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.