The report –Ripe for Retirement: The Case for Closing America’s Costliest Coal Plants – found that more than 350 coal generators in 31 states might no longer be economically viable if they implemented modern pollution controls required by the US Environmental Protection Agency.

The Union of Concerned Scientists (UCS) said about 18% of the nation’s coal generating capacity should be considered for closure because the produced electricity would be more expensive than that from natural gas or wind power.

The science-based, non-profit group said in its report, Ripe for Retirement: The Case for Closing America’s Costliest Coal Plants, that another projected 40,000 megawatts of coal generation was already scheduled for mothballing or gas conversion in the coming years.

Combined, this represented about a third of coal power generation in the US.

“Our analysis shows that switching to cleaner energy sources and investing in energy efficiency often makes more economic sense than spending billions to extend the life of obsolete coal plants,” UCS Midwest office director and report co-author Steve Frenkel said.

“Regulators should require utility companies to carefully consider whether ratepayers would be better off by retiring old coal plants and boosting electricity generation from natural gas and renewable energy sources like wind.

“Spending billions to upgrade old coal plants may simply be throwing good money after bad.”

Shutting down the coal plants, the UCS said, could “create an opportunity to accelerate our nation's transition to a cleaner energy future” and shift investment from coal to renewable energy resources as well as a modernized grid and more reliance on the natural gas-fired sector.

Looking at age, size, efficiency and emission controls, the group evaluated the competitiveness of those remaining plants after required pollution controls were introduced against the operating cost of a typical gas-fired plant.

The study considered the operating cost of coal plants after controlling the four major air pollutants - sulfur dioxide, nitrogen oxide, particulate matter and mercury.

Ripe for Retirement then ranked the states and utilities with the most coal-fired power capacity that should be considered for closure.

“Georgia tops the state list, followed by Alabama, Tennessee, Florida, Michigan, South Carolina, Wisconsin, Indiana, Mississippi and Virginia,” the report said.

“Southern Company, one of the nation’s largest private utilities, owns the most coal-fired capacity ready for retirement, followed by government-owned Tennessee Valley Authority. Duke Energy, American Electric Power and FirstEnergy.”

UCS estimates ranged from a low of 1.7% of US annual output to a high of 6.3%.

"Our analysis shows many of these ripe-for-retirement units may already be uneconomic even before considering the cost of pollution controls,” it said.

Even without pollution controls, 23,400MW were already more expensive to operate than existing natural gas plants, it said.

The US has 316,000MW of coal-fired generation, according to the scientists, which makes up about 30% of the nation's total generation fleet.

Coal-fired generators are forecast to produce about 37% of the nation's power supply in 2012, down from 42% in 2011.

According to Energy Information Administration figures for 2013, coal should produce 40% of the nation's electricity.

“This is an historic opportunity to accelerate the transition to a clean energy economy,” Frenkel said. “Decisions we will make in the next three to five years can improve public health, reduce global warming and create a more resilient energy system.”