propane

thumb|A 20 lb () steel propane cylinder. This cylinder is fitted with an overfill prevention device (OPD) valve, as evidenced by the trilobular handwheel. Propane () is a three-carbon chain alkane with the molecular formula . It is a gas at standard temperature and pressure, but becomes liquid when compressed for transportation and storage. A by-product of natural gas processing and petroleum refining, it is often a constituent of liquefied petroleum gas (LPG), which is commonly used as a fuel in domestic and industrial applications and in low-emissions public transportation; other constituents of LPG may include propylene, butane, butylene, butadiene, and isobutylene. Discovered in 1857 by the French chemist Marcellin Berthelot, it became commercially available in the US by 1911. Propane has lower volumetric energy density than gasoline or coal, but has higher gravimetric energy density than them and burns more cleanly.

Propane gas has become a popular choice for barbecues and portable stoves because its low −42 °C boiling point makes it vaporise inside pressurised liquid containers (it exists in two phases, vapor above liquid). It retains its ability to vaporise even in cold weather, making it better-suited for outdoor use in cold climates than alternatives with higher boiling points like butane. LPG powers buses, forklifts, automobiles, outboard boat motors, and ice resurfacing machines, and is used for heat and cooking in recreational vehicles and campers. Propane is also becoming popular as a replacement refrigerant (R290) for heat pumps as it offers greater efficiency than the current refrigerants: R410A / R32, higher temperature heat output and less damage to the atmosphere for escaped gases—at the expense of high gas flammability.