The regenerator reactor of the calcium looping (Ca-L) process requires the fuel combustion to provide heat for calcium carbonate (CaCO3) decomposition. In this study, the dried sewage sludge (DSS) and the hydrothermally treated sludge (HTS) were used as fuels. The effect of the hydrothermal temperature on the NOx emission characteristics of HTS samples was studied. Meanwhile, the effect of the sludge combustion on the decomposition characteristic of CaCO3 was also investigated in this study. Results showed that the hydrothermal treatment was an effective way to reduce the NOx emissions from sludge combustion in the O-2/CO2 atmosphere. The NOx reduction ratio significantly increases from 6.05 to 40.46% as the hydrothermal temperature increases from 200 to 280 degrees C. The good pore structure is formed during the CaCO3 decomposition process. This promotes the adsorption of organic matter released during the DSS combustion and decreases the relative decomposition ratio of CaCO3. However, the relative decomposition ratio under the effect of the HTS combustion is improved due to the smaller volatile content in HTS. The chemical reaction model and two-dimensional diffusion control model are the best fitting for the two stages of the CaCO3 decomposition.