A comprehensive review on biological methanation processes: from gaseous feedstocks to biomethane

Abstract

Biological methanation is a process that utilizes methanogenic archaea as catalysts to convert carbon dioxide (CO2) and hydrogen (H2) to methane (CH4). The process can be carried out in various ways: in-situ, i.e. within a biogas digestor fed with organic feedstocks and H2, ex-situ, i.e. in a reactor fed with CO2-rich gas and H2, or in a microbial electrosynthesis reactor, where the reducing equivalents for CO2 reduction are provided with cathode electrode. This review shortly presents the key metabolic pathways involved in biological methanation and the different process options, including microbial electrosynthesis, and examines in detail the significance of the CO2 and H2 sources and the availability and composition of these gas streams, and presents results from life cycle assessment (LCA) of biological methanation processes. The focus of the review is on biogenic CO2. When planning biological methanation, it is crucial to carefully evaluate the options of transferring CO2 and/or H2 or producing H2 where CO2 is produced in terms of cost and feasibility. The continuous versus intermittent availability of CO2 and H2 as well as the potential presence of impurities in the CO2-rich gases can impact the efficiency of the biological methanation process. Specifically, impurities such as nitrogen and sulfur oxides, hydrogen sulfide and heavy metals can negatively impact the biological methanation process. Several LCA studies have demonstrated that biological methanation significantly reduces the greenhouse gas emissions and improves climate impacts, when renewable energy is used for H2 generation and natural gas replacement is considered.