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Fluidized Bed Gasifiers

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In fluidized bed gasifiers, the biomass is brought into an inert bed of fluidized material (e.g. sand, char, etc.). Such systems are less sensitive to fuel variations but produce larger amounts of tar and dust. They are more compact but also more complex, and usually used at larger scales. Fluidized bed gasifiers are operated with significantly higher gas flow velocities than fixed bed gasifiers. The fuel bed and a carrier material (e.g. sand) are fluidized by the gas flow (fumigator and recirculated product gas). Thus, the gasification reaction takes place in a fluidized bed but only 5-10% wt of the bed is fuel.

fluidized bed gasifier

Fluidized bed Gasifier

The fuel is fed into the system either above-bed or directly into the bed, depending upon the size and density of the fuel and how it is affected by the bed velocities. During normal operation, the bed media is maintained at a temperature between 1000EF and 1800EF. When a fuel particle is introduced into this environment, its drying and pyrolyzing reactions proceed rapidly, driving off all gaseous portions of the fuel at relatively low temperatures. The remaining char is oxidized within the bed to provide the heat source for the drying and de-volatilizing reactions to continue. In those systems using inert bed material, the wood particles are subjected to an intense abrasion action from fluidized sand. This etching action tends to remove any surface deposits (ash, char, etc.) from the particle and expose a clean reaction surface to the surrounding gases. As a result, the residence time of a particle in this system is on the order of only a few minutes, as opposed to hours in other types of gasifiers. Thus, higher fuel throughput rates are achievable.

Since the fluidized bed allows an intensive mixing and a good heat transfer, there are no distinguished reaction zones. Hence, drying, pyrolysis, oxidation and reduction reactions take place simultaneously. The temperature distribution in the fluidized bed is relatively constant and typically ranges between 700°C and 900°C. The large thermal capacity of inert bed material plus the intense mixing associated with the fluid bed enable this system to handle a much greater quantity and, normally, a much lower quality of fuel.

 

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