Experimental and Theoretical Investigation on the Initiation Mechanism of Low-Rank Coal's Self-Heating Process

Abstract

Coal spontaneous combustion remains a safety concern during coal mining, transportation, and storage and requires further investigation to develop effective prevention and control strategies. While various intrinsic and extrinsic conditions may influence coal self-heating, low-rank coals are more prone to spontaneous combustion, especially in humid climates. Therefore, quantifying the effect of extrinsic moisture on the self-heating of low-rank coals is critical for understanding the high propensity of low-rank coals for spontaneous combustion. Many studies have qualitatively evaluated the role of extrinsic moisture on the coal self-heating. Despite this, few studies have quantified the contribution of extrinsic moisture on wetting and oxidation heat and their ultimate impact on coal's tendency for spontaneous combustion. This dissertation seeks to develop experimental and mathematical strategies for quantifying the contribution of extrinsic moisture to low-rank coal self-heating under various intrinsic conditions. As such, a modified R70 experiment setup is designed to effectively introduce moist oxygen and evaluate the impact of extrinsic moisture on coal self-heating. This experimental setup is capable of controlling the temperature and humidity of inlet oxygen while monitoring the relative humidity of the outlet gas in real-time. The comparison between inlet oxygen's enthalpy and coal oxidation heat shows that only a portion of the moist oxygen's enthalpy is consumed to sustain the coal's self-heating. Next, the wetting heat is identified as the differential enthalpy between the inlet and outlet gas and quantified by implementing the photogrammetry technique for the inlet gas’s specific humidity and the real-time measurement of the outlet gas's relative humidity. Following the results of these systematic testing, the quantification of wetting heat is comprehensively investigated by testing different low-rank coal samples under varying air temperatures and humidity levels. Furthermore, the combinatory effect of extrinsic moisture and coal particle size is determined by testing different types of low-rank coals with different particle size distributions. Finally, a mathematical model is developed based on the energy conservation law to simulate the self-heating curve of low-rank coal under various air humidity and temperature levels.