Thermodynamic characteristics and oxygen threshold limiting the Re-ignition of overfired coal residues in sealed fire zones

This study investigates the thermodynamic properties and oxygen threshold limiting the reignition of overfired coal residues within sealed fire zones under controlled oxygen conditions. Thermogravimetric experiments were conducted to analyze re-ignition behavior across varying heating rates and oxygen concentrations. The re-ignition process is delineated into three distinct phases: low-temperature pyrolysis, oxygen adsorption, and combustion. Elevated oxygen concentrations accelerate re-ignition and combustion by lowering ignition and burnout temperatures, shortening combustion duration, and increasing the maximum weight loss rate, indicating a more rapid and intense combustion process. The threshold oxygen concentration for re-ignition suppression was identified between 3 % and 5 %, effectively restricting combustion due to limited oxygen availability. Re-ignition mechanisms shift with oxygen levels, from the A-E model at 5 %, to Jander diffusion at 15 %, and to the shrinking spherical model at 21 %. Below 5 % oxygen, combustion becomes oxygen-limited, with pyrolysis predominating. In this phase, the activation energy fluctuates and significantly decreases around the 5 % threshold, marking a critical shift in the combustion process. These insights offer important guidance for safely reopening fire zones in coal mines and for reducing re-ignition risks.