Thermal Behavior of the Low-temperature Secondary Oxidation of Coal under Different Pre-oxidation Temperatures

Abstract

ABSTRACT To study the effect of pre-oxidation temperature on the thermal behavior of secondary oxidation of coal, this paper studies samples from a Jurassic coalfield that is susceptible to spontaneous combustion. Under an air atmosphere, pre-oxidized coal samples with different pre-oxidation temperatures (75°C, 115°C, 155°C, 195°C) were prepared by heating at a rate of 0.5°C·min−1. Samples of each, and a raw coal sample, were then heated from 30–300°C at heating rates of 0.2, 0.3, 0.4, and 0.5°C·min−1 in an air atmosphere of 120 mL·min−1. The micro-characteristics of pre-oxidized coal samples were identified by Fourier infrared spectroscopy method. A C80 microcalorimeter was employed to monitor the thermal behavior of low-temperature secondary oxidation of coal. The heat flow curves of raw coal and pre-oxidized coal samples during the heating process show similar trends. Characteristic temperatures determined from the data of samples from different endothermic stages increase with increasing pre-oxidation temperature. The heat absorption of pre-oxidized coal samples in different endothermic stages is significantly lower than that of raw coal, and decreases first and then increases with increasing pre-oxidation temperature. Due to the effect of pre-oxidation, the methyl, hydroxyl and carboxyl groups of pre-oxidized coal samples are obviously lower than that of raw coal. The apparent activation energy of different oxidation stages of the experimental coal samples was calculated with a multi-rate heating method. The secondary oxidation activation energy of coal increases first and then decreases with increasing pre-oxidation temperature. In the rapid endothermic stage and the slow endothermic stage, the activation energy ranges of the coal samples were 68.12–201.46 kJ·mol−1 and 26.21–135.58 kJ·mol−1.