The Tennessee Valley Authority sharply accelerated a shift away from coal as an energy source on Thursday, saying it would shut down eight electricity-generating units that together will burn nearly a fifth of its coal this year.
The closings are part of a long-term strategy, also announced Thursday, for the authority to generate 20 percent of its electricity from coal, instead of the current 38 percent. It also plans to increase the use of renewable energy sources like solar and hydropower to 20 percent, from the current 15.7 percent.
The authority’s chief executive, Bill Johnson, said experts were studying whether more coal-fired plants should be shut down later.
“These were difficult recommendations to make, as they directly impact our employees and communities,” he said in a release announcing the shutdowns. “But the plan is what’s best in terms of its positive impact on T.V.A.’s rates, debt and the environment, and it will bring the greatest benefit to the people of the valley.”
Officials did not say when the generating units at three plants in Alabama and Kentucky would be shut down. But they noted that new Environmental Protection Agency standards for emissions of mercury, sulfur dioxide and other power-plant pollutants would take effect in 2016. At least some of the units marked for closing would not meet those standards.
The authority also faced the prospect of new limits on carbon dioxide emissions at the units. President Obama has ordered the Environmental Protection Agency to propose a regulation capping carbon pollution from existing power plants by next summer.
Closing the eight generating units would significantly cut the authority’s emissions of greenhouse gases. In the mid-2000s, before recession crippled manufacturing and reduced electricity demand, the units jointly produced 22 million tons of carbon dioxide a year, nearly a fifth of the authority’s entire output and, by E.P.A. calculations, the equivalent of the output of 4.6 million vehicles per year.
Last year the units being closed produced a combined total of 14.1 million tons of carbon dioxide. “It’s a considerable chunk of our system,” the authority’s director of environmental policy, John Myers, said in an interview.
Two other developments hastened the shutdowns: the advent of cheap natural gas, which has turned coal into a costlier fuel, and falling demand for electricity.
The authority finished installing $500 million in pollution controls last year at two of the generating units to be closed at its Paradise power station in western Kentucky. Now the two will be replaced by one or more gas-burning units.
Electricity demand has fallen by about 9 percent in the last five years in the region that T.V.A. serves, Mr. Johnson said, as the authority altered its rates to promote conservation, and industrial customers — forced to cut costs during the recession — turned to power-saving techniques like automation.
Thursday’s announcement was the second and biggest step the authority had taken to reduce its appetite for coal. In 2011, T.V.A. agreed to retire 18 coal-fired generating units to settle a lawsuit by states and environmental groups charging violations of the Clean Air Act. Four of those 18 units have been shuttered so far.
Many of those generating units were comparatively small. Thursday’s closings include some of T.V.A.’s largest coal-fired units, and their use of coal and carbon emissions far exceed those of the plants chosen in 2011.
Eventually, the authority hopes to get a fifth of its power each from coal, natural gas and renewables and the remaining two-fifths from nuclear plants. In 1971, the authority got 80 percent of its electricity from coal.
Environmental groups applauded the announcement. “It’s a big deal,” said Bruce Nilles, who directs the Sierra Club’s Beyond Coal campaign. “The T.V.A. has been for decades one of the largest coal burners in the country.”
He said the closings symbolized “a huge opportunity to decarbonize the electricity sector in a very short time.”