CHARLESTON, W.Va. -- As many as 353 additional coal-fired power plants around the country should be considered for closure because operating them is more expensive than generating electricity with low-cost natural gas or wind, according to a new report from the Union of Concerned Scientists. The power plants, located in 31 states, may no longer be economical if they need to be upgraded with the latest air pollution controls and compete with other fuels, according to the report released Tuesday. Collectively, these "ripe-for-retirement" facilities -- as the group called them -- produce about 6 percent of the nation's power and represent nearly 18 percent of the nation's fleet of coal plants. "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," said Steve Frenkel, director of the UCS Midwest office and co-author of the report. Frenkel said that state-level utility regulators should require 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," Frenkel said. Plants targeted in the report as candidates for retirement are typically older, less efficient, underutilized and more polluting than the rest of the nation's coal fleet. They averaged 45 years old, well beyond the 30-year expected life of a typical coal plant, and 70 percent of them lacked the most modern pollution controls to limit emissions of sulfur dioxide, nitrogen oxides and mercury. The Union of Concerned Scientists is a science-based nonprofit group that combines research with citizen action to promote a healthy environment and a safer world. Experts from the group examined publicly available data on coal-fired power plant efficiency, economics and environmental performance to analyze which facilities could become uneconomical compared to natural gas and if forced to improve emissions control. The resulting report included two lists: One collection of 353 plants that could have trouble competing with existing natural gas plants that are already paid for, and another set of 153 plants that could be uneconomical compared to new gas plants whose costs have yet to be recovered by utilities. Both lists are in addition to the 288 coal-generating units -- with power plants themselves often including several generating "units" -- already announced for retirement over the next few years. In West Virginia, the largest of the two new lists included the relatively small Morgantown Energy Facility and the Grant Town power plant, but contained a surprise: American Electric Power's John E. Amos plant in Putnam County. Jeff Deyette, a UCS energy analyst and report co-author, said his group believes the largest of the plant's three units -- a 1,300-megawatt generator opened in 1973 -- would have trouble competing with natural gas if it were forced to add a bag-house to control particulate matter pollution and activated carbon injection, or ACI, to reduce mercury emissions. But Melissa McHenry, a spokeswoman for AEP, said her company is confident that the John Amos facility can meet the latest federal mercury and air toxics limits with its existing pollution controls, which include $1 billion sulfur dioxide scrubbers that serve all three of the plant's units. "We are fully committed to continuing to operate the Amos units long term," McHenry said. McHenry said that while natural-gas prices are low now, its costs have historically been volatile. Many coal-fired plants are going to close, she said, but AEP plans to maintain use of a diversity of fuels, including coal. "The real challenge that has happened is the very low price of natural gas," McHenry said. "But natural-gas prices have been very volatile, and it's not clear that shale gas has ended that volatility." Deyette acknowledged that if John Amos can meet the latest U.S. Environmental Protection Agency limits with its existing pollution equipment that could help keep it economical going forward. Even assuming AEP were to invest money for bag-houses and additional mercury controls, John Amos was close to not making the UCS list, Deyette said. "It's sort of a yellow-flag situation," Deyette said. "There is a risk that the Amos plant faces going forward." The Union of Concerned Scientists said that shuttering the 59 gigawatts of coal-fired generators the new study identified, along with the 41 gigawatts already slated for retirement would reduce the electric power sector's annual carbon dioxide emissions up to 16.4 percent, or 410 million tons. "This is an historic opportunity to accelerate the transition to a clean energy economy," the report 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." Reach Ken Ward Jr. at firstname.lastname@example.org or 304-348-1702.