Mercury Emissions from Electric Generating Units: A Review of EPA Analysis and MACT Determination

Publication Date: January 2005

Publisher: Library of Congress. Congressional Research Service

Author(s):

Research Area: Environment

Type:

Abstract:

The Environmental Protection Agency (EPA) has identified mercury as the hazardous air pollutant emitted from electric generating units that is of greatest public health concern. As a result, EPA proposed regulations limiting electric generating unit (EGU) mercury emissions. In December 2000, EPA committed to promulgating Maximum Achievable Control Technology (MACT) emissions limits under Section 112 of the Clean Air Act.

Section 112 sets specific requirements for MACT standards. For new facilities, the MACT standard must be at least as stringent as the degree of emissions control achieved at the best controlled similar source. For existing facilities, the MACT standard must generally achieve limits equal to the average performance of the best 12% of comparable sources. Determination of this performance is complicated by the lack of installed commercial technology specifically for capture of EGU mercury emissions, thereby necessitating data collection on other existing technologies and extensive analysis of potential control levels. To determine the level of allowable mercury emissions, EPA collected data regarding coal composition and mercury emissions from an 80 EGU sample. Analysis of these data led EPA to subcategorize EGUs and propose MACT standards for each subcategory in January 2004.

The proposed MACT standards have been criticized by a wide range of stakeholders on several criteria, including EPA methodology to determine the allowable emissions threshold. The EPA methodology incorporates two statistical treatments, the use of, first, a 97.5% upper confidence limit to account for the variability in input coal, and, second, another 97.5% upper confidence interval to account for variance in plant operation. The result is proposed standards that are substantially less stringent than the average emissions rate of the top 12% of the 80 unit sample. While EPA justifies these statistical treatments, others assert they unnecessarily weaken the proposed regulation. Indeed, the proposed MACT standard allows a greater amount of mercury emission than most stakeholders, from all viewpoints, had recommended prior to the EPA proposal.