1

Selective Catalytic Reduction System Performance and 

Reliability Review 

The 2006 MEGA Symposium, Paper # 121

By 

Clayton A. Erickson 1 and James E. Staudt 2 

1

 Babcock Power Environmental Inc., 5 Neponset Street, Worcester, MA 01615 

2

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

ABSTRACT 

Using the 2005 U.S. Environmental Protection Agency’s Electronic Data Reporting (EDR) site 

(www.epa.gov/airmarkets/emissions/raw/index.html) database of utility stack emissions, a review of 

installed SCR system NOx removal performance and reliability has been undertaken. The NOx

emissions for all plants have been determined based upon hourly emissions and gross heat input to 

determine the plants overall NOx removal efficiency and average outlet NOx for the 2005 Ozone 

season. The data analysis was performed for all operating hours, including low load and startup 

conditions. Analysis of the data showed that removal efficiencies of 90% and greater were obtained 

by greater than 30 units and that overall Ozone season average NOx emissions rates of less than 0.05 

lb/MMBtu were consistently achievable by SCR systems. The data also looks at the type of fuel and 

ammonia systems and their effect on the SCR system’s ability to meet high levels of reliability. Last 

the ability for plant with long term (greater than 3 years) of operating to improve their process is 

review for three selected plants. 

The review concludes that low NOx emissions rates can be achieved with very low hourly standard 

deviations. Further the data suggests that not all units with low emissions rate can obtain low 

standard deviations. The reason for this are investigated as related to boiler and SCR characteristics 

and system operation. 

BACKGROUND 

US SCR installations are unique from those of other countries in that the removal efficiencies of the 

systems are generally higher than in Europe or Japan. US installations also have been installed with 

full SCR bypass system allowing for the isolation of the system during non-Ozone season operation. 

These differences are largely due to the US regulatory system of trading NOx emissions that makes it 

economically preferable to achieve higher removal efficiencies and operate only during the Ozone 

season. Unit emission rate caps as practiced in Europe, on the other hand, do not create the same 

incentive for higher NOx removal efficiencies. 

Previous work examined the reliability of SCR systems on US coal-fired electric utility plants in 

achieving high NOx removal efficiencies, however a limit number of operational units were 

available. As more units have come on line and more data has become available, it is now possible 

to look at a fairly large population of units and find what trends are apparent and what lessons can be 
 2

learned. For some units, multiple years of data available allowing for an investigation of a plant 

ability to improve and maintain SCR performance. 

For example, in Cichanowicz1 and others examined data on twelve units that raised some questions 

regarding SCR reliability and ability to achieve 90% removal. In 2004, Staudt2 and others reported 

on the results of surveys taken of users regarding their views on the reliability of SCR and FGD 

systems for high removal efficiency. In this effort, all of those companies operating SCR’s installed 

in response to the NOx SIP Call that did respond indicated that overall reliability met or exceeded 

expectations. They also indicated that full load removal efficiencies were, on average 88%-89%, 

close to the guarantee levels of roughly 90%. However, the survey showed that the user’s estimate 

of the best removal rate they thought the SCR system could achieve on a regular basis if they had a 

reason to operate it at higher removal rates was generally greater than 90% - and in every case 

greater than the guaranteed NOx removal from the supplier. One of the conclusions of Reference 2 

was that operational choices result in some SCRs not being operated at their highest attainable 

removal efficiencies. 

In 2005, Erickson3

 surveyed a larger population of units for the 2004 Ozone season and examined 

the effects of catalyst type, ammonia source, technology supplier, and learning over time. Erickson 

examined the removal efficiency as determined by the average emission rate over the ozone season 

versus the first quarter NOx emission rate. Some conclusions reached by Erickson include: 

• Catalyst type does not appear to impact the removal efficiency of the SCR. 

• Ammonia source may have some impact on removal efficiency of the SCR, data set to small 

for conclusion. 

• 19 units achieved over 90% removal NOx removal. 

• Some units improved their SCR outlet NOx level over the period as well as the variability in 

the outlet NOx emissions. 

None of the previous efforts explicitly examined the ability of the SCR to provide consistent NOx

emissions. This effort expands on the previous work by Erickson in that it updates the analysis with 

2005 data and also explicitly examines variability in outlet NOx emissions. 

CURRENT EFFORT 

In this effort we have evaluated the population of coal SCRs and examined performance and 

reliability using EPA reported emissions data. Performance is measured on the basis of outlet NOx

emissions and NOx reduction. NOx reduction for seasonally controlled units was evaluated by 

comparing ozone season emissions to first quarter emissions for 2005. 

Reliability is more difficult to measure. In this effort we sought indications of reliability to maintain 

an emission rate. To this end, reliability was analyzed using two measures: 

Equation 1. Coefficient of Variation (CV) of the hourly outlet NOx during the ozone season, where 

CV% = (standard deviation of hourly NOx rate)*100/(average hourly NOx rate) 

The coefficient of variation is a dimensionless number that allows comparison of the variation of 

data that have significantly different mean values. If CV is greater than 100%, that means that the 
 3

standard deviation of the values exceed the average, in such a case the NOx emissions rates would be 

greater than the average. 

Equation 2. Load Effect (for lack of a better term), Load Effect (LE) was calculated, where 

LE% = (((average of hourly NOx rate over ozone season)/(overall ozone season NOx rate))*100) – 

100 

LE is another dimensionless parameter that indicates how much higher (or lower) the average of 

hourly NOx emission rates is compared to the overall rate for the period. Because the reported 

hourly NOx rate for any hour is treated equally when taking the average of these values, regardless of 

the heat input during the particular hour, the average of the hourly NOx emission rates will normally 

differ somewhat from the overall NOx emission rate for the entire season. Therefore, LE is an 

indication of how the average hourly NOx rate differs from the overall NOx emission rate for the 

period as a result of changes in NOx emission when unit load changes. If the average of hourly NOx

emission rates over the period exactly equals the overall NOx, then load changes do not have an 

effect on NOx emissions rates and LE will equal zero. For an SCR, LE is an important indicator. 

Because ammonia to an SCR may be secured at part load or during shutdown, the NOx emission rate 

during those periods will increase and LE will be a positive number. On the other hand, if NOx at 

part load is lower than at high load (for example, if the SCR and ammonia are left on at the same rate 

at low load), then LE will be negative. LE gives us a way to measure how important this effect was 

when analyzing the data for the period in question. As will be shown, some units will show high 

variability in terms of CV. LE provides a way to determine to what extent the variability is 

associated with changes in load. In calculating both CV and LE, NOx rate is measured in lb/MMBtu. 

Unfortunately, CV and LE do not fully capture reliability. High variability by either measure can 

result from normal operating practices, as a result of equipment choices the owner made that limit 

the load-following ability of the equipment, from other operating choices not associated with varying 

load, or from equipment problems that impact performance. So, theses measurement provide some 

insight, but not a complete picture of system reliability. 

Analysis Data Set 

In this current work, we looked at the following emissions data sets: 

1. 2005 hourly ozone season and first quarter 2005 emissions data for all units (including units 

that do not use SCR) with less than 0.15 lb/MMBtu average NOx emissions rate for the 2005 

Ozone season. After filtering for common stacks and missing data, this group included 219 

units. 

2. 2005 hourly ozone season and first quarter 2005 emissions data for units equipped with SCR 

for the 2005 Ozone season where SCR characteristics were known. This included 130 units. 

However, after filtering for units with missing data to determine variability, this group was 

reduced to 120 units. 

3. 2005 year round emissions data on selected units equipped with SCR. 

4. 2002 thru 2005 hourly Ozone season and first quarter 2002 thru 2005 emissions data for 

three selected units equipped with SCR. 
 4

The collection of units that provide the first data set include units that do not have SCR and use other 

technologies to achieve under 0.15 lb/MMBtu. The collection of units that provide the second data 

set are combined with our database of SCR installation information to enable us to evaluate if some 

SCR characteristics impact the performance of the SCR. This gave us a database of 120 units with 

SCR’s and their associated vendor, catalyst type, ammonia source and other unit-specific 

information. For each of the data sets, the average of the hourly Ozone season NOx emission rates 

were calculated, as was the standard deviation. These are used in calculating CV and LE as 

described earlier. 

Analysis of Units With NOx Emissions Below 0.15 lb/MMBtu During 2005 Ozone 

Season 

Figure 1 shows CV and LE during the 2005 Ozone Season versus 2005 Ozone Season NOx

Reduction (versus 2005 Q1) for the population of boilers with 2005 Ozone Season NOx emissions 

below 0.15 lb/MMBtu. Each data point shows the data for one unit. Most of the boilers that have 

relatively low NOx reductions are PRB-fired units with combustion controls that are not equipped 

with SCR. On the other hand, some of the low reduction units are annually operated SCRs. The 

units with high NOx removals can be presumed to be equipped with SCR. As shown, there is 

significant variability across the spectrum. However, there seems to be somewhat more variability – 

in general - at the higher removal rates. This suggests that there is greater variability with SCR than 

with combustion NOx controls. This probably is not surprising because SCR may be secured at 

times due to system design or operation desires. It is also noteworthy that some units nevertheless 

achieve high removal efficiencies with low variability. This demonstrates that SCR as a technology 

is capable of maintaining emissions levels very closely to a particular rate at high removal rates. 

Figure 2 shows average hourly Ozone season NOx emission rate plus/minus the standard deviation in 

hourly Ozone season NOx emission rate for these 219 units with 2005 ozone season NOx below 0.15 

lb/MMBtu. Like Figure 1, this Figure shows data from some units that are equipped with SCR and 

some units that are not equipped with SCR. Also shown on this graph is the overall 2005 Ozone 

season emission rate – determined by the total emissions divided by the heat input. Each data point 

and its associated range represent one unit. As shown, some units are achieving very low NOx

emissions rates with very low variability. However, some are not. Notably, the units with the 

highest variability are not the units with the lowest emissions. As shown on the graph, the average 

of the hourly NOx emissions rates does not always match the emissions rate for the season. This is 

due to low load operations having different NOx emissions rates than high load operation. In most 

cases where there is a significant difference, the average of the hourly emission rates is higher than 

the overall ozone season rate. This difference is what accounts for the LE as described above. In 

most cases, a larger difference between the average of the Ozone season hourly emission rates and 

the overall Ozone season rate corresponds with a high standard deviation in the hourly emission rate. 

This is not surprising because large variations in load that impact NOx emissions would invariably 

impact variability in NOx emission rate. This is illustrated further by Figure 3, which shows the 

relationship between CV (which is always positive by definition) and the absolute value LE. As 

shown, CV and the absolute value of LE show a significant degree of correlation, although they are 

not perfectly correlated. This correlation persists at the same level even if only units with SCR are 

screened or if CV is compared to LE. So, load changes that impact NOx emission rates are a 

significant part of the explanation in NOx emission rate variability during the Ozone season for all 

units with low NOx emission rates. But, load changes do not fully explain relationships shown. 
 5

-50%

0%

50%

100%

150%

200%

-20% -10% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

2005 Ozone season NOx reduction versus 2005 Q1

LE CV

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

1 13 25 37 49 61 73 85 97 109 121 133 145 157 169 181 193 205 217

Average Hrly Ozone NOx plus/minus std deviation

Average of Hourly Ozone Season NOx Emission Rates

2005 Ozone Season NOx Emission Rate

Figure 1. CV and LE versus 2005 Ozone Season NOx Reduction 

Figure 2. Average Hourly Ozone Season NOx Emission Rate Plus/Minus Standard Deviation 
 6

In short, there are two points to be made regarding variation in hourly NOx emissions during the 

ozone season. 

• The correlation between CV and LE indicates that some significant portion (but not all) of 

the variation in hourly NOx emission rates is attributable to changes in NOx associated with 

load changes, and may not be indicative of the reliability of the SCR, but simply how the unit 

is operated. 

• Since not all of the variability in outlet NOx emissions during the ozone season is associated 

with load changes, there are other factors that affect variability. 

Variability in NOx emission rates during the Ozone season that are not due to load changes may 

result from operating choices other than load changes, or they may result from other factors that may 

be associated with reliability. In the following sections we will attempt to isolate some of these 

specifically as they relate to SCR. 

y = 4.307x + 0.415

R2

 = 0.416

0%

50%

100%

150%

200%

250%

0% 10% 20% 30% 40% 50%

absolute value of Load Effect

CV

Analysis of SCR Operation with Different Coal Types 

Figure 4 shows CV of hourly NOx during the 2005 Ozone season versus Bituminous and Powder 

River Basin (PRB) coals. Nines units of each coal type were selected from the data set described 

above. Although some units operate on a year round basis, only the Ozone season data was 

considered. From the selected units the PRB units have an average CV of 48% while the nine 

bituminous units used for comparison have an average of 93%. The average CV for all 120 units in 

data set 2 is 43% - slightly below that of PRB units only. While the fuel comparison analysis does 

not have the large population of units to provide a high degree of statistical certainty it does suggest 

Figure 3. CV versus absolute value of LE for Units with Emissions Under 0.15 lb/MMBtu 

For 2005 Ozone Season 
 7

that SCR applications on PRB units offer no greater control or reliability issues than Bituminous 

coal. The Bituminous unit with the lowest CV of all units analyzed was included in the comparison, 

several PRB units compare within 50% of the lowest CV value and over half of the PRB units shown 

are within 25% of the lowest overall CV. 

0

20

40

60

80

100

120

140

160

180

200

Coal Type

CV of Hourly NOx

Bituminous Powder River Basin

Figure 5 shows Outlet NOx during the 2005 Ozone season versus Bituminous and Powder River 

Basin (PRB) coals. The same nine units of each coal type selected for Figure 4 were used in the 

same order. From the selected units, those fired with PRB have an average outlet NOx of 0.0554 

lb/MMBtu while the nine bituminous units used for comparison have any average of 0.0473 

lb/MMBtu. This comparison illustrates that both fuels are very similar in their attainable outlet NOx

values. Some of the PRB units benefit from combustion NOx controls providing furnace outlet NOx

emission rates significantly lower than those of bituminous units therefore requiring lower removal 

efficiencies for the same outlet rate. However, higher NOx removal rates with SCR are being 

practiced on bituminous units resulting in bituminous outlet NOx emission rates equal to those of 

PRB. The LE versus coal type was analyzed; the data indicates no clear trends and considering the 

small population and the large effect of plant design this data is not presented herein. 

The review of bituminous versus PRB SCR systems indicates two general conclusions: 

• SCR systems on PRB fired unit have no greater control or reliability issues compared to 

bituminous. 

• SCR systems on bituminous fired units can attain, with high removal efficiencies, outlet 

NOx emission limits in the same range or better than PRB units with combustion NOx

control systems. 

The above conclusions on the effect of fuel type are based on a significantly smaller population of 

data than other analyses present herein. Even with the smaller population these conclusions appear 

consistent with the basic theory of SCR removal and have been an industry wide concern. One 

Figure 4. CV of Hourly NOx During the 2005 Ozone Season versus Different Coal Types 
 8

interesting question raised by this analysis is: Why are PRB units employing combustion NOx

control not operating at high removal rates resulting in even lower outlet NOx emission rates? The 

CV of several PRB units appears low enough to support higher removal efficiencies. The low CV, 

combined with the lower sensitivity of PRB units to ammonia slip, leads the authors to believe that 

lower emission rates, and higher NOx removal rates, are attainable with high reliability on PRB units 

than are currently being practiced. 

0.00

0.02

0.04

0.06

0.08

0.10

Coal Type

Average Hourly Ozone NOx (lb/MMBtu)

Bituminous Powder River Basin

Analysis of SCR Operation by Catalyst Type, Ammonia Source, Year 

Commissioned and 2004 versus 2005 Ozone Season Emission 

In the work by Erickson3

 using 2004 Ozone season data, it was determined that catalyst type does 

not appear to significantly impact the removal efficiency of the SCR and that ammonia source may 

have some impact on removal efficiency of the SCR. In this effort we examined removal efficiency 

as well as variability in NOx emissions rates using 2005 data. Also, to see if there were trends 

indicating operational improvement, we examined reduction efficiency as well as variability in NOx

emission rates based on the year the unit was commissioned. For this analysis we used a population 

of 120 units equipped with SCR where the characteristics of the SCR – catalyst supplier, system 

supplier, ammonia source, and year commissioned – were known. 

Catalyst Type 

Figures 6, 7, and 8 show the results of a sort by removal efficiency, CV and LE, respectively, to 

illustrate the effects of catalyst type. Consistent with the previous findings of Erickson3 using 2004 

data, catalyst type does not appear to impact removal efficiency. Figures 7 and 8 also show that 

there does not appear to be an impact on variability in controlled NOx emission rates. Keep in mind 

that the data includes some annually controlled units that, because we are comparing Ozone season 

NOx emission rates to first quarter NOx emissions rates, will indicate low removal for these units. 

The conclusion that catalyst type does not affect removal efficiency, control variability and 

reliability implies that system design and operation have a greater effect than the type (plate, 

honeycomb, corrugated, etc.) of catalyst installed. 

Figure 5. Average Hourly Ozone NOx versus Different Coal Types 
 9

Ammonia Source 

Figures 9, 10 and 11 show the results of a sort by NOx removal efficiency, CV and LE, respectively, 

to investigate the effects of ammonia source. Consistent with the previous findings of Erickson3

using 2004 data, the units with aqueous ammonia tend to have lower removal efficiencies than for 

anhydrous ammonia or urea. The units with aqueous ammonia may show slightly less variability 

than for the other ammonia sources, but with similar load effect. Due to the small number of 

aqueous ammonia units relative to anhydrous and urea units, we cannot say that these results are 

statistically meaningful. Moreover, even if statistically meaningful, this does not mean that aqueous 

ammonia is the cause of lower NOx reduction rates on these units – it maybe coincidental that 

aqueous was used on units with lower NOx reduction rates by design and we don’t know the reason. 

Again, keep in mind that the data includes some annually controlled units that, because we are 

comparing Ozone season NOx emission rates to first quarter NOx emissions rates, will indicate low 

removal for these units. 

2005 Ozone Season Removal versus 2005 Q1

75%

80%

85%

90%

95%

100%

0 0.2 0.4 0.6 0.8 1

Fraction of Units 

Percent Removal

Corrugated

Honeycomb

Plate

includes some annually controlled 

units that show low removal due to 

screening method

Variability of 2005 ozone hrly NOx

0%

50%

100%

150%

200%

0 0.2 0.4 0.6 0.8 1

Fraction of Units 

CV

Corrugated

Honeycomb

Plate

Figure 6. NOx Removal Efficiency versus Catalyst Type 

Figure 7. CV versus Catalyst Type 
 10

Year Commissioned 

Figures 12, 13 and 14 show the results of a sort by removal efficiency, CV and LE, respectively, to 

see the effects of start up date. Disregarding the data of 2000 and 2005 because there were relatively 

few units in these dates (3 and 2, respectively), we see that there is little difference in removal 

efficiency except for possibly 2002, which seems a bit lower at the low end. This effect for 2002 

may be due to annually controlled units. Again, focusing on 2001 through 2004, 2004 seems to have 

higher variability (in both CV and LE) than 2001, 2002 and 2003. This may be indicative of a 

learning effect where operators take a year or more to develop operating practices at the plant that 

make the most of the SCR. Since the variability of the NOx emissions, measured in CV and LE, for 

years 2001, 2002, and 2003 are close, this may indicate that most of the benefits of learning are 

achieved in the first year. 

Load Effect during 2005 Ozone Season 

-30%

-20%

-10%

0%

10%

20%

30%

40%

0 0.2 0.4 0.6 0.8 1

Fraction of Units 

LE

Corrugated

Honeycomb

Plate

Figure 8. LE versus Catalyst Type 

2005 Ozone Season Removal versus 2005 Q1

75%

80%

85%

90%

95%

100%

0 0.2 0.4 0.6 0.8 1

Fraction of Units 

Percent Removal

Anhydrous

Aqueous

Urea

includes some annually 

controlled units that 

show low removal due 

to screening method

Figure 9. NOx Removal Efficiency versus Ammonia Source 
 11

Comparison of 2004 to 2005 

Comparison of the units analyzed by Erickson3

 for NOx removal efficiency in 2004 showed that 

between 2004 and 2005 71% (or 92 of 130) of the 130 units examined improved their NOx reduction 

percentage. Of these units 6% (or 8 of 130) went to annual controls (thus comparison of Q1 to ozone 

season NOx emission rate to estimate reduction is meaningless) and only 23% (30 of 130) had lower 

removal efficiency in 2005 than they did in 2004. Figure 15 compares the distribution of removal 

efficiency for these units in 2004 and in 2005 – sorted from highest to lowest removal efficiency for 

each year. In Figure 15 it is assumed that the 8 units that controlled annually in 2005 had similar 

removal efficiencies as in 2004. As shown, nearly 30% of the units achieved 90% or more removal 

in 2005 while that number was slightly over 10% in 2004. Roughly 70% of the units in 2005 

achieved 85% or better removal while in 2004 the percent that achieved 85% or better removal was 

about 50%. This shows a clear trend toward improved performance between 2004 and 2005 for 

these units. 

Variability of 2005 ozone hrly NOx

0%

50%

100%

150%

200%

0 0.2 0.4 0.6 0.8 1

Fraction of Units 

CV

Anhydrous

Aqueous

Urea

Load Effect for 2005 Ozone Season

-30%

-20%

-10%

0%

10%

20%

30%

40%

0 0.2 0.4 0.6 0.8 1

Fraction of Units 

LE

Anhydrous

Aqueous

Urea

Figure 10. CV versus Ammonia Source 

Figure 11. LE versus Ammonia Source 
 12

CV during 2005 Ozone Season

0%

20%

40%

60%

80%

100%

120%

140%

160%

180%

200%

0 0.2 0.4 0.6 0.8 1

Fraction of Units 

CV

2000 2001

2002 2003

2004 2005

2005 Ozone Season removal versus 1Q 2005

75%

80%

85%

90%

95%

100%

0 0.2 0.4 0.6 0.8 1

Fraction of Units 

NOx Reduction

2000 2001

2002 2003

2004 2005

includes some annually controlled 

units that show low removal due to 

screening method

Load Effect during 2005 Ozone Season 

-30%

-20%

-10%

0%

10%

20%

30%

40%

0 0.2 0.4 0.6 0.8 1

Fraction of Units 

LE

2000 2001

2002 2003

2004 2005

Figure 12. NOx Removal Efficiency versus Year Commissioned 

Figure 13. CV versus Year Commissioned 

Figure 14. LE versus Ammonia Source 
 13

 

Analysis of Operational Improvement and Stability Over Time 

Figure 16 shows CV of hourly NOx for three plants versus years of operation. All three plants fire 

bituminous coal and are greater than 600 MW in size. Plant one was the first SCR plant for the 

utility and has no SCR inlet temperature controls. Plants 2 and 3 are owned by the same utility, are 

the same size, and are not the first SCR systems for utility and employ steam side SCR inlet 

temperature control. Plant 1 uses anhydrous ammonia while plants 2 and 3 uses urea based 

ammonia. This illustrates variability of CV over time and between plants of similar design. This 

finding is in contradiction to the single plant analysis by Erickson3

, which concluded that once stable 

a plant remains stable. Figure 17 shows LE for the same plant over the same time period. This figure 

suggests that similar plants with the same design and SCR temperature control system can operate 

differently with respect to NOx removal as a function of load. The analysis of operational years 

suggest that operational characteristic of SCR are plant dependent. The cause of this dependence is 

unknown and has not been investigated at this time. 

 

The conclusions related to CV and LE as a function of years of operation are based on limited data 

and have not included a detailed investigation of each plant to determine the underlying reasons for 

the differences. This analysis does indicate that plant operation, even with similar plant and owners, 

has an effect on the SCR systems operation. 

Analysis of Year Round SCR System Operation 

Figure 18 shows the CV for 12 year round operating SCR systems; the CV is plotted for both the 

year and only the Ozone season. Plants 1 through 6 represent early US SCR retrofit plants, plants 7 

and 8 are units with the SCR designed as original equipment and the last four (4) plants (9-12) are 

units designed for Ozone operation that now operate year round. The graph shows considerable 

variation between plants regardless of above category. Plants with low variability during the ozone 

season showed low variability year round. The plot also shows increased CV during the Ozone 

season for most units. This was not expected since it was anticipated that better operation might be 

found during the Ozone season due to the value of NOx allowances. The increase in CV is most 

2004 vs 2005 removal efficiency

70%

75%

80%

85%

90%

95%

100%

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

Percent of units

Ozone Season Removal

2005 Rem Eff

2004 Rem Eff

Figure 15. Ozone Season Removal 2004 versus 2005 
 14

likely due to plants operating at higher removal efficiencies during the Ozone season resulting in 

lower outlet NOx emissions. 

0%

50%

100%

150%

200%

1234

Years of Operation

CV of Hourly NOx

Plant 1

Plant 2

Plant 3

LE versus Years of Operation

-20%

-10%

0%

10%

20%

30%

1234

Years of Operation

Load Effect (LE)

Plant 1

Plant 2

Plant 3

Figure 16. CV versus Years of Operation 

Figure 17. LE versus Years of Operation 
 15

CONCLUSIONS 

In this work we examined the performance and reliability of SCRs on US coal-fired utility boilers. 

Performance was measured in terms of NOx removal and in terms of outlet NOx levels. Reliability 

is more difficult to measure. However, we used measures of variability of outlet NOx as an 

indicator of the SCR’s reliability in providing NOx control. One of the twos measures of variability 

was used to determine the significance of load in the variability of outlet NOx level. We have 

reached the following conclusions from this work. 

• Ninety percent (90%) removal efficiency is currently being achieved by a significant portion 

of the coal-fired SCR fleet. And, performance measured in terms of NOx removal efficiency 

appears to be improving for the majority of units. 

• High levels of variability were demonstrated for units equipped with only combustion 

controls and for units equipped with SCR controls, although the highest variability was for 

units equipped with SCR. However, some of the units with SCR achieved high NOx 

reduction (over 90%) with low variability. 

• The units with the highest absolute variability in NOx emissions rate were not the units with 

the lowest outlet NOx emissions rate. In fact, the data showed some units with very low 

outlet NOx emissions rate (below 0.05 lb/MMBtu) and very low variability. This shows that 

low emissions rates can be achieved with high reliability. 

• A significant amount of variability, although not all, is associated with changes in load. So, 

some significant amount of variability in outlet NOx is associated with operating practices. 

• Bituminous units with SCR are achieving similar NOx emissions as PRB units with SCR, 

although the PRB units have a lower combustion NOx level, This, along with the low 

variability of PRB emissions, suggests that lower NOx emission rates (higher NOx removal 

rates) are possible from PRB units. 

• Catalyst type does not appear to have a significant impact on reduction or variability. 

0

20

40

60

80

100

120

Plant

1

Plant

2

Plant

3

Plant

4

Plant

5

Plant

6

Plant

7

Plant

8

Plant

9

Plant

10

Plant

11

Plant

12

CV of Hourly NO

x

Year

Ozone

Figure 18. CV versus Year Round SCR Operation 
 16

• The choice of anhydrous ammonia or urea as the ammonia source does not appear to impact 

reduction rate or variability. Aqueous ammonia may show different behavior, but it is 

difficult to determine since few units in this study used aqueous ammonia. 

• There does appear to be a learning curve that benefits both NOx removal and variability in 

controlled NOx emission rates. This learning has resulted in significant improvements in 

NOx removal performance across the fleet of SCRs. Reductions in variability appear to be 

occurring as well. 

• Annually controlled units that showed low variability, appeared to do so year round. 

Variability was usually higher in the ozone season, possibly due to higher NOx removal 

rates. 

FUTURE AREAS OF INTEREST/QUESTIONS 

This study examined reliability from the perspective of variability of NOx emissions rate. This may 

not be the best indicator of reliability. Future work may examine other measures of reliability. 

ACKNOWLEDGEMENTS 

The authors wish to acknowledge the contribution of Sue Erickson for downloading data and the 

significant data manipulation required to allow this analysis. 

REFERENCES 

 

1

 Cichanowicz, J. Edward, and Smith, Lowell, L., “SCR Performance Analysis Hints at Difficulty in 

Achieving High NOx Removal Targets”, Power Engineering, November 2002, pp 82-90 

2

 Staudt, J., Khan, S., and Oliva, M., “Reliability of Selective Catalytic Reduction (SCR) and Flue 

Gas Desulfurization (FGD) Systems for High Pollutant Removal Efficiencies on Coal Fired 

Utility Boilers”, 2004 Combined Power Plant Air Pollution Control Mega Symposium, 

August 30-September 2, 2004, Washington, DC. 

3

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Louisville, KY.