Gas and Solids Mixing in the Bottom Zone of a Circulating Fluidized Bed Cornelis Aue-Klett, Ali Al-Shawabkeh * and Joachim Werther ** Chemical Engineering I, Technical University Hamburg-Harburg D-21071 Hamburg, Germany * Chemical Engineering Dept., The University of Jordan, Amman 11942, Jordan Abstract – Gas and solids mixing in the bottom zone of a circulating fluidized bed (CFB) were investigated by means of tracer techniques. Gas mixing was studied by injecting gaseous CO2 into the CFB unit, and gas samples were continuously collected and analyzed. For solids mixing solid CO2 (dry ice) was injected, and gas concentrations and temperatures were measured. The experiments were performed in a cold model CFB riser with a rectangular cross section of 0.3 m  1.0 m and a height of 8.5 m. Quartz sand with a surface mean diameter of 150 μm was used as the bed material. The unit was operated at superficial gas velocities between 2.6 and 3.8 m/s with the riser pressure drop ranging from 7,000 to 10,000 Pa. The experimental findings indicate that the gas flow is not significantly influenced by the lateral flow of solids induced by the external recirculation of solids. However, the measurements indicate limited mixing in the lateral direction. Solids mixing turned out to be nearly ideal in the axial direction, whereas horizontal solids mixing in the bottom zone was found to be limited. A previously developed one-dimensional convection-diffusion model considering the convective solids transport originating from the external solids recirculation was used to calculate the horizontal solids dispersion coefficient Dx in the bottom zone. This latter dispersion coefficient was found to increase with increasing gas velocity. The numerical values of the horizontal solids dispersion coefficient were found to be much higher than solids dispersion coefficients measured in bubbling fluidized beds and in the upper dilute zone of circulating fluidized beds. Introduction In circulating fluidized bed (CFB) combustors the solids suspension density varies strongly with height above the gas distributor. A CFB of this type is therefore common- ly divided into regions with different solids concentrations and flow structures. In par- ticular, a distinction is often made between an upper dilute and a dense bottom region (Svensson et al., 1993). Solids mixing in the bottom zone is important for the operating behavior of a CFB combustor since it is responsible for the lateral distribution of the fuel feed. Schlichthaerle and Werther (2001) have recently investigated solids mixing and Schlichthaerle et al. (1998) investigated gas mixing in the bottom zone. Both inves- tigations were focused on a suitable experimental facility, the development of measur- ing methods and the development of models for the interpretation of the measured data. Due to time limitations it was not possible to perform a larger series of measurements and therefore several questions remained open. In particular, there were two main ques- tions waiting for answers: The first question was whether the horizontal convection flow initiated by the recir- culated solids flow in the bottom zone influences the flow and mixing of gas. The solids ** Corresponding author, Tel.: +49-40-42878-3039; fax: +49-40-42878-2678 E-mail : werther@tuhh.de