November 14, 2022

Most Boiler MACT Emission Limits Tightened

On October 6, 2022, EPA promulgated a final rule amending the boiler and process heater MACT standard at Part 63, Subpart DDDDD. [87 FR 60816] The rule finalized amendments to several emission limits in response to a 2016 court decision. The agency also bolstered its use of carbon monoxide (CO) as a surrogate for organic HAPs, and in response to a different court case from 2018, provided additional support for the minimum CO emission limit of 130 ppm for several boiler subcategories. Also finalized were some technical clarifications and corrections.

In U.S. Sugar Corp v. EPA (830 F. 3d 579, D.C. Cir. 2016), the court found EPA erroneously excluded certain units from its MACT floor calculations, even though those units were included in one of the standard’s subcategories. The result was a MACT floor higher than if EPA had included all of the best-performing sources in its emission limit calculations. The same court decision also concluded EPA failed to adequately address public comments on how CO can be an appropriate surrogate for organic HAPs. Two years later, in Sierra Club v. EPA (884 F.3d 1185, D.C. Cir. 2018), the same court found EPA failed to adequately justify its decision to set a higher CO emission limit (130 ppm) for some boiler subcategories.

In response to these court mandates, the agency has revised 34 separate emission limits for not only CO, but also PM, total suspended metals (TSM), HCl, and Hg, for 23 regulated subcategories of boilers and process heaters. Most of the updated emission limits are more stringent, though six PM (TSM) emission limits have been loosened after EPA factored in all sources in each given subcategory. The agency also provided additional support for using CO as a surrogate for organic HAPs. EPA explained combustion is a commonly applied option for controlling organic HAPs, and CO is a by-product of incomplete combustion; therefore, the presence of a low CO concentration in a flue gas stream is evidence the combustion process is efficient and effective.

As the agency mentioned in the 2010 boiler MACT proposal, setting emission limits for individual organic HAPs would be “impractical and costly.” [75 FR 32018] If the agency could not use CO as a surrogate in the boiler MACT, the consequences could extend to other standards using CO as a surrogate, such as the RICE standard of Part 63, Subpart ZZZZ.

EPA also provided additional justification, as ordered by the court, for the minimum 130 ppm CO limit it established for several boiler subcategories. Some technical corrections and clarifications were also made by the final rule, which is effective on December 5, 2022, and facilities have three years to achieve compliance with the revised emission limits.


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