Influence of Water Immersion Time on Re-Ignition Characteristics of High Temperature Oxidized Coal

Abstract

ABSTRACT After a coal seam is mined, the oxidized coal left in its goaf is prone to water immersion, and spontaneous combustion accidents involving water-immersed coal may easily occur when mining the underlying coal seam. Using scanning electron microscopy and nitrogen adsorption experimental methods, the physical structure of high-temperature pre-oxidized water-immersed coal samples was characterized. By means of infrared spectroscopy, electron spin resonance (ESR) and simultaneous thermal analysis, the microscopic characteristics of the coal samples and the endothermic and exothermic characteristics of the oxidative combustion process were studied. The results showed that a specific pre-oxidation soaking time generated more oxidation pores in the coal sample, causing more water to become trapped in the pores and increasing the secondary oxidation heat absorption. High-temperature oxidation promoted the conversion of coal sample functional groups (FG) and free radical active sites, Water immersion led to the formation and loss of FG, and the longer the water immersion time was, the greater the loss. The larger average pore size of the pre-oxidized water-immersed coal sample provided more coal-oxygen contact sites, and the breaking of chemical bonds in the FG during the secondary oxidation process accelerated the heat release and promoted the oxidative re-ignition of the coal sample. The spontaneous combustion of water-immersed coal in the goaf can be prevented by controlling the high oxidation temperature and water immersion time.