# Uniqueness of SCR Retrofits Translates into Broad Cost Variations

05/01/2003

By: Bill Hoskins, P.E., C.C.E., Washington Group International T he growing population of U.S. SCR systems provides an opportunity to examine the impact of various factors on installed costs. This analysis is base d on SCR retrofit installatio ns on balanced draft, pulverized coal boiler units with size ranges from 150-200 to 900-1000 MW. The anonymous units are in four states, east of the Mississippi River. The cost data are from various retrofits at varying levels of completion or installation. As a whol e, the cost results are at a stage of development that allows for meaningful and reasonablyconsistent comparison. W hen characterizing costs of SCR retrofits on a re gional or industry basis, their uniqueness and complexity suggest that it is better to consider them as a broad range ra ther than as an average cost. For the more than 20 units included in this analysis, SCR retr ofit costs ranged from about $80-160/kW, with about 70 percent of the units in this sample abov e $100/kW. The wide range of SCR retrofit costs is indicative of site-specific factors, scope differences, and site congestion. SCR retrofits are not only added to an existing plant site, but are al so incorporated within the existing flue gas path, adding significant pressure drop, which increases auxiliary power requirements. W hile constituting a modest sample compared to the entire population of SCR re trofits, the sample size is large enough to indicate with so me confidence that most previous predictions (nomina lly from the mid 1990s) understated SCR retrofit costs. Sites with hi ghly congested "back end" areas will have a much 

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...higher cost than sites with little congestion. Capital costs can be as much as 35 percent higher or 30 percent lower than the regression fit ofplants in the 400-900 MW size range. 

# Previous Cost Estimates During the mid to late 1990s, EPA, DOE, state envi ronmental agencies, and other groups estimated costs for SCR retrofits based on very lim ited U.S. utility experience. At th e time, a significant proportion of predictions led people to believe that nominal 80 percent removal SCR retrofit installations would cost $70-90/kW. The quotations below, from three public ations and five references, typify the nature of previous cost estimates for SCR re trofit systems on coal-fired units:

• It is most likely that a 67 percent reduction syst em will cost in the range of $50-60/kW and the 85 percent reduction system wi ll cost $70-80/kW [$1995]. 1 • The Institute of Clean Air Companies suggests thatin 1999 SCR will cost $50-80/kW for retrofit to units. 2 • The cost for low NOx rate SCR is $70/kW and th e cost for high NOx rate SCR is $72/kW [$1997]. 3 • Of the units evaluated, capital costs for installi ng a retrofit SCR range fr om $59/kW for an 880 MW unit to $87/kW for a 100 MW unit assumi ng no balanced draf t conversion [$1996]. 4 • The average capital cost of retrofitting SCR system s on 24 boilers is $86/ kW. The average for units with capacities greater than 175 MW is $75/kW. The data indicate the wide variation in capital cost depending on site-specific conditions – within a cl uster of units from 550 to 625 MW, any unit can vary by $30-50/kW from the average [$1996]. 5

W hile these references estimated capital costs for SC R retrofits in the range of $70-90/kW, all five references indicated that capital costs were site-s pecific and dependent on site constraints and other factors. However, only reference 5 provided quantitative si gnificance to the impact of site-specific conditions. In addition, it seems that lack of U.S. ex perience and agency cost algorithms5 from this era led many to predict overly optimistic(low) SCR retrofit costs. It also seems that during the early stages of U.S. SCR application, many outside the utility industry thought of $70-90/kW as the encompassing benchmark of SCR retrofit costs. 

# Recent Cost Estimate s Page 2 of 10 Uniqueness of SCR Retrofits Translates into Broad Cost Variations - Power Engineerin g

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Figure 1 compares total installed co sts for SCR retrofits on more than 20 units from the 1999 to 2001 period. The costs of all SCR systems in Figu re 1 are based on ammonia (NH3) reagent. Figure 1 shows that the costs range from about $80 to $160/kWnet ($2002). The wide cost range reflects site-specific conditions including co ngestion caused by existing equipmen t in the area of the retrofit, increase in pressure drop and the consequent pote ntial of costly impacts on the ID fans and plant electrical distribution system. The differences in capital costs also reflect differences in sparing philosophy and maintena nce access requirements of different utilities. 

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Figure 2 provides a generalized assessment of the ec onomy of scale and other cost factors for the SCR retrofits shown in Figure 1. No adjustments have been made for differences in site factors, labor rates or scope because the necessary data for all retrofits was not available. Figu re 2 provides a regression fit of these cost data, which indicates: (i) There is a "weak" economy of scale with increasing unit size; and (ii) T hat scope and retrofit difficulty are more important th an economy of scale, which is clearly reflected by the scatter of capital cost data. The wide variance is exemplified in the range of 400-900 MW, in which the cost can be as much as 30 percent lower or 35 percent higher than the regression curve. T he weak economy of scale and importance of scop e and retrofit difficulty is further exemplified by considering even larger SCR retrofits. Although not incl uded in the rest of the analysis herein, the Belews Creek project in North Carolina provides an importan t illustration when compared to the curve in Figure 2.
T he Belews Creek project involves one of th e largest SCR retrofit projects in the US. 6 SCR systems are being installed on twin 1120 MW units. The civil and structural challenges are significant in that the SCR structure/reactor will extend above the boiler building, more than 280 above ground level. The SCR framework is designed to accommoda te high wind loads, which signif icantly impacts the quantity, design, and cost of the structural steel (more than 12,000 tonsof structural steel will be required). In addition, the project will require relocation of items to ac commodate tie-ins, upgrades to the ID/FD fans, new 

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...secondary air heaters, removal of th e existing secondary air heaters, an d a new configuration of ductwork in the air heater area. T he reported total cost of this retrofit is about $145/kW. If this is compared to the regression curve in Figure 2, the $145/kW cost is about 30 percent grea ter than the corresponding point on the curve. This further reinforces the importance of retrofit difficulty and its influence on capital cost. T he high cost of SCR shown in the fi gures above and in the rest of this discussion is leading some utilities to consider less expensive altern ative processes. Some "second-gene ration" SNCR processes combined w ith high-energy overfire air have been deployed on nominal 150 MW units and/or are beginning demonstrations. Initial indications of NOx removal le vels in the range of SC R are generating utility interest. In order for such processes to be contenderswith SCR, the applicability to a variety and vintage of coal-fired boilers needs to be fu rther defined. In addition, several fu ll-scale applicatio ns are necessary to confirm the ability to consistently achieve high NOx removal and maintain re quired ammonia slip. In addition, autocatatalytic and ozone in jection processes are in earlier st ages of development. They offer the possibility/.potential of less expensive alternatesto SCR depending on plant configuration, other emission control systems and ultimate performance on full-scale units. 

# A ssessment Of Different Craft Labor Rate s

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A lthough not available for all of th e more than 20 units reflected in Figures 1 and 2, more detailed installation data and labor costs were available for five SCR retrofits: J, K, O, P, and S-average (S-average 

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...is average of multiple unitsof the same size at the same plant). Fi gure 3 compares the total craft labor (in hours per MW) and craft labor cost ($Thousands/MW) for the five units. Figure 3 shows the labor hours per MW for S-average are less than three of the five units. However, the labor cost per MW for S-average is more than three of th e five units. This is a resu lt of the all-in labor wage rate for S-average, which is in a different state and higher than any of the other retrofit locations. Since the total craft labor cost for these retrofits rangesfrom 22-26 percent of the total installed cost, increasing the labor rate of the ot her units to the S-average labor rate would increase the total project cost of the other units. The last section of this articleexamines all the retrofits with the same labor wage and reagent, allowing isolatio n of the impact of SCR syst em scope and site congestion. 

# Differences In Scope A lthough indications are anecdotal, it would appear th at early estimates of SCR retrofit cost (by EPA and others) were based on "standard" or simple SCR inst allations. However, all retrofits are unique and it is generally not possible to find units with scope, overa ll characteristics, and cost s that compare one-to-one. Areview of the specific project data for these five retrofits confirmed that cost s and cost differences are influenced significantly by various conditions and/or site -specific factors such as:

• Differences in craft (c onstruction) labor rates • Difference in reagent feed systems • The available space between and ar ound the economizer and air heater • The congestion downstream of the air heater (as applicable, space already occupied by buildings, conveyors, existing ESP, FGD system, ID fan, and/or stack) • The design margins of the existing ID fan, if an y (the need to upgrade or replace fan impellers, replace ID fans or add booster fans) • The capacity, condition, and design marg ins of the electrical distribution system • The design margins of the existin g structural steel support systems • The positive and negative design pres sure of the furnace and existing ESP • The number, nature, and type of existing abov e ground or underground items that must be relocated to accommodate SCR foun dations or associated systems • The accuracy of locations and dimensions ofthe existing structural steel, equipment, and underground items on "as-built" drawings • The differences in vintage and boiler manufacturer • Differences in sparin g philosophy and maintenance access requirements of different utilities. 

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A cknowledging the above influences, Fi gure 4 adjusts retrofits J, K, S, O and P to a more common footing. One difference in the projects is the reagent used – some use the urea-to-ammonia system, while others use the ammonia (NH3) system. To allow for a more co nsistent comparison, all costs in Figure 4 were adjusted to the urea system. The urea-to-ammonia sy stem is receiving more consideration and is being installed at a growing number ofSCR installations due to the potential health/safety concerns associated with storing large amounts of ammonia. In general, the total cost of an SCR system based on the urea-to-ammonia reagent system is about 2-5 perc ent more than an SCR system based on anhydrous NH3. In addition, Figure 4 reflects adjustment of the craft labor ra tes for retrofits J, K, O and P to the labor rates at the retrofit S-average plant. The costs we re adjusted by a method that inherently accounts for labor rates and productivity. Th ese adjustments allow the comparison to be focused more on the impact of SCR system scope and site congestion. Tota l costs after this adjustment resulted in a range from about $90-180/kWnet. It is important to note that Unit K is the lowest cost and is considered a 

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..."simple" retrofit. Industry experience has shown th at simple SCR retrofits ar e generally the exception rather than the rule. W ith a common reagent and labor rate, Figure 4 and th e discussion below can now be used to compare the significant cost impact resulting from the scop e and site-specific differences for retrofits J, K, S-average, O, and P. The discussion below itemizes important cost-influencing scope characteristics in contrast to what would be defined as a standard retrofit and shows si gnificant potential cost additions for:

• Retrofit J - new ID fans, various internal replacem ents and modifications to air pre-heater (APH), APH to precipitator (ESP) duct reinforcement, dr illed pier foundations, ESP reinforcement, and economizer duct bypass • Retrofit S – new ID fans (replacement of two half-size fans for each existin g ID fan), demolition of existing ID fan foundations, additio nal new ID fan foundations, more ductwork because of increase in number of ID fans, elevated SCR (significa ntly more structural steel to support SCR and accommodate wind loading), significant depth-dr illed piers due to diff icult soil conditions, relocation of existing flue gas conditioning system, and upgradesof electrical feeds (and associated electrical components) • Retrofit O - new APH, APH structural steel, APH ductwork, additional APH foundations, new booster fans, booster fan foundations, and elevated SCR (more structural steel to support SCR) • Retrofit P - drilled piers, new booster fans, ne w booster fan foundations, remove existing ESP and ESP foundations, relocation of existing flue gas co nditioning system, and economizer modifications • Retrofit K - relatively few scope additions resulting in the lowest cost of the five retrofits.

T he discussion below provides some general contrasts between these five retrofits (these contrasts should only be considered as illust rations of potential relative impacts). It would be expected that retrofit S (700-800 MW) wo uld have a lower cost than retrofit J (400-500 MW) due to economy of scale. However, the wide range ofcosts in Figure 2 would indicate that scope has a potentially greater impact on cost than economy of scale (esp ecially over the size range of 400-900 MW). Both locations require drilled piers, replacement ofexisting ID fans with ne w ID fans, and additional ductwork. However, retrofit J includes significant modi fications to the APHs, rein forcement of duct from A PH to ESP, and reinforcement of ESP, which are not re quired at retrofit S. Thes e are costly modifications, w hich indicates in part why retrofit J has a proportionally higher cost than retrofit S. Retrofit O (800-900 MW) is also more costly than retrofit S with retr ofit factors outweighing economy of scale. Retrofit O requires a new AP H and other significant APH-relate d items, which would increase the 

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...SCR retrofit cost compared to retrofit S. This should be offset somewhat by the fact that retrofit S requires new ID fans, while retrofit O requires prop ortionally less expensive booster fans. However, the new APH, APH structural steel, APH ductwork, and AP H foundations at retrofit O are quite costly and in part support why retrofit O has a proporti onally higher cost than retrofit S. Retrofit P (900-1000 MW) is less costlythan retrofit S with retrofit fa ctors also outweighing economy of scale. On the surface, re trofit P seems to have seve ral scope features similar to retrofit S, including drilled piers and relocation of the ESP flue gas conditioning system. Howeve r, retrofit P includes demolition of an existing ESP, althou gh demolition is not as costly asconstruction. On the other hand, retrofit S requires new ID fans, whil e retrofit P requires less expensive bo oster fans. In addition, retrofit P site space is not as constrained as the space at retrofit S. Therefore, th e SCR at retrofit P is not elevated as it is at retrofit S (P requiring relatively less SCR suppor t steel than S). Further, retrofit P does not require the ductwork complexity th at is necessary at retrofit S. A ll of the above discussion pointsto potential impact of scope and retrofit differences and their influence on total installed costs. Since scope-related capital cost brea kdowns were not available for all aspects of retrofits J, K, O, or P, it is not possibl e to quantitatively reconcile th e scope differences to the cost differences portrayed in Figure 4. However, the discussion above po ints to the significant differences in SCR retrofits. It also reinforces the co ntention that there is no on e-cost-fits-all or even a narrow range that should be used to characterize SCR retrofit costs. Rather, thisevaluation has indicated that SCR retrofit costs can vary from $80-$160/kWnet or –30 percent to +35 percent from the regression fit of the more than 20 units evaluated. It should also be noted that a recent EPA publication has coalfired algorithms that reportedlypredict SCR retrofit costs in th e range of $50-$110/kW ($2000). 7 

# Reference s 1 Northeast States for Coordinated Air Use Manage ment and Mid-Atlantic Regional Air Management A ssociation, "Status Report on NOx Control Technologiesand Cost Effectiveness for Utility Boilers," June 1998. 2 SCR Committee of Institute of Clea n Air Companies, "Selective Cataly tic Reduction Control for Utility Boilers," November 1997. 3 United States Environmental Protection Agency, "Tec hnical Bulletin, Nitrogen Oxides (NOx), Why and How They Are Controlled," EPA 456/F-99-006R, November 1999. 4 Cox, Healy, et al., "Economic Analysis of Selectiv e Catalytic Reduction Applie d to Coal-Fired Boilers," EPRI-DOE-EPA Combined Utility Air Control Symposium, August 1997. 

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...Copyright © 2007-2015. PennWell Corporat ion, Tulsa, OK. All Rights Reserved. PRIVACY POLICY | TERMS AND CONDITIONS 5 Cichanowicz, J., "SCR For Coal-Fired Boilers: A Surv ey of Recent Utility Cost Estimates," EPRI-DOE-EPA Combined Utility Air Contro l Symposium, August 1997. 6 "SCR = Supremely Complex Retrofit," Power Engineering,November 2002, p. 20. 7 .Cost of SCR Application for NOx Control on Coal -Fired Boilers, EPA/600/SR-01/087, January 2002. A uthor – Bill Hoskins, P.E., is a supervising process engine er at Washington Group International (Integrated Engineering, Construction and Management Solutions). He supervises or is responsible for scoping and feasibility studies for the power industry. Hoskins is a Certified Cost Engineer (CCE) and holds a bachelor's degree from New Mexico State University. 

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