The annular fluidized bed is a new technology consisting of a large central nozzle surrounded by a stationary fluidized bed. Due to the moderate primary gas fluidization of the annulus, the solids overflow at the upper edge of the central nozzle is mixed into the high upward velocity central secondary gas stream and transported into the mixing chamber. With this geometrical configuration, the process combines the advantages of the stationary fluidized bed (long solids residence time) and of the circulating fluidized bed (good heat and mass transfer). This paper presents the determination of the operating range of the system and the characterization of the solids mixing behaviour above the central nozzle in order to develop scale-up rules for the industrial application of the annular fluidized bed. The local radial solids concentration, velocity and mass flux profiles are investigated by capacitance probe measurements in a laboratory rig and a pilot plant of inside diameter 0.19 m and 0.74 m, respectively. The geometric and operating parameters varied are the diameter of the central nozzle, the gas velocity in the annulus and in the central nozzle and the pressure drop in the mixing chamber. (C) 2008 Published by Elsevier B.V.