Coal-fired Merrimack plant emerges from regional haze unscathed

The regional haze plan proposed by the New Hampshire Department of Environmental Services (NH DES) as part of a revision of its state implementation plan (SIP) should have relatively little impact on the coal-fired Merrimack power plant of Public Service Co. of New Hampshire.

In the Feb. 28 Federal Register, the U.S. Environmental Protection Agency proposed approval of the SIP revision, with public comment being taken until March 29. The revision addresses regional haze for the first planning period from 2008 through 2018. This revision addresses the requirements of the Clean Air Act (CAA) and EPA’s rules that require states to prevent any future, and remedy any existing, manmade impairment of visibility in mandatory Class I areas such as national parks.

The Merrimack plant has two coal-fired steam-generating boilers. Only one of the boilers (MK2) is subject to Best Available Retrofit Technology (BART) rules under the regional haze program. MK2 is a wet bottom, cyclone-type boiler with an electrical output of 320 MW. The unit is currently equipped with selective catalytic reduction (SCR) for NOx control, and two electrostatic precipitators (ESPs) operated in series to capture particulate matter (PM) in the flue gases.

The existing ESPs at MK2 were previously upgraded to include state-of-the-art electronic controls. Adding a third ESP was found to be unreasonable due to space limitations, EPA noted. The current permit limit for this unit is 0.227 lb of total suspended particulate (TSP)/MMBtu). Limited stack tests indicate that the actual TSP emission rate is much lower, averaging 0.034 lb TSP/MMBtu.

The NH DES model scenario of upgrading the current ESPs to 90% control resulted in a visibility improvement at nearby Class I areas. But, NH DES determined that the installation of additional PM controls is unlikely to result in substantial visibility improvement. However, based on the limited available stack test data, NH DES determined that the current emission limit of 0.227 lb/MMBtu was not reflective of the performance capabilities of the control equipment.

New Hampshire has adopted a new regulation which places Units MK1 and MK2 within a regulatory ‘‘bubble’’ for the purposes of TSP compliance. The revised emission limit is 0.08 lb TSP/MMBtu for both Units MK1 and MK2. New Hampshire defined this level of control as BART.

As for SO2 BART, emissions of SO2 from MK2 are currently controlled by a fuel sulfur limit of 2 lbs sulfur/MMBtu. The most stringent retrofit control technology for SO2 controls is wet flue gas desulfurization (FGD). New Hampshire law requires the installation of a wet FGD for mercury removal on MK1 and MK2. As a co-benefit, the FGD is required to achieve at least 90% SO2 control. Because this installation is already mandated and the removal rate approaches a recommended limit of 95%, New Hampshire determined that the BART SO2 emission limit for this unit is at least 90% control.

Current permit conditions require the facility to submit monthly SO2 emission rates for the preceding 12 months by Dec. 31, 2014. At that time, New Hampshire will determine the maximum sustainable rate of control. As specified by permit conditions, in no case may this rate be less than 90% control. In addition, emissions from MK1 will also be controlled via the FGD.

In the area of NOx BART review, PSNH currently operates SCR on MK2. It was installed in 1994 to meet other air quality requirements (ozone season NOx). Selective non-catalytic reduction (SNCR) is the only other major post-combustion control technology available for controlling NOx and is generally considered to be less effective. The existing SCR has received previous retrofits to improve performance. Additional upgrades would require major redesign and construction. Capital cost would be comparable to installing a new SCR and would achieve only marginal additional reduction.

Because MK2 has an existing SCR system that can be operated year-round at reasonable cost, full time operation of the existing SCR was determined by New Hampshire to be BART for NOx control. In addition, New Hampshire reduced the permitted NOx emission limit from a 0.86 lb/MMBtu annual average to a 0.3 lb/MMBtu 30-day rolling average.

EPA said in the Feb. 28 Federal Register notice that the New Hampshire decisions on PM, SO2 and NOx compliance for Merrimack are reasonable and plans to approve them.

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.