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

Best Available Retrofit Technology (BART) Analysis – TriState Craig Station Page 1 

Best Available Retrofit Technology (BART) Analysis of Control Options 

For 

Tri-State Generation & Transmission Association, Inc. – Craig Station Units 1 & 2 

I. Source Description

Owner/Operator: Tri-State Generation & Transmission Association, Inc. 

Source Type: Electric Utility Steam Generating Unit 

SCC (EGU): 10100222 

Boiler Type: Dry-Bottom Pulverized Coal-Fired Boilers, two opposed-wall-fired 

(Units 1 and 2) 

The Tri-State Generation & Transmission Association, Inc. (Tri-State) Craig Station is 

located in Moffat County approximately 2.5 miles southwest of the town of Craig, 

Colorado. This facility is a coal-fired power plant with a total net electric generating 

capacity of 1264 MW, consisting of three units. Units 1 and 2, rated at 4,318 

mmBtu/hour each (net 428 MW), were placed in service in 1980, and 1979, respectively. 

Units 1 & 2: Construction of Units 1 and 2 began in 1974; Unit 1 began operation in 

1980 and Unit 2 began operation in 1979. These units are equipped with fabric filter 

(baghouse) systems for controlling particulate matter (PM) emissions, and wet limestone 

Fuel Gas Desulfurization (FGD) systems for the control of sulfur dioxide (SO2) 

emissions. The boilers are equipped with ultra-low nitrogen oxide (NOx) dual register 

burners with overfire air for minimization of NOx emissions. The FGD and ultra low 

NOx burner systems were required to be installed and fully operational by December 31, 

2004 as a result of a consent decree with the Sierra Club (signed January 10, 2001). 

Unit 3: Construction of Unit 3 began in 1981 and the unit commenced operation in 1984. 

This unit is equipped with a baghouse system for controlling PM emissions, a dry lime 

system for control of SO2 and low-NOx burners with overfire air. 

All three units can use natural gas, propane, or fuel oil for start-up, shutdown, and for 

flame stabilization. All three units are subject to the requirements of Title IV, the Acid 

Rain Program, and were approved for Early Election for NOx limits, effective January 1, 

1997. Associated activities include two cooling towers, coal handling systems, ash 

handling systems, limestone handling system, and the staging/landfilling area. Unit 3 is 

not subject to BART. 

Error! Reference source not found. below lists the units at Tri-State Craig Station 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. 

Table 1: Craig Boilers Technical Information 

 Unit 1 Unit 2 

Placed in Service 1980 1979 

Gross Boiler 4,417 4,417 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – TriState Craig Station Page 2 

Rating, 

MMBtu/Hr for 

coal 

Electrical Power 

Rating, Net 

Megawatts 

428 428 

Description Babcock & Wilcox Pulverized Coal 

Opposed-Wall Dry Bottom, firing coal with 

natural gas, propane or No. 2 fuel oil used for 

startup, shutdown and/or flame stabilization. 

Babcock & Wilcox Pulverized Coal OpposedWall Dry Bottom, firing coal with natural gas, 

propane or No. 2 fuel oil used for startup, 

shutdown and/or flame stabilization. 

Air Pollution 

Control 

Equipment 

PM/PM10 – Pulse Jet Fabric Filter Baghouse 

NOx – Ultra-low NOx Burners with Over-Fire 

Air 

SO2 – Wet Limestone FGD 

All updated control equipment commenced 

full operations in 2004. 

PM/PM10 – Pulse Jet Fabric Filter Baghouse 

NOx – Ultra-low NOx Burners with Over-Fire 

Air 

SO2 – Wet Limestone FGD 

All updated control equipment commenced 

full operations in 2004. 

Emissions 

Reduction (%)* 

NOx – 23.8% /53.9% 

SO2 – 77.6% 

PM – 99.6% 

PM10 – 99.4% 

NOx – 29.5%/54.7% 

SO2 – 79.5% 

PM – 99.9% 

PM10 – 99.5% 

*Emissions Reduction estimated by comparing pre-control 2001 – 2002 CAMD data to controlled 2006 – 2008 data. 

The first NOx number compares the additional reduction achieved by the ultra-low NOx burners vs. the original lowNOx burners and the second NOx number compares uncontrolled AP-42 factor to actual average emission factor 

(2006 – 2008). For PM/PM10., uncontrolled AP-42 factor were compared to actual average emission factors (2006 – 

2008). See “Craig APCD Technical Analysis” for further details. Not based on actual testing.

Only Units 1 and 2 are 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. These boilers also cause or contribute to visibility impairment at 

a federal Class I area at or above a 0.5 deciview change. Tri-State submitted a BART 

Analysis to the Division on July 31, 2006 with revisions, updates, and/or comments 

submitted on October 25, 2007, December 31, 2009, May 14, 2010, June 4, 2010, July 

30, 2010, November 23, 2010, and December 8, 2010. The submittals are included as 

“TriState BART Submittals”. 

II. Source Emissions

Tri-State estimated that a realistic depiction of anticipated annual emissions for Units 1 

and 2, or “Baseline” Emissions”, to be conservative, was the average of two previous 

(2004, 2005) of emissions data in the July 31, 2006 analysis. Several years have 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. These emissions are summarized in Table 2. 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – TriState Craig Station Page 3 

Table 2: Tri-State Craig Units 1 and 2 Baseline Emissions 

Pollutant 

Unit 1 Unit 2 

Annual Emissions* 

(tpy) 

Average Emissions** 

(lb/MMBtu) 

Annual Emissions* 

(tpy) 

Average Emissions** 

(lb/MMBtu) 

NOx 5,190 0.278 5,372 0.271 

SO2 970 0.052 982 0.050 

PM10 80 0.006*** 40 0.005*** 

*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 factor is determined from the most recent Title V permit compliance stack tests 

(January 2004). 

III. Units Evaluated for Control

Tri-State notes that the Craig boilers burn Colorado coal that primarily comes from the 

Trapper mine, supplemented by ColoWyo coal, which are both high-ranking subbituminous coal. Limited amounts of coal from the Twentymile mine, ranked as 

bituminous, are also burned. All of these mines are located in northwestern Colorado. 

The Trapper contract expires in 2014. Future nearby coal supplies could come from 

sources such as Trapper, ColoWyo, or Twentymile. Accordingly, the trend of future coal 

supplies is such that in the context of NOx-forming characteristics, Craig 1&2 will 

continue to burn “bituminous-like” coal, plus, it is likely that additional quantities of 

bituminous coals will be burned at Craig 1&2 in the future. Similar to PSCo, Tri-State 

notes that these coals are ranked as sub-bituminous, but are closer in characteristics to 

bituminous coal in many of the parameters influencing NOx formation. The 

specifications for these coals are listed below in Table 3. Note that with the exception of 

moisture content, the coal characteristics are reasonably close for the two coals. 

Table 3: Craig Station Coal Specifications (2008) 

Coal Mine/Region Colowyo Trapper Twentymile 

Coal Rank Classification Sub-bituminous, Class A Sub-bituminous, Class A Bituminous 

H2O (Moisture %) 17.42 16.7 9.62 

Ash (%) 5.71 6.5 11.93 

Sulfur (%) 0.37 0.44 0.52 

Nitrogen (%) 1.35 ~1.5 1.57 

Heating Value (HHV Btu/lb) 10,392 9,800 11,084 

Uncontrolled emission factors are outlined in Table 4. The factors are based on firing 

bituminous coal as well as the highest ash and sulfur content from the two coals for 

conservative estimates. 

Table 4: Uncontrolled emission factors for Craig BART-eligible sources1

 Pollutant (lb/ton)* 

Emission Unit NOx SO2 PM 

(filterable) 

PM10

(filterable) 

Unit 1 12 16.9 73.9 17.0 

 

1

 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 – TriState Craig Station Page 4 

Unit 2 12 16.1 71.1 16.4 

*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, averaged from APEN data (2006 – 2008). 

Please refer to “Craig APCD Technical Analysis” for more details. 

IV. BART Evaluation of Units 1 and 2

A. Sulfur Dioxide (SO2)

Step 1: Identify All Available Technologies

Wet FGD Upgrades – As discussed in EPA’s BART Guidelines2

, electric generating units 

(EGUs) with existing controls achieving removal efficiencies of greater than 50 percent are not 

required to remove these controls and replace them with new controls. The Division interprets 

this to include fuel switching to natural gas, which would require significant boiler 

modifications, including removing the wet FGD. 

However, based on Appendix Y [70 FR 39171], the following dry scrubber upgrades should be 

considered for Craig Units 1 and 2 if technically feasible. These upgrades include: 

-Elimination of bypass reheat 

-Installation of liquid distribution rings 

-Installation of perforated trays 

-Use of organic acid additives 

-Improve or upgrade scrubber auxiliary equipment 

-Redesign spray header or nozzle configuration 

The current Operating Permit limits are depicted in Table 5. 

Table 5: Craig Units 1 & 2 SO2 Operating Permit Limits 

SO2 limits (lb/MMBtu) Reduction (%) Required

3-hr rolling 30-day rolling 90-day rolling 90-day rolling

Units 1 & 2 1.2 0.160 0.130 90

The current Operating Permit also requires that 100% of the flue gas in the FGD be treated 

(Conditions 1.3.3 and 2.3.3) and that the Craig Unit 1 and 2 FGDs be designed to meet at least a 

97.3% removal rate (Conditions 1.3.4 and 2.3.4). 

Step 2: Eliminate Technically Infeasible Options

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

common dry FGD system is a lime spray dry absorber uses that 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 

 

2

 EPA, 2005. Federal Register, 40 CFR Part 51. Regional Haze Regulations and Guidelines for Best Available 

Retrofit Technology (BART) Determinations: Final Rule. Pgs. 39133. 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – TriState Craig Station Page 5 

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

result 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. Colorado Ute Electric Association, which owned Craig before 

TriState, installed wet limestone FGD systems, on Craig Units 1 and 2 when the units began 

operations in 1980 and 1979, respectively. TriState upgraded these FGD systems in the 2003 – 

2004 timeframe. This system exceeds EPA’s presumptive limits stated in 40 CFR Part 51 

Appendix Y of 0.15 lb/MMBtu. 

At the Division’s request, TriState submitted a SO2 upgrade analysis to the Division on June 4, 

2010 regarding potential upgrades for the wet FGD systems at Craig Station Units 1 and 2. 

TriState examined potential upgrades to the Craig wet FGD systems, with the following results: 

-Elimination of bypass reheat: The FGD system bypass was redesigned to eliminate bypass of 

the FGD system except for boiler safety situations. After the Yampa Environmental Project 

(YEP) Upgrades (2003 – 2004), 100 percent of the flue gas now passes through the scrubber 

with no reheat and no bypassing. 

-Installation of liquid distribution rings: Liquid distribution rings were not installed during the 

YEP; however, TriState determined that installation of perforated trays, described below, 

accomplished the same objective. 

-Installation of perforated trays: Upgrades during the YEP included installation of a perforated 

plate tray in each scrubber module. The trays improve the absorption of SO2 by increasing the 

contact between the flue gas and the limestone slurry. The trays also function like Slurry 

Distribution Rings by redirecting slurry from running down the absorber wall back to the flue 

gas flow stream. 

-Use of organic acid additives: Organic acid additives such as Dibasic Acid (DBA) can be used 

to improve SO2 removal efficiency by increasing scrubbing liquor alkalinity. This option was 

considered for Craig Units 1 and 2 during YEP; however, it was not selected for the following 

reasons: 

1. DBA has not been tested at the very low inlet SO2 concentrations seen at Craig Units 1 and 2. 

2. DBA could cause changes in sulfite oxidation with impacts on SO2 removal and solids 

settling and dewatering characteristics. 

3. Installation of the perforated plate tray accomplished the same objective of increased SO2

removal. 

-Improve or upgrade scrubber auxiliary equipment: YEP included installation of the following 

upgrades on limestone processing and scrubber modules on Craig 1 and 2: 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – TriState Craig Station Page 6 

1. Two vertical ball mills were installed for additional limestone processing capability for 

increased SO2 removal. The two grinding circuit trains were redesigned to position the existing 

horizontal ball mills and the vertical ball mills in series to accommodate the increased quantity of 

limestone required for increased removal rates. The two mills in series also were designed to 

maintain the fine particle size (95% <325 mesh or 44 microns) required for high SO2 removal 

rates. 

2. Forced oxidation within the SO2 removal system was thought necessary to accommodate 

increased removal rates and maintain the dewatering characteristics of the limestone slurry. 

Operation, performance, and maintenance of the gypsum dewatering equipment are more reliable 

with consistent slurry oxidation. 

3. A ventilation system was installed for each reaction tank. 

4. A new mist eliminator wash system was installed due to the increased gas flow through the 

absorbers since flue gas bypass was eliminated, which increased demand on the mist eliminator 

system. A complete redesign and replacement of the mist eliminator system including new pads 

and wash system improved the reliability of the individual modules by minimizing down time for 

washing deposits out of the pads. 

5. TriState installed new module outlet isolation damper blades. The new blades, made of a 

corrosion-resistant nickel alloy, allow for safer entry into the non-operating module for 

maintenance activities. 

6. Various dewatering upgrades were completed. Dewatering the gypsum slurry waste is done 

to minimize the water content in waste solids prior to placements of the solids in reclamation 

areas at the Trapper Mine. The gypsum solids are mixed or layered with ash and used for fill 

during mine reclamation at Trapper Mine. The installed system was designed for the increased 

capacity required for increased SO2 removal. New hydrocyclones and vacuum drums were 

installed as well as a new conveyor and stack out system for solid waste disposal. 

7. Instrumentation and controls were modified to support all of the new equipment. 

-Redesign spray header or nozzle configuration: The slurry spray distribution was modified 

during YEP. The modified slurry spray distribution system improved slurry spray 

characteristics and was designed to minimize pluggage in the piping. 

Therefore, TriState and the Division concur that there are not any technically feasible upgrade 

options for Craig Station Units 1 and 2. However, the Division has evaluated the option of 

tightening the SO2 emission limit for Craig Units 1 and 2. 

Step 3: Evaluate Control Effectiveness of Each Remaining Technology

The control effectiveness of tightening the 30-day rolling emission limits on Craig Units 1 and 2 

have been evaluated by the Division. The Division analyzed the baseline period (2006 – 2008) 

to determine the maximum and average 30-day rolling emission rates, shown in Table 6, to 

determine potential control effectiveness, if any. This information allows the Division to set a 

more relevant emission limit for Craig Units 1 and 2 using representative actual emissions. 

Table 6: Craig Units 1 & 2 30-day rolling emission rates (baseline 2006 - 2008) 

Unit Maximum 30-day rolling emission rate 

(lb/MMBtu) 

Average 30-day rolling emission rate 

(lb/MMBtu) 

Craig Unit 0.081 0.052 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – TriState Craig Station Page 7 

1 

Craig Unit 

2 

0.093 0.079 

Step 4: Evaluate Impacts and Document Results

Since there are not any remaining control technologies available for Craig Station Units 1 and 2, 

there are not any impacts to evaluate or results to document. 

Step 5: Evaluate Visibility Results

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

emission limit tightening. 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 7 shows the number of days pre- and postcontrol. Table 8 depicts the visibility results (98th percentile impact and improvements). Cost 

effectiveness in $/deciview was not determined since there will minimal, if any, costs associated 

with emission limit tightening. 

Per the April 2010 modeling protocol3

, 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 both boilers with NOx emissions at 0.07 lb/MMBtu (SCR control) and SO2

emissions at 0.10 lb/MMBtu (wet FGD). 

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

SO2 

Control 

Scenario 

Boiler(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-

hour 

1 0.166 Mt. Zirkel 

Wilderness 207 --- --- 123 --- --- 

2 0.161 

 

3

 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 – TriState Craig Station Page 8 

Wet 

FGD 

1 0.150 207 206 1 123 123 0 

2 0.150 207 207 0 123 123 0 

Wet 

FGD 

1 0.120 207 204 3 123 123 0 

2 0.120 207 204 3 123 123 0 

Wet 

FGD 

1 0.110* n/a 

2 0.110* n/a 

Wet 

FGD 

1 0.100 207 203 4 123 123 0 

2 0.100 207 203 4 123 123 0 

Wet 

FGD 

1 0.070 207 202 5 123 122 1 

2 0.070 207 203 4 123 122 1 

Combo 

1 0.100 

207 57 150 123 12 111 

2 0.100 

* Denotes that output was interpolated by the Division and is not an actual modeled output. See “Craig BART 

Modeling Summary” for more details. 

Table 8: 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 

(dv) (∆ dv) (%) 

Max 24-

hour 

1 0.166 

3.73 --- --- 

2 0.161 

Wet FGD 

1 0.150 3.72 0.01 0% 

2 0.150 3.72 0.01 0% 

Wet FGD 

1 0.120 3.70 0.02 1% 

2 0.120 3.71 0.02 1% 

Wet FGD 

1 0.110* 3.70 0.03 1% 

2 0.110* 3.70 0.03 1% 

Wet FGD 

1 0.100 3.69 0.03 1% 

2 0.100 3.70 0.03 1% 

Wet FGD 

1 0.070 3.68 0.05 1% 

2 0.070 3.68 0.05 1% 

Combo 

1 0.070 

1.17 2.56 69% 

2 0.070 

* Denotes that output was interpolated by the Division and is not an actual modeled output. See “Craig BART 

Modeling Summary” for more details. 

Step 6: Select BART Control
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – TriState Craig Station Page 9 

There are no technically feasible upgrade options for Craig Station Units 1 and 2. However, the 

state evaluated the option of tightening the emission limit for Craig Units 1 and 2 and determined 

that a more stringent 30-day rolling SO2 limit of 0.11 lbs/MMBtu represents an appropriate level 

of emissions control for this wet FGD control technology. The tighter emission limits are 

achievable without additional capital investment. An SO2 limit lower than 0.11 lbs/MMBtu 

would likely require additional capital expenditure and is not reasonable for the small 

incremental visibility improvement of 0.02 deciview. 

B. Filterable Particulate Matter (PM10) 

Craig Units 1 and 2 are each equipped with pulse jet fabric filter (PJFF) 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. 

Table 9 shows the most recent stack test data (2004). Real-time data demonstrates that these 

baghouses are meeting >95% control. The Title V permit limit is 0.03 lb/MMBtu (Condition 

1.1.3). The most recent stack test data is used to determine compliance with the permit limit, 

which at a minimum, occurs every five years, and more frequently depending on the results. 

Table 9: Craig Units 1 and 2 Stack Test Results (2004) 

Pollutant Unit 1 (lb/MMBtu) Unit 2 (lb/MMBtu) 

Filterable PM10 0.006 0.005 

PM10 Control efficiency 99.23% 99.35% 

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 

above stack test results are well below the range of recent BACT determinations. Refer to 

“Division RBLC Analysis” for more details regarding BACT determinations. Both boilers must 

meet the PM emission standard of 0.03 lb/MMBtu in accordance with the Long-Term Strategy 

Review and Revision of Colorado’s SIP for Class I Visibility Protection Part I: Craig Station 

Units 1 and 2 Requirements (4/19/01), as approved by EPA at 66 FR 35374 (07/05/01). 

The Division has determined that the existing Unit 1 and 2 pulse jet fabric filter baghouses and 

the emission limit of 0.03 lb/MMBtu (PM/PM10) represents the most stringent control option. 

The units are exceeding a PM control efficiency of 95%, and the control technology and 

emission limits are BART for PM/PM10.

C. Nitrogen Oxide (NOx) 

Step 1: Identify All Available Technologies
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – TriState Craig Station Page 10 

TriState identified five options for NOx control: 

New/modified Low NOx Burners (LNBs) with Overfire Air (OFA) system (next generation) 

Advanced OFA system or Rotating overfire Air (ROFA) 

Neural network system combustion controls 

Selective Non-Catalytic Reduction (SNCR) 

Selective Catalytic Reduction (SCR) 

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

units: 

Electro-Catalytic Oxidation (ECO)® 

Rich Reagent Injection (RRI) 

Coal reburn +SNCR 

Craig Units 1 and 2 currently have ultra-low NOx burners with over-fire air (ULNBs+OFA) 

installed (2004) for NOx control purposes. 

Step 2: Eliminate Technically Infeasible Options

LNBs with OFA Upgrades: TriState contracted with ACT to modify the existing Craig 1&2 

burners and upgrade the OFA system. ACT determined that burners and OFA system could be 

upgraded. However, ACT has not modified ultra low-NOx Babcock & Wilcox 4Z burners such 

as those in use at Craig Units 1 and 2. In addition ACT stated that a complete plant inspection, 

data review, baseline testing, and computational fluid dynamics (CFD) modeling would be 

required for them to guarantee performance predictions. An amended proposal was submitted by 

ACT upon receipt of updated coal analyses that more closely represent the quality of coal being 

burned at Craig 1&2. In their amended proposal, ACT again reiterated that “to give a guaranteed 

NOx reduction, a lot more information is required.” LNBs modifications with OFA upgrades 

appear to be technically feasible for Craig Units 1 and 2. 

Advanced OFA system – rotating overfire air system (ROFA): ROFA® injects air into the 

furnace first to break up the fireball and then to create a cyclonic gas flow to improve 

combustion. ROFA® differs from OFA in that ROFA® utilizes a booster fan to increase the 

velocity of air to promote mixing and to increase the retention time in the furnace. To date, 

ROFA® has only been installed as a retrofit technology on units firing eastern bituminous coals. 

TriState contacted Motobec, the manufacturer of ROFA® technology, to determine if ROFA is 

feasible for Craig Units 1 and 2. Mobotec could not give TriState a definitive guarantee for 

reductions due to the variability in the quality of coals. 

Based on data published by the manufacturer, ROFA® technology has been reported as 

achieving NOx emission reductions from 45 to 65 % based on fuel load4

. While ROFA is 

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

expected to increase emissions reductions for Craig Units 1 and 2. The Division asserts that 

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

LNB+OFA baseline for these units, ROFA® technology is not considered further in this 

analysis. 

 

4

 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 – TriState Craig Station Page 11 

Neural network system combustion controls: TriState received a neural network proposal from 

NeuCo in April 2006. The proposal offers to enhance the existing Craig 1&2 control system by 

providing combustion optimization technology. For a given set of objectives, a neural network 

directs the unit’s distributive control system (DCS) or other control systems to optimize the 

boiler performance. 

Based on review of the Craig 1&2 current operations, NeuCo stated that Craig 1&2 appear to be 

good candidates for the optimization system. Key aspects to neural network success are the 

training support provided by the supplier, as well as achieving buy-in from plant operators. 

TriState states that it is important to note that the condition of the unit(s) and the manner in 

which the unit(s) is operated prior to the installation of the combustion optimization system also 

play an important role in determining potential NOx reductions. Neural network system 

combustion controls appear to be technically feasible for Craig Units 1 and 2. 

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. This 20-40% range includes units operating with LNB/combustion 

modifications. 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. SCNR is considered a 

technically feasible alternative for Craig Units 1 and 2. Tri-State conducted a site-specific 

SNCR study in October and November 2010. The Division received a summary of results on 

November 23, 2010 and the raw data on December 8, 2010. 

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

applications in new coal units, for Craig, two retrofit SCR systems, the 0.07 lb/MMBtu 

controlled NOx value is more expected, although TriState asserts that the units cannot achieve 

below 0.08 lb/MMBtu. See “TriState BART Submittals” for more details. The SCR reaction 

occurs within the temperature range of 550°F to 850°F where the extremes are highly dependent 

on the fuel quality. SCR is a technically feasible alternative for Craig Units 1 and 2. 

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

Best Available Retrofit Technology (BART) Analysis – TriState Craig Station Page 12 

been demonstrated on a full-size pulverized coal-fired boiler5

 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 boilers6

 and has not been demonstrated for other types of 

units. Therefore, RRI is considered technically infeasible for Units 1 and 2. 

LNB/SOFA/LNB+SOFA: Craig Units 1 and 2 are already equipped with ultra-low NOx burners 

with over-fire air (ULNB+OFA) as part of a consent decree. Requirements for these control 

systems were adopted into revisions to Colorado’s Visibility SIP, specified in a document 

entitled “Long-Term Strategy Review and Revision of Colorado’s State Implementation Plan for 

Class I Visibility Protection Part I: Craig Station Units 1 and 2 Requirements,” dated April 19, 

2001. Table 1 illustrates that these systems achieve 39.7% and 41.1% NOx reductions (based on 

actual emissions) on Units 1 and 2, respectively. 

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/MMBtu7

. 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

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

and other state BART proposals,8 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 Craig Units 1 and 2 to determine 

control efficiencies and annual reductions. 

LNBs with OFA Upgrades: TriState noted in the original BART submittal (July 31, 2006) that 

ACT proposed that a modified LNB with upgraded OFA system could achieve 10 – 15% NOx

reduction above current levels. Tri-State submitted additional information regarding combustion 

control refinement, which the Division assumes is upgrades of the existing ULNBs, on 

December 8, 2010. These control refinements consist mostly of more precise control of fuel and 

air for combustion. This study conducted by Black & Veatch (B&V) notes that these 

refinements could achieve approximately 0- 2 % control. B&V explains that the reduction in 

 

5

 Powerspan ECO®: Overview and Advantages, 2000 – 2010. http://www.powerspan.com/ECO_overview.aspx 6

 Fuel Tech: Air Pollution Control – Rich Reagent Injection (RRI), 1998 – 2009. http://www.ftek.com/apcRRI.php 7

 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 8

 State of North Dakota BART Determination for Leland Olds Station Units 1 and 2. Page 16. 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – TriState Craig Station Page 13 

control efficiency is due to the difference between “design criteria” versus permit limit. The 

Division notes that the Craig units already have ultra-low NOx burners (ULNBs) installed, and as 

there is very little to no information on improvements to ULNBs, the Division accepts the 

amended B&V study for combustion control refinements from December 8, 2010. 

 

Neural network system combustion controls: TriState noted in the original BART submittal (July 

31, 2006) that NeuCo provided a neural network proposal projecting that an optimization system 

could achieve 5 – 15% NOx reductions. Tri-State submitted additional information regarding 

neural network (NN) system combustion controls on December 8, 2010. This study, conducted 

by Black & Veatch (B&V), notes that the NN equipment will be minimal, consisting of a few 

computer servers that will interface with existing systems in the same location(s). NN system 

combustion controls could achieve approximately 0 – 5% control. B&V explains that the 

reduction in control efficiency is due to the difference between “design criteria” versus permit 

limit. The Division notes that although limited information is available regarding NN systems, 

this information is very specific to individual units and is still considered emerging by industry 

standards. Therefore, the Division accepts the amended B&V study control efficiency for NN 

system controls submitted on December 8, 2010. 

SNCR: TriState stated in the May 14, 2010 submittal that based on the boiler configuration, 

TriState could expect a continuous NOx reduction performance with SNCR technology in the 

range of 10 – 15%. This is based on TriState’s extensive research into the application of SNCR 

technology at Craig Station. The vast majority of the research was focused on system 

performance and impacts on plant performance. TriState staff conducted a visit to First Energy’s 

Eastlake and Sammis power plants in Ohio; this visit was specifically design to evaluate boiler 

designs due to the similarity in boiler/burner configurations similar to the Craig Station boilers. 

These estimates are lower than EPA’s SNCR Air Pollution Control Technology Fact Sheet, 

which estimates SNCR between 30 – 50% control. Other Colorado facilities estimated SNCR as 

achieving between 17 – 40% NOx control. Tri-State conducted a site-specific SNCR study in 

October and November 2010. The Division received a summary of results on November 23, 

2010 and the raw data on December 8, 2010. The results of this study varied significantly 

depending on what coal type was utilized and were applicable for Craig Unit 1. Control 

effectiveness has been historically noted to be lower for wall fired boilers similar to the Craig 

boilers; therefore the Divisions considers approximately 15% to be a reasonable control 

effectiveness for SNCR. 

SCR: TriState stated in the May 14, 2010 submittal the expected emission rates for Craig Units 1 

and 2 when applying SCR are 0.08 lb/MMBtu. TriState did not specify if this estimate was a 30-

day rolling averages, although, as stated in the December 31, 2009 submittal, the baselines are 

averages of 30-day averages. The Division notes that several other Colorado facilities have 

noted SCR expectations of 0.070 lb/MMBtu9

or even lower. Additionally, a recent AWMA study 

found similar-sized EGUs achieve NOx reduction efficiencies greater than 85% with emission 

 

9

 Public Service Company of Colorado (April 20, 2010), Colorado Energy Nations Company (November 12, 2009), 

Colorado Springs Utilities (February 20, 2009), and Platte River Power Authority (January 22, 2009) all note that 

their individual EGUs can achieve 0.070 lb/MMBtu or even lower on a 30-day rolling average basis. 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – TriState Craig Station Page 14 

rates between 0.04 and 0.07 lb/MMBtu (during the ozone season).10 EPA’s AP-42 emission 

factor tables estimate SCR as achieving 75 – 85% NOx emission reductions. However, an 

appropriate margin of error must be applied when evaluating SCR. The design goal emission 

rate may be lower than the permitted limit to ensure that unnecessary non-compliance periods do 

not become an issue, The Division may evaluate tighter emission limits in future RH planning 

periods if SCR is determined to be BART for either Craig Unit 1 or 2. At this time, the Division 

accepts Tri-State’s estimates of 0.08 lb/MMBtu on a 30-day rolling average. Table 10 depicts a 

comparison of SCR control efficiencies. The Division adjusted TriState’s estimate to 0.07 

lb/MMBtu based on the reasoning above. 

Table 10: SCR Control Efficiency Comparison 

Unit Baseline 

(lb/MMBtu)

Control Efficiency (%) Resultant Emissions (lb/MMBtu)

TriState 

Estimate

Division 

Estimate

TriState Estimate 

(annual average)

Division Estimate 

(annual average)

Craig 

Unit 1

0.278 71.4 74.9 0.080 0.070

Craig 

Unit 2

0.271 70.5 74.0 0.080 0.070

Table 11 summarizes each available technology and technical feasibility for NOx control. 

Table 11: Craig Units 1 and 2 NOx Technology Options and Technical Feasibility 

Technology Emission 

Reduction 

Potential (%)

Technically Feasible? 

(Y = yes, N = no)

Low NOx Burners/Ultra-low 

NOx burners (LNB/ULNB)

10-30% Y – installed 

LNB + OFA 25-45% Y – installed

Air Staging – overfire air 

(OFA)

5-40% Y – installed

Ultra-Low NOx Burner 

(ULNB) 

Upgrade/Refinements

0 – 2% 

(TriState)

Y

Neural network system 0 – 5% 

(TriState)

Y 

SNCR ~15% Y

Rotating overfire air (ROFA) 45 – 65% N

SCR 75 – 90% Y

Electro-Catalytic Oxidation 

(ECO)®

n/a N

Rich Reagent Injection (RRI) n/a N

Coal reburn+SNCR n/a N

Step 4: Evaluate Impacts and Document Results

Cost of Compliance

 

10 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 – TriState Craig Station Page 15 

Low NOx burner upgrades: Tri-State submitted additional information regarding combustion 

control refinement, which the Division assumes is upgrades of the existing ULNBs, on 

December 8, 2010. Through a literature review, the Division could not find any examples or 

support for upgrades on ultra-low NOx burners with overfire air. Ultra-low NOx burners are 

fairly new within the industry, so additional upgrades have not yet been researched. The first 

commercial application for these burners was documented in May 2000.11 Tri-State estimates 

that the initial cost of combustion control refinement at about $2,200,000 with an annualized 20-

year cost of $122,000. The Division notes that the Craig units already have ultra-low NOx

burners (ULNBs) installed, and as there is very little to no information on improvements to 

ULNBs, the Division accepts the amended B&V study for combustion control refinement cost 

estimates from December 8, 2010. 

Neural network system: TriState did not provide a quantitative evaluation of the application of a 

neural network system to the Division. There are three other facilities in Colorado alone using 

neural network systems from the same provider that TriState contacted.12 It is unknown why 

TriState will provide further analysis of this system. Costs for these systems are very specific to 

individual units, so the Division cannot estimate costs for this option. Tri-State submitted 

additional information regarding neural network (NN) system combustion controls on December 

8, 2010. Tri-State estimates that the initial cost of neural network systems (per unit) at about 

$800,000 with an annualized 20-year cost of $280,000. The Division notes that although 

limited information is available regarding NN systems, this information is very specific to 

individual units and is still considered emerging by industry standards. Therefore, the Division 

accepts the amended B&V study cost estimates for NN system controls submitted on December 

8, 2010. 

SNCR: A typical breakdown of annualized costs for SNCR on industrial boilers will be 15 – 25% 

for capital recovery and 65 – 85% for operating expenses.13 The TriState-estimated SNCR costs 

for operating expenses are 67% for Craig Units 1 and 2 (individually). 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.14 

 

The cost effectiveness for SNCR on Units 1 and 2 (at 15% control efficiency) is approximately 

$4,877 and $4,712 per ton, respectively. Recent NESCAUM studies estimate SNCR retrofits on 

wall fired boilers (similar to Units 1 and 2) achieving 0.50 – 0.65 lb/MMBtu and emission 

reductions of 30 – 50% as costing $590 - $1,100 per ton of NOx reduced, depending on initial 

 

11 Bryk and Kleisley, 2000. “First Commercial Application of DRB-4ZTM Ultra-Low NOx Coal-Fired Burner.” 

Presented to POWER-GEN International 2000. November 14-16, 2000. Orlando, Florida. 

12 NeuCo White Papers and Case Studies. http://www.neuco.net/library/case-studies/default.cfm and Platte River 

Power Authority January 22, 2009 submittal: “Rawhide Unit 101 NOx Emission Control Cost and Technical 

Feasibility Information.” 

13 ICAC, 2000. Institute of Clean Air Companies, Inc. “White Paper: Selective Non-Catalytic Reduction (SNCR) 

for Controlling NOx Emissions.” Washington, D.C. 2000. 14 EPA, 2003. “SNCR Air Pollution Control Technology Fact Sheet.” http://www.epa.gov/ttn/catc/dir1/fsncr.pdf 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – TriState Craig Station Page 16 

capital costs and capacity factor. 15,16 It should be noted that TriState is estimating resultant 

emission rates lower than 0.30 lb/MMBtu for both boilers, therefore costs will be higher. EPA’s 

SNCR Fact Sheet cites SNCR as costing from $400 - $2,500 per ton of NOx reduced.17 On a 

linear scale, based on the NESCAUM estimates and assuming an achieved rate of 0.23 

lb/MMBtu, the costs should be approximately $2,500 per ton. TriState and the Division’s 

revised estimates are above this range; the Division has inquired about the reagent and auxiliary 

power costs, but has not received feedback from TriState. The costs for these two items are 

higher than other Colorado facility estimates. Additionally, similar Colorado facility cost 

estimates fall within the EPA SNCR Fact Sheet range. The Division accepts TriState’s capital 

and operation/maintenance costs for this analysis.. 

SCR: Recent NESCAUM studies estimate SCR retrofits on wall fired boilers achieving NOx

emission rates of 0.15 – 0.25 lb/MMBtu and emission reductions of 75 – 85% as costing $1,700 - 

$3,200 per ton of NOx reduced, depending on initial capital costs and capacity factor.18,19 20,21 It 

should be noted that TriState is estimating resultant emission rates lower than 0.15 lb/MMBtu for 

both boilers, therefore costs will be higher. TriState’s estimates are above this range; on a linear 

scale (achieving 0.07 lb/MMBtu); the costs should be approximately $7,000 per ton. The 

Division’s revised cost estimates are close to this estimate; therefore, the Division concludes that 

these cost estimates are reasonable. 

Table 12, Table 13, Table 14, and Table 15 depict controlled NOx emissions and control cost 

comparisons.

Table 12: Craig Unit 1 Control Resultant NOx Emissions 

Alternative Control 

Efficiency (%)

Resultant Emissions 

Annual Emissions 

(tons/year) 

Annual Average 

(lb/MMBtu) 

30-day 

Rolling Average 

(lb/MMBtu) 

Baseline --- 5,190 0.278 

Combustion control 

refinements 2 5,087 0.273 0.31 

Neural network 

system 5 4,931 0.264 0.30 

 

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

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

16 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. 

17 EPA, 2003. “SNCR Air Pollution Control Technology Fact Sheet.” http://www.epa.gov/ttn/catc/dir1/fsncr.pdf 18 Neuffer, Bill – ESD/OAQPS, 2003. “NOx Controls for Existing Utility Boilers.” 

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

19 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. 

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

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

21 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 – TriState Craig Station Page 17 

SNCR 15 4,412 0.236 0.27 

SCR 74.9 1,305 0.070 0.08 

Table 13: Craig Unit 2 Control Resultant NOx Emissions 

Alternative Control 

Efficiency (%)

Resultant Emissions 

Annual Emissions 

(tons/year) 

Annual Average 

(lb/MMBtu) 

30-day 

Rolling Average 

(lb/MMBtu) 

Baseline --- 5,372 0.271

Combustion control 

refinements 

2 5,264 0.265 0.31 

Neural network 

system 

5 0.257 0.30 

SNCR 15 4,566 0.230 0.27 

SCR 74 1,397 0.070 0.07 

Table 14: Craig Unit 1 NOx Cost Comparisons 

Alternative Emissions 

Reduction (tpy)

Annualized 

Cost ($)

Cost Effectiveness 

($/ton)

Incremental Cost 

($/ton)

Baseline 0 $0 $0 --- 

Combustion control 

refinements 104 $122,000 $1,175 $1,175 

Neural network 

system 260 $280,000 $1,079 $1,015 

SNCR 779 $3,797,000 $4,877 $6,776 

SCR 3,893 $25,036,709 $6,432 $6,708 

Table 15: Craig Unit 2NOx Cost Comparisons 

Alternative Emissions 

Reduction (tpy)

Annualized 

Cost ($)

Cost Effectiveness 

($/ton)

Incremental Cost 

($/ton)

Baseline 0 $0 $0 --- 

Combustion control 

refinements 

107 $122,000 $1,136 $1,136 

Neural network 

system 

269 $280,000 $1,043 $980 

SNCR 806 $3,797,000 $4,712 $4,712 

SCR 3,975 $25,036,709 $6,299 $6,702 

Energy and Non-Air Quality Impacts

LNB Upgrades/Neural network system(s): There are no known non-air quality impacts 

associated with upgrades on low-NOx burner systems or neural network systems. Energy 

impacts are not significant. 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 – TriState Craig Station Page 18 

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. 

Post-combustion add-on control technologies such as SNCR do increase power needs 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. In particular, 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. 

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. 

Remaining Useful Life

TriState asserts that there are no near-term limitations on the useful of these boilers, so it can be 

assumed that they will remain in service for the 20-year amortization period. 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 16 shows the number of days pre- and 

post-control. Table 17 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 protocol22, 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 and other 

BART-eligible units are held constant at pre-control levels. For BART sources with more than 

 

22 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 – TriState Craig Station Page 19 

one BART unit, modeling the units individually would ignore important atmospheric chemical 

reactions that occur when units operate simultaneously. The combination scenario assumed both 

boilers with NOx emissions at 0.07 lb/MMBtu (SCR control) and SO2 emissions at 0.10 

lb/MMBtu (wet FGD control). 

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 16: 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 

1 0.352 

Mt. Zirkel 

Wilderness 

207 --- --- 123 --- --- 2 0.345 

SNCR 1 0.236 207 192 15 123 123 0 

2 0.230 207 194 13 123 123 0 

SCR 1 0.07 207 165 42 123 123 0 

2 0.07 207 166 41 123 123 0 

Combo 

1 0.07 

207 57 150 123 12 111 2 0.07 

Table 17: 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 

1 0.352 3.73 --- --- --- 

2 0.345 

SNCR 1 0.236 3.42 0.31 8% $12,327,922 

2 0.230 3.42 0.31 8% $12,327,922 

SCR 1 0.07 2.72 1.01 27% $24,887,384 

2 0.07 2.75 0.98 26% $25,652,365 

Combo 

1 0.07 

1.17 

2.56 69% 

$19,537,034 2 0.07 

 Step 6: Select BART Control
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – TriState Craig Station Page 20 

While potential modifications to the ULNB burners and a neural network system were also found 

to be technically feasible, these options did not provide the same level of reductions as SNCR or 

SCR, which are included within the ultimate BART Alternative determination for Units 1 and 2. 

Therefore, these options were not further considered in the technical analysis. 

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

NOx BART is SNCR controls at the following NOx emission rates: 

 Craig Unit 1: 0.27 lb/MMBtu (30-day rolling average) 

 Craig Unit 2: 0.27 lb/MMBtu (30-day rolling average) 

 For SNCR at Units 1 and 2, the cost per ton of emissions removed, 

coupled with the estimated visibility improvements gained, falls within the guidance criteria 

presented in Chapter 6 of the Regional Haze State Implementation Plan. 

• Unit 1: $4,877 per ton NOx removed; 0.31 deciview of improvement 

• Unit 2: $4,712 per ton NOx removed; 0.31 deciview of improvement 

The dollars per ton control costs, coupled with notable visibility improvements, leads the state to 

this determination. To the extent practicable, any technological application Tri-State utilizes to 

achieve these BART emission limits shall be installed, maintained, and operated in a manner 

consistent with good air pollution control practices for minimizing emissions. Although emission 

limits associated with SCR achieve better emissions reductions, the cost-effectiveness of SCR 

for this BART determination was determined to be excessive and above the cost guidance 

criteria presented above. The state reached this conclusion after considering the associated 

visibility improvement information and after considering the SCR cost information in the SIP 

materials and provided during the pre-hearing and hearing process by the company, parties to the 

hearing, and the FLMs. 

Per Section 308(e)(2) of EPA’s Regional Haze Rule, as an alternative to BART (or “BART 

alternative”) it was proposed and the state agreed to a more stringent NOx emissions control plan 

for these BART units that consists of emission limits assumed to be associated with the operation 

of SNCR for Unit 1 and the operation of SCR for Unit 2. These NOx emission rates are as 

follows: 

Craig Unit 1: 0.28 lb/MMBtu (30-day rolling average) 

Craig Unit 2: 0.08 lb/MMBtu (30-day rolling average) 

Unit 1’s 0.28 lb/MMBtu NOx emission rate equates to a 14% control and a NOx reduction of 

727 tons per year, which is slightly less than the 15% control and a NOx reduction of 779 tons 

per year associated with the 0.27 lb/MMBtu BART emission rate determination. 

Unit 2’s 0.08 lb/MMBtu NOx emission rate equates to a 74% control and a NOx reduction of 

3,975 tons per year, which is much greater than the 15% control and a NOx reduction of 806 tons 

per year associated with the 0.27 lb/MMBtu BART emission rate determination. 

The total NOx emission reduction resulting from the BART determination is 1,585 tons per year 

(779 + 806 = 1,585 tons per year). The total NOx emission reduction resulting from the BART 

Alternative is 4,702 tons per year (727 + 3,975 = 4,702 tons per year). Given the far greater 

emission reduction achieved by the BART Alternative when compared to the BART 

determinations for the individual units, the state determines, in accordance with the federal 

Regional Haze regulations, that the BART Alternative emission rates are appropriate for Craig 
Colorado Department of Public Health and Environment - Air Pollution Control Division

Best Available Retrofit Technology (BART) Analysis – TriState Craig Station Page 21 

Units 1 and 2 as providing greater reasonable progress than the application of BART as set forth 

in the federal BART Alternative regulation. 

The state also evaluated the NOx emission reduction associated with both units (Craig 1 & 2) in 

contrast to the existing NOx rates, presumptive BART NOx rate, source-by-source determination, 

and the final RH determination to determine the total NOx reduction benefit. In the below table, 

the existing NOx emissions from both units is 10,562 tons/year which is much lower than the 

existing presumptive BART emissions of 14,849 tons/year. The source-by-source BART 

determination resulted in NOx emissions of 8,978 tons/year which is well above the 5,860 

tons/year in NOx emissions calculated to result from application of the BART Alternative. These 

tons/year calculations provide an emissions based comparison to demonstrate that the Craig 

BART Alternative provides greater reasonable progress than, and is superior to, source by source 

BART for these units. The table below is illustrative for demonstration purposes only. The tons 

per year projections provide an emission based comparison and are not enforceable 

requirements.