# Andover Technology Partners

Measuring Ammonia Slip fro m Po st Combustion NOx Reduction Systems

By

Jam e s E. Staudt Andover Technology Partners

ABST RAC T

Unreacted ammonia, referred to as ammonia slip , can be a by -product of certain NOx reduction processes. Facilities with NOx reduction system s that experience substantial operating problem s from ammonia slip are the ex ception s ra ther than the rule. This is largely due to im proved understanding of tec hnology application and also due to the f a ct that these sy stem s are usually operated well within the operating envelopes in order to avoid prob le m s with ammonia slip. In m a ny f acilities there m a y be room f o r process im prove m e nt that could reduce operating costs or provide revenue enhancem ent opportunities. Operating SNCR or SCR tec hnology closer to its lim it offers these benefits but increases the risk of high ammonia slip. The ri sk could be ab ated with th e use of a mmonia m onitoring technology. But, except for gas-fired plants, continuous ammonia slip m onitoring technologies have not been available at a practical price to facilitate these cost-saving or revenue enha ncing im prove m e nts. Conditions that exist in high dust, high tem p erature, high N O x and/or high SO 2 environm ents make m o st analyzer m e thods of continuously m easuring ammonia unsuitable for application to coal plants.

In this p a per, the m o tivation to utilize ammonia slip m onitorin g technolog y will be analyzed an d quantif ied f o r an example plan t. As will be sho w n, the bene f its of monitoring ammonia slip are largely realized in the ability to operate a f a c ility in a

m a nner that perm its facility incom e e nhancem en t at a lower risk. Hence, the value is in risk reduction while revenue is enhanced.

Finally, a brief overview of the technologies available for m easuring ammonia in the h i gh dust region downstream of a NOx re duction syst em and the status of their developm ent will be p r ovided.

ICAC Forum 20002

Introductio n For Selec tiv e Cataly tic Reduction ( S CR) and Selec tive Non

- Cataly tic R e duction (S NCR) technologies, the level of ammonia slip is nor m a lly controlled to leve ls that do not create operating problem s or pollutant em ission is sues for users. Under the unlikely circum stances where hig h slip m a y occur, the types of problem s a mmoni a slip can potentially contribute to ar e well docum ented and include:

• Form ation of amm onium salts on air pr eheater and other do wnstream surfaces.

• Ammonium Chloride plum e from the stack.

• Ammonia odor in fly ash.

• Em ission of gaseous ammonia.

The first, second and third problem s are of m o st concern to boiler ope rators, particularly the first. The third problem is of gre a test c oncern to air regulators. This paper will not explore these problem s in detail, since they are mostly well unders tood and operators of facilities and air regulator s are well aware of them.

Reference 1 evaluated the use of NOx reduc tion technologies on several U.S. utility boilers and found that significant operating pr oblem s due to ammonia slip were rare, although there were units identified in the study that experienced som e air preheater buildup. The level of slip that needed to be m a intained to avoid problems varied. According to Reference 1, one unit experienced air heater plugging when ammonia slip exceeded 2 ppm while other p l ants could with s t and higher ammonia slip levels up to 10
ppm

. None of the facilities reported higher em issions than 10 ppm

. W h ile sulfur content of

the f u el plays a m a jor role in dete r m ining th e level of slip that is assoc i ated with an onset of pro b lem s , boiler design, fly ash char acteristics, and condition of the boiler (air in-leakage, etc.) all p l ay a significant role.

NOx reduction plants are norm a lly operated well within th e o p erating limits in o r de r to avoid amm o nia slip. In m a ny facilities there m a y be room for process im provem e nt that could reduce operating costs or even provide the opportuni ty for revenue enhancem ent under allowance trading scenarios. However, such operation m i ght risk higher slip. Ammonia slip m onitoring technology has not be en available to facil itate such benefits with suf f i cie n t conf idenc e that slip w on' t be a problem . To da te, amm onia m onitoring technology application has been limited largely to natural gas fired applications or other clean gas applications. Thes e applications could be addr essed with existing m onitoring techniqu es, such as UV photom etry, ion m obility spectro scop y or differen tial NOx analysis following conversion of ammonia to NOx. Conditions that exist in high dust, high tem p erature, hi gh NOx and/or high SO 2 environm ents have m a de these m e thods mostly unsu itab l e f o r ap plica tion to coal plants. Moreover, most commercial

em bodi m e nt s of these instrum e nts are such that a several m i nute tim e delay would exist, which m a y m a ke the m l e ss useful as process con t rol d e vices in SNCR or SCR syste m s. Therefore, other technologies m u st be used.

Andover Technology Partners 112 Tucker Fa rm Road, Nort h Andover, MA 01845 phone: (978 ) 683-9599, fax: (978 ) 683-3843, e-mail: staudt@andovertechnology.com web site: http://www.andovertechnology.com However, before em barking on a description of technology for m easuring ammonia, it is worth considering the reasons for monitoring it.3

Benefits o f Monitor i ng Ammonia The value of the benefit of m onitorin g amm onia slip con tinuo usly m u st exceed the cos t or there would be no reason to pay the price of an ammonia slip m onitorin g instrum e nt.

Others who have considered this issue have focussed on the benefits of process optim ization or avoidance of air preheater cleaning. These are good reasons to m onitor ammonia. However, as will be sho w n, the argu m e nt m a y not be com p elling in m a ny cases. However, when the facility is opera ted closer to the lim its of the technology for the purpose of revenue enhancem ent, the argum ent becom e s much m o re com p elling.

Experience with SNCR and SCR system s in th e U.S. on coal fired boilers docum ented in Reference 1 showed that only one of seventeen coal-fired boilers evaluate d experienced an ammonium bisulfate deposition problem that forced shutdown or a derate. This one case was an unusually challenging S N CR syste m that required special injectors because of the required level of reducti on, the boiler' s size and furnace characteristics. In other cases, where air heater b u ild up did o ccur, cleaning was perform e d during scheduled outages. Therefore, if S CR and SNCR system s continue to be operated as they have - well with in their c a pabilities - it is re asona bly un likely that an y particula r boile r will experience air heater plugging that is so seve re that it is forced to an unplanned shut down or take a derate. Som e units m a y experience an increase in air-preheater build up. But, it will rarely be so severe that forced out ages or derates result. Of course, air heater cleaning is a nuisance and costs m o ney, but it probably is not as costly as purchasing replacem ent power. The cost of periodic air heater cleaning, which is unlikely to go away

com p letely with an ammonia m onit o r, is like ly to be reduced with the help of a m m oni a monitoring technology. If, on aver age, cleaning is reduced from

, say, 2 tim e s per year to one tim e per year. This is worth in th e range of $20,000 to $40,000 per year (assum ing $20,000 per cleaning). It is notable that there was at least one unit in Reference 1 that operated an SNCR system for several year s without having any apparent ammonium bisulfate build-up in the air pr eheater. Of course, this too is probably an exceptional case. In su mmary, m a ny facilities d i d experience som e increase in air p r eheater deposition. However, experience shows that under current operating m e thods the likelihood of problem s s e rious enough to affect operation is very low.

SNCR Process Improv ement SNCR process chem istry is such th at there is a trade off between chem ical utilization and ammonia slip. Che m ical utiliz a tion is rela ted to the leve l of NOx reduction associated with a given trea tm ent rate. In ord e r to assu re that amm onia s lip will not be a problem, SNCR opera tors are forced to operate the system in such a w a y that chemical u t ilization is usually less than optim al. Figure 1 show s how this trade off between NOx reduction and ammonia slip occurs larg ely as a function of injecti on tem p eratu r e. T h e SNCR process ch em istry is very com p lex and it is sens itive to m a ny furnace variables. For this reason, it is very difficult to assess how m u ch of an im provem e nt in reagent consum ption

m a y possible through integration of an ammoni a analyzer into the SNCR controls without

som e sort of detailed test program or co m puter modeling. Reference 2 discusses how New England Power used an ammonia anal yzer and other control im provem e nts to reduce reagent consum ption by a total of 60%. Half of the improvem e nt was attributed Andover Technology Partners 112 Tucker Fa rm Road, Nort h Andover, MA 01845 phone: (978 ) 683-9599, fax: (978 ) 683-3843, e-mail: staudt@andovertechnology.com web site: http://www.andovertechnology.com4

by New England Power entirely to changes in th e way the bo ile r furnace wall blowers were operated as a result of an on-line furn ace tem p erature monitor th at was installed. It is unclear how m u ch of t h e rem a ining im prove ment could be attributed to the amm oni a analyzer because substantial changes were m a de to the ure a in jection sys t e m that lik ely accounted f o r m u ch of the im provem e nt. If an ammonia instrum e nt could be integrated into the controls of an SNCR syste m to im prove reagent consum ption by, say, 5%, then a unit with an annual urea b ill of $500, 000 (an approxim a te number for a 200 MW Group I boiler operating year-round at a capacity factor of 65%) would see a $25,000 im prove m e nt in the annual urea cost. A 10% im provem e nt could be worth $50,000 per year. On the other hand, if the boiler only uses its SNCR seasonally, the cost sav i ngs are much less, b ecause the u r ea bill will be m u ch less - around 4 0 % of the aforem entioned values.

On the other hand, an ability to iden tify th e onset of high ammonia slip and adjust the
process m a y offer the potential for reve nue enhancem ent through m o re aggressive operation of the SNCR s y stem and sale of excess NOx credits. The additional NOx reduction could potentially resu lt from process improvem e nt s at no additional reagent costs or fro m injecting more reagen t while m onitoring ammonia to en sure that high slip does not result.

What would it cos t to eq uip an SNCR system to accomm odat e the s i gnal of an amm o nia monitor to improve controls? This depends upon the current conf iguration of the SNCR controls. It could be anywhere from slightly m o re than the co st of the analyzer to several tim es the co st of the analyzer if the controls needed substa ntial changes. Putting values to this, this could be an expenditure of no less than $50,000-$100,000 to several hundred thousand dollars.

Improved SCR controls Because of the inherent efficiency of the SCR reaction, there is little or no room for im prove m e nt of reagent consum ption. An on line analyzer may enhance transient operation, avoiding high slip conditions th at m a y occur during rapid dow nward load changes. In the even t that the un it is exp ected to have frequent load ch an ges, an ammonia analyzer m a y be useful in helpi ng to avoid som e of the problem s identified earlier in th is paper. Th ese benefits are hard to quantify, and m a y not m a ke com p elling
argum ents for capital ex penditu res.

Improved SCR Asset Utiliz ation Asset utilization relates to the ability of an a sset to generate revenues. Therefore, if an SCR or SNCR system has the ab ility to im prove the rev e nues of a f acility, it can potentially improve the asset utilization of the facility. However, we will return to this financial topic shortly, af ter discussing t echnically how and why these processes can im prove revenues.

Andover Technology Partners 112 Tucker Fa rm Road, Nort h Andover, MA 01845 phone: (978 ) 683-9599, fax: (978 ) 683-3843, e-mail: staudt@andovertechnology.com web site: http://www.andovertechnology.com5

Figure 1. Effect of Temperature on NOx Reduction and Ammonia Slip

1800F N Ox REDUCTION

N H3 SLIP

N ote that higher che m ic al utilization (NOx reduction for the treat ment rate) risks higher amm o nia slip. HIGH NH3 SLIP
LOW NOx REDUCTION

TEMPERATURE

SCR Technical Overview SCR' s are designed to meet a guaranteed le vel of NOx reduction with a maxim u m level of a m m onia slip after a specifi ed number of operating hours.

Catalyst volum e, Vcat, is approxim a tely determ ined by the following equation, which captures the effects of chem ical ki netics at a constant tem p erature 3 :

Vcat = {F1 * Vfg * [-ln ( 1 - η )] / (Asp* k*F2)} + F3

where η = the fraction of NOx re moved, F1, F2, and F3 are correction factors based on experience, Vfg is the furnace gas volum e flowra te, k is the catalyst activ ity coefficien t, and Asp is the specific s u rface area of the cataly s t.

For a particular SCR reactor with a designe d v o lum e , flowrate, specific surface area and correction factors, the above equation can be reduced to:

(1 - η ) = e -k C

Of course, this is recognized by chem ical engineers as the f a m ous Arrhenius Rate expression from che m ic al kinetics with tem p er ature held constant, from which the first Andover Technology Partners 112 Tucker Fa rm Road, Nort h Andover, MA 01845 phone: (978 ) 683-9599, fax: (978 ) 683-3843, e-mail: staudt@andovertechnology.com web site: http://www.andovertechnology.com6

equation is o r igin ally d e rived. The cataly st reactor is des i gned to m a intain at least a certain level of activity after a num be r of opera ting hours. This is norm a lly referred to as k/k o , or the fraction of origin al activity. Figure 2 shows an exam ple plot of catalyst activity versus operating hours. The actual value of k/k o is determ ined by the particulars of the application and th e volum e of the reacto r

. In any event, when the catalyst is new, it can achieve greater reductions of NOx while

m a intaining within r e quired amm onia slip leve ls. The cata l ys t loses its ac tivity as a re sult of depositio n of i m purities from the fuel onto th e cata l yst su rface. It is importan t to no te that injectio n of amm oni a for the NOx re duction reaction does not in any way im pact catalyst degradation.

For exam ple, a catalyst with activity as is Figure 2, m i ght reduce NOx as shown in Figure 3 if enough ammonia is injected. The shaded area indicates the unutilized outlet NOx potential if the SCR is operated only at th e end of life design va lue of ~0.15 lb/MMB TU.

The Potential Value and Uncertain ty of Excess R e duction s This excess capacity of the SCR to reduce NOx early in it' s lif e is poten tially va luable during the ozone season in the Ozone Trans port R e gion, where em ission reduction credits can be generated and traded. In fact, Pub lic Serv ice Com p any of New Ham p shire used funds earned from the sale of NOx early reduction credits to pay for facility im prove m e nts. 4

Hence, for facilities in the Ozone Transpor t Region, there is the potential for revenue enhancem ent by overcontrolling NOx and se lling em ission reduction credits. The

m a rginal cost of creatin g these credits is larg ely the cos t of the addi tional amm onia. Bear in m i nd that at th is poin t in the an alys is we are assum i ng that the additional reductions are being done with the initial catalys t charg e , so that no a dditional catalyst is needed. That is why the potential bene f it dim i nishes with tim e as the cataly s t lo ses activ ity.

The value of these credits in the m a rketplace less the cost of generating them ( m ostly the cost of ammonia) will determ ine the degree of in come enhan cem ent that is possib l e with this approach. Figure 4 shows the potential va lu e of this app r oach for th ree consecu tive ozone seasons on a 500 M W boiler with an Oz one Season capacity factor of 70% while

assum i ng an ammonia cost of $360/ton. Assum p tions incorporated in to the analysis of Figure 4 include catalyst behavior exactly as in F i gure 2 and that every ton of NOx be low 0.15 lb/MMBTU can be sold.

There is, however, som e additional risk a ssociated with operating in this m a nner. Although experience has shown that few NO x reduction system s cause their owners serious difficulties from ammonia slip, these f acilities were no t try i ng to o p erate at the catalyst' s lim it. And, in reality, the catalyst activity m a y not behave exactly as expected in the design curve such as indicated in Figur e 2. There is an expected ran g e of activities expected over catalyst life, as shown in Fi gure 5. Figure 5 shows the exp e cted cataly st activ ity alon g with an as sum e d expected 95% co nfidence lev e l for the catalyst.

Andover Technology Partners 112 Tucker Fa rm Road, Nort h Andover, MA 01845 phone: (978 ) 683-9599, fax: (978 ) 683-3843, e-mail: staudt@andovertechnology.com web site: http://www.andovertechnology.comAndover Technology Partners 112 Tucker Fa rm Road, Nort h Andover, MA 01845 phone: (978 ) 683-9599, fax: (978 ) 683-3843, e-mail: staudt@andovertechnology.com web site: http://www.andovertechnology.com 7



### Figu r e 2

. S C R C a taly s t A c ti v i ty v e r s u s Ope r ati n g H o u r s

0. 5 0 0. 6 0 0. 7 0 0. 8 0 0. 9 0 1. 0 0 1. 1 0

0 4

, 0 00 8, 0 0 0 1 2

, 0 0 0 16, 0 0 0 2 0, 0 0 0 2 4, 00 0

o p erat i n g h o u r skok/

F i g u r e 3. S CR O u t l et NOx P o t e n t ial

0. 000 0. 010 0. 020 0. 030 0. 040 0. 050 0. 060 0. 070 0. 080 0. 090 0. 100 0. 110 0. 120 0. 130 0. 140 0. 150

0 4

, 0 0 0 8, 000 12, 00 0 1 6, 000 20

, 0 00 2 4

, 000 Ope r a t ing H o ur sNOx (lb/MMBTU)

P ote n t ia l

Out l e t N O x Le v e l Unutili zed SCR c apa city8

F i g u r e 4. P o t e n t ial I n co m e E n h a n c em en t fr o m Ozo n e S easo n N O x A l lo wan ce S a les th r o u g h Ov er co n t r o l o f S C R

$0 $5 0 0 ,0 00 $1, 0 00, 00 0 $1, 5 00, 00 0 $2, 0 00, 00 0 $2, 5 00, 00 0

123
Ozone S e a s on ( y e a r )IncomeperOzoneSeason $ 500/ t o n NO x
$ 1

, 000/ t o n
$ 2

, 000/ t o n
$ 3

, 000/ t o n
$ 4

, 000/ t o n 500 M W pl ant
cap. Fc t r = 70% 153 day se ason

r e qui r e d l e ve l 0

. 15 l b / MMB T U

The range of uncertainty in catalyst activ ity wi ll v a ry for each application. In som e cases it will be sm all, in others the uncertainty can be high. The range of uncertainty will be determ ined by the quality of the database of the catalyst supplier, the quality of the inform ation provided by the owner to the ca talyst designer, an d how closely actual operations m i rror the infor m ation provided by th e owner to the catal yst supplier. In general, the catalyst suppliers have very good databases and can predict perform a nce very well for certain, well-speci fied conditions. However, in the authors experience, the quality of infor m ation available during the design process and how closely the actual operations m a tch expected operations play a m u ch grea ter ro le in de term ining the lev e l of uncertainty in future catalyst performance.

Fortunately, the effects of this uncertain ty can be quantitatively assessed through

com puter simulation. F i gure 6 shows cum u la tive probability plots resulting from a Monte Carlo sim u lation of ozone season NOx a llowances availab l e for sale for each of seasons 1, 2, and 3 with the un certainty of Figure 5 included.

The results of Figure 6 indicat e the tons available for sale on a 500 MW plant with a capacity factor of 70% that only needs to control to 0.15 lb /MMBTU if the catalyst is operated at it' s lim it. As shown, in the firs t ozone season there is a 50% chance that 514 excess tons can be generated without ammonia slip problem s and the uncertainty is quite low regard in g how m a ny tons. This is because th e catalyst is relatively new and there is little un cer ta inty r e gard ing the perf o r m a nce it can offer. There is abou t a 95% chance that up to 505 tons can be generated. Yet, th ere is only a 5% chance that 523 tons can be

Andover Technology Partners 112 Tucker Fa rm Road, Nort h Andover, MA 01845 phone: (978 ) 683-9599, fax: (978 ) 683-3843, e-mail: staudt@andovertechnology.com web site: http://www.andovertechnology.com9

generated. In other words, risk can be s ubstantially reduced with a penalty of only 18 tons or 3.5% of the m ean expected value. F i g u r e 5. E x p ected k/ko p l u s an d m i n u s 2 x std

. Dev i at io n (95% co n f id en ce in ter v al)

0. 5 0. 6 0. 7 0. 8 0. 9

1 1. 1

0 4

, 0 0 0 8, 000 12

, 000 16, 0 0 0 20, 000 2 4

, 000

O p erat i n g Ho u r sk/ko k/ k o ( l o w )

k/ k o
k / k o ( h igh )

In the au thors ex perien ce , th e qu a lit y o f inform a t i on ava ilab l e during the design pr ocess and how closely th e actu a l operations ma t c h expected operations play the greatest ro le in d e ter m ining the le vel of uncer ta int y in future c a ta l y s t p e rform ance

.
In the second ozone season, 342 tons are the m e a n of the projections. There is about a 95% chance that up to 315 tons can be gene rated. There is only a 5% chance that 368
tons or m o re can be generated. Hence, the sp read is getting w i der, to 15% of the m ean expected v a lue.

For each year the uncertainty in the p r ojections in creas es. As stated earlier, this is larg ely due to unce r tain ty in ho w the f acility is ac tu ally operated versus ini tial expectations. In the third ozone season, 204 tons is the value of the m ean of the projections. There is about a 95% chance that 154 tons can be gene rated. There is only a 5% chance that 253
tons or m o re can be generated. The spread in uncertainty has incr eased to 98 tons in season three from 18 tons in season 1, or nearly 50% of the mean expected value.

Figure 7 translates the results of Figure 6 to incom e enhancem ent potential assum i ng NOx allowance prices of $1,000/ton a nd the price of a m m oni a is $360/ton.

As shown in Figure 7, the average (50% probab ility va lue) f o r the Fi rst O z one Season is $446,000 with a Standard Deviation of only $4,800 at an allowan ce price of $1,000/ton. There is a 95% probability that $438,000 in inco m e can be made and only 5% chance that $454,000 in incom e can be generated in the first ozone season. Andover Technology Partners 112 Tucker Fa rm Road, Nort h Andover, MA 01845 phone: (978 ) 683-9599, fax: (978 ) 683-3843, e-mail: staudt@andovertechnology.com web site: http://www.andovertechnology.com10

In Ozone Season 2, the 50% probability value for incom e enhancem ent is just under $300,000 (~$298,000) and standard deviation is $13,720 at an allowance price of $1,000/ton. Or, there is a 95% chance that $273,000 in incom e can be made and 5% chance that $319,000 can be generated. This is still a fairly narrow band of uncertainty, although it is m u ch increased from the first ozone season.

Finally, in Ozone Season #3, the 50% pr obability value is only $177,733 of incom e enhancem ent at an allow a nce price of $1,000/ton. Standard De viation is $26,175, or nearly 15% of the m ean. There is a 95 % chance that $134,000 in incom e can be generated and only 5% chance that $219,000 in incom e can be generated. Clearly, the uncertainty of available allowance s f o r trad ing an d the unce r tainty in inco m e potential has substantially increased to nearly 50% of the m ean expected value.

Finally, F i gure 8 plots the 5%, 50%, and 95% confidence values from Figure 7, which dem onstrates the dim i nishing value and increa sing uncertainty of incom e for each ozone

season.

Figure 6. Range of Potential Oz one Season Tons Available for Sale, Accounting for Uncertainty : 500 MW Plant, Cata lyst activity of Figure 5

0% 20% 40% 60% 80% 100% 120%

0 100 200 300 400 500 600 S e a s on 1 t ons
M e a n of S eas on 1 t ons S e a s on 2 t ons
M e a n of S eas on 2 t ons S e a s on 3 t ons
M e a n of S eas on 3 t ons S e as on 1 S eas on 2 S eas on 3CumulativeProbability

NO x A l l o w anc es A v ai l a bl e, t o n s

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Figure 7. Range of Potential Net I n come Enhancement, Accounting for Uncertainty 500 MW Plant, $1,000/ton allow a nce prices, Catalyst activity of F i gure 5

0% 20% 40% 60% 80% 100% 120%

$0 $50, 000 $100, 0 0 0 $15 0, 000 $2 00, 000 $250, 00 0 $300, 0 0 0 $35 0, 000 $ 400, 000 $450, 00 0 $500, 000 S e a s o n 1 @ $1

, 000/ t o n

M ean o f S eas on 1 @ $ 1

, 0 0 0 / t o n

S e a s o n 2 @ $1

, 000/ t o n

M ean o f S eas on 2 @ $ 1

, 0 0 0 / t o n

S e a s o n 3 @ $1

, 000/ t o n

M ean o f S eas on 3 @ $ 1

, 0 0 0 / t o n Se ason 1 Se ason 2 S e a s on 3CumulativeProbability

O z one S e aso n I n com e

F i g u r e 8. 95% , 50 % an d 5% C o n f ide n ce V a lu es o f I n co m e E n h a n c e m e n t fr om Fi gu r e 7

$1 00, 00 0 $1 50, 00 0 $2 00, 00 0 $2 50, 00 0 $3 00, 00 0 $3 50, 00 0 $4 00, 00 0 $4 50, 00 0 $5 00, 00 0

S eas on 1 S eas on 2 S ea s o n 3 95% Conf i den c e

50% Conf i den c e

5% C onf i d enc e

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Reducing the Uncertainty - The Value of Mon i toring Ammonia There is a large m o tivation to uti liz e the asse ts of utility p l ants as effectiv ely as poss i b l e. Therefore, when it is possible to enhance revenues and incom e through sale of NOx allowance credits, these opportunities should be explored. T h e analysis in this paper showed, quantitatively, what th e value of these opportunities m i ght be for a particular situation. T h e analysis also showed how ther e can be significant uncertainty in projecting what the value of the opportunity m i ght be. Un certainty in the projections is determ ined by several things. The price of NOx credits is one. But, one of the m o st significant for SCR syste m s is the behavior of the catalyst ove r tim e. Generally, this is more a facto r of how closely actual operation m a tches expected S CR operation than any uncertainty over how the catalyst would behave under absolute ly certain conditions. For S N CR syste m s, there are nu m e rous factors that affect the p r edictability of o p eration. M o st im portan t is the consistency of furnace condition s , since for SNCR catalyst degrad ation is not an issue.

Ammonia monitoring is one im porta nt tool in the effort to ad dress the uncertainty of NOx reduction system operation. Other tools incl ude periodic catalyst activity testing, period ic furnace testing, and m a intaining cons istent fuel and boiler conditions. However, ammonia slip is the single indicator that wi ll provide a direct m easure of whether th e NOx reduction system is being operated unde r, at, or beyond its lim itations and by how much. Therefore, the use of a m m oni a m on itoring technology (if available) can m a ke it possible for much of the risk to be addressed.

Unfortunately, for operators of coal-f ired power plants ammonia- m onitoring technology has not been available in a reliable and cost-effective form . Table 1 lists several continuous analyzer technologies . W h ile several m e thods have proven very useful for gas applications where low NOx and low SO 2 exist, infra-red m e thods appear to offer the greatest hope for addressing this issue for co al-fired plants. Multi-com ponent infra-red

m e thods tend to be expensive and are not am en able to in-situ m easur em e n t. One of the most difficult aspects of m easuring ammonia is getting the s a mple to the analyze r while

m a intaining sam p le integ r ity. In-s itu m eas ure m ent avoids the concern regarding lost

sam p le integ r ity. Tunab l e Diode Laser spectroscopy, because it offers th e potential f o r reasonable cost, in-situ m easure m ents, high m o lecular selectivity a nd rapid response, is the m o st prom ising technology. Interference fr om m o isture is a potential problem . But,

som e of the suppliers have developed m e thods for addr essing this concern while

m a intaining adequate sensitiv ity. T h e prim ar y disadvan t ages of TDL methods are th at there is little commercial e xperience for amm oni a applicat ions and that m a ny of the suppliers are sm all, overseas firm s. In m o st cases the ins t rum e nts are still developm entstage prototypes. Service and application suppo rt will be im portant features to users of these products.

Andover Technology Partners 112 Tucker Fa rm Road, Nort h Andover, MA 01845 phone: (978 ) 683-9599, fax: (978 ) 683-3843, e-mail: staudt@andovertechnology.com web site: http://www.andovertechnology.com13

Table 1. Technologies for Conti nuous Measurement From Ammonia Technique Advantages Disadvantages Well Suited For NOx dif f e rentia l • Experience & f a m iliarity with

m e thod • Measures a surrogate

• Sensitiv ity f o r high NOx situations can be
poor • Low NOx (g as turbin es or g a s boile rs)

UV photom etry • Experience & f a m iliarity with

m e thod • Strong Interference fro m SO 2 • Natural Gas applic ation s , or other low SO 2 Ion Mobility • Sensitiv ity

• Fairly inte rf erence free • Not well suited f o r Corrosive g a ses

• Slow response • Any clean, noncorrosive gas

TDL (IR) • Relativ ely interference free (excep t for water)

• Solid-s tate

• In-Situ - no sam p le handling required

• Sensitiv ity • Lim ited experien ce

• Service and Support (m ostly sm all, f o reign fi rms)

• Moderate moisture interference must be properly addressed • All applications, especially coal

IR (m ulticom po nent) • Multip le sp e c ies • Tends to be Expensive • All applications

Autom a ted Wet Chem istry • Experience & f a m iliarity with

m e thod

• Can be set up quickly • Labor intensive

• Requires reagents • Testing
program s

The a b ove are gene ral statements that reflec t the au t h or 's ov er all im p r essio n b a sed upo n h i s clo s e fam i liarity wit h NOx redu cti o n techn o l o g y an d amm o n i a m o n ito rin g tech no log y

. Th is is no t in tend ed to b e

a co m p lete list. Howev e r, it is a list o f th e mo st im p o r tan t ap pro ach es in t h e au thor's o p i n i o n

. In so m e

cases com p ani e s m a y cla i m to have address e d ce rtai n di sa dva nt age s

. Th e aut h o r nei t h e r di sp ut es no r confirm s their claim s

.

Andover Technology Partners 112 Tucker Fa rm Road, Nort h Andover, MA 01845 phone: (978 ) 683-9599, fax: (978 ) 683-3843, e-mail: staudt@andovertechnology.com web site: http://www.andovertechnology.comAndover Technology Partners 112 Tucker Farm Road, North Andover, MA 01845 phone: (978) 683-9599, fax: (978) 683-3843, e-mail: staudt@andovertechnology.com web site: http://www.andovertechnology.com 14

Summary There are large opportunities for SCR and SNCR NOx reduction system operators to enhance overall facility income. However, there are uncertainties and risks associated with operating the facility at close to the lim it of system capabilities. In this paper the values and uncertainties were analyzed and quantified for a sample plant. Computer simulation of one example showed that the un certainty can be substantial over time. Availability of ammonia m onitoring technology will permit th ese uncertainties and risks to be addressed more effectively. Technologies have been available for monitoring natural gas fired applications. However, unt il recently there have not been technologies available for coal-fired electric utility plan ts that are of reasonable cost, sufficient accuracy, and reliability. Some new technol ogies, particularly TDL spectroscopy, offer excellent potential to ad dress coal applications.

References

1 NESCAUM & MARAMA ,"Status Report on NOx: Cont rol Technologies and Cost Effectiveness for Utility Boilers", available from Northeast States for Coordinated Air Use Management, June 1998

2 Staudt, J.E., Kehrer, K., Poczynek, J., Cote, R., Pier ce, R., Afonso, R., Sload, A., Miles, D., "Optimizing Selective Non-Catalytic Reduction Systems for Cost -Effective Operation on Coal-Fired Electric Utility Boilers", ICAC Forum 1998, Raleigh-Durham, March 1998.

3. Siemens, Katt Treff USA 1997 , June 3-5, 1997

4 Pickering, R., "Developing Flex ible, Least-Cost Plans for NOx Co mpliance: PSNH's Experiences in Implementing SCR and SNCR Technology", Infocast: New NOx Regulations and Power Generation In Competitive Markets, Crystal City, VA, November 3-5, 1999