CFD-DEM study on mixing and segregation characteristics of binary particles in a fluidized bed with secondary air

Secondary air intake has been suggested to be an efficient tar reduction method for fluidized bed biomass gasification. However, the influence mechanism of secondary air intake on the particle motion in the fluidized bed has not been fully understood. A CFD-DEM model was developed to investigate the segregation and mixing characteristics of binary mixture for a fluidized bed with secondary air injection. It was found that the intake of secondary air could change the original bubble pattern in the fluidized bed and form vortexes in the bed, accelerating the circulation rate of particles. The injection of secondary air has the potential to improve the speed and index of mixing, especially in the area above the nozzle plane. For the studied conditions, there is an optimum secondary air to total air ratio (SATR) for the mixing performance of binary mixture in a fluidized bed. On the other hand, the introduction of secondary air and relevant reduction in primary air velocity can lead to a better segregation of the binary mixture below the nozzle plane thus creating two layers with very different mixing/segregation characteristics.