Coal-mine methane is a by-product of coal formations in subterranean coal seams, and is released before or during active coal mining. Coal mines worldwide emit volumes of methane gas approximately equivalent to 400 million tons (28 billion cubic meters) of carbon dioxide. This corresponds to the consumption of 818 million barrels of oil or carbon dioxide emissions of 64 million passenger vehicles.
Gas is released from coal seams in mining activities. Four types of gas are divided into four zones: unexplored coalbed methane (CNM), active mining methane (CMM), abandoned mine methane (AMM) and lean gas. CBM, CMM and AMM can be used in gas engines because their methane content is as high as 30% to 95%. Only lean gas can not be used for power generation because of its low methane content (0.2% - 1.5%).
The sweaty methane in the environment can be converted into energy by using gas containing up to 30% hexane.