Wood gas is formed from the thermal gasification of biomass or other carbon containing materials such as coal in a gasifier. It is the result of a high temperature reaction (>700 °C), where carbon reacts with steam or a limited amount of air or oxygen producing carbon monoxide, hydrogen, and carbon dioxide.
In some gasifiers the actual gasification process is preceded by pyrolysis, where the biomass or coal turns into char releasing hydrocarbon rich tars and methane. Other gasifiers are fed with previously pyrolysed char in the form of charcoal.
Wood gas is flammable gas because of the carbon monoxide, hydrogen and methane content.
Before and during the Second World War there were many commercial suppliers of gas producers for motor vehicles due to the rationing of petrol. Today there are a growing band of enthusiasts converting cars to wood gas due to increasing fuel prices.
Other uses for wood gasifiers include cooking stoves that use less wood than a conventional wood stove and heating of domestic hot water supply.