On August 21, 2018, the U.S. Environmental Protection Agency proposed the Affordable Clean Energy rule (ACE) which would establish emission guidelines for states to develop plans to address greenhouse gas emissions from existing coal-fired power plants. ACE would replace the 2015 Clean Power Plan, which EPA has proposed to repeal because it exceeded EPA's authority. The Clean Power Plan was stayed by the U.S. Supreme Court and has never gone into effect.

ACE has several components: a determination of the best system of emission reduction for greenhouse gas emissions from coal-fired power plants, a list of “candidate technologies” states can use when developing their plans, a new preliminary applicability test for determining whether a physical or operational change made to a power plant may be a “major modification” triggering New Source Review, and new implementing regulations for emission guidelines under Clean Air Act section 111(d). 

• Press Release: EPA Proposes Affordable Clean Energy (ACE) Rule
• Fact Sheets
  • Overview of the proposed ACE rule
  • CO₂ emissions trends
  • Costs and benefits 
  • Legal overview
  • Permitting improvements under New Source Review program
  • Side-by-side comparison of ACE and CPP
• Federal Register Notice: Affordable Clean Energy rule proposal
• Regulatory Impact Analysis: Affordable Clean Energy rule proposal RIA
  • Power Sector Analysis of the proposed ACE rule
• Public Hearing