A circulating fluidized bed (CFB) was used to study the particle motion in the riser section to simulate flow regimes in a cold gasifier. High speed imaging was used in order to capture the dynamics of the particles in motion. The shadow sizing technique was used that allowed the determination of particle volume fraction and estimating the trajectories at various flow rates in the riser. The formation of clusters in the riser section of the CFB was detected by analyzing the deviation of the particle volume fraction from the mean value. The shadow sizing technique was observed to be an effective method for detecting dynamics of particles in motion and formation of clusters when supported with high-speed imaging capability. (c) 2011 Elsevier B.V. All rights reserved.