The 1990 Clean Air Act Amendments (CAAA) brought about new awareness regarding the overall health-effects of stationary source fossil combustion emissions. Title III of the CAAA identified 189 pollutants as potentially hazardous or toxic that required EPA to evaluate their emissions by source, health and environmental implications, and the need to control these emissions. These pollutants have collectively been referred to as air toxics or hazardous air pollutants (HAPs). The provisions in Title III specific to electric power generation units were comprehensively addressed by DOE’s National Energy Technology Laboratory (NETL) and the Electric Power Research Institute (EPRI) in collaborative air toxic characterization programs conducted between 1990 and 1997.
This work provided most of the data supporting the conclusions found in EPA's Congressionally mandated reports regarding air toxic emissions from coal-fired utility boilers: the Mercury Study Report to Congress (1997) and the Study of Hazardous Air Pollutant Emissions from Electric Utility Steam Generating Units -- Final Report to Congress (1998). The first report identified coal-fired power plants as the largest source of human-generated mercury emissions in the U.S. and the second concluded that mercury from coal-fired utilities was the HAP of "greatest potential concern" to the environment and human health that merited additional research and monitoring.

Subsequent to these findings, data was gathered during EPA’s 1999/2000 Information Collection Request (ICR), in cooperation with NETL, to refine the total mercury emission inventory from coal-fired plants and ascertain the mercury control capabilities of existing and potential emission control technologies. Results of this work, and an independent evaluation of mercury health impacts by the National Academy of Sciences (NAS), finally culminated in EPA’s decision in December 2000 [PDF-18KB] to regulate the mercury emissions of coal-fired power plants. In their regulatory determination, EPA concluded that there was a "plausible link" between emissions of mercury from coal-fired electric utility steam generating units and the bioaccumulation of methylmercury in fish and other animals that eat fish. Since human exposure to mercury occurs primarily through consumption of contaminated saltwater or freshwater fish, further control of coal- and oil-fired power plants was deemed necessary.

2005 Clean Air Mercury Rule
On March 15, 2005, the Environmental Protection Agency (EPA) issued the Clean Air Mercury Rule (CAMR), the first federal rule designed to reduce mercury emissions from coal-fired power plants. CAMR builds upon the Clean Air Interstate Rule (CAIR) which was designed to significantly reduce sulfur dioxide (SO2) and nitrogen oxide (NOx) emissions from power plants. Combined, these two rules are expected to reduce U.S. mercury emissions by 70 percent, from 48 tons per year to 15 tons per year. CAMR creates a market-based cap-and trade program that will reduce national mercury emissions through two distinct phases. In 2010, phase I caps mercury emissions at 38 tons and is achieved as a “co-benefit” of controlling SO2 and NOx emissions under CAIR. Under full implementation in 2018, a 15 ton cap must be achieved. In addition to the phase I and II caps, new coal-fired plants will have to meet new source performance standards.

Units Subject to the Rule
According to CAMR, coal-fired units capable of firing more than 73 megawatts (MW) (250 million Btu/hour) heat input of fossil fuel and industrial cogeneration facilities that sell over 25 MW of electrical output and more than one-third of their potential output capacity to any utility power distribution system are subject to the rule.

Standards of Performance
CAMR will be regulated under CAA section 111, which covers the promulgation of standards of performance for both new and existing sources. Section 111 (b) provides EPA authority to establish new source performance standards (NSPS) for new sources while Section 111 (d) provides EPA the authority to establish guidelines for states to develop standards of performance for existing sources. For existing sources, the term ‘standards of performance’ refers to the cap-and-trade program as described in the following section.

The NSPS under CAA section 111(b) apply to new plants only—units for which construction, modification, or reconstruction is commenced after January 20, 2004. The mercury standards of performance for new coal-fired plants, which are based on a 12-month rolling average, are listed in Table 1. EPA has established six subcategories for mercury limits based on a combination of coal rank and process type. These five categories are bituminous coal, subbituminous coal wet FGD, subbituminous coal dry FGD, lignite coal, coal refuse, and Integrated Gasification Combined Cycle (IGCC). These standards are based on gross energy output. The information collection request (ICR) stack test data was used to establish NSPS limits.

Table 1: New Source Standards of Performance*

Monitoring Requirements
Compliance with final standards of performance for mercury will be on a 12-month rolling average. Mercury emissions are determined by continuously collecting data from each affected unit by installing and operating a certified continuous emission monitoring system (CEMS) or an appropriate long-term sorbent trap method that can collect an uninterrupted, continuous sample of the mercury in the flue gases emitted from the unit.   For units that commence commercial operation before July 2008, mercury monitoring systems must be installed and certified by January 1, 2009.

The rolling average is weighted according to the number of hours of valid mercury emissions data collected each month, unless insufficient valid data are collected in a month. The rule requires that valid mercury mass emissions data be obtained for a minimum of 75 percent of the unit operating hours each month.  If this is not met, the data for the month are discarded.  If there are any months in the 12-month cycle for which the data capture is not met, data substitution is required.  The first time it occurs, the mean mercury emission rate for the last 12 months is reported, and for any subsequent occurrences, the maximum emission rate from the past 12 months is reported.  Whenever a substitute emission rate is reported, the substitute emission rate is weighted according to the number of unit operating hours in that month when the 12-month rolling average is calculated.

The owner/operator is required to prepare a unit-specific monitoring plant and submit it to the Administrator for approval.   The plan must address certain aspects with regard to the monitoring system, installation performance and equipment specifications, performance evaluations, operation and maintenance procedures, quality assurance techniques, and recordkeeping and reporting procedures.   

For new cogeneration units, the process steam must also be considered in determining compliance with the output-based standard.  The owner/operator must calculate emission rates based on electrical output to the grid plus half the equivalent electrical output energy in the unit’s process steam. 

A new coal-fired unit that burns a blend of fuels will develop a unit-specific mercury emission limitation.  The unit-specific mercury emission rate will be used for the portion of the compliance period in which the unit burned the blend of fuels.

In addition to the standards of performance for new units, both new and existing units will be subject to the cap-and-trade provisions. 

Cap and Trade Program
The mercury cap-and-trade-program established under CAMR is similar to EPA’s Acid Rain Program.  The program will be implemented in two phases and reduce overall U.S. mercury emissions from coal-fired power plants.  Phase I is a 38 ton cap effective in 2010.  Mercury reductions achieved under Phase I are a result of co-benefit reductions under CAIR.  Coal-fired utilities must then meet the Phase II cap of 15 tons in 2018. Under the rule, each state is assigned an annual electrical generating unit (EGU) mercury budget as listed in Table 2 States that have no coal-fired utility units are assigned mercury emission budgets of zero tons.  State emission budgets are permanent regardless of electricity sector growth.    

The rule includes a model cap-and-trade program that states can choose to adopt in order to achieve mercury emissions budgets.  States have the flexibility to choose whether they will adopt the model trading rule or separate, more stringent state regulations.  States then must each submit a State Implementation Plan (SIP) describing how they will meet the budget for reducing mercury.  Under the cap-and-trade program, plants must demonstrate compliance with the standard by holding one allowance for each ounce of mercury emitted in a year.  Allowances are transferable among regulated facilities.

Table 2 : State Mercury Allowance Allocations*