Faced with a reality they have fought for months, Pennsylvania's coal-plant operators now have to figure out how they can vastly reduce the mercury emissions from their plants.

Under a new rule that went into effect Saturday, they must cut mercury emissions 80 percent by 2010 and 90 percent by 2015.

With the rule, Pennsylvania joins Illinois among the country's major coal-producing states that have adopted measures tougher than federal legislation. That legislation requires cuts of about 86 percent in emissions over the next 20 years.

The state has 36 coal-fired power plants and is second only to Texas in mercury emissions. The new rule would stop an estimated 3.6 tons of mercury from flowing into the air per year.

"This is great news for the people and children of Pennsylvania," said Gov. Rendell's press secretary, Kate Philips.

Mercury becomes airborne when coal is burned. Once it falls into waterways, it becomes methylmercury, which is more toxic and accumulates in fish. It can cause nervous-system damage in a developing fetus and young children.

But Douglas L. Biden, president of the Electric Power Generation Association, predicted a catastrophic fallout among older, smaller plants.

Unable to afford the new technology, "they're going to go out of business," he said. "They're going to shut down."

The rest, Biden said, will explore which technologies - or combinations of technologies - will reduce their emissions by what he said would be closer to 98 percent, given the intricacies of overlapping federal and state rules.

He said that in addition to smokestack equipment, plant operators would consider switching to coal that has less mercury to begin with, or washing it with chemicals to remove some of the mercury before the coal is burned.

One problem, Biden said, is that not all the technology is proven, so a plant might go to the expense of installing it and still not get the reductions it needs.

He also said that if plants have to spend too much money to meet the new limits, they will not be competitive in the wholesale market. Then, he said, PJM Interconnection LLC, the company that presides over the power grid for 13 states and Washington, might tell Pennsylvania plants to power down and shift production to Ohio plants "to the west, upwind of us."

Exelon Generating spokesman Ben Armstrong said it was too early to tell what effect the rule would have on the company, which depends on coal for 9 percent of its electric generation.

Exelon's two wholly owned coal units - in Eddystone and Phoenixville - have scrubbers that let them meet existing federal regulations for mercury, nitrogen oxide, sulfur dioxide and carbon dioxide, he said.

Allentown-based PPL Corp. is building scrubbers at its coal-burning plants in Montour and York Counties.

Designed to reduce sulfur dioxide emissions, the scrubbers also should reduce mercury, said spokesman George Lewis, although just how much is unclear. "We expect that we'll have to do something in addition."

The company has warned shareholders that the tab for extra mercury controls could exceed $500 million, Lewis said.

Like many in the industry, PPL had hoped the Pennsylvania rule would allow trading, so companies able to reduce pollution more than required could sell "credits" to plants that could not make the reductions without great expense.

That may yet happen.

State Sen. Mary Jo White (R., Venango), chairwoman of the Senate Environmental Resources and Energy Committee, who battled with the Rendell administration over the initiative for a year, said yesterday that she planned to introduce legislation that would incorporate the Rendell plan but also allow interstate trading of emissions.

PennEnvironment's energy and clean-air advocate, Nathan Wilcox, said his group would be on the lookout for such efforts.

Calling the rule a "tremendous victory for Pennsylvania's environment and public health," he said nearly 11,000 citizens submitted comments in support of the rule.

"So there is a definite sentiment that the public's wishes are being carried out," he said.

Contact staff writer Sandy Bauers at 215-854-5147 or sbauers@phillynews.com.

Inquirer staff writer Amy Worden contributed to this article.