Responses of pore structure parameters and functional group compositions of coals to irradiation power: implication to coalbed methane production via microwave irradiation

Microwave irradiation could alter pores and surface chemistry properties of coal matrix, thereby affecting coalbed methane (CBM) production. The impacts of irradiation power on dominant pore structure parameters and functional group compositions of coals were elaborated in this study. The results show that the responses of micropore structure parameters of coals to irradiation depend on coal rank. Generally, micropore surface area and micropore volume of low-rank coals decrease after microwave irradiation. Moreover, a U-shaped trend exists between micropore parameters of low-rank coals and irradiation power. As for middle- and high-rank coals, micropore surface area and micropore volume increase after microwave irradiation. The irradiation power dependence of micropore parameters of middle-rank coal shows a decreasing trend, while that of high-rank coal shows an inverse U-shaped trend. Microwave irradiation decreases mesopores with diameter range of 2.00-32.00 nm for all coals, thereby reducing adsorption space of coal matrix for CH4. Besides, irradiation reduces C-O, C=O, and COOH of low-rank coals, while increases C-O and C=O of middle- and high-rank coals. Furthermore, microwave irradiation increases pyridine-N but decreases pyrrole/pyridone-N of all coals. Such alterations become pronounced with rising irradiation power. Overall, responses of pores and functional groups of coals to irradiation power rest with coal rank, therefore requiring further consideration in CBM production.