Power plant emissions of sulfur dioxide and nitrogen oxides continue to decline in 2012

Emissionsof sulfur dioxide (SO₂) and nitrogen oxides (NO_x) from the electric power sector in 2012 declined to their lowest level since the passage of the Clean Air Act Amendments of 1990. The decline in emissions is due primarily to an increasing number of coal-fired units retrofitted with flue-gas desulfurization (FGD), or scrubbers, to coal plants switching to lower sulfur coal, and to selective catalytic reduction (SCRs), selective noncatalytic reduction (SNCR), or low NO_x burners to limit NO_x emissions. In recent years the decreased use of coal for electric power generation because of cheaper natural gas has also played a significant role in the SO₂ and NO_x emissions declines.

The decline in SO₂ and NO_x emissions began soon after enactment of the 1990 Clean Air Act Amendments, which established a national cap-and-trade program for SO₂ and required other controls for NO_x emissions from fossil-fueled electric power plants. The program was primarily motivated by concerns regarding acid rain affecting areas downwind of plants emitting these compounds. Because coal-fired units accounted for a large share of SO₂ and NO_x emissions, the program provided an economic incentive for coal-fired power plants to reduce emissions by installing pollution control systems, burning lower sulfur coal, or, for high-emitting plants, dispatching less electricity.

In 2005, EPA developed the Clean Air Interstate Rule, a cap-and-trade program intended to reduce SO₂ and NO_x emissions beyond the levels defined by the acid rain program in the eastern half of the United States. While that rule and several proposed alternatives have been challenged in the courts, 91 GW of coal-fired power capacity was retrofitted with FGD scrubbers between 2005 and 2011. By the end of 2011, 60% of the U.S. coal fleet had FGD scrubbers installed, and 67% had either a SCR or a SNCR installed.

Additional state requirements and settlements under the Clean Air Act's New Source Review program also contributed to the increases in coal plant environmental retrofits. The Mercury and Air Toxics Standards, issued in 2011 and set to take effect in 2015, do not directly regulate SO₂ or NO_x but the standards should lead coal-fired power plants to install equipment that further limits SO₂ emissions.

The reduction in SO₂ and NO_x emissions through the late 2000s was primarily the result of coal plants employing strategies to reduce emissions, not a reduction in the use of coal overall. In fact, coal generation set record highs in 2007 when significant SO₂ reduction was already underway. Two major contributors of declining SO₂ and NO_x emissions since late 2008 have been:

• Lower overall generation from coal in 2012
• Historically low natural gas prices, which have contributed to shifting some generation from coal to natural gas