The groups, Boone-based Appalachian Voices and the Canary Coalition of Sylva, claim the government illegally awarded the credits without studying the plants' environmental impact.
The groups filed suit in U.S. District Court for the District of Columbia.
They named as plaintiffs Energy Secretary Samuel Brodman, Treasury Secretary Henry Paulson and two other Energy officials. In 2006 the department gave $62.5 million in clean-coal credits to Duke's Cliffside plant, which will add an advanced-design generating unit, and $133.5 million to Duke's new coal-gasification plant in Edwardsport, Ind.
Other plants getting credits are n Mississippi, Kentucky, California, Texas and Florida. The groups argue that the tax credits constitute "major federal actions" that require the government to hold environmental studies of the plants. Environmentalists say coal mining and coal-fired power plants do widespread damage, from scraping Appalachian mountaintops to releasing planet-warming carbon dioxide. Recently, they fought permits to expand the Cliffside plant.
"There's no such thing as clean coal as long as our mountains are getting clear-cut, blown up and bulldozed down," Appalachian Voices director Mary Anne Hitt said in a statement.
The Energy Department did not return calls. Duke spokeswoman Marilyn Lineberger said the company is "confident that the Department of Energy and Department of the Treasury carefully evaluated our applications" before awarding the tax breaks to Duke's plants.