Oxy-fuel combustion with many advantages such as high combustion efficiency, low flue gas flow rate and low NOx emission has emerged as a promising CCS technology for coal combustion facilities. In this study, the effects of the direct sulfation reaction on SO2 removal efficiency were evaluated in a drop tube furnace under typical oxy-fuel combustion conditions represented by high concentrations of CO2 and SO2 formed by gas recirculation to control furnace combustion temperature. The effects of the operating parameters including the reaction temperature, CO2 concentration, SO2 concentration, Ca/S ratio and humidity on SO2 removal efficiency were investigated experimentally. SO2 removal efficiency increased with reaction temperature up to 1,200 due to promoted calcination of limestone reagent particles. And SO2 removal efficiency increased with SO2 concentrations and the humidity of the bulk gas. The increase of SO2 removal efficiency with CO2 concentrations showed that SO2 removal by limestone was mainly done by the direct sulfation reaction under oxy-fuel combustion conditions. From the impact assessment of operation parameters, it was shown that these parameters have an effects on the desulfurization reaction by the order of the Ca/S ratio > residence time > O2 concentration > reaction temperature > SO2 concentration > CO2 concentration > water vapor. The semi-empirical model equation for to evaluate the effect of the operating parameters on the performance of in-furnace desulfurization for oxy-fuel combustion was established.