Changes in Thermal Kinetics Characteristics during Low-Temperature Oxidation of Low-Rank Coals under Lean-Oxygen Conditions

In the present work, we derive the heat-flow and mass-loss curves for the oxidation of three low-rank coals under Oxygen-Lean conditions using thermogravimetry and differential scanning calorimetry. Relationships between the exothermic character of the low-temperature oxidation (the stage before the volatile combustion of coal) and the oxygen concentration were analyzed. Kinetic parameters for low-temperature coal oxidation under oxygen-lean conditions were obtained, based on a derived Arrhenius equation. The results demonstrate that there is a critical oxygen concentration above which the full oxidative combustion reaction of low-rank coal will be reached under oxygen-lean conditions of continuous ventilation; if the oxygen concentration exceeds the critical value, the temperature at which coal oxidative combustion reaction is completed is pushed back as the oxygen concentration decreases. The amount of heat released during low-temperature oxidation decreases with decreasing oxygen concentration, but the influence on total quantity of heat released is small. During the low-temperature oxidation of low-rank coal, three stages of kinetic parameters were identified: During stages 1 and 2, the apparent activation energy and pre-exponential factor fluctuated slightly at average values as the oxygen concentration changed; however, during stage 3, the apparent activation energy decreased in a linear manner as the oxygen concentration decreased, and the pre-exponential factors for XJ-H and XJ-Z coals decreased in a linear manner whereas that for PS coal decreased in an exponential manner.