Theoretical evaluation on the thermal response of coal during the conventional crossing-point temperature measurement

Abstract

The crossing-point temperature measurement is a classical thermal test method for ranking the propensity of coal towards self-heating and spontaneous combustion. Despite its longstanding use, it is still considered an empirical approach due to a lack of understanding on its working mechanism, preventing its wide use and standardization globally. In this paper, efforts were exerted to investigate the formation mechanism of the crossing-point temperature (CPT) and the impact of experimental settings on this parameter, aiming at the consolidation of the physical basis of this test method and paving the way for its further development. In light of the principles of heat and mass transfer, the thermal response of a coal sample stored in a cylindrical reactor exposed to linear heating environment was monitored. Fine coal particles were prepared and packed in the reactor with very thin wall, while the moisture content of a sample varied between 5 % and 20 %. The traced temperature histories are in agreement with the available experimental data. Observations indicated that the volume element at the sample center heats up by offsetting the heat sink term resulting from water evaporation. When the central temperature of the sample catches up with the environmental temperature, the heat sink term from water evaporation disappears, and the direction of heat flow via conduction is reversed, indicating the development of a self-heating domain. The impact of sample attributes and experimental settings on the measurement process is the actual reflection of the self-heating performance of a coal sample with specific physical properties and under defined environmental conditions. During the CPT measurement, the status of zero conductive heat flow at the central volume element can be monitored simultaneously, thereby extracting the apparent kinetic parameters of the sample oxidation at the same time. The established understanding sheds light on the broader application of this test method.