Effect of carbon dioxide on the high temperature transformation of siderite under low oxygen conditions

•In 7% O2/93% N2 and 7% O2/93% CO2, pure siderite only produces magnetite.•Under the above conditions, excluded siderite produces wustite and magnetite.•Replacing N2 with CO2 leads to more magnetite produced from excluded siderite.•In the same atmosphere, coal addition causes more extensive particle melting.•For excluded siderite, replacing N2 with CO2 suppresses particle melting.

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 O2 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 °C in a drop tube furnace. Two conditions, i.e. O2/N2 = 7/93 and O2/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 O2/CO2 = 7/93 is dominated by irregular particles with rounded edges, while that generated in O2/N2 = 7/93 is dominated by spherical particles. Wustite and magnetite are detected in both products, but the production of magnetite is much higher in O2/CO2 = 7/93 than that in O2/N2 = 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.