Editing Wood gas (section)

== Production ==
[[Image:Holzvergaser_Güssing.jpg|thumb|[[Fluidized bed]] gasifier in [[Güssing]], [[Austria]], operated on wood chips]]
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]].<ref name=Nagel-2008>
{{cite thesis
 |first=Florian |last=Nagel
 |year=2008
 |title=Electricity from wood through the combination of gasification and solid oxide fuel cells
 |degree=Ph.D.
 |publisher=Swiss Federal Institute of Technology
 |place=Zurich, CH
 |url=http://e-collection.ethbib.ethz.ch/view/eth:41553
 |access-date=October 8, 2023
}}
</ref>
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.'', "[[Cracking (chemistry)|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 the World War&nbsp;II gasifiers, can be engineered to produce essentially tar-free gas (less than 1&nbsp;mg/m<sup>3</sup>), while single-reactor [[fluidized bed]] gasifiers may exceed 50,000&nbsp;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 (1985)<ref name=Taylor-1985>
{{cite book
 |last=Taylor |first=Charles Fayette
 |year=1985
 |title=Internal-Combustion Engine in Theory and Practice
 |publisher= The MIT Press |location=Cambridge, MA
 |isbn=978-0-262-70027-6
 |volume=1 |pages=46–47
}}
</ref>
reports that producer gas has a lower [[heat of combustion]] of 5.7&nbsp;MJ/kg versus 55.9&nbsp;MJ/kg for natural gas and 44.1&nbsp;MJ/kg for gasoline. The heat of combustion of wood is typically 15–18&nbsp;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:
[[File:Nambassa 1981 Alt Energy centre Gas Producer Photographer Michael Bennetts.jpg|thumb|A charcoal gas producer at the [[Nambassa]] alternative festival in New Zealand in 1981]]

::{|  style="textalign:left;"
 |+ {{big|'''"Producer gas" composition'''}}<ref name=Taylor-1985/>
 |-
 ! {{left|Chemical name}} &nbsp; !! {{center|Formula}} !! &nbsp; {{right|Fraction}}
 |-
 | [[Nitrogen]]               || {{center|N{{sub|2}} }}        || {{right|50.9%}}
 |-
 | [[Carbon monoxide]] || {{center|CO}}                 || {{right|27.0%}}
 |-
 | [[Hydrogen]]               || {{center|H{{sub|2}} }}        || {{right|14.0%}}
 |-
 | [[Carbon dioxide]]         || {{center|CO{{sub|2}} }}       || {{right|4.5%}}
 |-
 | [[Methane]]                || {{center|CH{{sub|4}} }}       || {{right|3.0%}}
 |-
 | [[Oxygen]]                 || {{center|O{{sub|2}} }}        || {{right|0.6%}}
 |}

The composition of the gas 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.<ref name=Nagel-2008/><ref>
{{cite report
 |title=Handbook of Biomass Downdraft Gasifier Engine Systems
 |year=1988
 |at=Section&nbsp;5.2, paragraph&nbsp;2, page&nbsp;30
 |publisher=U.S. [[Department of Energy]]
 |department=Solar Technical Information Program
 |series=Solar Energy Research Institute
 |url=http://www.nrel.gov/docs/legosti/old/3022.pdf
}}
</ref>

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.