Uniqueness of SCR Retrofits Translates 

into Broad Cost Variations

05/01/2003 

By: Bill Hoskins, P.E., C.C.E., Washington Group International

The growing population of U.S. SCR systems provides an opportunity to examine the impact of various 

factors on installed costs. This analysis is based on SCR retrofit installations 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 whole, the cost results are at a stage of development that allows for 

meaningful and reasonably consistent comparison.

When characterizing costs of SCR retrofits on a regional or industry basis, their uniqueness and 

complexity suggest that it is better to consider them as a broad range rather than as an average cost. For 

the more than 20 units included in this analysis, SCR retrofit costs ranged from about $80-160/kW, with 

about 70 percent of the units in this sample above $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 also incorporated within the existing flue gas path, adding significant 

pressure drop, which increases auxiliary power requirements.

While constituting a modest sample compared to the entire population of SCR retrofits, the sample size 

is large enough to indicate with some confidence that most previous predictions (nominally from the mid 

1990s) understated SCR retrofit costs. Sites with highly 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 of plants in the 400-900 MW size range.

Previous Cost Estimates 

During the mid to late 1990s, EPA, DOE, state environmental agencies, and other groups estimated costs 

for SCR retrofits based on very limited U.S. utility experience. At the 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 publications and five references, typify the nature of 

previous cost estimates for SCR retrofit systems on coal-fired units:

• It is most likely that a 67 percent reduction system will cost in the range of $50-60/kW and the 85 

percent reduction system will cost $70-80/kW [$1995].1

• The Institute of Clean Air Companies suggests that in 1999 SCR will cost $50-80/kW for retrofit to 

units.2

• The cost for low NOx rate SCR is $70/kW and the cost for high NOx rate SCR is $72/kW [$1997].3

• Of the units evaluated, capital costs for installing a retrofit SCR range from $59/kW for an 880 

MW unit to $87/kW for a 100 MW unit assuming no balanced draft conversion [$1996].4

• The average capital cost of retrofitting SCR systems 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 cluster of units from 550 to 625 MW, any unit can 

vary by $30-50/kW from the average [$1996].5

While these references estimated capital costs for SCR retrofits in the range of $70-90/kW, all five 

references indicated that capital costs were site-specific and dependent on site constraints and other 

factors. However, only reference 5 provided quantitative significance to the impact of site-specific 

conditions. In addition, it seems that lack of U.S. experience 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 Estimates

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Figure 1 compares total installed costs for SCR retrofits on more than 20 units from the 1999 to 2001 

period. The costs of all SCR systems in Figure 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 congestion caused by existing equipment in the area of the retrofit, 

increase in pressure drop and the consequent potential of costly impacts on the ID fans and plant 

electrical distribution system. The differences in capital costs also reflect differences in sparing 

philosophy and maintenance access requirements of different utilities. 

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Figure 2 provides a generalized assessment of the economy 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. Figure 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) 

That scope and retrofit difficulty are more important than 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.

The weak economy of scale and importance of scope and retrofit difficulty is further exemplified by 

considering even larger SCR retrofits. Although not included in the rest of the analysis herein, the Belews 

Creek project in North Carolina provides an important illustration when compared to the curve in Figure 

2.

The Belews Creek project involves one of the 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 accommodate high wind loads, which significantly impacts the quantity, design,

and cost of the structural steel (more than 12,000 tons of structural steel will be required). In addition, 

the project will require relocation of items to accommodate tie-ins, upgrades to the ID/FD fans, new 

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secondary air heaters, removal of the existing secondary air heaters, and a new configuration of ductwork 

in the air heater area. 

The 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 greater than the corresponding point on the curve. This 

further reinforces the importance of retrofit difficulty and its influence on capital cost.

The high cost of SCR shown in the figures above and in the rest of this discussion is leading some utilities 

to consider less expensive alternative processes. Some "second-generation" SNCR processes combined 

with high-energy overfire air have been deployed on nominal 150 MW units and/or are beginning 

demonstrations. Initial indications of NOx removal levels in the range of SCR are generating utility 

interest. In order for such processes to be contenders with SCR, the applicability to a variety and vintage 

of coal-fired boilers needs to be further defined. In addition, several full-scale applications are necessary 

to confirm the ability to consistently achieve high NOx removal and maintain required ammonia slip. In 

addition, autocatatalytic and ozone injection processes are in earlier stages of development. They offer 

the possibility/.potential of less expensive alternates to SCR depending on plant configuration, other 

emission control systems and ultimate performance on full-scale units.

Assessment Of Different Craft Labor Rates

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Although not available for all of the 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 units of the same size at the same plant). Figure 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 the five units. This is a result 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 ranges from 22-26 percent of the total installed cost, 

increasing the labor rate of the other units to the S-average labor rate would increase the total project 

cost of the other units. The last section of this article examines all the retrofits with the same labor wage 

and reagent, allowing isolation of the impact of SCR system scope and site congestion.

Differences In Scope 

Although indications are anecdotal, it would appear that early estimates of SCR retrofit cost (by EPA and 

others) were based on "standard" or simple SCR installations. However, all retrofits are unique and it is 

generally not possible to find units with scope, overall characteristics, and costs that compare one-to-one. 

A review of the specific project data for these five retrofits confirmed that costs and cost differences are 

influenced significantly by various conditions and/or site-specific factors such as:

• Differences in craft (construction) labor rates

• Difference in reagent feed systems

• The available space between and around 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 any (the need to upgrade or replace fan impellers, 

replace ID fans or add booster fans)

• The capacity, condition, and design margins of the electrical distribution system

• The design margins of the existing structural steel support systems

• The positive and negative design pressure of the furnace and existing ESP

• The number, nature, and type of existing above ground or underground items that must be 

relocated to accommodate SCR foundations or associated systems

• The accuracy of locations and dimensions of the existing structural steel, equipment, and 

underground items on "as-built" drawings

• The differences in vintage and boiler manufacturer

• Differences in sparing philosophy and maintenance access requirements of different utilities.

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Acknowledging the above influences, Figure 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 consistent comparison, all costs in Figure 4 were 

adjusted to the urea system. The urea-to-ammonia system is receiving more consideration and is being 

installed at a growing number of SCR 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 percent more than an SCR system based on anhydrous 

NH3. In addition, Figure 4 reflects adjustment of the craft labor rates for retrofits J, K, O and P to the 

labor rates at the retrofit S-average plant. The costs were adjusted by a method that inherently accounts 

for labor rates and productivity. These adjustments allow the comparison to be focused more on the 

impact of SCR system scope and site congestion. Total 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 that simple SCR retrofits are generally the exception 

rather than the rule. 

With a common reagent and labor rate, Figure 4 and the discussion below can now be used to compare 

the significant cost impact resulting from the scope 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 significant potential cost additions 

for: 

• Retrofit J - new ID fans, various internal replacements and modifications to air pre-heater (APH), 

APH to precipitator (ESP) duct reinforcement, drilled pier foundations, ESP reinforcement, and 

economizer duct bypass 

• Retrofit S – new ID fans (replacement of two half-size fans for each existing ID fan), demolition of 

existing ID fan foundations, additional new ID fan foundations, more ductwork because of increase 

in number of ID fans, elevated SCR (significantly more structural steel to support SCR and 

accommodate wind loading), significant depth-drilled piers due to difficult soil conditions, 

relocation of existing flue gas conditioning system, and upgrades of 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, new booster fan foundations, remove existing ESP and 

ESP foundations, relocation of existing flue gas conditioning system, and economizer modifications 

• Retrofit K - relatively few scope additions resulting in the lowest cost of the five retrofits. 

The discussion below provides some general contrasts between these five retrofits (these contrasts 

should only be considered as illustrations of potential relative impacts). 

It would be expected that retrofit S (700-800 MW) would have a lower cost than retrofit J (400-500 MW) 

due to economy of scale. However, the wide range of costs in Figure 2 would indicate that scope has a 

potentially greater impact on cost than economy of scale (especially over the size range of 400-900 MW). 

Both locations require drilled piers, replacement of existing ID fans with new ID fans, and additional 

ductwork. However, retrofit J includes significant modifications to the APHs, reinforcement of duct from 

APH to ESP, and reinforcement of ESP, which are not required at retrofit S. These are costly modifications, 

which 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 retrofit factors outweighing economy of 

scale. Retrofit O requires a new APH and other significant APH-related 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 proportionally less expensive booster fans. However, the 

new APH, APH structural steel, APH ductwork, and APH foundations at retrofit O are quite costly and in 

part support why retrofit O has a proportionally higher cost than retrofit S. 

Retrofit P (900-1000 MW) is less costly than retrofit S with retrofit factors also outweighing economy of 

scale. On the surface, retrofit P seems to have several scope features similar to retrofit S, including 

drilled piers and relocation of the ESP flue gas conditioning system. However, retrofit P includes 

demolition of an existing ESP, although demolition is not as costly as construction. On the other hand, 

retrofit S requires new ID fans, while retrofit P requires less expensive booster fans. In addition, retrofit P 

site space is not as constrained as the space at retrofit S. Therefore, the SCR at retrofit P is not elevated 

as it is at retrofit S (P requiring relatively less SCR support steel than S). Further, retrofit P does not 

require the ductwork complexity that is necessary at retrofit S. 

All of the above discussion points to potential impact of scope and retrofit differences and their 

influence on total installed costs. Since scope-related capital cost breakdowns were not available for all 

aspects of retrofits J, K, O, or P, it is not possible to quantitatively reconcile the scope differences to the 

cost differences portrayed in Figure 4. However, the discussion above points to the significant 

differences in SCR retrofits. It also reinforces the contention that there is no one-cost-fits-all or even a 

narrow range that should be used to characterize SCR retrofit costs. Rather, this evaluation 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 reportedly predict SCR retrofit costs in the range of $50-$110/kW ($2000).7

References

1 Northeast States for Coordinated Air Use Management and Mid-Atlantic Regional Air Management 

Association, "Status Report on NOx Control Technologies and Cost Effectiveness for Utility Boilers," June 

1998.

2 SCR Committee of Institute of Clean Air Companies, "Selective Catalytic Reduction Control for Utility 

Boilers," November 1997.

3 United States Environmental Protection Agency, "Technical 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 Selective Catalytic Reduction Applied to Coal-Fired Boilers," 

EPRI-DOE-EPA Combined Utility Air Control Symposium, August 1997.

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Copyright © 2007-2015. PennWell Corporation, Tulsa, OK. All Rights Reserved.   PRIVACY POLICY | TERMS AND 

CONDITIONS

5 Cichanowicz, J., "SCR For Coal-Fired Boilers: A Survey of Recent Utility Cost Estimates," EPRI-DOE-EPA 

Combined Utility Air Control 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.

Author –

Bill Hoskins, P.E., is a supervising process engineer 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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