Thermal Treatment

Types of gasifiers

Gasification is still a developing technology. The fixed-bed gasifiers are mostly small-scale ones and come in two types, down-draft type (<2 MW) or up-draft type (<10 MW). The fluidized-bed gasifiers are in the higher MW range from tenth of MW to a few hundred MW. Another type of gasifiers, the entrained-flow gasifiers, is lagging the other two types in development due to the higher costs.

Fixed-bed gasifier

In fixed-bed gasifiers, the feedstock is fed in at the top of the gasifier in solid form and goes through different stages of the gasification process. Depending on the directions of the flow of fuel and air, fixed-bed gasifiers can be further divided into up-draft type and down-draft type.

• In up-draft gasification, the feedstock is fed at the top of a gasifier while air is drawn from the bottom. The air passes through the heating zone at a rate that is insufficient to support full combustion but is adequate for a proportion of feedstock to burn and raise the temperature of the remainder to that at which pyrolysis occurs. The gas leaves at the upper part of the gasifier.
• In down-draft gasification, air is drawn in through the feedstock into the gasification zone, and the resulting gases pass in sequence through the pyrolysis zone, the oxidation zone and the reduction zone in the gasifier. Gases produced leave the gasifier at its lower part.

Fluidized-bed gasifier

In fluidized-bed gasifier, air and feedstock are mixed up in a hot bed of solid materials, e.g. quarry sand. The mixture is kept in suspension above the bed such that it is almost fluid-like. The temperature is the same throughout the bed and there is no clear differentiation of the pyrolysis zone, oxidation zone and reduction zone. This enables the combustion process to occur at stable temperature and ensures optimal combustion.