The future of coal-generated electricity, some say, sits in a boxy, blue steel complex at the edge of the Delaware River south of Trenton.

The huge Mercer Generating Station is three times as big as it used to be. That's because it now bristles with more than half a billion dollars in air-pollution equipment that has reduced emissions 90 percent or more.

"This is how coal in the 21st century looks," said Mark Brownstein, an energy expert at the Environmental Defense Fund, the national advocacy nonprofit.

A more traditional coal plant sits just across the river in Pennsylvania, a little farther north.

New Jersey officials say the Portland plant, in Northampton County, emitted nearly three times as much sulfur dioxide in 2009 as all of New Jersey's plants combined.

The emissions were wafting east, fouling the air in four North Jersey counties. Regulators want the plant to reduce sulfur dioxide 81 percent.

The two plants show the present and the promise of coal power. Burning coal is both common and dirty, and while some plants have cleaned up their act, others have hunkered down, betting they can squeeze out more energy before regulators clamp down.

A hearing in Philadelphia on Tuesday will help chart the course of coal, which provides 45 percent of the nation's electricity and is expected to remain dominant for the next quarter-century.

The Environmental Protection Agency wants public comment on a rule that would curb plant emissions of mercury, arsenic, lead, nickel, chromium, and acid gases.

Other rules in progress would limit ozone, particulates, sulfur dioxide, and nitrogen oxides.

All can harm people's lungs.

The issue resonates in this region because Pennsylvania is a coal powerhouse; the output of its 40 plants ranks it among the top coal-burning states.

Yet all breathe the emissions that can spread hundreds of miles downwind.

While four of New Jersey's five coal-fired plants have modernized to meet new standards (the fifth is considering a change to oil), officials estimate that half of Pennsylvania's geriatric fleet will have to make major investments.

That or switch to cleaner fuels, as PPL did with a 1950s-vintage plant in Northampton County, Martins Creek, which now burns natural gas.

Or shut down. Exelon Corp. has already announced the end for its aged plants in Eddystone and Phoenixville.

Some, such as Portland, got enmeshed in legal battles as they resisted installing more pollution controls.

Even if coal remains part of America's energy mix for the next generation, "what do you do with these old plants?" asked Joseph O. Minott, executive director of the Clean Air Council in Philadelphia. "They just go trudging along."

Officials "sort of skirt the requirements in the hopes that they'll be able to squeeze a few more years out of them," he said.

The industry says the new rules would be too costly and onerous.

PSEG actually pays a penalty, of sorts, for its upgrades in New Jersey. Its electricity is now more expensive. In the regional grid, plants that are cheaper (usually older and more polluting) are brought online first.

But PSEG says Mercer is proof that a plant can clean up its stacks and be profitable, pointing the way the nation's coal-fired plants must go.

Companies have seen the rules coming and have had time to plan, said Eric B. Svenson Jr., a PSEG vice president. "We think there's limited excuse for not getting this thing done."

In announcing the new mercury rules that will be debated Tuesday - the first for the nation - EPA Administrator Lisa P. Jackson said they would save 10 times the cost of implementing them in health benefits.

It all comes down to lungs.

Healthy adults might cough or wheeze on a smoggy day, said Paul Billings, a vice president of the American Lung Association. "For someone with asthma, it could trigger an attack.

"For someone with heart or lung disease, it could land them in the emergency room or even lead to their premature death."

The EPA has estimated the mercury rule would cost the typical residential customer $3 to $4 a month and save 17,000 lives a year.

"That's a very large body count," Billings said.

'New source review'

Even in the 1990s, Mercer was old.

Its first unit came online in 1960, the year Elvis Presley recorded "Are You Lonesome Tonight?"

PSEG did some maintenance upgrades, which triggered a provision of the Clean Air Act called "new source review."

In general, coal-fired power plants have been exempt from new rules. But under this provision, any that made substantial changes would be considered a new source of power, subject to new rules.

Then, as now, the climate was rancorous.

The Clinton administration wanted to cut emissions, but the industry was balking.

PSEG took advantage of the fact that the administration was eager to negotiate.

The company "ran the numbers, and we looked at any number of different scenarios," Brownstein said. Ultimately, it concluded that it was best to invest and retrofit.

PSEG agreed to some upgrades in 2002. Then, after the deadline passed unmet, it consented in 2006 to even more pollution-control equipment, $6 million in fines, and $3.25 million for a regional pollution-reduction program.

The EPA's Jackson has said the new rules would spur job growth. During peak construction at Mercer, more than 800 construction workers were on site. Now that it's up and running, 20 people have been added to a staff of 130.

Now just one third of the plant is tied to electricity production, Svenson said. "The other two-thirds is a chemical plant for cleaning up the emissions."

About $100 million went into a device that works like a catalytic converter on a car; it reduced nitrogen oxides, which contribute to smog, 95 percent.

An additional $10 million funded a carbon injection system that reduced emissions of mercury, a neurological poison that accumulates in fish, more than 90 percent.

A $265 million "scrubber" reduced sulfur-dioxide emissions by 90 percent.

A $225 million baghouse helped reduce soot, which also contributes to smog, 99 percent.

PSEG met all terms of the agreement in December, an EPA spokesman said.

Some environmental advocates still criticize the move, saying the plant should have converted to a cleaner fuel.

"In this day and age," said Matt Elliott, clean-energy advocate for Environment New Jersey, "we should be retiring and replacing them with cleaner alternatives, not figuring out a way to string them along into the future."

But William O'Sullivan, air-quality director with the New Jersey Department of Environmental Protection, said the latest Mercer test results impressed him.

His staff is still reviewing the data, but the emissions "are likely amongst the lowest in the country for coal burners and lower than from oil-fired units," O'Sullivan said. "Some of the emission levels are in the same range as burning natural gas."

'Health risk'

Pennsylvania's Portland plant, built two years earlier, in 1958, is owned by GenOn Energy.

"It has poor air-pollution controls every way you look at it," said O'Sullivan. "While PSEG has all the bells and whistles, Portland has none."

"Our residents have a right to be protected from this health risk," DEP Commissioner Bob Martin said in 2010, when he petitioned the EPA to take action.

In March this year, the agency proposed requiring the plant to reduce sulfur-dioxide emissions 81 percent over three years.

A public-comment period continues until Friday.

GenOn spokesman Mark Baird said the plant had complied with all Pennsylvania and federal standards. Once new federal standards are in place, "one way or another, we'll comply."

Much farther west is Homer City, Pennsylvania's second-largest coal plant.

David Hawkins, a former EPA air administrator and a climate expert with the Natural Resources Defense Council, called Homer City "a prime example of a big, old-fashioned plant. The operator strategy is to run it as long as possible."

New Jersey, New York, Pennsylvania, and the U.S. government contend that its 1,217-foot smokestack - 25 percent higher than Philadelphia's Comcast Center - spews pollution hundreds of miles away.

In January, the EPA filed a federal lawsuit against the plant and its operators, with the states joining in. The suit says the sulfur-dioxide pollution alone - more than 100,000 tons a year - makes the plant "one of the largest air-pollution sources in the nation."

Like PSEG, the owners had made plant modifications that the EPA said triggered the "new source review," requiring them to control pollution.

"The owners of this power plant have repeatedly thumbed their noses at clean-air laws," New York Attorney General Eric Schneiderman said.

Charley Parnell, a spokesman for Edison Mission Energy, which has operated the plant since 1999, said the company should not be responsible for the actions of a previous owner.

"We haven't fought against installing new controls," he said. The company installed some, and is "waiting to find out what the new rules of the road were going to be" before installing others.

Contact staff writer Sandy Bauers at 215-854-5147 or sbauers@phillynews.com. Visit her blog at http://go.philly.com/greenspace