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The horizontal diffusion of coarse particles in dense fluidized beds is commonly used for particle mixing and discharging in CFB boilers. However, numerical modeling of such flows is hard at present because of the flow complexity and a lack of reliable methods. In this work, a TFM-DEM hybrid model is used to study the diffusion behavior of coal particle in a dense fluidized bed. The gas phase and the solid medium phase are modeled by the TFM, while the coal particles are modeled by the DEM. The TFM can give a reliable description of the dense medium flow while the DEM can readily designate the size and density distributions of the coarse particles. The diffusion pattern of the particles with different size and density is mainly studied. Influence factors such as the fluidized air velocity and the coal particle number are also investigated. To explain the mechanisms of particle diffusion in the dense bed, main forces exerted on the coal particles are analyzed in the Lagrangian framework.