An advanced simulator is newly developed to predict desulfurization behavior by direct injection of Ca fine particles in coal combustion furnaces. The Ca fine Particles, the main component of which is CaCO3, are a byproduct of cement manufacturing process This simulator is built on the coal combustion/gasification module based on the commercial computational fluid dynamics (CFD). software of STAR CD V3.26. Regarding the desulfurization reaction model, it is based on two physical models. of thermal decomposition reaction of CaCO3 and of desulfurization reaction by CaO. We improve both models in order to simulate desulfurization behavior of Ca fine particles in coal combustion. A case study is carried out With a bench scale experimental furnace. According to the comparison. between calculated result and experimental one this simulator can predict the tendency of the increase of desulfurization ratio with an Increase of feeding rate of Ca fine particles and the influence of the temperature at injection part of Ca fine particles Moreover, the calculated result shows that the desulfurization reaction does not finish in the boiler and it continues at the exhaust pipe, since the Ca fine particle's temperature in the pipe is still higher than 1073 K As a consequence, this simulator well predicts the tendency of the desulfurization process successfully and Can be used for improvement of the operation condition of the furnaces.