Also called LPG, GPL, LP Gas, is a mixture of hydrocarbon gases used as a fuel in heating appliances and vehicles, and increasingly replacing chlorofluorocarbons as an aerosol propellant and a refrigerant to reduce damage to the ozone layer. LPG is a low carbon emitting hydrocarbon fuel, emitting 19 percent less CO2 per kWh than oil, 30 percent less than coal and more than 50 percent less than coal-generated electricity distributed via the grid. Being a mix of propane and butane, LPG emits more carbon per joule than propane and LPG emits less carbon per joule than butane. Varieties of LPG bought and sold include mixes that are primarily propane, mixes that are primarily butane, and the more common, mixes including both propane (60%) and butane (40%), depending on the season—in winter more propane, in summer more butane. Propylene and butylenes are usually also present in small concentration.
LPG is synthesised by refining petroleum or 'wet' natural gas, and is usually derived from fossil fuel sources, being manufactured during the refining of crude oil, or extracted from oil or gas streams as they emerge from the ground. LPG has a typical specific calorific value of 46.1 MJ/kg compared to 42.5 MJ/kg for diesel and 43.5 MJ/kg for premium grade petrol (gasoline). At normal temperatures and pressures, LPG will evaporate. Because of this, LPG is supplied in pressurised steel bottles. In order to allow for thermal expansion of the contained liquid, these bottles are not filled completely; typically, they are filled to between 80% and 85% of their capacity. Large amounts of LPG can be stored in bulk tanks and can be buried underground if required. Alternatively, gas cylinders can be used.