Responses of Physico-chemical Properties and CH4 Adsorption/Desorption Behaviors of Diverse Rank Coal Matrices to Microwave Radiation

To assess feasibility of microwave radiation in CH4 production, temperature rise of coals under microwave field and resultant alterations in physico-chemical properties were addressed. Results indicated that the temperature rise of coals is dominated by mineral compositions. The bulk temperature containing strong microwave-absorbing minerals such as pyrite, smectite, and kaolinite raises markedly. The elevated temperature breaks alkyl side chains and aromatic rings of coals. Moreover, the total oxygen-containing functional groups decrease by radiation. Generally, the radiation reduces the micro- and mesopores of coals. Besides, the pore surface of low-rank coals after radiation becomes smooth, whereas that of medium- and high-rank coals becomes rough. Finally, the radiation decreases CH4 maximum adsorption capacity by 6.74-36.69%. The radiation also diminishes CH4 adsorption/desorption hysteresis for low-rank coals, while an opposite trend exists in medium- and high-rank coals. Such variations rely on pore structure responses to microwave. Overall, microwave radiation could promote CH4 recovery.