Fixed-Bed Gasifier
Fixed-bed gasifier has a bed of solid fuel particles through which the gasifying media and gas moves. This design can be divided into three groups depending on the gas flow direction as gas moving up (updraft), moving down (downdraft) or introduced from one side of the reactor and released from the other side on the same horizontal level (cross-draft). Fixed-bed gasifier is the simplest type of gasifier, typically consisting of a cylindrical space for fuel and gasifying media with a fuel-feeding unit, an ash-removal unit and a gas exit. The plan of an updraft fixed-bed gasifier is shown in Figure 11.2. In this gasifier design the fuel bed moves slowly down the reactor as the gasification take place, and air is fed from the bottom, whereas the syngas is removed from the top of the chamber.
In fixed-bed gasifiers, the char bed gasification zone where char is converted into syngas plays a major role in terms of efficiency and control of the process. Teixeira and coworkers have investigated the mechanical and thermochemical behavior of the char bed gasification zone and focused particularly on bed compaction when wood pellets are used as the biomass feed. In this study they found that pelletization has no effect on char bed compaction, final char conversion and syngas quality [8]. Generally, fixed-bed gasifiers are simple to construct and normally operate with high carbon conversion, long solid residence time, low gas velocity and low ash carry-over.