Kinetic and thermodynamic evolution driven by oxygen gradients in underground coal gasification

Underground coal gasification (UCG) is essentially the multi-field coupled thermochemical conversion process. One of the most significant characteristics during the UCG process is the formation of the pronounced oxygen concentration gradient along the gasification channel, which leads to significant zoning characteristics in the underground reaction zone. In this work, the coal reaction behavior driven by oxygen concentration gradient from 0% to 100% (10 levels) is focused. Meanwhile, the numerical model combining reaction kinetics and empirical component distribution is used to compare the gas production performance under different oxygen concentrations. Under the low oxygen concentrations (<21% O 2 ), the maximum generation rate and content of CO are relatively high, reaching 0.23 mol/(kg·K) and 0.75, respectively. In contrast, under the high oxygen concentration (21% ∼ 100% O 2 ), the maximum generation contents of CO 2 , CH 4 , and H 2 are higher. As the oxygen concentration increases from 0% to 100%, the apparent activation energy of coal firstly increases from 31.56 to 90.69 kJ/mol, and then decreases exponentially to 20.62 kJ/mol. Meanwhile, compared to the oxygen-lean conditions, the enthalpy change, entropy change, and Gibbs free energy change are lower under the oxygen-enriched conditions. The critical oxygen concentration for the kinetic-thermodynamic parameters of coal is 8%. The gradient distribution of oxygen along the gasification channel is the core driver behind the zonal evolution in UCG. Therefore, in the actual UCG application, the precise regulation of oxygen concentration is essential to optimize syngas composition, improve gasification reaction intensity, and achieve stable evolution of the reaction three zones.