The study’s lead author Lincoln F Pratson said the stricter regulations on sulfur dioxide, particulate matter, nitrogen oxide and mercury might make 65% of the nation’s coal-fired power plants as expensive to run as their natural gas competitors.

“Because of the cost of upgrading plants to meet the EPA’s pending emissions regulations and its stricter enforcement of current regulations, natural gas plants would become cost-competitive with a majority of coal plants – even if natural gas becomes more than four times as expensive as coal,” Pratson said in a statement.

“This is a much higher fraction of economic vulnerability than has previously been reported.”

To conduct the study, Pratson and his team assessed the cost of electricity generation at plants producing 95% of the nation’s coal-fired electricity and 70% of its natural gas-powered electricity.

The researchers estimated costs for both types of plants over a wide range of fuel prices and under both existing and pending emissions standards.

Under current standards and at current fuel prices, 9% of US coal-fired plants are more costly to run than a median-cost natural gas plant, the research found. Even a modest jump in gas prices could erase this advantage.

“If the ratio of natural gas-to-coal prices rises to 1.8 from its recent level of around 1.5, coal plants would again become the dominant least-cost generation option,” Pratson said.

However, with the tougher emissions standards the EPA plans to enact and enforce, another 56% of US coal plants will become as costly to run as natural gas plants.

The regulations would make 65% of coal plants nationwide as expensive as natural gas, even if gas prices rise significantly.

“Most natural gas plants typically produce only one emission – nitrogen oxide – that is in excess of the proposed new EPA thresholds but many coal plants may exceed all of the thresholds, making it more expensive for them to come into compliance,” Pratson said.

“This has spurred legal and political debates over whether the pending regulations unfairly disadvantage the US coal industry.”

US electricity sector emissions have dropped considerably in recent years, due mainly to a recent surge in domestic shale gas production and the lower prices that followed.

But Pratson noted that the proposed EPA standards were not the only factor in the equation, saying that in order for natural gas to become a fully reliable replacement for coal, a large network of infrastructure would be required.

The Duke team’s peer-reviewed study was published this week in the online edition of Environmental Science & Technology.

Drew Haerer and Dalia Patino-Echeverri co-authored the study with Pratson, while funding came from the Bank of America Foundation and the National Science Foundation’s Center for Climate and Energy Decision Making.