Two of the US's largest petroleum refiners - British Petroleum and the Koch Petroleum Group, have agreed to spend almost $600 million on eliminating almost 60,000 tonnes of nitrogen oxide, sulphur dioxide and other emissions.British Petroleum, (BP), the nation's second largest refiner, and Wichita based Koch Petroleum Group, which together account for 15 percent of total U.S. refining capacity, took the initiative to begin talks with the US Government's Environmental Protection Agency, (EPA), rather than wait for possible enforcement action by them.