Fine powders (Geldart's group Q are added to a circulating fluidized bed (CF13) of coarse particles (Geldart's group A) and the solid circulation rate (SCR) is investigated with addition of fine powders of different sizes and different fractions (different hold-ups) to the bed. Experiments were carried out in a CFB of 2 m in height and 0.052 m in diameter, using FCC catalyst particles of 66 mu m as the coarse particles and cohesive aluminum hydroxide powders of 0.5-15 mu m as the fine powders. The effects of hold-up of fine powders in the bed, fine powders size, and superficial gas velocity on the SCR were investigated. The SCR strongly depended on the hold-up of fine powders of 0.5-1.0 mu m in size and noticeably decreased with increasing the hold-up of fine powders under constant gas velocity. This dependency disappeared when the size of fine powders was larger than 3.0 mu m. Thus, depending on the size of fine powders added to the CFB, two distinct regions for the changes of SCR could be clearly identified. Published by Elsevier Ltd.