1

Reliability of Selective Catalytic Reduction (SCR) and Flue 

Gas Desulfurization (FGD) Systems for High Pollutant 

Removal Efficiencies on Coal Fired Utility Boilers 

The 2004 MEGA Symposium, Paper # 04-A-56-AWMA 

Prepared by 

James E. Staudt1, Sikander R. Khan2, and Manuel J. Oliva2

1

 Andover Technology Partners, 112 Tucker Farm Road, North Andover, MA 01845 

2

 Office of Air and Radiation, United States Environmental Protection Agency 

ABSTRACT 

SCR and FGD are making a significant contribution to the control of NOx and SO2 emissions 

from coal-fired generating units. These technologies are expected to contribute to further 

reductions in pollutant emissions in response to proposed emission reduction rules. The ability 

of these technologies to make significant contributions to pollution reduction in a cost-effective 

manner depends, in part, on the reliability of these technologies in providing high levels of 

pollution reduction. 

A study of reliability of these technologies in providing high levels of pollutant reduction was 

conducted. The study included a review of CEMS data and input from operators of facilities. 

Operators of facilities that use these technologies were provided a questionnaire to collect their 

input on the reliability of these technologies. Information collected included emissions 

reductions guaranteed and achieved, reliability overall, and reliability by system component. 

The information provided by users was compiled and analyzed for statistical trends. In this 

paper we will present summary results of this study. 

INTRODUCTION 

Selective Catalytic Reduction (SCR) and Flue Gas Desulfurization (FGD) are technologies 

used to reduce emission of air pollution from power plants, especially, coal-fired plants. These 

technologies are currently in use at many coal-fired generating plants and are likely to be 

installed on many more in response to proposed emission regulations.1 The reliability of these 

technologies plays an important role in influencing: 1) the ability of these technologies to 

achieve the necessary reduction in pollution emissions, and 2) the cost of achieving the 

necessary reduction in pollution emissions. Articles by others have raised questions about the 

reliability of SCR systems in providing high levels of NOx control, i.e., approaching 90% NOx 

removal.2

 Reference 2 explored reliability of operational SCR systems through an analysis of 

continuous emission monitoring system (CEMS) data. However, Reference 2 acknowledges 

that various plant operations – such as unit start up or shut down - can introduce artifacts in 

CEMS data that will affect the inferred level of NOx reduction. Reference 2 describes the 

filtering techniques used by the authors in an effort to compensate for these artifacts. 
2

In this effort, we explore some of the important considerations for estimating effectiveness of 

control equipment through a review of CEMS data. Examples are used to provide insight on 

how to best screen and interpret this data. 

A major part of this effort, however, was associated with collecting and evaluating input from 

facility operators. Information on the operating approaches they use and their experience with 

reliability of SCR and FGD systems were of interest. Questionnaires were prepared and sent to 

many utility companies. Some companies chose not to participate, but several did choose to 

participate. Data was collected for 23 SCR systems and 14 FGD systems. These represent a 

fraction of the total installations in the US, and may not be statistically representative. But, 

certain trends can be observed from the data. 

REVIEW OF CEMS DATA 

The Clean Air Markets Division of the US Environmental Protection Agency (EPA) collects 

hourly air emissions data from sources subject to reporting requirements of Part 75 under 

programs such as the Acid Rain Program. This hourly data, along with unit characteristics is 

made available to the public via the EPA website.3

 Although EPA provides this data for use by 

the public, certain operational characteristics such as the efficiency of add-on control devices is 

difficult to derive from this data alone. For example, facilities often monitor emissions at 

common stacks for multiple units where emissions from uncontrolled and controlled units are 

monitored together. In such cases distinguishing the contribution of emissions of the 

controlled unit from those of the uncontrolled unit may not be possible. 

Figure 1 provides the NOX emissions over a three-year period for a facility in the eastern 

United States where two tangential coal-fired units of similar size share a common stack with a 

single CEMS monitoring the common stack. As shown in Figure 1, the reported NOX

emissions for both units follow a very close pattern, even though one unit has an operational 

SCR and actually has much lower NOx emissions during the ozone season (May 1 – 

September 30). An SCR was installed on Unit 1 in June 2001 and is operated during the ozone 

season. Unit 2 does not have an add-on NOX emissions control device. The incorrect 

representation of NOx emissions results from the methodology used by the CEMS in allocating 

emissions between the units. Total NOx emissions from the common stack are allocated to the 

individual units on the basis of heat input without consideration to the actual emissions from 

each unit – only the total is measured. As a result, the hourly data from the CEMS at this plant 

cannot be used to determine SCR characteristic information for the unit with the SCR. 

In addition to common stack issues, hourly data may not always be from CEMS as certain 

sources may use alternative Part 75 methods to develop hourly emissions. For example, during 

periods of CEMS downtime units may use acceptable substitute data. Also, a limited number 

of units do not need to install CEMS and may use other methods to determine hourly emission 

such as default emission rates. Finally, units that install add-on control devices, such as an 

SCR, under Part 75 requirements are installing the device to meet NOX allowance demands. 

That is to say that these units have an emissions “cap” that they are required to meet and can 

reduce emissions to meet their “cap” by using add-on controls or by purchasing additional 
3

allowances. Economics is a powerful driver in this type of compliance system, and facilities 

that have add-on controls rarely use these controls to their maximum capacity unless there is an 

economic incentive to do so. Therefore, use of EPA hourly data from sources subject to Part 75 

reporting requirements should not be used to characterize add-on control efficiencies without 

carefully analyzing how the data are derived and what they represent. 

Figure 1. Combined NOX Emissions from a unit with SCR and a unit without SCR 

 

Although care must be taken to assure CEMS hourly data accurately reflects the operations of a 

unit, it is possible to estimate add-on control performance for carefully screened data sets. For 

example, Figure 2 provides a comparison of hourly NOX emission rates of two coal-fired units 

(each from a different facility) which installed SCR prior to the ozone season of 2003. As 

shown in this figure, the NOx emission rates during the month of August 2002 for each unit 

were approximately 0.42 lb/MMBtu prior to SCR installation (dashed lines). The NOx 

emission rates during the month of August 2003 for each unit were approximately 0.04 lb/ 

MMBtu after SCR installation and optimization (solid lines). Both units averaged a percent 

reduction of approximately 89% in NOx emission rates after SCR installation. Similar data 

analysis for other CEMS-monitored units has demonstrated NOx achievable emission rates of 

0.04 lb/ MMBtu with SCR. 

0

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Heat Input (mmBtu/hr)

Unit 1 NOX Emissions Unit 2 NOX Emissions Unit 1 Heat Input Unit 2 Heat Input

2000 2001 2002

NOTE: Unit 1 has SCR, Unit 2 is uncontrolled,

 both share a common stack.
4

Figure 2. Example of NOX Emission Reductions with SCR Installation 

A screen of all CEMS monitored (under Part 75) coal fired units equipped with SCR that were 

operating in the third quarter of 2003 resulted in 22 units with NOX rates equal to or less than 

0.07 lb/MMBtu. These 22 units included only coal-fired boilers that either had single stacks or 

shared a common stack or multiple stacks with other coal-fired boilers also equipped with SCR 

control devices. The average emissions rate of the 22 units during the third quarter of 2003 is 

0.058 lb/MMBtu with a standard deviation of 0.010 lb/MMBtu. Of these 22 units, 15 units had 

emission rates from 0.04 to 0.06 lb/MMBtu. Without information on the value of the SCR 

inlet NOX level, it is not possible to estimate the percent reduction resulting from SCR. But, if 

the average emission rate into the SCR were assumed to be 0.50 for the 22 units, the average 

NOx reduction level for these units would be approximately 88 percent. For the 15 units with 

0.06 lb/MMBtu or lower emissions, the average NOx reduction level would be approximately 

90 percent. 

As mentioned earlier, CEMS data may not reflect the maximum capabilities of the SCR 

installations. In some cases, the operating decisions may have an impact on the SCR NOx 

removal rates estimated from such data. For example, due to operating or economic reasons, 

operators might choose to get less NOX removal than is possible from their SCR systems. To 

better understand plant specific situations, facility owners were surveyed. 

0.00

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28-Jul 2-Aug 7-Aug 12-Aug 17-Aug 22-Aug 27-Aug 1-Sep 6-Sep

NOX Rate (lb/mmBtu)

Unit A, SCR - 2003 Unit A, Uncontrolled - 2002

Unit 2, SCR - 2003 Unit 2, Uncontrolled - 2002

NOTE: SCRs Installed in June 2003
5

RESPONSES TO SCR QUESTIONS 

Because the focus of this effort was to evaluate reliability of high efficiency SCR’s, the 

discussion will initially focus on high NOx reduction SCRs (85% or greater NOx reduction). 

Subsequently, results from such SCRs will be compared with results from older SCRs built for 

lower removal efficiencies. Data was collected on 23 high efficiency SCRs (85% removal or 

higher) and from owners of two older SCRs that are not designed for high removal efficiencies. 

Of the high removal efficiency SCRs, the average unit size was 814 MW, the largest unit was 

1300 MW, the smallest unit was 349 MW and the standard deviation in size was 265 MW. 

Fuels fired included bituminous coals, subbituminous coals (Powder River Basin, PRB) and 

bituminous/subbituminous coal blends. 

OPERATION 

Although most of the high removal efficiency facilities must meet total mass emission 

requirements over a period of time, about half of the facilities responded that they control to 

specific emission rates rather than tonnage over a period of time. 52% of the respondents 

indicated that they control to a specific emissions rate and the remainder (48%) reported that 

they control to a total tonnage level. Since a review of state permits for each unit was outside 

the scope of this work, it is not possible to determine whether the state or local requirements 

differed from the SIP Call requirements. But, it is most likely that units that are controlled to 

specific levels are operated in this manner with the intent of targeting a cumulative mass 

emission for the period. However, this could not be confirmed. 

Most of these SCRs were designed to achieve an emissions rate for a particular coal and firing 

condition. The impact is that if the NOx emissions from the boiler are less than the design 

basis, the percent reduction will likely be less than the design or guaranteed rate. 

The average operating hours for these high removal efficiency units was about 5,300 hours, 

which is within the warranty period in most cases. The unit with the most operating hours was 

10,000 hours and the unit with the least had only 1,500. Standard deviation of the operating 

hours was 2,475 hours. So, it could be argued that these responses may reflect initial operating 

experience and it will be necessary to wait for long-term experience. In a later section these 

results are compared to units with longer operating experience but lower NOx removal. 

NOx REDUCTION 

The operators were asked to provide the typical full-load NOx reduction, the designed or 

guaranteed NOx reduction, and an estimate of the best removal rate they thought the system 

could achieve on a regular basis if they had a reason to operate it at higher removal rates. 

As shown on Figure 3, the SCRs, on average are currently providing between 88% and 89% 

NOx reduction (This is consistent with the results of Reference 2). The error bars show plus 

and minus one standard deviation. The guaranteed reduction on average is nearly 90% NOx 

reduction, with a narrower deviation than the actual operating reduction. And, if necessary, 

these units could provide, on average, close to 91% NOx reduction on a regular basis. The 

highest NOx reduction reported was 93%, by five units, or over 20% of those responding. In 
6

Figure 3. SCR NOx Reduction

82

84

86

88

90

92

94

Full Load Guaranteed Possible

NOx Reduction, %

avg +/- std dev

every case the highest regular reduction 

estimated to be possible was equal to or 

greater than the guaranteed NOx 

reduction. Therefore, all of the operators 

are confident of the SCR’s ability to 

achieve the guaranteed reduction on a 

regular basis. The fact that the operating 

full-load NOx reduction rates are slightly 

less than the guaranteed rates is likely 

because the NOx levels into the SCR are 

probably less than the maximum expected 

for the design conditions. If the units are 

being operated to maintain a particular 

outlet NOx emissions rate, as many are, 

then the operating NOx removal rate is going to be lower than the design or guaranteed rate. 

Thus, if SCR NOx reduction performance were inferred from CEMS data, this could cause an 

artifact suggesting poorer NOx reduction than is actually possible. 

All of the high removal efficiency units responded that initial performance tests were 

performed and on all of the units the initial performance tests were passed. 

All of the units responded that the SCR is operated at part load conditions and about one third 

of them responded that they operate during start up or shut down, but only while gas 

temperature is adequate for SCR operation. During start up or shut down the gas temperature 

can drop below the necessary gas temperature for SCR operation. As a result, if the unit is 

being shut down for start up, the outlet emissions rate may indicate higher than normal on the 

CEMS. Thus, if SCR NOx reduction performance were inferred from CEMS data, this could 

introduce an artifact suggesting poorer NOx reduction than is actually occurring during normal 

SCR operation. 

RELIABILITY RELATIVE TO EXPECTATIONS 

The survey also polled the reliability of the SCR and its components versus expectations. The 

following categories were evaluated: 

• Performance in general 

• Reagent Storage/Handling 

• Ammonia Injection 

• Instrumentation and Controls 

• Catalyst Activity 

• Ash Depostion/Pluggage 

• SCR Reactor and Structural 

• Dampers and Ductwork 
7

Figure 4. Overall Reliability versus 

Expectations

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50%

60%

1. Much

Worse

2. Worse

3. As

Expected

4. Better

5. Much

Better

Fraction of Responses

Figure 5. Reagent Storage/Handling 

Reliability versus Expectations

0%

10%

20%

30%

40%

50%

60%

70%

1. Much

Worse

2. Worse

3. As

Expected

4. Better

5. Much

Better

Fraction of Responses

Respondents could indicate one of the following for each category: 

1. Much worse than expectations 

2. Worse than expectations 

3. Met expectations 

4. Better than expected 

5. Much better than expected 

As shown on Figure 4, all of the high 

removal SCRs generally performed as 

expected or better than expected (either 3 

or 4). Responses for SCR components 

varied. 

In the following paragraphs we will 

examine the reliability of SCR 

components. 

Figure 5. shows the distribution of 

responses regarding Reagent Storage and 

Handling. Ammonia or Urea storage and 

Handling was very problematic for some 

respondents. Fortunately, all of those 

respondents that indicated a performance 

shortcoming in this system (answered 1 or 

2) also indicated that the problem was 

short term and correctable. Some 

indicated separately that the long-term 

reliability of some urea conversion 

systems remains a question. 
8

Figure 6. Ammonia Injection Reliability 

versus Expectations

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90%

1. Much

Worse

2. Worse

3. As

Expected

4. Better

5. Much

Better

Fraction of Responses

Figure 7. I&C Reliability versus 

Expectations

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1. Much

Worse

2. Worse

3. As

Expected

4. Better

5. Much

Better

Fraction of Responses

Figure 8. Catalyst Activity Reliability 

versus Expectations

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80%

1. Much

Worse

2. Worse

3. As

Expected

4. Better

5. Much

Better

Fraction of Responses

Figure 6 shows the distribution of 

responses regarding ammonia injection 

equipment. Fortunately, most 

respondents found this equipment to meet 

or exceed expectations. Furthermore, in 

the small number of cases where 

reliability of ammonia injection 

equipment was worse than expected, the 

problems were short term and correctable. 

So, these problems should not adversely 

affect reliability in the long term. 

Figure 7 shows the distribution of responses 

regarding instrumentation and controls. 

Fortunately, most respondents found this 

equipment to meet or exceed expectations – in 

several cases much better than expectations. 

Furthermore, in the small number of cases where 

reliability of instrumentation and controls was 

worse than expected, the problems were short term 

and correctable. So, these problems should not 

adversely affect reliability in the long term. 

Figure 8 shows the distribution of 

responses regarding catalyst activity. All 

respondents found catalyst activity 

meeting or exceeding expectations – in 

some cases much better than 

expectations. However, it is important to 

note that since most of these units had 

only a few thousand operating hours, 

these responses may not be indicative of 

long-term catalyst performance. Longterm catalyst behavior can only be 

determined with more time. 
9

Figure 9. Ash Deposition/Pluggage 

versus Expectations

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1. Much

Worse

2. Worse

3. As

Expected

4. Better

5. Much

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Fraction of Responses

Figure 10. SCR Reactor/Structural 

Reliability versus Expectations

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1. Much

Worse

2. Worse

3. As

Expected

4. Better

5. Much

Better

Fraction of Responses

Figure 11. Dampers and Ductwork 

Reliability versus Expectations

0%

10%

20%

30%

40%

50%

60%

1. Much

Worse

2. Worse

3. As

Expected

4. Better

5. Much

Better

Fraction of Responses

 Figure 9 shows the distribution of 

responses regarding ash deposition and 

plugging. Although most respondents 

found ash deposition and plugging 

meeting or exceeding expectations, a 

significant number of respondents found 

ash deposition and plugging to be much 

worse than expectations. Moreover, all 

of those responding that ash deposition 

and plugging were much worse than 

expected also indicated that this is 

expected to be a long-term problem. 

Therefore, this is likely to impact future 

reliability and a solution to this problem 

would be very desirable. 

Figure 10 shows the distribution of 

responses regarding the reliability of the 

SCR reactor and other structural 

components. Most respondents found 

the reliability of this equipment to meet 

or exceed expectations. Furthermore, in 

the small number of cases where 

reliability of the SCR reactor or other 

structural components was much worse 

than expected, the problems were short 

term and correctable. So, these problems 

should not adversely affect reliability in 

the long term. 

Figure 11 shows the distribution of 

responses regarding the reliability of the 

SCR dampers and ductwork. Included in 

this is experience with expansion joints. 

Experience was mostly either very good 

or bad – little in between. Fortunately, 

for the facilities that reported that 

reliability of the SCR dampers or 

ductwork or expansion joints was worse 

than expected, the large majority of them 

reported that the problems were short 

term and/or correctable. So, in some 

cases there will be a long-term impact on 

reliability. But, these are likely to be the 

minority of cases. 
10

Figure 12. SCR Overall Reliability

1

2

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4

5

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Percent Reduction

Reliability

Figure 13. SCR Overall Reliability

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Operating Hours

Reliability

In summary, all high NOx reduction SCR respondents found that their systems met or 

exceeded expectations overall. Where shortcomings in performance were noted, in most cases 

the shortcomings were short term or correctable. However, ash plugging and dampers and 

ductwork may be an ongoing reliability problem. And, the long-term reliability of urea 

conversion systems may also be a question. 

Comparisons with Lower Removal Units 

Although the focus of this effort was evaluating the performance of high NOx removal SCRs, 

in the process of collecting information, information was received from two facilities designed 

to achieve lower removal rates than what has been achieved from the retrofit units that were 

discussed so far. These two units also have been in operation for several years, unlike most of 

the high removal rate retrofit units that were retrofit onto existing units in recent years. 

 Figure 12 shows the results of overall 

reliability for all respondents as a 

function of the NOx reduction that they 

are actually delivering. Note that some 

of the data points represent more than 

one response. The only unit that was 

rated below expectations (1 or 2) 

provides 75% reduction (which is also 

higher than the design NOx reduction 

for that unit). So, there’s no apparent 

trend that higher reduction results in 

worse reliability, except possibly if you 

operate significantly above the design 

removal rate. 

Figure 13 shows the results of overall 

reliability for all respondents as a 

function of the operating hours. The 

higher NOx reduction units have fewer 

operating hours. As in Figure 10, there 

is not a clear trend toward poorer 

reliability over time. Of course, there 

are only a few data points at higher 

operating hours and it would be better 

to have more data. Of possible 

significance is the fact that the two 

units with high operating hours were among the first SCR installations on coal-fired boilers in 

the United States. So, there could be “first of a kind” issues with these two units, especially the 

one with more hours that showed reliability below the operator’s expectations (and is also 

operated well above its designed NOx removal rate). In this one installation some issues have 

been corrected, but efforts continue to address others. 
11

Summary on SCR Reliability 

In this effort, responses were received for 25 SCR systems – 23 of them high NOx removal 

systems of 85% removal or better. Based upon operation of a few thousand hours, all are 

capable of achieving their design NOx removal rate if necessary and all are meeting or 

exceeding expectations for reliability. Some units have had reliability issues with specific 

components of the SCR system. But, overall reliability is satisfactory. Because these high 

removal SCRs have generally operated for a few years or less, these results should be 

considered representative for initial operation. Longer operation may reveal other issues. 

Comparison against two lower removal systems that have been operating for several years 

showed no clear trend in reliability as a function of removal efficiency or operating hours. Of 

course, since there are only a few data points for longer operating hours or lower efficiency, it 

is difficult to reach any firm conclusions. 

RESPONSES TO FGD QUESTIONS 

In this effort information was gathered on reliability of FGD systems. There are three basic 

types of FGD systems that were evaluated: 1) Limestone-based wet FGD; 2) Lime-based wet 

FGD, and ; 3) Lime-based Spray Dryer Absorber (SDA). Unfortunately, it was not possible to 

collect as many responses for FGD technology as for SCR technology. This is partly because 

many FGD systems were installed much earlier than SCR systems. So, while most SCR 

systems in the United States reflect the current state-of-the-art, most FGD systems were 

installed many years ago when technology was less advanced than today’s state-of-the-art and 

also when prevailing regulations did not require or encourage higher SO2 removal efficiencies. 

Nevertheless, information was obtained on Limestone-based wet FGD systems serving five 

boilers (some with multiple boilers served by a single FGD tower), on seven different Limebased wet FGD systems serving seven boilers (some serving only a portion of a boiler’s flue 

gas), and on only two Lime-based Spray Drier Absorbers serving two boilers. 

The largest boiler reviewed of all of the FGD systems was 750 MW, the smallest was 80 MW. 

In one case a single limestone FGD system served multiple boilers. In another case a lime wet 

FGD system scrubbed only a portion of the boiler gases. 

OPERATION 

Almost all of the FGD systems are operated with the intention of maintaining a particular 

emissions rate or percent reduction. Since a review of the state permits for each unit was 

outside the scope of this work, it is not possible to determine whether the state or local 

requirements differed from the Federal requirements. Because all or most of these units are 

Title IV affected units, it is likely units are controlled to specific levels with the intent of 

targeting a cumulative mass emission for the period. However, this was not confirmed. 

Unlike the high NOx reduction SCRs, most of these FGD systems have been in operation for at 

least a few years. The average operating history of Limestone-based FGD systems was 44,000 

operating hours, the maximum was 60,000 operating hours and the minimum was 40,000 

operating hours. The average operating history of Lime-based FGD systems was 114,000 

operating hours, the maximum was 254,000 operating hours and the minimum was estimated 

20,000 operating hours. The two SDA systems had about 70,000 to 80,000 operating hours. 
12

Figure 14. Limestone-based Wet 

FGD SO2 Reduction

92

93

94

95

96

97

98

99

Full Load Guaranteed Possible

SO2 Reduction, %

avg +/- std dev

Figure 15. Full Load SO2 Reduction

Lime-based wet FGD

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Operating hours

SO2 Reduction, %

Figure 16. Lime SDA SO2 

Reduction

85

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95

Full Load Guaranteed Possible

SO2 Reduction, %

avg +/- std dev

SO2 REDUCTION 

All of the Limestone wet FGD systems 

are designed and guaranteed for 95% SO2

reduction, and all operate at or better than 

that level of SO2 removal. Respondents 

indicated that some of the systems could 

operate at higher SO2 removal 

efficiencies, as indicated in Figure 14. 

Not all systems had acceptance tests upon 

commissioning. But, those that had, 

passed the acceptance test. 

For the lime-based wet FGD systems, of 

the seven systems that responded, only 

three of them were designed to 

achieve 90% or more SO2

removal for the flue gas they 

treated.1 The other facilities 

were older facilities that were 

designed for lower SO2

removal rates – on the order of 

70-80% removal. As shown in 

Figure 15, which shows full 

load SO2 reduction of limebased wet FGD plotted against 

operating hours, more recently 

installed systems operate at 

much higher flue gas SO2 removal rates 

than previously installed systems. 

Figure 16 shows the SO2 removal 

efficiencies reported for the two SDAequipped facilities. As shown, both 

systems normally operate at removal 

efficiencies in the range of 85% to over 

90% removal efficiency. Both were 

designed for guaranteed SO2 removal 

efficiencies of somewhat over 90% 

removal and facility operators project that 

about 92% removal is achievable on a 

regular basis with these systems if 

necessary. 

 

1 One of the systems treated a fraction of the total flue gas for the boiler. We consider the percent reduction for 

the flue gas treated here. Also, two installations have same percent reduction and operating hours, which explains 

why there are apparently only six points displayed on Figure 13. 
13

RELIABILITY RELATIVE TO EXPECTATIONS 

The survey also polled the reliability of the FGD and its components versus expectations. The 

following categories were evaluated: 

• Performance in general 

• Reagent Storage/Handling 

• Slurry Injection/Atomization 

• Instrumentation and Controls 

• Pumps, Tanks, chemical hardware 

• Corrosion or scale build up 

• Waste disposal or handling 

• FGD reactor vessel or structural 

• Dampers or Ductwork 

• Fabric Filter, for SDA 

Respondents could indicate one of the following for each category: 

1. Much worse than expectations 

2. Worse than expectations 

3. Met expectations 

4. Better than expected 

5. Much better than expected 

Because for each SO2 control technology – Limestone-based wet FGD, Lime-based wet FGD, 

and Spray Drier Absorber (SDA) technology – there are a limited number of responses, the 

results of this analysis will be presented in a different way than for the SCR analysis. 

Figure 17 shows the results of responses to questions regarding reliability versus expectations – 

in general and for specific components - for the limestone-based wet FGD. As shown, for all 

cases the reliability in general met or exceeded expectations. Only in the case of waste 

disposal and handling did reliability fail to meet expectations. Fortunately, in every case the 

reliability shortcoming was short term and correctable. 

Figure 18 shows the results of responses to questions regarding reliability versus expectations – 

in general and for specific components - for the lime-based wet FGD. As shown, for all cases 

the reliability in general met or exceeded expectations. As in the case of limestone wet FGD, 

waste disposal and handling did not exhibit reliability meeting expectations. In some cases 

dampers and ductwork failed to meet expectations. In this case it was unclear to the 

respondents whether or not the reliability shortcomings are a short-term or long-term problem. 

Figure 19 shows the results of responses to questions regarding reliability versus expectations – 

in general and for specific components - for the lime-based Spray Drier Absorber technology. 

Only two facilities responded in this case. As shown, in one case the reliability in general and 

for all components met reliability expectations. In the other case, the facility operator reported 

that performance in general failed to meet expectations. And the same user also reported that 

several components of the SDA system failed to be as reliable as expected. This user also 
14

reported that all of the areas where reliability failed to meet expectations were long-term 

issues, except for corrosion/scale build up and reactor structural which were short-term 

problems. It should also be noted that with only two facilities responding, it would be very 

desirable to collect more information in the future from other facilities to compare with these 

results. 

Figure 17. Limestone wet FGD Reliability Versus 

Expectations

1

2

3

4

5

In General

 Reagent

Storage/Handling

 Slurry

Injection/Atomization

 Instrumentation

and Controls

 Pumps, Tanks,

chemical hardware

 Corrosion or

scale build up

 Waste disposal

or handling

 FGD reactor

vessel or structural

 Dampers or

Ductwork

Avg

Max

Min

Figure 18. Lime Wet FGD Performance Versus 

Expectations

1

2

3

4

5

In General

 Reagent

Storage/Handling

 Slurry

Injection/Atomization

 Instrumentation

and Controls

 Pumps, Tanks,

chemical hardware

 Corrosion or

scale build up

 Waste disposal

or handling

 FGD reactor

vessel or structural

 Dampers or

Ductwork

Avg

Max

Min

Reliability Key 

5 = Much Better Than Expected 

4 = Better Than Expected 

3 = As Expected 

2 = Worse Than Expected 

1 = Much Worse Than Expected 
15

Figure 20. Overall Reliability

1

2

3

4

5

70 80 90 100

Percent Reduction

Reliability

LSFO Lime w et FGD SDA

Figure 19. SDA Reliability Versus Expectations

1

2

3

4

5

In General

 Reagent

Storage/Handling

 Slurry

Injection/Atomization

 Instrumentation

and Controls

 Pumps, Tanks,

chemical hardware

 Corrosion or

scale build up

 Waste disposal

or handling

 FGD reactor

vessel or structural

 Dampers or

Ductwork

 Fabric Filter, for

SDA

Avg

Max

Min

Summary on FGD Reliability 

Figure 20 is a plot of overall reliability 

reported against reported full-load SO2

reduction. With one exception – an SDA 

system – all systems met or exceeded 

expectations regarding reliability. In some 

cases there were individual components 

that failed to meet expectations regarding 

reliability, but overall reliability met 

expectations. 

As shown on Figure 20, reliability for 

systems with under 80 percent reduction 

were reported to exceed expectations. For 

those systems operating above 80 percent, all but one were reported to have met or exceeded 

expectations for reliability. One of the two SDA systems that we received information on 

reported reliability that was below expectations. But, in both cases the responses showed that 

over 90% SO2 removal was possible on a regular basis if necessary. Thus, there is no clear 

trend toward poor reliability at high SO2 removals. 

Figure 21 shows Overall Reliability for the different technologies reported against Operating 

Hours. There is no clear trend toward increasing or decreasing reliability versus operating 

hours. 

However, it would be very beneficial to collect more data on FGD systems, especially SDA 

technology. 
16

Figure 21. Overall Reliability

1

2

3

4

5

0 100,000 200,000 300,000

Operating Hours

Reliability

LSFO Lime w et FGD SDA

CONCLUSIONS 

This effort explored the reliability of SCR 

systems at providing high levels of NOx 

removal. CEMS data was evaluated. But, 

while CEMS data provided some useful 

information regarding the emission rates being 

achieved, it is not possible to ascertain the 

actual emissions reductions accurately because 

of artifacts associated with the way CEMS 

data is collected and reported. These artifacts 

are complicated by operating decisions that 

facility owners make that will affect the data. 

For this reason, facility owners were contacted 

for their input on how they operate the facilities. 

The results of an effort to collect information from facility operators on the reliability of SCR 

and FGD systems at providing high levels of NOx and SO2 reduction are summarized in this 

paper. It is always desirable to collect information from as many members of a population as 

possible to ensure that the sample is as representative of the population as possible. To this 

end, numerous facility owners were contacted. Only a portion of those facility owners chose to 

participate in this survey. Nevertheless, some very important insights have been garnered from 

this effort. All of the high NOx reduction SCRs that are represented in this survey met or 

exceeded expectations regarding reliability in general. Some facilities experienced less-thanexpected reliability on some specific SCR system components; however, these were in most 

cases short term or correctable. And, because other aspects of the SCR exceeded expectations 

for reliability, reliability in general was viewed by the respondent as meeting or exceeding 

expectations. 

Since the high NOx reduction SCR systems that are represented in this survey generally had 

only a few thousand operating hours, it could be argued that the results of this survey may not 

be representative for long-term operation. However, because an SCR has only a limited 

number of wear parts, it is likely that the experience reflected here can provide useful insights 

for all of the SCR components except possibly for the SCR catalyst, which is known to degrade 

over time, or possibly the urea conversion technology, for which there is limited long-term 

experience. There are high NOx removal facilities in the US with more operating hours. 

Many of these facilities were invited to participate; however, the facility owners chose not to. 

Nevertheless, the fact that all of the high NOx removal facilities that responded were operating 

over 85% reduction regularly, were typically guaranteed for about 90% reduction and all 

replies indicated that at least guaranteed NOx reductions could be reliably achieved if desired, 

indicates that 90% NOx reduction is a reasonable estimate to use for the NOx removal that is 

possible from SCR. Whether facility owners choose to achieve such removal rates is a 

business decision that they make. 

With regard to FGD, a fewer number of facilities were available to collect information from, 

particularly with regard to high levels of removal. As shown in Figure 15, high removal 
17

systems have only been installed in more recent years. So, removal rates on systems installed 

20 or more years ago are not representative of what is possible with current technology. But, 

the more recently installed wet (lime and limestone) systems were mostly guaranteed at 95% 

reduction or better with higher reductions in the range of 97-98% reported as being possible on 

a reliable basis, if needed. Only two dry FGD systems were evaluated, and both of them were 

reported to be capable of over 90% SO2 removal, if needed. 

With regard to FGD reliability, in every case except for one SDA system, performance overall 

met or exceeded owner expectations. Therefore, there is reason to believe that the FGD 

systems will perform as expected (albeit, with some specific components that may perform 

more or less reliably than other components). That only two responses were received for SDA 

systems raises the question regarding how representative this sample is for SDA systems. 

Therefore, there may be some benefit in collecting more information in the future for all three 

FGD technologies, but especially for SDA technology. 

 

DISCLAIMER 

The views and opinions expressed here are those of the author(s) alone and do not necessarily 

represent the policies of the U.S. Environmental Projection Agency. 

REFERENCES 

 

1

 EPA, Clean Air Markets Division, “Economic & Energy Analysis for the Proposed Interstate 

Air Quality Rulemaking”, January 28, 2004

2

 Cicanowicz, J. Edward, and Smith, Lowell, L., “SCR Performance Analysis Hints at 

Difficulty in AchievingHigh NOx Removal Targets”, Power Engineering, November 

2002, pp 82-90 

3

 EPA Website - http://www.epa.gov/airmarkets