Product gases from fixed-bed versus fluidized bed gasifier configurations vary
significantly. Fixed-bed gasifiers are relatively easy to design and operate and
are best suited for small to medium-scale applications with thermal
requirements of up to a few mega watts thermal (MWt). For large scale
applications, fixed bed gasifiers may encounter problems with bridging of the
biomass feedstock and non-uniform bed temperatures. Bridging leads to
uneven gas flow, while non-uniform bed temperature may lead to hot spots
and ash deformation and slagging. Large scale applications are also
susceptible to temperature variations throughout the gasifier as a result of poor
mixing in the reaction zone. Most fixed-bed gasifiers are air-blown and
produce low-energy pgas, although oxygen-blown designs have been tested.
HGI’s assessment indicates that fixed-bed gasifiers are not ideal for producing
a syngas of sufficient quality for conversion to liquid hydrocarbons, and such
gasifier technology was not included in the study.