Boosting the efficiency of sustainable aviation fuel production

Fuels like kerosene can be produced in a climate-friendly way from CO₂, water and green electricity using power-to-liquid processes. Researchers from Karlsruhe Institute of Technology (KIT) have already demonstrated this with systems in actual operation.

Now, researchers working on the Kopernikus P2X project have succeeded for the first time in coupling—at an industrial 220-kilowatt scale—the highly efficient co-electrolysis process with fuel synthesis.

In order to achieve its climate targets, Europe needs green alternatives for applications that do not easily lend themselves to electrification. “The aviation sector in particular will rely on sustainably produced kerosene for the time being,” says Professor Roland Dittmeyer from KIT’s Institute for Micro Process Engineering (IMVT).

“Synthetic fuels that are produced by means of power-to-liquid processes with CO₂ from the atmosphere or biogenic sources, water, and green electricity are particularly suitable.”

Dittmeyer is the spokesperson for the Kopernikus P2X project and heads the corresponding research activities at KIT. The project has now reached an important technological milestone on the way to sustainable aviation fuel: For the first time in the world, the innovative, highly efficient water vapor/CO₂ co-electrolysis technology from industrial partner Sunfire was coupled directly with a synthesis process at an industry-relevant scale (220 kilowatts of electrolysis output).

Co-electrolysis makes power-to-liquid more efficient

For the production of synthetic kerosene at KIT’s Energy Lab, a multi-stage process distributed to modular facilities is used. First, syngas—a mixture of hydrogen and carbon monoxide—is produced from CO₂ and water.

In principle, there are several ways to generate syngas. The new configuration uses a co-electrolysis module with an output of 220 kilowatts from industry partner Sunfire, which simplifies this process step and, above all, boosts its efficiency.

“Co-electrolysis stands out in that it is a highly efficient process that electrochemically converts water vapor and CO₂ directly into syngas in a single step. Up to 85% of the electrical energy used for this process can be recovered as chemical energy in the syngas.

“In addition, we could demonstrate with this coupling that our co-electrolysis method features a very high plant availability and reliability and has the potential to produce syngas with the desired quality at any time,” says Hubertus Richter, Senior Engineer R&D Project Management & Process Engineering at Sunfire.

“This eliminates the traditionally separate hydrogen production process with downstream syngas production, significantly increasing the efficiency of the overall process for the production of synthetic fuels.”

For the coupled operation of co-electrolysis and fuel synthesis, the syngas needs to be brought to reaction pressure. This job is done by a compressor with safety devices the researchers added to the process chain. In a microstructured reactor, the syngas is then converted to long-chain hydrocarbons—known as syncrude—using Fischer-Tropsch synthesis. These hydrocarbons can be used directly to produce fuels such as kerosene or other chemical products.

Scientists at KIT developed this reactor technology, which is already being commercialized by INERATEC, a KIT spin-off. In the future, it is planned to use the heat released as vapor during synthesis for the co-electrolysis. This would further reduce the energy demand of the entire process and demonstrate that the product preparation to finally obtain kerosene is feasible at this scale.

By combining these process steps, it is possible to fully utilize the carbon dioxide provided and achieve the highest possible energy conversion efficiency, as this process chain allows efficient recycling of material flows in addition to the energy flows.

The next step: A ton of kerosene per day

Researchers at KIT successfully tested the integration of co-electrolysis in campaign operation under real conditions, producing up to 100 liters of syncrude per day. Coupled operation marks an important milestone in the second funding phase of the Kopernikus P2X project. The facility is now being expanded for a capacity of up to 300 liters syncrude per day.

In the third and final funding phase, the research team has INERATEC additionally build a larger Fischer-Tropsch production facility in the Höchst Industrial Park near Frankfurt. “For the first time, ton-scale production will be realized there,” says Dittmeyer.

The product, which will eventually be processed into kerosene, will be used by aircraft engine manufacturers and research partners for testing. Accompanying analyses will ensure that the fuel meets the strict aviation standards.