Experimental research on the features of spontaneous combustion during secondary oxidation of coal with various particle sizes

Abstract

The secondary oxidation of remaining coal in the process of secondary mining of coal seams poses a major threat to the safety of personnel and property, as well as the effortless re-mining of relict coal. The grain size composition of the relict coal in the mining area is complex, and it is an essential component determining the coal's spontaneous combustion characteristics, particularly the more complex secondary oxidation reaction. The secondary oxidation characteristics of coal treated with various pre-oxidation temperatures at different grain sizes were investigated using TG-DSC coupled experiments and electron paramagnetic resonance (EPR) experiments. The results showed that as the particle size of the coal samples decreased, the ignition point temperature T6 and the maximum burning rate temperature T7 decreased gradually; the burnout index and the comprehensive combustibility index increased gradually; the endothermic ‘ladder’ on the DSC curves are more pronounced, with the peaks shifting to lower temperatures; the activation energy of coal samples gradually declines from the ignition point temperature T6 to the maximum burning rate temperature T7 during the combustion process, as determined by Coats-Redfern and Achar-Brindley-Sharp-Wendworth respectively; the g-factor of coal samples tends to decrease, while the concentration of free radicals declines first and subsequently increases. With the same particle size, the g-factor of the coal samples remained virtually constant as the pre-oxidation temperature increased, whereas the free radical concentration gradually increased.