LEAF technology: aircraft powered by 100% hydrogen with no NOx emissions

Hydrogen represents a unique opportunity to achieve clean energy transition for a secure energy future. This fuel, which can be produced emission-free using renewable energy (green hydrogen), will be a vital part of the European Union’s path towards climate neutrality. But the transition to flights powered by this decarbonized hydrogen is based on the use of gas turbines modified with “flame technologies” new and innovative for propulsion. The project Clean Sky 2 LEAFINNOX has the answer: a technology based on flameless oxidation called Lean Azimuthal Flame, or LEAF.

The potentially high temperatures of hydrogen combustion cause emissions of NOx. This product is a pollutant that can increase global warming in addition to causing negative effects on air quality. The physicochemical characteristics of hydrogen also result in fundamentally different flame behavior than kerosene, the fuel on which modern air travel depends.

All this means that the combustion chamber of the gas turbine must be redesigned to operate with hydrogen. The project based on flameless oxidation called LEAF, can be implemented in aircraft fueled 100% with hydrogen. “Our LEAF concept is based on putting air and fuel in very intimate contact with hot products, and we achieve this through a specially designed sequential combustion idea,” explains Cambridge University professor Epaminondas Mastorakos.

LEAF technology is flexible in fuels and can also be applied to work with sustainable aviation fuels (SAF), such as the LEAFINNOX, successfully demonstrating NOx and soot reduction when operated with them. The results obtained both in Cambridge and in the Federal Polytechnic School of Zurich (ETH) with 100% hydrogen, have shown the high degree of versatility of this technologywhich can be optimized to operate with sustainable aviation fuels in the liquid phase or with highly reactive gaseous fuels such as hydrogen, explains Dr. Pedro de Oliveira, one of the project’s researchers.

combustion chamber leaf hydrogen-propelled aircraft interior1
Lean Azimuthal Flame (LEAF) combustion chamber operated with kerosene and hydrogen. Image L. Miniero – CAPS Laboratory.

The LEAF concept has also been extensively investigated by the ETH group (Luigi Miniero, Dr. Khushboo Pandey and Professor Nicolas Noiray): “We have shown that the LEAF combustion chamber is capable of change from kerosene to pure hydrogen during operation. Such a capability will likely play a key role in future clean aviation technologies,” says Professor Noiray.

Preliminary tests carried out in parallel at the University of Cambridge and at the ETH Zurich show very low NOx emissions in the LEAF using 100% hydrogen, with values ​​that are in line with CAEP regulation trends and stricter NOx emission limits for future aeronautical engines. Upcoming experiments will focus on validating the excellent NOx performance measured under laboratory atmospheric conditions, to one higher pressure, closer to the actual operating conditions of aero engines.

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