Andover Technology Partners 

Measuring Ammonia Slip from Post Combustion NOx Reduction Systems 

By 

James E. Staudt 

Andover Technology Partners 

ABSTRACT

Unreacted ammonia, referred to as ammonia slip, can be a by-product of certain NOx 

reduction processes. Facilities with NOx reduction systems that experience substantial 

operating problems from ammonia slip are the exceptions rather than the rule. This is

largely due to improved understanding of technology application and also due to the fact 

that these systems are usually operated well within the operating envelopes in order to 

avoid problems with ammonia slip. In many facilities there may be room for process 

improvement that could reduce operating costs or provide revenue enhancement 

opportunities. Operating SNCR or SCR technology closer to its limit offers these 

benefits but increases the risk of high ammonia slip. The risk could be abated with the 

use of ammonia monitoring technology. But, except for gas-fired plants, continuous 

ammonia slip monitoring technologies have not been available at a practical price to 

facilitate these cost-saving or revenue enhancing improvements. Conditions that exist in 

high dust, high temperature, high NOx and/or high SO2 environments make most 

analyzer methods of continuously measuring ammonia unsuitable for application to coal 

plants. 

In this paper, the motivation to utilize ammonia slip monitoring technology will be 

analyzed and quantified for an example plant. As will be shown, the benefits of

monitoring ammonia slip are largely realized in the ability to operate a facility in a 

manner that permits facility income enhancement at a lower risk. Hence, the value is in 

risk reduction while revenue is enhanced. 

Finally, a brief overview of the technologies available for measuring ammonia in the high 

dust region downstream of a NOx reduction system and the status of their development 

will be provided. 

ICAC Forum 2000 
2

Introduction

For Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR) 

technologies, the level of ammonia slip is normally controlled to levels that do not create 

operating problems or pollutant emission issues for users. Under the unlikely 

circumstances where high slip may occur, the types of problems ammonia slip can 

potentially contribute to are well documented and include: 

• Formation of ammonium salts on air preheater and other downstream surfaces. 

• Ammonium Chloride plume from the stack. 

• Ammonia odor in fly ash. 

• Emission of gaseous ammonia. 

The first, second and third problems are of most concern to boiler operators, particularly 

the first. The third problem is of greatest concern to air regulators. This paper will not 

explore these problems in detail, since they are mostly well understood and operators of 

facilities and air regulators are well aware of them. 

Reference 1 evaluated the use of NOx reduction technologies on several U.S. utility 

boilers and found that significant operating problems due to ammonia slip were rare, 

although there were units identified in the study that experienced some air preheater 

buildup. The level of slip that needed to be maintained to avoid problems varied. 

According to Reference 1, one unit experienced air heater plugging when ammonia slip 

exceeded 2 ppm while other plants could withstand higher ammonia slip levels up to 10 

ppm. None of the facilities reported higher emissions than 10 ppm. While sulfur content 

of the fuel plays a major role in determining the level of slip that is associated with an

onset of problems, boiler design, fly ash characteristics, and condition of the boiler (air 

in-leakage, etc.) all play a significant role. 

NOx reduction plants are normally operated well within the operating limits in order to 

avoid ammonia slip. In many facilities there may be room for process improvement that 

could reduce operating costs or even provide the opportunity for revenue enhancement 

under allowance trading scenarios. However, such operation might risk higher slip. 

Ammonia slip monitoring technology has not been available to facilitate such benefits 

with sufficient confidence that slip won't be a problem. To date, ammonia monitoring 

technology application has been limited largely to natural gas fired applications or other 

clean gas applications. These applications could be addressed with existing monitoring 

techniques, such as UV photometry, ion mobility spectroscopy or differential NOx 

analysis following conversion of ammonia to NOx. Conditions that exist in high dust, 

high temperature, high NOx and/or high SO2 environments have made these methods 

mostly unsuitable for application to coal plants. Moreover, most commercial 

embodiments of these instruments are such that a several minute time delay would exist, 

which may make them less useful as process control devices in SNCR or SCR systems. 

Therefore, other technologies must be used. 

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However, before embarking on a description of technology for measuring ammonia, it is 

worth considering the reasons for monitoring it. 
3

Benefits of Monitoring Ammonia

The value of the benefit of monitoring ammonia slip continuously must exceed the cost

or there would be no reason to pay the price of an ammonia slip monitoring instrument. 

Others who have considered this issue have focussed on the benefits of process 

optimization or avoidance of air preheater cleaning. These are good reasons to monitor 

ammonia. However, as will be shown, the argument may not be compelling in many 

cases. However, when the facility is operated closer to the limits of the technology for 

the purpose of revenue enhancement, the argument becomes much more compelling. 

Experience with SNCR and SCR systems in the U.S. on coal fired boilers documented in 

Reference 1 showed that only one of seventeen coal-fired boilers evaluated experienced 

an ammonium bisulfate deposition problem that forced shutdown or a derate. This one 

case was an unusually challenging SNCR system that required special injectors because 

of the required level of reduction, the boiler's size and furnace characteristics. In other 

cases, where air heater build up did occur, cleaning was performed during scheduled 

outages. Therefore, if SCR and SNCR systems continue to be operated as they have - 

well within their capabilities - it is reasonably unlikely that any particular boiler will 

experience air heater plugging that is so severe that it is forced to an unplanned shut 

down or take a derate. Some units may experience an increase in air-preheater build up. 

But, it will rarely be so severe that forced outages or derates result. Of course, air heater 

cleaning is a nuisance and costs money, but it probably is not as costly as purchasing 

replacement power. The cost of periodic air heater cleaning, which is unlikely to go away 

completely with an ammonia monitor, is likely to be reduced with the help of ammonia 

monitoring technology. If, on average, cleaning is reduced from, say, 2 times per year to 

one time per year. This is worth in the range of $20,000 to $40,000 per year (assuming 

$20,000 per cleaning). It is notable that there was at least one unit in Reference 1 that 

operated an SNCR system for several years without having any apparent ammonium

bisulfate build-up in the air preheater. Of course, this too is probably an exceptional 

case. In summary, many facilities did experience some increase in air preheater 

deposition. However, experience shows that under current operating methods the 

likelihood of problems serious enough to affect operation is very low. 

SNCR Process Improvement 

SNCR process chemistry is such that there is a trade off between chemical utilization and 

ammonia slip. Chemical utilization is related to the level of NOx reduction associated 

with a given treatment rate. In order to assure that ammonia slip will not be a problem, 

SNCR operators are forced to operate the system in such a way that chemical utilization 

is usually less than optimal. Figure 1 shows how this trade off between NOx reduction 

and ammonia slip occurs largely as a function of injection temperature. The SNCR 

process chemistry is very complex and it is sensitive to many furnace variables. For this 

reason, it is very difficult to assess how much of an improvement in reagent consumption 

may possible through integration of an ammonia analyzer into the SNCR controls without 

some sort of detailed test program or computer modeling. Reference 2 discusses how 

New England Power used an ammonia analyzer and other control improvements to 

reduce reagent consumption by a total of 60%. Half of the improvement was attributed 

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by New England Power entirely to changes in the way the boiler furnace wall blowers 

were operated as a result of an on-line furnace temperature monitor that was installed. It 

is unclear how much of the remaining improvement could be attributed to the ammonia 

analyzer because substantial changes were made to the urea injection system that likely

accounted for much of the improvement. If an ammonia instrument could be integrated 

into the controls of an SNCR system to improve reagent consumption by, say, 5%, then a 

unit with an annual urea bill of $500,000 (an approximate number for a 200 MW Group I 

boiler operating year-round at a capacity factor of 65%) would see a $25,000 

improvement in the annual urea cost. A 10% improvement could be worth $50,000 per 

year. On the other hand, if the boiler only uses its SNCR seasonally, the cost savings are 

much less, because the urea bill will be much less - around 40% of the aforementioned 

values. 

On the other hand, an ability to identify the onset of high ammonia slip and adjust the 

process may offer the potential for revenue enhancement through more aggressive 

operation of the SNCR system and sale of excess NOx credits. The additional NOx 

reduction could potentially result from process improvements at no additional reagent 

costs or from injecting more reagent while monitoring ammonia to ensure that high slip 

does not result. 

What would it cost to equip an SNCR system to accommodate the signal of an ammonia 

monitor to improve controls? This depends upon the current configuration of the SNCR 

controls. It could be anywhere from slightly more than the cost of the analyzer to several 

times the cost of the analyzer if the controls needed substantial 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 

improvement of reagent consumption. An on line analyzer may enhance transient 

operation, avoiding high slip conditions that may occur during rapid downward load 

changes. In the event that the unit is expected to have frequent load changes, an 

ammonia analyzer may be useful in helping to avoid some of the problems identified 

earlier in this paper. These benefits are hard to quantify, and may not make compelling 

arguments for capital expenditures. 

Improved SCR Asset Utilization 

Asset utilization relates to the ability of an asset to generate revenues. Therefore, if an 

SCR or SNCR system has the ability to improve the revenues of a facility, it can 

potentially improve the asset utilization of the facility. However, we will return to this 

financial topic shortly, after discussing technically how and why these processes can 

improve revenues. 

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

 1800F 

NOx 

REDUCTION

NH3 SLIP 

Note that higher chemical 

utilization (NOx reduction 

for the treatment rate) risks 

higher ammonia slip. 

HIGH NH3 SLIP 

LOW NOx REDUCTION

TEMPERATURE 

SCR Technical Overview 

SCR's are designed to meet a guaranteed level of NOx reduction with a maximum level 

of ammonia slip after a specified number of operating hours. 

Catalyst volume, Vcat, is approximately determined by the following equation, which 

captures the effects of chemical kinetics at a constant temperature 3

: 

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

where η = the fraction of NOx removed, F1, F2, and F3 are correction factors based on 

experience, Vfg is the furnace gas volume flowrate, k is the catalyst activity coefficient, 

and Asp is the specific surface area of the catalyst. 

For a particular SCR reactor with a designed volume, flowrate, specific surface area and

correction factors, the above equation can be reduced to: 

(1 - η) = e-kC

Of course, this is recognized by chemical engineers as the famous Arrhenius Rate 

expression from chemical kinetics with temperature held constant, from which the first 

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equation is originally derived. The catalyst reactor is designed to maintain at least a 

certain level of activity after a number of operating hours. This is normally referred to as 

k/ko , or the fraction of original activity. Figure 2 shows an example plot of catalyst 

activity versus operating hours. The actual value of k/ko is determined by the particulars 

of the application and the volume of the reactor. 

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

maintaining within required ammonia slip levels. The catalyst loses its activity as a result 

of deposition of impurities from the fuel onto the catalyst surface. It is important to note 

that injection of ammonia for the NOx reduction reaction does not in any way impact 

catalyst degradation. 

For example, a catalyst with activity as is Figure 2, might 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 the end of life design value of ~0.15 lb/MMBTU. 

The Potential Value and Uncertainty of Excess Reductions

This excess capacity of the SCR to reduce NOx early in it's life is potentially valuable 

during the ozone season in the Ozone Transport Region, where emission reduction credits 

can be generated and traded. In fact, Public Service Company of New Hampshire used 

funds earned from the sale of NOx early reduction credits to pay for facility 

improvements.4

Hence, for facilities in the Ozone Transport Region, there is the potential for revenue 

enhancement by overcontrolling NOx and selling emission reduction credits. The 

marginal cost of creating these credits is largely the cost of the additional ammonia. Bear 

in mind that at this point in the analysis we are assuming that the additional reductions 

are being done with the initial catalyst charge, so that no additional catalyst is needed. 

That is why the potential benefit diminishes with time as the catalyst loses activity. 

The value of these credits in the marketplace less the cost of generating them (mostly the 

cost of ammonia) will determine the degree of income enhancement that is possible with 

this approach. Figure 4 shows the potential value of this approach for three consecutive 

ozone seasons on a 500 MW boiler with an Ozone Season capacity factor of 70% while 

assuming an ammonia cost of $360/ton. Assumptions incorporated into the analysis of 

Figure 4 include catalyst behavior exactly as in Figure 2 and that every ton of NOx below 

0.15 lb/MMBTU can be sold. 

There is, however, some additional risk associated with operating in this manner. 

Although experience has shown that few NOx reduction systems cause their owners 

serious difficulties from ammonia slip, these facilities were not trying to operate at the 

catalyst's limit. And, in reality, the catalyst activity may not behave exactly as expected 

in the design curve such as indicated in Figure 2. There is an expected range of activities 

expected over catalyst life, as shown in Figure 5. Figure 5 shows the expected catalyst 

activity along with an assumed expected 95% confidence level for the catalyst. 

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Figure 2. SCR Catalyst Activity versus Operating Hours

0.50

0.60

0.70

0.80

0.90

1.00

1.10

0 4,000 8,000 12,000 16,000 20,000 24,000

operating hours

ko k/

Figure 3. SCR Outlet NOx Potential

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 4,000 8,000 12,000 16,000 20,000 24,000

Operating Hours

NOx (lb/M

MBTU)

Potential 

Outlet NOx

Level

Unutilized 

SCR capacity
8

 

Figure 4. Potential Income Enhancement from Ozone 

Season NOx Allowance Sales through Overcontrol of SCR

$0

$500,000

$1,000,000

$1,500,000

$2,000,000

$2,500,000

123

Ozone Season (year)

Inco

m

e p

er O

z

o

n

e Seaso

n

$500/ton NOx

$1,000/ton

$2,000/ton

$3,000/ton

$4,000/ton

500 MW plant

cap. Fctr = 70%

153 day season

required level 0.15 

lb/MMBTU

The range of uncertainty in catalyst activity will vary for each application. In some cases 

it will be small, in others the uncertainty can be high. The range of uncertainty will be 

determined by the quality of the database of the catalyst supplier, the quality of the 

information provided by the owner to the catalyst designer, and how closely actual 

operations mirror the information provided by the owner to the catalyst supplier. In 

general, the catalyst suppliers have very good databases and can predict performance 

very well for certain, well-specified conditions. However, in the authors experience, the 

quality of information available during the design process and how closely the actual 

operations match expected operations play a much greater role in determining the level of

uncertainty in future catalyst performance. 

Fortunately, the effects of this uncertainty can be quantitatively assessed through 

computer simulation. Figure 6 shows cumulative probability plots resulting from a 

Monte Carlo simulation of ozone season NOx allowances available for sale for each of 

seasons 1, 2, and 3 with the uncertainty of Figure 5 included. 

The results of Figure 6 indicate 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 limit. As shown, in the first ozone season there is a 50% chance that 514 

excess tons can be generated without ammonia slip problems and the uncertainty is quite 

low regarding how many tons. This is because the catalyst is relatively new and there is 

little uncertainty regarding the performance it can offer. There is about a 95% chance 

that up to 505 tons can be generated. Yet, there is only a 5% chance that 523 tons can be 

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generated. In other words, risk can be substantially reduced with a penalty of only 18 

tons or 3.5% of the mean expected value. 

 Figure 5. Expected k/ko plus and minus 2 x std. Deviation

(95% confidence interval)

0.5

0.6

0.7

0.8

0.9

1

1.1

0 4,000 8,000 12,000 16,000 20,000 24,000

Operating Hours

k/ko

k/ko(low)

k/ko

k/ko (high)

In the authors experience, the quality of information available during the design process and how closely the actual operations match 

expected operations play the greatest role in determining the level of uncertainty in future catalyst performance. 

In the second ozone season, 342 tons are the mean of the projections. There is about a 

95% chance that up to 315 tons can be generated. There is only a 5% chance that 368 

tons or more can be generated. Hence, the spread is getting wider, to 15% of the mean 

expected value. 

For each year the uncertainty in the projections increases. As stated earlier, this is largely 

due to uncertainty in how the facility is actually operated versus initial expectations. In 

the third ozone season, 204 tons is the value of the mean of the projections. There is 

about a 95% chance that 154 tons can be generated. There is only a 5% chance that 253 

tons or more can be generated. The spread in uncertainty has increased 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 income enhancement potential assuming 

NOx allowance prices of $1,000/ton and the price of ammonia is $360/ton. 

As shown in Figure 7, the average (50% probability value) for the First Ozone Season is 

$446,000 with a Standard Deviation of only $4,800 at an allowance price of $1,000/ton. 

There is a 95% probability that $438,000 in income can be made and only 5% chance that 

$454,000 in income can be generated in the first ozone season. 

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In Ozone Season 2, the 50% probability value for income enhancement 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 income can be made and 5% 

chance that $319,000 can be generated. This is still a fairly narrow band of uncertainty, 

although it is much increased from the first ozone season. 

Finally, in Ozone Season #3, the 50% probability value is only $177,733 of income

enhancement at an allowance price of $1,000/ton. Standard Deviation is $26,175, or 

nearly 15% of the mean. There is a 95% chance that $134,000 in income can be 

generated and only 5% chance that $219,000 in income can be generated. Clearly, the 

uncertainty of available allowances for trading and the uncertainty in income potential 

has substantially increased to nearly 50% of the mean expected value. 

Finally, Figure 8 plots the 5%, 50%, and 95% confidence values from Figure 7, which 

demonstrates the diminishing value and increasing uncertainty of income for each ozone 

season. 

Figure 6. Range of Potential Ozone Season Tons Available for Sale, Accounting for 

Uncertainty: 500 MW Plant, Catalyst activity of Figure 5

0%

20%

40%

60%

80%

100%

120%

0 100 200 300 400 500 600

Season 1 tons

Mean of Season 1 tons

Season 2 tons

Mean of Season 2 tons

Season 3 tons

Mean of Season 3 tons

Season 3 Season 2 Season 1

Cumulativ

e Probabilit

y

NOx Allowances Available, tons

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Figure 7. Range of Potential Net Income Enhancement, Accounting for Uncertainty 

500 MW Plant, $1,000/ton allowance prices, Catalyst activity of Figure 5 

0%

20%

40%

60%

80%

100%

120%

$0 $50,000 $100,000 $150,000 $200,000 $250,000 $300,000 $350,000 $400,000 $450,000 $500,000

Season 1 @ $1,000/ton

Mean of Season 1 @ $1,000/ton

Season 2 @ $1,000/ton

Mean of Season 2 @ $1,000/ton

Season 3 @ $1,000/ton

Mean of Season 3 @ $1,000/ton

Season 3 Season 2 Season 1

Cumulative Probability

Ozone Season Income

Figure 8. 95%, 50% and 5% Confidence Values of Income

Enhancement from Figure 7

$100,000

$150,000

$200,000

$250,000

$300,000

$350,000

$400,000

$450,000

$500,000

Season 1 Season 2 Season 3

95% Confidence

50% Confidence

5% Confidence

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Reducing the Uncertainty - The Value of Monitoring Ammonia

There is a large motivation to utilize the assets of utility plants as effectively as possible. 

Therefore, when it is possible to enhance revenues and income through sale of NOx 

allowance credits, these opportunities should be explored. The analysis in this paper 

showed, quantitatively, what the value of these opportunities might be for a particular 

situation. The analysis also showed how there can be significant uncertainty in projecting 

what the value of the opportunity might be. Uncertainty in the projections is determined 

by several things. The price of NOx credits is one. But, one of the most significant for 

SCR systems is the behavior of the catalyst over time. Generally, this is more a factor of 

how closely actual operation matches expected SCR operation than any uncertainty over 

how the catalyst would behave under absolutely certain conditions. For SNCR systems, 

there are numerous factors that affect the predictability of operation. Most important is 

the consistency of furnace conditions, since for SNCR catalyst degradation is not an

issue. 

Ammonia monitoring is one important tool in the effort to address the uncertainty of NOx 

reduction system operation. Other tools include periodic catalyst activity testing, 

periodic furnace testing, and maintaining consistent fuel and boiler conditions. However, 

ammonia slip is the single indicator that will provide a direct measure of whether the 

NOx reduction system is being operated under, at, or beyond its limitations and by how 

much. Therefore, the use of ammonia monitoring technology (if available) can make it 

possible for much of the risk to be addressed. 

Unfortunately, for operators of coal-fired power plants ammonia-monitoring technology 

has not been available in a reliable and cost-effective form. Table 1 lists several 

continuous analyzer technologies. While several methods have proven very useful for 

gas applications where low NOx and low SO2 exist, infra-red methods appear to offer the 

greatest hope for addressing this issue for coal-fired plants. Multi-component infra-red 

methods tend to be expensive and are not amenable to in-situ measurement. One of the 

most difficult aspects of measuring ammonia is getting the sample to the analyzer while 

maintaining sample integrity. In-situ measurement avoids the concern regarding lost 

sample integrity. Tunable Diode Laser spectroscopy, because it offers the potential for 

reasonable cost, in-situ measurements, high molecular selectivity and rapid response, is 

the most promising technology. Interference from moisture is a potential problem. But, 

some of the suppliers have developed methods for addressing this concern while 

maintaining adequate sensitivity. The primary disadvantages of TDL methods are that 

there is little commercial experience for ammonia applications and that many of the 

suppliers are small, overseas firms. In most cases the instruments are still developmentstage prototypes. Service and application support will be important features to users of 

these products. 

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Table 1. Technologies for Continuous Measurement From Ammonia 

Technique Advantages Disadvantages Well Suited For 

NOx 

differential 

• Experience & 

familiarity with 

method 

• Measures a surrogate 

• Sensitivity for high 

NOx situations can be 

poor 

• Low NOx (gas 

turbines or gas 

boilers) 

UV 

photometry 

• Experience & 

familiarity with 

method 

• Strong Interference 

from SO2

• Natural Gas 

applications, or 

other low SO2

Ion Mobility • Sensitivity 

• Fairly interference 

free 

• Not well suited for 

Corrosive gases 

• Slow response 

• Any clean, noncorrosive gas 

TDL (IR) • Relatively 

interference free 

(except for water) 

• Solid-state 

• In-Situ - no sample 

handling required 

• Sensitivity 

• Limited experience 

• Service and Support 

(mostly small, foreign 

firms) 

• Moderate moisture 

interference must be 

properly addressed 

• All applications, 

especially coal 

IR 

(multicompo

nent) 

• Multiple species • Tends to be Expensive • All applications 

Automated

Wet

Chemistry 

• Experience & 

familiarity with 

method 

• Can be set up 

quickly 

• Labor intensive 

• Requires reagents 

• Testing 

programs 

The above are general statements that reflect the author's overall impression based upon his close 

familiarity with NOx reduction technology and ammonia monitoring technology. This is not intended to be

a complete list. However, it is a list of the most important approaches in the author's opinion. In some

cases companies may claim to have addressed certain disadvantages. The author neither disputes nor 

confirms their claims. 

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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 limit 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 uncertainty can be substantial over time. 

Availability of ammonia monitoring technology will permit these uncertainties and risks 

to be addressed more effectively. Technologies have been available for monitoring 

natural gas fired applications. However, until recently there have not been technologies 

available for coal-fired electric utility plants that are of reasonable cost, sufficient 

accuracy, and reliability. Some new technologies, particularly TDL spectroscopy, offer 

excellent potential to address coal applications. 

References 

 

1

 NESCAUM & MARAMA ,"Status Report on NOx: Control 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., Pierce, 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 Flexible, Least-Cost Plans for NOx Compliance: 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