BASIN ELECTRIC POWER COOPERATIVE

171 7 EAST INTERSTATE AVENUE BISMARCK. NORTH DAKOTA 58503-0564 PHONE 701 -223-0441 FAX: 7011224-5336

February 25,2009

Mr. David Finley, Administrator Air Quality Division WY Department of Environmental Quality 122 West 25th Street Herschler Building Cheyenne, WY 82002

Dear Mr. Finley,

The Department of Environmental Quality (DEQ) notified Basin Electric in June 2006 that the Laramie River Station (LRS) was a Best Available Retrofit Technology (BART) applicable source, which required a BART engineering and modeling analysis for reducing visibility impacts in accordance with the Environmental Protection Agency's Guidelines for BART Determinations under the Regional Haze Rules (40 CFR Part 51). Visibility impacts for LRS were evaluated at two Federal Class 

### I areas 

## -- Badlands National Park and Wind Cave National Park.

A BART review was required to identify the best retrofit technology for the reduction of nitrogen oxides (NOx), sulfur dioxide (SO2), and particulate matter (PM) emissions from Laramie River Station Units 1, 2 and 3. Basin Electric contracted Black & Veatch to conduct a BART analysis to identify technically feasible and cost-effective technologies following the BART Guidelines. A modeling analysis was completed to evaluate the impact on visibility in the two identified Class I areas. Reports and subsequent revisions of the BART analyses were submitted to your office on February 28, 2007; September 25, 2007; February 14, 2008; and July 24, 2008.

As a result of several discussions between Basin Electric and DEQ, Basin Electric proposes a BART limitation for NOx emissions on a plant-wide 30-day rolling average based on a pound-perhour limitation of 4,082 pounds per hour for the station.

We are proceeding with our plans to install over-fire air (OFA) on Unit 1 this spring, Unit 2 in the spring of 2010, and on Unit 3 in the spring of 201 1. In addition, we plan to upgrade to Low-NOx Burners (LNB) on Units 1, 2, and 3 in 2012, 2013, and 2014, respectively. While we anticipate emissions of 0.18 IbImmBtu under ideal operating conditions, normal emissions are likely to be higher. Please see the attached letter from our OFA contractor, Burns & McDonnell. A letter is also attached from Black 

### & Veatch, who performed our BART analysis, with their company's perspective
on NOx emission rates with LNBIOFA.

After Basin Electric and DEQ agree on a plan, we will present it to the Missouri Basin Power Project (MBPP) management and ask approval from the project owners.

A Touchstone Energym Cooperative 

# 63 

# - Equal Employment opporruniiy EmployerFebruary 25, 2009 Page 2

If you have any questions, please contact me at 701-557-5654 or Lyle Witham at 701-557-5652. Sincerely,

Robert 

### L. Eriksen, 

### P.E. Sr. Environmental Compliance Administrator


### /g mj Enclosures cc: Chad Schlichtemeier, 

### DEQ Mike Fluharty David Cummings Lyle WithamBLACK & VEATCH

MEMORANDUM

Basin Electric Power Co operative B&V Project 145423 Laramie River Station B&V File 30.0000 LNB and OFA Control Effectiveness February 24, 2009



### On September 27, 2007, Basin Electric Powe r Cooperative (BEPC) submitted the Best Available Retrofit Technology (BART) report for Laramie River Station (LRS), which identifies the control effectiveness for NOx after the retrofit of low NOx burners (LNB) and over-fire air (OFA) at the three LRS un its. The BART analysis was prepared by Black & Veatch (B&V) for BEPC.

LRS Units 1, 2, and 3 each have a 550 MW (net) capacity and are equipped with Babcock & Wilcox (B&W) sub-critical boiler and opposed-wall fired burners. In the BART report, LNB with OFA have been id entified as a technically available and applicable control technology for the reduction of NOx emi ssions from the LRS units.
The baseline NOx emission rate from the LRS units based on continuous emissions monitoring (CEM) annual emissions aver ages from 2001 to 2003 is 0.27 lb/MBtu.

In the BART report, the c ontrol effectiveness for LNB with OFA of 0.15 lb/MBtu was determined based on the results of a comput ational fluid dynamics (CFD) modeling study commissioned by BEPC and completed by a th ird party combustion modeling expert in 2004. The CFD model study simulated the eff ects of adding and optimizing OFA ports specifically for the LRS units.

Wall fired units are typically fitted with numerous burners mounted on the front and/or
rear wall of the furnace. Each burner is fed coal, primary air and secondary air to safely and efficiently burn the pulverized coal fed to that burner. Primary air is the carrier for the pulverized coal fed to that individual burne r and secondary air is typically fed to the periphery of the coal and primary air str eam to provide sufficient oxygen for complete
combustion of the coal.BLACK & VEATCH

MEMORANDUM Page 2

Basin Electric Power Co operative B&V Project 145423 Laramie River Station February 24, 2009 LNB and OFA Control Effectiveness



### To reduce the production of unintended byproducts of combustion such as NO x 

### , the temperature of combustion at the burner as well as the timing a nd mixing rate of the secondary air with the primary combus tion products are cont rolled to limit NO x 

### production. In addition to reducing the peak burner flame temperatures, the general philosophy for reducing the NO x 

### generated from a wall fired burner is to reduce the oxygen and nitrogen that is available in the highest temperature region of the burner flame to limit the rate the fuel bound nitrogen as well as the atmospheric nitrogen is
oxidized to form NO x 

### . Once this initial combustion has taken place at the burners, the balance of the combustion air is provided by over-fire air ports to enable complete combustion of the vaporized fuel components prior to being cooled by the radiant and
convective heat transfer su rfaces of the boiler. NO x 

### production can be minimized in the lower furnace area around the burners by limiting the total combustion air and maintaining a reducing atmosphere. This reduc ing atmosphere especially in the presence of sulfur in the flue gas stream can cause si gnificant corrosive attack of the furnace heat transfer surfaces. This potential for rapi d corrosion of the furnace walls, limits the staging of combustion in the lower furnace ar ea and the effectiveness of LNB to limit
NO x 

### production.

As well as the increased potential for fire-s ide corrosion of the furnace walls in the case of insufficient oxygen in the lower furnace, in a typical coal-fired boiler, there may be as many as 49 individual burners th at must operate in concert to provide for the combustion
of coal in this manner. To minimize NO x 

### formation, the quantities of coal, primary air and secondary air must be supplied in a uniform quantity to each burner. The performance of each individual burner must be maintained to minimize NO x 

### production to the lowest level. Due to the physical di fficulty of continuously providing this equal
balance of air and fuel as well as fuel fine ness to each burner, this theoretical peakBLACK & VEATCH

MEMORANDUM Page 4

Basin Electric Power Co operative B&V Project 145423 Laramie River Station February 24, 2009 LNB and OFA Control Effectiveness



# Wall Fired

Figure 2 - Controlled and Uncontrolled NOx Emissions from PC Wall-Fired Boilers



### In addition to optimizing the LNB-OFA to ensure complete burn-out of char, evaluation
of NOx control effectiveness resulting from LNB-OFA system installed in an opposedwall system needs to take into account seve ral key boiler and combustion characteristics: • Composition of fuel combusted • Ability of unit to monitor air and fuel flow (for combustion optimization) • Boiler physical design and arrangement
• Burner fuel and air balance
• Mill fineness • Boiler management system (e.g. neural networks) • Boiler cleaning technologies.BLACK & VEATCH

MEMORANDUM Page 5

Basin Electric Power Co operative B&V Project 145423 Laramie River Station February 24, 2009 LNB and OFA Control Effectiveness



### While the CFD model study of the burner with OFA at LRS units considered the items listed above, it assumes a steady-state conditio n with all these process variables that might impact the production of NOx from th e LRS units. At steady-state operation, and with operation to optimize all boiler and co mbustion characteristics to minimize the production of NOx, an outlet NOx of 0.15 lb/M Btu was determined to be achievable based on the design of the optimized OFA ports.

Therefore, this outlet NOx value is depe ndent on the LRS boilers/combustion process operating at a steady-state c ondition, while maintaining an even/optimum combustion air and fuel distribution, with little variation in the composition of the fuel and the fineness out of the coal mills. However, in actua l operating conditions, this optimized, steadystate condition can seldom be maintained. Th erefore, the controll ed outlet NOx of 0.15
lb/MBtu should be classified as a theoreti cal, lowest achievable outlet NOx when the operating conditions approach the optimum conditions used for the modeling inputs.

In addition to that, a review of the CFD m odel study recently identified an error with the use of sea-level conditions (for combustion air barometric pressure), when the LRS units
are located in an area with an elevation much higher than sea-level. This resulted in the CFD model study based on combustion air that is denser than actual. This impacts the
modeled NOx outlet of 0.15 lb/MBtu since denser combustion air is more favorable to minimize the production of NOx during combustion. The correction to the lower flue gas density also reduces the furnace residenc e time so will increase the potential for
excessive unburned char from combustion. To reduce the char and associated slagging, fouling and corrosion, the level of staging in th e lower furnace will be reduced as well as the quantity of over fire air fl ow. This will reduce the perf ormance of the LNB system.BLACK & VEATCH

MEMORANDUM Page 6

Basin Electric Power Co operative B&V Project 145423 Laramie River Station February 24, 2009 LNB and OFA Control Effectiveness



### The CFD model expert estimated that a 5% error was introduced into the CFD model study results.

Since the assumptions used to develop th e CFD model study was based on optimum and steady-state conditions, which cannot be mainta ined during normal operations of the LRS boilers, with the impacts of the error in troduced by the use of a wrong basis on the density of combustion air, the controlled NOx outlet after the retrofit of LNB-OFA is expected to be higher than the result of the original CFD model study of 0.15 lb/MBtu. For BART, the control effec tiveness for NOx should be based on a demonstrated and achievable level. Since BEPC is currently considering LNB with OFA retrofits at the LRS units, the most recent CFD combustion model study performed in 2008 will be more representative than the values reported in the BART analysis report submitted in 2007. It
was identified that the controlled outlet NOx from this recent model is 0.18 lb/MBtu with an accuracy of 20%. This range of accura cy is consistent with predicted model information when verified by an actual test from our experience. In addition to the results of this recent combustion model, an evaluation of reported CEM data from 2005
to 2006 indicated that several wall-fired bo ilers combusting subbituminous coal equipped with LNB-OFA demonstrated a 30-day rolling average NOx ranging from 0.18 to 0.21
lb/MBtu.