Microstructure changes induced by adsorption/desorption of low rank coals and its desorption hysteresis mechanism

Adsorption/desorption behavior and its hysteresis phenomenon of coalbed methane (CBM) make a critical contribution to estimate CBM recovery. Hence, to investigate the adsorption/desorption mechanism of low rank coals, this study compared the adsorption/desorption characteristics of methane in different rank coals through isotherm adsorption/desorption experiments and then analyzed the changes in microstructure before and after adsorption/desorption. Thus, the coupling relationship among chemical structure, nanopores and gas adsorption/desorption and the desorption hysteresis mechanism for low rank coals were revealed. Results show that the isosteric adsorption heat and desorption hysteresis coefficient present a decreasing first and then increasing trend with coalification from isotherm adsorption/desorption curves and thermodynamic characteristics. Specially, for low rank coals, as the degree of thermal evolution increases, the desorption hysteresis coefficient increases from 0.27 to 0.39 during low metamorphism stage. Then, compared the nanopore structure distribution and micro-crystalline structure characteristics before and after adsorption/desorption, methane adsorption/desorption behavior not only promotes the pore expansion effect of ultra-micropores, but also leads to a swelling effect of basic structure unit (BSU). Furthermore, it can be found that as the BSU increases, the specific surface area (SSA) and pore volume (PV) of nanopores in coal increase, which directly controls the adsorption space. Meanwhile, the adsorption/desorption behavior can also induce the BSU swelling to change nanopore structure distribution, especially micropores and ultra-micropores. Finally, concerning desorption hysteresis effect, its esssence can actually be explained by the energy difference and microstructure. All these findings would provide the theorical guidance for CBM development in low rank coals.