Calcium-Bearing Minerals Transformation during Underground Coal Gasification

Abstract

Calcium-bearing minerals are one of the main typical minerals in coal and coal ash. In the process of coal thermal conversion, calcium-bearing minerals undergo different morphological transformation in which the reaction temperature, pressure, and atmosphere are important factors affecting their transformation. The reaction process of underground coal gasification (UCG) could be clearly divided into pyrolysis, reduction, and oxidation and the typical calcium-bearing minerals are expected to indicate the actual reaction conditions of UCG. A high-calcium coal, Zhundong coal, was used in this research. The products of UCG were prepared and the minerals were identified by X-ray diffraction (XRD) and a scanning electron microscope coupled with an energy-dispersive spectrometer (SEM-EDS). The thermodynamic calculation was used to assist in understanding the transformation behaviors of calcium-bearing minerals. The experimental results show that the calcium-bearing mineral is gradually converted from gypsum (CaSO4·2H2O) in the raw coal into anhydrite (CaSO4) during the pyrolysis process. In the reduction stage, anhydrite reacts with the reducing gas (CO) to produce oldhamite (CaS), and the oldhamite is stably present in the reduction ash. During the oxidation process, oldhamite is first transformed into CaSO4, and then CaSO4 is converted into CaO. Finally, CaO reacts with Al2O3 and SiO2 to produce gehlenite (Ca2Al2SiO7) at 1100 °C. As the oxidation temperature rises to 1400 °C, gehlenite is transformed into the thermodynamically stable anorthite (CaAl2Si2O8). With the further progress of the reaction, anorthite will co-melt with iron-bearing minerals above 1500 °C. The ternary phase diagram of SiO2–Al2O3–CaO system proves that anorthite and gehlenite are the typical high-temperature calcium-bearing minerals when the mole fraction of SiO2 is higher than 0.6. Moreover, the gehlenite is converted to anorthite with the temperature rise, which is consistent with experimental results. This study provides a scientific basis for understanding the UCG reaction conditions.