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The stainless steel tube reactor forms the central component of the test rig for biological methanation.
© DBI Freiberg

The image shows the plant control system for the biological methanation testing facility.
© DBI Freiberg

Storing surplus electricity in the natural gas network

Surplus electricity can be used in Power-to-Gas systems to generate hydrogen by means of electrolysis. Downstream methanation provides a possibility for storing the remaining gas that cannot be directly used or fed into the natural gas network. The initial, highly promising results for a newly developed biological process have now been presented. The test facility has a low energy requirement and the process is continuous. The system is suitable for decentralised and small systems.

In the future, more storage potential will be needed in the electricity grid, particularly during times when wind and solar power systems are feeding maximum amounts of electricity into the grid. A technical possibility is provided by Power-to-Gas (PtG) systems. These generate hydrogen (H2) with the aid of the surplus electricity. This can be directly added to the natural gas up to a maximum concentration. Subsequent methanation is possible for any additionally produced hydrogen. To achieve this, carbon dioxide (CO2) is added to the hydrogen: methane is then formed in a chemical or biological reaction. This synthetic natural gas can be fed into the natural gas network in any quantity and replaces natural gas there. The CO2 originates, for example, from the exhaust gas from power stations or from biogas plants.

In the BioRePow research project, scientists from DBI - Gastechnologisches Institut gGmbH in Freiberg are developing a biochemical methanation process together with collaborative partners. This enables biomethane to be produced based on a fermentative approach. The Freiberg-based researchers have developed a special tube reactor for this purpose that dispenses with energy-intensive components such as agitators, circulation or gas separation. This improves the energy efficiency of the process. Methanation occurs in the tube reactor by feeding the CO2 and H2 gases through it once. The project manager in Freiberg, Ronny Erler, explains: "Our process is suitable for small and decentralised systems, but can also be scaled up as desired. The results from the test rig produced during the now completed initial phase are highly promising."

Continuously operating the bioreactor

In the case of biological methanation, hydrogen can be added in an existing biogas reactor – known as an in-situ concept – or can be converted fermentatively together with CO2 in a separate reactor (ex-situ concept). The latter concept corresponds to the facility in Freiberg.

Two factors are particularly important for the quality of the methane formation: there must be optimal conditions for the micro-organisms in regards to, for example, the temperature and nutrient supply, and the gas solubility is also significant. The type of gas supply and the associated increase in biological availability are therefore key issues in the project.

On the test rig, the scientists in Freiberg investigated the influence of basic parameters such as the temperature, pressure, composition and volumetric flow of the feed gas. The results were incorporated into a mathematical model. With the optimised parameters determined there, the researchers have adapted the gas supply system with a view to achieving high conversion rates and methane levels. The H2 and CO2 only spend about three seconds in the tube reactor, and a conversion rate of more than 80 per cent has already been achieved in the first project phase. The conversion rate is based on the proportion of the source gases (H2 and CO2) actually converted into methane, whereby the tube reactor is operated continuously. Recirculation of the starting gases through the rector is not necessary. The reaction proceeds exothermically and thermophilic mixed cultures are used.

Project manager Ronny Erler explains: "In the second phase of the project we are planning to further increase the conversion rate to increase the methane content to 95 per cent and more. The work will focus on constructing a demonstrator." 

Status of the methanation and project participants

The large-scale chemical methanation of hydrogen, known as catalytic methanation, is expensive and energy-intensive. High pressures and temperatures are required. Such systems therefore have to convert large quantities of hydrogen in order to be able to operate economically. There are already batch processes on the market for biological methanation. These processes handle the gas discontinuously. Here a vessel is filled, the process takes place in it, and the vessel is completely emptied. The next cycle then follows. In such batch processes, the reaction gas or recirculated material usually has to flow through the vessel several times or is kept in motion by agitators until a corresponding methane content in the product gas is reached.

In addition to the DBI, the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT and the Engler Bunte Institute (EBI) from Karlsruhe are also working on the new tube reactor for biological methanation. The German Federal Ministry for Economic Affairs and Energy (BMWi) has funded the project as part of the Cooperative Industrial Research (AiF) programme. Together with four others, the project was selected from 50 proposals two years ago as an AiF flagship project. It forms part of the programme entitled "Development of innovative, highly efficient technologies for the processing of biogas / biomethane across the entire value-added and exploitation chain (inTeBi)".



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WOMBAT project
In the WOMBAT project, researchers are pursuing the idea of combining the electricity and gas networks. The intention is to utilise surplus electricity from renewable sources for generating hydrogen. This is synthesised with carbon dioxide in biogas plants to form methane, which is the main component of natural gas.

Energy storage systems
Recent reports on research, development and demonstration of energy storage systems

Research funding

The project is registered with the funding code Nr 22 LBG for the German Gas and Water Industry Association (DVGW) research association and was funded by the AiF as part of its programme for promoting cooperative industrial research.

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