Char combustion is on a special reducing condition in the dense bed of a circulating fluidized bed combustor. Experimental findings were used to develop a comprehensive mathematical model to simulate the hydrodynamic and combustion processes in a circulating fluidized bed combustor. In the model, gas solid interaction was used to account for the mass transfer between the bubble phase and the emulsion phase in the dense bed, which contributes to the reducing atmosphere in the dense bed. A core annular structure was assumed in the dilute area rather than a one dimensional model. The submodels were combined to build the comprehensive model to analyze the combustion in a circulating fluidized bed combustor and the effect of operating parameters on the coal combustion. The model predictions agree well with experimental results.  Char combustion is on a special reducing condition in the dense bed of a circulating fluidized bed combustor. Experimental results were used to develop a comprehensive mathematical model to simulate the hydrodynamic and combustion processes in a circulating fluidized bed combustor. In the model, gas solid interaction was used to account for the mass transfer between the bubble phase and the emulsion phase in the dense bed, which contributes to the reducing atmosphere in the dense bed. A core annular structure was assumed in the dilute area rather than a one dimensional model. The submodels were combined to build the comprehensive model to analyze the combustion in a circulating fluidized bed combustor and the effect of operating parameters on the coal combustion.  model predictions agree well with experimental results. 