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Ignition and Reactive-Structure Transition of Ammonia/Hydrogen Blends under MILD Combustion Conditions

This study employs numerical simulations of counterflow flamelets to investigate the ignition dynamics and reactive-structure transitions of ammonia/hydrogen blends under MILD combustion conditions, revealing how fuel dilution and hydrogen enrichment govern the evolution from localized ignition to distributed reaction zones while elucidating the resulting impacts on temperature profiles and NO formation pathways.

Original authors: Biagio Cassese, Giancarlo Sorrentino, Pino Sabia, Raffaele Ragucci, Mara de Joannon

Published 2026-10-27

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CC BY 4.0

Original authors: Biagio Cassese, Giancarlo Sorrentino, Pino Sabia, Raffaele Ragucci, Mara de Joannon

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The global push to decarbonize the energy sector has turned scientific attention toward fuels that burn without releasing carbon dioxide. Two such candidates are hydrogen and ammonia. Hydrogen is often seen as the ultimate clean fuel, but storing and moving it is difficult and expensive. Ammonia, a molecule made of nitrogen and hydrogen, offers a practical solution: it is easier to store and transport, and it carries a high density of hydrogen within its chemical bonds. However, burning ammonia is tricky. It is not very reactive, meaning it is hard to ignite and burns slowly. In standard engines or furnaces, it struggles to stay lit and can produce harmful nitrogen oxides. To overcome these hurdles, engineers are exploring a technique called MILD combustion. This method involves preheating the air and mixing it with a large amount of recycled exhaust gas before the fuel is introduced. This creates a hot, diluted environment where the chemical reactions happen more gently and spread out, rather than in a single, intense flame front. This approach promises to make difficult-to-burn fuels like ammonia stable and clean, but scientists needed to understand exactly how the ignition process works in these specific conditions before building real-world systems.

A team of researchers from the National Research Council in Italy set out to map this process using detailed computer simulations. They focused on how ammonia, and mixtures of ammonia with hydrogen, ignite and stabilize when exposed to a stream of hot, diluted air. Instead of building a physical burner, they modeled a thin layer where the fuel and hot air meet, allowing them to watch the chemical reactions unfold in extreme detail over time. Their goal was to see how the reaction zone forms, how it moves, and how the addition of hydrogen changes the behavior of the ammonia. They tested a wide range of conditions, varying the amount of nitrogen used to dilute the fuel, the temperature of the incoming air, and the speed at which the gases were mixed.

The simulations revealed that ignition always begins on the side where the air is most abundant, known as the lean side. This is where the hot oxidizer first triggers the chemical activity. From there, the story diverges depending on how much the fuel is diluted. When the fuel is only slightly diluted, the reaction starts as a localized spot that quickly spreads toward the fuel-rich side. It behaves somewhat like a traditional flame, forming distinct, layered zones of heat release. However, when the fuel is heavily diluted with nitrogen, the reaction behaves differently. The intense, localized spots fail to form. Instead, the heat release spreads out into a broad, gentle region that covers a wide area. This is the signature of MILD combustion, where the reaction is distributed rather than concentrated. The researchers found that adding hydrogen to the ammonia mix makes the fuel ignite much faster, but it does not fundamentally change this shift from a localized flame to a distributed reaction as the dilution increases.

A key discovery in the study concerns the relationship between the reaction zone and the ideal mixing ratio of fuel and air. In conventional flames, the hottest part of the fire sits exactly where the fuel and air are perfectly balanced. In these highly diluted conditions, the researchers observed that this connection breaks down. As the dilution increases past a certain point—around seventy-five to eighty percent nitrogen—the location of the most intense chemical activity drifts away from the ideal mixing ratio. The reaction zone becomes decoupled from the stoichiometric point, meaning the fire is no longer anchored to the specific place where fuel and air are perfectly matched. This shift marks the transition into the true MILD regime. The study showed that while adding hydrogen makes the chemistry more reactive, the physical movement of the molecules still pushes the reaction zone toward the lean side, creating a complex balance between chemical speed and physical diffusion.

The researchers also traced the path of nitrogen atoms to understand how harmful nitrogen oxides are formed. They found that in less diluted conditions, adding hydrogen increases the production of these pollutants because it speeds up the chemical reactions that convert fuel nitrogen into oxides. However, in the highly diluted, distributed regime, the opposite occurs. The broader, cooler reaction zones suppress the formation of nitrogen oxides, even when hydrogen is added. The study identified specific chemical pathways that dominate the process, showing that the route nitrogen takes to become a pollutant changes as the environment becomes more diluted. In the most diluted cases, the chemical network simplifies, and the formation of nitrogen oxides is naturally limited by the conditions of the MILD combustion itself.

These findings provide a clear picture of how ammonia and ammonia-hydrogen blends behave when ignited in a hot, diluted environment. The work confirms that while hydrogen helps ammonia ignite more easily, the unique, distributed nature of MILD combustion only emerges when the fuel is sufficiently diluted. The transition from a traditional, anchored flame to a widespread, gentle reaction is a critical threshold that engineers must navigate. By understanding exactly where this transition happens and how the chemical pathways shift, the study offers a roadmap for designing future combustion systems that can safely and cleanly utilize ammonia as a carbon-free fuel source. The simulations suggest that with the right balance of dilution and temperature, it is possible to burn these challenging fuels without the high temperatures that typically create pollution, paving the way for a cleaner energy future.

Technical Summary: Ignition and Reactive-Structure Transition of Ammonia/Hydrogen Blends under MILD Combustion Conditions

Problem Statement
The decarbonization of the energy sector has intensified interest in ammonia ( $NH_3$ ) as a carbon-free energy carrier, often blended with hydrogen ( $H_2$ ) to overcome its low reactivity, high autoignition temperature, and narrow flammability limits. While Moderate or Intense Low-Oxygen Dilution (MILD) combustion has shown promise for stabilizing ammonia flames with low $NO_x$ emissions, existing literature predominantly focuses on steady-state conditions. A critical knowledge gap remains regarding the transient behavior of ammonia-fueled MILD combustion, specifically the ignition dynamics and the evolution of reactive structures under diffusion-controlled, highly diluted conditions. Understanding these transient phenomena is essential for the safe and reliable design of burners for gas turbines, furnaces, and incineration applications where low-heating-value fuels operate in hot product environments.

Methodology
The study employs numerical simulations using the 1D reactive flow solver CHEM1D to model counterflow diffusion flamelets under Hot-Oxidizer Diluted-Fuel (HODF) conditions. This configuration mimics MILD combustion by exposing a fuel stream (pure ammonia, pure hydrogen, or blends) to a hot, highly diluted oxidizer stream.

Parameters: Simulations cover oxidizer inlet temperatures of 1400, 1600, and 1800 K; strain rates of 50, 150, and 500 $s^{-1}$ ; and nitrogen dilution levels ranging from 0% to 93%. Hydrogen blending ratios vary from 0% to 100%.
Chemistry and Transport: The chemical kinetic mechanism by Manna et al. (2023), validated for high-dilution MILD conditions, is utilized. Both mixture-averaged diffusion and unity-Lewis-number approximations are tested to assess transport sensitivity.
Analysis: A total of 252 transient simulations were performed. Post-processing involves tracking the temporal evolution of the mixture fraction at maximum heat release ( $Z(\dot{q}_{max})$ ) and maximum temperature rise ( $Z(\Delta T_{max})$ ) relative to the stoichiometric mixture fraction ( $Z_{st}$ ). Nitrogen-oxide ( $NO_x$ ) formation is analyzed through domain integrals and detailed pathway decomposition (including graph-based analysis of $NH_3$ -to- $NO$ conversion).

Key Contributions and Results

Transient Ignition and Reactive Structure Evolution:
- Ignition is consistently triggered on the lean side of the mixing layer where the hot oxidizer initiates chemical activation.
- Low/Moderate Dilution: The reaction zone evolves into a localized, stratified structure. After initial lean ignition, a secondary autoignition kernel often forms and propagates toward the rich side, creating multi-peak heat-release structures.
- High Dilution: Strong dilution suppresses secondary rich-side ignition. The reactive zone evolves into a broader, weaker, and more spatially distributed "igni-diffusive" regime characteristic of MILD combustion, remaining confined to leaner mixture fractions without crossing $Z_{st}$ .
Ignition Delay and Stabilization:
- Increasing nitrogen dilution systematically increases ignition delay. Hydrogen addition significantly accelerates ignition, reducing delay by an order of magnitude even at low blending levels (5% $H_2$ ). However, the relative speed-up effect of hydrogen diminishes at very high dilution levels.
- The stabilization of the dominant oxidation zone is primarily controlled by dilution. At low dilution, the zone stabilizes on the rich side; as dilution increases, it shifts to the lean side. Hydrogen blending modulates the sharpness of this transition but does not fundamentally alter the dilution threshold for regime change.
Stoichiometric Decoupling and Transition Criterion:
- Under weakly diluted conditions, the peak reactive region remains anchored to the stoichiometric mixture fraction ( $Z_{st}$ ).
- Under highly diluted conditions, a progressive spatial decoupling occurs where the location of maximum chemical activity ( $Z(\Delta T_{max})$ ) diverges from $Z_{st}$ .
- The study defines a normalized displacement metric ( $D_T$ ) to quantify this transition. For the baseline configuration, the transition from a stoichiometrically anchored diffusion structure to a distributed MILD regime occurs approximately at 75–80% nitrogen dilution. Hydrogen enrichment has competing effects: enhanced reactivity tends to preserve stoichiometric anchoring, while high hydrogen diffusivity promotes displacement toward leaner mixtures.
Nitrogen Chemistry and $NO_x$ Formation:
- Pathways: The dominant routes for $NO$ formation are mediated by HNO and NH species. A robust backbone of $NH_3 \to NH_2 \to NH$ initiates conversion, followed by competition between direct oxidation to $NO$ , oxidation via HNO, and (under highly diluted, low-hydrogen conditions) via $H_2NO$ .
- Emissions Trends: At moderate dilution, $NO$ increases with hydrogen content due to enhanced radical pools. At high dilution, $NO$ decreases with hydrogen enrichment due to the reduced availability of fuel-bound nitrogen and cooler, distributed reaction zones.
- Dilution Effect: Dilution suppresses both $NO$ production and consumption pathways. However, for highly hydrogen-enriched mixtures, dilution effectively shifts the balance toward reduced net $NO$ formation. $N_2O$ formation is favored at low temperatures and high dilution, peaking at modest hydrogen enrichment.

Significance
This work provides fundamental insights into the ignition kinetics and stability limits of ammonia and ammonia/hydrogen blends under MILD conditions, addressing a gap in transient analysis. The findings demonstrate that the conventional concept of a stoichiometrically anchored flame front becomes less representative under highly diluted conditions, where the reactive zone becomes kinetically controlled and spatially distributed. The identification of a transition dilution threshold (75–80% $N_2$ ) and the characterization of competing kinetic and transport effects of hydrogen enrichment offer critical data for the design and optimization of future carbon-free combustion systems utilizing ammonia-based fuels. The study confirms that MILD combustion is a viable strategy for stabilizing ammonia flames with low emissions, provided that the transition to distributed reactive structures is properly managed.

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