The United States Court of Appeals for the District of Columbia has vacated the Affordable Clean Energy (ACE) rule, issued in 2019, which gave States more time and authority to decide how to implement the best new technology to reduce CO2 emissions from coal-fired power plants.
In 2019, the ACE rule set emission guidelines for States when developing plans to cut CO2 emissions at their coal-fired power plants and identified heat rate improvements as the best system of emission reduction (BSER) for CO2 at coal-fired power plants. It allowed States to decide the best emissions control technologies for coal-fired plants within their boundaries, making it harder for future administrations to enact national standards for emissions. The ACE rule replaced the Clean Power Plan (2015), which aimed to cut emissions from the power sector by 32% by 2030 compared to 2005 levels. Most notably, the Clean Power Plan directed every state to prepare plans to eliminate carbon emissions from power plants by phasing out coal and increasing the generation of renewable energy; the regulation was never enacted because of lawsuits by some states and was stayed by the Supreme Court in 2016.