BFB and CFB reactors

The bubbling fluidized beds operate at relatively low gas velocities, typically below 1 m/s. In the BFB gasifier most of the conversion of the feedstock to product gas takes place in the dense bed region in the bottom of the gasifier, even though some conversion continues in the freeboard section owing to reactions associated with entrained (small) particles. Because of the relatively low gas velocities in the BFB gasifier, freeboard gasifier elutriation is minimal and the addition of new bed material limited. A schematic of a BFB gasifier is shown in Figure 6.7a. The circulating fluidized beds, illustrated in Figure 6.7b, operate at much higher gas velocities, 3-10 m/s, and are significantly different in their hydrodynamics, compared to a BFB reactor. The solids are dispersed all over the tall riser, allowing for a long residence time for both the gas and the fine particles. In the CFBG, the particles are separated from the gas in a cyclone and recycled back to the bottom of the reactor. For fly ash/ dust removal are in both configurations a particle filter employed.

The inert bed material will enhance the heat exchange between the fuel particles, and therefore the fluidized beds will operate under almost isothermal conditions. For both configurations, the maximum operating temperature is limited by the ash-induced melting point of the bed material that typically will lie between 800 and 900°C. At these relatively low temperatures, coupled with the prevailing relatively short gas residence times, will the (slow), especially heterogeneous, gasification reactions normally not reach chemical equilibrium. This is especially true for the faster CFB gasifier. Thus, for example, methane concentrations tend to be much higher than those suggested by the chemical equilibrium. Additionally, tar levels are normally between those of the downdraft and the updraft fixed bed gasifiers.

The conversion of the feedstock is high in a fluidized bed gasifier, almost 100%. This only, however, if the carryover of fines is limited, which predominantly may occur in top-feeding configurations.

Due to the geometry and the excellent mixing properties, fluidized beds may be scaled up with confidence. However, fuel distribution may become problematic in large beds, although multiple feeding can solve the problem partly. The energy throughput per unit of reactor cross­sectional area is higher for a CFB gasifier than for the BFB gasifier. Both configurations can be operated under pressurized conditions, which will further increase the energy throughput. Pressurized conditions are also beneficial if the eventual subsequent downstream upgrading calls


Figure 6.7. Schematics of fluidized bed gasifiers: (a) bubbling fluidized and (b) circulating fluidized bed (Olofsson, 2005).

for pressurized conditions, as for instance the Fischer-Tropsch synthesis. The intense mixing allows the reactor to perform well over a broader fuel particle size distribution, starting already with relatively fine particles. Furthermore, in contrast to other reactor systems, the fluidized bed gasification allows for the possibility to use additives, e. g., for in-situ removal of pollutants (like sulfur) or primary measures to increase tar conversion via employment of catalytically active bed materials. Alternatively, a subsequent catalytic or thermal reactor can be added.

Loss of fluidization in fluidized beds due to bed sintering is one commonly encountered prob­lem, depending on the thermal characteristics of the ash. Alkali compounds from the biomass ash form low-melting eutectics with the silica sand, which is the usual bed material. This may result in agglomeration and bed sintering, resulting in eventual defluidization and subsequent shutdown of the gasifier. The presence of chlorine will amplify this problematic effect, as often alkali and chlorine go together. By applying proper counter measures, such as adding additives with alkali — attracting properties may part of this problem handled though. With biomass of high ash/alkali content, it may be advisable to use alternative bed materials such as alumina or magnesite. The main drawback with these more sophisticated bed materials is that of cost.

The CFB can run with most kinds of fuels, from coal to waste, including biomass. For larger CFB gasifiers, it is often preferable to employ a few smaller cyclones in parallel in contrast to only one large cyclone.