EPA Admits Error in Proposed Mercury MACT Rule

The Environmental Protection Agency (EPA) has acknowledged in a letter to non-profit power trade organization Utility Air Regulatory Group (UARG) that it made a conversion error in the way mercury emissions data was calculated to set limits for the agency’s mercury maximum achievable control technology (MACT) floor in the proposed Mercury and Air Toxics Standards (MATS) rule.

EPA Assistant Administrator for Air and Radiation Gina McCarthy wrote that the error would be corrected and results presented within a week, but that the corrections were not expected to have an “appreciable impact” on the pollution controls required to meet the new standards.

The group had said in a May 6 letter to the EPA that the MACT rule is one of the "most far-reaching and expensive rules" the agency had ever proposed under the Clean Air Act. The EPA asked for information of "unprecedented size and scope," including historic information on hazardous air pollutants that required electric generating units to conduct more than $100 million of stack sampling for emissions.

The UARG said it was concerned that the EPA had not allotted enough time to review all the data collected. A review of the information in the "limited time" made available for public comment showed several errors that "warrant withdrawal of the current proposal, re-analysis of the data, and re-proposal once the errors have been addressed."

One glaring error regarding MACT standards for mercury concerned how the EPA converted historical emissions data to common emission units. "Specifically, the conversion factor EPA used to convert measurements reported in terms of lb/GWh to lb/MWh is incorrect by a factor of 1,000," the group asserted. "As a result, the emissions of the unit chosen as the single best performing mercury source are actually 1,000 times higher than those EPA assumed in setting the new source MACT limit for mercury."

The group also charged that the conversion error pervades EPA’s mercury MACT floor analysis for existing units. "At least half of the 40 units identified as ‘best performing’ have actual emissions 1,000 times higher than EPA used in its MACT floor analysis." 

The UARG also found errors in the EPA’s MACT limits for mercury for units designed to burn coal. For units burning coal of more than 8,300 BTH/hour, the limit is 1.0 lb/10 BTU or 0.008lb/GWh. But the "required heat rate to convert from the proposed input limit to the proposed output limit is 8,000 BTU/KWh, which is unrealistically low value for a coal-fired power plant," the group said.

Addressing that concern, McCarthy said in her letter on Thursday that revised calculations resulted in only a slight increase of that limit, however, from 1.0 to 1.2 pounds per BTU. "We estimate that this change will lessen our expected mercury reductions by about half of a ton, not changing our projected 90 percent reduction from the mercury in the coal used by power plants."

McCarthy also acknowledged the UARG’s concerns as "possible discrepancies" regarding the EPA’s calculations, and she said the EPA would "fully consider" the UARG’s comments and other comments provided during the comment period.

"We have revised these mercury calculations and will include them in the docket this week to ensure that this information is publicly available," she wrote. "We do not expect this change to have any appreciable impact on the controls that will be needed for compliance or on the analyses underlying the proposal."

Sources: POWERnews, EPA, UARG,

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