Renewable energies will become the most important energy source for the energy transition in Germany. Developing conversion and storage technologies is crucial in ensuring a sustainable energy supply. An approach for flexible and demand-oriented energy storage is the generation of methane (CH4) from hydrogen (H2) and carbon dioxide (CO2) (power-to-methane). In this process, H2 is produced from unused renewable electricity by means of electrolysis, and CO2 from wastewater treatment plants, biogas plants, or industry can be used directly at the point of generation.
The conversion of H2 and CO2 to CH4 under anaerobic conditions by methanogenic microorganisms is known as a sub-process from biogas plants or digesters at wastewater treatment plants. A particularly efficient reactor concept is the gas-filled trickle bed reactor, in which the microorganisms are immobilized on carrier materials. Carrier materials with a large surface area increase the contact area between the gas (H2 and CO2) and the microorganisms in the liquid phase.
Thermophilic anaerobic trickle bed reactors at lab-scale already demonstrated a high performance with a CH4 production of 15.4 m3CH4/(m3reaction volume·d) at CH4 concentrations in the product gas above 96%. This would allow direct injection of the biomethane into the natural gas network without the need for further gas purification.
With an active reaction volume of 0.8 m3, the applicability of the reactor concept was demonstrated on a semi-industrial scale. This makes the pilot reactor at the wastewater treatment plant Garching (Figure 16) one of the largest anaerobic trickle bed reactors in the world. Biogas upgrading at the point of origin has a holistic potential as all resources required for the operation of the reactor can be found locally.
After inoculating the reactor with digested sludge, the reactor was operated with raw biogas as CO2 source from the local digester for a total of 450 days. With a stable CH4 production of 6 m3CH4/(m3reaction volume·d) at gas grid injection quality, which corresponds to a product gas flow of 17 m3CH4/(m3reaction volume·d), taking the inert CH4 content in the biogas into account, the potential of the energy conversion technology was demonstrated.