The knowledge of siderite transformation is critical to the understanding of ash deposition behavior in oxy-fuel combustion. This work aims to explore the effect of CO2 on the transformation of siderite in different modes under low O-2 conditions, which are common in real boilers. Two samples, i. e. pure siderite and excluded siderite simulated by the addition of siderite into a demineralized coal sample, were combusted at 1350 degrees C in a drop tube furnace. Two conditions, i.e. O-2/N-2 = 7/93 and O-2/CO2 = 7/93 were tested for elucidating the effect of CO2. The combustion products were analyzed by scanning electron microscopy and X-ray diffraction. The results show that, for pure siderite, the products generated in both conditions have similar morphology and mineralogy, suggesting negligible effects of CO2. They are dominated by irregular particles with sharp edges, and only magnetite is detected. However, for excluded siderite, the products generated in both conditions are distinctly different in morphology and mineralogy, which indicates significant effects of CO2 on the transformation of excluded siderite. The product generated in O-2/CO2 = 7/93 is dominated by irregular particles with rounded edges, while that generated in O-2/N-2 = 7/93 is dominated by spherical particles. Wustite and magnetite are detected in both products, but the production of magnetite is much higher in O-2/CO2 = 7/93 than that in O-2/N-2 = 7/93, which is attributed to the presence of a high concentration of CO2. Compared with pure siderite, excluded siderite experiences more reducing conditions and higher temperatures due to coal combustion, which results in the survival of wustite and more extensive melting. (C) 2015 Elsevier Ltd. All rights reserved.