ROCKLAND, Maine FMC, the communitys largest taxpayer and a major employer, has filed an application with the city to convert its fuel from oil to compressed natural gas. The conversion is expected to cost $2.5 million and take a year to complete, according to paperwork filed with the citys code enforcement office. The company has been looking to make its carrageenan plant in Rockland more energy efficient. In April, FMC officials announced plans to convert from oil to liquefied natural gas. That project would have consisted of four 35-foot-tall, 18,000-gallon tanks to store the fuel.