Transformation characteristics of sodium, sulfur and chlorine during the high-sodium Zhundong coal combustion were investigated by performing both air combustion and O-2/CO2 combustion experiments in a 50 kW circulating fluidized bed testing system. Via analyzing the chemical compositions and crystalline phases of ash samples, the following results were obtained. In comparison with air combustion, sodium release into gas phase was inhibited under O-2/CO2 combustion atmosphere at the identical oxygen concentrations. While increasing the inlet oxygen concentration under O-2/CO2 combustion could promote the release of sodium. Chlorine almost finished total release from the coal under all experimental conditions. Besides, sulfation reaction affected the transformation of sodium, sulfur and chlorine markedly, for it could alter the chemical form of sodium from chloride to sulfate and further influence the release and condensation process of sodium during the combustion process. In particular, elevating the inlet oxygen concentration could promote the sulfation reaction and reduce the content of chlorine in deposits significantly. (C) 2019 Elsevier Ltd. All rights reserved.