Oxidative kinetic characteristics of dried soaked coal and its related spontaneous combustion mechanism

Abstract

Spontaneous combustion of the lone-term water soaked coal has been frequently taken place during air drying after water discharge, however the reason for the easier spontaneous combustion of water-immersed coal is still not clear. In this study, raw and coal sample soaked for nearly seven years was selected, low-temperature oxidation and crossing point temperature tests were first analyzed. The increased gaseous products concentration and the decreased crossing point temperature indicated that water immersed coal was more prone to oxidation and spontaneous combustion. Besides, the species and concentration of cations, pore structure, functional groups and thermal kinetic parameters were measured by ICP-MS, N2 adsorption, IR and TG analysis. The concentration of alkali cations that had the inhibitory effect on spontaneous combustion was found to decrease in the water-immersed coal, the pore size and specific surface area were enlarged, thus providing suitable internal conditions for the occurrence of coal spontaneous combustion. Moreover, the variation of functional groups and activation energies during the oxidation process showed that numbers of active sites were accumulated in the process of long-term water immersion and the water immersed coal can be oxidized at low temperatures, leading to the shorting of the side chain and the generation of oxygen-containing functional groups. Therefore, it is proposed that the water immersion process enhances the concentration of active sites in coal and provide physical channels for its oxidation, which led to rapid oxidation and even uncontrollable spontaneous combustion of water-immersed coal. The results help with our understanding of the mechanism of long-term water immersion on spontaneous combustion of coal, in addition to prevent the coal fires of water immersed coal by reducing the concentration of primary active sites.