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Gasifier Types

There are four main types of Gasifiers:
-Downdraft or Imbert
-Updraft
-Fluidized bed
-Cross Draft

There have been many variations on these designs, some with very surprising results. The main reason for these gasifier modifications is the need for optimal results with a particular fuel or gas requirement.

For internal combustion use, the Imbert gasifier is ideal, as it produces the least amount of tar and particulates. The operation of the Imbert will be described in detail here, although other gasifier types work on similar principles.



Operation process of an Imbert:
Diagram of imbert down draft gasifier
A. The upper section is the Fuel Bunker section, which receives some residual heat from the lower zones and also serves as a drying zone (FAO).

B. Then comes the Pyrolysis Zone where fuel begins to burn, break down and produce small amounts of combustible gas (FAO), ie: methane and hydrogen, along with inert gases (mostly nitrogen). This mixture is then pulled down to the next process.

C. The main “gasification” component happens in the Oxidation Zone; this is where outside air containing the key component of oxygen is introduced. This is usually done through the use of nozzles, sized to match the engine and to obtain the right intake velocity. This process typically takes place at temperatures ranging from 1832°-2732°F (1000°-1500°C)(FAO). The resulting hot gases are then pulled through to the next zone.

D. The Reduction Zone: this happens just as the name implies, with a restricted opening at the bottom of the hearth, this is sized to the engine and/or fuel types. The charcoal is converted into gas and breaks down into ash and smaller coals.

E. The ash and coals drop down to the bottom of the gasifier. This area usually employs a grate to help further break down any coals that pulled through the reduction zone.

F. The resulting gas from the processes explained above are pulled up and around the inner section/tank to a gas outlet, typically located toward the top. By extracting the gases higher up, results in a larger quantity of particulates that settle to the bottom and it also helps preheat the inner tank.

Sources:
Forestry Department. “Woodgas as engine fuel.”, FAO 1986 3/8/08 http://www.fao.org/DOCREP/T0512E/T0512e0c.htm