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 Ag ency’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 NO x removal performance and reliability has been undertaken. The NO x emissions for all plants have been determined ba sed upon hourly emissions and gross heat input to determine the plants overall NO x removal efficiency and average outlet NO x for the 2005 Ozone season. The data analysis was performed for a ll operating hours, including low load and startup conditions. Analysis of the data showed that rem oval efficiencies of 90% and greater were obtained by greater than 30 units and that overall Ozone seas on average NOx emissions rates of less than 0.05 lb/MMBtu were consistently achieva ble 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 leve ls 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 sugge sts that not all units with lo w emissions rate can obtain low standard deviations. The reason fo r this are investigated as relate d 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 isolatio n of the system during non-Ozone season operation. These differences are largely due to th e US regulatory system of trading NO x emissions that makes it economically preferable to achieve higher removal efficiencies an d 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 NO x removal efficiencies.

Previous work examined the reliability of SCR syst ems on US coal-fired electric utility plants in achieving high NO x removal efficiencies, however a lim it number of opera tional units were available. As more units have co me on line and more data has beco me available, it is now possible to look at a fairly large population of units and find what trends are apparent and what lessons can be2 learned. For some units, multiple years of data av ailable allowing for an investigation of a plant ability to improve and maintain SCR performance.

For example, in Cichanowicz 1 and others examined data on twelve units that raised some questions regarding SCR reliability and ability to achieve 90% removal. In 2004, Staudt 2 and others reported on the results of surveys taken of users regardi ng their views on the reliability of SCR and FGD systems for high removal efficiency. In this effo rt, all of those companies operating SCR’s installed in response to the NOx SIP Call th at did respond indicated that ove rall reliability met or exceeded expectations. They also indica ted that full load removal efficiencies were, on average 88%-89%, close to the guarantee levels of roughly 90%. Howeve r, the survey showed th at the user’s estimate of the best removal rate they thought the SCR syst em could achieve on a regular basis if they had a reason to operate it at higher removal rates was ge nerally greater than 90% - and in every case greater than the guaranteed NO x removal from the supplier. One of the conclusions of Reference 2 was that operational choices result in some SCRs not being operated at th eir highest attainable removal efficiencies.

In 2005, Erickson 3 surveyed a larger population of units for the 2004 Ozone season and examined the effects of catalyst ty pe, 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 NO x emission rate. Some conclusi ons 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 NO x removal.

• Some units improved their SCR outlet NO x level over the period as well as the variability in the outlet NO x emissions.

None of the previous efforts explicitly examined the ability of the SCR to provide consistent NO x emissions. This effort expands on the previous work by Erickson in th at it updates the analysis with 2005 data and also explicitly exam ines variability in outlet NO x emissions.



### CURRENT EFFORT

In this effort we have evaluated the populati on of coal SCRs and examined performance and reliability using EPA reported emissions data. Performance is measured on the basis of outlet NO x emissions and NO x reduction. NO x 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 e ffort we sought indications of reliability to maintain an emission rate. To this end, reliab ility was analyzed using two measures:

Equation 1. Coefficient of Variation (CV) of the hourly outlet NO x during the ozone season, where

CV% = (standard deviation of hourly NO x rate)*100/(average hourly NO x rate)

The coefficient of variation is a dimensionless nu mber that allows comparison of the variation of data that have significantly different mean values. If CV is greater than 100%, that means that the3 standard deviation of the values exc eed the average, in such a case the NO x 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 parame ter that indicates how much hi gher (or lower) the average of hourly NO x emission rates is compared to the overall rate for the period. Because the reported hourly NO x rate for any hour is treated equally when taki ng the average of these values, regardless of the heat input during the particular hour, the average of the hourly NO x emission rates will normally differ somewhat from the overall NO x emission rate for the entire season. Therefore, LE is an indication of how the average hourly NO x rate differs from the overall NO x emission rate for the period as a result of changes in NO x emission when unit load changes. If the average of hourly NO x emission rates over the period exactly equals the overall NO x , then load changes do not have an effect on NO x 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 NO x emission rate during those periods will increa se and LE will be a positive numb er. On the other hand, if NO x 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 ne gative. LE gives us a way to meas ure how important this effect was when analyzing the data for the period in questi on. As will be shown, some units will show high variability in terms of CV. LE provides a way to determine to wh at extent the variability is associated with changes in load. In calculating both CV and LE, NO x rate is measured in lb/MMBtu.

Unfortunately, CV and LE do not fully capture reli ability. 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 perf ormance. 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 fo r all units (including units that do not use SCR) with less than 0.15 lb/MMBtu average NO x emissions rate for the 2005 Ozone season. After filteri ng for common stacks and missing data, this group included 219 units. 2. 2005 hourly ozone season and firs t quarter 2005 emissions data for units equipped with SCR for the 2005 Ozone season where SCR characteristi cs 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 qua rter 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 ga ve us a database of 120 units with SCR’s and their associated ve ndor, catalyst type, ammonia source and other unit-specific information. For each of the data sets, the average of the hourly Ozone season NO x emission rates were calculated, as was the standard deviation. These are used in calculating CV and LE as described earlier.


### Analysis of Units With NO x 

### 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 NO x Reduction (versus 2005 Q1) for the population of boilers with 2005 Ozone Season NO x emissions below 0.15 lb/MMBtu. Each data point shows the data for one unit. Most of the boilers that have relatively low NO x reductions are PRB-fired units with com bustion 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 NO x removals can be presumed to be eq uipped with SCR. As shown, there is significant variability across the spectrum. However, there seems to be some what more variability – in general - at the hi gher removal rates. This suggests that there is greater variability with SCR than with combustion NO x 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 variabili ty. This demonstrates th at SCR as a technology is capable of maintaining emissions levels very cl osely to a particular rate at high removal rates.

Figure 2 shows average hourly Ozone season NO x emission rate plus/minus the standard deviation in hourly Ozone season NO x emission rate for these 219 units with 2005 ozone season NO x below 0.15 lb/MMBtu. Like Figure 1, this Figu re shows data from some units that are equipped with SCR and some units that are not equipped with SCR. Al so shown on this graph is the overall 2005 Ozone season emission rate – determined by the total emissi ons divided by the heat i nput. Each data point and its associated range repres ent one unit. As shown, some units are achieving very low NO x emissions rates with very low variability. Howe ver, some are not. Notably, the units with the highest variability are not the uni ts with the lowest emissions. As shown on the graph, the average of the hourly NO x emissions rates does not always match th e emissions rate for the season. This is due to low load operations having different NO x emissions rates than high load operation. In most cases where there is a significant difference, the average of the hour ly 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 averag e of the Ozone season hourly emission rates and the overall Ozone season rate corre sponds with a high standard deviat ion in the hourly emission rate. This is not surprising because large variations in load that impact NO x emissions would invariably impact variability in NO x emission rate. This is illustrated further by Figure 3, which shows the relationship between CV (which is always positive by definition) a nd the absolute value LE. As shown, CV and the absolute valu e 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 NO x emission rates are a significant part of the explanation in NO x 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 217Average 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 NO x Reduction

Figure 2. Average Hourly Ozone Season NO x Emission Rate Plus/Minus Standard Deviation6 In short, there are two points to be made regarding variation in hourly NO x emissions during the

ozone season.

• The correlation between CV and LE indicates that some significant portion (but not all) of the variation in hourly NO x emission rates is attributable to changes in NO x 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 NO x emissions during the ozone season is associated with load changes, there are other fa ctors that affect variability.

Variability in NO x emission rates during the Ozone season that are not due to load changes may result from operating choices other than load changes, or they ma y result from other factors that may be associated with reliability. In the following s ections we will attempt to isolate some of these specifically as they relate to SCR.

y = 4.307x + 0.415 R

2 = 0.416

0% 50% 100% 150% 200% 250%

0% 10% 20% 30% 40% 50%
absolute value of Load EffectCV



### Analysis of SCR Operation with Different Coal Types

Figure 4 shows CV of hourly NO x during the 2005 Ozone season ve rsus 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 Oz one season data was considered. From the selected units the PRB un its have an average CV of 48% while the nine bituminous units used for comparison have an averag e 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 stat istical certainty it does suggest Figure 3 . CV versus absolute value of LE for Units with Emissions Under 0.15 lb/MMBtu For 2005 Ozone Season7 that SCR applications on PRB units offer no great er 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 TypeCV of Hourly NOx Bituminous Powder River Basin

Figure 5 shows Outlet NO x during the 2005 Ozone season versus Bituminous and Powder River Basin (PRB) coals. The same nine units of each co al type selected for Fi gure 4 were used in the same order. From the selected units, thos e fired with PRB have an average outlet NO x of 0.0554 lb/MMBtu while the nine bitu minous units used for comparison have any average of 0.0473 lb/MMBtu. This comparison illustrates that both fuel s are very similar in their attainable outlet NO x values. Some of the PRB units benefit from combustion NO x controls providing furnace outlet NO x emission rates significantly lower than those of bituminous units therefore requiring lower removal efficiencies for the same outlet rate. However, higher NO x removal rates with SCR are being practiced on bituminous units re sulting in bituminous outlet NO x emission rates equal to those of PRB. The LE versus coal type was analyzed; the da ta indicates no clear trends and considering the small population and the large e ffect of plant design this da ta 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 c ontrol or reliability issues compared to bituminous.

• SCR systems on bituminous fired units can atta in, with high removal efficiencies, outlet NO x emission limits in the same range or better than PRB units with combustion NO x 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 w ith the smaller population th ese conclusions appear consistent with the basic theory of SCR removal and have been an industry wide concern. One Figure 4. CV of Hourly NO x During the 2005 Ozone Season versus Different Coal Types8 interesting question raised by this analysis is: Why are PRB units employing combustion NO x control not operating at high removal ra tes resulting in even lower outlet NO x emission rates? The CV of several PRB units appears low enough to s upport 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 NO x 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 TypeAverage Hourly Ozone NOx (lb/MMBtu)

Bituminous Powder River Basin



### Analysis of SCR Operation by Cata lyst Type, Ammonia Source, Year Commissioned and 2004 versus 2005 Ozone Season Emission

In the work by Erickson 3 using 2004 Ozone season data, it was de termined that catalyst type does not appear to significantly impact the removal e fficiency 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 NO x emissions rates using 2005 data. Al so, to see if there were trends indicating operational improvement, we examined reduction efficiency as well as variability in NO x 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 characte ristics 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. Consis tent with the previous findings of Erickson 3 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 im pact on variability in controlled NO x emission rates. Keep in mind that the data includes some annually controlled units that, because we are comparing Ozone season NO x emission rates to first quarter NO x emissions rates, will indicate low removal for these units. The conclusion that catalyst type does not aff ect 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 NO x versus Different Coal Types9 Ammonia Source Figures 9, 10 and 11 show the results of a sort by NO x removal efficiency, CV and LE, respectively, to investigate the effects of ammonia source. C onsistent with the previous findings of Erickson 3 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 sim ilar 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 NO x reduction rates on these units – it maybe coincidental that aqueous was used on units with lower NO x reduction rates by design and we don’t know the reason. Again, keep in mind that the da ta includes some annually contro lled units that, because we are comparing Ozone season NO x emission rates to first quarter NO x 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 UnitsPercent 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 UnitsCV Corrugated Honeycomb Plate Figure 6. NO x Removal Efficiency versus Catalyst Type

Figure 7. CV versus Catalyst Type10 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 th e 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 b it lower at the low end. This effect for 2002 may be due to annually controlle d 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 opera ting practices at the plant that make the most of the SCR. Since the variability of the NO x emissions, measured in CV and LE, for years 2001, 2002, and 2003 are close, this may indicate that most of the bene fits 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 UnitsLE 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 UnitsPercent Removal Anhydrous Aqueous Urea

includes some annually controlled units that show low removal due to screening method Figure 9. NO x Removal Efficiency versus Ammonia Source11 Comparison of 2004 to 2005 Comparison of the units analyzed by Erickson 3 for NO x removal efficiency in 2004 showed that between 2004 and 2005 71% (or 92 of 130) of the 130 units examined improved their NO x 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 lowestremoval 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 achieve d 90% or more removal in 2005 while that number was slightly over 10% in 2004. R oughly 70% of the units in 2005 achieved 85% or better removal while in 2004 the pe rcent 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 UnitsCV 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 UnitsLE Anhydrous Aqueous Urea Figure 10. CV versus Ammonia Source

Figure 11. LE versus Ammonia Source12 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 UnitsCV 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 UnitsNOx Reduction 2000 2001 2002 2003 2004 2005

includes some annually controlled units that show low removal due to screening method

Load Effect durin g 2005 Ozone Season

-30% -20% -10%

0% 10% 20% 30% 40%

0 0.2 0.4 0.6 0.8 1

Fraction of UnitsLE 2000 2001
2002 2003
2004 2005 Figure 12. NO x Removal Efficiency versus Year Commissioned

Figure 13. CV versus Year Commissioned

Figure 14. LE versus Ammonia Source13 

### Analysis of Operational Impr ovement and Stability Over Time

Figure 16 shows CV of hourly NO x 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 system s for utility and employ steam side SCR inlet temperature control. Plant 1 uses anhydrous a mmonia 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 contradict ion to the single plant analysis by Erickson 3 , which concluded that once stable a plant remains stable. Figure 17 sh ows LE for the same plant over the same time period. This figure suggests that similar plants with the same design and SCR temper ature control system can operate differently with respect to NO x 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 functi on 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 plan t 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 designe d as original equipment and the la st four (4) plants (9-12) are units designed for Ozone operation that now opera te year round. The grap h shows considerable variation between plants regardless of above cate gory. Plants with low variability during the ozone season showed low variability year round. The pl ot also shows increased CV during the Ozone season for most units. This was not expected since it was anticipa ted 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 unitsOzone Season Removal 2005 Rem Eff
2004 Rem Eff Figure 15. Ozone Season Removal 2004 versus 200514 likely due to plants operating at higher removal efficiencies during the Ozone season resulting in lower outlet NO x emissions.

0% 50% 100% 150% 200%

1234 Years of OperationCV of Hourly NOx Plant 1 Plant 2 Plant 3

LE versus Years of Operation

-20% -10%

0% 10% 20% 30%

1234

Years of OperationLoad Effect (LE) Plant 1 Plant 2 Plant 3 Figure 16. CV versus Years of Operation

Figure 17. LE versus Years of Operation15 

### CONCLUSIONS

In this work we examined the performance and relia bility of SCRs on US coal -fired utility boilers. Performance was measured in term s 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 m easured in terms of NOx removal efficiency appears to be improving for the majority of units.

• High levels of variability were demonstrat ed for units equipped with only combustion controls and for units equipped with SCR cont rols, 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 ach ieved with high reliability.

• A significant amount of vari ability, although not all, is associated with ch anges 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 combusti on 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 si gnificant impact on re duction 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 12CV of Hourly NOx Year
Ozone Figure 18. CV versus Year Round SCR Operation16 • The choice of anhydrous ammonia or urea as th e ammonia source does not appear to impact reduction rate or variability. Aqueous ammoni a 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 remova l and variability in controlled NOx emission rates. This learning has resulted in signifi cant 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 contributio n of Sue Erickson for downloading data and the significant data manipulation requi red to allow this analysis.



### REFERENCES

1 Cichanowicz, J. Edward, and Smith, Lowell, L., “SCR Performance Analysis Hints at Difficulty in Achieving High NO x 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 Po llutant Removal Efficiencies on Coal Fired Utility Boilers”, 2004 Combined Power Plan t Air Pollution Control Mega Symposium, August 30-September 2, 2004, Washington, DC. 3 Erickson, C., “Availability of the US S CR Fleet”, EPRI SCR Wo rkshop, November, 2005, Louisville, KY.