Stage Development Characteristics of Oxygen-Lean Combustion of Coal in Fire Zone: Transition Characteristics of the Ignition Mechanism

Oxygen-lean combustion of coal in fire zones is a global disaster. To enhance underground coal management and prevent its re-ignition, this study employed Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) experiments to investigate the transition characteristics of the ignition mechanism in three typical low-rank coals under the competing effects of pyrolysis and oxidation, and quantitatively defined the suffocating oxygen concentration for coal oxygen-lean combustion. The results indicated that the ignition behavior of low-rank coals under pyrolysis-oxidation competition can be classified into three ignition mechanisms. With increasing coal rank, the transition point of the ignition mechanism shifted forward from around 3% oxygen concentration (lignite) to greater than 5% (gas coal). Furthermore, the transition characteristics of lignite were insensitive to time-scale effects, whereas those of long-flame coal and gas coal were relatively sensitive. At 1% oxygen concentration, the ignition mechanism of gas coal transitioned from transitional ignition to semi-coke multiphase ignition when the heating rate increased from 5°C/min to 10°C/min. DSC heat flow measurements revealed that at 1% oxygen concentration, the heat release of lignite decreased by approximately 52.2%, while that of gas coal plummeted by up to 89.1%, confirming that higher-rank coals experienced earlier (3% oxygen concentration) inhibition of energy feedback under low-oxygen conditions. Based on this, the suffocating oxygen concentration for the oxygen-lean combustion of typical low-rank coals was determined to be within the 3%–1% oxygen concentration range. The higher the coal rank, the closer the suffocating oxygen concentration was to 3%; the lower the rank, the closer it was to 1%. These findings provide a critical theoretical foundation for formulating coal fire management strategies and establishing engineering control indicators to prevent coal fire re-ignition.