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 gas producer. It is the result of two high-temperature reactions (above 700 °C (1,292 °F)): 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.
Usage in engines
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.he 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. Normal catalytic converters do perfectly well on wood gas but emission level less than 20 ppm HC and 0,2 % CO can be easily achieved by most automobile engines with out catalytic converter . Combustion of wood gas generates no particulates and the gas thus renders very little carbon black amongst motor oil. Normal catalytic converters are reported to suit perfectly well for wood gas operation.
Use in stoves, cooking and furnaces
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.
