Montgomery County Executive Douglas M. Duncan endorsed state legislation yesterday to limit emissions from coal-burning power plants, including a northern Montgomery facility that environmentalists call one of the county's biggest polluters.

Duncan (D) said one of his top priorities next year will be to persuade the General Assembly to pass a bill mandating that coal-burning power plants reduce certain emissions by up to 90 percent over the next eight years.

"It is time for the state of Maryland to aggressively regulate air pollution," Duncan said at an event in Silver Spring sponsored by the Clean Energy Partnership.

County officials said Duncan, who has had a checkered relationship with environmental leaders, will become more vocal in coming weeks on environmental issues.

Duncan's decision to target the Dickerson Generating Station, owned by the Atlanta-based Mirant Corp., and 14 other coal-fired power plants in the state, comes as he tries to burnish his environmental credentials in preparation for a possible bid for governor in 2006.

But several environmental activists dismissed Duncan's overtures, saying his ties to Montgomery County developers and support for the intercounty connector, the proposed east-west highway connecting Montgomery and Prince George's counties, make it unlikely he can win them over.

"This is a calculated political nibble at the environmental community to portray Mr. Duncan as an environmental leader, but on one critical issue -- the ICC -- he has failed, and I think people know that," said Neal Fitzpatrick, executive director of the Audubon Naturalist Society.

Betsy Johnson, chairman of the Maryland chapter of the Sierra Club, said her organization also is skeptical of Duncan's motives. "He has a very mixed record on the environment," Johnson said.

Other environmental leaders said Duncan actually has a strong record on energy-related issues. Earlier this year, he announced that the county would head a group of local governments in buying wind-generated electricity for some county buildings.

"He has always been a leader on this issue," said Gary Skulnik, founder of the Clean Energy Partnership.

The legislation, sponsored by Del. James W. Hubbard (D-Prince George's), would require power companies to equip their plants with technology that reduces the release of nitrogen, sulfur, carbon dioxide and mercury.

Hubbard said the legislation, which died in committee last year, is in response to the Bush administration's tepid effort at enforcing provisions of the federal Clean Air Act dealing with power plants and refineries.

Duncan said yesterday that forcing the Dickerson plant to use pollution control technology would be "equivalent to removing every car and truck in Montgomery County."

Steve Arabia, spokesman for Mirant, said the proposal would cost the region's power industry "hundreds of millions of dollars" and would lead to higher utility bills and possible layoffs at plants.

"I think people are well-intentioned, but they are shooting from the hip without a full appreciation of the complexity and the enormous costs of their proposals," Arabia said. He said company officials were surprised by Duncan's decision to endorse the legislation.

Emissions from the Dickerson plant have been a growing source of frustration for environmentalists. Last week, six protesters were arrested for blocking the entrance to the plant.