, fax: (978) 683-3843, website: www.andovertechnology.com Fig. 3. Ut ilizat ion v s . FEGT dat a

18.50% 19.00% 19.50% 20.00% 20.50% 21.00% 21.50% 22.00% 22.50% 23.00%

2160.0 2165.0 2170.0 2175.0 2180.0 2185.0

FE GT , de g FUtilization, %

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Fig. 3. Ut ilizat ion v s . FEGT dat a

18.50% 19.00% 19.50% 20.00% 20.50% 21.00% 21.50% 22.00% 22.50% 23.00%

2160.0 2165.0 2170.0 2175.0 2180.0 2185.0

FE GT , de g FUtilization, %9


## Results of Optimization Programs

Delmarva Pow er, Edge Moor #3 Delm arva Power Com p any operates an 84 MW e (gross) tangentially-fired pulverized coal boiler (unit #3) at its Edge Moor generating station. The boiler is a natural circulation, single drum radiant boiler with a dry bottom

. The furnace has four corners and four levels of tilting, tangentially fired coal buckets. The boiler can also fire oil or gas. Unit #3 operates on dispatch typically in a load following m ode.

In cooperation with the Delaware Natural Resource Environm ental Control Agency, in 1995 Delm arva voluntarily installed a NOxOUT urea-based Selective Non-Catalytic Reduction system to dem onstrate this technology. Optim ization during initial

com m i ssioning dem onstrated that NOx could be m a intained below 0.38 lb/MMBTU under all design operating conditions with low levels of am m onia slip and reagent

consum ption at expected design levels 1 . The NOxOUT system has been operated interm ittently during the Ozone Season since com m i ssioning f o r the purpose of dem onstrating the operation of the system . Optim ization of the furnace during the

sum m e r of 1996 dem onstrated that NOxOUT reagent consum ption could be im proved from what was achieved during initial com m i ssioning. However, Delm arva believed that further reductions in operating cost m i ght be possible and another optim ization program was perform e d in the sum m e r of 1997.

Results of Optimization - In addition to injector optim ization, during the Edge Moor #3 optim ization program several furnace firing conditions were evaluated against the conditions for original SNCR vendor optim ization. The controlled NOx objective was 0.40 lb/MMBTU. Under the firing conditions that the SNCR was originally designed to operate at, the baseline NOx was 0.70 lb/MMBTU at full load (85 MW g) and 0.60 lb/MMBTU at m i d load (65 MW g). The objective of m odifying the firing conditions was to reduce baseline NOx while also considering the cost of im pact on the plant heat rate.

Under the varied firing conditions, data was collected to characterize the m a jor cost

com ponents of operating the NOxOUT system at these different firing conditions.

Full Load Testing - Figure 5 shows the operating cost breakdown of controlling NOx under different firing conditions expressed as a percentage of the total operating cost of controlling NOx at the baseline conditions for the SNCR system . Unfortunately, data on the heat rate penalty was not available for the baseline condition at 100% coal. Therefore, the total cost at the baseline condition m i ght actually be higher.

As shown, gas cofiring with vertical air staging in the prim ary com bustion zone resulted in over 80% reduction in urea consum ption with a m oderate heat rate penalty. Also, the gas cofiring condition produces a substantial fuel prem ium cost that raises the total cost of controlling NOx with this firing condition to nearly the cost of the baseline urea cost.

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During this test period it was not possible to operate at full load with 100% coal due to
perm it lim itations. Therefore, testing the vertical air staging condition at 100% coal could not be perform e d. It is believed that this firing condition would produce a m u ch

m o re cost-effective m e thod of operation than the baseline condition. For the sam e reason, simulated Close-Coupled Over Fire Air (CCO FA) with 100% coal was not tested at this load.

The furnace was also tested in a simulated close-coupled gas reburn m ode (CCGR), which resulted in the lowest cost of any condition tested at this load. Coal m ils B, C, and D were operating, gas was applied to the top gas burner, and air dam p ers were set for sim u lated CCOFA. Under this condition it was possible to achieve below 0.40 lb/MMBTU without operating the SNCR system . A m i nim a l heat rate penalty was experienced under this condition with a significant fuel prem ium

. The fuel prem ium is based upon gas prices at the tim e of testing. The total operating cost of reducing NOx in this m a nner is just under 50% of the operating cost of the baseline SNCR. This condition shows that the operator can benefit from low gas prices when available by using gas to very effectively reduce the SNCR reagent cost.

bas el ine S NCR-ori g

.

v endor optim iz at ion gas c o f i r i ng , air

s t agin g & S NCR CCGR 0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0% 90.0% 100.0%

bas el ine S NCR-ori g

.

v endor optim iz at ion gas c o f i r i ng , air

s t agin g & S NCR CCGR Fig. 5. No rmalized Op er. Cost of R e ducin g NOx, Ed ge Mo or #3, 85 MW

Fuel Pr em ium
HR P enalty
S NCR cost Did not hav e heat r a te da ta f r om or iginal tes t ing. Bas e li ne S NCR cost s m i ght actual l y be higher w i th this inc l ud ed.% of baseline cost

Mid Load Testing - Testing at 65 MW showed that simulated close-coupled over fire air (CCOFA) could be used to cut the urea consum ption by 70%, as shown in Fig. 6. There is a m odest heat rate penalty (~19% of baseline SNCR cost) so that, overall, operating cost of reducing NOx is cut by just over 50%. Testing of this condition at 85 MW was not possible due to operating lim itations and due to fuel lim itations discussed earlier.

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bas eline S NCR-ori g

.

v endor op t i m i z a tio n CCOF A & S NCR 0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0% 90.0% 100.0%

bas eline S NCR-ori g

.

v endor op t i m i z a tio n CCOF A & S NCR Fig. 6. No rmalized Op er. Cost s of R e ducin g NOx, Ed ge Mo or #3, 65 MW g

Fuel Pr em HR P enalty
S NCR Cost Did not hav e heat r a te data f r om or iginal te s t ing. Ori g i n a l co sts m i g h t

ac tually be h i gher w i th this inc l uded.% of baseline cost



### Montaup Electric, Somerset Boiler #8 Montaup Electric Com p any operates a 112 MW e tangentially-fired pulverized coal boiler (Boiler #8) at its Som e rset, MA generating station. The boiler is a natural circulation, reheat, single drum radiant boiler with a dry bottom that is rated at 800,000 lbs/hr steam flow at the superheater outlet at 1,975 psig and 1,000 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.

In 1994 and 1995, Boiler #8 operated on dispatch typically in a load following m ode. During the day and during other high dem a nd periods the boiler operated at or near full capacity. In the evening and on weekends the boiler was frequently reduced to 35% to 55% MCR. In 1996, Boiler #8 has operated as a base loaded unit.

In 1995 Montaup Electric installed a NOxOUT urea-based Selective Non-Catalytic Reduction system to reduce the NOx em issions to levels that com p ly with the Com m onwealth of Massachusetts Reasonably Available Control Technology (RACT) regulations, which required NOx em issions from Boiler #8 to be controlled to below 0.38 lb/MMBTU at all tim es. The system has successf ully m a intained NOx levels below required levels with low levels of am m onia slip. Experience with the operation is discussed in previous literature 2, 3

.

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Results of OptimizationDuring initial startup and during subsequent optim ization, Montaup operators have always m a intained an objective of not adversely im pacting heat rate. Furtherm ore, adherence to the turbine m a nufacturer’s operating requirem e nts regarding steam
tem p erature do not perm it operating under conditions where heat rate can be adversely im pacted in a significant way. This im poses a constraint on what can be done to optim ize SNCR operations. Nevertheless, Montaup undertook an aggressive optim ization program directed toward re ducing their SNCR operating cost while not changing steam tem p erature.

The fuel and furnace firing conditions of the Som e rset #8 boiler are such that it is not necessary to operate furnace wall blowers. Occasional operation of soot blowers is necessary to m a intain proper balance between turbine inlet and reheat steam
tem p eratures. Air in-leakage to the boiler is low. Moreover, Som e rset #8 is equipped with an Autom a ted Burner Managem e nt System (ABMS) and Distributed Control System (DCS). As a result of these features of the facility, Furnace Exit Gas Tem p erature (FEGT), furnace oxygen, baseline NOx, and other furnace conditions critical to SNCR perform a nce can be m a intained at a very consistent level.

It was recognized that a reduction in NOx baseline would be beneficial to urea

consum ption. However, a reduction in baseline had to be achieved without capital expense and without adversely im pacting steam tem p eratures. In order to reduce the NOx baseline, the furnace operation was m odified for air staging with reduced oxygen to about 2.0-2.5% oxygen at the furnace exit. As expected, LOI and CO increased

som e what; however, they have been m a intained within acceptable levels. To m a intain
steam tem p eratures, burner tilts were put in m a nual and raised. This raised the furnace slag level, and FEGT was raised to about 2,350 ° F im m e diately prior to the secondary superheater pendant. Af ter a f e w weeks of m a nual control of burner tilts, the slag level increased suf f i ciently that the burner tilts could be put back in autom a tic. The f acility has operated in this condition for over one year. It has m a intained an uncontrolled full-load NOx baseline of about 0.45-0.49 lb/MMBTU as opposed to the original, uncontrolled, full-load baseline of 0.65 lb/MMBTU.

Operation of the SNCR system to reduce em issions below 0.38 lb/MMBTU has substantially changed since im plem enting these changes to operation. First, injection at full load had previously been in zones 3 and 4 of this four-zone NOxOUT system

. Current operation requires operation of only four of the zone 4 injectors at full load, which reduces the air and dilution water requirem e nt by m o re than 60%. Recent characterization testing of this operating m ode showed that FEGT is currently 2350 ° F, approxim a tely 150 ° F higher than the FEGT under the original baseline SNCR conditions. Am m onia slip continues to be extrem ely low. The urea consum ption has been reduced about 66% (see Figure 7), without an adverse im pact to heat rate.

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bas eline S NCR wi t h opti m iz ed f u r nac e 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

bas eline S NCR wi t h opti m iz ed f u r nac e Fig. 7. No rmalized O p er. Co s t of r e d u cing NOx, M ont aup Electric, Som e r s et boiler #8% of baseline cost



### New England Electric, Salem Harbor #3 New England Power (NEP) operates four units at Salem Harbor. The three coal-fired units were the f i rst com m e rcial applications of SNCR on coal-f ired utility units in the U.S. Salem Harbor #3 is a 150 MW , wall-fired dry-bottom boiler. It is equipped with urea SNCR (NOxOUT), Low NOx Burners (LNB) and Separated Over Fire Air (SOFA) for control of NOx to 0.30 lb/MMBTU. NEP installed the LNBs and SOFA after they installed the SNCR to SH #3. Prior to installation of the LNB’s and SOFA or NOxOUT, NOx em issions averaged 1.00 lb/MMBTU. The LNB’s and SOFA reduced NOx to the NOxOUT system from 1.00 to 0.43 lb/MMBTU. The site does not have natural gas. As a result, there are no additional options for further reducing NOx baseline to the SNCR system at SH #3.

Unlike the previous two exam ples, the SH #3 furnace experiences significant slagging in the furnace with m u ch FEGT variability. Furnace wall blowing was historically perform e d on a schedule for the purpose of controlling steam tem p eratures. Figure 8 shows data taken over a six-hour period in 1994, prior to the recent optim ization
program . W ith this level of FEGT variability, and considering the general trends indicated on Figures 2 and 3, high chem ical u tilization with low am m onia slip would not be possible without a control system that could rapidly sense the changes in FEGT and adjust the injection rate and location accordingly. At the tim e the SNCR system was originally installed (1993), such a control system was not available. In order to avoid high am m onia slip conditions, the SNCR system was operated in a m a nner that produced lower reagent utilization than would be possible in a unit without the FEGT variability.
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Fig. 8. Furnace Exit Gas Temperature (FEGT) at Salem Harbor over six hours 4

.

The SNCR system that was originally installed at Salem Harbor #3 incorporated the state-of-the-art for the technology at the tim e. Because sensors were not available for
tem p erature or am m onia m easurem ent, the original SH #3 NOxOUT system was controlled prim arily in a feed-forward m a nner based upon the m easured steam flow and the coal m ills in operation. The NOx signal from the CEMS was com p ared with a setpoint for a slow-adjusting feedback trim

.

W h en the NOxOUT system was originally installed in 1993 - prior to the LNB and SOFA - the reagent consum ption was rather high as a result of the need to reduce NOx from 1.00 lb/MMBTU to about 0.30 lb/MMBTU. About eight m onths after the NOxOUT system was installed, the LNB and SOFA retrof it reduced the baseline to the NOxOUT system to about 0.43 lb/MMBTU. This, of course, had the expected benefit with regard to reagent consum ption, reducing reagent consum ption about 60%. This also had a relatively m i nor im pact on heat rate because it was m o re difficult to m a intain steam
tem p eratures as a result of the com bustion system retrofit.

It was believed that there was still signif i cant room to im prove reagent consum ption if a

m eans to address the variable FEGT could be im plem ented. 4 However, a com m e rcially available approach did not exist during the first years of system operation. In 1996, a new control system - im plem enting sensors and control m e thods previously unavailable for SNCR control - was installed and dem onstrated at Salem Harbor #3.

The new control system - provided by the original SNCR system vendor, Nalco Fuel Tech, - utilizes signals f r om optical FEGT instrum e nts and steam f

l ow to control the injection zones and injection rate. 5 Two FEGT instrum e nts were supplied to m onitor both sides of the split furnace. Piping m odifications were also m a de to the NOxOUT system in order to provide the system independent injection zone control, which had not been provided on the original system . W ith the use of the FEGT instrum e nts, the control system could m o re accurately track the proper injection point in the furnace and respond
Andover Technology Partners 112 Tucker Farm Road, North Andover, MA 01845 phone: (978) 683-9599, fax: (978) 683-3843, website: www.andovertechnology.com15

to rapid system changes. W ith the piping changes, the NOxOUT system could effectively bias the reagent injection rate between levels in response to changes in
tem p erature. Another feature of the control system is the use of a continuous am m onia slip signal that indicates transients of high am m onia slip. W ith this signal, the NOxOUT control system can detect and respond to conditions that would otherwise produce high

am m onia slip.

The benefits of this control system are presented in Figure 9, which shows the total operating cost of NOx reduction, including reagent consum ption rate and heat rate penalty (norm a lized to the historical 1994, 1995 and early 1996 values). It is notable that the FEGT instrum e nt was installed prior to placing the new control algorithm s in operation. W ith the FEGT instrum e nt, the operators m odified their soot blowing operations, which created a benefit to the reagent consum ption and elim inated the heat rate penalty that previously existed. W ith the full controls in place, reagent consum ption was about 33% of the historical 1994, 1995, and early 1996 values. In addition to the im provem e nt in reagent consum ption, Salem Harbor #3 experienced a significant reduction in am m onia slip 5

.

L N B+ SO F A +

NO xOUT W i th FEG T W i th FE GT + N e w

Contr o ls 0. 0 % 10 .0% 20 .0% 30 .0% 40 .0% 50 .0% 60 .0% 70 .0% 80 .0% 90 .0% 100 .0%

L N B+ SO F A + NO xOUT W i th FEG T W i th FE GT + N e w

Contr o ls Fig u re 9. Norm alized Oper. C o st o f Red u cing NOx, Salem Harb or #3

Heat Rate P enalty

SNCR Cost% of historical cost

Andover Technology Partners 112 Tucker Farm Road, North Andover, MA 01845 phone: (978) 683-9599, fax: (978) 683-3843, website: www.andovertechnology.com16



## Summary

In this report, operators of three dif f e rent f acilities equipped with urea SNCR (NOxOUT) found ways of operating the facility in a m a nner that significantly reduced the total operating cost of reducing NOx when using the SNCR system . In all cases the system s were initially providing the designed and guaranteed perform a nce that the vendor originally prom ised. However, with experience, the operators found m e thods of im proving on the perform a nce of the system . And, although each situation involved the

sam e technology on coal-fired boilers, signifi cantly different m e thods were em ployed to im prove the perform a nce. Below, the key features of these program s and lessons learned are discussed.

• Reducing the baseline NOx will usually produce the greatest im provem e nt in urea

consum ption. This can be done through com bustion adjustm e nts, reburning, or fuel substitution. However, these m e thods will frequently increase other operating costs -

m o st notably, fuel cost - which m u st be considered in any analysis. At Edge Moor, Som e rset and Salem Harbor the operators found m e thods of operating the boilers that significantly reduced baseline NOx. In all cases, the im proved reagent consum ption was balanced against other costs, such as increased heat rate and/or the additional cost of f

i ring a prem ium f

u el. Edge Moor #3’s gas availability gives it certain f l exibility during sum m e r m onths when gas is less expensive. Som e rset’s ABMS enables it to m a intain furnace conditions that offer low NOx and good heat rate, enabling large reductions in reagent consum ption. At Salem Harbor, the LNB + SOFA retrofit provided a substantial reduction in NOx with a sm all penalty to heat
rate.

• In cases where NOx has been m i nim i zed, efforts to im prove reagent utilization should be taken. Of the param e ters the operator can im pact, reagent utilization is m o st affected by reaction tem p erature and dist ribution. Providing that distribution is optim ized, the operator m u st optim ize utilization with respect to tem p erature, which requires balancing reagent savings against the risk of high am m onia slip and/or increased heat rate. On furnaces such as at Salem Harbor #3 that have highly variable FEGT’s at any fixed load due to slagging, oxygen control, or other reasons, this balance can be very delicate. Newly developed instrum e ntation and control m e thods used at Salem Harbor of f e r the ability to m a intain this balance and realize the dual benefits of reduced reagent consum ption - lower operating cost with lower am m onia slip - while avoiding the risk of high am m onia slip during transients. In the cases of Edge Moor and Som e rset, unlike Salem Harbor, FEGT is fairly stable. Therefore, at these f acilities it is easier to optim ize reagent injection with respect to tem p erature without additional instrum e ntation. At these two facilities the control system im provem e nts used at Salem Harbor - if im plem ented at Edge Moor or Som e rset - would provide operating benefits, but not as great as those experienced at Salem Harbor.

Andover Technology Partners 112 Tucker Farm Road, North Andover, MA 01845 phone: (978) 683-9599, fax: (978) 683-3843, website: www.andovertechnology.comAndover Technology Partners 112 Tucker Farm Road, North Andover, MA 01845 phone: (978) 683-9599, fax: (978) 683-3843, website: www.andovertechnology.com 17



## References

1. Ciarlante, V, Zoccola, M. “Application of Urea SNCR on a Tangentially Fired 84 MW e Pulverized Coal Boiler”, Conference on Selective Catalytic and NonCatalytic Reduction for NOx Control, May 15-16, 1997.

2. Staudt, J.E., Casill, R. P., Tsai, T. S., Ariagno, L. J., “Com m e rcial Application of Urea SNCR for NOx RACT Com p liance on a 112 MW e Pulverized Coal Boiler”, EPRI/EPA 1995 Joint Sym posium on Stationary Com bustion NOx Control, Kansas City, MO, May 16-19, 1995.

3. Tsai, T.S., Ariagno, L., Cote, R., Staudt, J.E., Casill, R.P., “Living with Urea Selective Non-Catalytic NOx Reduction at Montaup Electric’s 112 MW PC Boiler”, ICAC Forum ’96, March 1996.

4. Frish, M.B., Johnson, S., Com e r, J., Afonso, R., Sload, A., “Integrated NOx Control at New England Power, Salem Harbor Station”, EPRI-EPA 1995 Joint Sym posium on Stationary Com bustion NOx Control, May 1995

5. O’Leary, J.H., Sun, W

. H., Afonso, R., Sload, A., “SNCR Reagent Reduction Through Innovative System Controls at Salem Harbor Station Unit 3”, EPRI-DOE-EPA Com b ined Utility Air Pollution Control Sym posium
, August 25-29, 1997