Binary particle circulation rates in circulating fluidized beds have been investigated to relate with loading of superfine powders and with optimum water contents of the bed particles. Circulation rates of binary Geldart A-C mixtures of FCC particles (66 mu m mean size) and superfine Al(OH)(3) powders (3.0 mu m mean size), were investigated in a semi-batch circulating fluidized bed. Circulation rates were affected not only by gas humidity but also by the equilibrium water contents of the bed particles. Circulation rates were measured at different loadings of superfine powders, different equilibrium water contents of FCC particles and different gas velocities. The circulation rates increased greatly using FCC particles humidified to higher equilibrium moisture contents as 0.061-0.067 wt(H2O)/wt(dry FCC). FCC particles humidified to lower equilibrium moisture contents as 0-0.038 wt(H2O)/wt(dry FCC) proved not to be practically suitable for CFBs as extremely low circulation rates were obtained even at higher loadings of superfine powders. High relative humidity such as 85% could not increase the circulation rates in the presence of totally dry bed particles. Particles have to humidify prior to fluidization. (C) 2014 Elsevier B.V. All rights reserved.