Structural changes in coal at elevated temperature pertinent to underground coal gasification: A review

Underground coal gasification (UCG) has been identified as an environmentally friendly technique for in-situ gasification of deep unmineable coal seams. As coal is gasified, a cavity is created which grows with time. Cavity evolution along with high temperature imposes changes to the coal and surrounding strata. Understanding structural changes in the coal during drying/vaporization, pyrolysis, and gasification in UCG is a key factor in studying growth of the gasification zone and helps in optimizing the UCG process in order to minimize syn- and post-gasification risks to the strata and groundwater. The main objective of this study is to elaborate structural impacts of UCG process on coal and to review the current state of knowledge in the area of influence of elevated temperature on transport and mechanical properties of coal in the context of UCG. Published high-pressure high-temperature experimental studies on coals are very scarce; hence, this study reviews and compares the behavior of different rank coals from different parts of the world in order to develop a pathway for future high-pressure high-temperature geomechanical experiments of coals. Impact of elevated temperature on weight loss, thermal deformation, microcrack generation, pore volume, average pore aperture, porosity, permeability, tensile, compressive and shear stress-strain responses, elastic and shear moduli, and Poisson's ratio of the coals under study are discussed and compared.