Recent progress towards in-situ biogas upgrading technologies

Biogas is mainly produced from the anaerobic fermentation of biomass, containing methane with an extensive range between about 50% and 70%. Higher methane content biogas has higher energy and heat value, which needs biogas upgrading. There are mainly two types of biogas upgrading technologies (ex-situ and in-situ). This manuscript presents a review of technologies on in-situ biogas upgrading. These technologies comprise H-2 addition technology (e.g., continuous stirring tank reactor (CSTR), hollow fiber membrane (HFM), nano bubble (NB) technology, upflow anaerobic sludge blanket (UASB)), high-pressure anaerobic digestion (HPAD), bioelectrochemical system (BES), and additives (e.g., ash, biochar, and iron powder). The results confirm the excellence of H-2-addition technology, with the highest average CH4 content obtained (HFM: 92.5%) and one of the few full-scale cases reported (Danish GasMix ejector system: 1110 m(3)). Meanwhile, newly pop-up technology such as HPAD delivers appropriate CH4 content (an average of 87%) and is close to the full-scale application (https://bareau.nl/en/for-professionals/). More importantly, the combo between HPAD and H-2-addition technology is prominent as the former improves the low gas-to-liquid obstacle confronted by the latter. Additionally, recently emerging BES can't stand out yet because of limited efficiency on CH4 content or constraint full-scale application behaviors (disability to operate at high current density). However, its combination with H-2 addition technology to form the Power to Gas (PtG) concept is promising, and its commercial application is available (http://www.electrochaea.com/). Hydrogenotrophic methanogens are imperative players in all reviewed technologies for the generation of upgraded CH4. (C) 2021 Elsevier B.V. All rights reserved.