In a semi-industrial desulfurization activation reactor, the high-CaO coal ash is directly used as desulfurization sorbent and activated by spray water. As a result of the inertial impactions between ash particles and water droplets, concentrated slurry droplets are formed. In order to numerically investigate the process, a new model of particle-flow-passage is deduced to simulate the impaction humidification process between particles and droplets. Computation results show that the spray water flow rate and droplets size have obvious influence on droplets catching efficiency, and the ash particles are caught primarily in the vicinity of the atomizer nozzle. With the models of gas phase turbulence, heat and mass transfer, and chemical reactions being considered, a comprehensive 3D model for FGD process (Flue Gas Desulfurization) is established, and several operation conditions are simulated by the model. The predicted results such as SO2 absorption efficiency and the flue gas temperature at the outlet are in good agreement with experimental results. (C) 2004 Elsevier Ltd. All rights reserved.