Principles of a novel multistage circulating fluidized bed reactor for biomass gasification

In this paper a novel multistage circulating fluidized bed reactor has been introduced. The riser of this multistage circulating fluidized bed consists of several segments (seven in the base-case design) in series each built-up out of two opposite cones. Due to the specific shape, a fluidized bed arises in the bottom cone of each riser segment. Back-mixing of gas and solids between the segments is prevented effectively. The absence of back-mixing combined with the enlarged solids residence time in each segment (each segment is a fluidized bed) creates the opportunity to operate, spatially divided, separate process steps in a single reactor. The benefit of a concept in which different processes are carried out in separate segments of the same reactor has been demonstrated for the specific case of biomass gasification. In the novel reactor it is possible to create oxidation segments in which O2 reacts exclusively with char (carbon). This results in an increased carbon conversion and consequently improved gasification efficiency. Creating an exclusive char combustion zone, aimed at improving both the carbon conversion and the thermal efficiency, has also been applied successfully in a conventional CFB biomass gasifier (ECNʹs CFB= ~100 kg wood/h) by building a flow restriction in the riser between the primary air nozzles and the biomass feed point.