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A model for the fluidized bed freeboard region is developed to describe reactions which are important in many fluidized bed chemical reactors including fluidized bed coal combustor. Particles in freeboard region originate from the bursting of bubbles at the bed surface. In this work, the rate of ejection of solid particles at the bed surface is correlated with hydrodynamic parameters of the bed, such as the bubble diameter and the excess gas velocity. Those entrained particles for which the terminal velocity is larger than the actual gas velocity in the freeboard will reach a given height within the freeboard, then will fall back into the bed. However, those particles for which the terminal velocity is smaller than the actual velocity will be elutriated. The elutriation rate of fines is related to basic parameters by equations derived from the solid material balance and particle force balance. It is shown that the effect of column diameter on the elutriation rate is significant and has affected all previous correlations. A particle entrainment mechanism is proposed for both large and small particles to explain the apparent exponential decrease in the entrainment rate of the ejected particles along the freeboard height. A mechanistic model is proposed for calculating the solid hold-up in the freeboard. This entrainment model is also utilized in the overall reaction model to describe catalytic and non-catalytic solid-gas reaction which can take place in the freeboard region. A comprehensive model incorporating elutriation, entrainment and reaction is presented for describing the chemical reaction and hydronamics occurring in the freeboard. A comparison of experimental data on the amount of particles elutriated and the concentration profiles of SO(,2) and NO(,x) obtained from the freeboard of fluidized bed coal combustion (FBC) by Babcock & Wilcox Company has been made with the calculation based on the proposed model. Good agreement has been shown.