Thermoelectric effect in process of gas adsorption in different particle sizes of coal

Coal strongly adsorbs methane and during the adsorption process there is always a thermal effect that leads to some changes in the coal structure and temperature. These changes have an electrical effect, such as a change in coal resistivity. The investigation of the thermoelectric effect of the adsorption process of coal has long been a hot research issue to industry experts and scholars. It is important that this thermoelectric effect be well understand with respect to the energy transfer and translation in the adsorption process, as well as to support geophysical electrical prospecting and disaster prediction. Both domestic and international scholars have conducted many studies on this issue. The heating and electrical effects in the process of coal adsorption are interrelated. Previous research has primarily addressed the single characteristic of gas adsorption or its application, with little attention being paid to their correlation. Most studies have used adsorption quantity to reflect the coal adsorption ability, and few have focused on the thermoelectric effect to study coal adsorption. Studies regarding coal particle size and the thermoelectric effect are even rarer. In this study, we investigated the temperature change and the current vs. time curves of three different metamorphic grade briquette coals, which consist of five different coal particle sizes, at a temperature of 25℃ and pressure of 3 MPa in the coal gas adsorption process. To do so, we used a SH-X multi-channel temperature tester and CHI660E electrochemical workstation. To determine the adsorption ability of coal with respect to the thermoelectric effect, we used the Clausius-Lapeyron equation to analyze the thermoelectric effect mechanism of coal and its correlation in the coal gas adsorption process of different particle sizes. The results show that an obvious thermoelectric effect accompanies the coal gas adsorption process. The temperature of the coal increases from 0.93℃ to 8.74℃ and the resistivity of the coal decreases 0.14-0.16 times from that of its stability when it reaches adsorption equilibrium. We found the temperature of the coal to increase as the particle size decreased and the gas adsorption quantity increased, but the coal resistivity change was the opposite. Coal temperature and resistivity change are strongly correlated with the gas adsorption quantity, with the correlation coefficients r_d and r_w ranging between 0.9502-0.9899 and -0.9316 to -0.9916, respectively, which are close to±1. Therefore, the heat adsorption process can reflect the adsorption ability of coal. When reaching equilibrium, the greater the temperature change, the higher the temperature and the lower the resistivity, which means that the adsorption capacity is greater, and in contrast, the adsorption capacity is weaker.