Two local power plants where environmentalists have long complained of excessive pollution will probably stop burning coal, the facilities’ new owner has indicated.

Apparently responding to the same trends that have forced the industry to retire so many other coal plants around the country, NRG Energy requested permission from a federal agency recently to deactivate the coal units at two of its Maryland facilities. The generators at Dickerson in Montgomery County and Chalk Point in Prince George’s County will continue to burn oil and natural gas.

Environmentalists were pleased with the news. “It’s a forward-thinking move,” said Diana Dascalu-Joffe, a lawyer at the Chesapeake Climate Action Network.

Dave Gaier, a spokesman for the company, said that the deactivation request came in response to increasingly stringent regulations imposed by the state. For the coal-fired boilers, all of them about 50 years old, to comply, NRG would have to make costly investments in new scrubbers and other equipment.

“Those would simply not be economical to install at those particular coal units,” Gaier said.

Yet the plants have run afoul of existing rules as well, according to regulators. The two facilities are among a group of energy properties in the region that have changed hands frequently in recent years and have often been a source of legal trouble for their owners.

NRG is trying to settle a lawsuit brought by the state alleging violations of wastewater rules at Dickerson and Chalk Point. That suit was filed in June, shortly after Maryland settled another case with the company over the disposal of the ash the facilities produce.

The Chalk Point plant was the site of a major spill in 2000, when a ruptured pipeline leaked an estimated 111,000 gallons of oil into the Patuxent River.

“You can still stick an oar into the muck and it comes out black with oil,” said Fred Tutman, who leads Patuxent Riverkeeper.

He said that the termination of the coal units at Chalk Point would be “a step in the right direction” but that the oil and gas units might remain a danger to the watershed.

NRG, which is based in Princeton, N.J., and owns several dozen plants around the country, took control of the Chalk Point and Dickerson facilities when it acquired their previous owner last year.

NRG’s decision to stop burning coal at the two plants is not yet final, and Gaier declined to discuss the company’s plans in detail. NRG would deactivate the units in 2017.

Yet Tutman and Dascalu-Joffe think that not only are the closures at Chalk Point and Dickerson inevitable, but also that other coal-fired facilities in Maryland will be forced into retirement.

They noted forthcoming restrictions on carbon dioxide emissions from the Obama administration and the decreasing cost of natural gas resulting from new extraction methods.

A Western Maryland coal plant, the R. Paul Smith facility in Williamsport, shut down last year, but the small facility was aging and rarely used. Closures at Dickerson and Chalk Point would be more significant. The coal units at the two plants have a capacity of 1.2 gigawatts in total, enough to power about 1 million residences.