America's fleet of coal-fired power plants is aging. Having built those plants in an era of expensive natural gas and less stringent clean air regulations, much of the industry now finds itself at odds with the economic and political realities of the 21st century.

With three-quarters of coal plants already in excess of their original 30-year life span, some 41 gigawatts of coal-fired power capacity -- 288 units, or roughly 4 percent of total U.S. electrical capacity -- is currently slated for closure.

In 2009, the utility Public Service of New Hampshire decided to retrofit rather than close down the 52-year-old Merrimack Station power plant. Three years and $422 million later, the plant is often idle due to cheaper gas-fired power elsewhere in the state. Photo courtesy of Public Service of New Hampshire.

Yet even those numbers may underestimate the true economic vulnerability of the U.S. coal-powered fleet. A new study by the Union of Concerned Scientists (UCS) identifies an additional 59 GW of coal-fired power in 31 states that, saddled with the costs of state-of-the-art pollution controls, could become financially uncompetitive with other energy sources.

Of those 59 GW, "About 40 percent are economically vulnerable even before you factor in the cost of new pollution control technologies," said Steve Frenkel, director of the Midwest office for UCS and co-author of the report. "In all of the areas we looked at, there is sufficient underutilized gas and renewable energy potential to more than replace uncompetitive coal plants."

Even more plants would become vulnerable with the creation of a carbon price, including the one proposed by Reps. Henry Waxman (D-Calif.) and Edward Markey (D-Mass.) in 2009, and a carbon tax currently being weighed by Washington think tanks as one solution to the "fiscal cliff" problem.

Retrofitted only to idle

One example: For much of the past year, the Merrimack power station near Bow, N.H., has sat idle. The 500,000 customers it once supplied now receive power purchased on the open market, much of it from gas-fired plants elsewhere in the region.

The lost time is costly. Just three years ago, Public Service of New Hampshire (PSNH), the utility operating the plant, decided to retrofit Merrimack to control for harmful nitrous oxide and sulfur dioxide emissions. The retrofits cost $422 million, costs that are still being recouped from the utility's customers.

The UCS report says the Merrimack plant, which PSNH suspended indefinitely last May, is illustrative of a problem that is now endemic to coal-fired power -- many plants simply cannot operate for the same price as natural gas.

And natural gas isn't the only competition squeezing coal. "For at least 353 coal-fired generators -- nearly a third of those across the nation -- it would be less expensive to build and operate a new array of wind turbines than it would be to retrofit and run these old coal units," the report says.

The report does not estimate the cost of meeting current and pending U.S. EPA regulations, but rather the cost of installing pollution controls for specific pollutants, like sulfur dioxide, at plants currently lacking such controls.

"Our analysis is an analysis of a modernization of the coal fleet rather than compliance with any particular measure," said Frenkel. The report's estimates are based on the costs of pollution controls that are standard practice within the industry, he said.

Many of the plants identified by the UCS report are among the oldest -- and, for that reason, most underperforming -- in the country. Already, many are operated well below capacity, meaning their closure would not necessarily be accompanied by a proportional reduction in capacity.

"We found that 18 percent of the nation's coal-fired capacity is vulnerable. When you look at how much actual power those plants produce, it's only 6 percent of energy use," said Frenkel.

"We're not advocating for the closure of any particular plant," he added. "We just want utilities to take a hard look at the economics before they decide to retrofit, and ask themselves -- does this really make sense? Are other alternatives available?"

Pending clean air rules could drive more closures

Utilities have by and large decided on their closures for the next several years, and will probably not deviate much from the current plan of retiring 41 GW of coal-fired power, said Luke Popovitch, vice president of communications for the National Mining Association.

"The utilities' current road map is likely to be the real driver in terms of destroying existing capacity," he said.

Although the UCS report does not look at the cost of specific clean-air regulation, a pending rule over emissions standards may play a significant role in future plant closures.

The Mercury and Air Toxics Standards (MATS), issued in 2011 and currently facing challenges in the U.S. Federal Court of Appeals, could be the biggest driver of plant closure in the short term, said Jeff Holmstead, a former assistant administrator of air and radiation at EPA under the George W. Bush administretion and current head of environmental strategies at the law firm Bracewell & Giuliani.

"While we won't know for sure until the middle of next year, we've seen a number of pretty fair analytical pieces put out in the lead-up of MATS that put the number of closures higher than previous estimates," he said.

A revised version of the MATS rule is expected in March 2013.