Cold flow model investigations of the countercurrent flow of a dual circulating fluidized bed gasifier

Abstract

A novel fluidized bed gasification concept with enhanced gas–particle interaction combining two circulating fluidized bed reactors is proposed. Cold flow model results show the feasibility of the concept with regard to fluid dynamics. The aim of the design is to generate a nitrogen (N2) free product gas with low tars and fines contents. Therefore, the system is divided into an air/combustion and a fuel/gasification reactor. Two gas streams are obtained separately. The two reactors are interconnected via loop seals to assure the global circulation of bed material and to avoid gas leakages from one reactor to the other. The global circulation rate is driven by the gas velocity in the air/combustion reactor. Furthermore, the fuel/gasification reactor itself is a circulating fluidized bed with the special characteristic of almost countercurrent flow conditions for the gas phase and bed material particles. By simple geometrical modifications, it is possible to achieve well-mixed flow conditions in the fuel/gasification reactor along the full height. The gas velocity and the geometrical properties in the fuel/gasification reactor are chosen in such a way that the entrainment of coarse particles is low at the top. Due to the dispersed downward movement of the bed material particles and the feedstock input at defined locations of the fuel/gasification reactor, no volatiles are produced in the upper regions and the problems of insufficient gas phase conversion and high tar contents are avoided.