Underground bio-methanation: Concept and potential

As a major part of the energy turn around, the European Union and other countries are supporting the development of renewable energy technologies to decrease nuclear and fossil energy production. Therefore, efficient use of renewable energy resources is one challenge, as they are influenced by environmental conditions and hence, the intensity of resources such as wind or solar power fluctuates. To secure constant energy supply, suitable energy storage and conversion techniques are required. An upcoming solution is the utilization and storage of hydrogen or hydrogen-rich natural gas in porous formations in the underground. In the past, microbial methanation was observed as a side effect during these gas storage operations. The concept of underground bio-methanation arised, which uses the microbial metabolism to convert hydrogen and carbon dioxide into methane. The concept consists of injecting gaseous hydrogen and carbon dioxide into an underground structure during energy production peaks which are subsequently partly converted into methane. The resulting methane-rich gas mixture is withdrawn during high energy demand. The concept is comparable to engineered bio-reactors which are already locally integrated into the gas infrastructure. In both technologies, the conversion process of hydrogen into methane is driven by hydrogenotrophic methanogenic archaea present in the aqueous phase of the natural underground or above-ground engineered reactor. Nevertheless, the porous medium in the underground provides, compared to the engineered bio-reactors, a larger interface between the gas and aqueous phase caused by the enormous volume in the underground porous media. The following article summarizes the potential and concept of underground methanation and the current state of the art in terms of laboratory investigations and pilot tests. A short system potential analysis shows that an underground bio-reactor with a storage capacity of 850 Mio. Sm3 could deliver methane to more than 600,000 households, based on a hydrogen production from renewable energies.