Sequential mineral transformation during underground coal gasification with the presence of coal partings

Abstract

Underground coal gasification (UCG) can be adopted to convert unminable coal resources into synthesis gas for use in the production of electricity, fuels, and chemicals. UCG is a promising technology with many health, safety, and environmental advantages over conventional mining techniques. It is of great scientific significance to study the mineral transformation and identify the typical minerals in certain process conditions, because it may help to ensure the stable operation of the gasification processes and improve the utilization efficiency of coal seams. The presence of coal partings has an important impact on mineral transformation. In this paper, the UCG process has been subdivided into pyrolysis, reduction, and oxidation stages, and the progressive coal conversion products were prepared. Ulanqab lignite was used as the test coal. The minerals in the coal transformation products were identified by X-ray diffraction (XRD) and scanning electron microscopy coupled with an energy-dispersive spectrometer (SEM-EDS). The thermodynamic equilibrium simulation software, FactSage 7.2, was used to calculate the relevant phase diagram to assist in understanding the mineral transformations. The experimental results indicate that the different iron-bearing minerals occur in raw coal and non-coal partings which are nontronite (Na0.3Fe2Si4O10(OH)2.4H2O) and glauconite (K (Fe, Al)2(Si, Al)4O10(OH)2) respectively. Pyrrhotite is the main iron-bearing mineral in the pyrolysis products. In the reduction stage, the relative content of Al2O3 is increased due to the presence of non-coal partings, which reacts with FeO to produce hercynite (FeAl2O4) at a temperature above 1100 °C. When the temperature increases to 1300 °C, hercynite is converted to the thermodynamically-stable cordierite (Fe2Al4Si5O18). In the oxidation stage, mullite (Al6Si2O13) is produced when the oxidation temperature reaching 1300 °C, which could be used to reflect the underground reaction conditions in the coal seam. The phase diagram of (SiO2)6.2Al2O3-CaO-Fe2O3 proves that mullite and anorthite (CaAl2Si2O8) are the dominant minerals when the mole fraction of (SiO2)6.2Al2O3 is above 0.7. In addition, the ash melting point rises with an increase of Si and Al content in the presence of non-coal partings.