A Houston-based electric power generator plans to upgrade two coal-fired power plants in southwestern Pennsylvania to remove 207,000 tons of annual sulfur dioxide emissions from the atmosphere.

Reliant Energy, which operates 21 power plants in Pennsylvania, Maryland and New Jersey, plans to install new scrubbers in its Cheswick Generating Station in Springdale and the Keystone Generating Station, which it partially owns, in Indiana County.

The company also will upgrade scrubbers this year at the Elrama plant in Washington County and a power plant it owns in Niles, Ohio.

The new emission control systems, known as "wet flue gas desulfurization systems," will reduce sulfur dioxide by 98 percent of total emissions at Cheswick and Keystone.

They also will cut emissions of nitrogen oxide, or NOX, particulates and mercury, but the size of those reductions has yet to be determined. The company said it's investigating other technologies to reduce smokestack pollution.

Ed Feith, the company's environmental director, said the company will invest $350 million on environmental upgrades at Cheswick and Keystone that will create 1,100 construction jobs. The scrubbers will go into operation in 2009. The new technology will help Reliant Electric comply with federal clean air regulations that take effect in 2010.

"This is part of a strategy for complying with future requirements involving sulfur dioxide, mercury and nitrogen oxide," Mr. Feith said. "It's not mandated by any regulations to add a scrubber, but we're doing that to capture the economic benefits.

"The requirements become more stringent in 2010, so we'll be in compliance," he said.

The plants in question are older ones. Cheswick, which produces 580 megawatts of electric power, went into service in 1970. Keystone, which Reliant owns with six other entities, came online in 1968 and produces 1,700 megawatts.

Elrama was commissioned in the 1950s and produces 465 megawatts, while the Niles plant, which opened in 1954, produces 208 megawatts.

Had the company decided against the upgrades, Mr. Feith said, it would have been forced to buy environmental credits from compliant companies in 2010 to meet federal regulations. But those credits would have been expensive, making the investment in pollution control technology a wise alternative.

"This will be good for the environment, good for us and good for the communities we're in," Mr. Feith said.

Charles McPhedran, senior attorney with Citizens for Pennsylvania's Future, agreed that the company's action "is a good thing." He said it proves that stricter environmental regulations work to reduce acid rain caused by sulfur dioxide emissions, create jobs and improve the economy.

In past years, local power plants found it cheaper to buy environmental credits from compliant companies rather than reduce emissions. That policy, he said, left Pennsylvania with some of the worst air pollution problems in the nation.

"I'm encouraged by this action, but will it have enough of an effect?" Mr. McPhedran said. "Maybe we'll finally see those acid rain levels nudge downward."