Biomethanation of syngas

Anaerobic digestion is a well-established technology that generates renewable energy in the form of biogas, as well as a nutrient-rich digestate. Lignocellulosic biomass is one of the huge global renewable resources with great biogas potential. However, due to their limited applicability during the conventional digestion process, such materials are currently underutilized as a substrate in biogas processes(Hendriks and Zeeman 2009). One way to tap the potential of such type of biomass is the combined process of thermal gasification to syngas (mainly H2, CO and CO2), followed by conversion to methane. This conversion can be mediated by microbes (biomethanation) and can be operated under ambient conditions (low pressure, low temperature).

To establish efficient microbiological activity and growth while converting syngas, it is crucial to provide sufficient nutrients with the liquid medium. Biomethanation of syngas has in several previous studies been investigated with various defined nutrient media. However, for process upscaling, more accessible and economically viable nutrient sources are required, e.g. digestate from anaerobic digestion or reject water from dewatered digested sewage sludge. 

To circumvent gas-liquid mass transfer limitations, the trickle-bed reactor (TBR) has been proven to be a viable reactor design for biomethanation. TBRs are gas-tight columns filled with carrier material covered by microbes, over which a nutrient liquid is trickled at different frequencies. The carriers provide a high specific surface (in relation to the reactor volume) for the biofilm to grow on, strengthening the gas-liquid phase boundary interaction.

Regarding future implementation in production scale, there is a synergy when establishing syngas biomethanation from e.g. forest residues integrated with the utilization of digestate from existing co-digestion biogas plants and reject water WWTP with biogas production from sewage sludge (reject water). Furthermore, the combination of conventional biogas and biomethanised syngas will increase the total gas flow, which can lead to a more cost-effective upgrading process to biomethane.

The overall aim is to gain new knowledge regarding biological methanation of syngas with trickle bed reactors (TBRs). The focus is on the utilization of digestate and reject water as non-defined nutrient sources to determine limitations in composition and supply for the microbial community.  Different process parameters such as liquid hydraulic retention time, trickling regimes and syngas composition, and their combinations, are studied to maximize the methane production under stable process conditions and high conversion of hydrogen and carbon monoxide.