The rule's goals for NOx, however, fall short, Epps and others contend. Though emissions would drop 52 percent overall, Pennsylvania plants would reduce them just 40 percent. They "must not be allowed to backslide," Epps said.
Companies have said that the EPA underestimates the cost and that they may be unable to meet the deadlines.
Meanwhile, the new rule is not the only one to take aim at an industry that historically has been one of the nation's biggest air polluters.
The EPA has three more rules in the pipeline that will further tighten limits on interstate transport, ozone pollution, and the emission of toxics, including mercury.
"There has never been a period of time even remotely similar to this one in terms of the number of different requirements coming down the pike," said Jeff Holmstead, a Clean Air Act expert with the law firm Bracewell & Giuliani L.L.P., which often represents electric companies.
For coal, the cascade of regulations presents a kind of high-stakes poker game. A company may ante up with a $100 million scrubber to remove sulphur dioxide, but then have to fold if new limits on emissions of toxic chemicals are just too expensive.
Companies need "clear, coordinated, and flexible regulations that provide certainty . . . to plan multibillion-dollar investments," said John Kinsman, environmental director at the Edison Electric Institute, an industry group.
Even companies that have upgraded or sold their plants - and that are largely supportive of the rule - are wary of what may come next.
PPL Corp. recently spent $1.4 billion for pollution controls on its Brunner Island plant in York County, built in 1961, and Montour in Montour County, built in 1972. Sulphur dioxide emissions at Montour fell 88 percent - from 128,000 tons in 2007 to 15,000 tons in 2009.
Both are massive "baseload" plants, running pretty much around the clock to help produce the 55 percent of electricity in this region that comes from coal.
Spokesman George Lewis said the company decided to install the equipment in advance of deadlines because "we wanted to get in the front of the line" for labor and materials, whose costs would likely only increase.
PPL also intended to sell pollution credits to plants that did not upgrade. But such a trading plan is not a certainty.
Whatever form the final rule takes, many of those who testified at Thursday's EPA hearing emphasized that the plan is meant to address health, not the bottom line.
"Otherwise," said Nick Rogers, a Philadelphia cyclist and asthmatic, "I and people like me will cost the government and private insurance companies billions of dollars in health-care costs."
