The Massachusetts Department of Environmental Protection has given final state approval to a Somerset power plant to use a new technology called coal plasma gasification.

State environmental officials say the new process is a cleaner alternative than Somerset Power LLC’s current emissions at its Riverside Avenue coal-burning plant. The technology uses high heat to convert coal to synthetic gas, which is then used as fuel.

The company says it will reduce some air pollutants by 95 percent.

But the state’s action Thursday is being criticized by some environmentalists, who say it allows the plant to continue releasing carbon dioxide for decades to come. Opponents have 21 days to file an appeal.

The Fall River Herald News reported that community activists called the decision unacceptable and said it significantly undermined the state’s policy against global warming.

The Somerset plant was set to be shut down or powered with cleaner emissions by 2010 under the state’s “Filthy Five” regulation, intended to cut down on the region’s global warming pollution.

“We are outraged by this decision,” said Westport resident Dave Dionne. “This community has endured the presence of coal-fired power plants for decades. Prolonging the life of one of these plants in our backyard is a clear example of environmental injustice. Public opposition is really mounting — this fight is far from over.”

Earlier this month, the state’s Executive Office of Energy and Environmental Affairs declined to order a Massachusetts Environmental Policy Act review of the plant.

The Conservation Law Foundation, which lobbies on environmental issues in New England, had called for the review. “In a final permit issued by DEP, the commonwealth said it would allow the power plant to adopt experimental coal plasma gasification technology and continue releasing carbon dioxide at current levels,” said the foundation’s spokesman Colin Durrant. “The decision runs contrary to a 2007 policy adopted by the Patrick administration to reduce global warming pollution.”

State officials say the plant won’t emit more carbon dioxide from coal than it already does. Somerset Power, which is owned by NRG Energy, the 10th largest U.S. power company, wants to retrofit its 50-year-old boiler to allow for the plasma gasification process, which breaks down coal into its component parts before converting it into energy.

The Conservation Law Foundation claims the process will product 28 million tons of carbon dioxide emissions that could be avoided if the plant were closed. NRG Energy has said the foundation’s pollution estimate is skewed because it compares emissions from gasification to a total shutdown of the plant, not to continued operation as a coal-fired facility.

Somerset Selectmen William Meehan, Lorne Lawless and Eleanor Gagnon reiterated their support of the power plant’s project after the state refused the CLF’s request for a full review of the plant.

Earlier this month, Energy and Environment Secretary Ian Bowles rejected a petition filed by the CLF, Clean Water Action, the Toxics Action Center and Environment Massachusetts requesting a full-scale environmental review to assess the impact of the Somerset proposal on greenhouse-gas emissions and global warming.

“This decision is a clear step backward for clean energy in Massachusetts,” said Katy Krottinger, clean energy organizer at Clean Water Action. “The failure of the Department of Environmental Protection to hold the plant owners to their original commitment to either re-power with cleaner fuel or shut down is deeply disappointing, particularly since the proposed technology is untested and has never been applied to coal burning in this country,” she said, the newspaper reported.