This article reviews the critical role of material selection and design in ensuring efficient performance and safe operation of gas turbine engines fuelled by ammonia–hydrogen. As these energy fuels present unique combustion characteristics in turbine combustors, the identification of suitable materials becomes imperative. Detailed material characterisation is indispensable for discerning defects and degradation routes in turbine components, thereby illuminating avenues for improvement. With elevated turbine inlet temperatures, there is an augmented susceptibility to thermal degradation and mechanical shortcomings, especially in the high-pressure turbine blade—a critical life-determining component. This review highlights challenges in turbine design for ammonia–hydrogen fuels, addressing concerns like ammonia corrosion, hydrogen embrittlement, and stress corrosion cracking. To ensure engine safety and efficacy, this article advocates for leveraging advanced analytical techniques in both material development and risk evaluation, emphasising the interplay among technological progress, equipment specifications, operational criteria, and analysis methods.

Ammonia is increasingly seen as a promising fuel for combustion applications, including gas turbines, due to its hydrogen content and ease of storage, making it a potential method for storing renewable energy. However, using ammonia directly poses challenges in controlling NOx emissions, especially for retrofitting existing gas turbines. Cracking ammonia to produce hydrogen and nitrogen could mitigate this issue, although uncracked ammonia traces may remain due to inefficiencies. Therefore, this paper evaluates the impacts on swirling flames, representative of gas turbine combustors, when highly cracked ammonia (17.5% H2, 1.0% NH3, and 81.5% N2) is used. Experiments were conducted at pressures ranging from 1.1 to 6 bar absolute, with air preheated to 500 K and a constant power output of 22.7 kW maintained under lean conditions (equivalence ratio ∼ 0.545) throughout the tests. NH2 chemiluminescence intensity increased monotonically with pressure from 1.1 to 6 bar, with a peak intensity observed at 6 bar due to enhanced radical formation at higher collision frequencies. NOx emissions rose from 90 ppmv at 1.1 bar to 189 ppmv at 4 bar before stabilizing, indicating a balance between thermal NOx formation and ammonia-mediated reduction pathways at higher pressures. NH* intensity decreased with increasing pressure, while OH* radicals remained relatively constant, providing insights into flame structure and reaction zone characteristics. A chemical reactor network model complemented the experimental findings, capturing flame zone dynamics and revealing consistent NO formation pathways through NH3, NH2, and NNH dissociation across different pressures. These findings demonstrate that pressure significantly influences radical distribution and NOx formation mechanisms in highly cracked ammonia combustion, with implications for gas turbine combustor design and emission control strategies. To the authors’ knowledge, this is the first systematic study of pressure-dependent chemiluminescence behavior in highly cracked ammonia swirl flames, providing critical insights for the development of low-emission gas turbine combustors using ammonia-derived fuels.

This study examines the high-temperature degradation of Hastelloy C276, a corrosion-resistant nickel-based alloy, during exposure to combustion products generated by methane and 99% cracked ammonia. Using a high-pressure optical combustor (HPOC) at 4 bar and exhaust temperatures of 815–860 °C, standard tensile specimens were exposed for five hours to fully developed post-flame exhaust gases, simulating real industrial turbine or burner conditions. The surfaces and subsurface regions of the samples were analysed using scanning electron microscopy (SEM; Zeiss Sigma HD FEG-SEM, Carl Zeiss, Oberkochen, Germany) and energy-dispersive X-ray spectroscopy (EDX; Oxford Instruments X-MaxN detectors, Oxford Instruments, Abingdon, United Kingdom), while mechanical properties were evaluated by tensile testing, and the gas-phase compositions were tracked in detail for each fuel blend. Results show that exposure to methane causes moderate oxidation and some grain boundary carburisation, with localised carbon enrichment detected by high-resolution EDX mapping. In contrast, 99% cracked ammonia resulted in much more aggressive selective oxidation, as evidenced by extensive surface roughening, significant chromium depletion, and higher oxygen incorporation, correlating with increased NO_x in the exhaust gas. Tensile testing reveals that methane exposure causes severe embrittlement (yield strength +41%, elongation −53%) through grain boundary carbide precipitation, while cracked ammonia exposure results in moderate degradation (yield strength +4%, elongation −24%) with fully preserved ultimate tensile strength (870 MPa), despite more aggressive surface oxidation. These counterintuitive findings demonstrate that grain boundary integrity is more critical than surface condition for mechanical reliability. These findings underscore the importance of evaluating material compatibility in low-carbon and hydrogen/ammonia-fuelled combustion systems and establish critical microstructural benchmarks for the anticipated mechanical testing in future work.