Research progress of temperature field and heat transfer characteristics in process of underground coal gasification

Underground coal gasification (UCG) technology, as an environmentally-friendly coal mining method, can be used to develop deep coal seams and coal resources left over from mines. At the time of underground gasification, as the gasification time passes, coal “burns” inside the coal seam to gradually form a gasification furnace, and the maximum temperature in the furnace can reach 1 200 ℃. Starting from the exothermic reaction in the UCG process, this paper summarized the heat source, the change of temperature field and its heat transfer characteristics, and summarized the important methods for studying the temperature field. It is believed that UCG is a complex process of dynamic changes in temperature and heat, during which the reversible reactions of oxidation-reduction reaction, endothermic and exothermic proceed simultaneously, resulting the complexity of the temperature field of the gasifier and its heat transfer mecha nism. There are four main methods for studying the heat transfer mechanism of the gasification process: physical simulation, experimental monitoring, theoretical calculation and numerical simulation, each with different adaptability. Among them, the physical simulation has strong maneuverability, but it is difficult to solve the contact thermal resistance error caused by the contact relationship between the formations; the experimental monitoring can truly reflect the changes in the temperature field, but the actual operation is not economical; the theoretical calculation passes the separation of the gasification process, qualitatively quantified the temperature field characteristics of UCG from a theoretical point of view, but theoretical calculations are difficult to consider the influence of temperature seepage on temperature; numerical simulation uses computers to conduct multi-field coupling of temperature fields, and comprehensively consider seepage, deformation, and temperature. However, the calculation of the physical coupling of multiple fields is too difficult, and the thermophysical properties of coal seams and rocks will vary greatly with temperature changes, which increases the computational difficulty of numerical modeling and restricts numerical simulation methods.In summary, the existing UCG temperature field research methods cannot truly reflect the heat transfer characteristics of the gasifier, and various research methods can only study temperature changes under specific conditions and are difficult to relate to actual conditions. Therefore, combining field test data andlarge-scale physical simulation experiments with numerical simulation methods to further develop the coupling mechanism research between multiple physical fields will be the main research direction of subsequent underground coal gasification. In addition, the changing law of rock thermophysical properties at high temperature and its influence on the temperature field will also be one of the research directions worthy of attention.