U.S. power plant emissions from the nation's largest generators continued to fall in 2011 even as overall electricity output rose "largely due to increased use of natural gas and growing reliance on renewable energy," according to a May 15 report issued by Ceres, the Natural Resources Defense Council and others.

Since Congress passed major amendments to the Clean Air Act in 1990, power plant NOx emissions have fallen 70% and SO2 emissions have dropped 72%. Carbon dioxide emissions have declined 7% from 2008 to 2011, according to the report, titled "Benchmarking Air Emissions of the 100 Largest Electric Power Producers in the United States."

The highest CO2 emission rates came from states that are heavily reliant on coal, including Wyoming and Kentucky, said the report, which was prepared from 2011 generation and emissions data from the U.S. Energy Information Administration and the U.S. EPA.

The groups attributed the steep emissions cuts to a shift away from coal-fired generation toward lower-emitting energy sources, such as renewables and natural gas. Between 2000 and 2011, natural gas generation rose 69% in the U.S. and renewable generation grew 44%. Coal generation dropped 12%, but still represented 44% of the power generated by the 100 largest generators, by far the largest.

"The electric power industry is moving to cleaner sources of energy, demonstrating that cleaner power generation is achievable. Stronger regulations will reinforce those trends and stimulate further investment in low-carbon, low-risk resources like renewable power and energy efficiency," said Mindy Lubber, president of Ceres, which helped sponsor the report. M.J. Bradley & Associates authored the report.

The top 100 power producers own more than 2,600 power plants and account for 86% of domestic electricity generation.

Air pollution emissions from power plants, while declining overall, are highly concentrated primarily among three power producers: Southern Co., American Electric Power Co. Inc. and NextEra Energy Inc., the report found.

Columbus-based AEP, the nation's largest producer of coal-fired generation in 2011, was listed in the report as the largest emitter of SO2, NOx and CO2, followed by Southern Co.

AEP and Southern have been preparing to retire thousands of megawatts of coal-fired capacity, convert coal units to natural gas or installing emission control equipment on certain units, in order to reduce emissions and meet new federal air regulations.

AEP spokeswoman Melissa McHenry told SNL Energy on May 15 that the report clearly shows the company is making progress in cutting emissions. "AEP has invested more than $7 billion since 1990 to cut sulfur dioxide and nitrogen oxide emissions by more than 80 percent. We will invest another $4 billion to $5 billion through 2020 to comply with the new EPA regulations that take effect in 2015," she said.

AEP plans to retire about 6,000 MW of coal-fired capacity and install additional pollution controls on more than 10,000 MW of capacity by 2020 "as we transition to a more diverse fuel mix for our generation fleet," McHenry said.

McHenry's comments were echoed by American Coalition for Clean Coal Electricity spokeswoman Lisa Camooso Miller, who said power companies have spent decades cleaning up their coal plants. "Over the past 30 years, the coal industry has invested over 100 billion dollars in technologies that have contributed to the reduction of emissions by 90 percent," Miller said.

While the report found power plants have made significant progress in reducing NOx and SO2 emissions through the installation of pollution controls at coal burning plants, it found less progress has been made in reducing mercury and CO2 emissions. Coal-fired plants were responsible for 81% of CO2 emissions, compared to 18% for gas-fired plants, the report said.

The EIA projects mercury emissions will drop beginning in 2016 with the adoption of the EPA's Mercury and Air Toxics Standards. And the EPA is planning to implement emission standards targeting greenhouse gas-emitting CO2 emissions for new and existing power plants.

On a state-by-state basis, the report found Texas was the leading emitter of CO2, followed by Pennsylvania, Florida, Ohio and Indiana.