Investigation of macro-kinetics of coal-oxygen reactions under varying oxygen concentrations: Towards the understanding of combustion characteristics in underground coal fires

Abstract

The typical combustion mode of underground coal fires (UCFs) is smoldering during which the reaction zone is not exposed to a constant oxygen concentration. Understanding the macro-kinetics of coal-oxygen reactions under varying oxygen concentrations, especially the extremely oxygen-depleted condition, is of both theoretical importance and practical relevance to the control and extinguishment of UCFs. Considering the actual conditions of UCFs, thermal analysis tests under four oxygen concentrations (from 21% to 3%) and three heating rates (1, 2 and 5 °C /min) were carried out. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results were obtained for a bituminous coal sample from Inner Mongolia, China. With the global reaction assumption, the macro-kinetics parameters (the apparent activation energy, the pre-exponential factor and the kinetics model function) were determined. On the profiles of the apparent activation energy (Ea), three peak values were observed, physically interpreted as the depletion of volatiles, the formation of plastic mass and the depletion of char, respectively. This interpretation was verified by characteristic temperatures extracted from the experimental data. With the decrease of the oxygen concentration from 21% to 9%, two peak values diminish gradually. The case with 3% oxygen concentration gives a nearly monotonically declining Ea, indicating that under that particular condition, oxygen diffusion stands as the only limiting factor across all stages of coal-oxygen reactions. The best-fit kinetics model functions suggest that the char oxidation stage falls into the kinetics-controlled regime when the oxygen concentration is as low as 9%. For the volatiles burning stage, the universal ignition index (Fz) is found to be effectively related to the reaction regime for a variety of coal ranks. The quantitative results obtained can be integrated into any CFD multi-physics models as a sub-model for chemical kinetics.