The in-furnace desulfurization technique is applied to the O 2/CO2 combustion system for the carbon capture and storage (CCS) process because this combustion system does not need an additional chamber for the desulfurization. CaCO3 sorbent particles, which have a wide range in size from a few nanometers to several tens of micrometers, are used for this process. In this study, an experimental system which can simulate the O2/CO2 combustion system was developed. CaCO 3 sorbent particles were exposed to the high temperature reactor at 1200°C with various residence times (0.33-1.46 s) in air and CO2 atmospheric conditions, respectively. The sorbent particles were men sampled at the inlet and outlet of the reactor and analyzed qualitatively/quantitatively using SMPS, XRD, TGA, and SEM. The results showed that the residence time and atmospheric condition in a high temperature reactor can affect the characteristics of the CaCO3 sorbent particles used in the in-furnace desulfurization technique, such as the calcination rate and reaction mechanism.