Combustion Behavior and Thermal Disaster Quantification of Weathered Water-Saturated Coal in an Oxygen-Poor Environment of Goaf

Weathered water-saturated coal (WWSC) reserves in oxygen-poor environments in a goaf are present in large amounts, dispersed and pose a high risk of spontaneous combustion (SC). To determine the thermodynamic behavior and disaster-causing tendency of WWSCs stored in oxygen-poor environments, WWSCs with different weathering cycles were prepared. The oxidative–thermal behaviors of WWSCs in atmospheres with different oxygen concentrations were analyzed by using thermogravimetric analysis–differential scanning calorimetry (TG–DSC), and systematic combustion thermodynamic analyses were carried out. The results showed that the weathering time and environmental oxygen concentration synergistically affected the conversion rate of WWSC, thus affecting the length of the reaction stage. The reaction and transformation ability of WWSC weathered for 27 days at the low-temperature stage was better; the heat production of WWSC with short-term weathering (O_15-3d) was higher in the oxygen-poor environment, with maximum heat release and heat flow of 15751.5 J and 15 W/g, respectively. Different coal temperature stages of the WWSCs have different reaction dynamic models; these included low temperature–first-order reaction model and high temperature–two-dimensional diffusion Valensi model. The treatment of high oxygen concentration–long weathering time and low oxygen concentration–short weathering time caused a decrease in the E, ΔH and ΔG of WWSC and an increase in the D_f and H_F of coal. The synergistic effect of weathering time and oxygen concentration led to the greater SC tendency of the water-saturated coal with high oxygen concentration–long weathering time and low oxygen concentration–short weathering time, and the risk of thermal disaster was high. Our research results provide an important theoretical basis for goaf fire prevention and resource and environmental protection in deep coal mining and goaf remining and other projects.