Increasing methane yield from biogas plants with a new process

Germany is on the path to climate neutrality and aims to reduce carbon dioxide emissions by 65% ​​by 2030 compared to 1990 levels. Biogas plants play an important role in defossilization: bacteria from these plants break down biomass in the absence of oxygen to form biogas which, on average, includes up to 60% methane and more than 40% CO2. While biogas is used to generate electricity and heat in combined heat and power units or can be upgraded to natural gas quality and injected into the natural gas grid, CO2 has not been used to date, writes the Fraunhofer Institute in a press release.

Ensuring the full use of biogas

Fraunhofer IMM researchers are now working on the technology to use CO2. “We convert CO2 into methane using green hydrogen”, explains Dr. Christian Bidart, one of the scientists at Fraunhofer IMM, explaining the principle of the new process. This means that the biogas produced can now be used in full and not just around 60%, as in the past. The underlying chemical reaction was discovered over a hundred years ago, but to date has not been used for direct biogas upgrading. In the context of the energy transition, however, the chains of use of CO2 come into focus.

In the ICOCAD I project, the research team developed a demonstration plant that converts one cubic meter of biogas per hour into one cubic meter of methane with a thermal output equivalent to ten kilowatts of the electrolyser needed to produce the hydrogen of the process. In the follow-up ICOCAD II project, researchers are now expanding this demonstrator by a factor of five – to a thermal output of 50 kilowatts. One of the challenges of this project is the highly dynamic nature of the process. The amount of electricity generated by wind and photovoltaic systems fluctuates considerably – which means that the amount of green hydrogen obtained from water using electricity in electrolyzers is also subject to considerable fluctuations. The demonstration plant must therefore be able to react quickly to varying amounts of hydrogen. Hydrogen storage would be technically possible but would be complicated and expensive. “We are therefore working to make the entire system flexible in order to avoid the storage of hydrogen as much as possible”, specifies Bidart. CO2 storage tanks are part of this plan, because the amount of CO2 produced in biogas plants remains constant.

Develop effective catalysts

Developing efficient catalysts for the reaction was another challenge. The solution proposed by the Fraunhofer IMM researchers was to use a microcoating based on precious metals. The underlying principle is that hydrogen and carbon dioxide pass through a large number of microchannels – the walls of which are coated with the catalyst – where they react with each other. “This way, we can increase the contact surface between the gases and the catalyst material and reduce the amount of catalyst needed,” explains Bidart. Many such microstructures are stacked on top of each other in the reactor.

Additional scaling plans

Researchers are currently working on implementing the largest demonstrator and achieving dynamic operation. They hope to be able to commission this plant in 2023 in order to test it in real conditions on a biogas plant. However, this is by no means the limit of their scaling plans – given the large volumes of CO2 produced by biogas plants. The researchers therefore have new plans to increase to 500 kilowatts by 2025 and again to one to two megawatts by 2026.

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