Feasibility study of enhanced biogenic coalbed methane production by super-critical CO 2 extraction Hongguang Guo a, c, * , Yujie Zhang a, c, 1 , Yiwen Zhang a, c, 1 , Xingfeng Li a, c, 1 , Zhigang Li a, c, ** , Weiguo Liang b, c , Zaixing Huang d, e , Michael Urynowicz e , Muhammad Ishtiaq Ali f a College of Safety and Emergency Management and Engineering, Taiyuan University of Technology, Taiyuan, 030024, China b College of Mining Technology, Taiyuan University of Technology, Taiyuan, 030024, China c Key Lab of In-situ Property-improving Mining of Ministry of Education, Taiyuan University of Technology, Taiyuan, 030024, China d School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou, 221116, China e Department of Civil & Architectural Engineering, University of Wyoming, Laramie, WY, 82071, USA f Environmental Microbiology Lab, Department of Microbiology, Quaid-I-Azam University, 8 Islamabad, 45320, Pakistan article info Article history: Received 31 July 2020 Received in revised form 18 September 2020 Accepted 22 September 2020 Available online 25 September 2020 Keywords: Microbially enhanced coalbed methane Super-critical CO 2 Organics Functional group Nitrogen adsorption abstract Super-critical CO 2 enhanced coalbed methane (Sc-CO 2 -ECBM) and microbially enhanced coalbed methane (MECBM) are environment-friendly technologies that can improve CBM recovery and gener- ation. In this study, a new approach of MECBM based on Sc-CO 2 extraction is presented. The Sc-CO 2 pretreatment experiments of anthracite and bituminous coal were conducted to produce biomethane. The extracted organics and the changes of coal structure caused by Sc-CO 2 were also analyzed to discuss the mechanism of methane stimulation. The results indicated that methane yields have been greatly improved after Sc-CO 2 extraction by 734.85% and 148.15% for anthracite and bituminous coal, respec- tively. The extractions observed by GC-MS analysis is also favored to generate methane by microor- ganisms. The little increment of methane production from coal treated by subcritical CO 2 indicated that the special characteristics of Sc-CO 2 were critical for the stimulation of methane production. More functional groups and even new functional groups were formed to increase coal bioavailability after Sc- CO 2 extraction. The specific surface area and total pore volume of bituminous coal increase after Sc-CO 2 extraction that could provide more action sites for microorganisms and enzymes. These results strongly proved the feasibility of enhancing CBM by microbial degradation based on Sc-CO 2 extraction. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction Driven by the development of low-carbon economy, the global energy consumption patterns are changing greatly [1]. British Pe- troleum (BP) energy outlook 2019 reported that natural gas con- sumption has risen by 74% in the past two decades and will continue to increase over the next two decades [2]. Coalbed methane (CBM), an unconventional natural gas and a major strategic supplement of conventional natural gas, has become important in many countries [3]. However, the development of CBM is limited by the low recovery and the future of CBM exploi- tation remains uncertain [4,5]. Therefore, the development of new technologies for improving CBM recovery has attracted increasing attention [6]. In recent years, microbially enhanced CBM (MECBM) has become a research hotspot due to its unique properties in coal modifications, enhanced CBM generation and recovery, and envi- ronmentally friendly. MECBM was proposed by Scott in 1999 that was originally intended to increase CBM production and improve coal perme- ability [7]. In that, new methane can be generated by injecting anaerobic microbial communities and nutrients into the coal seam to degrade organics in coal. Since then, many studies indicated that organics in coal can be transformed into simple organics such as long-chain alkanes, fatty acids, and single ring aromatics under the * Corresponding author. College of Safety and Emergency Management and En- gineering, Taiyuan University of Technology, Taiyuan, 030024, China. ** Corresponding author. College of Safety and Emergency Management and En- gineering, Taiyuan University of Technology, Taiyuan, 030024, China. E-mail addresses: guohg_tyut@163.com (H. Guo), zhiganglee2009@hotmail.com (Z. Li). 1 These authors contribute equally to the paper. Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy https://doi.org/10.1016/j.energy.2020.118935 0360-5442/© 2020 Elsevier Ltd. All rights reserved. Energy 214 (2021) 118935