Air Emissions and Electricity Generation at U.S. Power Plants

GAO-12-545R: Published: Apr 18, 2012. Publicly Released: May 18, 2012.

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David C. Trimble
(202) 512-3841


Franklin W. Rusco
(202) 512-3841


Office of Public Affairs
(202) 512-4800

What GAO Found

Older electricity generating units—those that began operating in or before 1978—provided 45 percent of electricity from fossil fuel units in 2010 but produced a disproportionate share of emissions, both in aggregate and per unit of electricity generated. Overall, in 2010 older units contributed 75 percent of sulfur dioxide emissions, 64 percent of nitrogen oxides emissions, and 54 percent of carbon dioxide emissions from fossil fuel units. For each unit of electricity generated, older units collectively emitted about 3.6 times as much sulfur dioxide, 2.1 times as much nitrogen oxides, and 1.3 times as much carbon dioxide as newer units. The difference in emissions between older units and their newer counterparts may be attributed to a number of factors. First, 93 percent of the electricity produced by older fossil fuel units in 2010 was generated by coal-fired units. Compared with natural gas units, coal-fired units produced over 90 times as much sulfur dioxide, twice as much carbon dioxide and over five times as much nitrogen oxides per unit of electricity, largely because coal contains more sulfur and carbon than natural gas. Second, fewer older units have installed emissions controls, which reduce emissions by limiting their formation or capturing them after they are formed. Among coal-fired units—which produce nearly all sulfur dioxide emissions from electric power generation—approximately 26 percent of older units used controls for sulfur dioxide, compared with 63 percent of newer units. Controls for nitrogen oxide emissions were more common among all types of fossil fuel units, but these controls vary widely in their effectiveness. Among older units, 14 percent had installed selective catalytic reduction (SCR) equipment, the type of control capable of reducing the greatest amount of nitrogen oxides emissions, compared with 33 percent of newer units. In addition, approximately 38 percent of older units did not have any controls for nitrogen oxides, compared with 6 percent of newer units. Third, lower emissions among newer units may be attributable in part to improvements in the efficiency with which newer units convert fuel into electricity. Nonetheless, older units remain an important part of the electricity generating sector, particularly in certain regions of the United States.

Why GAO Did This Study

This report responds in part to a Congressional request for information on electricity generation and emissions at U.S. electricity generating units and the implementation of NSR. Our objective is to provide information on how older fossil fuel electricity generating units compare with newer units in terms of their air emissions and electricity generation.

The United States depends on a variety of fuels to generate electricity, including fossil fuels (coal, natural gas, and oil), nuclear power, and renewable sources. Power plants that burn fossil fuels provide about 70 percent of U.S. electricity, but they also produce substantial amounts of harmful air emissions. In particular, electricity generating units at fossil fuel power plants are among the largest emitters of sulfur dioxide and nitrogen oxides, which have been linked to respiratory illnesses and acid rain, as well as of carbon dioxide, the primary greenhouse gas contributing to climate change. Of the three fossil fuels, coal is the most widely used fuel in the United States, providing about 45 percent of electricity in 2010, followed by natural gas, which provided about 24 percent. Coal plays a critical role in the reliability of the electricity grid, especially in certain geographic areas, but coal-fired units also generally emit more air pollution than units burning natural gas or oil.

Under the Clean Air Act, the Environmental Protection Agency (EPA) establishes national ambient air quality standards for six pollutants that states are primarily responsible for attaining. States attain these standards, in part, by regulating emissions of these pollutants from certain stationary sources, such as electricity generating units. Numerous Clean Air Act requirements apply to electricity generating units, including New Source Review (NSR), a permitting process established in 1977. Under NSR, owners of generating units must obtain a preconstruction permit that establishes emission limits and requires the use of certain pollution control technologies. NSR applies to (1) generating units built after August 7, 1977, and (2) to existing generating units—regardless of the date built—that seek to undertake a “major modification,” a physical or operational change that would result in a significant net increase in emissions of a regulated pollutant. Units built before August 7, 1977, are not required to undergo NSR unless they undertake a major modification. For the purposes of this report, we refer to units that began operation in or before 1978—the first full year after NSR was established—as “older units” and those that began operating after 1978 as “newer units.”

In limiting NSR’s requirements to facilities built or undertaking major modifications after August 7, 1977, Congress allowed existing facilities to defer installation of pollution controls until they made a major modification, with the expectation that over time all facilities would either install such equipment or shut down, thereby lowering overall emissions. According to EPA data, 1,485 older units (43 percent of fossil fuel units) were still in operation in 2010. Some research suggests that many of these older units continue to operate without emissions controls, and in June 2002, we reported that older fossil fuel electricity generating units emitted air pollution at higher rates than newer units.

For more information, contact David Trimble at (202) 512-3841 or or Frank Rusco at (202) 512-3841 or

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