Heat Release Characteristics of L-Malic Acid Inhibited-Coal Oxidation

Abstract

ABSTRACT Transition metal ions in coal can catalyze the oxidation of coal, reduce the activation energy of the coal-oxygen reaction, and promote spontaneous combustion. Adding a metal chelating agent to coal can form chelates with metal ions so that the metal ions lose their catalysis. A green metal chelating agent, L-malic acid (L-MA), was proposed in this study. A C600 microcalorimeter was used to study the inhibitory effect of L-MA on spontaneous coal combustion. The results show that under the same temperature-programmed conditions, the heat absorption capacity of coal treated with L-MA solution is higher than that of raw coal, but the heat release capacity of treated coal is lower. With increase in the concentration of the L-MA solution, the inhibitory effect first increased and then decreased. The maximum heat flow of raw coal is 292 mW, and the maximum heat flow of coal samples treated with malic acid is 246 mW, 237 mW, 232 mW, 240 mW and 255 mW, respectively. The maximum heat flow of 15 wt% coal sample is 20.55% lower than that of raw coal. Based on the thermal analysis, kinetic theory, and experimental data, the activation energy of each coal sample can be calculated. The activation energy of raw coal is the lowest, and the activation energy of each group first increases and then decreases with increasing L-MA concentration, which is in agreement with the law of heat flow data. Combined with the microscopic process of coal oxidation at different temperature stages, the mechanism of L-MA inhibiting spontaneous coal combustion was explored. In summary, L-MA can effectively reduce the concentration of related free radicals in the chain reaction of spontaneous coal combustion and inhibit spontaneous coal combustion.