For years, Luminant’s Big Brown coal-fired plant has been described as one of the worst air polluters in the state, sitting at or near the top of the EPA’s list of plants spewing nitrogen oxide, which can cause respiratory illnesses. So, it would seem a ripe target for a Clean Air Act lawsuit filed by the Sierra Club.

Taking a shot at Big Brown, however, could wind up costing the environmental group more than $6 million. Turns out, Big Brown may pump out a lot of nasty stuff into the air, but no more than its operating permit allows. U.S. District Judge Walter Smith ruled that the Sierra Club knew that when it filed an unsuccessful Clean Air Act suit against the plant’s owner, Luminant, and last week he ordered the Club to pay the power company’s legal bills for what he called a “frivolous” suit.

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