Effect of acid-base pretreatment on the production of active sites and coal self-heating behavior

The effect of inorganic salt ions on coal spontaneous combustion has been well revealed. However, the effect of organic metals on coal self-heating process has been paid little attention, which deserves further study. In this study, coals with different content of carboxylate were prepared by acid treatment, alkali treatment and re-acid treatment. The effect of acid-base pretreatment on the generation and oxidation of active sites was measured by pyrolysis-room temperature oxidation experiments. Thermal analysis and kinetic calculation, SEM-EDX, XPS, ESR were used to analyze the internal mechanism. The results show that the increase of free radicals (ΔNg) for the acid -pretreated coal during pyrolysis is 14.52 × 1018 spins/g, less than that of raw coal. The conversion of carboxylate structures to carboxylic acid structures by acid treatment significantly reduced the production of active sites, thus inhibiting coal self-heating behavior. The activation energy of alkali treated coal was 33.57 kJ/mol lower than that of raw coal, attributing to the conversion of carboxylic structures to carboxylate structures. The carboxylate structures can form coal matrix and Na binding structures (CM-Na) during the thermal decomposition process. The structure was able to constantly break and recombine due to its weak stability at low temperatures, accompanied by the generation of active sites, resulting in higher free radical concentration of 20.66 × 1018 spins/g. The research results have great value for investigating the mechanism of coal spontaneous combustion and providing an effective pathway to restrain the coal self-heating process.