After more than two years of review, Massachusetts energy officials began using stricter standards Friday to help ensure wood-burning power plants operate more efficiently and reduce emissions.

Under the new rules, wood-burning plants would receive partial subsides under state green energy programs if 50 percent of the wood they burn generates electricity, rather than escaping through stacks as emissions. They would get a full subsidy if 60 percent of the wood they burn generates electricity, according to the Massachusetts Department of Energy Resources. Many of these so-called biomass operations struggle to achieve a 25 percent efficiency rate.

The stricter standard is a reversal of support for a power source the state once promoted as critical to its efforts to use renewable resources to combat climate change. That changed when a 2010 study commissioned by the state revealed that power plants that burn wood released more greenhouse gases into the atmosphere per unit of energy than fossil fuels, and that those heat-trapping gases took longer for forests to absorb than previously thought.