A twin fluidized bed solid circulation system, in which two adjacent fluidized beds exchange solids, was developed for coal gasification to produce middle heating value gas. The effects of bed material, operation velocity, and bed structure on the solid circulating rate were tested at a small-scale test facility. Experimental results showed that the solid circulation rate of the system could be adjusted by changing gas velocities of two beds and could attain 30-40 times of the fuel feed rate, which would meet the demands of heat supply to an endothermic process of a gasifier. On the basis of experiments, reasonable operation and design parameters were put forward, which can be used to as a reference for the commercial gasifier design. A mathematical model was erected to calculate the solid circulation rate of the system, and it could predict well the solid flow rate through a horizontal orifice by comparison with experimental data. (C) 2003 Elsevier Science B.V. All rights reserved.