Insight into the low-temperature oxidation behavior of coal undergone heating treatment in N2

Abstract

Heat treatment of coal under nitrogen is an effective technology to reduce water content and upgrade lignite, but the subsequent oxidation behavior and spontaneous combustion characteristic of dried coal are unclear, possibly posing a threat to lignite transportation and storage. In this study, raw coal and dried coal at different temperatures were selected and oxidation parameters were first analyzed by temperature-programmed experiment. The carbon oxides emissions and oxygen consumption of dried coals are obviously higher than that of raw coal, and increase with rising dehydration temperature, indicating coal after drying is more prone to oxidation and spontaneous combustion. Then, the physical structure, chemical groups and free radicals of coal were determined by nitrogen adsorption, FTIR and ESR technology. During drying process, gas release and water evaporation increase pore diameter and pore volume of coal, thus improving O2 transport capacity and oxidation rate. The hydroxyl content in coal decreases, but the CO content increases due to the polycondensation and dehydration reaction of hydroxyl groups after high-temperature drying. Meanwhile, carbonyl compounds and carboxyl groups undergo thermal decomposition and generate free radical active sites on the coal surface. The subsequent oxidation of accumulated free radicals not just release large amounts of carbon oxides, but also contribute to the formation of oxygen-containing groups inside coal with the rise of temperature, accelerating coal-oxygen reaction. Therefore, it is proposed that heat treatment expands physical channels, increases the content of CO groups, and enhances the content of active sites, which leads to vigorous oxidation of dried coal and even uncontrolled coal fires.