The Affordable Clean Energy Rule (ACE) provides a plan for states to make realistic assessments and upgrade their power plants with clean technologies so that they can operate in a more environmentally friendly manner. ACE refocuses the Environmental Protection Agency (EPA) on sound science, transparency, supply diversity, and the rule of law, rather than bureaucratic overreach and politics. Obama’s Clean Power Plan, which ACE replaces, was never implemented. The United States Supreme Court stopped the rule, partly due to claims from over half the states that they would suffer “irreparable injury” without intervention. The stay on implementation was an extraordinary step for the Supreme Court, which many Court-watchers at the time said indicated displeasure with a vast rule with enormous consequences.

Affordable Clean Energy Rule

On June 19, 2019, EPA issued ACE, replacing the Obama administration’s Clean Power Plan with a rule that establishes emission guidelines for states to use when developing plans to limit carbon dioxide at their coal-fired electric generating units. ACE regulates the electric industry on a plant-by-plant basis, allowing older plants to keep operating as they adopt improved, more-efficient technology. The new plan removes the federal government’s power to set emissions-reduction targets across the industry, regulating only “inside the fence line of” each plant. It creates a menu of technological options the plants can choose from to boost efficiency, using less coal to generate the same amount of electricity. States have the authority to design plans for power plants within their borders, with three years to develop and submit the plans to the EPA for approval.

More specifically, ACE establishes heat rate improvement—a measure of the amount of energy required to generate a unit of electricity—as the best system of emissions reduction for carbon dioxide from coal-fired generating units. ACE provides six “candidate technologies:”

• Neural Network/Intelligent Sootblowers
• Boiler Feed Pumps
• Air Heater and Duct Leakage Control
• Variable Frequency Drives
• Blade Path Upgrade (Steam Turbine)
• Redesign/Replace Economizer

ACE does not set a specific target for the power sector to reduce carbon dioxide emissions, giving states the authority to write their own plans for reducing carbon dioxide at individual plants. The Trump EPA estimates that carbon dioxide emissions would be reduced through both market forces and ACE by as much as 35 percent below 2005 levels in 2030, which is similar to reduction estimates calculated by Obama’s EPA for the Clean Power Plan. Trump’s EPA estimates ACE will directly reduce carbon dioxide emissions by 11 million metric tons by 2030—a 0.7 percent reduction compared to no regulation. There would also be benefits from reducing sulfur dioxide, nitrogen oxide, particulate matter, and mercury emissions, which are already controlled by other regulations and have been declining rapidly.

New Source Review

EPA is also proposing separately to change New Source Review (NSR)—a Clean Air Act rule that requires plant owners, who make a change that significantly increases a plant’s annual emissions to install pollution controls. EPA is proposing to change NSR so that it is triggered by increases in emission rates rather than increases in overall emissions. Under current rules, power plants must undergo new pollution reviews when they upgrade facilities, making it expensive to make changes that would improve plant efficiency and in turn, allow more production with less emissions per unit produced. Generally, it would cost less to upgrade an existing coal plant to make it cleaner and more efficient than to invest in a new facility were it not for NSR.


ACE succeeds in replacing the onerous and legally dubious Clean Power Plan and providing emission standards compatible with a “dependable, diverse supply of electricity that all Americans can afford.” ACE gives the states authority to design plans for plants within their borders, submitting them to EPA for approval within 3 years. The plan is much more realistic and far less costly than the Clean Power Plan, which would have increased electricity prices, causing hardship for American families. Further, that hardship would be unwarranted because reducing U.S. power plant carbon dioxide emissions to zero by 2030 would only reduce the global concentration of carbon dioxide by 0.7 percent in 2050 and have immeasurable impacts on temperatures according to the Obama EPA’s own analysis.

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