Three area power plants rank among the “100 dirtiest” in the country, a new report by an environmental group says.

The Homer City and Conemaugh plants in Indiana County are examples of aging, coal-fired facilities that contribute significantly to the global-warming problem, Philadelphia-based PennEnvironment said Tuesday.

The organization’s report also cites the Keystone Generating Station on the border of Indiana and Armstrong counties.

“It’s time for the oldest and dirtiest power plants to clean up their act,” said Nathan Willcox, PennEnvironment’s energy and clean air advocate.

The organization analyzed 2007 data – the latest numbers available – from the U.S. Environmental Protection Agency in formulating its report, titled “America’s Biggest Polluters.”

Among the report’s findings:

n Pennsylvania ranked fifth of all states in terms of carbon dioxide pollution from power plants.

n Five plants in the Keystone State were among the 100 oldest such facilities in the country.

n Seven power plants in Pennsylvania were ranked among the top 100 “dirtiest” in the nation.

The report focused on carbon dioxide emissions, which PennEnvironment called “the main pollutant that fuels global warming.”

The Homer City Generating Station, which is 40 years old, produced more than 13.5 million tons of carbon dioxide in 2007, the report says. That’s equivalent to the carbon dioxide emitted by nearly 2.4 million cars, PennEnvironment said.

The plant, which ranked 34th on PennEnvironment’s list, is operated by Edison Mission Group of Irvine, Calif.

Edison spokesman Charley Parnell acknowledged that coal-fired power plants “are a significant contributor to CO2 emissions,” but he said there is little Edison can do.

“There’s no current, commercially available technology that would allow us to remove CO2 from our emissions,” Parnell said. “The only way to reduce those emissions is not running.”

However, Parnell said plant operators have taken steps to substantially reduce other pollutants.

That includes an investment of more than $270 million to cut nitrogen oxide emissions, as well as installation of a “scrubber” to cut sulfur dioxide emissions by 98 percent.

Mercury emissions also have been addressed, Parnell said.

Elsewhere in Indiana County, the Conemaugh plant in New Florence emitted 12.1 million tons of carbon dioxide in 2007, PennEnvironment said.

That equals the CO2 generated by about 2.1 million cars.

The facility, which dates to 1970, was ranked 44th on the organization’s list.

Houston-based RRI Energy Inc. operates the plant and has implemented “emission control upgrades” to reduce nitrogen oxide, sulfur dioxide and mercury emissions, the company’s Web site said.

But Ed Feith, the company’s vice president of environmental safety and health, echoed Edison’s spokesman in saying there is no “deployable technology” available to control carbon dioxide emissions.

RRI Energy also operates the Shelocta-based Keystone plant.

It emitted almost 11.9 million tons of carbon dioxide in 2007 and ranked 45th on PennEnvironment’s “dirtiest” list.

Together, RRI administrators said, the Conemaugh and Keystone plants churn out 3,500 megawatts of power.

The Edison-operated plant located in Homer City generates 1,884 megawatts.

Overall, coal supplies nearly half of the country’s electricity.

So Feith said coal-fired power plants are critical to the nation’s infrastructure.

“Coal will continue to have a role. We can’t just take coal out of the mix,” he said. “We can’t just turn off half of the power plants in the country.”

Industry representatives and PennEnvironment administrators also noted that there are no governmental regulations regulating carbon-dioxide emissions from power plants.

Feith said his company supports development of a “well-crafted energy policy” by Congress.

“It is a very big and complex issue,” he said.

But PennEnvironment’s Willcox contends that coal plants “must stop polluting with impunity” and urged lawmakers to support a “strong clean-energy bill” at the federal level.

“Clean energy holds amazing promise for America – to make our nation energy independent, create millions of new jobs and stop the worst effects of global warming,” Willcox said.