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. Staudt 1 , Sikander R. Khan 2 , and Manuel J. Oliva 2

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

2 Office of Air and Radiation, United St ates Environmental Protection Agency



### ABSTRACT

SCR and FGD are making a significant cont ribution to the control of NOx and SO 2 emissions from coal-fired generating units . These technologies are expect ed 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 pollu tion reduction in a cost-effective manner depends, in part, on the reliability of th ese technologies in pr oviding high levels of pollution reduction.

A study of reliability of these t echnologies in providing high le vels of pollutant reduction was conducted. The study included a review of CEMS da ta 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 ov erall, and reliability by system component. The information provided by users was compiled a nd analyzed for statistical trends. In this paper we will present summary results of this study.



### INTRODUCTION

Selective Catalytic Reduction (SCR) and Flue Gas Desulfuriz ation (FGD) are technologies used to reduce emission of air pollution from powe r plants, especially, coal-fired plants. These technologies are currently in use at many coal-f ired 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 ro le in influencing: 1) the ab ility of these technologies to achieve the necessary reduction in pollution em issions, and 2) the cost of achieving the necessary reduction in pollution emissions. Article s by others have raised questions about the reliability of SCR systems in providing high leve ls of NOx control, i.e., approaching 90% NOx

removal. 2 Reference 2 explored reliability of ope rational SCR systems through an analysis of continuous emission monitoring system (CEMS) data. However, Reference 2 acknowledges that various plant operations – such as unit star t up or shut down - can introduce artifacts in

CEMS data that will affect the inferred leve l 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 revi ew 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 associ ated 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 intere st. 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 E nvironmental Protection Agency (EPA) collects hourly air emissions data from sources subj ect 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 ad d-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 distingu ishing the contribution of emissions of the

controlled unit from those of the uncont rolled unit may not be possible.

Figure 1 provides the NO X emissions over a three-year period for a facility in the eastern United States where two tangential coal-fired uni ts of similar size share a common stack with a single CEMS monitoring the common stack. As shown in Figure 1, the reported NO X emissions for both units follow a very close pa ttern, 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 NO X 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 measur ed. As a result, the hourly data from the CEMS at this plant

cannot be used to determine SCR characteris tic 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 th at install add-on contro l devices, such as an SCR, under Part 75 requirements ar e installing the device to meet NO X allowance demands. That is to say that these units have an emissi ons “cap” that they are required to meet and can reduce emissions to meet their “cap” by usi ng add-on controls or by purchasing additional3 allowances. Economics is a powerful driver in th is 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 EP A hourly data from sources subject to Part 75 reporting requirements should not be used to ch aracterize add-on control efficiencies without carefully analyzing how the data are derived and what they represent.

Figure 1. Combined NO X Emissions from a unit with SCR and a unit without SCR

Although care must be taken to assure CEMS hourly data accurately reflec ts 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 NO X emission rates of two coal-fired units (each from a different facility) which insta lled SCR prior to the ozone season of 2003. As shown in this figure, the NOx emission rate s 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 (so lid 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 de monstrated NOx achievable emission rates of

0.04 lb/ MMBtu with SCR. 0 50 100 150 200 250 300

JAN MAR MAY JUL SE P NOV JA N M A R MAY JUL SEP NO V JA N M AR M A Y JUL SEP NOV

HoursNOX Emissions (ton)

0 200,000 400,000 600,000 800,000 1,000,000 1,200,000 1,400,000Heat 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 NO X 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 NO X rates equal to or less than 0.07 lb/MMBtu. These 22 units included only coal-fir ed boilers that either had single stacks or shared a common stack or multiple stacks with ot her 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. W ithout information on the value of the SCR

inlet NO X level, it is not possible to estimate the pe rcent 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 appr oximately 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 re flect 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 NO X removal than is possible from their SCR systems. To better understand plant specific situations , facility owners were surveyed. 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

28-Jul 2-Aug 7-Aug 12-Aug 17-Aug 22-Aug 27-Aug 1-Sep 6-SepNOX Rate (lb/mmBtu)

Unit A, SCR - 2003 Unit A, Uncontrolled - 2002
Unit 2, SCR - 2003 Unit 2, Uncontrolled - 2002 NOTE: SCRs Installed in June 20035 

### RESPONSES TO SCR QUESTIONS

Because the focus of this effort was to evalua te 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 co mpared 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 av erage unit size was 814 MW , the largest unit was 1300 MW, the smallest unit was 349 MW and th e standard deviation in size was 265 MW. Fuels fired included bituminous coals, subbitu minous 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 ove r a period of time. 52% of the respondents indicated that they control to a specific emissi ons rate and the remainder (48%) reported that they control to a total tonnage level. Since a review of stat e 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 requirement s. But, it is most likely th at units that are controlled to specific levels are operated in this manner w ith the intent of targeting a cumulative mass emission for the period. However, this could not be confirmed.

Most of these SCRs were designe d to achieve an emissions rate for a particular coal and firing condition. The impact is that if the NOx emi ssions from the boiler ar e 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 re moval efficiency units was about 5,300 hours, which is within the warranty period in most cas es. 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 ty pical 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 ar e currently providing be tween 88% and 89% NOx reduction (This is consistent with the result s of Reference 2). The error bars show plus and minus one standard deviation. The guara nteed 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 av erage, close to 91% NOx reduc tion on a regular basis. The highest NOx reduction reported was 93%, by five un its, or over 20% of those responding. In6 Figure 3. SCR NOx Reduction

82 84 86 88 90 92 94

Full Load Guaranteed PossibleNOx 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 in ferred from CEMS data, this could cause an artifact suggesting poorer NOx reduc tion than is actually possible.

All of the high removal efficiency units res ponded that initial pe rformance tests were performed and on all of the units the in itial performance tests were passed.

All of the units responded that the SCR is operate d at part load conditi ons and about one third of them responded that they operate during start up or s hut down, but only while gas temperature is adequate for SCR operation. Duri ng start up or shut down the gas temperature can drop below the necessary gas temperature fo r SCR operation. As a result, if the unit is being shut down for start up, the outlet emissi ons 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 reduc tion than is actually occurring during normal SCR operation.

RELIABILITY RELATIVE TO EXPECTATIONS The survey also polled the reliability of the S CR 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 Ductwork7 Figure 4. Overall Reliability versus
Expectations

0% 10% 20% 30% 40% 50% 60%1. MuchWorse2. Worse3. AsExpected4. Better5. MuchBetterFraction of Responses

Figure 5. Reagent Storage/Handling
Reliability versus Expectations

0% 10% 20% 30% 40% 50% 60% 70%1. MuchWorse2. Worse3. AsExpected4. Better5. MuchBetterFraction 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 th at 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

0% 10% 20% 30% 40% 50% 60% 70% 80% 90%1. MuchWorse2. Worse3. AsExpected4. Better5. MuchBetterFraction of Responses

Figure 7. I&C Reliability versus
Expectations

0% 5% 10% 15% 20% 25% 30% 35% 40% 45%1. MuchWorse2. Worse3. AsExpected4. Better5. MuchBetterFraction of Responses

Figure 8. Catalyst Activity Reliability
versus Expectations

0% 10% 20% 30% 40% 50% 60% 70% 80%1. MuchWorse2. Worse3. AsExpected4. Better5. MuchBetterFraction 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 instrumenta tion 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 oflong-term catalyst performance. Longterm catalyst behavior can only be

determined with more time.9 Figure 9. Ash Deposition/Pluggage
versus Expectations

0% 10% 20% 30% 40% 50% 60%1. MuchWorse2. Worse3. AsExpected4. Better5. MuchBetterFraction of Responses

Figure 10. SCR Reactor/Structural

Reliability versus Expectations

0% 10% 20% 30% 40% 50% 60%1. MuchWorse2. Worse3. AsExpected4. Better5. MuchBetterFraction of Responses

Figure 11. Dampers and Ductwork Reliability versus Expectations

0% 10% 20% 30% 40% 50% 60%1. MuchWorse2. Worse3. AsExpected4. Better5. MuchBetterFraction 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 3 4 5

50 55 60 65 70 75 80 85 90 95

Percent ReductionReliability

Figure 13. SCR Overall Reliability

1 2 3 4 5

0 20,000 40,000 60,000 80,000 Operating HoursReliability In summary, all high NOx reduction SCR re spondents found that their systems met or exceeded expectations overall. Where shortcomi ngs in performance were noted, in most cases the shortcomings were short term or correct able. 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 evaluati ng 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 b een in operation for severa l years, unlike most of the high removal rate retrofit units that were retrofit onto existing un its 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 re duction results in worse reliability, ex cept possibly if you

operate significantly above the design

removal rate.

Figure 13 shows the re sults 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 firs t SCR installations on co al-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 fo r 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 reliabil ity. Some units have had relia bility issues with specific components of the SCR system. But, overall re liability is satisfactor y. Because these high removal SCRs have generally operated for a few years or less, these results should be considered representative for initial operati on. 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 reli ability 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 curr ent 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 di d not require or encourage higher SO 2 removal efficiencies. Nevertheless, information was obtained on Lime stone-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 (s ome 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 possi ble 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 cont rolled 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 thes e FGD systems have been in operation for at least a few years. The average operating hi story 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 hi story of Lime-based FGD systems was 114,000 operating hours, the maximum was 254,000 opera ting hours and the minimum was estimated 20,000 operating hours. The two SDA system s 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 PossibleSO2 Reduction, %

avg +/- std dev

Figure 15. Full Load SO 2 Reduction

Lime-based wet FGD

70 75 80 85 90 95 100

0 50,000 100,000 150,000 200,000 250,000 300,000

Operating hoursSO2 Reduction, %

Figure 16. Lime SDA SO2

Reduction

85 86 87 88 89 90 91 92 93 94 95

Full Load Guaranteed PossibleSO2 Reduction, %

avg +/- std dev SO 2 REDUCTION All of the Limestone wet FGD systems are designed and guaranteed for 95% SO 2 reduction, and all operate at or better than

that level of SO 2 removal. Respondents indicated that some of the systems could operate at higher SO 2 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 SO 2 removal for the flue gas they

treated. 1 The other facilities

were older facilities that were designed for lower SO 2 removal rates – on the order of

70-80% removal. As shown in Figure 15, which shows full

load SO 2 reduction of limebased wet FGD plotted against operating hours, more recently installed systems operate at

much higher flue gas SO 2 removal rates than previously installed systems.

Figure 16 shows the SO 2 removalefficiencies 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 SO 2 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 F GD 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 SO 2 control technology – Li mestone-based wet FGD, Lime-based wet FGD, and Spray Drier Absorber (SDA) technology – th ere 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 questi ons 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 questi ons 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 expect ations. 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 expecta tions. In this case it was unclear to the respondents whether or not the reliability shortc omings are a short-term or long-term problem.

Figure 19 shows the results of responses to questi ons regarding reliability versus expectations – in general and for specific components - for th e lime-based Spray Drier Absorber technology. Only two facilities responded in this case. As shown, in one case the re liability in general and for all components met reliability expectations. In the other case, the f acility operator reported that performance in general failed to meet exp ectations. And the same user also reported that several components of the SDA system failed to be as reliable as expected. This user also14 reported that all of the areas wh ere 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 res ponding, it would be very

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

results.

Figure 17. Limestone wet FGD Reliability Versus Expectations

1 2 3 4 5In GeneralReagentStorage/HandlingSlurryInjection/AtomizationInstrumentationand ControlsPumps, Tanks,chemical hardwareCorrosion orscale build upWaste disposalor handlingFGD reactorvessel or structuralDampers orDuctwork Avg

Max

Min

Figure 18. Lime Wet FGD Performance Versus Expectations

1 2 3 4 5In GeneralReagentStorage/HandlingSlurryInjection/AtomizationInstrumentationand ControlsPumps, Tanks,chemical hardwareCorrosion orscale build upWaste disposalor handlingFGD reactorvessel or structuralDampers orDuctwork 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 Expected15 Figure 20. Overall Reliability

1 2 3 4 5

70 80 90 100

Percent ReductionReliability LSFO Lime w et FGD SDA Figure 19. SDA Reliabili ty Versus Expectations

1 2 3 4 5In GeneralReagentStorage/HandlingSlurryInjection/AtomizationInstrumentationand ControlsPumps, Tanks,chemical hardwareCorrosion orscale build upWaste disposalor handlingFGD reactorvessel or structuralDampers orDuctworkFabric Filter, forSDA Avg

Max

Min

Summary on FGD Reliability Figure 20 is a plot of overall reliability reported against reported full-load SO 2 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 tw o SDA systems that we received information on reported reliability that was belo w expectations. But, in both cases the responses showed that

over 90% SO 2 removal was possible on a regular basis if necessary. Thus, there is no clear trend toward poor reliability at high SO 2 removals.

Figure 21 shows Overall Reliability for the diffe rent technologies report ed against Operating

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

hours.

However, it would be very beneficial to coll ect 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 HoursReliability

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 re ported. 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 providi ng high levels of NOx and SO 2 reduction are summarized in this paper. It is always desirabl e to collect information from as many members of a population as possible to ensure that the sample is as repres entative of the population as possible. To this end, numerous facility owners were contacted. On ly a portion of those faci lity owners chose to participate in this survey. Neve rtheless, 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 ge neral. Some facilities experienced less-thanexpected reliability on some specific SCR syst em 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 ar e represented in this survey generally had only a few thousand operating hours, it could be argued that the resu lts of this survey may not be representative for long-term operation. Ho wever, because an SCR has only a limited number of wear parts, it is lik ely 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 tec hnology, 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 particip ate; however, the facility owners chose not to. Nevertheless, the fact that all of the high NOx removal faciliti es that responded were operating over 85% reduction regularly, were typically guaranteed for about 90% reduction and all replies indicated that at least guaranteed NOx re ductions could be reliably achieved if desired,

indicates that 90% NOx reduction is a reasonable estimate to us e for the NOx removal that is possible from SCR. Whether facili ty 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 removal17 systems have only been installed in more recent years. So, removal rates on systems installed 20 or more years ago are not representative of wh at is possible with curre nt technology. But, the more recently installed wet (lime and limes tone) systems were mostly guaranteed at 95% reduction or better with higher reductions in th e range of 97-98% reported as being possible on a reliable basis, if needed. Only two dry F GD systems were evaluated, and both of them were reported to be capable of over 90% SO 2 removal, if needed.

With regard to FGD reliability, in every case except for one SDA system, performance overall met or exceeded owner expectati ons. Therefore, there is reas on to believe that the FGD systems will perform as expected (albeit, wi th 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 repres entative 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