Catalytic calcium-looping reforming of biogas: A novel strategy to produce syngas with improved H 2 /CO molar ratios Sicong Tian a, * , Xiaoxia Yang a , Xuejing Chen b , Guangshi Li c , Aihemaiti Aikelaimu b , Yuan Meng b , Yuchen Gao b , Candace Lang a , Maohong Fan d a School of Engineering, Macquarie University, Sydney, New South Wales, 2109, Australia b School of Environment, Tsinghua University, Beijing, 100084, PR China c State Key Laboratory of Advanced Special Steel & School of Materials Science and Engineering, Shanghai University, No. 99 Shangda Road, Shanghai, 200444, PR China d Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, WY, 82071, USA article info Article history: Received 11 March 2020 Received in revised form 27 May 2020 Accepted 28 May 2020 Available online 8 June 2020 Handling Editor: Bin Chen Keywords: Biogas Calcium looping Syngas H 2 /CO molar ratio Nickel abstract Anaerobic digestion to acquire biogas is currently the most widely used approach to dispose of vast anthropogenic biowastes. However, effective strategies, except for power generation in the energy sector, to utilize the waste-derived biogas resource are lacking. Herein, we propose a new thermochemical biogas transformation process, catalytic calcium-looping reforming of biogas, to produce syngas with a controllable molar ratio of H 2 to CO. In this process, the conventional CH 4 dry reforming reaction is decoupled into the separate CH 4 dissociation reaction to acquire H 2 and reverse Boudouard reaction to acquire CO, respectively. Results showed that the proposed process was able to produce a H 2 -rich stream at a yield of 175.4mmol H2 =g Ni with a H 2 /CO molar ratio of 5.6 at 700  C, followed by a high-purity CO stream at a yield of 155.4 mmol CO /g Ni at 800  C in one biogas reforming cycle. It is shown that the re- action between CO 2 and the carbon species with a graphitic ordering is the rate-determining step of the proposed biogas reforming process. Despite the scope for improvement of process stability, the proposed catalytic calcium-looping reforming of biogas has been experimentally demonstrated to be a promising strategy to inherently control the molar ratio of H 2 to CO during syngas production. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction Biogas, produced from anaerobic digestion of organic matter, is a potentially carbon-neutral source of renewable energy (Tabatabaei et al., 2020a, 2020b). Accordingly, developing biogas energy is a promising option to mitigate the increasing energy depletion due to global industrialization and rapid population growth, and to combat global climate change. Owing to the advantages of (I) a wide range of agroindustrial and municipal biowastes as available feedstocks (Zhang et al., 2016; Achinas et al., 2017; Aghbashlo et al., 2019), (II) being free from the impact of geographical and climatic conditions, and (III) reducing mankind’s dependence on conven- tional fossil fuels, the biogas industry has experienced rapid development during the past decades. In the European Union, biogas production has kept increasing to approach 20 billion m 3 of methane in 2015, leading to wide applications of the biogas product in transport as well as electricity and heat delivery (Scarlat et al., 2018). Major countries in other regions, including China, America, and India, have also shown significant potential in the production and utilization of biogas resources (Bao et al., 2019; Shen et al., 2015; Mittal et al., 2018, 2019). From a chemical point of view, biogas is mainly composed of CO 2 (30e50%), CH 4 (50e70%), H 2 O (5e10%), N 2 (0e3%), O 2 (0e1%), and a trace of impurities, and can thus be directly used on-site for heat recovery (Mu~ noz et al., 2015). However, on account of the limited calorific value of the raw biogas as compared to natural gas, direct heat recovery does not represent a highly efficient option to utilize the biogas resources. Therefore, upgrading of the raw biogas to increase the content of high calorific components is required for its versatile utilization in the energy sector. Most attention on biogas upgrading has been paid to the two major components (CO 2 and CH 4 ), which can be basically divided into physical/chemical upgrading to remove CO 2 from biogas (Khan et al., 2017), biological upgrading to transform CO 2 into CH 4 (Angelidaki et al., 2018), and * Corresponding author. E-mail address: sicong.tian@outlook.com (S. Tian). Contents lists available at ScienceDirect Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro https://doi.org/10.1016/j.jclepro.2020.122504 0959-6526/© 2020 Elsevier Ltd. All rights reserved. Journal of Cleaner Production 270 (2020) 122504