Mercury Emissions from Electric Generating Units: A Review of EPA Analysis and MACT Determination Page: 2 of 21
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Mercury Emissions from Electric Generating Units: A
Review of EPA Analysis and MACT Determination
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. The proposed MACT
standards (pounds of mercury per trillion Btu) are shown, along with the average
mercury emission and the results of the first statistical treatment, in the table below.
Coal Average Measured Average 97.5% Upper Proposed MACT
Subcategory Mercury Emission Confidence Limit Level
Bituminous 0.1 1.1 2.0
Subbituminous 0.8 3.1 5.8
Lignite 5.0 7.8 9.2
IGCC 5 6 19
Waste Coal 0.09 0.14 0.38
The EPA has agreed to promulgate final mercury emission standards by March
15, 2005 for EGUs. Whether these standards should follow the proposed regulation
in its current form, an alternate form, or through an alternate mechanism is a topic
of likely congressional interest. This report will not be updated.
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Mercury Emissions from Electric Generating Units: A Review of EPA Analysis and MACT Determination, report, January 21, 2005; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc808863/m1/2/: accessed October 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.