Biogas is a well-known energy source produced during anaerobic digestion with a variety of downstream uses. Amongst them, biogas introduction into the natural gas grid has raised interest in recent years both from industrials and from the administration, also promoted through EU and national policies towards the production and use of renewable, cleaner energies. However, raw biogas cannot be introduced into the gas grid, mainly because of its high carbon dioxide content and the presence of traces of other compounds in the raw biogas. Biogas upgrading through sustainable, cutting-edge clean technological innovations is currently being developed.
In this sense, NIMBUS targets the development of efficient, robust and environmentally friendly biotechnology based on the use of H2 as a powerful electron donor for the recovery of energy from biogas. NIMBUS will upgrade biogas using hydrogenotrophic methanogens as a key step towards a sustainable process to produce green energy. Hydrogenotrophic methanogenic archaea are microorganisms capable of converting CO2 into CH4 under anaerobic conditions using H2 through the following reaction:
Due to the low solubility of H2 in water, the main drawback identified to the successful development of the technology at the industrial scale is to dissolve H2 to a biofilm where microorganisms develop their activity (Eq. 1). NIMBUS is implementing the process in a bioreactor configuration able to achieve marketable CH4 production yields and rates under biomethane grid specifications (CH4 content above 97%). In this bioreactor configuration (Figure 1), the biogas and H2 mixture is fed into a pressurized tower where H2 and CO2 are transferred from the gas phase to the microbial culture that grows as a biofilm on the surface of a carrier material. In the biofilm, the biological reaction in Eq. 1 occurs, leading to a CH4 flow that is collected from the top of the tower. Mild pressures will be used in NIMBUS to not affect the microbial culture but to improve H2 transport.
Figure 1. Schematic of the essential parts and mechanisms of a hydrogenotrophic methanogenic bioreactor
The pressurized NIMBUS bioreactor (figure 2) is the first of its kind, being fed with process biogas and H2, the latter produced from an electrolyzer plus that from a bioelectrochemical pilot plant producing H2 from the primary effluent of the WWTP of El Prat de Llobregat. The unit will be started up along the first trimester of 2023.
Figure 2. H2 -trophic methanogenic bioreactor (column) and ancillary equipment