In the latest dust-up over regional coal power plants, a national environmental group is accusing the Big Stone I plant in South Dakota of violating federal clean air laws.

In a lawsuit filed Tuesday in U.S. District Court in South Dakota, the Sierra Club alleges the plant failed to install modern pollution controls during three renovations and expansions in the past 13 years, as required by the U.S. Clean Air Act for modifications that extend the life of an old plant.

It asks the court to order immediate installation of the controls and a fine of as much as $25,000 for each day the plant was not in compliance.

Cris Kling, a spokeswoman for Otter Tail Power, the plant's major owner, said all changes complied with the law and were approved by state environmental regulators.

The suit steps into the middle of a contentious and continuing battle over the proposed construction of a Big Stone II coal plant at the same site, near the Minnesota border. South Dakota regulators have approved the plant, but its fate is tied to two unapproved transmission lines to carry nearly half the generated power to Minnesota.

Last Thursday, after four years of consideration, the Minnesota Public Utilities Commission voted to delay a decision. The six regional power companies that want to build the plant, including Fergus Falls-based Otter Tail, argue that it is needed to meet their customers' growing demand for electricity. Environmentalists argue against it because coal plants are a major source of carbon dioxide emissions, which are blamed in large part for global warming.

Bruce Nilles, a Sierra Club attorney based in Madison, Wis., said his group filed the suit after complaints to South Dakota and the U.S. Environmental Protection Agency got no response.

Modern pollution controls could have cut Big Stone I's emissions by more than 90 percent had they been included in the plant's modifications, which included an adjustment to burn a new kind of coal and addition of an ethanol plant, the Sierra Club said.

"The vast majority of the pollution winds up in Minnesota," Nilles said. "The regulators in South Dakota would be getting more pressure from their constituents if it were hitting them."

H.J. Cummins • 612-673-4671