(Adds details of ruling)
By Lawrence Hurley
WASHINGTON, April 18 (Reuters) - A U.S. appeals court on Friday largely upheld new emissions standards aimed at curbing air pollution from the nation's cement plants.
The U.S. Court of Appeals for the District of Columbia Circuit rejected a challenge by environmental groups, including the Natural Resources Defense Council and Sierra Club, who said the emissions standards were not stringent enough.
But the court handed the challengers a partial victory by ruling that the U.S. Environmental Protection Agency could not carve out a special legal defense for plant operators to cite if they are sued by members of the public in the event of a plant malfunction.
The 2013 regulation was the second time the agency had tried to craft new standards. The previous attempt was thrown out by the appeals court in 2011.
The regulation curbs various pollutants, including mercury and hydrochloric acid. Plant operators are not required to comply with the new rule until September 2015.
Judge Brett Kavanaugh wrote in the decision that the EPA exceeded its authority in setting up the new legal defense for plant operators.
When a civil lawsuit is filed, the agency can intervene in cases to express its views but "under the statutory scheme, the decision whether to accept the defendant's argument is for the court in the first instance, not for EPA," he wrote.
In a separate ruling issued on Tuesday, the same court upheld hazardous air pollution regulations for coal and gas-fired power plants.
The case is Natural Resources Defense Council v. EPA, U.S. Court of Appeals for the District of Columbia Circuit, 10-1371. (Reporting by Lawrence Hurley; Editing by Sandra Maler and Sofina Mirza-Reid)