Experiments were carried out in a pilot-scale entrained flow reactor (EFR) to investigate the reaction of SO3 with Ca(OH)(2) as a method of dry sorbent injection (DSI) for SO3 removal from flue gas. The results indicate that SO3 can be removed by Ca(OH)(2) with an efficiency that can reach 80%, and it was found that the molar ratio of Ca(OH)(2) to SO3 ([Ca]/[S]) and reaction temperature have a significant effect on SO3 removal efficiency. The experimental data measured inside the EFR were analyzed with a computing fluid dynamic (CFD) simulation, in which the Euler-Lagrangian frames were used for gas- and discrete-phase modeling. The CFD models were validated and applied to analyze the effects of certain parameters on SO3 removal efficiency, such as particle velocity, [Ca]/[S], temperature and residence time. It was found that the sorbent diameter has a significant influence on SO3 removal efficiency, with an obvious decrease in efficiency if the Ca(OH)(2) particle diameter increases. For example, if the sorbent diameter increases from 3 to 10 mu m, the SO3 removal efficiency at the reactor outlet will decrease from 99% to 55%. A detailed comparison and theoretical analysis indicated that external diffusion of SO3 from the gas phase to the particle surface is the rate controlling step for larger Ca(OH)(2) particles, and more attention should be paid to the competition between external diffusion and surface reaction when applying the DSI method for removing SO3 from flue gas.