June 20, 2023

Tighter Standards for EGUs Proposed

The mercury and air toxics standards (MATS) NESHAP for coal- and oil-fired electric utility steam generating units (EGUs) was first promulgated in 2012. [77 FR 9304] In 2020, the residual risk and technology review (RTR) for the MATS was finalized, with EPA not only determining it was unnecessary to update the rule but also stating it was not appropriate and necessary to regulate coal-and oil-fired EGUs under CAA Section 112. [85 FR 31286] Under Executive Order 13990, EPA was instructed to review the “appropriate and necessary” finding and the 2020 RTR. The agency reaffirmed it remains appropriate and necessary to regulate HAPs from coal-and oil-fired EGUs under the CAA in February 2023. [88 FR 13956] More recently, on April 24, 2023, EPA proposed stricter standards based on its review of the 2020 RTR for Part 63, Subpart UUUUU. [88 FR 24854]

Since the initial 2012 rule, EPA has found the majority of sources outperform the MACT standards with cheaper and more effective control technologies than what the agency originally forecast. Accordingly, the April 2023 proposal tightens the emission limits for certain new and existing affected sources subject to Subpart UUUUU, depending upon which of the seven regulated subcategories they fall into.

Strengthening MATS

EPA’s proposal makes six key changes to Subpart UUUUU:

The non-mercury (non-Hg) metal surrogate of filterable particulate matter (fPM) standard for existing coal-fired EGUs would be lowered from 0.03 lb/MMBtu to 0.01 lb/MMBtu. EPA is also specifically seeking comment on whether the fPM emission standard should be reduced to an even more stringent level of 0.006 lb/MMBtu.
All coal-fired EGUs would be required to demonstrate compliance with the fPM emission standard via a PM continuous emission monitoring system (CEMS).
The current rule sets individual emission limits for ten non-Hg metals but allows affected sources to meet an overall emission limit for “total non-Hg metals” by summing the emission rates of each of the non-Hg metals. As an alternative, sources can also meet an fPM emission limit as a surrogate for those non-Hg metals. Since most sources have chosen to demonstrate compliance via the fPM surrogate, and since the PM CEMS would be required, EPA proposes removing the individual and total non-Hg metals emission limits.
Existing lignite-fired EGUs have a current Hg emission standard of 4.0 lb/Tbtu though sources can opt for an alternative output-based emission standard of 0.04 lb/GWh. The proposal would require lignite-fired EGUs to meet the same Hg emission standard as bituminous- and subbituminous-fired EGUs, which is 1.2 lb/Tbtu or an alternative output-based standard of 0.013 lb/GWh. [Note: lignite coal has less heating value per ton than bituminous coal.]
Two definitions of “startup” exist in Subpart UUUUU. The definition of “startup” that ends four hours after the EGU generates electricity that is sold or used for any other purpose (including on-site use), or four hours after the EGU makes useful thermal energy for industrial, commercial, hearing, or cooling purposes, whichever is earlier, would be removed. The more-stringent definition of “startup” that ends when any steam from the boiler is used to generate electricity for sale over the grid or for other purposes, such as on-site use, would remain.
EPA is not aware of any new coal- or oil-fired EGUs in development and did review the MATS for new sources. However, because of the proposed strengthening of the existing source requirements, instances would arise where new sources have a less stringent requirement than an existing source. As such, the rule set the corresponding new source standards to be at least as stringent as the proposed existing source standards mentioned above.

EPA has provided additional information on the proposed rule online. Comments may be submitted on the proposal until June 24, 2023 via Docket ID No. EPA-HQ-OAR-2018-0794.

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