Effect of Moisture on Enhanced Coalbed Methane Recovery by Microwave Irradiation

Microwave irradiation enhances coalbed methane (CBM) recovery under dry conditions, but moisture always exists in coal seams. It is still unknown how moisture affects this technique. This work analyzed the adsorption and desorption patterns of methane (CH₄), the main component of CBM, in moist versus dry coals induced by microwave irradiation. These patterns were further interpreted with critical physicochemical properties of the coals. Results indicated that although moisture intensifies the initial thermal response of the coals to microwave because of the strong polarity of H₂O, it decreases the final temperature through vaporization as well. In this case, the heating behaviors of the diverse-rank coals are not considerably sensitive to moisture, being primarily controlled by coal grade and dielectric property. However, moisture can still enhance CBM recovery by stimulating pore structures of the coals. The small pores were squeezed, and the large pores and cracks were generated, resulting in compromised adsorption and improved desorption of CH₄, respectively. These alterations could be related to the microwave-driven moisture vaporization and the more pronounced mineral decomposition, transformation, and generation. As a consequence, the microwave irradiation reduces the maximum adsorption capacity of CH₄ on the moist coals by 36.36-50.39% relative to that of the dry coals (3.37-36.43%); moisture further weakens the adsorption and desorption hysteresis index by 47.44-83.43%. Our findings demonstrate the benefits of moisture in enhancing CBM recovery via microwave irradiation.