Natural gas is a gaseous fossil fuel consisting primarily of methane but including significant quantities of ethane, propane, butane, and pentane. These are heavier hydrocarbons removed prior to use as a consumer fuel, as well as carbon dioxide, nitrogen, helium and hydrogen sulfide. When methane-rich gases are produced by the anaerobic decay of non-fossil organic material, these are referred to as biogas. Sources of biogas include swamps, marshes, and landfills, as well as sewage sludge and manure by way of anaerobic digesters, in addition to enteric fermentation particularly in cattle.
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