The ruling awards the company a reprieve on tight sulfur dioxide emission restrictions from its Joliet, Romeoville, Powerton and Waukegan plants that otherwise would have had to be met by 2015 and 2016.

At a January public hearing, officials with Midwest Generation and its parent company Edison Mission Energy told the board that the bankrupt company did not have the financial means to make the required emission control upgrades by the deadlines.

Company officials said the plants would be back on schedule by 2017, when they expected a more positive financial outlook including the resolution of their Chapter 11 bankruptcy proceedings.

"This deferral gives us the best possible opportunity to maintain a viable fleet of coal plants for the long term but we still face difficult case-by-case decisions on retrofitting generating units over the next several years," Midwest Generation president Doug McFarlan said in a statement.

In recent financial filings, the company cited competition with low natural gas prices for its financial woes and signaled it could close some of its coal plants or shut down individual units rather than clean them up.

Environmental leaders and local residents have expressed doubt that Midwest Generation would keep its promises to install pollution controls and will just use the deferral to operate the plants for slightly longer before shutting them down.

In a lawsuit filed late last year, the Sierra Club accused Midwest Generation of repeatedly violating sulfur dioxide limits at its four plants.

The company faces a separate lawsuit filed by the US Environmental Protection Agency, Illinois Attorney General Lisa Madigan and a Will County environmental group.

Midwest Generation and Edison Mission Energy filed for bankruptcy protection in December and agreed on a reorganization plan of its $3.7 billion in debt.

Company officials said environmental upgrades of the Joliet, Romeoville and Powerton plants would cost at least $628 million.