After a relatively quiet 2021, the U.S. Environmental Protection Agency is gearing up for a flurry of key power-sector rulemakings that could advance the Biden administration's energy and climate agenda.

Under former U.S. President Donald Trump, the EPA sought to largely dismantle former President Barack Obama's climate legacy by weakening or rescinding dozens of rules affecting the U.S. electricity industry. Now, U.S. President Joe Biden's EPA is reversing the Trump administration's signature rollbacks, with plans to go even further than some Obama-era regulations.

As 2021 draws to a close, here are five EPA rulemakings to watch in 2022.

ACE rule replacement

At the top of the EPA's power-sector agenda is a replacement regulation for the Trump administration's Affordable Clean Energy, or ACE, rule — itself a replacement for the Obama-era Clean Power Plan.

During his Senate confirmation hearing, EPA Administrator Michael Regan said the agency will start with a "clean slate" after the U.S. Court of Appeals for the District of Columbia vacated and remanded the ACE rule due to what it found to be an impermissibly narrow reading of the Clean Air Act.

The EPA plans to issue a proposed replacement for the ACE rule by July 2022 with a final rule by July 2023, according to the agency's latest unified regulatory agenda. That could give the agency sufficient time to respond to a Supreme Court review of the EPA's power-sector authority under the Clean Air Act.

In November, the high court granted and consolidated four petitions for review of the D.C. Circuit's ACE rule decision brought by the coal-producing states of West Virginia and North Dakota as well as Westmoreland Mining Holdings LLC and The North American Coal Corp.

The court's review is expected to delve into the legal issues surrounding the Clean Power Plan's focus on "outside the fence line" emission reduction measures such as coal-to-gas switching and renewable energy development. But legal experts have also noted it could also result in the Supreme Court's conservative majority reviving the nondelegation doctrine, a line of legal reasoning that holds the U.S. Congress cannot delegate rule-writing authorities to federal agencies.

Clean Air Act co-benefits

Most of the Clean Power Plan's projected benefits were tied to public health savings, or co-benefits, associated with reductions in fine particulate matter emissions from coal-fired electricity generation.

With Trump as president, the EPA subsequently issued multiple final rules targeting how the agency calculates public health co-benefits in Clean Air Act rulemakings. One of those rules, issued in December 2020, requires the EPA to separately report co-benefits related to reductions in pollutants that are not the direct focus of Clean Air Act regulations.

Critics worried that the rule would frustrate efforts by future administrations to justify climate and energy regulations that cut air pollution.

But the Biden EPA is now in the process of rescinding that regulation after concluding it is "untethered" to the statute. After issuing an interim rule in May, the EPA plans to promulgate a final rescission rule in February 2022.

MATS finding

Public health advocates also blasted a Trump EPA rule that scrapped the legal basis for the Obama-era Mercury and Air Toxics Standards, or MATS, rule.

While the MATS rule targeted mercury emissions from oil- and coal-fired power plants, it also relied heavily on monetized co-benefits flowing from reductions in fine particulate matter. In rescinding a supplemental finding for the MATS rule, the Trump administration effectively found that regulating those generators for mercury emissions is not "appropriate and necessary."

The Biden EPA is now in the process of reversing that rule as well.

The administration had planned to issue a replacement proposal by November, although that deadline has since passed. The EPA plans to issue a final rule reaffirming that regulating the U.S. power sector for mercury emissions is appropriate and necessary by September 2022.

National Ambient Air Quality Standards review for particulate matter

The Trump EPA further angered public health advocates by declining to strengthen the National Ambient Air Quality Standards for fine particulate matter, also known as PM 2.5.

Measuring 2.5 microns or less in diameter — smaller than the width of a human hair — inhaled PM 2.5 can make its way from the lungs into the bloodstream and brain, causing a range of severe health problems.

In September 2019, EPA career staff released a draft policy assessment estimating that approximately 50,000 premature deaths linked to PM 2.5 are occurring annually under the EPA's primary standard of 12 micrograms per cubic meter. Staff said that moving to a 9-microgram threshold could save between 9,050 and 34,600 lives a year.

The Biden EPA expects to complete a fresh review of its PM 2.5 standards by August 2022 followed by a final rule in March 2023. Some legal experts have suggested tighter PM 2.5 standards could force additional coal-fired units — one of the largest sources of fine particulate emissions — offline as part of states' Clean Air Act implementation plans.

Coal ash regulations

The Biden EPA also plans to respond next year to an August 2018 decision by the D.C. Circuit that faulted the agency for exempting inactive coal ash impoundments from a 2015 coal ash rule.

The Obama-era rule set the first minimum safety requirements for active coal ash ponds and landfills but exempted inactive units, known as "legacy" impoundments, because the EPA claimed at the time that it lacked sufficient data — an argument rejected by the D.C. Circuit.

In response to the court's ruling, the Trump EPA issued an advanced notice of proposed rulemaking in October 2020 seeking additional data on legacy impoundments. But the agency did not specify when it planned to issue a final rule, a move blasted by critics as "grossly inadequate and illegal."

However, the Biden EPA now plans to issue a proposed rule governing coal ash disposal at legacy impoundments by September 2022. And the agency expects to issue a final rule by September 2023, according to its regulatory agenda.