Ammonia Combustion toward Decarbonized Energy Systems

Overview

Ammonia has attracted significant attention not only as a hydrogen carrier but also as a direct fuel owing to its carbon-free nature, ease of liquefaction, and high hydrogen volumetric energy density. In addition to well-recognized challenges such as low combustibility and fuel-NOx emissions, which have been intensively studied in recent years, nitriding of metal surfaces induced by ammonia flames has emerged as another critical issue in combustion systems, including gas turbines, industrial furnaces, and boilers. In this study, we aim to achieve a deeper understanding of ammonia combustion and the associated nitriding phenomena, and to develop predictive models that capture these behaviors by accounting for both homogeneous and heterogeneous reaction kinetics as well as nitrogen atomic diffusion within metals.


Ammonia pyrolysis on iron-based metal walls

Tubular reactors made of quartz and stainless steel were heated using an electric furnace. By varying the inner diameter of the reactors, different surface-to-volume ratios were achieved, allowing the effects of heterogeneous reactions to be separated from those of homogeneous reactions. Mixtures of ammonia and nitrogen were supplied to the reactors with a constant residence time, independent of reactor temperature and diameter. The ammonia decomposition rate was measured at the reactor outlet using gas chromatography.

Ammonia decomposition in the quartz reactor starts at approximately 1050 K, whereas in the stainless steel reactor it begins at around 673 K due to catalytic heterogeneous reactions on the surface. Surface ammonia pyrolysis on stainless steel walls was modeled assuming a one-step global reaction mechanism. This model was shown to reasonably estimate ammonia and radical concentration distributions near the wall in the ammonia flame field, highlighting the importance of properly accounting for the surface-effect to accurately predict flame structures in ammonia-fueled combustors. We further aim to model detailed surface reaction mechanisms to elucidate ammonia pyrolysis kinetics on metal walls.

Experimental setup for measuring the NH3 decomposition rate in reactors

NH3 decomposition rate on the stainless steel and quartz surface as a function of tempearture (Feng et al., 2023)


Nitriding effect induced by ammonia flames

To systematically investigate nitriding effects induced by ammonia flames on iron-based materials, ammonia premixed flames were impinged on stainless steel test plates under well-controlled temperature and surrounding environment conditions. After several hours of exposure, the test plates showed marked increases in both surface hardness and nitrogen content, demonstrating that ammonia flames can induce significant nitriding effects comparable to those achieved by conventional gas nitriding processes. Laser diagnostics reveal that the near-wall ammonia concentration, rather than the canonical nitriding potential, is the dominant parameter governing nitriding behavior, while nitrogen-containing radicals may play a secondary role.

The effects of wall temperature and water vapor, which are crucial in ammonia-fueled systems, were further quantified. Elevated wall temperatures and increased H2O concentrations were found to significantly suppress nitriding. Although heterogeneous ammonia pyrolysis is thermally promoted, the surface nitrogen concentration decreases at high temperatures due to rapid recombination into molecular nitrogen. In addition, water vapor strongly inhibits ammonia pyrolysis through surface oxidation, reducing nitriding, while the resulting oxide layers can undergo subsequent re-nitriding.

These effects are further investigated for different gas/metal compositions and under elevated pressures, in combination with numerical modeling. The resulting insights support material selection and lifetime assessment for securing the performance and durability of ammonia-fueled systems.

Experimental setup for NH3 flame nitriding

Radial distributions of surface hardness (left) and atomic nitrogen content (right) after 5 hours of exposure to ammonia flames at temperatures of 550–850 ºC (Wang et al., 2024)


Advanced laser diagnostics for high-fidelity ammonia kinetics

Previous ammonia kinetic models still exhibit large uncertainties in predicting intermediate species concentrations, particularly under low-dilution conditions that result in high flame temperatures, for which experimental data remain scarce. To address this issue, the structures of ammonia flames are directly investigated under both impinging flame and counterflow burner configurations. Spatial distributions of OH and NH radicals are measured by laser-induced fluorescence (LIF), while NH3 and atomic H concentrations are quantified using the two-photon LIF (TALIF) technique. In addition, gas temperatures are measured using Rayleigh scatterings. Based on these experimental measurements, ammonia reaction kinetics are systematically evaluated and further refined.

(a) Flame structure with spatial distributions of (b) NH3, (c) NH2*, and (d) NH in the wall-impinging flame (Wang et al., 2024)

Rayleigh scattering measurements in NH3 counterflow diffusion flame (Xu et al., 2025)


Modeling of the nitriding process

We address the challenge of developing a predictive, multiscale modeling framework to elucidate ammonia-induced nitriding in iron-based materials. Ab initio molecular dynamics (AIMD) simulations based on a density functional theory (DFT) are employed to generate high-fidelity training data for a machine-learning interatomic potential. Using this potential, large-scale atomistic simulations are performed to quantitatively determine both concentration- and temperature-dependent nitrogen diffusion coefficients in iron and iron nitrides over wide temperature and composition ranges. These diffusion data are integrated with microkinetic modeling of ammonia decomposition on nitrided metal surfaces and a continuum-scale diffusion model to predict nitride layer growth. The resulting framework provides fundamental insight into the coupled roles of surface reaction kinetics and bulk diffusion, offering a robust tool for designing durable materials for ammonia-fueled energy systems.

Preliminary AIMD simulation results of NH3 pyrolysis on iron surfaces (left) and internal diffusion of atomic N in iron (right) (Feng et al., 2023)


Collaborators:
Dr. Yong Fan (National Institute of Advanced Industrial Science and Technology)
Prof. Yiguang Ju (Princeton University)
Prof. Waruna Kulatilaka (Texas A&M University)
Decarbonized Industrial thermo-System Center
Mitsubishi Heavy Industries, Ltd.

Recent Reports

Nitriding effect induced by ammonia flames

  • Xing, Y., Lee, M., and Suzuki, Y.,
    “Effect of Water Vapor on Nitriding of Stainless Steel Walls Induced by Ammonia Flames,”
    Proc. Combust. Inst., Vol. 41, (2025), 105831.
    (doi.org/10.1016/j.proci.2025.105831)
  • Wang, D., Xing, Y., Lee, M., and Suzuki, Y.,
    “Effects of Wall Temperature and Water Vapor on the Nitriding of Stainless Steel Induced by Ammonia Flames,”
    Proc. Combust. Inst., Vol. 40, (2024), 105562.
    (doi:10.1016/j.proci.2024.105562)
  • Wang, D., Lee, M., and Suzuki, Y.,
    “Nitriding Effects of Ammonia Flames on Iron-based Metal Wall,”
    J. Ammonia Energy, Vol. 1, (2023), pp. 1-10.
    (doi:10.18573/jae.8)

Modeling of ammonia decomposition and nitrogen diffusion

  • Feng, P., Lele, A. D., Lee, M., Ju, Y., and Suzuki, Y.,
    “Molecular dynamics simulation of nitrogen diffusion in iron and iron nitrides using ab initio data trained machine learning potentials,”
    Phys. Chem. Chem. Phys., (2026). (under review)
  • Feng, P., Lee, M., Wang, D., and Suzuki, Y.,
    “Ammonia Thermal Decomposition on Quartz and Stainless Steel Walls,”
    Int. J. Hydrog. Energ., Vol. 48, Issue 75, (2023), pp. 29209-29219.
    (doi:10.1016/j.ijhydene.2023.04.106) (Corrigendum)

Laser diagnostics on ammonia flames

  • Xu X., Lee M., Akiba T., and Suzuki Y.,
    “Quantitative measurements of hydrogen atom concentrations in ammonia diffusion flames using two-photon absorption laser-induced fluorescence,”
    15th Asia-Pacific Conference on Combustion (ASPACC 2025), Singapore, #460, (2025).
  • Xu, X., Lee, M., Akiba, T., and Suzuki, Y.,
    "Quantitative Measurements of H-atom and OH in Ammonia Counterflow Diffusion Flames,"
    4th Symposium on Ammonia Energy, Minnesota, (2025).
  • Xu, X., Lee, M., and Suzuki, Y.,
    “Investigation of temperature and species distributions in ammonia diffusion flames,”
    19th International Conference on Numerical Combustion (ICNC 2024), Kyoto, ICNC2024-1606, (2024).
  • Xu, X., Akiba, T., Lee, M., and Suzuki, Y.,
    “Investigation of ammonia counterflow diffusion flames through direct measurements of species distribution,”
    CI’s 40th Int. Symp. - Emphasizing Energy Transition (Combustion 2024), Milan, WiPP-2P095, (2024).
  • Xu, X., Lee, M., and Suzuki, Y.,
    “Measurements of NH3 and NH2* Profiles in Ammonia Counterflow Diffusion Flames,”
    2nd Symp. on Ammonia Energy, Orleans, (2023).
  • Fan, Y., Wang, Z., Wang, Y., Lee, M., Kulatilaka, W. D., and Suzuki, Y.,
    “Species Structures in Preheated Ammonia Micro Flames,”
    Proc. Combust. Inst., Vol. 39, (2023), pp. 4427-4436.
    (doi:10.1016/j.proci.2022.07.267)

Last update: 2026-01-22