Development of fluidized bed gasifier with triple-beds and dual circulation

A new type of circulating fluidized bed gasifier was proposed. The main features of this proposed gasifier are the adoption of a triple-beds structure (comprising pyrolyzer, gasifier, and combustor), the separation of a circulation path for tar-absorbing material and that for the fuel and silica sand. Independent circulation systems are employed for the fuel system and for the tar-absorbing particles, and the pyrolyzer and gasifier each have a two-stage fluidized bed: the lower stage is for the fuel system and the upper stage is for the tar-absorbing particle system. The two circulation systems each have an independent combustor. In the pyrolyzer, the tar formed together with the gas produced during the pyrolysis of the fuel in the lower stage is absorbed by tar-absorbing particles in the upper stage. By adopting this two-stage fluidized bed approach, mixing of the tar-absorbing particles, which is rather expensive, with the ash contained in the fuel can be avoided. Accordingly, accumulation of ash in the circulation path for tar-absorbing material can be avoided, and the quantity of the discharge of absorption particles can be reduced. Therefore, the quantity used can be greatly reduced. The quantity of costly tar-absorbing particles used can be further minimized by optimizing the quantity of particles with respect to the quantity of tar generated by the fuel.As the first stage towards development of the proposed new type gasifier, the fundamental experiments that simulated a two-stage pyrolyzer and gasifier were carried out. As a result, the amount of H2 formed by pyrolysis resulted in a marked increase by using porous alumina as the tar-absorbing particles than the conventional conditions of using only silica sand. The amount of H2 formed was more increased because the coke was not only cracked but also reformed by steam gasification.