Fossil fuels such as coal, oil and gas have been indispensable commodities for the economy since long ago. Electricity, which is essential to daily life, is mostly generated from these fossil fuels. Fuels for automobiles such as gasoline, diesel oil and gas are also fossil fuels. Steam and heat required for industrial activities are also largely produced from these fossil fuels. In fact, large part of all energy sources, that are crucial to economic activities in households, transportation and industry, comes from fossil fuels. However, among various fossil fuels, coal and oil are basically composed of carbon and contain a large quantity of impurities. Therefore, no matter how high their combustion efficiency may be, air pollutants, including particulate matters(PM), sulfur dioxide(SO2) and nitrogen dioxide(NO2), and global warming substances such as carbon dioxide(CO2) are emitted during the process of combustion. On the other hand, although such gases like liquefied natural gas (LNG) and liquefied petroleum gas (LPG) are fossil fuels, their components are different from those of oil and coal and their contents of impurities are lower. Naturally, in the process of combustion, they emit much less air pollutants and global warming substances than coal and oil. Korea has achieved rapid economic growth since the 1970s, mainly driven by heavy and chemical industries. However, because the nation primarily relied on coal and oil as its energy sources, it suffered from air pollution triggered by SO2 and PM.