Substituent positions and types for the inhibitory effects of phenolic inhibitors in coal spontaneous combustion

Abstract

This study is aimed at investigating the effects of substituent positions and types on the activities of phenolic inhibitors for inhibiting coal spontaneous combustion (CSC). To achieve this aim, six phenolic inhibitors with different substituent positions (ortho-, meta- and para-) and types (methyl, ethyl, isopropyl and tert butyl) were selected for testing their inhibitory effects on Xilinhe coal. First, the temperature-programmed oxidation experiment was performed on the raw coal sample and the inhibited coal samples to explore their CO generation volumes and crossing-point temperatures under low-temperature oxidation conditions. Meanwhile, the corresponding inhibition rates, oxygen consumption rates and apparent activation energy values were calculated. Furthermore, the weight loss processes, surface morphologies and active functional group variations of the raw coal sample and the inhibited coal samples were analyzed with the aid of a thermogravimetric analyzer, a scanning electron microscope and a Fourier transform infrared spectrometer. The test results showed that the phenolic inhibitors can not only infilter into coal but also accumulate on coal surface to form an inhibitor layer. Besides, they can inhibit CSC in both physical and chemical ways during low-temperature oxidation of coal, and their inhibitory effects exhibit varying characteristics in different stages. When the temperature is below 120 °C, the physical inhibitory effects of oxygen isolation and water retention dominate. When it exceeds 120 °C, the chemical inhibitory effect of capturing active free radicals in coal plays a dominant role. The types of substituents have a more notable effect on the activities of phenolic inhibitors than the positions of substituents. The inhibitor with smaller substituent steric hindrance and stronger infiltration ability is more capable of inhibiting CSC physically and chemically. Among the inhibitors with different substituent types, m-cresol boasts the optimal inhibitory effect. Its inhibition rate is 84.4% at 170 °C; the oxygen consumption rate is only 1/4 of that of raw coal; the crossing-point temperature is raised by 30.3 °C to 181.7 °C; and the apparent activation energy values are increased by 7.74 kJ/mol, 16.18 kJ/mol and 20.61 kJ/mol in the slow oxidation stage, the accelerated oxidation stage and the intense oxidation stage, respectively.