EPA rebuts critics of haze rule findings for Big Stone coal unit

The U.S. Environmental Protection Agency said April 26 that it is granting final approval a revision to the South Dakota State Implementation Plan (SIP) addressing regional haze, with the agency also rebutting a number of criticisms over how emissions from the coal-fired Big Stone power plant were handled under that revision.

EPA has determined that the plan submitted by South Dakota satisfies the requirements of the Clean Air Act (CAA) and EPA’s rules that require states to prevent any future and remedy any existing man-made impairment of visibility in mandatory Class I areas caused by emissions of NOx, SO2 and particulate matter (PM). The final rule, for the first haze implementation period running through 2018, is effective May 29. The proposed rule was published in the Dec. 8, 2011, Federal Register, with the final approval posted in the April 26 edition of that publication.

South Dakota has one source, the coal-fired Big Stone Unit 1, which is subject to the best available retrofit technology (BART) requirements. The state has identified various BART requirements including emission limits for Big Stone Unit 1.

During the public comment period on the draft SIP approval, EPA received written comments from the state of South Dakota, CREDO Action, the National Parks Conservation Association (NPCA), the Sierra Club, and the National Park Service (NPS).

EPA noted that one unnamed commenter stated that South Dakota is not excused from following a reasonable analysis in evaluating BART and setting BART emission limits because Big Stone Unit 1 has a generating capacity less than a 750-MW threshold. Because South Dakota did not consider the capabilities of various pollution controls in its BART analysis for Big Stone I, its cost impact analysis is skewed in favor of low-cost equipment, and does not evaluate cost impacts in terms of pollution reduced, the commenter said.

“We agree with the commenter that the Regional Haze Rule requires states to consider the most stringent level of control,” EPA responded. “However, we disagree with the statements that South Dakota’s BART analysis is skewed in favor of low-cost equipment for Big Stone I, and that the analysis does not evaluate cost impacts in terms of pollution reduced. South Dakota did describe the range of control efficiencies possible for the various technically feasible control options in its BART determinations. While we acknowledge that South Dakota did not select the highest control efficiency option in every case (e.g., South Dakota selected semi-dry instead of wet flue gas desulfurization (FGD or ‘scrubber’ controls) for SO2 control), we find the state was reasonable in its selection of controls considering the five statutory factors and did not unreasonably reject any control options based on cost as further explained in our responses to other comments in this action.”

Two commenters stated that the SO2 emission limit for Big Stone I is too high as a result of the baseline emission rate used in the analysis. The commenters stated that Otter Tail Power, the operator of Big Stone, and the state both incorrectly assumed an uncontrolled SO2 emission rate of 0.86 lb/MMBtu for the Big Stone I BART determination. The Sierra Club recommended a baseline emission rate of 0.70 lb/MMBtu, which is the maximum annual average SO2 emission rate at Big Stone I over the last ten years, or at the very least recommends the highest 30-day average uncontrolled SO2 emission rate. The NPCA stated that it is unclear where the 0.86 lb/MMBtu baseline originates.

EPA said it found that the state did not underestimate the baseline emissions in its BART analysis. “Based on our review of all the information, we find that South Dakota acted reasonably in establishing the SO2 BART emission limit for Big Stone,” the agency added.

One commenter stated that it is unclear where the baseline rate of 0.86 lbs/MMBtu for NOX originated, because the thirty-day rolling values for NOX only reached 0.85 lbs/MMBtu during the baseline period. The commenter noted that the thirty-day rolling values for NOX have been at or below 0.71 lbs/MMBtu since 2007 because of the installation of overfire air. The commenter asserted that 0.71 lbs/MMBtu should therefore be the starting point for additional NOX reductions from selective catalytic reduction (SCR).

EPA said it found that the state did not underestimate the baseline emissions in its BART analysis.

One commenter stated that the NOX BART analysis at Big Stone I is flawed because it fails to consider the level of control available with SCR, resulting in an inflated NOX emission limit. The state’s proposed NOX emission rate of 0.10 lb/MMBtu reflects 85.9% NOX control with the installation of SCR based on emission data showing that the highest monthly emission rate of NOX in 2009 was 0.71 lb/MMBtu.

“Because the control efficiency of SCR is dependent on the NOX inlet concentration, it is more appropriate to assess the control effectiveness of SCR relative to the performance rate,” EPA responded. “Although we acknowledge that other SCR retrofits have resulted in lower NOX emission levels than 0.10 lb/MMBtu, we find that South Dakota’s limit is reasonable using SCR plus separated overfire air at Big Stone I.”

One commenter stated that the state’s proposed particulate matter (PM) BART emission limit of 0.012 lb/MMBtu is not reflective of the limits achievable by fabric filter baghouses, and is inconsistent with some lower PM limits required as BACT. The commenter cited a permit for a planned coal plant in Georgia, Plant Washington, with a PM limit of 0.010 lb/MMBtu to argue that Big Stone’s PM emission limit should be no higher than this level.

“As noted in the proposal, the 0.012 lb/MMBtu PM emission limit ‘represents a stringent level of control that is consistent with recent Best Available Control Technology determinations for PSD [prevention of significant deterioration] permits,’” EPA responded. “Also, performance test data for the baghouse indicates that the actual emission rate is 0.011 lb/MMBtu. Therefore, we find the emission limit set by South Dakota is commensurate with the actual performance of the control device. Moreover, there is no indication that a more stringent level of control would lead to meaningful visibility benefits.”

Otter Tail Power already seeking haze retrofit approvals

“The South Dakota Board of Minerals and Environment approved rules implementing the South Dakota Regional Haze State Implementation Plan (SIP) on September 15, 2010,” Otter Tail Power said on a special section of its website. “The rules and the South Dakota SIP will require that Big Stone Plant install and operate a new BART-compliant air quality control system (AQCS) to reduce emissions of particulate matter, sulfur dioxide, and nitrogen oxides. The new air quality control system must be installed within five years of the EPA’s approval of the South Dakota State Implementation Plan. We project that compliance with the new rule and the Regional Haze State Implementation Plan could be required as early as 2016.”

Capacity expansion modeling submitted as part of the company’s most recent integrated resource plan filing to the Minnesota Public Utilities Commission (PUC) demonstrates that installing new air quality control systems at Big Stone is part of a least-cost resource plan, the website added. Making environmental equipment upgrades at an existing plant, versus building a new generation resource, is less costly, takes advantage of existing transmission infrastructure, maintains (even adds to) job opportunities, and significantly reduces emissions from current levels.

In December 2011, the Minnesota PUC granted an advance determination of prudence for the costs of the air quality control system at Big Stone plant that are attributable to serving its Minnesota ratepayers. Otter Tail has also filed for project approvals in North Dakota and South Dakota.

Big Stone is a 475-MW plant that fires sub-bituminous coal that has been on-line since 1975. Otter Tail Power owns 53.9%, NorthWestern Energy owns 23.4% and Montana-Dakota Utilities owns 22.7%.

About Barry Cassell 20414 Articles
Barry Cassell is Chief Analyst for GenerationHub covering coal and emission controls issues, projects and policy. He has covered the coal and power generation industry for more than 24 years, beginning in November 2011 at GenerationHub and prior to that as editor of SNL Energy’s Coal Report. He was formerly with Coal Outlook for 15 years as the publication’s editor and contributing writer, and prior to that he was editor of Coal & Synfuels Technology and associate editor of The Energy Report. He has a bachelor’s degree from Central Michigan University.