The use of hydrogen as an aviation fuel allows to eliminate the direct CO₂ emissions from aircraft engines completely and thus offers the potential to contribute substantially to the ambition of a net-zero carbon emission aviation as defined in Waypoint 2050^[1], and to the European goal to “go climate neutral” by 2050 as described in the European Green Deal. Of the available technologies and the targeted market ranges, a direct burn hydrogen combustion system for the short- and medium-range market will be the preferred configuration because currently 67 % of the global CO₂ emissions from aviation are being emitted by 70 % of the global fleet in this segment while only 30 % of the global CO₂ emissions are being emitted by long-range wide-body turbofan engines and only 3 % of the global CO₂ emissions are being emitted by regional jets (Hydrogen-powered aviation study, p. 16)^[2] to which hydrogen fuel cells are currently limited because of their relatively low energy density. Therefore, direct burn hydrogen combustion systems applied to the short- and medium-range market will play the dominant role in decarbonising aviation by using hydrogen as a fuel.

Moreover, in addition to the decarbonisation initiatives, increased focus has recently been put on non-CO₂ emissions, especially NO_x, in order to drive for climate neutrality. Therefore, without the development of specialised ultra-low NO_x combustion technologies, direct burn hydrogen combustion systems are prone to experience higher NO_x emissions than current, state-of-the-art combustion systems operated with Jet-A1 because of higher flame temperatures and the high reactivity of hydrogen when burnt in air. Increased NO_x emissions would have a global warming effect from non-CO₂ emissions and would impact the local air quality around airports which would endanger the acceptance of direct burn hydrogen combustion systems and thus would limit its ability to utilise the available decarbonisation potential. The development of ultra-low NO_x combustion technologies is then an essential requirement for direct burn hydrogen combustion systems.

Project results are expected to contribute to all the following outcomes:

• deliver technologies for Airbus’ ZEROe game-changing concepts for future commercial passenger aircraft using hydrogen as the primary energy supply (Airbus ZEROe, 2022^[3])

Project results are expected to contribute to the following objectives and KPIs of the Clean Hydrogen JU SRIA as well as to the following additional KPIs:

• Low NOx emitting hydrogen turbines
• Dry low NOx emissions across all engine operating conditions at least as low as current state-of-the-art which is around 50% below regulation in force (CAEP/8).
• Dry low NOx technology with potential to reduce NOx emissions further by no less than 30% compared to state-of-the-art.

The scope of the topic is to develop a direct burn hydrogen combustion system with low NO_x emissions compatible with aero engine specifications and progress it up to TRL 4. Because of the specific thermo-physical characteristics of hydrogen (very high flame speed, high diffusivity, high reactivity, high flame temperatures, etc.) there are many technological hurdles to overcome in order to realise a reliable and successful low NO_x combustion system. The most difficult and important ones are:

• Effective temperature control in the primary zone to avoid excessive thermal NO_x production
• Reliable and safe ignition and flame stabilisation without autoignition and flash-back
• Wide stability limits necessary to cover typical engine operating ranges
• No overheating of flame holding and combustor structures (dome & liners)
• Control of thermoacoustic instabilities

Therefore, the scope for the development of the low NO_x combustion technology should include the following steps:

• Development of a new innovative fuel injection system capable of creating a homogeneous fuel/air distribution, reliable and safe ignition, and flame stabilisation without autoignition and flash-back, no overheating of flame holding and combustor structures, and effective control of the NO_x production across typical aero engine operating ranges;
• Demonstration of the low NO_x technology in single cup tests with optical access and thermoacoustic measurement capabilities up to relevant operating pressures and temperatures (T₃ = 950 Kelvin, P₃ = 20 bar);
• Demonstration of the low NO_x technology in multi-cup sector combustor tests with different thermoacoustic boundary conditions (open exit and exit restriction), and thermoacoustic as well as emission measurement capabilities up to relevant operating pressures and temperatures (T₃ = 950 Kelvin, P₃ = 40 bar);
• Demonstration of reliable and safe operation across relevant operating range (T₃, P₃, FAR: 950 Kelvin, 40 bar, at least 0.004 - 0.04) without flash-back, auto-ignition, blow-out, and over-heating of combustor hardware;
• Proof of thermoacoustic stability without excessive pressure amplitudes across relevant operating range;
• Contribution to development of EU competitiveness for low NO_x hydrogen direct burn combustion technologies.

The project should address the following requirements and specifications:

• Reliable and safe combustor ignition at ground start conditions with 100% hydrogen;
• Lean blow-out within limits necessary for typical aero engine operation (FAR_LBO ≤ 0.004);
• Dry low NO_x emissions across all engine operating conditions at least as low as current state-of-the-art which is around 50% below regulation in force (CAEP/8).
• Dry low NOx technology with potential to reduce NO_x emissions further by no less than 30% compared to state-of-the-art.
• Operability and emission performance fulfilled for hydrogen temperatures in the range of 200 – 420 Kelvin;
• Efficient fuel/air mixing without flashback and autoignition across typical aero engine operating range;
• Temperatures of fuel injectors, dome, and combustor liners within limits for targeted engine life;
• Dynamic combustion pressures (P₄) within limits for targeted engine life;
• Combustor length no longer than current state-of-the-art aero engine combustors.

Proposals are expected to co-operate and seek synergies with the projects and activities of the Clean Aviation JU. In particular:

• In Clean Aviation JU Phase 1, an existing turbofan engine will be adapted for the operation with 100% hydrogen and will be demonstrated in a ground test demonstrator in order to prove full system feasibility, starting from the liquid hydrogen tank, through the hydrogen fuel and control system including vaporiser/conditioner, up to the adapted combustor. The successful 100% hydrogen turbofan engine ground test demonstrator is a prerequisite for the planned flight test demonstrator, including contrails measurements by a chasing airplane, at the beginning of Clean Aviation JU Phase 2. Because of the very tight schedule and the challenging tasks in Clean Aviation Phase 1, only limited effort can be put on the development of a low NO_x combustion technology (limited to residence time, mixing and dilution). On the other hand, the time in Clean Aviation JU Phase 2 will be too short to develop the low NO_x combustion technology on time up to the TRL level necessary for the launch of a product development after Clean Aviation JU if no pre-development work has been performed in parallel to Clean Aviation JU Phase 1;
• Therefore, the development of the low NO_x combustion technology is proposed to be performed in this topic up to TRL4 in order to be able to further mature the technology in Clean Aviation JU Phase 2 up to TRL 6. The low NO_x combustion technology developed in the current project will be complementing the “conventional” hydrogen combustion technology developed for the early ground test demonstrator in Phase 1 of Clean Aviation JU. In order to exploit synergies between the two Work Programs, applicants are expected to exchange information with projects selected from the Clean Aviation JU call topic HORIZON-JU-CLEAN-AVIATION-2022-01-HPA-01^[4]: ‘Direct Combustion of Hydrogen in Aero-engines’, including (but not limited to) engine and/or combustion chamber geometries and specifications, and forecast emissions profiles;
• The development activities in this topic should be closely aligned with the projection of a hydrogen direct burn innovation as defined in Clean Aviation JU in order to develop a targeted low NO_x combustion technology for this application.

Applicants are encouraged to address sustainability and circularity aspects in the activities proposed.

Activities are expected achieve TRL 4 by the end of the project - see General Annex B.

The JU estimates that an EU contribution of maximum EUR 8.00 million would allow these outcomes to be addressed appropriately.

The conditions related to this topic are provided in the chapter 2.2.3.2 of the Clean Hydrogen JU 2023 Annual Work Plan and in the General Annexes to the Horizon Europe Work Programme 2023–2024 which apply mutatis mutandis.

[1] https://aviationbenefits.org/media/167187/w2050_full.pdf

[2] https://www.clean-hydrogen.europa.eu/media/publications/hydrogen-powered-aviation_en

[3] https://www.airbus.com/en/innovation/zero-emission/hydrogen/zeroe

[4] https://ec.europa.eu/info/funding-tenders/opportunities/portal/screen/opportunities/topic-details/horizon-ju-clean-aviation-2022-01-hpa-01;