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A Summary of Proposed Updates to the Mercury and Air Toxics Standards for Power Plants

Posted: April 19th, 2023

Authors: Michael L. 

On April 5, 2023, the U.S. Environmental Protection Agency (U.S. EPA) signed proposed revisions to the National Emission Standards for Hazardous Air Pollutants (NESHAP) for Coal- and Oil-Fired Electric Utility Steam Generating Units (EGUs), also known as the Mercury and Air Toxics Standards (MATS). The current MATS established standards to limit emissions of mercury; acid gas hazardous air pollutants (HAP) such as hydrogen chloride and hydrogen fluoride; non-mercury HAP metals such as nickel, lead, and chromium; and organic HAP such as formaldehyde and dioxin/furan from coal- and oil-fired power plants.

The proposed new pollution standards advance more stringent requirements to reduce mercury and other HAP emissions and represent the most significant update since the MATS were first issued in 2012. These more stringent standards are based on U.S. EPA’s determination that technologies and/or methods of operation are currently available to achieve additional HAP control from coal-fired EGUs at reasonable costs. The proposed standards affect EGUs with a capacity of more than 25 megawatts.

This proposal follows the February 2023 U.S. EPA announcement that reaffirmed the scientific, economic, and legal underpinnings of the standards, reversing a rule issued by the Trump administration that undermined the legal basis for the rule. The proposed rule is intended to reflect the significant improvements in pollution control technologies since the original MATS rule was passed and satisfy the U.S. EPA’s requirement to periodically review emissions standards.

The proposed updates to the MATS further limit the emission of non-mercury HAP metals from existing coal-fired power plants by significantly reducing (by 67 percent) the emission standard for filterable particulate matter (fPM) and eliminating the option to comply with the HAP metals standard. As part of this proposed change, U.S. EPA is eliminating the low-emitting EGU (LEE) compliance option for fPM/non-mercury metals.  U.S. EPA is also proposing to tighten the emission limit for mercury for existing lignite-fired power plants by 70 percent, a level that is aligned with the mercury standard that other coal-fired power plants have been achieving under the current MATS. The proposal would also change emissions monitoring and compliance by requiring solid oil-fired and coal-fired EGUs to comply with the fPM standard using a particulate matter (PM) continuous emissions monitoring system (CEMS) instead of parameter monitoring or quarterly stack testing. The new standards would take effect three years after the effective date of the final rule.

Finally, U.S. EPA is proposing to revise startup requirements in MATS to assure better emissions performance during startup. Subject facilities will no longer be able to choose paragraph (2) of the ‘startup’ definition in § 63.10042 and use the associated work practice standards as compliance. The new startup requirements are proposed to take effect 180 days from the effective date of the final rule.

U.S. EPA is requesting public comment on all aspects of this proposed rule, including the evaluation of the costs and efficacy of control option assumptions. U.S. EPA will accept public comment on the rule for 60 days after publication in the Federal Register and also plans to hold a virtual public hearing.  Further details will be announced here.

The proposed revisions to the MATS will require subject facilities to potentially purchase new control devices and/or upgrade existing control devices. The proposed revisions would also require subject facilities to implement fPM monitoring systems (PM CEMS) with written monitoring plans. ALL4 can assist subject facilities with the review of which control devices would be most applicable to your site and help with the implementation of PM CEMS monitoring systems and written monitoring plans. If you have questions about how the proposed MATS revisions could affect your facility’s program, or what your next steps should be once the rule is finalized, please reach out to me at mliebert@all4inc.com. ALL4 is monitoring all updates published by the U.S. EPA on this topic, and we are here to answer your questions and assist your facility with any aspects of MATS compliance.


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