On the way to CO₂-neutral flying

Kerosene from air and water

Traveling by air - and still not emitting any additional CO2. This would be possible with synthetic fuels obtained from water and ambient air using renewable energies. However, enormous quantities would have to be produced. A new production process from the KEROGREEN research project uses an innovative plasma technology that could make this possible. The research partners have built an initial plant at the Karlsruhe Institute of Technology (KIT) Germany.

Organizing air traffic in a CO2-neutral way is a major challenge: "Batteries, hydrogen, and hybrid solutions are unsuitable due to their low energy density," says Professor Peter Pfeifer from the KIT Institute of Microprocess Engineering and one of the spokespersons of the KEROGREEN research project. "Biofuels, in turn, compete with food production and the natural environment due to the required cultivation areas." To make CO2-neutral flying possible nonetheless, Pfeifer and the partners involved in KEROGREEN have been researching another way: kerosene from air and water. "Using renewable energy and CO2 directly from the atmosphere, this creates a closed carbon cycle. We can even continue to use existing infrastructure for storage, transportation, aircraft refueling and, most importantly, engine technology." In addition, synthetic green kerosene would emit no sulfur and less soot and nitrogen oxides (NOx).

To be able to produce the fuel in sufficient quantities, the partners in the EU KEROGREEN project have spent four and a half years developing a scalable process based on a new plasma technology that fits into a container module. The work was coordinated by the Dutch Institute for Fundamental Energy Research (DIFFER) in Eindhoven, and a research facility was set up at KIT. The technology is thus in the final phase of system integration, in which the individual elements are already connected to form a closed unit, but are still at different stages of development. "The new manufacturing process is particularly resource-saving because no rare raw materials are used," says Pfeifer.

Innovative plasma technology for CO2 splitting

The process is essentially based on three steps: The CO2 from the ambient air is first fed into a reactor, where it is broken down into carbon monoxide (CO) and oxygen by a plasma generated with microwave radiation. The oxygen is then removed, while the CO is partially converted to hydrogen in a second reactor by means of a water-gas shift reaction. This hydrogen and the remaining CO (referred to as syngas in the combination) is converted to hydrocarbons in a third reactor using Fischer-Tropsch synthesis. High molecular weight hydrocarbons that cannot be used for kerosene production are cracked in-process at the plant. The final product is the basic component of fuels commonly used in aviation. This raw material can then be refined into the desired kerosene or stored directly as energy storage.

Ideal for decentralized use with renewable energies

According to the researchers, plasma technology could be used for plants in the megawatt range. But it would also be suitable for use in small, decentralized production plants in container format: "Future plants will be modular and scalable and could therefore be easily integrated into an offshore wind farm or a solar farm in the desert," says Pfeifer. "Then, if wind or sun are ever unavailable, the plasma reactor would temporarily shut down and simply start up again with available energy." The results from the project are now being carefully analyzed and, in particular, are already being used by the industrial partners to implement individual process steps.