Natural gas is one of the major combustion fuels used throughout the country. It is mainly used. to generate industrial and utility electric power, produce industrial process steam and heat, and heat. residential and commercial space. Natural gas consists of a high percentage of methane (generally above. 85 percent) and varying amounts of ethane, propane, butane, and inerts (typically nitrogen, carbon. dioxide, and helium). The average gross heating value of natural gas is approximately 1,020 British. thermal units per standard cubic foot (Btu/scf), usually varying from 950 to 1,050 Btu/scf.