A discrete particle motion-collision decoupled model has been developed based on gas molecular dynamics and gas-solid two-phase fluid dynamics. Particle collision is modeled by means of the direct simulation Monte Carlo (DSMC) method. The Newtonian equations of motion are solved for each simulated particle in the system. The interaction between gas phase and simulated particle is determined by means of Newtonian third law. The flow behavior of gas and particles phases were numerically simulated in a circulating fluidized bed. The distributions of gas and particle velocity and particle concentration in the risers were analyzed. The duration time, averaged solid concentration in cluster, the ratio of total cluster duration time to total observation time and the cluster frequency were obtained in the circulating fluidized bed. The simulated results are in agreement with experimental measurements by Manyele et al. [S.V. Manyele, J.H. Parssinen, J.X. Zhu, Characterizing particle aggregates in a high-density and high-flux CFB riser, Chem. Eng. J. 88 (2002) 151-161] and Sharma et al. [A.K. Sharma, K. Tuzla, J. Matsen, J.C. Chen, Parametric effects of particle size and gas velocity on cluster characteristics in fast fluidized beds, Powder Technol. 111 (2000) 114-122] in the circulating fluidized beds. The wavelet multi-resolution analysis was used to analyze the simulated data of instantaneous particle concentration. From the random-like particle concentration fluctuations, the cluster frequency can be extracted based on the wavelet multi-resolution analysis over a time-frequency plane. (c) 2004 Elsevier B.V. All rights reserved.