Overview and Outlook on Utilization Technologies of Low-Concentration Coal Mine Methane

Coal mine methane (CMM) with the worldwide reserves of 2.6 × 1014 m3 is one of the important unconventional natural gases. However, most of the low-concentration coal mine methane (LC-CMM, cCH4 < 30 vol %) is not utilized and directly emitted to the atmosphere, producing up to about 28 billion m3 CH4 emissions annually and causing a serious greenhouse effect and energy loss. Evidently, resource utilization of LC-CMM is of vital significance to achieve energy transition and carbon neutrality. Herein, we review the recent advances of LC-CMM utilization technologies in terms of low-concentration drainage gas (1 vol % < cCH4 < 30 vol %) and ventilation air methane (VAM, cCH4 < 0.75 vol %). Especially, the great progresses of LC-CMM utilization in China are extensively introduced. A large amount of the low-concentration drainage gas with cCH4 > 6 vol % has been utilized in China due to the wide application of specially designed internal combustion engines (ICEs) and safe pipeline transport technologies. To avoid the shortages of low efficiency and high pollution of ICEs, solid oxide fuel cells (SOFCs) fueled by LC-CMM were recently proposed and modified for improving safety, efficiency, and stability. Also, methane enrichment such as pressure swing adsorption (PSA) and direct burning are key technologies to improve the utilization ratio of the drainage gas with cCH4 < 6 vol %. The modifications to improve the safety and efficiency of the PSA process and the new metal fiber combustion have been developed and applied in China. Furthermore, the recent advances of VAM utilization technologies are introduced. A thermal flow reverse reactor (TFRR) for VAM utilization has been applied in some coal mines. VAM enrichment and a catalytic flow reverse reactor (CFRR) create opportunities to efficiently use VAM with much lower CH4 concentration. Finally, the technical and economic challenges as well as the coping strategies of LC-CMM utilization are described. It is believed that LC-CMM with all various CH4 concentrations can be effectively and cleanly utilized in the near future, contributing to the new energy structure establishment and environment protection.