Optimization of char and tar conversion in fluidized bed biomass gasifiers

Abstract

In fluidized-bed gasification (FBG) of biomass and waste the temperature is maintained relatively low to prevent agglomeration. This slows down carbon conversion in conventional FBG, and a gas with relatively high concentration of tar is generated. Then the gasification efficiency is reduced and utilization of the gas is difficult in applications where the gas is cooled or compressed. In the present work the conversion of char and tar is studied to identify the main factors hindering complete conversion of the fuel into a product gas that is free from tar. It is concluded that char conversion can be increased by solids recirculation in directly heated FBG (stand-alone units) or by burning the char in a separate chamber in indirectly heated FBG. However, the tar content of the gas remains high, making gas cleaning necessary. Downstream cleaning of gas by catalytic cracking and/or scrubbing is complex and/or expensive for small to medium gasification plants, so conversion of tar within the gasifier is preferred. The optimization of conventional directly heated FBG by use of in-bed catalyst and distribution of the gasification agent to various zones of the gasifier, although improving the process, is not sufficient to attain the gas purity required for cold gas applications. Staged gasification is a suitable way to reach high char conversion, while yielding a gas with low concentration of heavy tar. Most of the staged-gasification developments proposed up to date have been based on fixed-beds, thus having relatively small capacity. A recently proposed concept to achieve almost complete tar and char conversion in fluidized bed is presented.