Steam gasification of woody biomass in a circulating dual bubbling fluidized bed system

Partial oxidation is generally applied for woody biomass gasification. However, the calorific value of gaseous products is relatively low. To increase the calorific value of the gaseous product, a circulating dual bubbling fluidized bed (CDBFB) system for woody biomass gasification based on a concept of separation of combustion zone from gasification zone was developed in this study. The CDBFB consisted of two bubbling fluidized beds, with one serving as a gasifier and the other as a combustor. Two kinds of sawdusts were gasified with steam in the CDBFB at 773, 873, 973, and 1073 K. Almost all the tar evolved from the sawdust samples was captured by the porous gamma-alumina particle bed material in the gasifier. The tar deposited on the alumina, referred to herein as coke, and the resulting char were gasified with steam in the gasifier. Since the residence time of solid in the gasifier can be controlled, coke as well as char was effectively gasified with steam in comparison with the conventional circulating fluidized bed. A high carbon conversion and hydrogen yield were achieved using the CDBFB, resulting in high cold gas efficiency.