Sub-Product

Propane is a three-Carbon Alkane with the molecular formula C₃H₈. It is a gas at Standard temperature and pressure, but compressible to a transportable liquid. A by-product of Natural Gas processing and Petroleum refining, it is commonly used as a fuel in domestic and industrial applications and in low-emissions public transportation. Discovered in 1857 by the French chemist Marcellin Berthelot, it became commercially available in the US by 1911. Propane is one of a group of liquefied petroleum gases (LP gases). The others include Butane, Propylene, Butadiene, Butylene, Isobutylene, and mixtures thereof. Propane has a lower energy density but burns more cleanly than gasoline and coal.

Source: Wikipedia, Propane