Everything about Wood Gas totally explained
Wood gas, also known as
holzgas,
air gas or
blue gas, is the product of thermal
gasification of
biomass or other carbon containing materials such as
coal in a gasifier or
wood gas generator. 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 (CO), molecular
hydrogen (H
2), and
carbon dioxide (CO
2).
In several gasifiers the actual gasification process is preceded by
pyrolysis, where the biomass or coal turns into char releasing
polycyclic aromatic hydrocarbon (PAH) rich
tar and
methane (CH
4). Other gasifiers are fed with previously pyrolysed char. Wood gas is flammable because of the carbon monoxide, hydrogen and methane content.
Usage
Wood gas 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 refuelling 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 engine. 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 [2]. This can be considered to be a good result, because no other refining of the fuel is required. The study also includes all possible losses of the wood gas system like pre-heating of the system and carrying of the extra weight of the gas generating system [2].
Production
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 constantly from the gas) and wood gas. The wood gas can then be filtered for tars and soot/ash particles, cooled and directed to for example an
internal combustion engine,
gas turbine,
Stirling engine or
fuel cell to produce electricity. Most of these devices have severe requirements to the purity of the wood gas, so the gas often has to pass through extensive gas cleaning in order to remove or convert (for example to "crack") tars and particles. 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. 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 for example the WWII gasifiers) can be engineered to produce essentially tar-free gas (<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.
History
The first wood gasifier was apparently built by
Bischof in
1839. The first vehicle powered by wood gas was built by
Parker
in
1901.
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.
Further Information
Get more info on 'Wood Gas'.
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