Annual U.S. electric power industry emissions of sulfur dioxide (SO₂) and nitrogen oxides (NO_x) have declined by 88% and 76%, respectively, between their peaks in 1997 and 2017.

The Energy Department's Energy Information Administration (EIA) said that during the period, coal-fired electric power generation was responsible for 90% of SO₂ emissions and 76% of NO_x emissions. Declining coal-fired generation and implementation of environmental regulations under the Clean Air Act Amendments (CAAA) of 1990 both contributed to the decrease in electric power sector SO₂ and NO_x emissions.

Emissions of SO2 and NOx have declined as coal use for electric power generation has fallen. Credit: Wikimedia Commons by joelbeebEIA said that coal-fired generation peaked in the US in 2007 at 2,016 million megawatthours (MWh) and fell to 1,206 million MWh by 2017. The decline was driven by increased competition from natural gas and renewable generation, along with environmental regulations.

Coal accounted for more than 50% of U.S. electricity generation in 1997, and natural gas and renewables together accounted for 26% of generation. By 2017, EIA said that coal-fired generation had declined to 30% of electricity generation while generation from natural gas and renewables grew to nearly 50% of electric power industry generation.

Credit: EIAIn addition to burning less coal to generate electricity, EIA said that the rate at which coal-fired power plants emit SO₂ and NO_x has also declined during the past two decades, primarily as a result of plants complying with environmental regulations under the CAAA. In 1997, each megawatthour of coal-fired electricity generation produced 14.6 pounds of SO₂ and 6.4 pounds of NO_x. By 2017, those rates had fallen to 2.4 pounds per megawatthour (lbs/MWh) and 1.5 lbs/MWh, respectively.

Emission-Reduction Programs

EIA said that the CAAA required several regulations that reduced emissions of SO₂ and NO_x. The Acid Rain Program imposed a cap on emissions of SO₂ and NO_x from coal and residual-fuel oil-fired power plants starting in 1995. The main compliance approach by electric generators for Mercury and Air Toxics Standards (MATS), with an April 2015 initial deadline, was to install flue-gas desulfurization (scrubber) or dry sorbent injection equipment. Both technologies also remove SO₂ and NO_x in addition to the targeted air pollutants regulated under MATS.

Credit: EIA27 States

In 2005, the Clean Air Interstate Rule (CAIR) addressed regional interstate transport of contributors to ground-level ozone (smog). It required 27 eastern states to file implementation plans to reduce SO₂ and NO_x emissions. CAIR was replaced by the Cross-State Air Pollution Rule (CSAPR) in 2015.

EIA said that the 27 states regulated under CAIR accounted for more than 80% of both coal-fired electricity generation and national SO₂ and NO_x power sector emissions in 1997. The decline in emissions between 1997 and 2017 mostly occurred in these 27 states. Ohio, Pennsylvania, and Indiana experienced the greatest decline in SO₂ emissions between 1997 and 2017, EIA said. Ohio, Indiana, and Kentucky saw the greatest decline in NO_x emissions.