Periodic Reporting for period 2 - FLEX4H2 (Flexibility for Hydrogen)
Reporting period: 2024-07-01 to 2025-12-31
The EU has committed to achieving climate neutrality by 2050 and reducing greenhouse gas emissions by at least 55% by 2030. To reach these ambitious targets, there is a need for deploying renewable energy sources and improving energy efficiencies. Hydrogen (H2) offers reliability, independence from weather conditions, and potential for decarbonization, but challenges arise with fuels with high hydrogen content , such as flashback risk, complying with emission limits, and material challenges. To tackle these challenges, FLEX4H2 aims at moving technological frontiers for low-emission hydrogen combustion in modern gas turbines at high firing temperatures and pressures. FLEX4H2 will design, develop, and validate a highly fuel-flexible sequential combustion system operating with any concentration of hydrogen admixed with natural gas (NG) up to 100% at H-Class operating temperatures, maximizing power and efficiency, while meeting emission targets. FLEX4H2 will tackle the challenges related to H2 combustion by maturing the sequential combustion technology through a combination of design optimisation, analytical research, and validation in a relevant environment. FLEX4H2 will validate scaled and full-size prototypes of the combustor through dedicated atmospheric and high-pressure test campaigns up to TRL6. During the tests, the combustor’s capability of operating at any mixture of hydrogen and natural gas will be demonstrated, without diluents while complying with emission limits. In addition, the possibility to start-up the engine on any amount of H2 in natural gas will be demonstrated.
During RP2, FLEX4H2 achieved a major step forward in the development and validation of a hydrogen-ready sequential combustion system capable of operating with natural gas/hydrogen blends up to 100% H2 under relevant gas turbine conditions
The reporting period focused on the design, optimisation, and validation of the Gen2 combustor prototypes, addressing the main Gen1 limitations—namely flashback margin and restricted firing temperature.
In WP1, Gen2 first- and second-stage burners were redesigned to enhance hydrogen flexibility. Key improvements included optimised fuel injectors for faster mixing at reduced pressure drop, high-speed low-swirl aerodynamics for improved flashback resistance, enhanced damping solutions for thermoacoustic robustness, and improved temperature profile homogeneity at the first stage outlet.
Additive manufacturing (SLM) enabled rapid prototyping of complex injection geometries, integrating instrumentation for detailed thermo-mechanical monitoring.
In WP2, high-fidelity LES simulations coupled with advanced turbulence-chemistry interaction models were further validated against new high-pressure experimental datasets using pure hydrogen at reheat conditions.
Numerical investigations improved understanding of autoignition-driven flame stabilisation, fuel–air mixing behaviour, and hydrogen-induced flashback mechanisms. These results directly informed Gen2 hardware optimisation and strengthened confidence in predictive modelling for hydrogen combustion.
In WP3, thermoacoustic characterisation advanced significantly. Mode shape-dependent flame transfer functions were developed and incorporated into interconnected system models, enabling prediction of transverse instability growth rates with strong agreement to experimental data.
New broadband acoustic dampers were experimentally validated, demonstrating improved absorption over relevant frequency ranges and enhanced system stability margins.
In WP4, two full-scale test campaigns (atmospheric and high-pressure) were successfully executed, leveraging the experience and the flame insights gained from the small-scale high-pressure tests. Gen2 prototypes demonstrated stable operation across the full natural gas–hydrogen blending range, including successful operation at 100% hydrogen under both half- and full-pressure conditions.
Flashback limits were systematically mapped, ignition procedures for 100% H2 were validated, and stable fuel switching was demonstrated. NOx emissions remained within project targets at adjusted firing temperatures.
While maximum firing temperature remains constrained at high hydrogen levels, broader flashback margins and extended operating windows were achieved compared to Gen1.
Overall, RP2 results confirm the technical feasibility of operating a sequential heavy-duty gas turbine combustor with up to 100% hydrogen at full engine pressure, representing a decisive step toward TRL6 demonstration.
The reporting period focused on the design, optimisation, and validation of the Gen2 combustor prototypes, addressing the main Gen1 limitations—namely flashback margin and restricted firing temperature.
In WP1, Gen2 first- and second-stage burners were redesigned to enhance hydrogen flexibility. Key improvements included optimised fuel injectors for faster mixing at reduced pressure drop, high-speed low-swirl aerodynamics for improved flashback resistance, enhanced damping solutions for thermoacoustic robustness, and improved temperature profile homogeneity at the first stage outlet.
Additive manufacturing (SLM) enabled rapid prototyping of complex injection geometries, integrating instrumentation for detailed thermo-mechanical monitoring.
In WP2, high-fidelity LES simulations coupled with advanced turbulence-chemistry interaction models were further validated against new high-pressure experimental datasets using pure hydrogen at reheat conditions.
Numerical investigations improved understanding of autoignition-driven flame stabilisation, fuel–air mixing behaviour, and hydrogen-induced flashback mechanisms. These results directly informed Gen2 hardware optimisation and strengthened confidence in predictive modelling for hydrogen combustion.
In WP3, thermoacoustic characterisation advanced significantly. Mode shape-dependent flame transfer functions were developed and incorporated into interconnected system models, enabling prediction of transverse instability growth rates with strong agreement to experimental data.
New broadband acoustic dampers were experimentally validated, demonstrating improved absorption over relevant frequency ranges and enhanced system stability margins.
In WP4, two full-scale test campaigns (atmospheric and high-pressure) were successfully executed, leveraging the experience and the flame insights gained from the small-scale high-pressure tests. Gen2 prototypes demonstrated stable operation across the full natural gas–hydrogen blending range, including successful operation at 100% hydrogen under both half- and full-pressure conditions.
Flashback limits were systematically mapped, ignition procedures for 100% H2 were validated, and stable fuel switching was demonstrated. NOx emissions remained within project targets at adjusted firing temperatures.
While maximum firing temperature remains constrained at high hydrogen levels, broader flashback margins and extended operating windows were achieved compared to Gen1.
Overall, RP2 results confirm the technical feasibility of operating a sequential heavy-duty gas turbine combustor with up to 100% hydrogen at full engine pressure, representing a decisive step toward TRL6 demonstration.
FLEX4H2 addresses key technological challenges for achieving 100% hydrogen operation in gas turbine combustion systems. The project further develops and optimises Ansaldo Energia’s sequential combustion technology to improve control of auto-ignition and flame stabilisation at hydrogen contents up to 100%, ensuring broader flashback margins, low NOx emissions, maintained GT-class firing temperatures, and no performance loss.
Through high-pressure experimental campaigns and advanced numerical modelling, FLEX4H2 generates unique datasets on hydrogen-enriched flames (70–100% H2) under realistic sequential combustor conditions, enabling accurate model validation and deeper understanding of hydrogen combustion behaviour. Full-scale demonstrations confirm fuel flexibility across the entire natural gas–hydrogen blending range, supported by optimised fuel splits, injection strategies, mixing design, residence time control, and start-up procedures suitable for any H2 content.
The project paves the way for higher TRL development and commercialisation, pushing gas turbine fuel and load flexibility beyond the state of the art. By enabling carbon-free, dispatchable power and retrofitting existing assets, FLEX4H2 supports grid balancing, increased renewable integration, reduced fossil fuel dependency, and new commercial opportunities.
A roadmap toward commercialisation by 2030 assesses portfolio integration, retrofit potential, possible large-scale demonstrations, supply chain development, and regulatory implications, including contributions to future European NOx emission standards. Further research will support extension of sequential combustion to smaller engine platforms to maximise impact.
Through high-pressure experimental campaigns and advanced numerical modelling, FLEX4H2 generates unique datasets on hydrogen-enriched flames (70–100% H2) under realistic sequential combustor conditions, enabling accurate model validation and deeper understanding of hydrogen combustion behaviour. Full-scale demonstrations confirm fuel flexibility across the entire natural gas–hydrogen blending range, supported by optimised fuel splits, injection strategies, mixing design, residence time control, and start-up procedures suitable for any H2 content.
The project paves the way for higher TRL development and commercialisation, pushing gas turbine fuel and load flexibility beyond the state of the art. By enabling carbon-free, dispatchable power and retrofitting existing assets, FLEX4H2 supports grid balancing, increased renewable integration, reduced fossil fuel dependency, and new commercial opportunities.
A roadmap toward commercialisation by 2030 assesses portfolio integration, retrofit potential, possible large-scale demonstrations, supply chain development, and regulatory implications, including contributions to future European NOx emission standards. Further research will support extension of sequential combustion to smaller engine platforms to maximise impact.