The BCL gasifier is indirectly heated by a heat transfer mechanism as shown in Figure 2.3. Ash, char, and sand are entrained in the product gas, separated using a cyclone, and sent to a second bed where the char or additional biomass is burned in air to reheat the sand. The heat is transferred between the two beds by circulating the hot sand back to the gasification bed. This allows one to provide heat by burning some of the feed, but without the need to use oxygen, because combustion and gasification occur in separate vessels.

Because of the atmospheric pressure, the BCL gasifier produces a gas with a low CO2 content, but consequently containing a greater number of heavier hydrocarbons. Therefore, tar cracking and reforming are logical subsequent steps in order to maximize CO and H2 production. The reactor is fast fluidized allowing throughputs equal to the bubbling fluidized IGT, despite the atmospheric opera­tion. The atmospheric operation decreases cost at smaller scale, and the BCL has some commercial experience (demo in Burlington, VT (Paisley et al. 1998)). Because biomass gasification temperatures are relatively low, significant depar­tures from equilibrium are found in the product gas. Therefore, kinetic gasifier modelling is complex and different for each reactor type (Consonni et al. 1994;

Li et al. 2001). The main performance characteristics of both gasifiers are given in Table 2.1.