Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 1 

Best Available Retrofit Technology (BART) Analysis of Control Options 

For 

Colorado Springs Utilities – Drake Plant 

I. Source Description

Owner/Operator: Colorado Springs Utilities 

Source Type: Electric Utility Steam Generating Unit 

SCC (EGU): 10100202 

Boiler Type: Three Pulverized Coal, Dry-Bottom, Front-Fired, firing coal and 

natural gas (Units 5, 6, and 7) 

The facility is located at 700 South Conejos Street in Colorado Springs. This facility 

consists of three (3) steam driven turbine/generator units (Units 5, 6, and 7) and the 

associated equipment needed for generating electricity. These units fire coal as the 

primary fuel and use natural gas for backup and startup. The facility also includes the 

various processes necessary to handle the coal and ash. The coal and flyash handling 

systems are provided with baghouses for air pollution emission control of PM and PM10

at appropriate point sources. In addition, the coal is treated with chemical additives to 

reduce fugitive emissions. Table 1 depicts technical information for each boiler at the 

Drake Plant. 

Table 1: Drake Boilers Technical Information 

Unit 5 Unit 6 Unit 7 

Placed in 

Service 

October 28, 1962 July 27, 1968 June 14,1974 

Boiler Rating, 

MMBtu/Hr for 

coal 

548 861 1,336 

Electrical 

Power Rating, 

Gross 

Megawatts 

51 85 142 

Description Riley Pulverized Coal 

Front Fired Dry Bottom, 

firing natural gas and 

coal. 548 MMBtu/Hr w/ 

coal, 514 MMBtu/Hr w/ 

NG. 

Babcock and Wilcox 

Pulverized Coal Front Fired 

Dry Bottom, firing natural 

gas and coal. 861 

MMBtu/Hr w/ coal 850 

MMBtu/Hr w/ NG. 

Babcock and Wilcox 

Pulverized Coal Front Fired 

Dry Bottom, firing natural 

gas and coal. 1336 

MMBtu/Hr w/Coal, 1310 

MMBtu/Hr w/ NG. 

Air Pollution 

Control 

Equipment 

Reverse-Air Fabric Filter 

Baghouse- installed in 

May 1998 

Reverse-Air Fabric Filter 

Baghouse – installed in 

September 1978 

Reverse-Air Fabric Filter 

Baghouse– installed in 

November 1993 

Inherent 

Special 

Features 

Low NOx burners – 

placed in service in May 

1998 

Low NOx burners – placed 

in service in March 1998 

Low NOx burners – placed 

in service in October 1999 

Monitoring 

Equipment 

COM 

CEMs for SO2 , NOx, 

COM 

CEMs for SO2 , NOx, 

COM 

CEMs for SO2 , NOx, 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 2 

CO2, and stack gas 

flow rate 

CO2, and stack gas 

flow rate 

CO2, and stack gas 

flow rate 

Emissions 

Reduction 

(%)* 

NOx – 54.7% 

SO2 – None 

PM – 99.7% 

PM10 – 98.6% 

NOx – 52.8% 

SO2 – None 

PM – 99.6% 

PM10 – 98.2% 

NOx – 57.7% 

SO2 – None 

PM – 99.8% 

PM10 – 99.1% 

*Emissions Reduction estimated by comparing uncontrolled AP-42 factor to actual average emission factor 

for PM/PM10. For NOx estimates, CAMD data was used to calculate reduction. See “Drake APCD 

Technical Analysis” for further details. Not based on actual testing. 

Boilers 5, 6, and 7 are considered BART-eligible, being fossil-fuel steam electric plants 

of more than 250 MMBtu/hr heat input with the potential to emit 250 tons or more of 

haze forming pollution (NOx, SO2, PM10), and commenced operation in the 15-year 

period prior to August 7, 1977. The combined emissions of these boilers also cause or 

contribute to visibility impairment at a federal Class I area at or above a 0.5 deciview 

change; consequently, all three boilers are subject-to-BART. Initial air dispersion 

modeling performed by the Division demonstrated that the Martin Drake Plant 

contributes to visibility impairment (a 98th percentile impact equal to or greater than 0.5 

deciviews) and is therefore subject to BART. Colorado Springs Utilities (CSU) 

submitted a BART Analysis to the Division on August 1, 2006 with updated cost 

information submitted on March 29, 2007. CSU also provided information in “NOx and 

SO2 Reduction Cost and Technology Updates for Colorado Springs Utilities Drake and 

Nixon Plants” Submittal provided on February 20, 2009 as well as additional information 

upon the Division’s request on February 21, 2010, March 21, 2010, May 10, 2010, May 

28, 2010, June 2, 2010, and June 15, 2010. These documents are all provided as “CSU 

Drake BART Submittals”. 

Regulations that apply to these boilers are as follows: 

• State Regulation No. 1, III.A.1.c limits particulate matter emissions to 0.1 

lb/MMBtu. 

• State Regulation No. 1, VI.A.3.a.(ii) limits sulfur dioxide emissions to 1.2 

lb/MMBtu. 

• 40 CFR, Part 76-Acid Rain Nitrogen Oxides Emission Reduction Program limits 

NOX emissions to 0.46 lb/MMBtu of heat input on an annual average basis. 

• No other annual emission limitations or State Regulations since units are 

Grandfathered1

. 

II. Emissions for Units 5, 6, & 7

CSU estimated that a realistic depiction of anticipated annual emissions for Boilers 5, 6, 

and 7, or “Baseline Emissions”, to be conservative, was the average of two previous 

years (2004, 2005) of emissions data in the August 1, 2006 analysis. Several years have 

 

1

 Colorado Department of Public Health and Environment Air Quality Control Commission Regulation Number 3 

Stationary Source Permitting and Air Pollutant Emission Notice Requirements 5 CCR 1001-5 Part G.IV states: “A 

source existing before the adoption of the first Colorado Air Quality Control Act and the date of its implementing 

regulations of February 1, 1972, is not required to obtain a permit. This revision is intended to clarify the date prior 

to which existing sources are considered “grandfathered” and exempt from permit requirements.” 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 3 

passed since the original BART submittal, in which the Division has updated modeling 

and technical analyses. Therefore, the Division used years 2006 – 2008 (annual averages 

and 30-day rolling) for baseline emissions for reduction and cost calculations. The 

highest 24-hour peak emission rate during this timeframe was used for modeling 

visibility results. The Division verified these emissions using Colorado’s Air Pollutant 

Emission Notices and EPA’s CAMD database as applicable. These emissions are 

summarized in Table 2. 

Table 2: CSU Boilers 5, 6 and 7 Baseline Emissions 

Pollutant 

Boiler 5 Boiler 6 Boiler 7 

Annual 

Emissions* 

(tpy) 

Annual 

Emissions** 

(lb/MMBtu) 

Annual 

Emissions* 

(tpy) 

Annual 

Emissions** 

(lb/MMBtu) 

Annual 

Emissions* 

(tpy) 

Annual 

Emissions** 

(lb/MMBtu) 

NOx 768 0.38 1,413 0.42 2,081 0.39 

SO2 1,269 0.63 2,785 0.82 4,429 0.83 

PM10 27 0.01*** 58 0.02*** 55 0.01*** 

*Using daily CEMs data from 2006 – 2008 calendar years (CAMD data).

**The Division calculated average emission rate (lb/MMBtu) from the 2006 - 2008 calendar years (CAMD 

data) based on average daily reported data for each unit for NOx and SO2 emissions. 

***The PM10 emission rate is determined from the Title V permit compliance stack test. These values are 

as follows: Drake #5 – 0.0132 lb/MMBtu; Drake #6 – 0.0186 lb/MMBtu; Drake #7 – 0.0111 lb/MMBtu. 

III. Units Evaluated for Control

As documented by CSU, these boilers fire a variety of coal types, including coal from the 

southern Powder River Basin (PRB, located in Wyoming), ColoWyo coal (from 

northwestern Colorado), 20-Mile Foidel Creek coal (northwestern Colorado), and West 

Elk coal (western Colorado). The specifications for these coals are listed below in Table 

3 (averaged from 2006 – 2008). Table 4 lists the 2006 – 2008 averaged APEN-reported 

coal characteristics for each boiler. Table 4 is not based on percent of various coals fired, 

but instead based on the Division’s Air Pollutant Emission Notice (APEN) database. 

Sources submit annual emissions data using APENs. Due to equipment limitations, these 

boilers cannot achieve full load on PRB-sourced coal and instead fire a blend of the 

above listed coals. The ratio of PRB was discussed in the initial BART analysis 

submitted by CSU in an effort to demonstrate that firing sub-bituminous coal may have a 

minimal effect (if any) on a boiler’s NOx emissions. In fact the data suggested at that 

time that 100% sub-bituminous coal had no effect on NOx emissions for some of the 

boilers. CSU notes that this effect may be boiler specific. The difference in sulfur 

content and resultant SO2 emissions was not discussed in the initial BART analysis. 

Colorado’s BART guidance (Regulation No. 3, Part F, Section IV.B.1.f) states that 

sources may include an evaluation of representative characteristics of coals from sources 

they reasonably expect to use, so that these characteristics may be considered in a 

particular BART limit. 

Table 3: Drake Plant Coal Specifications (2004 – 2005) 

Coal Mine/Region Southern PRB Colowyo 20-Mile Foidel 

Creek 

West Elk 

Coal Rank 

Classification 

Sub-bituminous Sub-bituminous, 

Class A 

Bituminous Bituminous 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 4 

As Received Analysis 

H2O (Moisture %) 27.11 17.42 9.62 7.55 

Ash (%) 4.64 5.71 11.93 8.71 

Sulfur (%) 0.21 0.37 0.52 0.45 

Nitrogen (%) 0.69 1.35 1.57 1.30 

Heating Value 

(HHV Btu/lb) 

8,805 10,392 11,084 12,266 

Table 4: Coal Specifications (2006 – 2008 Averaged APEN data) 

 Specifications 

Emission Unit Fuel Heating Value 

(Btu/lb) 

Sulfur (% by weight) Ash (% by weight) 

Boiler #5 9,798 0.36 8.14 

Boiler #6 10,749 0.47 10.38 

Boiler #7 11,117 0.50 11.14 

Table 1 lists the units at Colorado Springs Utilities Drake Plant that the Division 

examined for control to meet BART-eligible requirements. Controlled and uncontrolled 

emission factors and CAMD data were used to evaluate the control effectiveness of the 

current emission controls. Uncontrolled emission factors are outlined in Table 5. The 

factors are based on firing bituminous coal for conservative estimates. 

Table 5: Uncontrolled emission factors for CSU Drake BART-eligible sources2

 Pollutant (lb/ton)* 

Emission Unit NOx SO2 PM 

(filterable) 

PM10

(filterable) 

Boiler #5 22 13.6 81.5 18.7 

Boiler #6 22 18.0 103.8 23.9 

Boiler #7 22 18.8 111.4 25.6 

*SO2 and PM/PM10 factors are determined by the applicable AP-42 equation, where %S and %A are the % of 

sulfur and ash present in the coal supply, respectively, determined from Table 4. 

A. Sulfur Dioxide (SO2)

Step 1: Identify All Available Technologies

CSU identified one control option for Units 6 and 7: 

Semi-dry flue gas desulfurization (dry FGD) aka lime spray drying (LSD/SDA) 

CSU identified two control options for Unit 5: 

Semi-dry flue gas desulfurization (dry FGD) aka lime spray drying (LSD/SDA) 

Dry sorbent injection – Trona (DSI) 

The Division also identified and examined additional control options for these units: 

Lime/limestone-based wet FGD – all units 

Emission limit tightening – Unit 5 (no control) 

Step 2: Eliminate Technically Infeasible Options

 

2

 EPA AP-42, Fifth Edition, Volume I, Chapter 1, Section 1.1, Tables 1.1-3 and 1.1-4. 

http://www.epa.gov/ttn/chief/ap42/ch01/final/c01s01.pdf 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 5 

FGD: Flue gas desulfurization removes SO2 from flue gases by a variety of methods. Wet 

scrubbing uses a slurry of alkaline sorbent, either limestone or lime, to scrub the gases. The most 

common dry FGD system is a lime spray dry absorber that uses slaked lime slurry sprayed into 

the flue gas, which is subsequently dried by the heat of the flue gas, and then collected in a 

particulate control device. Generally, FGD control systems need to be located in close proximity 

to the boiler exhaust gas stream to prevent condensation (e.g. cooling of the exhaust gases) that 

results in acidic precipitation in the duct which results in corrosion issues. 

Wet FGD: Wet FGD control systems must be located after the baghouse because the moist 

plume resulting from the wet scrubber system would create baghouse plugging issues if the 

control is placed ahead of the baghouse. Each absorber tower requires a similar “foot print” area, 

along with additional space for support equipment access, slurry preparation, mixing, associated 

tanks, dewatering and a chimney. 

Dry FGD: Dry FGD systems are commonly known as spray dry absorbers (SDA) or lime spray 

dryers (LSD), and currently make up about 12% of FGD systems at U.S. power plants3. SDA 

systems are typically utilized at smaller units that burn lower-sulfur coal in the western U.S., 

where water resources are limited. Additionally, Controlling SO2 Emissions: A Review of 

Technologies4 evaluates various SO2 control technologies and shows that for low-sulfur coal 

applications, LSDs can meet comparable emission rates to wet systems. 

A SDA system must be located before the boiler flue gases enter the baghouse. Each reactor 

vessel requires a “foot print” area comprising about 2,000 to 4,000 square feet (depending on 

volume of flue gas treated) along with additional space for support equipment access, slurry 

preparation, mixing and associated tanks. The plant is bounded to the north by West Cimarron 

Street, to the west by federal Interstate Highway 25 and Fountain Creek, to the east by Conejos 

Street, and the south by Fountain Creek (as the Interstate and the Creek curve to the southeast). 

Train tracks (the Drake rail spur) also bound the facility to the north, south, and west. Along the 

east side of the plan (immediately east of Conejos Street) is the main railroad line. Figure 1 

illustrates these boundaries. Figure 2, depicting a detailed view of the boilers, respective 

baghouses, and available spaces for FGD systems, indicates that available physical space is 

severely constrained at the Drake Plant, due to locations as well as pollution control retrofits for 

particulate matter. As figure 2 indicates, the square footage available to accommodate a FGD for 

Unit 5 is 3,025 ft2

 and Units 6 & 7 is 8,346 ft2 (or about 4,000 ft2 per unit). The entire site is 

very congested, with limited access and limited room for major retrofits of new capital 

equipment. Demolition and site reconfiguration would be required for FGD systems on these 

units and has been included in the cost analysis provided by Drake. CSU determined that it is 

technically feasible to install a dry FGD on Unit 5, Unit 6 and Unit 7. 

 

3

 Electric Power Research Institute: A Review of Literature Related to the Use of Spray Dryer Absorber Material – 

Production, Characterization, Utilization Applications, Barriers, and Recommendations, Technical Report, 

September 2007. University of North Dakota: Energy & Environmental Research Center – Coal Ash Resources 

Research Consortium. 15 North 23rd Street, Stop 9018. Grand Forks, ND, 58202. Pg. v. 

4 Srivastava, R.K. Controlling SO2 Emissions: A Review of Technologies. U.S. Environmental Protection Agency, 

Washington, DC, EPA/600/R-00/093 (NTIS PB2001-101224), 2000.
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 6 

Figure 1: Drake Plant Physical Boundaries 

Figure 2: Drake Plant Detailed View 

Cimarron Street

Fountain Creek

Conejos Street

Interstate 25 

Drake Plant

Train Tracks
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 7 

The Division conducted site visits and determined: 

• Unit 5 

o CSU determined dry FGD controls are technically feasible although available 

physical space was severely constrained and some demolition and site reconfiguration 

would be required; the Division conducted a site visit and determined that dry FGD 

controls were not appropriate considering the space constraints, shown in Figure 1 

and Figure 2. Therefore control effectiveness and impacts for dry FGD are not 

evaluated in this analysis. 

o Traditional wet FGD controls are possible considering that there is adequate space 

near the baghouse to allow for the installation of controls, but is being eliminated 

based on other considerations within the five factors (i.e. energy and non-air quality 

impacts). Refer to the energy and non-air quality impact section for the Division 

review regarding wet FGD controls for Unit 5. 

• Units 6 and 7 

o Dry FGD controls are technically feasible for Units 6 and 7. 

o Traditional wet FGD controls are possible considering that there is adequate space 

near the baghouse to allow for the installation of controls, but is being eliminated 

based on other considerations within the five factors (i.e. energy and non-air quality 

impacts). Refer to the energy and non-air quality impact section for the Division 

review regarding wet FGD controls for Units 6 and 7. 

It is worth noting that CSU-Drake is currently testing a new, innovative non-traditional wet 

scrubber control system that appears to be as effective, if not more effective, at controlling SO2

emissions with much less pressure drop (less parasitic load from increased fan demands) and 

requires a much smaller operational foot print area in comparison to traditional wet scrubbing.. 

The pilot-scale wet scrubber control system, called the NeuStream-S FGD process, is presently 

being tested on a 20 MW flue gas stream. CSU anticipates scaling the non-traditional wet 

scrubber control to full scale pending successful outcome of the current testing. This new wet 

scrubber technology uses a unique contacting vessel that makes it different from traditional wet 

scrubbers. It affords a higher liquid to gas contact ratio and so uses much less water / has lower 

pressure drop. It also uses a dual alkali system that is somewhat unique when compared to most 

traditional wet scrubbers. In comparison to traditional wet and LSD scrubbers, this new 

technology will have smaller water and energy requirements. There are several non-air quality 

aspects of the NeuStream-S process that compare favorably to traditional scrubbers, described in 

Step 4. Regarding the applicability of the NeuStream process to Drake Unit 5, the Division notes 

that this technology is not commercially available at this time. CSU has not determined if this 

technology is feasible for this smaller unit. However, the Division will re-assess this technology 

in the next Regional Haze planning period. 

Although the technology being tested by CSU does not technically meet the definition of 

“available” as set forth in the BART rules, the Division is willing to allow CSU the opportunity 

to prove the technology and if successful, the opportunity to install the NeuStream-S FGD 

scrubber. This process will be required to meet the emission limits established for the LSD 

technology established in this BART determination. Regardless of the technology utilized, 

Drake has to meet the LSD-based BART limits within 5 years of EPA approval of the BART 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 8 

SIP. CSU will test the NeuStream system until December 2011, and at that time, determine the 

control technology that will be used to comply with the specified SO2 BART limits for Units 6 

and 7. 

DSI: Dry sorbent injection involves the injection of typically a sodium based reagent, either the 

mineral trona (sodium sesquicarbonate) or refined sodium bicarbonate, into the flue gas. The 

injected reagent reacts with the SO2 present in the flue gas to create sodium sulfate, which is then 

collected in the particulate control device as in the case of the Drake boilers. CSU asserts that 

the flue gas temperatures present downstream of the Unit 5 airheater are in the appropriate range 

to allow for DSI application. A very important factor in DSI application is the ability for the 

boiler’s particulate control device to accommodate the added particulate loading of the DSI 

reagent in addition to the flyash loading. CSU’s preliminary review indicates that even with the 

added loading of DSI reagent, the Drake baghouses would be operating within the design 

specification for particulate loading. The flue gas is not cooled nor saturated with water, so 

reheating of desulfurized flue gas is not required. No gas-sorbent contacting vessel is required to 

be installed. DSI requires less capital equipment, less physical space, and less medication to 

existing ductwork compared to a SDA system. However, reagent costs are much higher and 

depending upon the absorbent and amount of sorbent injected, control efficiency is lower when 

compared to a SDA system. Lime, soda ash, and Trona (sodium sesquicarbonate) are possible. 

Lime is the least reactive reagent resulting in low efficiencies even at high injection rates. Trona 

is a very reactive reagent that can be used to achieve a range of efficiencies depending on the 

amount of sorbent injected, and would likely be the chosen reagent. 

One major challenge of DSI systems is the possibility of converting the NOx present in the flue 

gas from NO which is colorless to NO2 which has a reddish-brown color. This conversion of NO 

to NO2 can create a brown plume from the stack which could create opacity compliance issues. 

Due to variability of boiler configurations, coal composition, NOx to SO2 ratios, and other 

factors, it is difficult to arrive at a precise estimate of the maximum SO2 removal rate that is 

achievable while minimizing the brown plume condition. However, based on literature review, 

CSU estimated the maximum SO2 removal rate that can be achieved while minimizing the 

creation of the brown plume condition to be 60% SO2 removal. In practical application, a higher 

SO2 removal rate may be possible, while it is also possible that a lower SO2 removal rate may be 

necessary to limit the brown plume formation. This determination would require actual SO2

removal real-time testing. Therefore, DSI is technically feasible for Drake Plant Unit 5. The 

Division assumes that this same technology is also then technically feasible for Unit 6 and Unit 

7. 

Emission limit tightening (unit 5 only): The Division conducted technical analyses to determine 

whether the current SO2 emission limit could be more stringent based on actual emissions (2006 

– 2008) from the units. This option is technically feasible for all units. However, the Division 

only examined this option for Unit 5 since when this option was examined; preliminary SO2

determinations had already been established for all units. Unit 5 was the only unit where the 

emission limit could potentially be achieved with the assumption of no control. 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 9 

Step 3: Evaluate Control Effectiveness of Each Remaining Technology

CSU provided the Division annual average control estimates. In the Division’s experience, 30-

day SO2 rolling average emission rates are expected to be approximately 5% higher than the 

annual average emission rate. The Division projected a 30-day rolling average emission rate 

increased by 5% for Units 5, 6, and 7 to determine control efficiencies and annual reductions. 

The Division has reviewed the data supplied by CSU as well as other control techniques applied 

to pulverized coal boilers. A Division review of the EPA’s RBLC revealed recent BACT SO2

determinations range from 0.06 – 0.167 lbs/MMBtu. The Division narrowed down this range 

depending on the averaging time, permit type, facility size, and fuel type. This narrowed range is 

0.095 – 0.161 lbs/MMBtu, with an average of 0.119 lbs/MMBtu rounded to 0.12 lbs/MMBtu. 

While determinations made by other states do not dictate the emissions rate choice made by the 

Division, they do provide information on the range to validate the emissions rate chosen by the 

Division. Refer to “Division RBLC Analysis” for more details. 

Dry FGD (LSD): Controlling SO2 Emissions: A Review of Technologies5 indicates that the 

median control efficiency for dry FGD processes, such as LSD, is 90%. Typically dry FGD 

technology is applied to units that fire coal with a sulfur content below 1.0% to 1.5%. However, 

when concentrations of pollutants are low, as is the case with low-sulfur western coal, the 

achievable control efficiency will drop. Due to the very low sulfur content of the coal burned at 

the Drake Power Plant, typically <0.5% as detailed in Table 3, a 90% removal rate is at the upper 

end of what may reasonably be expected in practice. Additionally, achievement of a 90% 

removal rate on a long-term basis would require levels of equipment redundancy that may not be 

feasible to locate at a congested site such as the Drake Power Plant. 

DSI: Based on literature review, CSU estimated the maximum SO2 removal rate that can be 

achieved to be 60% SO2 removal. The Division concurs that this control efficiency is reasonable 

for retrofit on these units. 

Emission limit tightening: Since emission limit tightening is based on actual data, there will be 

minimal, if any, reductions from baseline period (2006 – 2008) SO2 emissions. The Division 

found that the maximum 30-day rolling emission rate for Unit 5 was 0.83 lb/MMBtu. As 

explained above, the Division projects 30-day rolling SO2 emission rates to be approximately 5% 

higher than annual average emission rates. The uncertainty of evaluating a “maximum” emission 

rate warrants a similar 5% buffer to be applied in this case, especially due to the fact that the 

Drake facility has limited coal storage capacity and blends four different types of coals. 

Therefore, an appropriate SO2 emission limit assuming no control technology for Unit 5 is 0.9 

lb/MMBtu on a 30-day rolling average. 

Table 6 summarizes each available technology and technical feasibility for SO2 control. 

 

5 Srivastava, R.K. Controlling SO2 Emissions: A Review of Technologies. U.S. Environmental Protection Agency, 

Washington, DC, EPA/600/R-00/093 (NTIS PB2001-101224), 2000.
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 10 

Table 6: Drake Units 5, 6, and 7 SO2 Technology Options and Technical Feasibility 

Technology Emission 

Reduction 

Potential (%) 

Technically Feasible? 

(Y = yes, N = no) 

Wet FGD 95% Y 

Dry FGD (LSD) 81 – 90% N – Unit 5 

Y – Units 6 & 7 

DSI 60% (CSU) Y 

Step 4: Evaluate Impacts and Document Results

Cost of Compliance

Wet FGD: The significant cost issue associated with securing sufficient water supplies (a costly 

and scarce resource in the Front Range) to support a wet FGD control system along with the cost 

of disposing the sludge byproduct at an approved landfill since on-site storage is not an option. 

There are other costs and environmental impacts that the Division also considers undesirable 

with respect to wet scrubbers. 

LSD/DSI: CSU submitted cost estimates for LSD systems on Units 5, 6 and 7 in the original 

BART submittal on August 1, 2006 and updated refined cost estimates on March 29, 2007. CSU 

provided cost estimates for the DSI system evaluated on Unit 5 on May 10, 2010. 

The application of LSD or DSI would remove nearly all of the halogens in the flue gas, thus 

improving the acid gas removal of the baghouse. However, it is anticipated that LSD or DSI 

would also lower the inherent mercury removal in the baghouses. Recent mercury tests at the 

Drake Plant have shown that the amount of mercury leaving the stack is approximately 60 – 90% 

less than what would have been expected based on coal analysis. It is believed that the halogens 

present in the flue gas are oxidizing the mercury, which is subsequently removed in the 

baghouse. The application of LSD or DSI would remove the halogens in the flue gas, which 

may lead to reduced mercury control. Due to this possibility, the provision of adding mercury 

control via activated carbon injection as part of a LSD or DSI system has been included in the 

estimated cost of LSD/DSI application. 

The Division compared CSU’s updated cost information to the study that EPA conducted in 

developing presumptive BART limits,6

 shown in Table 7. 

Table 7: CSU-Drake SO2 LSD Control Cost Comparison 

Unit 

Capacity 

(MW) 

EPA’s Calculated Cost Effectiveness 

for MW Group ($/ton SO2 Removed) 

CSU Refined Cost Estimate 

($/ton SO2 Removed (Control 

System)) 

Cost Differential 

Unit 6 – $2,399 $2,579 - $2,981 + 8% – 24% 

 

6

 EPA, 2005. Technical Support Document for the Best Available Retrofit Technology (BART) Notice of Final 

Rulemaking: Setting BART SO2 Limits for Electric Generating Units: Control Technology and Cost-Effectiveness. 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 11 

85 MW 

Unit 7 – 

142 MW 

$1,796 $2,140 - $2,694 + 19% - 50% 

EPA’s study was published in 2005 whereas CSU sent the Division updated cost analyses for 

LSD systems on Units 6 and 7 using various cost updates from the 2006 timeframe. Drake has 

reflected the costs of retrofitting a facility that is already congested with limited room and access 

for major retrofits of new capital equipment in the retrofit multiplier that is applied to the cost of 

new equipment. Therefore, the Division considers CSU’s updated cost information for the LSD 

controls on these units to be reasonable estimations for the cost of control. 

The Division considers this cost to be within a reasonable cost range that is comparable to other 

Colorado facility submittals.7 Therefore, the Division did not adjust CSU’s cost estimates. CSU 

only submitted DSI cost information for Unit 5. The Division scaled this cost information for 

Units 6 and 7 in Table 9, Table 10, and Table 11. Please see “Drake APCD Technical Analysis” 

for more details. 

For dry FGD, CSU estimated a removal rate of 83.3% based on a worst-case coal sulfur 

concentration of 0.9 lb/MMBtu, baseline years 2004 and 2005, and a resulting emission rate at 

the BART presumptive limits of 0.15 lb/MMBtu. The Division adjusted this removal rate using 

the baseline SO2 emissions from Table 2 (lb/MMBtu and tons/year) for each unit and using a 

realistic removal rate of 76 – 90%that meets or exceeds BART presumptive limits for Units 6 

and 7, and exceeds the limits for Unit 5. This range allows the Division to determine the most 

reasonable BART limit for this control option, if applicable. The Division scaled costs linearly 

for the LSD systems for higher control efficiencies as applicable. See “Division APCD 

Technical Analysis” for more details. 

Emission limit tightening: There are no costs associated with this option for unit 5. This option 

is considered equivalent to the “baseline” row in the tables below, and is not considered as a 

separate cost option. 

Table 8 illustrates resultant SO2 emissions for each technically feasible control option. Table 9, 

Table 10, and Table 11 show the SO2 control cost comparisons for each unit based on the 

detailed cost analyses. The Division used baseline emissions from Table 2. The Division 

analyzed both annual and 30-day rolling average limits. The Department’s experience with 

power plants suggests that the maximum 30-day rolling average SO2 emission rate is 

approximately 5% higher than the annual average emission rate. 

 

7

 ENSR, 2006. BART Analysis for the TriGen Colorado Energy Corporation Facility in Golden, Colorado. 

Prepared for Trigen. Document No: 10279-017-700. 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 12 

Table 8: Units 5, 6, and 7 Control Resultant SO2 Emissions 

Alternative Control 

Efficiency 

(%) 

Resultant Emissions 

Unit 5 Unit 6 Unit 7 

Annual 

Emissions 

(tons/year) 

Annual 

Average 

(lb/MMBtu) 

30-day 

rolling 

Average 

(lb/MMBtu) 

Annual 

Emissions 

(tons/year) 

Annual 

Average 

(lb/MMBtu) 

30-day 

rolling 

Average 

(lb/MMBtu) 

Annual 

Emissions 

(tons/year) 

Annual 

Average 

(lb/MMBtu) 

30-day 

rolling 

Average 

(lb/MMBtu) 

Baseline --- 1,269 0.63 2,785 0.82 4,429 0.83 

DSI 60 508 0.25 0.26 1,114 0.33 0.34 1,771 0.33 0.35 

Dry FGD 

(LSD) 

82 501 0.15 0.15 797 0.15 0.16 

Dry FGD 

(LSD) 

85 418 0.12 0.13 664 0.12 0.13 

Dry FGD 

(LSD) 

90 279 0.08 0.09 433 0.08 0.09 

Table 9: Drake Unit 5 SO2 Cost Comparison 

Alternative Emissions Reduction 

(tpy) 

Annualized Cost 

($) 

Cost Effectiveness 

($/ton) 

Incremental Cost 

($/ton) 

Baseline 0 $0 $0 ---

DSI 762 $1,340,663 $1,760 $1,760 

Table 10: Drake Unit 6 SO2 Cost Comparison 

Alternative Emissions 

Reduction (tpy) 

Annualized Cost 

($) 

Cost Effectiveness 

($/ton) 

Incremental Cost 

($/ton) 

Baseline 0 $0 $0 ---

DSI 1,671 $2,234,438 $1,337 $1,337

Dry FGD (LSD) @ 

82% control 

2,284 $6,186,854 $2,709 $6,540

Dry FGD (LSD) @ 

85% control 

2,368 $6,647,835 $2,808 $5,517

Dry FGD (LSD) @ 

90% control 

2,507 $7,452,788 $2,973 $5,780

Table 11: Drake Unit 7 SO2 Cost Comparison 

Alternative Emissions 

Reduction (tpy) 

Annualized Cost 

($) 

Cost Effectiveness 

($/ton) 

Incremental Cost 

($/ton) 

Baseline 0 $0 $0 ---

DSI 2,657 $3,732,826 $1,405 $1,405

Dry FGD (LSD) @ 

82% control 

3,632 $8,216,863 $2,263 $4,602

Dry FGD (LSD) @ 

85% control 

3,764 $8,829,321 $2,345 $4,610

Dry FGD (LSD) @ 

90% control 

3,986 $9,898,382 $2,483 $4,828
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 13 

Energy and Non-Air Quality Impacts

Traditional Wet FGD: Based upon its experience, and as discussed in detail below, the Division has 

determined that wet scrubbing has several negative energy and non-air quality environmental 

impacts, including massive water usage. This is a significant issue in Colorado, where water is a 

costly, precious and scarce resource. In the arid West, securing sufficient water supplies to support a 

wet FGD control system is a difficult undertaking that precludes other beneficial uses for such water. 

In Colorado, water law is based upon the doctrine of prior appropriation or “first in time - first in 

right,” and the priority date is established by the date the water was first put to a beneficial use. Thus, 

depending upon whether and when a power plant first secured a water appropriation and whether 

such appropriation is adequate to supply the demand, there may be insufficient water appropriations 

available in some areas of the state, particularly in the Front Range, to accommodate the added 

demands of wet FGD controls. At a minimum, the water demands of wet FGDs will compete for 

what is already a scarce resource needed for Colorado’s domestic, agricultural and industrial 

demands. Wet scrubbers consume approximately 23% more water than LSD scrubbers, 

depending on boiler size.8

There are other environmental impacts that the Division also considers undesirable with respect to 

wet scrubbers. Potential on-site storage of wet ash is an increasing regulatory concern, as evidenced 

by the recent Tennessee Valley Authority spill. In addition, the steam plume resulting from a wet 

FGD control system in such a confined river valley will produce a noticeable cloud that will 

hang over a densely populated area (City of Colorado Springs). The Division has received 

complaints regarding the more visible plumes associated with wet scrubbing; a potential irony in 

light of the visibility issues at the heart of the BART program. The Division largely focused its 

BART SO2 control technology consideration on commercially available once-through dry FGD 

controls, specifically, “lime spray dryers” (LSD), that have an established record of reliable 

performance on boilers burning low-sulfur coal. Generally, wet FGD controls can achieve a higher 

level of SO2 control on a percent capture basis that exceeds the capabilities of LSDs but, as noted 

above, there are a number of non-air quality and other environmental impacts including increased 

water usage, sludge disposal and wet plume issues that often overshadow any incremental 

improvement in SO2 emission reductions. Recent PSD applications in Colorado have demonstrated 

lime spray dryer systems to be BACT. 

The Division finds that the non-air quality environmental impacts outweigh the visibility benefits 

from this technology. Therefore, the State has eliminated this option as BART. 

Semi-dry FGD (LSD): CSU notes that there are a number of non-air quality environmental 

impacts with regard to lime spray dryer systems. Application of a dry scrubber will tend to 

remove halogens from the flue gas (primarily chlorine) that are important to the removal of 

mercury from the flue gas. Several sources of speciated mercury stack test data, including EPA’s 

own ICR stack test data, show that an unscrubbed plant with a baghouse burning western coal 

will remove more mercury from the flue gas when compared to a similar plant with a scrubber. 

 

8

 2008. “Revised BART Analysis for Unit 1 & 2 Gerald Gentleman Station Sutherland, Nebraska: Nebraska Public 

Power District.” Prepared by: HDF 701 Xenia Avenue South, Suite 600 Minneapolis, MN 55416 With control 

technology costs provided by: Sargent & Lundy. 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 14 

There will be a greater volume of material being landfilled. A LSD scrubber consumes a 

significant amount of water, as detailed in Table 12. 

Table 12: LSD Water Requirements 

Unit Water required for LSD (gpm) Water required for LSD (Mg/year) 

6 68 35.7 

7 100 53.0 

CSU states that the direct energy cost of the LSD systems due to additional auxiliary loads on the 

plant, as well as increased headloss through the scrubber, is the primary energy impact. These 

loads reduce the net output of each unit; therefore, both the lost energy production, as well as the 

reduced capacity, must be replaced. CSU estimates energy costs for replacement capacity and 

differential cost between existing MW-h of output and a replacement MW-h in Error! 

Reference source not found.. This is the incremental cost of a unit of replacement energy, and 

does not double count the direct energy cost already included in the operating cost. The reduced 

unit output will consequently reduce unit efficiency, thereby increasing emissions of CO2 when 

measured on a per MW-h basis. 

Table 13: LSD Energy Replacement Costs 

Unit Replacement capacity 

cost ($/kW-yr) 

Differential energy 

cost ($/MW-h) 

6/7 44 35 

This information, including detailed capital and annual cost data, are provided as “CSU Drake 

BART Submittals”. CSU originally generated costs using EPRI’s FGD Cost model.9

 This 

model uses specific unit data to calculate the cost of controlling emissions, and is considered to 

be accurate within ± 30%. The refined cost estimates from March 2007 were further 

extrapolated to account for retrofit difficulties, annual inflation, and also hyperinflation of certain 

construction commodities and energy. The March 2007 submittal also incorporates budgetary 

quotes from vendors for the major pieces of equipment as well as noting the need for a nonrecycling LSD due to the ash removal system’s operation at a very high capacity factor. As 

depicted in Figure 3, a non-recycling LSD would eliminate slurry solids; instead the FGD solids 

(removed in the baghouse) are immediately disposed. 

 

9

 EPA’s BART Guidelines recommend that the OAQPS Control Cost Manual be used to develop cost estimates, 

where possible. Unfortunately, the Control Cost Manual does not contain a section for SO2 removal equipment as of 

the date of this report. The Fifth edition (EPA 453/B-96-001) of the Control Cost Manual is referenced in the BART 

guideline; however, the Sixth edition (EPA 452/B-02-001, 7-22-2002) is now available.
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 15 

Figure 3: Lime Spray Dryer (LSD) Schematic10

Although these non-air quality/energy impacts have been identified, the State has determined 

that these impacts are not significant or unusual enough to warrant elimination of this control 

option. 

DSI: CSU documents additional collateral impacts of applying DSI include enhanced removal of 

halogenated acid gases, and reduced mercury capture in the baghouse. DSI ahead of the 

baghouse would contaminate the flyash with sodium sulfate, rendering the ash unsalable as a 

replacement for concrete and render it landfill material only. Application of DSI would be 

effective in further enhancing the removal of halogenated acid gases in the baghouse. Currently, 

there is moderate removal of acid gases in the baghouse due to the alkaline nature of the flyash. 

The dry sorbent injection system does result in an ash by-product. This by-product does not 

require additional treatment before being deposited in a landfill. However, a study conducted by 

the Department of Energy found arsenic and methylene chloride in the ash at some plants,11

which could become a problem if more stringent regulations are imposed in the future. 

However, it is not known yet if these levels are considered hazardous or if the levels vary 

depending on the ash; therefore, this issue requires future research. Otherwise, the DSI does not 

have any negative energy or non-air quality related impacts. Thus, this factor (regarding DSI) 

does not influence the selection of controls. 

Emission Limit Tightening: There are no known non-air quality or energy impacts associated 

with emission limit tightening. Thus, this factor does not influence the selection of this option. 

 

10 EPA, 2000. “Controlling SO2 Emissions: A Review of Technologies.” Prepared by Ravi K. Srivastava for Office 

of Research and Development, Washington, D.C. 20460. Pg. 12. 

11 Department of Energy, 2001. LIFAC Sorbent Injection Desulfurization Demonstration Project: A DOE 

Assessment. U.S. Department of Energy: National Energy Technology Laboratory. P.O. Box 880, 3610 Collins 

Ferry Road Morgantown, WV 26507-0880. 

http://www.netl.doe.gov/technologies/coalpower/cctc/resources/pdfs/lifac/LIFAC_PPA.pdf 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 16 

Remaining Useful Life

CSU asserts that the remaining useful life of Drake Units 5, 6, and 7 are each in excess of 20 

years, which is the maximum amortization period allowed in the BART analysis. Thus, this 

factor does not influence the selection of controls. 

Step 5: Evaluate Visibility Results

CALPUFF modeling was used to determine the projected visibility improvement associated with 

various control technologies. The modeling guideline requires that modeled baseline emission 

rate is the 24-hour peak emission rate. The modeling guideline also requires that, at a minimum, 

the presumptive emission rate scenario be modeled. Table 14 shows the number of days pre- and 

post-control. Table 15 depicts the visibility results (98th percentile impact and improvements) as 

well as cost effectiveness in $/deciview and the calculation methodology utilized by the 

Division. 

Per the April 2010 modeling protocol12, to isolate the effects of a given unit for controls on a 

given pollutant, the Division has judiciously constructed each emissions scenario to isolate the 

impact of a given BART control on a given unit. For example, to determine the effect of a SO2

BART control technology on a given unit, emission rates for the other pollutants (NOx and 

PM/PM10) and other BART-eligible units are held constant at pre-control levels. For BART 

sources with more than one BART unit, modeling the units individually would ignore important 

atmospheric chemical reactions that occur when units operate simultaneously. The combination 

scenario assumed all units with NOx emissions at 0.07 lb/MMBtu and SO2 emissions at 0.12 

lb/MMBtu for Units 6 and 7 and at 0.32 lb/MMBtu for Unit 5. The Division modeled Drake 

Unit 5 for 0.12 lb/MMBtu as a theoretical examination of the potential impacts of lower emission 

limits on that unit. 

In situations where the BART-eligible units at a given BART-eligible source operate 

simultaneously, the sulfate and nitrate estimates from the modeling system will be more realistic, 

in general, if all BART units and all pollutants at a BART-eligible source are modeled together. 

The combined unit approach has the added benefit of allowing Colorado to estimate the net 

degree of visibility improvement from the simultaneous operation of BART controls on multiple 

units for multiple pollutants at a given BART-eligible source. 

Table 14: Visibility Results – Change in Days >0.5 dv and >1.0 dv at highest affected Class I Area 

SO2

BART 

Control 

Limit 

Unit(s) 

SO2 

Emission 

Rate 

(lb/MMBtu) 

Class I 

Area 

Affected 

3-year totals 3-year totals 

PreControl 

Days 

>0.5 dv 

PostControl 

Days 

>0.5 dv 

∆days 

PreControl 

Days 

>1.0 dv 

PostControl 

Days 

>1.0 dv 

∆days 

Max 24-

hr SO2

rates 

5 0.943 Rocky 

Mountain 

National 

Park 

6 0.997 34 --- --- 17 --- --- 

7 0.994 

DSI 5 0.251 34 32 2 17 14 3 

 

12 Colorado Air Pollution Control Division, Technical Services Program, 2010. “Supplemental BART Analysis 

CALPUFF Protocol for Class I Federal Area Visibility Improvement Modeling Analysis.” 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 17 

6 0.328 34 32 2 17 14 3 

7 0.333 34 31 3 17 13 4 

dry FGD 

(LSD) 

5 0.120 n/a 

6 0.120 34 31 3 17 14 3 

7 0.120 34 28 6 17 12 5 

dry FGD 

(LSD) 

6 0.100 34 31 3 17 14 3 

7 0.100 34 28 6 17 12 5 

dry FGD 

(LSD) 

6 0.070 34 31 3 17 14 3 

7 0.070 34 28 6 17 12 5 

Combo 

5 0.321 

6 0.120 34 1 33 17 0 17 

7 0.120 

Table 15: Visibility Results – SO2 Control Options 

SO2 

Control 

Scenario 

Boiler(s) 

SO2 Emission 

Rate 

(lb/MMBtu) 

Output (@ 

98th

Percentile 

Impact) 

98th Percentile 

Impact 

Improvement 

98th Percentile 

Improvement from 

Maximum 

Cost 

Effectiveness 

(dv) (∆ dv) (%) ($/dv) 

Max 24-hr 

SO2 rates 

5 0.943 

6 0.997 1.84 --- --- --- 

7 0.994 

DSI 

5 0.251 1.72 0.12 6% $14,673,714 

6 0.328 1.65 0.18 10% $15,903,206 

7 0.333 1.55 0.29 16% $16,765,140 

dry FGD 

(LSD) 

5 0.120 n/a 

6 0.120 1.59 0.24 13% $27,470,391 

7 0.120 1.45 0.39 21% $22,697,484 

dry FGD 

(LSD) 

6 0.100 1.59 0.25 14% n/a 

7 0.100 1.44 0.40 22% n/a 

dry FGD 

(LSD) 

6 0.070 1.58 0.26 14% $28,999,176 

7 0.070 1.42 0.41 22% $23,967,026 

Combo 

5 0.321 

6 0.120 0.25 1.59 86% n/a 

7 0.120 

Step 6: Select BART Control

Based upon its consideration of the five factors summarized herein, the state has determined that 

SO2 BART for Unit 5 is the following SO2 emission rate: 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 18 

Drake Unit 5: 0.26 lb/MMBtu (30-day rolling average) 

The state assumes that the BART emission limit can be achieved through the installation and 

operation of dry sorbent injection. Other alternatives are not feasible. 

Based upon its consideration of the five factors summarized herein, the state has determined that 

SO2 BART for Unit 6 and Unit 7 is the following SO2 emission rates: 

 

 Drake Unit 6: 0.13 lb/MMBtu (30-day rolling average) 

 Drake Unit 7: 0.13 lb/MMBtu (30-day rolling average) 

The state assumes that the BART emission limits can be achieved through the installation and 

operation of lime spray dryers (LSDs). A lower emissions rate for Units 6 and 7 was deemed to 

not be reasonable as increased control costs to achieve such an emissions rate do not provide 

appreciable improvements in visibility (0.02 delta deciview for both units respectively). 

The emission rates for Units 6 and 7 provide 85% SO2 emission reduction at a modest cost per 

ton of emissions removed and result in a meaningful contribution to visibility improvement. 

• Unit 6: $2,808 per ton SO2 removed; 0.24 deciview of improvement 

• Unit 7: $2,345 per ton SO2 removed; 0.39 deciview of improvement 

B. Filterable Particulate Matter (PM10) 

Drake Units 5, 6, and 7 are each equipped with reverse-air fabric filter baghouses to control 

PM/PM10 emissions. Baghouses, or fabric filters, operate on the same principle as a vacuum 

cleaner. Air carrying dust particles is forced through a cloth bag. As the air passes through the 

fabric, the dust accumulates on the cloth, providing a cleaner air stream. The dust is periodically 

removed from the cloth by shaking or by reversing the air flow. The layer of dust, known as dust 

cake, trapped on the surface of the fabric results in high efficiency rates for particles ranging in 

size from submicron to several hundred microns in diameter. Additionally, fabric filters are the 

best PM control for western coals, due to the higher electrical resistivity. 

Colorado Operating Permit 95OPEP107 Condition 2.4.2 requires Units 5, 6, and 7 to conduct 

performance testing for PM10 annually. While the emission in Condition 2.4 is set at 0.1 

lb/MMBtu, the annual performance test must be used as an emission factor in determining 

emissions. 

Table 16 shows the most recent stack test data (June 14, 2006). It is important to note that the 

most recent stack test, which at a minimum, occurs every five years, and more frequently 

depending on the results, demonstrates that these baghouses are meeting >95% control. 

Table 16: Drake 2006 Stack Test Results 

Pollutant Unit 5 (lb/MMBtu) Unit 6 (lb/MMBtu) Unit 7 (lb/MMBtu) 

Filterable PM10 0.0132 0.0186 0.0111 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 19 

PM10 Control efficiency 98.6% 98.3% 99.0% 

A Division review of EPA’s RBLC revealed recent BACT PM/PM10 determinations ranging 

from 0.010 – 0.1 lbs/MMBtu, which are dependent on a number of factors, including PSD 

netting, EGU type and age, coal type, and adjacent controls (i.e. wet and dry FGD systems). The 

current stack test results above are well below the range of recent BACT determinations. While 

determinations made by other states do not dictate the emissions rate choice made by the 

Division, they do provide information on the range to validate the emissions rate chosen by the 

Division. Refer to “Division RBLC Analysis” for more details. 

The State determines that the existing regulatory emissions limit of 0.03 lb/MMBtu (PM/PM10) 

for the three units represents the most stringent control options. The units are exceeding a PM 

control efficiency of 95%, and the control technology and emission limits are BART for 

PM/PM10. The state assumes that the BART emission limit can be achieved through the 

operation of the existing fabric filter baghouses. Thus, as described in EPA’s BART Guidelines, 

a full five-factor analysis for PM/PM10 is not needed for Drake Units 5, 6, and 7. 

C. Nitrogen Oxide (NOx) 

Step 1: Identify All Available Technologies

CSU identified four NOx control options: 

 Overfire air (OFA) 

 Ultra-low NOx burners (ULNBs) 

 Selective Catalytic Reduction (SCR) 

 Ultra-low NOx burners and SCR (ULNBs + SCR) 

 

The Division also identified and examined the following additional control options for these 

units: 

 Electro-Catalytic Oxidation (ECO)® 

 Rich Reagent Injection (RRI) 

 Selective Non-Catalytic Reduction (SNCR) 

 Ultra-low NOx burners and Over-fire air (ULNB+OFA) 

 Coal reburn +SNCR 

Rotating overfire air (ROFA) was not considered in this analysis because ROFA® technology 

has been reported as achieving NOx emission reductions from 45 to 65 % based on fuel load13. 

While ROFA is considered superior to SOFA alone, ROFA alone is not superior to LNB+OFA 

and cannot achieve the predicted 70% or greater NOx reduction for Units 5, 6, and 7. Since 

ROFA® technology would not be expected to provide better emissions performance than the 

LNB+OFA baseline for this unit, ROFA® technology is not considered further in this analysis. 

Step 2: Eliminate Technically Infeasible Options

 

13 Nalco-Mobotec, ROFA Technology, 1992-2009, http://www.nalcomobotec.com/technology/rofa-technology.html
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 20 

OFA: Air staging or two-stage combustion, is generally described as the introduction of overfire 

air into the boiler or furnace. Staging the air in the burner (internal air staging) is generally one 

of the design features of low NOx burners, such as those already present on Units 5, 6, and 7. 

Furnace overfire air (OFA) technology requires the introduction of combustion air to be 

separated into primary and secondary flow sections to achieve complete burnout and to 

encourage the formation of N2 rather than NOx. Primary air (70-90%) is mixed with the fuel 

producing a relatively low temperature; oxygen deficient, fuel-rich zone and therefore moderate 

amounts of fuel NOx are formed14. The secondary (10-30%) of the combustion air is injected 

above the combustion zone through a special wind-box with air introducing ports and/or nozzles, 

mounted above the burners. Combustion is completed at this increased flame volume. Hence, the 

relatively low-temperature secondary-stage limits the production of thermal NOx. The location of 

the injection ports and mixing of overfire air are critical to maintain efficient combustion. 

Retrofitting overfire air on an existing boiler involves waterwall tube modifications to create the 

ports for the secondary air nozzles and the addition of ducts, dampers and the wind-box. OFA is 

a technically feasible option for Units 5, 6, and 7. 

ULNBs: Each unit has low NOx burners installed, shown in Table 1 . These LNBs can be 

replaced with ULNBs. Burner designs have improved in recent years to improve flame stability 

and combustion control schemes for increased NOx emission reductions with these ultra-low 

NOx burners. ULNBs are a technically feasible option for Units 5, 6, and 7. 

ULNB+OFA: Since ULNB and OFA are each technically feasible options and would be installed 

separately for Units 5, 6, and 7, it stands to reason that ULNB+OFA is technically feasible 

option for Units 5, 6, and 7. 

SCR: SCR systems are the most widely used post-combustion NOx control technology. In 

retrofit SCR systems, vaporized ammonia (NH3) injected into the flue gas stream acts as a 

reducing agent, achieving NOx emission reductions as low as 0.07 lb/MMBtu when passed over 

an appropriate amount of catalyst as demonstrated by recent determinations found in the EPA’s 

RBLC database. The NOx and ammonia reagent form nitrogen and water vapor. The reaction 

mechanisms are very efficient with a reagent stoichiometry of approximately 1.0 (on a NOx

reduction basis) with very low ammonia slip. 

While lower controlled NOx emission values have been demonstrated by SCR system 

applications in new coal units, for CSU, a retrofit SCR, the 0.07 lb/MMBtu controlled NOx value 

is more expected. The SCR reaction occurs within the temperature range of 600°F to 750°F 

where the extremes are highly dependent on the fuel quality. There are three different types of 

SCR arrangements – high-dust, low-dust, and tail-end. The pre-dominant arrangement applied in 

the United States has been high-dust. In most circumstances, a high-dust SCR system is the most 

economical arrangement alternative and would likely be the arrangement for Unit 5, 6, and 7 if 

applicable. For high- and low-dust arrangements, the catalyst, because of its location directly 

downstream of the boiler and upstream of the air heater, can impact the boiler through its effect 

on the air heater. The magnitude of this effect is dependent on the power plant configuration, air 

 

14 IEA Clean Coal Centre: Clean Coal Technologies – Air Stating for NOx control (overfire air and two-stage 

combustion), 2010. http://www.iea-coal.org/site/ieacoal_old/clean-coal-technologies-pages/air-staging-for-noxcontrol-overfire-air-ofa-or-two-stage-combustion? 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 21 

quality control components, type of fuel, and overall emission control requirements. For retrofit 

applications, adequate space between the economizer outlet and the air heater inlet to allow 

boiler outlet and air heater return duct is a prerequisite for the installation of a high-dust system 

and is the case at the Drake Plant. Therefore, high-dust SCR is a technically feasible alternative 

for Drake Units 5, 6, and 7. 

ULNBs/SCR layered: A layered approach of installing ULNBs pre-combustion and SCR postcombustion is technically feasible for Drake Units 5, 6, and 7. This scenario considers that less 

NOx would enter the SCR system and reduce aqueous ammonia storage, handling, and injection. 

CSU considered this scenario to determine if this option would be more economically and 

technically feasible for the three boilers at the Drake Plant. 

ECO®: The Powerspan ECO® system is installed downstream of a coal-fired power plants’ 

existing baghouse. The ECO® Reactor then oxidizes pollutants, which are removed downstream 

in an absorber vessel during cooling and saturation of the flue gas. This technology has not 

been demonstrated on a full-size pulverized coal-fired boiler15 and thus, is considered technically 

infeasible. 

RRI: Rich reagent injection is the process of adding NOx reducing agents in a staged lower 

furnace to reduce the formation of NOx, accomplished by injecting urea into the fuel-rich region 

of a furnace, where the reducing conditions in the lower furnace make RRI ideal for NOx

reductions. The combustion process is then completed with the use of overfire air. Rich reagent 

injection was developed for cyclone boilers16 and has not been demonstrated for other types of 

units. Therefore, RRI is considered technically infeasible for Units 5, 6, and 7. 

SNCR: Selective non-catalytic reduction is generally utilized to achieve modest NOx reductions 

on smaller units. With SNCR, an amine-based reagent such as ammonia or urea is injected into 

the furnace within a temperature range of 1,600°F to 2,100°F, where it reduces NOx to nitrogen 

and water. NOx reductions of up to 60% have been achieved, although 20-40% is more realistic 

for most applications. Reagent utilization, a measure of the efficiency with which the reagent 

reduces NOx, can have a significant impact on economics, with higher levels of NOx reduction 

generally resulting in lower reagent utilization and higher operating cost. 

It should be noted that selective non-catalytic reduction (SNCR) was not considered in CSU’s 

BART analysis because CSU asserts that SNCR achieves full-load NOx removal in the same 

range as ULNB at a higher levelized cost ($/ton NOx removed), and therefore should be ruled out 

due to a “least-cost envelope” analysis as detailed in the BART rule. The higher cost is primarily 

due to much higher operating costs, with most of the operating costs being for the reagent. 

Additionally, the chemical reaction required for SNCR to work is temperature sensitive. The 

CSU Drake boilers often operate below full load, when the temperature is no longer conducive to 

optimal NOx removal, resulting in NOx removal declines. The weighted average NOx removal 

over an annual load range can be less than ULNB depending on the portion of time the units 

operate at partial load. Therefore, SNCR was eliminated from consideration by CSU because of 

higher costs and efficiency losses at partial loads. However, the Division considers SNCR a 

 

15 Powerspan ECO®: Overview and Advantages, 2000 – 2010. http://www.powerspan.com/ECO_overview.aspx 16 Fuel Tech: Air Pollution Control – Rich Reagent Injection (RRI), 1998 – 2009. http://www.ftek.com/apcRRI.php 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 22 

technically feasible alternative for Drake Units 5, 6, and 7. Similar Colorado facilities evaluated 

SNCR as an option and it is recognized nationally as a NOx control option for EGUs, so the 

Division included SNCR in the full four-factor analysis. 

Coal Reburn + SNCR: Several research and development efforts in the United States evaluated 

using a combination of technologies to reduce NOx emissions, including combining coal reburn 

and SNCR. A novel injection procedure into the fuel-rich, post-combustion zone with staged, 

fuel-rich primary combustion and SNCR injection was found to reduce NOx emissions by 93% 

or well below 0.1 lb/MMBtu17. However, this procedure has not been performed on a full-size 

pulverized coal-fired boiler yet and thus, is considered technically infeasible. 

Step 3: Evaluate Control Effectiveness of Each Remaining Technology

CSU provided the Division annual average control estimates. In the Division’s experience and 

other state BART proposals,18 30-day NOx rolling average emission rates are expected to be 

approximately 5-15% higher than the annual average emission rate. The Division projected a 

30-day rolling average emission rate increased by 15% for Drake Units 5, 6, and 7 to determine 

control efficiencies and annual reductions. 

OFA: CSU estimated that overfire air, in conjunction with the existing low-NOx burners, is 

capable of reducing NOx emissions approximately an additional 20% from existing conditions in 

the original BART submittal (August 1, 2006). EPA’s AP-42 emission factor tables estimate 

low-NOx burners controlling 35 – 55% and LNB with OFA controlling 40 – 60% of NOx

emissions.19 The low NOx burners currently achieve about 50 – 56% control. However, in a 

more recent AWMA study, it is noted that OFA achieves an additional 10 – 25% control with the 

installed low NOx burners.20 Therefore, the Division concurs with CSU’s additional 20% NOx

control estimate. 

ULNBs: CSU asserts that additional NOx reductions of 20 – 30% are possible with 

implementation of some or all of the modifications that will be needed to retrofit ULNBs at the 

Drake boilers. These additional NOx reductions could be achieved while meeting acceptable CO 

levels. The ULNBs are estimated to control approximately 75% of uncontrolled NOx emissions, 

which is consistent with a U.S. Department of Energy Study which estimated NOx emissions 

reductions between 75 – 85%.21 Therefore, the Division concurs with CSU NOx reduction 

estimat3es for ULNBs. 

 

17 Coal Tech. Corp, 2002. “Tests on Combined Staged Combustion, SNCR & Reburning for NOx Control and 

Combined NOx/SO2 Control on an Industrial & Utility Boilers.” 

http://www.netl.doe.gov/publications/proceedings/04/NOx/summary/h11.50zauderer-summary.pdf 18 State of North Dakota BART Determination for Leland Olds Station Units 1 and 2. Page 16. 19 EPA AP-42, Fifth Edition, Volume I, Chapter 1, Section 1.1, Table 1.1-2. 

http://www.epa.gov/ttn/chief/ap42/ch01/final/c01s01.pdf 20 Srivastava et. al, 2005. Nitrogen Oxides Emission Control Options for Coal-Fired Electric Utility Boilers. Journal 

of Air & Waste Management Association 55:1367 – 1388. 

21 U.S. Department of Energy, 2004. Office of Fossil Energy, National Energy Technology Laboratory. 

http://www.netl.doe.gov/publications/factsheets/project/Proj294.pdf 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 23 

ULNB+OFA: The Division used information from CSU regarding ULNBs and OFA control 

efficiencies as described above. CSU noted in the February 2009 submittal that ULNB are 

assumed to achieve 20% efficiency assuming OFA is already installed (at 0.35 lb/MMBtu for 

each unit). The Division is employing a different baseline that CSU originally utilized (e.g. NOx 

emissions prior to consideration of OFA). The Division requested additional information from 

CSU to verify that the 20% ULNB assumption is still valid for all units. CSU noted that Units 6 

and 7 will likely be able to achieve the 20% reduction (using the Division’s higher NOx emission 

baseline). However, Unit 5 has an older technology coal mill and other technical issues and 

would not be able to achieve 20% reduction. Unit 5 has an older mill (ball-type pulverizers vs. 

the hammermills present at Units 6 and 7), which limits the level of coal fineness. In addition, 

Unit 5 is a smaller boiler than the other units. In light of these specific technical feasibility 

issues, the Division used 10% additional reduction efficiency for ULNBs for Unit 5. Therefore, 

the overall control efficiencies for ULNB+OFA in combination for the three units are 28% for 

Unit 5 and 36% for Units 6 and 7 respectively. 

SNCR: Other Colorado facilities have noted a variety of control ranges for SNCR. The Division 

used a variety of information, including similar Colorado facility estimates, EPA’s SNCR Air 

Pollution Control Fact Sheet and a recent AWMA study22 to conservatively approximate that the 

Drake boilers can achieve 30% control when SNCR is applied. 

SCR:CSU approximates that SCR can achieve an approximate 80% NOx reduction using 2004 – 

2005 baseline emissions (or 0.07 lb/MMBtu), determined by URS WD using a survey of a large 

collection of photographs, and experience in developing retrofit factors for many types of units 

and configurations at numerous facilities. The Division adjusted the control efficiency percent 

reduction to reflect the 2006 – 2008 baseline emissions, but kept the resultant 0.07 lb/MMBtu 

constant. This control efficiency is consistent with EPA’s AP-42 emission factor discussion, 

which estimates SCR as achieving 75 – 85% NOx emission reductions and also with a recent 

AWMA study citing SCR as achieving 80 – 90% reduction.23,24

ULNBs/SCR layered approach: CSU evaluated a layered approach of installing ULNBs 

upstream of the combustion process to reduce NOx entering the boiler and thus reducing 

subsequent SCR reduction requirements. This approach will achieve the same NOx emission 

reductions as SCR alone and is deemed to be appropriate by the Division. 

Table 17 summarizes each available technology and technical feasibility for NOx control. 

Table 17: Drake Units 5, 6, and 7 NOx Technology Options and Technical Feasibility 

Technology Emission 

Reduction 

Potential (%)

Technically Feasible? 

(Y = yes, N = no)

Low NOx Burners (LNB) 50 -56% Y – installed

 

22 Srivastava et. al, 2005. Nitrogen Oxides Emission Control Options for Coal-Fired Electric Utility Boilers. Journal 

of Air & Waste Management Association 55:1367 – 1388. 

23 EPA AP-42, Fifth Edition, Volume I, Chapter 1, Section 1.1, Table 1.1-2. 

http://www.epa.gov/ttn/chief/ap42/ch01/final/c01s01.pdf 24 Srivastava et. al, 2005. Nitrogen Oxides Emission Control Options for Coal-Fired Electric Utility Boilers. Journal 

of Air & Waste Management Association 55:1367 – 1388. 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 24 

LNB + OFA 60 – 81% Y (LNBs are installed 

on each unit)

Overfire air (OFA) 10 – 25% (alone) Y

Ultra-low NOx burners (ULNBs) 26 – 32% Y

ULNB+OFA 28 – 36% Y 

Selective non-catalytic reduction 

(SNCR)

~ 30% Y

Selective catalytic reduction (SCR) 75 – 90% Y

ULNB/SCR layered approach 75 – 90% Y

ECO® n/a N

RRI n/a N

Coal reburn +SNCR n/a N

Step 4: Evaluate Impacts and Document Results

Cost of Compliance

OFA: Washington Group International Inc. estimated the cost of overfire air during the course of 

a pollution control study for the Drake boilers in 2004. The cost estimates were generated using 

EPRI’s IECCOst model. This model uses specific unit data to calculate the cost of controlling 

emissions and is typically considered to be accurate within ±30%. Overfire air will not require 

large pieces of new equipment, but instead the costs consist primarily of labor and materials 

related to modifying the boiler waterwall tubes to allow for new air injection ports and the 

necessary ductwork, dampers, and instrumentation and control to supply the air from the existing 

secondary air duct. In a technical support document issued by the Northeast States for 

Coordinated Air Use Management (NESCAUM) entitled “NOx Controls for Existing Utility 

Boilers,”25 OFA alone ranges from $410 - $1,100 per ton NOx reduced annually for units 

estimating 15 – 30% NOx control, which is within the range of Drake’s estimated OFA NOx

reductions (20%). The estimates in Table 18, Table 20, and Table 22 are within this range. 

Therefore, the Division concurs with the OFA cost estimates. 

ULNBs: CSU’s cost estimate includes the burners, oil or gas lighter systems and controls at 

burner front, automatic air register adjustment and control drives, flame scanners and controls, 

all wind box controls including control drawings, all control and burner logic drawings. The 

estimates do not include burner wind box extensions or stove pipe, ducts installed on top of 

existing wind boxes, furnace water wall openings, structural steel support for ULNBs beyond 

supplemental support steel, cost for engineering, supply and construction of wind box 

extensions, physical modeling, math modeling, or wind box baffling, pulverizer upgrades, burner 

piping or classifiers for improved coal fineness and required size distribution. CSU notes that 

some or all of the items must be determined by boiler modeling and pulverizer testing. If all of 

these are needed, the capital costs could increase by 40 – 70% compared to the base scope listed 

in Table 19, Table 21, and Table 23. The Division considers CSU’s estimated costs more than 

 

25 Neuffer, Bill – ESD/OAQPS, 2003. “NOx Controls for Existing Utility Boilers.” 

http://www.epa.gov/ttn/nsr/gen/u3-26.pdf 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 25 

reasonable, with ULNBs under $1,000/ton which is comparable or lower than LNB costs 

presented in recent NESCAUM papers.26, 27

ULNB+OFA: The Division based cost estimates for this control option assuming that OFA and 

ULNBs will be installed separately; therefore, the cost for this layering option is a summation of 

individual annualized costs for OFA and ULNBs for each unit. The Division checked this 

assumption with CSU on November 8, 2010. 

SNCR: The difficulty of SNCR retrofit on smaller boilers significantly increases, with the 

primary concern being that there is adequate wall space within the boiler for installation of 

injectors. Movement and/or removal of existing watertubes and asbestos from the boiler housing 

may be required, as in the case of the Drake boilers. 

A typical breakdown of annual for industrial boilers will be 15 – 35% for capital recovery and 65 

– 85% for operating expense.28 A similar Colorado facility estimated operating expenses at 

approximately 81 – 86%.29 Since SNCR is an operating expense-driven technology, its cost 

varies directly with NOx reduction requirements and reagent usage. There is a wide range of cost 

effectiveness for SNCR due to different boiler configurations and site-specific conditions, even 

with a given industry. Cost effectiveness is impacted primarily by uncontrolled NOx level, 

required emission reductions, unit size and thermal efficiency, economic life of the unit, and 

degree of retrofit difficulty.30 

The Division used information from a similar facility submittal to determine approximate SNCR 

costs for the Drake boilers since CSU did not have SNCR information.31 The Division consulted 

with CSU on this decision to ensure that these boilers are roughly equivalent to the Drake boilers 

in scope and retrofit difficulty. 

The resultant cost effectiveness for SNCR on Units 5, 6, and 7 ranges from $2,700 to $4,400 per 

ton. Recent NESCAUM studies estimate SNCR retrofits on tangentially fired boilers achieving 

NOx emission rates of 0.30 – 0.40 lb/MMBtu and emission reductions of 30 – 50% as costing 

$630 - $1,300 per ton of NOx reduced, depending on initial capital costs and capacity factor.32,33 

 

26 Amar, Praveen, 2000. “Status Report on NOx Controls for Gas Turbines, Cement Kilns, Industrial Boilers, 

Internal Combustion Engines: Technologies & Cost Effectiveness.” Northeast States for Coordinated Air Use 

Management, 129 Portland Street, Boston, MA 02114. www.nescaum.org/documents/nox-2000.pdf 27 Neuffer, Bill – ESD/OAQPS, 2003. “NOx Controls for Existing Utility Boilers.” 

http://www.epa.gov/ttn/nsr/gen/u3-26.pdf 28 ICAC, 2000. Institute of Clean Air Companies, Inc. “White Paper: Selective Non-Catalytic Reduction (SNCR) 

for Controlling NOx Emissions.” Washington, D.C. 2000. 29 CENC, 2009. “NOx Technical Feasibility and Emission Control Costs for Colorado Energy Nations, Golden, 

Colorado.” Prepared by AECOM. 30 EPA, 2003. “SNCR Air Pollution Control Technology Fact Sheet.” http://www.epa.gov/ttn/catc/dir1/fsncr.pdf 31 CENC, 2009. “NOx Technical Feasibility and Emission Control Costs for Colorado Energy Nations, Golden, 

Colorado.” Prepared by AECOM. 

32 Neuffer, Bill – ESD/OAQPS, 2003. “NOx Controls for Existing Utility Boilers.” 

http://www.epa.gov/ttn/nsr/gen/u3-26.pdf

33 Amar, Praveen, 2000. “Status Report on NOx Controls for Gas Turbines, Cement Kilns, Industrial Boilers, 

Internal Combustion Engines: Technologies & Cost Effectiveness.” Northeast States for Coordinated Air Use 

Management, 129 Portland Street, Boston, MA 02114. 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 26 

EPA’s SNCR Fact Sheet cites SNCR as costing from $400 - $2,500 per ton of NOx reduced. 34 

Although the resulting cost estimates for the Drake boilers are greater than these ranges, the 

small size of the boilers as well as the difficulty of the retrofit leads the Division to the 

conclusion that the estimated cost estimates for SNCR are reasonable. 

SCR: CSU estimated the cost for the SCR system(s) using the IECCOST program. This estimate 

includes the cost of a new ID booster fan, since CSU/URS noted that the current ID fan does not 

have sufficient capacity to accommodate the additional pressure drop of the SCR retrofit. Recent 

NESCAUM studies estimate SCR retrofits achieving NOx emission rates of 0.05 – 0.15 

lb/MMBtu and emission reductions of 65 – 85% as costing $2,600 - $7,400 per ton of NOx

reduced, depending on initial capital costs and capacity factor.35,36 The SCR system estimates for 

the CSU Drake boilers range from approximately $5,000 - $7,100, which is within the 

NESCAUM estimates. The Division concurs that CSU cost estimates for SCR controls are 

reasonable. 

ULNBs/SCR layered approach: CSU chose to examine the ULNB/SCR layered approach 

because the cost of the SCR would be reduced somewhat in this scenario. The reduced costs 

would be noted in the reactor housing, amount of catalyst required, and the aqueous ammonia 

storage, handling, and injection. Therefore, this option was examined to determine the 

significance of the potential cost differential. The Division concurs that this is an appropriate 

option and may possibly reduce costs. 

Table 18, Table 20, and Table 22 illustrate resultant NOx emissions for each technically feasible 

control option. Table 19, Table 21, and Table 23 show the NOx control costs for each unit based 

on detailed cost analyses. The Division estimated resultant NOx using annual average reductions 

for tons of NOx reduced per year, as noted in Table 2. The Division’s experience with power 

plants suggest that the maximum 30-day rolling average NOx emission rate is 5-15% higher than 

the annual average emission rate. 

Table 18: Drake Unit 5 Control Resultant NOx Emissions 

Alternative Control 

Efficiency (%) 

Resultant Emissions 

Annual 

Emissions 

(tons/year)

Annual 

Average 

(lb/MMBtu)

30-day Rolling 

Average 

(lb/MMBtu)

Baseline --- 768 0.38

Overfire air (OFA) 20 615 0.30 0.35

Ultra-low NOx burners (ULNBs) 26 569 0.28 0.32

ULNBs+OFA 28 553 0.27 0.31 

Selective Non-Catalytic 

Reduction (SNCR)

30 538 0.26 0.30

 

34 EPA, 2003. “SNCR Air Pollution Control Technology Fact Sheet.” http://www.epa.gov/ttn/catc/dir1/fsncr.pdf 35 Neuffer, Bill – ESD/OAQPS, 2003. “NOx Controls for Existing Utility Boilers.” 

http://www.epa.gov/ttn/nsr/gen/u3-26.pdf

36 Amar, Praveen, 2000. “Status Report on NOx Controls for Gas Turbines, Cement Kilns, Industrial Boilers, 

Internal Combustion Engines: Technologies & Cost Effectiveness.” Northeast States for Coordinated Air Use 

Management, 129 Portland Street, Boston, MA 02114. 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 27 

ULNBs/SCR layered approach 81.5 142 0.070 0.080

Selective Catalytic Reduction 

(SCR) 

81.5 142 0.070 0.080

Table 19: Drake Unit 5 NOx Cost Comparison 

Alternative Emissions 

Reduction (tpy) 

Annualized Cost 

($) 

Cost 

Effectiveness 

($/ton) 

Incremental Cost 

($/ton) 

Baseline 0 $0 $0 ---

Overfire air (OFA) 154 $141,844 $923 $923

Ultra-low NOx burners 

(ULNBs) 200 $147,000 $736 $112

ULNBs+OFA 215.2 $288,844 $1,342 $9,230 

Selective Non-Catalytic 

Reduction (SNCR) 231 $1,011,324 $4,387 $47,011

ULNB/SCR layered 

approach 626 $4,467,000 $7,133 $8,732

Selective Catalytic 

Reduction (SCR) 626 $4,580,349 $7,314 ---

Table 20: Drake Unit 6 Control Resultant NOx Emissions 

Alternative Control 

Efficiency (%) 

Resultant Emissions 

Annual 

Emissions 

(tons/year)

Annual 

Average 

(lb/MMBtu)

30-day Rolling 

Average 

(lb/MMBtu)

Baseline --- 1,413 0.42

Overfire air (OFA) 20 1,130 0.33 0.38

Selective Non-Catalytic 

Reduction (SNCR)

30 989 0.29 0.33

Ultra-low NOx burners (ULNBs) 32 961 0.28 0.32

ULNBs+OFA 36 904 0.27 0.31 

ULNBs/SCR layered approach 83.2 237 0.070 0.080

Selective Catalytic Reduction 

(SCR) 

83.2 237 0.070 0.080

Table 21: Drake Unit 6 NOx Cost Comparison 

Alternative Emissions 

Reduction (tpy) 

Annualized 

Cost ($) 

Cost 

Effectiveness 

($/ton) 

Incremental Cost 

($/ton) 

Baseline 0 $0 $0 ---

Overfire air (OFA) 283 $104,951 $371 $371

Selective Non-Catalytic 

Reduction (SNCR) 

424 $1,208,302 $2,851 $7,810

Ultra-low NOx burners 

(ULNBs) 

452 $232,800 $515 ($34,525)

ULNBs+OFA 509 $337,751 $664 $1,857 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 28 

ULNBs/SCR layered 

approach 

1,175 $6,182,800 $5,260 $8,226 

Selective Catalytic 

Reduction (SCR) 

1,175 $6,340,797 $5,395 ---

Table 22: Drake Unit 7 Control Resultant NOx Emissions 

Alternative Control 

Efficiency (%) 

Resultant Emissions 

Annual 

Emissions 

(tons/year)

Annual 

Average 

(lb/MMBtu)

30-day Rolling 

Average 

(lb/MMBtu)

Baseline --- 2,081 0.39

Overfire air (OFA) 20 1,665 0.31 0.36

Ultra-low NOx burners (ULNBs) 28 1,498 0.28 0.33

Selective Non-Catalytic 

Reduction (SNCR)

30 1,457 0.28 0.32

ULNBs+OFA 36 1,332 0.25 0.29 

ULNBs/SCR layered approach 80.1 372 0.070 0.080

Selective Catalytic Reduction 

(SCR) 

80.1 372 0.070 0.081

Table 23: Drake Unit 7 NOx Cost Comparison 

Alternative Emissions 

Reduction (tpy) 

Annualized 

Cost ($) 

Cost 

Effectiveness 

($/ton) 

Incremental Cost 

($/ton) 

Baseline 0 $0 $0 ---

Overfire air (OFA) 416 $75,217 $181 $181

Ultra-low NOx burners 

(ULNBs) 

583 $386,000 $662 $1,867

Selective Non-Catalytic 

Reduction (SNCR) 

624 $2,018,575 $3,233 $39,226 

ULNBs+OFA 749 $461,217 $616 ($12,473) 

ULNBs/SCR layered 

approach 

1,708 $8,196,000 $4,797 $5,698 

Selective Catalytic 

Reduction (SCR) 

1,708 $8,510,067 $4,981 ---

Energy and Non-Air Quality Impacts

OFA: Overfire air does not have any significant energy or non-air quality related impacts. Thus, 

this factor does not influence the selection of this control. 

ULNBs: The additional energy required to further pulverize coal is relatively small and is 

accounted for in CSU’s February 2009 submittal. Therefore, ULNBs do not have any significant 

energy or non-air quality related impacts. Thus, this factor does not influence the selection of 

this control. 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 29 

SNCR /SCR: SCR retrofit impacts the existing flue gas fan systems, due to the additional 

pressure drop associated with the catalyst, which is typically a 6- to 8-inch water gage increase 

for the high temperature applications, and potentially somewhat lower for the low temperature 

alternatives. In addition, any flue gas reheat requirements for the low temperature applications 

may require significant energy input to heat the flue gas. SCR reagent injection systems have 

minimal power requirements. 

Post-combustion add-on control technologies like SNCR do increase power needs, in the range 

of 100 – 300 kilowatts (kW) depending on the boiler size, to operate pretreatment and injection 

equipment, drive the pumps and fans necessary to supply reagents, overcome additional pressure 

drops caused by the control equipment, and provide steam in some cases. 100 – 300 kW is less 

than 1.0% of the power generated by the Drake Unit 7 boiler annually, or enough energy to 

power about 10 homes for a year. These energy requirements are minimal. 

SCR systems require additional auxiliary power or power from the existing flue gas fan systems 

to overcome the pressure loss across the catalyst, to supply dilution air for mixing with the 

ammonia, and to pump ammonia into the vaporizer. For example, CSU estimates that on Drake 

7, the power consumption for a SCR system will be over 700 kW. These energy requirements 

are moderate (0.5% of Drake 7’s gross output). 

Installing SNCR or SCR increases levels of ammonia, and may create a ‘blue plume’, if 

ammonia rates are not adequately controlled. Other environmental factors include ammonia 

storage and transportation, particularly for anhydrous ammonia. Anhydrous ammonia is clear in 

the liquid state and boils at a temperature of -28°F. With its low boiling point, liquid anhydrous 

ammonia must be stored under pressure at ambient temperatures to remain a liquid. With 

anhydrous ammonia, an invisible vapor or gas is formed as the liquid evaporates during 

depressurization. Accidental atmospheric release of anhydrous ammonia vapor can be 

hazardous; therefore, stringent requirements for safety are enforced, and obtaining the permits to 

allow the storage of large quantities of anhydrous ammonia may prove difficult in densely 

populated areas. CSU has indicated to the Division that they would prefer to use aqueous 

ammonia instead if applicable to ensure personnel and surrounding community safety, and based 

the capital and operating costs of a SCR system on an aqueous ammonia reagent versus an 

ammonia reagent. 

Remaining Useful Life

CSU asserts that the remaining useful life of Drake Units 5, 6, and 7 are each in excess of 20 

years, which is the maximum amortization period allowed in the BART analysis. Thus, this 

factor does not influence the selection of controls. 

Step 5: Evaluate Visibility Results

CALPUFF modeling was used to determine the projected visibility improvement associated with 

various control technologies. The modeling guideline requires that modeled baseline emission 

rate is the 24-hour peak emission rate. The modeling guideline also requires that, at a minimum, 

the presumptive emission rate scenario be modeled. Table 24 shows the number of days pre- and 

post-control. Table 25 depicts the visibility results (98th percentile impact and improvements) as 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 30 

well as cost effectiveness in $/deciview and the calculation methodology utilized by the 

Division. 

Per the April 2010 modeling protocol37, to isolate the effects of a given unit for controls on a 

given pollutant, the Division has judiciously constructed each emissions scenario to isolate the 

impact of a given BART control on a given unit. For example, to determine the effect of a NOx

BART control technology on a given unit, emission rates for the other pollutants (SO2 and 

PM/PM10) and other BART-eligible units are held constant at pre-control levels. For BART 

sources with more than one BART unit, modeling the units individually would ignore important 

atmospheric chemical reactions that occur when units operate simultaneously. The combination 

scenario assumed all units with NOx emissions at 0.07 lb/MMBtu and SO2 emissions at 0.12 

lb/MMBtu for Units 6 and 7 and at 0.32 lb/MMBtu for Unit 5. 

In situations where the BART-eligible units at a given BART-eligible source operate 

simultaneously, the sulfate and nitrate estimates from the modeling system will be more realistic, 

in general, if all BART units and all pollutants at a BART-eligible source are modeled together. 

The combined unit approach has the added benefit of allowing Colorado to estimate the net 

degree of visibility improvement from the simultaneous operation of BART controls on multiple 

units for multiple pollutants at a given BART-eligible source. 

Table 24: Visibility Results – Change in Days >0.5 dv and >1.0 dv at highest affected Class I Area 

NOx Control 

Scenario Boiler(s) 

NOx 

Emission 

Rate 

(lb/MMBtu)* 

Class I 

Area 

Affected 

3-year 

totals 

3-year 

totals 

PreControl 

Days 

>0.5 dv 

PostControl 

Days 

>0.5 dv 

∆days

PreControl 

Days 

>1.0 dv 

PostControl 

Days 

>1.0 dv 

∆days

Max 24-hour 

NOx rates 

5 0.619 

Rocky 

Mountain 

National 

Park 

34 

--- --- 17 --- --- 

6 0.827 

7 0.710 

NOx Control 

Scenario 

5 0.390 34 34 0 17 15 2 

6 0.390 34 31 3 17 14 3 

7 0.390 34 31 3 17 14 3 

OFA

5 0.300* n/a

6 0.330* n/a

7 0.310* n/a

ULNBs

5 0.280* n/a

6 0.282* n/a

7 0.283* n/a

ULNBs+OFA

5 0.272* n/a 

6 0.266* n/a 

 

37 Colorado Air Pollution Control Division, Technical Services Program, 2010. “Supplemental BART Analysis 

CALPUFF Protocol for Class I Federal Area Visibility Improvement Modeling Analysis.” 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 31 

7 0.251* n/a 

SNCR 

5 0.265* n/a

6 0.291* n/a

7 0.275* n/a

NOx Control 

Scenario 

5 0.234 34 34 0 17 14 3 

6 0.234 34 31 3 17 14 3 

7 0.234 34 28 6 17 14 3 

SCR 

5 0.070 34 32 2 17 14 3 

6 0.070 34 27 7 17 14 3 

7 0.070 34 26 8 17 13 4 

Combo 

5 0.070 34 1 33 17 0 17 

6 0.070 

7 0.070 

* Denotes that output was interpolated by the Division and is not an actual modeled output. See “CSU Drake BART 

Modeling Summary” for more details. 

Table 25: Visibility Results – NOx Control Options 

NOx Control 

Scenario Boiler(s) 

NOx Emission 

Rate 

(lb/MMBtu)* 

Output (@ 

98th

Percentile 

Impact) 

98th Percentile 

Impact 

Improvement 

98th Percentile 

Improvement 

from Maximum 

Cost 

Effectiveness 

(dv) (∆ dv) (%) ($/dv) 

Max 24-hour 

NOx rates 

5 0.619 

1.84 --- 

--- 

6 0.827 --- 

7 0.710 

NOx Control 

Scenario 

5 0.390 1.79 0.05 3% n/a 

6 0.390 1.68 0.16 9% n/a 

7 0.390 1.66 0.18 10% n/a 

OFA

5 0.300* 1.76 0.08 4% $1,970,053 

6 0.330* 1.66 0.18 10% $583,061 

7 0.310* 1.61 0.22 12% $335,791 

ULNB

5 0.280* 1.76 0.08 4% $1,934,212 

6 0.282* 1.64 0.197 11% $1,181,727 

7 0.283* 1.60 0.24 13% $1,615,062 

SNCR 

5 0.265* 1.76 0.08 4% $12,641,549 

6 0.291* 1.64 0.19 11% $6,228,362 

7 0.275* 1.59 0.24 13% $8,272,850 

ULNBs+OFA 5 0.272* 1.76 0.08 4% $3,703,128 

 6 0.266* 1.63 0.20 11% $1,663,798 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – Colorado Spring Utilities Drake Page 32 

 7 0.251* 1.58 0.26 14% $1,794,618 

NOx Control 

Scenario 

5 0.234 1.75 0.24 5% n/a 

6 0.234 1.62 0.24 12% n/a 

7 0.234 1.57 0.24 15% n/a 

SCR 

5 0.070 1.71 0.12 7% $36,024,194 

6 0.070 1.56 0.27 15% $22,647,619 

7 0.070 1.47 0.37 20% $22,091,644 

Combo 

5 0.070 

6 0.070 0.25 1.59 86% n/a 

7 0.070 

* Denotes that output was interpolated by the Division and is not an actual modeled output. See “CSU Drake BART 

Modeling Summary” for more details. 

Step 6: Select BART Control

Based upon its consideration of the five factors summarized herein, the state has determined that 

NOx BART for Units 5, 6, and 7 is the following NOx emission rates: 

 Drake Units 5 and 6: 0.31 lb/MMBtu (30-day rolling hour average) 

 Drake Unit 7: 0.29 lb/MMBtu (30-day rolling hour average) 

 

The state assumes that the BART emission limits can be achieved through the installation and 

operation of ultra low-NOx burners (including over-fire air). 

• Unit 5: $1,342 per ton NOx removed 

• Unit 6: $664 per ton NOx removed 

• Unit 7: 616 per ton NOx removed 

The extremely low dollars per ton control costs, leads the state to selecting this emission rate for 

each of the Drake units. SNCR is not selected as that technology provides an equivalent 

emissions rate, similar level of NOx reduction coupled with equivalent visibility improvement at 

a much higher cost per ton of pollutant removed along with potential energy and non-air quality 

impacts. SCR is not selected as the cost/effectiveness ratios for Units 5 and 6 are too high and 

the visibility improvement does not meet the criteria guidance described in Chapter 6.4.3 of the 

Regional Haze SIP (e.g. less than 0.50 Δdv)