Re-ignition characteristics of the long-flame coal affected by high-temperature oxidization & water immersion

Alongside coal seam mining, the residue coal in overlying goaf is prone to primary oxidation due to air leaking through the geological fissures, which could cause spontaneous combustion accidents for water-immersed coal. Within scanning electron microscope (SEM) and nitrogen adsorption method, the physical structure of long-flame coal under pre-oxidation and water-immersion was characterized. By infrared spectroscopy and synchronous thermal analysis, the functional groups of coal samples and the heat release characteristics were also discussed during the re-ignition process. The results show that pre-oxidation could result in the increase of oxidized pores numbers compared with the raw coal, and the amount of small pores/mesopores are went up by water immersion. Pre-oxidation and water-immersion could increase the proportion of large pores, expand the average diameter of pore, improve the coal oxygen reaction capacity, increase the content of functional groups. Meanwhile, pre-oxidation and water-immersion would convert the active groups into active sites, simultaneously the secondary oxidation and the conversion of active-functional groups could result in bond breakage, the bond energy determines the change in heat release when the bond is broken. Oxidization at 200 °C and water immersion (O200I200) could lower the ignition point and the water-locking capacity, increase the heat release by 660.5 J/g. The coal sample (O200I200) is more prone to re-ignition than the other samples. The research results could be useful and meaningful for further understanding the characteristics of re-ignition for long-flame coal as mining, and also could provide theoretical guidance of the prevention for coal spontaneous combustion in overlying goaf caused by pre-oxidization and water-immersion.