In the present study, the carbonation reaction of hydrated lime in semi-dry condition is investigated experimentally in a laboratory-scale spouted bed reactor. Results show that for operating conditions where the concentration of CO2 is low, the capture efficiency is raised by increasing the inlet CO2 concentration. Additionally, because of the inconsistency between the experimental reaction rate and the calculated values based on the previous proposed equations, a new rate equation is introduced that considers the dependency of CO2 concentration too. To validate the proposed equation, its predictions were compared with another set of experimental data. (C) 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.