Four years ago, the Homer City Generating Station in Indiana County, Pennsylvania faced an existential choice: Either clean up to comply with new EPA air rules or close its doors. To comply, the coal-fired power plant would have to install extensive controls on mercury, sulfur dioxide and particulate matter emanating from its smokestacks. It would not be easy. The plant was one of the biggest sulfur dioxide emitters in the country. But $750 million later, Homer City has found a way to survive.