Fixed-Bed Gasifiers

The most successful of the fixed-bed designs is the updraft gasifier, in which the biomass is fed from the top of the gasifier and successively undergoes drying, pyrolysis, char gasification, and char combustion as it settles to the bottom of the gasifier. The product gas is removed from the top of the gasifier and the ash from the bottom.

Blast air and steam are injected into the gasifier to keep the ash below melting temperatures (in a dry-ash gasifier) and to facilitate char conversion. The product gas from this process has a low velocity and low temperature. The low operating temperature creates a considerable amount of condensable oils and tars in the product gas. However, the filtering effect of the bed and low stream velocities create a product gas with low particulate concentrations.

Because of the volatility of biomass, the excellent heat transfer design, and high peak temperatures found in the fixed-bed design, carbon conversion efficiency is typically 99%; the hot gas efficiency is in the range of 90-95%. Furthermore, the design and operation of the fixed-bed is relatively simple and is the most widely used commercially. The fixed-bed gasifier, however, requires large, dense, uniformly sized fuels. Thus, agricultural residues would generally require densification, thereby increasing fuel handling costs.