Environmental Permitting

Most Misunderstood Regs: The Mercury ‘Beyond-the-Floor’ Standard

Under the CAA’s maximum achievable control technology (MACT) approach to reducing emissions of air toxics, the MACT floor must be at least as stringent as the average emission limitation achieved by the best performing 12 percent of existing sources in the category (for which EPA has emissions information) or the best performing 5 sources for source categories with less than 30 sources.   The CAA prohibits EPA from considering the cost of compliance in setting the MACT floor.


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EPA must also determine if BTF standards are appropriate.  However, with a BTF standard EPA must consider costs, energy, and nonair quality health and environmental impacts.  In other words, EPA must perform a justification exercise specific to BTF.  For example, in the case of MATS, the cost justification involved use of EPA’s Integrated Planning Model (IPM), a computer program developed to project a wide range of costs for the electric power sector.  In terms of air pollutants, IPM can be used to evaluate the cost of policies to limit emissions of sulfur dioxide, nitrogen oxides, carbon dioxide, and mercury from electric power plants.

In the MATS, the BTF regulation applies to power plants that combust lignite coal.  EPA characterizes lignite as “low rank,” meaning virgin coal having a calorific value of 8,300 Btu/lb or less.  Lignite coal, which is commonly used in Texas power plants, has highly variable mercury concentrations.   The final BTF standard is 4 pounds of mercury per trillion BTUs fuel input (lb/TBtu) in contrast to the mercury MACT floor of 11 lb/TBtu.

Industry objected to the BTF standard when it was proposed, stating that EPA based it on insufficient data, specifically three samples from a single test conducted at only one power plant.  In the final rule, EPA disagreed, stating that sufficient data were available to back up the BTF decision.  According to the Agency, all low-rank virgin coal electric generating units (EGU) for which data were submitted in response to an information collection request (ICR) were meeting the MACT floor of 11 lb/TBtu.  Further, says the Agency, four of those units were equipped with activated carbon injection (ACI) systems and three of the four were achieving greater than 95 percent control levels for mercury emissions, which satisfied the 4 lb/TBtu BTF standard.  EPA states that control of mercury emissions in the range of 80 to 90 percent would be needed to meet the BTF standard.  Further, according to the Agency, one low-rank virgin coal-fired unit achieved 75 percent control with no mercury-specific control technology. 


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While reported data indicated that five units were meeting the BTF limit, the Agency points out that the CAA does not require that a specified percentage of sources in a category or subcategory be able to meet the MACT standard that is established.

“This is even truer for beyond-the-floor standards, which are set a levels beyond what the average of the best performing sources are achieving in practice and are instead based on what is achievable,” states EPA.

In general, the Agency believes that control levels that are being achieved still fall short of what could be achieved if ACI were used to its fullest extent.  Accordingly, EPA decided to base the BTF standard on use of ACI at low-rank coal-fired EGUs.