Overview of recent progress towards in-situ biogas upgradation techniques

Biogas, as derived from the anaerobic digestion process, offers a versatile possibility of renewable and sustainable energy usage. When enriched, upgraded biogas can yield high levels of biomethane, allowing its use as an alternative to natural gas via existing natural gas grids or being directly consumed by transport vehicles as fuel. Currently, biogas upgrading is experiencing a golden period of rapid development where many enrichment techniques are being revisited, modified or strengthened, and contemporary novel technologies are being proposed. Mainly, two broad categories of upgrading techniques are present in which conventional method primarily focuses on ex-situ approaches, treating produced biogas to methane by employing catalytic conversion ( biological and chemical), membrane gas-permeation, desulphurization, physical and chemical scrubbing, absorption and adsorption. Over the years, a considerable effort has been made to improve efficiency and to enhance the economic viability of the above techniques and many commercial plants worldwide use ex-situ approaches as options to enrich biogas as biofuel for direct utilization to vehicles. Coupled with the ex-situ method, in-situ techniques, such as CO2 desorption, pressurized reactor, H-2 addition (deployed to anaerobic digesters directly) and electromethanogenesis has also been gained significant attention recently. Comparative studies between in-situ and ex-situ method suggest that the former provides an increased economic performance for small to medium and small-scale facilities, allowing the upgrading of biogas above 85% v/v of methane. Additionally, innovations in bacterial species that are capable of direct exchange of electrons, escalating the biological conversion of CO2 to CH4 has also been demonstrated. This paper enlightens some of these aspects and reviews the state-of-the-art of biogas enriching techniques emphasizing in-situ approaches.