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Article

Residual Ammonia Effects on NO Formation in Cracked Ammonia/Air Premixed Flames

by
Donghyun Kim
1,
Jiwon Kim
1 and
Sungwoo Park
2,*
1
Department of Smart Air Mobility, Korea Aerospace University, Goyang 10540, Republic of Korea
2
School of Aerospace and Mechanical Engineering, Korea Aerospace University, Goyang 10540, Republic of Korea
*
Author to whom correspondence should be addressed.
Energies 2025, 18(23), 6334; https://doi.org/10.3390/en18236334 (registering DOI)
Submission received: 23 October 2025 / Revised: 18 November 2025 / Accepted: 29 November 2025 / Published: 2 December 2025
(This article belongs to the Section I2: Energy and Combustion Science)
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Abstract

Cracked ammonia is attracting attention as a carbon-free energy carrier, yet trace residual ammonia after reforming can significantly affect nitrogen oxide (NOx) emissions. This study quantifies how residual ammonia from 0 to 10,000 ppm affects NOx formation using one-dimensional premixed flame simulations under gas turbine-relevant conditions (ϕ = 0.5 and 2.0, 673 K, 20 atm). NO formation is evaluated using integrated rate of production (ROP) analysis, reaction pathway analysis, and A-factor sensitivity analysis (defined as sensitivity to the pre-exponential factor in the Arrhenius rate expression). Under lean conditions (ϕ = 0.5), NO increases approximately linearly with residual ammonia. Even at 100 ppm, the dominant NO formation route shifts rapidly from thermal and N2O mechanisms to fuel NO chemistry led by HNO. In contrast, under rich conditions (ϕ = 2.0), the final NO level remains below 10 ppm. Under rich conditions, residual ammonia and the higher flame temperature raise gross NO production in the reaction zone, yet strong DeNOx reactions in the post-flame region consume most of it, resulting in low net NO emissions. These mechanistic results inform cracking targets and the design of staged combustion strategies to minimize NOx formation when deploying cracked ammonia in practical gas turbine systems.

1. Introduction

Since industrialization, increasing greenhouse gas emissions have driven significant climate change [1,2,3,4,5]. In response, the Paris Agreement aims to achieve a balance between anthropogenic greenhouse gas emissions and removals later this century [6,7]. Accordingly, the development of sustainable, carbon-free energy sources to replace existing hydrocarbon fuels is essential. In parallel with these efforts, energy system studies have also advanced pollutant mitigation technologies for trace species in combustion exhausts. Recent work on MOF-based sorbents for elemental mercury removal [8,9] exemplifies this broader multi-pollutant control landscape, complementing decarbonization strategies. Hydrogen (H2) and ammonia (NH3) are attracting attention as representative carbon-free fuels [10,11,12,13]. Hydrogen offers significant advantages, including high gravimetric energy density and no fuel NOx formation, due to its nitrogen free nature during combustion. However, hydrogen still poses technological challenges in storage, transportation, and practical combustion applications, including cryogenic liquefaction at −253 °C and severe flashback risks due to its high reactivity [14,15,16]. In contrast, ammonia can leverage mature production and transportation infrastructure, and its high hydrogen storage density (17.6 wt%) makes it one of the most promising hydrogen carriers [17,18,19,20]. Due to these practical advantages, ammonia is being actively studied as a candidate fuel for carbon neutral combustion systems, including gas turbines for power generation.
However, pure ammonia exhibits relatively unfavorable combustion characteristics, including a low heating value, a slow burning velocity, and a narrow flammability range. These properties can cause flame instability in practical combustors [21,22,23]. To address these limitations, various strategies have been investigated, most notably co-firing with other highly reactive fuels [24,25,26,27] and cracking, which decomposes ammonia into a hydrogen-rich fuel [28,29]. In particular, partial cracking in situ within the combustor has attracted attention because it provides hydrogen on demand without separate storage [30]. Among ammonia-cracking technologies, thermochemical recuperation (TCR) is particularly noteworthy [31]. This technology uses turbine exhaust heat to drive the endothermic decomposition of ammonia. This converts waste heat into chemical energy and improves overall thermal efficiency, while the produced hydrogen increases fuel reactivity and enhances combustion stability [32,33]. As reviewed by Lucentini et al. [34], ammonia can achieve nearly complete cracking at moderate temperatures (around 400 °C) and near atmospheric pressure under appropriate catalytic conditions, emphasizing the practical potential of TCR systems.
Previous studies have explored the NOx reduction potential and system-level constraints associated with partially cracked ammonia combustion in gas turbines. Mazzotta and co-workers [35] conducted large eddy simulations (LESs) coupled with a chemical reaction network (CRN) to predict NOx formation from a gas turbine combustor fueled by hydrogen/ammonia blends and partially cracked ammonia. They reported that a fuel with an 80% cracked ammonia reduced NOx by 25% compared to a hydrogen/ammonia blend, indicating the potential of cracked ammonia for NOx mitigation. Zhou and Duan [36] used a CRN to assess a rich–quench–lean (RQL) strategy, applying a high cracking ratio for ignition and a lower ratio for the main stage. This reduced the reformer size by 80% while maintaining NOx below 50 ppm, suggesting a practical approach for staged combustors. Recently, Park and Lee [37] employed a perfectly stirred reactor (PSR) to investigate the effect of residual ammonia on NOx formation. They simulated residual ammonia concentrations from 10 to 10,000 ppm at lean equivalence ratios and found a pressure-dependent crossover in NOx trends, suggesting that the optimal cracking ratio of ammonia should account for pressure effects.
While these studies demonstrate the potential of partially cracked ammonia, a clear research gap remains. Most prior studies have focused on cracking ratios of roughly 50–90% (residual NH3 on the order of 104–105 ppm). In contrast, modern systems increasingly target near-complete decomposition to maximize efficiency. For example, Pashchenko [38] reports sub-percent residual NH3 in a solar-assisted decomposition system (e.g., ≈1.18% at 500 °C and 10 bar). Following this direction, the present work targets the ppm level residual regime (0–10,000 ppm, ≥99% cracking) for both lean and rich flames. A second gap exists because the limited studies in the ppm regime have mainly focused on lean conditions and do not provide systematic comparison across equivalence ratios that include fuel-rich flames. Supporting this need, Kim et al. [39] found that in fuel-rich NH3/H2 premixed flames, NO first increases steeply once the hydrogen mole fraction exceeds a threshold value and then decreases again as the mixture approaches pure H2, underscoring the importance of examining cracked ammonia flames under both lean and rich conditions. Accordingly, this study quantifies NO formation mechanisms for 0–10,000 ppm residual NH3 at ϕ = 0.5 and ϕ = 2.0 under gas turbine-relevant conditions and provides a mechanistic context for setting cracking targets and designing staged combustion strategies.
To address this research gap and support the development of combustion strategies for highly cracked ammonia fuels, it is necessary to clarify NO formation behavior in cracked ammonia flames containing ppm level residual ammonia. This study quantifies the effect of residual ammonia on NO formation in laminar premixed ammonia/hydrogen/air flames at gas turbine-relevant pressure and inlet temperature using a detailed chemical kinetic model. Simulations are performed at equivalence ratios 0.5 and 2.0 (673 K, 20 atm) for cracking rates of 98–100% (corresponding to 0–10,000 ppm NH3 in the cracked fuel). NO production and consumption pathways are evaluated using integrated rate of production (ROP) and A-factor sensitivity analyses. These results provide a mechanistic comparison between lean and rich flames, providing a quantitative basis for determining optimal cracking ratios and designing combustion strategies such as staged combustion in cracked ammonia systems.

2. Numerical Methods

We used the detailed chemical kinetic model developed and validated in our previous work [39]. The mechanism contains 33 species and 216 elementary reactions. It was assembled based on a nitrogen chemistry [40] combined with subsets from AramcoMech 3.0 [41] and then selectively refined to improve predictions for ammonia flames at elevated pressures. As documented in Ref. [39], rate constants for reactions identified through sensitivity analyses were updated without degrading NO predictions. The detailed modification procedure and validation results are described in Ref. [39].
Freely propagating laminar ammonia/hydrogen/nitrogen/air premixed flames were analyzed. All simulations were conducted using the ANSYS Chemkin-Pro 2021 R1 PREMIX module [42] with a mixture of averaged transport properties and thermal diffusion. An adaptive grid was used with gradient = 0.03 and curvature = 0.05. These mesh control parameters follow the validated settings from our previous high-pressure ammonia flame study [39], where mesh convergence was confirmed under similar thermochemical conditions (20 atm, NH3/H2 flames). To emulate gas turbine-relevant inlet conditions, the unburned mixture temperature and pressure were fixed at 673 K and 20 atm. This pressure represents typical operation of modern heavy-duty gas turbines, which commonly operate in the 15–30 atm range [43,44]. Accordingly, 20 atm is widely adopted as a standard high-pressure condition in experimental and numerical kinetic studies of gas turbine combustion [45]. The cracked ammonia fuel was formulated according to the specified cracking ratio, in which decomposed NH3 yields H2 and N2 in a 3:1 molar ratio, and the remaining NH3 was added to complete the fuel mixture. The resulting cases (98−100% cracking) correspond to 0−10,000 ppm residual ammonia, as shown in Table 1. Two equivalence ratios were examined, ϕ = 0.5 and ϕ = 2.0, representing lean and rich conditions. For each case, the maximum flame temperature remained nearly constant across the residual ammonia sweep, varying by less than about 10 K at ϕ = 0.5 and about 7 K at ϕ = 2.0.
For each flame, we quantified NO emission trends with varying residual ammonia and performed reaction pathway analyses by integrating ROP terms for NO production and consumption. Pathways were grouped into thermal, N2O, and fuel NO routes. The residence time was calculated using the axial distance and velocity, with time zero at the location of the maximum heat release rate in the reaction zone. NO concentrations were sampled at a residence time of 20 ms, which is relevant to industrial gas turbine combustors [46,47,48] and reported on a dry basis, corrected to 15% O2.

3. Results and Discussion

Figure 1 summarizes the overall effects of residual ammonia concentration on flame temperature and NO formation across equivalence ratios from ϕ = 0.5 to 2.0. As shown in Figure 1a, the maximum flame temperature remains almost unchanged over the entire examined range of equivalence ratios, confirming that the presence of ppm-level ammonia has little thermal influence on the flame structure or overall heat release characteristics.
In contrast, Figure 1b reveals a pronounced sensitivity of NO formation to residual ammonia, particularly under fuel-lean conditions. The NO concentration increases sharply from ϕ = 0.5, peaking slightly below stoichiometry (ϕ = 0.8–0.9), and then rapidly decreases toward rich mixtures. As the residual NH3 increases from 0 to 10,000 ppm, NO levels rise significantly in the lean region, whereas the change in peak NO near ϕ = 0.8–0.9 is comparatively small. Beyond stoichiometry, the curves remain nearly constant, indicating that the contribution of residual NH3 becomes negligible under rich conditions.
Two representative cases were selected for detailed analysis: ϕ = 0.5 for the lean condition where NO formation is most sensitive to residual ammonia and ϕ = 2.0 for the rich condition where NO formation is relatively insensitive. The small variation in flame temperature across these cases indicates that thermal effects are minimal, ensuring that the observed NO behavior primarily reflects chemical kinetic influences.

3.1. Residual Ammonia’s Effects on NO Formation in Lean Flames

Figure 2 illustrates the effect of residual ammonia in cracked ammonia flames under lean conditions (ϕ = 0.5). Figure 2a shows NO concentration on a 15% dry O2 basis at a residence time of 20 ms, while Figure 2b presents the maximum mole fractions of key ammonia-derived intermediate species, namely HNO, NH2, and NH. As shown in Figure 2a, NO increases almost linearly with residual ammonia, reaching approximately 1300 ppm at 10,000 ppm residual ammonia. Consistently, Figure 2b exhibits an approximately linear growth in the maximum mole fractions of HNO, NH2, and NH with increasing residual ammonia. The simultaneous rise of these intermediates highlights their strong coupling with the dominant NO formation pathways [39], indicating that identical chemical routes govern NO production even when residual ammonia is present only at ppm levels. To further clarify the chemical environment, Figure 2c presents the maximum mole fractions of the H, O, and OH radicals. The radical pool is dominated by O and OH, while the H radical concentration remains comparatively low.
To assess the temporal evolution of NO formation in the 1D flame, Figure 3 shows NO concentration on a 15% dry O2 basis as a function of residence time under fuel-lean conditions (ϕ = 0.5). For the perfectly cracked case (0 ppm NH3), NO remains very low throughout the entire residence time range. When residual ammonia is present, NO rises sharply within the reaction zone (t ≈ 0 ms) and then remains nearly constant, indicating that the NO generated in the reaction zone largely determines the final emission level. This behavior demonstrates that NO is formed primarily at the flame front, while thermal NO production in the post-flame zone is minimal under lean conditions. The absence of any noticeable decay following the initial rise further confirms that DeNOx reactions are not active to a measurable extent. These results confirm that the increase in NO with residual ammonia originates mainly within the reaction zone, rather than from downstream thermal processes.
Figure 4 illustrates the NO formation pathways in cracked ammonia flames under lean conditions (ϕ = 0.5). The diagrams are derived from the ROP results at the location of maximum heat release, and the percentage contribution of each formation pathway was obtained by integrating the ROP over the residence time interval from −1 ms to 20 ms. Figure 4a corresponds to the case without residual ammonia (0 ppm) and Figure 4b to the case with 10,000 ppm residual ammonia. Major NO formation routes are indicated by bold arrows, while consumption routes are shown in gray.
In Figure 4a, NO originates from nitrogen in the mixture and is produced mainly through the N2O and the thermal NO routes. Quantitatively, the N2O pathway contributes about 27.5%, and the thermal NO route (i.e., the extended Zeldovich mechanism) accounts for approximately 63.9%, which is consistent with previous hydrogen/air combustion studies [49]. In contrast, Figure 4b shows that the fuel NO routes derived from ammonia become dominant when residual ammonia is present. The pathway mediated by HNO, with H2NO as a precursor, is strongly activated, with additional contributions from routes involving NH and NH2 radicals. Under these conditions, approximately 70.2% of NO forms via HNO, 18.3% via NH radicals, and 4.8% via N radicals. Although NO consumption pathways involving NH and NH2 exist, they exert little influence on the net NO concentration, as shown in Figure 3. Overall, the pathway analysis shows that the presence of residual ammonia shifts the dominant NO formation mechanism from thermal/N2O routes to fuel NO mechanisms driven by ammonia-derived intermediates such as HNO and NH.
To identify the dominant routes of NO formation under lean conditions, the reactions were classified into six groups, namely thermal, N2O, NNH, HNO, NH, and Others. The thermal NO routes are important at high temperatures (typically above 1800 K) and include the reactions of the extended Zeldovich mechanism. The N2O route becomes relevant at relatively lower temperatures and higher pressures than the thermal route [50] and is thus pertinent to lean premixed gas turbine conditions. The NNH route, which proceeds through reactions of NNH with O atoms, is negligible under the present lean conditions. Within the fuel NO routes, NO production is dominated by the HNO pathway, primarily through the reaction of HNO with H radicals, whereas the NH route proceeds mainly via reactions of NH with O radicals and O2. The Others group includes all remaining NO-related reactions.
Figure 5 shows the percentage contributions of the major NO formation routes under lean conditions for residual ammonia level of 0, 100, 1000, 5000, and 10,000 ppm. The contributions were obtained by integrating the ROP over a residence time interval from −1 to 20 ms. At 0 ppm, the thermal and N2O routes account for 67% and 27%, respectively, consistent with hydrogen flame behavior shown in Figure 4a. With 100 ppm residual ammonia, the combined contribution of HNO and NH routes rises to 69%, while the thermal and N2O routes decrease to 23% and 7%, indicating a rapid shift toward fuel NO formation. From 1000 ppm onward, the combined HNO and NH contribution increases gradually to about 91%, consolidating their dominance over this range. Overall, residual ammonia shifts NO formation toward fuel NO routes led by HNO, and above several hundred ppm, the HNO and NH pathways together account for roughly 90% of total NO production, confirming that ammonia-derived routes become predominant.
Figure 6 presents the A-factor sensitivity analysis of NO at a residence time of 20 ms for residual ammonia levels of 0, 1000, and 10,000 ppm. For each reaction, we perturbed the A-factor and computed the sensitivity coefficient as the relative change in NO, which identifies the most influential reactions and how their sensitivities vary with residual ammonia. As the residual ammonia level increases, several reactions exhibit pronounced changes in sensitivity, including sign reversals. In particular, the thermal/N2O reactions R72 (N + NO → N2 + O) and R161 (N2O + O → NO + NO), which have strong positive sensitivities at 0 ppm, decrease sharply. For R72, the sensitivity even becomes negative at 10,000 ppm, indicating a transition from the reverse direction that produces NO to the forward direction that consumes NO. This behavior signifies suppression of thermal NO formation and is consistent with the contribution shifts shown in Figure 5. DeNOx reactions involving NH and NH2, notably R65 (NH + NO → N2O + H) and R52 (NH2 + NO → N2 + H2O), show increasingly negative sensitivities with rising residual ammonia levels, indicating enhanced NO consumption primarily downstream of the flame front. Overall, as residual ammonia increases, the dominant NO formation routes shift from thermal/N2O mechanisms toward NH/HNO-related pathways, consistent with the results of Figure 4 and Figure 5. A tabulated list of the top five reactions and their corresponding sensitivity coefficients for both lean and rich conditions is provided in the Supplementary Material (Table S1) to support reproducibility and for future reference.

3.2. Residual Ammonia’s Effects on NO Formation in Rich Flames

Figure 7 illustrates the effect of residual ammonia under fuel-rich conditions (ϕ = 2.0). Figure 7a shows the NO concentration on a 15% dry O2 basis at a residence time of 20 ms, and Figure 7b presents the maximum mole fractions of ammonia-derived intermediates HNO, NH2, and NH. Figure 7c shows the maximum mole fractions of the key radicals (H, O, OH) to illustrate the chemical environment. In sharp contrast to the lean case (Figure 2c), the radical pool is overwhelmingly dominated by H atoms, while the O and OH radical concentrations remain several orders of magnitude lower. As shown in Figure 7b, the mole fractions of these intermediates increase almost linearly with residual ammonia, while Figure 7a displays that NO remains very low (below 10 ppm) and exhibits diminishing increases as residual ammonia rises. Therefore, in contrast to the lean case, where both intermediates and NO increase in parallel, the rich flame exhibits a weak and nonlinear NO response despite the linear growth of precursors. The H-dominant, O-poor environment (Figure 7c) intuitively suppresses O-dependent NO production routes such as thermal and N2O mechanisms. This suggests that under rich conditions, additional factors beyond ammonia-derived intermediates control the balance between NO production and consumption, highlighting the need for further mechanistic resolution of rich flame chemistry.
Figure 8 shows the variation of NO concentration with residence time under rich conditions for different residual ammonia levels. In the reaction zone, a noticeable amount of NO is produced, which is similar to the lean flame behavior in Figure 3. However, the NO mole fraction decreases rapidly in the post-flame region, indicating strong DeNOx reactions. Although higher residual ammonia levels increase the peak NO near the reaction zone, the final NO concentration remains low for all cases. These results suggest that under rich conditions, NO consumption in the post-flame region plays a major role together with its production in the reaction zone. This contrasts with the lean case, where NO generated within the flame front largely determines the final emission level.
To quantify NO production, consumption, and the resulting net balance under rich conditions, Figure 9 displays the integrated ROP values as a function of residual ammonia level. Net NO was obtained by integrating the ROP from −1 to 20 ms, with gross production and gross consumption evaluated separately by integrating the positive and negative contributions, respectively. As residual ammonia increases, gross NO production rises significantly, but gross consumption also increases substantially, resulting in only a small net gain. This trend agrees with Figure 7, where the final NO level remains below 10 ppm. These results indicate that under rich conditions, DeNOx reactions offset most of the NO formed in the reaction zone within the residence time considered, yielding an approximate balance between production and consumption.
Figure 10 compares NO formation pathways under rich conditions for (a) 0 ppm and (b) 10,000 ppm residual ammonia. In Figure 10a, the NNH route is strongly activated, in contrast to the lean case where thermal and N2O mechanisms dominate. This indicates that abundant H radicals in rich flames promote the association of N2 with H to form NNH, followed by reactions of NNH with O radicals that generate NO. In Figure 10b, fuel NO routes become prominent, especially those involving HNO and NH intermediates. The higher flame temperature under rich conditions (2160 K versus 1850 K for the lean case) enhances the thermal mechanism, particularly reactions of N with OH, whereas pathways involving H2NO are suppressed at such temperatures. Overall, residual ammonia activates fuel NO pathways, while the elevated temperature simultaneously strengthens certain parts of the thermal route, so that both mechanisms contribute jointly to NO formation under rich conditions. As also shown in Figure 10b, NH2 can react with NO to form NNH, while NH can consume NO to form N2 and N2O. These DeNOx reactions become increasingly important in rich flames and effectively suppress net NO formation.
To identify the specific DeNOx reactions responsible for the strong NO consumption quantified in Figure 9, Figure 11 presents the detailed contribution analysis for (a) NO production and (b) NO consumption under rich conditions. The consumption analysis in Figure 11b provides a quantitative breakdown of the dominant DeNOx pathways, while the production analysis follows the same procedure as in Figure 5. For consumption, pathways are grouped according to the species reacting with NO, namely N which reduces NO to N2, NH which converts NO to N2 or N2O, and NH2 which converts NO to N2 or NNH. The Radicals group represents interactions with H, O, and OH that consume NO, and the Others category includes remaining minor routes.
At 0 ppm residual ammonia, the distribution differs markedly from the lean case. The combined contribution of thermal and N2O routes is about 33%, while NNH and Others contribute about 36% and 26%, respectively, indicating that the NNH route leads NO formation under rich, fully cracked ammonia conditions, consistent with Figure 10a. Once residual ammonia is introduced (100 ppm or higher), the production profile stabilizes, with HNO, NH, and thermal routes together accounting for about 99% of total production. These contributions remain nearly unchanged as residual ammonia increases from 100 to 10,000 ppm, showing that HNO and thermal mechanisms dominate NO production, while N2O, NNH, and minor routes play secondary roles.
In Figure 11b, the 0 ppm case is omitted because NO consumption is negligible. With residual ammonia present, the contribution of ammonia-derived species (N, NH, and NH2) increases steadily, while that of the Radicals group decreases. As residual ammonia rises, ammonia-derived pathways become the main channels for NO removal in the post-flame region, replacing radical driven consumption. Consequently, in incompletely cracked ammonia flames, NO consumption is primarily governed by DeNOx chemistry involving N, NH, and NH2, explaining the weak net NO increase observed under rich conditions.
Figure 12 shows the A-factor sensitivity analysis of NO under rich conditions. A pronounced shift is observed between 0 ppm and 10,000 ppm. At 0 ppm, H2/O2 related reactions such as R1, R166, R2, R24, and R25 exhibit large positive sensitivities, whereas at 10,000 ppm, they become negative, reflecting that the expanded radical pool promotes DeNOx reactions when residual ammonia is present. The 1000 ppm case already resembles the 10,000 ppm case, indicating a much faster transition than that observed in lean flames. These results suggest a rapid shift toward an ammonia flame type mechanism in which DeNOx reactions dominate, and even small amounts of residual ammonia are sufficient to reverse the sensitivity pattern, consistent with the pathway and contribution analyses. In addition, the NH2 recombination reaction (R166) becomes one of the most influential reactions once residual ammonia is introduced. The recombination of NH2 radicals to form N2H2 and H2 competes with DeNOx routes that consume NO through NH2, and thus variations in this channel redistribute the NH2 pool and directly modulate NO consumption. These findings reinforce that even ppm-level residual ammonia can fundamentally alter the kinetic control of NO by inducing a rapid transition toward NH3 dominated chemistry in fuel-rich cracked ammonia flames. Practically, these trends indicate conditional tolerance to imperfect cracking in rich-premixed RQL stages, provided that post-flame residence time and mixing are sufficient to sustain NH/NH2-driven DeNOx.

4. Conclusions

In this study, the effect of residual ammonia in cracked ammonia fuel on NO formation was investigated using one-dimensional premixed flame simulations in gas turbine-relevant conditions. The main findings are as follows.
Under lean conditions, residual ammonia directly increases NO emissions. NO forms rapidly within the flame front and remains almost constant in the post-flame region, leading to an approximately linear increase in final NO with residual ammonia concentration. Contribution and pathway analyses reveal a shift in the dominant formation route from thermal and N2O mechanisms to fuel NO chemistry governed by HNO and related intermediates.
Under rich conditions, NO emissions remain low, typically below about 10 ppm, even with increasing residual ammonia. Although a substantial amount of NO is generated in the reaction zone, strong DeNOx reactions in the post-flame region consume most of the NO formed, resulting in small net emissions.
These results indicate that when residual ammonia is present, the active NO chemistry more closely resembles that of an ammonia flame than a hydrogen flame. The findings provide a mechanistic basis for defining cracking targets and for designing staged combustion strategies to control NOx in practical cracked ammonia systems.
This work is limited to detailed kinetic simulations and does not include combustor-scale effects or residence time distributions. Specifically, the present 1D model assumes a spatially homogeneous distribution of residual NH3 with perfect premixing. In practical combustors, however, local inhomogeneities due to incomplete mixing or non-ideal reforming may create regions with elevated NH3, potentially leading to locally enhanced NO formation. Accordingly, the NO emissions predicted here should be viewed as a lower-bound estimate corresponding to idealized, perfectly premixed conditions.
Even so, the rich flame results suggest that a rich premixed stage in an RQL combustor could be relatively tolerant to imperfect cracking provided that post-flame residence time and mixing are sufficient, and the actual tolerance will depend on mixing and quench histories. As an intermediate step toward full CFD and experiments, CRNs can represent staged combustion, capture mixing and quenching effects, and test whether the DeNOx potential observed here persists under realistic RQL conditions.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en18236334/s1, Table S1: Top 5 reactions and their sensitivity coefficients for NO production at lean (ϕ = 0.5) conditions with 0 ppm, 1000 ppm and 10,000 ppm residual NH3. Table S2: Top 5 reactions and their sensitivity coefficients for NO production at rich (ϕ = 2.0) conditions with 0 ppm, 1000 ppm and 10,000 ppm residual NH3. Figure S1: Final NO mole fraction as a function of residual NH3 in the low-concentration (0–100 ppm) range under fuel-lean (ϕ = 0.5) conditions at 20 atm and 673 K. Figure S2: Final NO mole fraction as a function of residual NH3 in the low-concentration (0–100 ppm) range under fuel-rich (ϕ = 2.0) conditions at 20 atm and 673 K.

Author Contributions

Conceptualization, S.P.; methodology, D.K. and S.P.; software, D.K. and J.K.; validation, D.K., S.P. and J.K.; formal analysis, D.K.; investigation, D.K.; resources, S.P.; data curation, D.K.; writing—original draft preparation, D.K.; writing—review and editing, D.K., S.P.; visualization, D.K.; supervision, S.P.; project administration, S.P.; funding acquisition, S.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments

During the preparation of this manuscript, the author(s) used Gemini2.5 (Google) for the purposes of refining language and improving the manuscript’s structure. The authors have reviewed and edited the output and take full responsibility for the content of this publication.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
NOxNitrogen oxides
ROPRate of production
TCRThermochemical recuperation
CFDComputational fluid dynamics
CRNChemical reaction network
LESLarge eddy simulation
RQLRich–quench–lean
IDTIgnition delay time
LBVLaminar burning velocity
PSRPerfectly stirred reactor

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Figure 1. One-dimensional premixed cracked ammonia/air flames at varying residual ammonia levels as a function of equivalence ratio: (a) maximum flame temperature and (b) NO concentration (15% dry O2 basis).
Figure 1. One-dimensional premixed cracked ammonia/air flames at varying residual ammonia levels as a function of equivalence ratio: (a) maximum flame temperature and (b) NO concentration (15% dry O2 basis).
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Figure 2. (a) NO concentration (15% dry O2 basis) at a residence time of 20 ms and (b) maximum mole fraction of key intermediate species (HNO, NH2, and NH), and (c) maximum mole fraction of key radicals (H, O, OH) as a function of residual ammonia at a fuel-lean equivalence ratio (ϕ = 0.5).
Figure 2. (a) NO concentration (15% dry O2 basis) at a residence time of 20 ms and (b) maximum mole fraction of key intermediate species (HNO, NH2, and NH), and (c) maximum mole fraction of key radicals (H, O, OH) as a function of residual ammonia at a fuel-lean equivalence ratio (ϕ = 0.5).
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Figure 3. NO concentration (15% dry O2 basis) as a function of residence time at a fuel-lean equivalence ratio (ϕ = 0.5) with varying residual ammonia contents.
Figure 3. NO concentration (15% dry O2 basis) as a function of residence time at a fuel-lean equivalence ratio (ϕ = 0.5) with varying residual ammonia contents.
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Figure 4. Reaction pathway diagrams for NO formation in fuel-lean (ϕ = 0.5) premixed cracked ammonia/air flames at residual ammonia concentrations of (a) 0 ppm, and (b) 10,000 ppm.
Figure 4. Reaction pathway diagrams for NO formation in fuel-lean (ϕ = 0.5) premixed cracked ammonia/air flames at residual ammonia concentrations of (a) 0 ppm, and (b) 10,000 ppm.
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Figure 5. NO formation contribution of major reaction groups in fuel-lean (ϕ = 0.5) premixed cracked ammonia/air flames at varying residual ammonia concentrations.
Figure 5. NO formation contribution of major reaction groups in fuel-lean (ϕ = 0.5) premixed cracked ammonia/air flames at varying residual ammonia concentrations.
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Figure 6. A-factor sensitivity analysis for NO concentration at a residence time of 20 ms in premixed cracked ammonia/air flames (ϕ = 0.5) for residual ammonia (0, 1000, and 10,000 ppm).
Figure 6. A-factor sensitivity analysis for NO concentration at a residence time of 20 ms in premixed cracked ammonia/air flames (ϕ = 0.5) for residual ammonia (0, 1000, and 10,000 ppm).
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Figure 7. (a) NO concentration (15% dry O2 basis) at a residence time of 20 ms and (b) maximum mole fraction of key intermediate species (HNO, NH2, and NH), and (c) maximum mole fraction of key radicals (H, O, OH) as a function of residual ammonia at a fuel-rich equivalence ratio (ϕ = 2.0).
Figure 7. (a) NO concentration (15% dry O2 basis) at a residence time of 20 ms and (b) maximum mole fraction of key intermediate species (HNO, NH2, and NH), and (c) maximum mole fraction of key radicals (H, O, OH) as a function of residual ammonia at a fuel-rich equivalence ratio (ϕ = 2.0).
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Figure 8. NO concentration (15% dry O2 basis) as a function of residence time at a fuel-rich equivalence ratio (ϕ = 2.0) with varying residual ammonia contents.
Figure 8. NO concentration (15% dry O2 basis) as a function of residence time at a fuel-rich equivalence ratio (ϕ = 2.0) with varying residual ammonia contents.
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Figure 9. Cumulative NO from production and consumption and the resulting net NO in fuel-rich flames (ϕ = 2.0) for varying residual ammonia concentrations, integrated over a residence time from −1 ms to 20 ms.
Figure 9. Cumulative NO from production and consumption and the resulting net NO in fuel-rich flames (ϕ = 2.0) for varying residual ammonia concentrations, integrated over a residence time from −1 ms to 20 ms.
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Figure 10. Reaction pathway diagrams for NO formation in fuel-rich (ϕ = 2.0) premixed cracked ammonia/air flames at residual ammonia concentrations of (a) 0 ppm, and (b) 10,000 ppm.
Figure 10. Reaction pathway diagrams for NO formation in fuel-rich (ϕ = 2.0) premixed cracked ammonia/air flames at residual ammonia concentrations of (a) 0 ppm, and (b) 10,000 ppm.
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Figure 11. NO formation and consumption contribution of major reaction groups in fuel-rich (ϕ = 2.0) premixed cracked ammonia/air flames at varying residual ammonia concentrations: (a) production contribution; and (b) consumption contribution.
Figure 11. NO formation and consumption contribution of major reaction groups in fuel-rich (ϕ = 2.0) premixed cracked ammonia/air flames at varying residual ammonia concentrations: (a) production contribution; and (b) consumption contribution.
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Figure 12. A-factor sensitivity analysis for NO concentration at a residence time of 20 ms in premixed cracked ammonia/air flames (ϕ = 2.0) for residual ammonia (0, 1000, and 10,000 ppm).
Figure 12. A-factor sensitivity analysis for NO concentration at a residence time of 20 ms in premixed cracked ammonia/air flames (ϕ = 2.0) for residual ammonia (0, 1000, and 10,000 ppm).
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Table 1. Simulation conditions and key properties of 1D premixed cracked ammonia/air flames with varying residual ammonia contents.
Table 1. Simulation conditions and key properties of 1D premixed cracked ammonia/air flames with varying residual ammonia contents.
Residual NH3 [ppm]01001000500010,000
Cracking Ratio [%]100.0099.9899.899.0098.02
FuelXNH300.00010.0010.0050.01
XH20.750.7499250.749250.746250.7425
XN20.250.2499750.249750.248750.2475
Flame Temp. [K]ϕ = 0.51850.01849.41848.51846.81840.7
ϕ = 2.02164.82164.82164.22162.02158.0
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Kim, D.; Kim, J.; Park, S. Residual Ammonia Effects on NO Formation in Cracked Ammonia/Air Premixed Flames. Energies 2025, 18, 6334. https://doi.org/10.3390/en18236334

AMA Style

Kim D, Kim J, Park S. Residual Ammonia Effects on NO Formation in Cracked Ammonia/Air Premixed Flames. Energies. 2025; 18(23):6334. https://doi.org/10.3390/en18236334

Chicago/Turabian Style

Kim, Donghyun, Jiwon Kim, and Sungwoo Park. 2025. "Residual Ammonia Effects on NO Formation in Cracked Ammonia/Air Premixed Flames" Energies 18, no. 23: 6334. https://doi.org/10.3390/en18236334

APA Style

Kim, D., Kim, J., & Park, S. (2025). Residual Ammonia Effects on NO Formation in Cracked Ammonia/Air Premixed Flames. Energies, 18(23), 6334. https://doi.org/10.3390/en18236334

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