In February, a federal appeals court in Washington ruled that the US Environmental Protection Agency violated the Clean Air Act in not setting mandatory cuts for mercury emissions of power plants.

The suits would be filed in the federal districts where the proposed power plants would be located, Nilles said. The suits would seek to require the plants to go back to state permitting agencies for new permits that meet the tougher emission standards, Nilles said.

Coal-fired power plants are the largest source of mercury emissions in the United States as well as 40 percent of US carbon dioxide (CO2) emissions. CO2 is the by far the largest contributor to greenhouse gases that cause global warming.

Coal-fired power plants also are seen by most national politicians as essential because they make half the electricity used in the United States.

The Sierra Club said there are alternatives to coal power plants and that until capturing and sequestering carbon dioxide emissions is proven feasible and affordable, no more coal plants should be built.

Nilles said the Sierra Club has helped stop 63 of the 150 coal-fired power plants that were in the planning stages since 2002, including 31 last year.

"There are affordable technologies widely available today that can substantially reduce mercury and other toxic pollution," said Pat Gallagher, director of the Sierra Club's environmental law program. "In their rush to build new coal plants, developers have turned a blind eye to these technologies, and correspondingly the health of children everywhere."

About 30 coal-fired plants may be affected by the Sierra Club suits, Nilles said.

Owners of three plants under construction have already been notified of the intent to sue by the Sierra Club -- Entergy for a plant in Louisiana, Peabody Energy for a plant in Kentucky, and Louisiana Generating, a unit of NRG Energy, for a plant in Louisiana.

Another eight letters were sent Tuesday, for plants in Arizona, Georgia, Kentucky, North Carolina, Texas and Wyoming, the Sierra Club said. Among the plants involved are Duke Energy's Cliffside plant in North Carolina and Energy Future Holdings, formerly TXU Corp, for its proposed Oak Grove plant in Texas.

The Sierra Club said it is considering whether to send intent to sue letters to owners of a dozen more plants in Kentucky, Louisiana, Texas, Wyoming, Iowa, Massachusetts, Missouri, Nebraska, North Dakota, Pennsylvania and South Carolina.