Chapter 3: Biological Conversion of Syngas into Methane

In this chapter, two biological pathways from syngas towards methane are proposed. With the indirect acetate pathway, the methane producing bacteria do not have contact with the syngas and the methane producing bacteria (for instance M. barkeri and Methanothrix soehngenii) use acetate as a substrate only. Several pathways are described with micro-organisms, followed by a process design and economical analysis. The calculated conversion efficiency from wood towards SNG for the two proposed biological pathways may reach around 66 energy-%. It is also noted that from an energy efficiency perspective, it is important to select a gasifier type that exhibits both a high cold gas efficiency and a favourable syngas composition, i.e. a high theoretical maximum syngas conversion efficiency towards methane. For both the syngas fermentation pathway towards methane and the previously described pathway towards ethanol, a lot of attention is given towards technological and engineering aspects like gasifier selection, syngas cleaning, process design, reactor configuration and product upgrading (be it either ethanol distilling and dehydration or biogas purification). Within Europe, there are currently six biological methanation demonstration projects. The technologies are located at existing anaerobic digestion plants and have been developed by German companies like Microbenergy, Electroarchaea, Micropyros and Gicon. These projects are sometimes also referred to as “Power to Gas projects” as the hydrogen is intended to be produced from low-cost electricity. The methane produced is often injected into the national natural gas grid and as such could represent a cost-effective way for long term storage of electricity surpluses.