It began with compulsory replacement of fuels used in commercial boilers in Seoul Metropolitan City and by the Incheon Thermal Power Plant. It has been applied to an increasing number of regions and facilities, and since 2000, the use of clean fuels has become compulsory for commercial boilers above a certain size, multi-unit dwellings, regional heating and cooling facilities, and power generation facilities in a total of 37 regions, including the seven special and metropolitan cities.