Biological Conversion of Syngas into Methane

Abstract

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 (f.i. 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 which 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).