Wood gas is a syngas also known as producer gas which is produced by thermal gasification of biomass or other carbon containing materials such as coal in a gasifier or wood gas generator or producer gas. It is the result of two high-temperature reactions (above ): an exothermic reaction where carbon burns to CO2 but is then reduced partially back to CO (endothermic); and an endothermic reaction where carbon reacts with steam, producing carbon monoxide (CO), molecular hydrogen (H2), and carbon dioxide (CO2).
In several gasifiers, the actual gasification process is preceded by pyrolysis, where the biomass or coal turns into char, releasing methane (CH4) and tar rich in polycyclic aromatic hydrocarbons (PAH). Other gasifiers are fed with previously pyrolysed char. Wood gas is flammable because of the carbon monoxide, hydrogen, and methane content.
A wood gasifier takes wood chips, sawdust, charcoal, coal, rubber or similar materials as fuel and burns these incompletely in a fire box, producing solid ashes and soot (which have to be removed periodically from the gasifier) and wood gas. The wood gas can then be filtered for tars and soot/ash particles, cooled and directed to an engine or fuel cell to produce electricity. Most of these engines have severe purity requirements of the wood gas, so the gas often has to pass through extensive gas cleaning in order to remove or convert (i.e. to "crack") tars and particles. The removal of tar is often accomplished by using a water scrubber. Running wood gas in an unmodified gasoline-burning internal combustion engine may lead to problematic build-up of unburned compounds.
The heat of combustion of producer gas is rather low compared to other fuels. Taylor  reports that "producer gas" has a lower heating value of 5.7 MJ/kg versus 55.9 MJ/kg for natural gas and 44.1 MJ/kg for gasoline. The heating value of wood is typically 15-18 MJ/kg. Presumably, these values can vary somewhat from sample to sample. The same source reports the following chemical composition by volume which most likely is also variable:
The quality of the gas from different gasifier varies a great deal. Staged gasifiers, where pyrolysis and gasification occur separately (instead of in the same reaction zone as was the case in e.g. the WWII gasifiers) can be engineered to produce essentially tar-free gas ( less than 1 mg/m³), while single reactor fluid-bed gasifiers may exceed 50,000 mg/m³ tar. The fluid bed reactors have the advantage of being much more compact (more capacity per volume and price). Depending on the intended use of the gas, tar can be beneficial as well by increasing the heating value of the gas.
Wood gasifiers can power either spark ignition engines where 100% of the normal petrol can be replaced with little change to the carburation, or in a diesel engine by simply feeding the gas into the air inlet when the diesel governor automatically cuts back the diesel to fuel to between 15% - 40%. In the latter case the diesel fuel is still needed to ignite the gas. Wood can be used to power cars with ordinary internal combustion engines if a wood gasifier is attached. This was quite popular during World War II in several European and Asian countries because the war prevented easy and cost-effective access to oil. In more recent times, wood gas has been suggested as a clean and efficient method to heat and cook in developing countries, or even to produce electricity when combined with a gas turbine or internal combustion engine. Compared to the WWII technology, gasifiers have become less dependent on constant attention due to the use of sophisticated electronic control systems, but it remains difficult to get clean gas from them. Purification of the gas and feeding it into the natural gas pipelines is one variant to link it to existing refueling infrastructure. Liquification by the Fischer-Tropsch process is another possibility.
Efficiency rate of the gasifier system is relatively high. The gasification stage converts about 75% of fuel energy content into a combustible gas that can be used as fuel for internal combustion engines. Based on long term practical experiments and over 100 000 km driven with wood gas powered car, the energy consumption has been 1.54 times more compared to the energy demand of the same car on petrol (not including the energy needed to extract, transport and refine the oil from which petrol is derived). This means that 1000 kg of wood combustible matter has been found to substitute 365 litres of petrol during real transportation in similar driving conditions and with the same otherwise unmodified vehicle  . This can be considered to be a good result, because no other refining of the fuel is required. This study also considers all possible losses of the wood gas system like preheating of the system and carrying of the extra weight of the gas generating system.
Gasifiers have been built for remote Asian communities using rice husk, which in many cases has no other use. One installation in Burma uses an 80kW modified diesel for about 500 people who are otherwise without power. The ash can be used as fertilizer so this can be considered a renewable fuel. It has been suggested that the rice husk ash, which is particularly pure and high in silica can be used at high economic value in e.g. semiconductor manufacture.
Against general belief, exhaust gas emission level of internal combustion engine is significantly lower on wood gas than on petrol. Especially low are HC emissions. A normal catalytic converter works well with wood gas but even without it, emission level less than 20 ppm HC and 0,2 % CO can be easily achieved by most automobile engines. Combustion of wood gas generates no particulates and the gas thus renders very little carbon black amongst motor oil.
Certain designs of stove, are in effect a gasifier working on the updraft principle - the air passes up through the fuel, which can be a column of rice husks, and is combusted, then reduced to carbon monoxide by the residual char on the surface. The resulting gas is then burnt by heated secondary coming up a concentric tube. Such a device behaves very much like a gas stove. This arrangement is also known as a Chinese burner.
However, one must take into account that the carbon monoxide, which is part of the wood gas, is very toxic, while being difficult to notice due to its odorless and colorless nature.
Blue gas was used as a secondary fuel source for some zeppelin designs of the early 20th century, stored in gas cells within the envelope, just below the hydrogen gas cells. The engines could use either the blue gas or liquid petroleum-based fuel for power, but the former, having a density similar to air, required little change in ballast as it was consumed.
Around 1900, many cities delivered wood gas (centrally produced typically from coal) to residences. At this time also, Rudolf Diesel and Georges Imbert were also developing their various engines. It has been theorized that all of these internal combustion engines had been inspired by observing the operation of the fire piston fire making device which had been discovered in New Guinea and Sumatra early in the 1800s. Natural gas began to be used only in 1930.
Wood gasifiers are still manufactured in Singapore, China and Russia for automobiles and as power generators for industrial applications.