The pseudo-fluid model based on the closure of modified kinetic theory of granular flow was used to simulate the clustering phenomena in a circulating fluidized bed riser. The simulation results show that the clustering is a rather dynamic process. The formation of clusters reduces the slip velocity within the particle clusters

and hence some particles are lifted by air while the others fall down along the wall. The dynamic equilibrium of these two types of movements leads to the characteristic sigmoid profile of solid concentration along the axial direction.