Green hydrogen is considered to be the hope for the energy and mobility transition, but it is still in short supply. But that will soon be over - at least according to researchers at Graz University of Technology. Together with the Graz-based start-up Rouge H2 Engineering, they have produced high-purity hydrogen from real biogas - including all impurities present in the gas - in an industry-oriented demonstration plant as part of the Biogas2H2 project.
Project based on "chemical-looping hydrogen method”
The project is based on a process developed by the 2020 team, the so-called "chemical-looping hydrogen method". Now, for the first time, it has been integrated into an existing biogas plant, according to a TU Graz press release. "High-purity hydrogen for fuel cells is produced from real biogas, not only in the laboratory but actually on an industrial scale," explains Viktor Hacker from the university's Institute of Chemical Process and Environmental Engineering. The real biogas - methane gas from pig slurry, glycerine phase, silo maize and grain residues - comes from the South Styrian company Ökostrom Mureck GmbH.
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At the company site in Mureck, Rouge H2 Engineering and Graz University of Technology built the demonstration plant in summer 2021 and operated it for testing purposes until the end of October. The 10-kilowatt plant separates about one per cent of the biogas flow (about 30 litres per minute) and mixes the gas with water vapour, the researchers describe the process. The mixture flows into the plant's reactor. There, the biogas is reformed, and synthesis gas is produced. This gas subsequently reduces iron oxide to iron. Then steam enters the reactor, which re-oxidises the iron back to iron oxide. This releases hydrogen with a purity level of 99.998 per cent.
Technology is ready for industrial application
With this iron-water vapour process, an efficiency of 75 per cent is achieved, he says: "If instead of the one per cent, we were to pass the entire biogas flow of the Mureck biogas plant, about 480 cubic metres per hour, through an appropriately scaled-up chemical looping plant, we would actually come up with a 3-megawatt hydrogen production plant," Rouge H2 project manager Gernot Voitic emphasises. "This means the technology is now ready for commercial use." It would also be possible to produce decentralised hydrogen from real biogas on a large scale. All it needs is a little space for the plant.
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This kind of decentralised production also has a positive effect on the production price and thus on the purchase price of hydrogen. Hacker says: "At the moment, hydrogen is offered at 10 euros/kg at the filling station. The techno-economic analyses that are part of our research project forecast a competitive hydrogen price of 5 euros/kg for decentrally produced hydrogen for our process." This makes the process competitive with other technologies such as electrolysis (5-12 euros/kg hydrogen).
Pressure in the plant too low for refuelling
But questions still remain. For example, hydrogen-powered vehicles currently have to be refuelled at 700 bar pressure, "in order to get as much hydrogen as possible into as small a tank as possible and thus achieve an attractive range", explains Viktor Hacker. The chemical looping system produces hydrogen at a pressure of up to 100 bar, which is not enough for refuelling. Compressing the hydrogen to 700 bar is both tricky and expensive, he says. "Somewhere this compression has to take place, either directly at the production site or at the latest at the filling station, which could of course also be supplied with bottled hydrogen. The costs will be incurred, and that brings us back to the price of the tank."
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This compression is not technically necessary: in principle, fuel-cell vehicles can also run with only 2 bar pressure - but not very far, according to Hacker. Therefore, decentralised hydrogen production directly at biogas plants would make sense for shorter distances, for example for hydrogen tractors - which are currently not even available on the market - or for hydrogen-powered storage vehicles such as forklifts.
Other possibilities for using hydrogen "from the biogas plant" would be to fill it into gas cylinders for further transport, to lay hydrogen pipes directly to homes equipped with fuel cells, or to use it in industrial processes. (mfo)
