CHARLESTON, W.Va. -- Coal-fired power plants around the country may face much greater financial risks than previously projected from a combination of low natural gas prices and stronger air quality rules, according to a new Duke University study.The economic viability of as many as two-thirds of the nation's existing coal plants could be threatened in the years ahead, according to Duke researchers who examined operating costs for hundreds of coal- and gas-fired plants nationwide."This is a much higher fraction of economic vulnerability than has previously been reported," said Duke geologist and energy expert Lincoln Pratson, the lead author of the paper, published late last week in the journal Environmental Science and Technology.The paper is apparently the first peer-reviewed study to look closely at issues that caused great controversy in coalfield communities and fueled an ultimately unsuccessful coal industry campaign to defeat President Obama's re-election bid last year.But the researchers take no clear side on the issue of whether ongoing coal industry woes should be blamed strictly on the U.S. Environmental Protection Agency or are mostly caused by a wave of low-priced natural gas production. Instead, they say that both play a role, and the two factors interact in complex ways."Our hope is that this analysis helps people identify what the potential economic mix would be for coal and natural gas for different price ratios," Pratson said in an interview Tuesday. "We hope that the study provides people on both sides of the debate with more information."Jeremy Richardson, a Union of Concerned Scientists analyst who is studying Appalachian coal trends, noted that the Duke paper was not an examination of the broad variety of factors affecting the regional industry, including the mining out of the highest quality reserves and competition from other coal basins."The way I look at it is that coal faces a sort of 'death from a thousand cuts'," Richardson said. "It's not just one or two factors."Already, the boom in shale-gas production has been reshaping the nation's energy supply, prompting significant shifts among utilities from coal to natural gas. Previous reports -- by government agencies and industry analysts, but not in peer-reviewed journals -- have blamed natural gas at least in part for the announced closures of 8.5 percent of the nation's fleet of coal-fired power plants. Those reports also projected that figure could double if natural gas prices remain low.The new Duke study reached similar conclusions about current pressures on existing plants.Pratson and his team assessed the cost of electricity generation at 304 coal plants and 358 natural gas plants. They estimated costs for both types of plants over a wide range of fuel prices and under both existing and pending emissions standards, and developed a ratio to express the cost differences between natural gas and coal.Authors found that that at current fuel prices, 9 percent of U.S. coal-fired plants are more costly to run than a median-priced natural gas plant. But, they found, even a modest jump in gas prices -- from the current natural gas-to-coal price ratio of 1.5 to 1.8 -- could erase coal's current disadvantage.The analysis "partially supports the findings of previous economic modeling studies that low natural gas prices could indeed be driving a significant fraction of coal plants to close," the study says. However, it also shows that "the relative competitiveness of coal vs. natural gas is highly sensitive" and that moderate changes in either prices for either fuel could result in "a majority of coal plants once again becomes significantly cheaper than the lowest cost natural gas plant."However, with new EPA rules on mercury, interstate pollution and other emissions, another 56 percent of U.S. coal plants could become as costly to run as natural gas plants, the Duke researchers found.The study found that, with new EPA rules, the majority of natural gas plants would remain cost competitive with a majority of coal plants unless or until the gas-to-coal price ratio increase from the current 1.5 to 4.3."Even if natural gas prices rise, the U.S. electricity sector could still continue its shift away from coal plants and toward greater use of lower [carbon dioxide]-emitting natural gas plants," the Duke study said. "This is because the economics of natural gas vs. coal plants depends not only on the price of natural gas, but also on the price of coal and on the expense of meeting the stricter EPA regulations."The Duke researchers made a number of assumptions, some of which could have inflated their estimates of regulatory costs, and others of which might have low-balled those costs.For example, the report assumes that power plants that don't currently meet new EPA standards would have to install separate control equipment for each individual pollutant. But, some power companies have said expensive controls for one pollutant, such as sulfur dioxide, will also help them control for other emissions, such as mercury.Also, the report takes into account only air pollution rules, not other potential regulations, such as coal-ash handling and disposal restrictions, which could affect coal-fired utilities.A spokeswoman for the National Mining Association said her group is still reviewing the Duke study.Melissa McHenry, a spokeswoman for American Electric Power, said the study outlines potential risks to power plants that are "much higher than the EPA projections of the impacts."We can't speak to the accuracy of their analysis, but this study certainly supports the concerns that we have been expressing for some time about the EPA regulations and the potential for regional reliability impacts that the closure of a significant amount of electricity generating units may cause," McHenry said in an e-mail message.John Coequyt, an energy analyst with the Sierra Club, noted that the Duke study did not take into account any health or climate change benefits from reducing coal plant pollution."We believe that Congress told companies to clean up their coal plants a very long time ago," Coequyt said. "Once you start to deal with the air and water pollution problems from these coal plants, you have to start thinking about whether these plants can be competitive."Reach Ken Ward Jr. at email@example.com or 304-348-1702.