Coalbed methane emissions and drainage methods in underground mining for mining safety and environmental benefits: A review

Coalbed methane (CBM) drainage in underground coal mining is of significance to guarantee mining process safety and bring valuable environmental benefits, as the drainage could help to eliminate methane-related accidents, reduce greenhouse gas (GHG) emissions and provide a source of clean energy. Due to the manifold benefits, extensive investigations have been conducted on methane emissions and drainage methods. However, few review studies have been reported in analysing and summarizing those previous investigations. Therefore, in this paper, a broad review on CBM emissions and drainage methods is conducted. First, the methane emissions in underground coal mines are reviewed, including methane generation, storage and migration characteristics, emission sources, emission estimation, and the factors influencing emissions such as coal properties, geological conditions and mining-process parameters. Then, based on the borehole trajectory, three methane drainage methods for reducing methane emissions are categorized: surface to inseam (SIS) methane drainage, underground to inseam (UIS) methane drainage and cross-measure borehole methane drainage. Each drainage method is analysed from perspectives of borehole design and application function. Particularly, the popular UIS drainage borehole design and sealing for preventing ventilation air leakage are emphasized. Finally, summary and conclusions on previous works are provided. Outcomes of this review work are expected to provide relevant researchers and mining engineers with effective contents on methane emissions and drainage methods in underground coal mines. Based on the site-specific methane conditions in their coalmines, mining engineers could determine the most appropriate methane-drainage methods to increase the drainage efficiency and reduce methane emissions for a safer and more efficient mining process.