Wood gas



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 (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 polycyclic aromatic hydrocarbon (PAH) rich tar and methane (CH4). 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 countries because the armies active in the war did not always have 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 the other possibility.

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 e.g. 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 (i.e. to "crack") tars and particles. If you plan to run wood gas in an internal combustion engine, get to know your local cylinder head repair shops.

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:
 * Nitrogen N2       50.9%,
 * Carbon monoxide CO        27.0%,
 * Hydrogen H2      14.0%,
 * Carbon dioxide CO2     4.5%,
 * Methane CH4     3.0%,
 * Oxygen O2      0.6%.

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 (<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 your intended use of the gas, tar can be beneficial as well; increasing the higher 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. 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.