page_title: Wood gas
section_title: Production
hierarchical_section_title: Wood gas / Production
caption_attribution_description: Deutsch: Holzvergaser Güssing 2006 eigene Aufnahme
context_page_description: Wood gas is a syngas fuel which can be used as a fuel for furnaces, stoves and vehicles in place of gasoline, diesel or other fuels. During the production process biomass or other carbon-containing materials are gasified within the oxygen-limited environment of a wood gas generator to produce hydrogen and carbon monoxide. These gases can then be burnt as a fuel within an oxygen rich environment to produce carbon dioxide, water and heat. In some gasifiers this process is preceded by pyrolysis, where the biomass or coal is first converted to char, releasing methane and tar rich in polycyclic aromatic hydrocarbons.
context_section_description: A wood gasifier takes wood chips, sawdust, charcoal, coal, rubber or similar materials as fuel and burns these incompletely in a fire box, producing wood gas, solid ash and soot, the latter of which have to be removed periodically from the gasifier. The wood gas can then be filtered for tars and soot/ash particles, cooled and directed to an engine or fuel cell. Most of these engines have strict 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., "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 accumulation of unburned compounds.
  The quality of the gas from different gasifiers 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 World War II gasifiers, can be engineered to produce essentially tar-free gas (less than 1 mg/m³), while single-reactor fluidized bed gasifiers may exceed 50,000 mg/m³ tar. The fluidized bed reactors have the advantage of being much more compact, with more capacity per unit 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.
  The heat of combustion of "producer gas" — a term used in the United States meaning wood gas produced for use in a combustion engine — is rather low compared to other fuels. Taylor reports that producer gas has a lower heat of combustion of  5.7 MJ/kg versus 55.9 MJ/kg for natural gas and 44.1 MJ/kg for gasoline. The heat of combustion 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:
  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%.
  It is pointed out that the gas composition is strongly dependent on the gasification process, the gasification medium (air, oxygen or steam) and the fuel moisture. Steam-gasification processes typically yield high hydrogen contents, downdraft fixed bed gasifiers yield high nitrogen concentrations and low tar loads, while updraft fixed bed gasifiers yield high tar loads.
  During the production of charcoal for blackpowder, the volatile wood gas is vented.  Extremely-high-surface-area carbon results, suitable for use as a fuel in black powder.
caption: Fluidized bed gasifier in Güssing, Austria, operated on wood chips
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