Wood gasification types

Wood carburetors can be designed differently. Depending on the structure and mode of operation, slightly different gases are formed. Which types of wood gasifiers can be distinguished, and how they work exactly, can be read in this article.

Allotherm or autothermic gasification?

These two complicated terms indicate nothing other than the way the heat is supplied to the carburetor.

Either a part of the biomass is burned to reach the necessary temperature for the gasification - in this case one speaks of autothermal gasification.

On the other hand, if all the heat necessary for gasification comes from the outside, that is called technical allothermic gasification.

In the case of wood gasification boilers in the home, autothermal gasification is used almost exclusively - part of the wood is first burned, then the rest of the wood is gasified. Only when producing process gas is occasionally resorted to allothermal gasification, because it produces gas with a significantly higher energy content (12,000 kJ / m³ instead of only 8,500 kJ / m³ in autothermal gasification).

The wood gasifier boiler in the household is less concerned with the energy content of the gas itself, but above all with the increase in efficiency that is achieved when wood and wood gas are burned separately from each other in time and space. Compared to the sole burning of wood, the efficiency is then slightly more than half higher.

carburetor assembly

Wood gasifiers can have a different technical structure. The simplest structure has a fixed bed carburetor. Fluidized bed and entrained flow gasifiers are more complicated and accordingly also less common.

Fixed bed gasifier

The fuels lie here like on a normal oven on a grid. The wood burns slowly and thus generates the necessary heat for the process.

By means of a blower, the air is now sucked upwards by the burning wood and sucked off above the wood. Through the exclusion of air the wood layers begin to fade. The resulting in this process wood gas has a very high water vapor content, and a high amount of organic components but a very low temperature of only about 100° C. For the production of wood gas condensate so only a small cooling is necessary.

Alternatively, the air may also be exhausted within the combustion zone. Here you get much hotter, but also much cleaner gases with less organic components. The pH of the wood gas condensate is here rather in the basic range.

Since in the first method, the exhaust air moves in the opposite direction to the sinking wood, this type of process is also called countercurrent gasification, in the second case is technically DC gasification.

Fluidized bed gasifier

This is the preferred technology used in wood gas power plants. The wood must be finely ground here and mixed with sand. The heat is generated by a fluidized bed firing, usually allothermic. Here, the resulting gas has the highest temperature for all carburettors, namely around 900° C.

Entrained flow gasifier

In the case of an afterburner gasification happens very fast and in a gas cloud. The fuels have to be blown into the combustion chamber as dust, where they gas very quickly.

Video Board: Downdraft Gasification