In this study, the combustion characteristics of coal in CO2/O-2 and steam/CO2/O-2 atmospheres were investigated during limestone decomposition in a continuously operating fluidized bed reactor for CO2 capture. The results show that the variations and concentrations of CO, CH4, and H-2 in the exhaust gas of the reactor in the steam/CO2/O-2 atmosphere were smaller than those in the CO2/O-2 atmosphere. Because of steam dilution, the CO2 concentration in the bed was lower in the steam/CO2/O-2 atmosphere than that in the CO2/O-2 atmosphere, and the effect of differential pressure variation on limestone decomposition in the fluidized bed was less pronounced in the steam/CO2/O-2 atmosphere than that in the CO2/O-2 atmosphere. Additionally, N2O emission was detected only in the CO2/O-2 atmosphere, and the conversion of N to NO in the steam dilution atmosphere was of a smaller magnitude than that observed in the CO2/O-2 atmosphere. We also found that the conversion Of S to SO2 in the steam/CO2/O-2 atmosphere was lower than that observed in the CO2/O-2 atmosphere. The contents of sulfur, SiO2, and Al2O3 were much higher in solid samples located in the cyclone than in the overflow holder in both steam/CO2/O-2 and CO2/O-2 atmospheres. Finally, the hydration and carbonation reactivities of CaO produced in the steam/CO2/O-2 atmosphere were better than those produced in the CO2/O-2 atmosphere.