International audienceThe article presents findings of a study on CO2/CH4 exchange sorption taking place in a coal briquette, initiated by the injection of a CO2 into a briquette that was previously saturated with CH4. The briquette was subjected to multiple cycles consisting of vacuum outgassing, CH4 saturation under 10 bar pressure, and an exchange sorption process. In each experiment input CO2 pressure and output gas mixture pressure were stabilized. The chosen pressure levels were 12, 13 and 11 bar at the input and 10 bar at the output of the briquette. Flow rate of gas streams flowing to and flowing out of the briquette, as well as gas concentration at the output of the briquette, was measured. During the process of exchange sorption, the flow rate of gas streams at the input and output of the briquette varied, even though the input and output pressures were constant. In all the experiments, the entire CH4 was displaced from the briquette after injecting about twice the rate of the CO2 mass in relation to the previously sorptive bonded CH4. The results thus obtained were confronted with results obtained by other researchers, available in relevant sources, for the purpose of their comparison

Traditionally, physicochemical processes whose result is the release of methane from a coal sample—often referred to as desorption—are discussed in relation to two phenomena, i.e., the transportation of gas inward (or outward) the porous substance, and the desorption (or sorption) proper of gas from the coal surface. The complex pore structure of coal necessitated the adoption of an arbitrary division of the gas transportation into: the diffusion of gas within the pore network in grains (understood as a molecular phenomenon) and the filtration of gas through the system of fissures and large pores (understood as a phenomenon related to the fluid mechanics). The filtration-type transportation of gas through a porous medium is most typically discussed in the context of the flow of gas through coal beds, or other porous rocks. In laboratory analyses carried out on granular samples obtained from the porous material, the filtrational flow of gas among the sorbent grains and within the fissure system is so fast (as compared to the diffusion occurring inside the grains) that the decisive factor influencing the kinetics of gas release is, most often, diffusion. In the case of such observations, it is assumed that the process of the sorption proper is almost instantaneous in its nature. There are no theoretical premises that would challenge an assumption thus formed, however, it would be very interesting to carry out an experiment confirming their existence. The present paper discusses an attempt to estimate, under laboratory conditions, the kinetics of the sorption proper. It provides a description of the specially built measuring equipment, as well as the results of the performed experiments, together with their analysis. In the performed experiments, the obtained time constants of sorption were smaller than 50 ms. The lowest values did not exceed 10 ms. Since the obtained time constant values are very low—in spite of the fact that it was impossible to completely eliminate the delays connected with the transportation of molecules to the sorption sites—the authors are of the opinion that the process of sorption proper shall be assumed to be instantaneous.

The aim of this paper was to investigate the repeatability of sorption processes occurring in the coal-methane system during multiple measurement series. For research purposes, three granular samples of coal sorbent were used. The samples were subjected to a series of three measurements, each time performed under the same conditions. During each series of measurements, the following stages took place: outgassing the sample in a vacuum, and then saturating it with methane under the pressure of 1, 3, and 10 bar. Sorption capacities and the values of effective diffusion coefficients were compared. The studies into the repeatability of measurements of sorption capacities, conducted by means of the IGA-001 (Intelligent Gravimetric Analyzer) gravimetric instrument for three measurement cycles, showed that the obtained measurement discrepancies in relation to the mean value did not exceed 1.23%. The discrepancies in question approximated the measurement uncertainty of the IGA-001 device. The evaluation of the recurrence of determining the values of effective diffusion coefficients showed that the obtained discrepancies were no larger than 10%, which might have resulted from problems with fulfilling most of the assumptions of the applied unipore diffusion model. The authors did not observe any changes of the sorption parameters of hard coal during consecutive cycles of methane saturation.