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• A new corrosion-resistant experimental system was designed to study SOx absorption reaction with alkaline sorbents. • The SO 3 absorption performance of the three alkaline sorbents was Na 2 CO 3 > Ca(OH) 2 ≈ Mg(OH) 2. • The influence of operating parameters on SO 3 removal had been deeply studied. • The SO 3 absorption variation above 300 °C was similar to the condition without vapor. • The external diffusion control model was established for SO 3 absorption rate. Selective Catalytic Reduction (SCR) system can efficiently remove NOx in coal-fired power plant, while raise SO 3 concentration in the flue gas. Alkaline sorbent injection is a major approach to remove SO 3. Experiments were conducted to study the SO 3 absorption performance of alkaline sorbent. The SO 3 absorption performance of the three alkaline sorbents was Na 2 CO 3 > Ca(OH) 2 ≈ Mg(OH) 2. The ratio of SO 3 absorption rate to SO 2 absorption rate for Na 2 CO 3 increased with the increasing temperature. The key parameters on the SO 3 absorption performance of Na 2 CO 3 were studied. The SO 3 absorption by Na 2 CO 3 was increased by 72.7% with increasing temperature from 150 °C to 300 °C, and the temperature had little influence on SO 3 absorption above 300 °C. The increasing SO 3 concentration and decreasing particle size of Na 2 CO 3 enhanced SO 3 absorption. However, when the average particle diameter was less than 50 μm, the SO 3 absorption was no longer significantly increased. The increasing CO 2 concentration slightly reduced the SO 3 absorption. After adding vapor into the flue gas, the SO 3 absorption was obviously increased below 300 °C. The SO 3 absorption variation above 300 °C was similar to the condition without vapor. When reaction temperature was 300 °C or above and reaction time was less than 20 min, external diffusion became the control step for SO 3 absorption. On the basis of external diffusion control model, the mass transfer coefficient slightly increased from 1.68 × 10−3 m/s to 1.75 × 10−3 m/s when the reaction temperature increased from 300 °C to 400 °C. [ABSTRACT FROM AUTHOR]