An advanced simulator is newly developed to predict fly ash modification behavior by direct injection of Ca fine particles in coal combustion furnaces. The Ca fine particles are a by-product 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 fly ash modification reaction model, it is based on the solid particle coagulation model. We improve this coagulation model to apply the fly ash modification reaction. The parameters of this particle coagulation model consist of the number density of fly ash and Ca fine particles and of the coagulation coefficients caused by turbulence flow and Brownian dynamics. In addition to these parameters, fly ash melting degree and char reaction ratio are taken into account. These parameters relate to preventation of coagulation from re-dispersion caused by strong swirl flow in combustion furnaces and moreover to progress of the modification reaction in the co-melting phase between fly ash and Ca fine particles. Case study is done for a bench scale experimental furnace. The calculation results show 60–70 wt% of the modification reaction occurs near the burner zone. This is caused by the high temperature and by the long residence time of the particles in circulation flow near the burner. According to the experimental results, the modification ratio increases along with the increase in Ca fine particle’s mixing ratio. This simulator well predicts this tendency of the modification ratio successfully.