Gasification & Syngas

Gasification and Syngas (see... )

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. 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...


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 solid ashes and soot (which have to be removed periodically from the gasifier) and wood gas. The wood gas can then be filtered for tars and soot/ash particles, cooled and directed to an engine or fuel cell[1]. Most of these engines have severe 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. to "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 build-up 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 WWII gasifiers), can be engineered to produce essentially tar-free gas (less than 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.

The heat of combustion of producer gas is rather low compared to other fuels. Taylor [2] 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. The heating value 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:

A charcoal gas producer at the Nambassa alternative festival in New Zealand in 1981

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.

More detailed information can be found in [3].