Study on the heat and mass transfer mechanisms of coal under sub-/supercritical water environment with co-injected oxygen

By utilizing a sub-/supercritical water-assisted oxygen injection experimental system, real-time temperature monitoring at different measurement points within the coal body is performed during the sub-/supercritical water environment with co-injected oxygen. Combined with theoretical analysis and numerical simulation, this study investigates the heat and mass transfer mechanisms of coal under sub-/supercritical water environment with co-injected oxygen. The main research findings are as follows: Firstly, the critical temperature point for rapid oxidation of coal with oxygen in a sub-/supercritical water environment is 350 °C. Secondly, reaction heat at different locations shows no variation below 350 °C, diverges sharply at the critical point, and remains significantly different above it with little sensitivity to initial oxygen injection temperature. Thirdly, among positions equidistant from the injection well, those along the injection-to-production well direction exhibit significantly higher reaction heat during oxygen injection. Furthermore, below 350 °C, extensive incomplete oxidation occurs between coal and oxygen in the subcritical water environment, with significant residual oxygen present. Above 350 °C, oxidation becomes more complete in the supercritical water environment, with little to no residual oxygen remaining. Finally, when the temperature of the coal in the sub-/supercritical water environment exceeds the critical temperature of 350 °C before oxygen injection, oxygen concentrations vary significantly with distance from the injection point. Due to the combined effects of oxygen transport and oxidation reactions, three sequential zones are formed: a fully oxidized exothermic zone, an incompletely oxidized exothermic zone, and a pyrolysis zone under low oxygen concentration.