Experimental and Kinetic Study of Laminar Burning Velocities for NH₃/CH₄/O₂/NO/CO₂ Premixed Flames

Abstract

Ammonia, as a promising carbon-neutral fuel, has attracted growing attention for blended combustion applications from academia to industry. Low-NO_x-combustion strategies such as staged combustion, oxygen-enriched combustion, and exhaust gas recirculation may lead to ammonia combustion in CO₂-rich and NO-rich environments. In this work, the laminar burning velocities (S_L) in NH₃/CH₄/O₂/NO/CO₂ flames with various ammonia blended ratios under atmospheric pressure were investigated using the heat flux method. The addition of NO to the oxidizer significantly enhances S_L, with the enhancement factor ξ proportional to the NO fraction in the oxidizer and strongly dependent on the fuel composition. Chemical effects rather than thermal-diffusion effects dominate the enhancement of S_L. Kinetic analysis shows that NO actively participates in the reaction network during the early flame stage, promoting the formation of key radicals such as H and OH through pathways like NH₂ + NO = NNH + OH and NNH = N₂ + H, thereby accelerating chain-branching and sustaining flame propagation.

1. Introduction

In recent years, ammonia (NH₃) has attracted growing attention as a promising carbon-neutral fuel for blended combustion applications. However, one of the major challenges associated with its utilization is the formation of nitrogen oxides (NO_x) during combustion, which poses a significant barrier to its practical deployment. To address this issue, various combustion strategies aimed at reducing NO_x emissions have been extensively investigated, including staged combustion [1], oxygen-enriched combustion [2,3], and exhaust gas recirculation (EGR) [4,5]. These techniques often lead to ammonia combustion occurring in flue gas environment rich in CO₂ and NO_x, resulting in flame propagation behaviors that differ significantly from those in conventional fuel-air combustion systems. To better exploit these low-NO_x ammonia combustion technologies and develop reliable chemical kinetic models, it is essential to understand the flame propagation characteristics of ammonia under CO₂-diluted conditions and to evaluate the influence of NO addition on the combustion process.

The effect of CO₂ dilution on hydrocarbon flame propagation characteristics has been extensively studied in the context of carbon capture and storage (CCS) research [6,7,8,9]. Studies under both atmospheric and high-pressure conditions have shown that CO₂ dilution and N₂ dilution both reduce combustion rates. However, CO₂ exhibits a more pronounced inhibitory effect on laminar burning velocity (S_L) due to its higher specific heat capacity and its participation in chemical reactions such as the reverse of CO + OH = CO₂ + H, which competes with the chain-branching reaction H + O₂ = O + OH for H radicals. In contrast, investigations into the impact of CO₂ dilution on ammonia flame propagation characteristics remain relatively limited. Shi et al. [10] conducted experimental and kinetic modeling studies on the laminar burning velocities of pure ammonia and NH₃/DME mixtures in O₂/CO₂ atmospheres. Zhu et al. [11] further investigated the synergistic effects of CO₂ dilution and H₂ addition on the laminar combustion characteristics of NH₃/CH₄ mixtures under high-temperature and high-pressure conditions. Their results revealed that CO₂-involved reduction reactions, such as NH + CO₂ = HNO + CO and the reverse of CO + OH = CO₂ + H, significantly suppress the S_L by interfering with key chain-branching pathways involving H and NH radicals.

The effect of NO addition in the oxidizer on the laminar burning velocity of ammonia fuels has been less extensively studied compared to other aspects of ammonia combustion. Mei et al. [12] first investigated the propagation characteristics of NH₃/NO/N₂ laminar flames at ambient temperature and pressure. Subsequently, Monnier et al. [13] and Liu et al. [14] extended these measurements to elevated temperatures and pressures, enabling a more realistic assessment of the kinetic interactions between NH₃ and NO under combustion-relevant conditions. Their results indicate that NH₃ primarily undergoes oxidation through the NH₃-NH₂-NNH-N₂ pathway in NO/N₂ atmospheres, with the reaction NH₂ + NO = NNH + OH playing a crucial role. However, these studies all treated NO as the sole oxidant, which significantly deviates from practical application scenarios, and no work has previously been reported on laminar burning velocities under conditions involving both of CO₂ dilution and NO addition.

Building upon the aforementioned background, the laminar flame propagation of NH₃/CH₄ in an O₂/CO₂ atmosphere without and with NO addition was investigated in this work using the heat flux method. The specific objectives of the present study are as follows: (1) to extend the laminar burning velocity dataset of NH₃/CH₄ flames under O₂/CO₂ conditions, thereby providing essential data for the validation of chemical kinetic models; (2) to quantify the thermal-diffusive, dilution, and chemical effects induced by CO₂ dilution and NO addition; and (3) to investigate the influence mechanism of CO₂ dilution and NO addition on NH₃/CH₄ flame propagation through sensitivity analysis.

2. Methodology

2.1. Heat Flux Method and Test Conditions

The heat flux method, originally proposed by Bosschaart and de Goey [15], has been widely adopted for measuring laminar burning velocities under a wide range of temperature and pressure conditions. Figure 1 presents the schematic diagram of the heat flux measurement setup. A porous plate with a diameter of 30 mm is installed at the center of the top burner disk, featuring hole diameters and center-to-center spacing of 0.3 mm and 0.4 mm, respectively. This optimized configuration effectively reduces local flame stretch and suppresses cellular flame instability, thereby promoting the formation of a smooth and spatially uniform flame front. In the heat flux method, the heat loss from the flame to the porous plate is counterbalanced by the convective heat gain of the incoming unburnt premixed gas flowing through the plate. When thermal equilibrium is achieved, indicated by a zero radial temperature gradient across the plate, an adiabatic, one-dimensional, stretch-free flat flame is established, and the laminar burning velocity can be determined from the incoming unburnt gas flow rate. Given that the flames stabilized on the heat flux burner propagate downwardly, the denser, unburned mixture is located below the lighter, burned flue gas. This stratification is inherently gravitationally stable, meaning that buoyancy forces act to suppress hydrodynamic instabilities rather than promote them. Consequently, the influence of buoyancy on the measured burning velocities is negligible.

Detailed operational conditions of the present laminar burning velocity measurements are summarized in

Table 1. Experimental conditions of NH₃/CH₄ at T_u = 353 K.

Fuel	Oxidizer Components				Equivalence Ratio (ϕ)	Tu (K)	Pu (atm)
	O2 (%)	NO (%)	N2 (%)	CO2 (%)
0% NH3/100% CH4	21	-	79	-	0.6–1.4	353	1
	30–40	-	-	60–70	0.6–1.4	353	1
30% NH3/70% CH4	21	-	79	-	0.6–1.4	353	1
	30–40	-	-	60–70	0.6–1.4	353	1
	30–40	0.5/1.0/2.0	-	58–69.5	0.6–1.4	353	1
50% NH3/50% CH4	21	-	79	-	0.6–1.4	353	1
	30–40	-	-	60–70	0.6–1.4	353	1
	30–40	0.5/1.0/2.0	-	58–69.5	0.6–1.4	353	1

. To prevent the formation of ammonium salts via the reactions NH₃ + CO₂ + H₂O = NH₄HCO₃ and 2NH₃ + CO₂ + H₂O = (NH₄)₂CO₃ from affecting the composition of the gas mixture, all measurements were conducted at atmospheric pressure and an unburned mixture temperature (T_u) of 353 K, under which NH₄HCO₃ is nearly completely decomposed. No powder deposits were observed in the gas delivery lines before or after the experiments, confirming the absence of solid-phase formation. Furthermore, to mitigate the flame edge wrinkling and tilting commonly observed in ammonia-containing flames, which may affect the stability of the planar flame front, the burner plate was heated to 443 K using dimethicone as the heating medium. The gas flow rates were precisely controlled by mass flow controllers (Alicat, Tucson, AZ, USA). High-purity gases (NH₃, CH₄, CO₂, NO, O₂, and N₂: 99.999%) were used in this study.

2.2. Assessment of the Measurement Uncertainties

The uncertainty of S_L (${∆S}_L$) in this study was evaluated following the methodology established in previous investigations [16]. Two primary contributors to the overall uncertainty were considered: (1) ${∆S}_L^{TC}$, originating from burner plate temperature measurements using thermocouples, and (2) ${∆S}_L^{MFC}$, arising from gas flow controller. The total measurement uncertainty is, therefore, expressed as follows:

$${∆S}_L={∆S}_L^{TC}+{∆S}_L^{MFC}={\sigma }_c/s+S_L·{\left({\sum \left({∆Q}_i\right)}^2\right)}^{0.5}/Q_{tot}$$

(1)

where $s$ is the measurement sensitivity defined as $s={\displaystyle \frac{dC}{du}}{|}_{u = S_L}$, with $C$ being the parabolic coefficient that was interpolated to infer S_L; ${∆Q}_i$ is the uncertainty of the $i$ th mass flow controller; $Q_{tot}$ is the total flow rate of all gases; ${\sigma }_c$ is the mean standard deviation of the thermocouple temperature, which has a specific constant value for a given porous plate, and can be calculated as the following expression:

$${\sigma }_c=sqrt\left[{\displaystyle \frac{\sum {\left(T_i-C{r}_i^2-T_{center}\right)}^2}{\left(N-2\right)\sum {\left(r_i^2-r^2\right)}^2}}\right]$$

(2)

2.3. Kinetic Simulation

Numerical simulations were conducted using the PREMIX module of Chemkin-Pro, which is specifically designed to model the propagation characteristics of one-dimensional, steady-state, adiabatic premixed flames. The chemical kinetic mechanisms employed in the simulations are the CEU-NH3-Mech 1.1 mechanism [17] (CEU-2022), and the mechanisms from Okafor et al. [18] (Okafor-2019), Shrestha et al. [19] (Shrestha-2021), Zhang et al. [20] (Zhang-2023), and Konnov et al. [21,22] (Konnov-2023). These parenthetical serve as abbreviated names for the mechanisms and will be used consistently throughout the subsequent analysis. Among them, the CEU-2022 mechanism developed by our group is an updated version of the original CEU-NH₃-Mech mechanism, which has been widely used in studies [23,24] on laminar flame propagation of ammonia-hydrocarbon blended fuels and has demonstrated accurate predictive capability for burning velocities. And the updated mechanism was validated according to the comparison with experimental data [17] obtained under elevated pressure conditions. The Okaafor-2019, owing to its highly reduced reaction network and high computational efficiency, has been widely adopted in CFD simulations of ammonia-fueled combustion. In addition, mechanisms such as the Shrestha-2021, the Zhang-2023, and the Konnov-2023, have been developed through iterative refinement based on extensive validation against experimental laminar burning velocity data for NH₃, NH₃/H₂, and NH₃/C₁ fuel systems. These mechanisms exhibit robust predictive performance across a range of operating conditions and are now recognized as representative chemical kinetic models in the field of ammonia combustion research.

For each case studied, a minimum of 800 grid points was utilized to ensure sufficient spatial resolution, and the adaptive grid refinement strategy was applied with the values of GRAD and CURV set to 0.01 to ensure grid independence. The thermal diffusion (Soret effect) and multicomponent transportation were considered in all simulation cases. Additionally, the effect of radiative heat loss was also considered as suggested by Nakamura and Shindo [25].

3. Results and Discussion

3.1. Effects of CO₂ Dilution on S_L of NH₃/CH₄ Flames

Figure 2 presents the measured and predicted laminar burning velocities of NH₃/CH₄ mixtures as a function of equivalence ratio for various ammonia blended ratios under different oxidizer compositions. It should be pointed out that the laminar burning velocities obtained in this work agree well with those under similar CO₂-diluted conditions in the literature [26,27]. A significant reduction in laminar burning velocity (S_L) is observed when the diluent in the oxidizer is changed from N₂ to CO₂, and the S_L only exceeds that under air atmosphere when the oxygen concentration in the O₂ + CO₂ mixture is increased to 40%. It is primarily attributed to the higher specific heat capacity of CO₂ and its role as a combustion product of CH₄ oxidation, both of which effectively reduce the flame temperature and inhibit the forward progress of the methane oxidation reaction. Consistent with the trends observed in CH₄ flames, the equivalence ratio corresponding to the peak S_L of NH₃/CH₄ flames shifts significantly toward the fuel-lean side when the oxidizer is changed from O₂/N₂ to CO₂-diluted conditions. Specifically, the peak S_L occurs at an equivalence ratio of 1.05–1.10 under air atmosphere, whereas it shifts to 0.95–1.05 under CO₂ dilution. Moreover, ammonia blending further accentuates this shift in the peak burning velocity toward the fuel-lean side.

The Shrestha-2021 significantly overpredicts the laminar burning velocities across all tested conditions, while the Okafor-2019 tends to underpredict them. Although the Konnov-2023 accurately captures the burning velocities of NH₃/CH₄ flames under air-fired conditions, it exhibits a stronger underprediction of burning velocities under CO₂-diluted atmospheres, yielding simulation results even lower than those obtained with the Okafor-2019. In contrast, the simulations using the CEU-2022 and Zhang-2023 mechanisms produce nearly identical predictions and show excellent agreement with the experimental data in NH₃/CH₄ flames: only the S_L near stoichiometric equivalence ratio is slightly underpredicted, but all other simulated values fall within the experimental uncertainty bounds.

Nonetheless, the overall agreement confirms that the heat flux method employed in the present study enables accurate measurement of laminar burning velocities for ammonia-containing fuels with an unburned gas temperature of 353 K under CO₂ dilution. The CEU-2022 and Zhang-2023 provide quantitatively reliable predictions of S_L of NH₃/CH₄ flames under both air and CO₂-diluted conditions, with minor deviations observed only under specific blending and dilution combinations. However, the CEU-2022 consists of only 91 species and 444 elementary reactions, whereas the Zhang-2023 includes 152 species and 1388 reactions. Given its significantly higher computational efficiency and improved convergence behavior, subsequent numerical investigations in this work are based on the CEU-2022 mechanism.

3.2. Effects of NO Addition on S_L of NH₃/CH₄/O₂/NO/CO₂ Flames

Given that ammonia combustion inherently promotes the formation of fuel-NO_x, the exhaust gas from ammonia-containing fuels typically contains a non-negligible concentration of NO, in addition to CO₂. This residual NO can be recirculated in systems employing exhaust gas recirculation (EGR) or oxy-fuel combustion with CO₂ recycle, thereby reintroducing NO into the fresh mixture. The presence of NO in the reactants can significantly influence flame chemistry through redox interactions with NH₃, which play a crucial role in the combustion characteristics of ammonia-containing fuels and may alter the reaction pathways, flame stability, and overall burning velocity.

As shown in Figure 3, the addition of NO in the oxidizer leads to a significant enhancement in S_L across all equivalence ratios and oxygen concentrations tested. For example, at ϕ = 1.0 and 35% O₂, the introduction of 2% NO increases S_L by up to 15.4%/18.0% to the NO-free case for 30% and 50% NH₃ blended, respectively. Despite this enhancement, the overall trend of S_L versus equivalence ratio remains unchanged, and the peak burning velocity continues to occur within the ϕ = 0.95–1.0 range, consistent with the lean-shifted behavior induced by CO₂ dilution. The CEU-2022 mechanism captures the general trend of S_L increase with NO addition and reproduces the location of the peak velocity. However, it systematically underpredicts the magnitude of S_L, particularly in the equivalence ratio range of 0.95–1.0, where the burning velocity reaches its maximum, and the deviation exceeds the experimental uncertainty bounds. While on the fuel-lean and fuel-rich sides of the equivalence ratio (ϕ < 0.85, ϕ > 1.15), the deviations between simulation and experiment generally fall within the measurement uncertainty.

To quantify the effect of NO addition on the laminar burning velocity of NH₃/CH₄/O₂/CO₂ flames, a normalized enhancement parameter $\xi$ defined in previous work [28] was employed:

$$\xi =\left(S_{L,NO}-S_L^0\right)/S_L^0\times 100\%$$

(3)

where $S_{L,NO}$ is the S_L with NO addition and $S_L^0$ represents the corresponding value without NO addition under identical conditions.

Figure 4 presents the experimental and simulation results of S_L enhancement at 353 K and 1 atm, as a function of equivalence ratio for various ammonia blended ratios, oxygen concentrations, and NO contents. Despite significant scatter in the measured data points, particularly under off-stoichiometric conditions, the enhancement factor $\xi$ still provides a more intuitive representation of the promoting effect of NO on the laminar burning velocity, which is positively corrected with both the ammonia blended ratio and the NO concentration, with $\xi$ exhibiting an approximately linear dependence on the NO content. A clear trend is observed that the maximum enhancement occurs at fuel-lean conditions, and the enhancement effect weakens with increasing equivalence ratio, reaches a minimum near stoichiometric conditions, and then increases again under fuel-rich conditions. Such non-monotonic trend may be attributed to the lower reaction rates and flame temperatures in fuel-lean and fuel-rich conditions compared to stoichiometric conditions. Additionally, as the oxygen concentration increases from 30% to 40%, the $\xi$ under 2% NO condition at the stoichiometric equivalence ratio decreases from 25.88% and 21.39% to 16.36% and 15.43% for 50% and 30% NH₃ blended, respectively. This reduction in enhancement at higher O₂ levels is attributed to the elevated baseline reactivity and flame temperature, which diminishes the relative promoting effect of NO. In other words, as the intrinsic combustion intensity increases with O₂, the incremental benefit of NO addition becomes less pronounced.

Considering the experimental uncertainties, the CEU-2022 mechanism shows excellent agreement with the measured enhancement factors on the fuel-lean side and near the stoichiometric condition. However, noticeable discrepancies are observed on the fuel-rich side (ϕ > 1.20), where the model tends to underpredict or overpredict $\xi$ depending on oxygen concentration. These discrepancies may be partially attributed to experimental uncertainties associated with flame instability caused by flame front wrinkling or cellular flame formation during the measurement of burning velocities under fuel-rich conditions. Nevertheless, the overall trend of $\xi$, including the U-shaped dependence on equivalence ratio and its positive correlation with NO concentration and ammonia blending ratio, is well captured by the CEU-2022 mechanism.

To further evaluate and compare the influence of NO addition versus direct oxygen enrichment on the laminar burning velocity of NH₃/CH₄/O₂/CO₂ flames, two additional experimental cases were designed, and the results are presented in Figure 5. In these cases, the oxidizer compositions were designed to maintain either a constant oxygen mole fraction (36% O₂ vs. 35% O₂ + 2% NO) or a constant dilution level (37% O₂ vs. 35% O₂ + 2% NO). As shown in Figure 5, the addition of NO results in a significantly higher S_L compared to the 36% O₂ condition, with the enhancement becoming increasingly pronounced as the ammonia blended ratio increases from 30% to 50%. For the 30% NH₃ blend, the values of $\xi$ are comparable to those under the 37% O₂ condition, and for the 50% NH₃ blend, the enhancement even surpasses that achieved under the 37% O₂ condition. This indicates that NO is not merely a passive diluent or minor contaminant, but an active chemical promoter that can outperform additional oxygen in enhancing flame propagation, particularly in ammonia-rich conditions. The superior promoting effect of NO can be attributed to the high reactivity of NO in the reaction of NH₃/CH₄ oxidation. Unlike O₂, which primarily acts as an oxidizing agent and typically requires reactions such as R1 to generate reactive species like O and OH radicals before participating in the oxidation process, NO can directly participate in chain-branching and radical-propagating reactions (e.g., R2–R7) and promote the formation of key radicals such as OH, NH₂, HNO, and HONO. The introduction of NO effectively activates alternative reaction pathways that sustain the radical pool and accelerate the overall oxidation process, particularly under fuel-lean and fuel-rich conditions where flame temperatures are relatively low and radical chain reactions are inherently weaker. This mechanistic behavior explains the previously observed trend that the enhancement factor $\xi$ is higher under off-stoichiometric conditions compared to stoichiometric mixtures.

H + O₂ = O + OH

(R1)

NH₃ + NO = NH₂ + HNO

(R2)

NH₂ + NO = NNH + OH

(R3)

NH + NO = N₂ + OH

(R4)

CH₃ + HNO = NO + CH₄

(R5)

NO + H = HNO

(R6)

NO + OH = HONO

(R7)

3.3. Contribution of Thermal-Diffusion and Chemical Effects on S_L of NH₃/CH₄ Flames

For flames in the O₂/CO₂ atmosphere, the impact of CO₂ dilution is typically investigated considering three distinct effects: the dilution effect, the thermal-diffusion effect, and the chemical effect. Similarly, the influence of NO addition can also be decomposed into these three perspectives. Following the methodology used in previous studies on hydrocarbon fuels [29], the conventional fictitious diluent gas method was employed in the numerical simulations using the CEU-2022 mechanism to isolate and quantify each individual effect. Specifically, two hypothetical inert diluent gases, FCO₂ and FNO, whose thermodynamic and transport properties are identical to those of CO₂ and NO, respectively, were introduced in the laminar flame propagation simulations. Among all the simulated cases involving NO or FNO addition, the NO or FNO concentration in the oxidizer was consistently fixed at 2%.

As shown in Figure 6a,b, the laminar burning velocities simulated using different diluent gases are compared, enabling a clear distinction between the three contributions of CO₂ dilution and NO addition. Since the concentrations of N₂, FCO₂, and CO₂ are identical across the relevant cases, the dilution effect of CO₂ can be considered negligible. Consequently, the difference between simulated S_L of N₂ dilution and FCO₂ dilution reflects the thermal-diffusion effect. Similarly, the chemical effect can be derived from the difference between the simulated S_L of FCO₂-diluted and CO₂-diluted conditions. These contributions are quantified using the following normalized contribution factors:

$${CF}_{thermal-diffusion, { CO}_2}={\displaystyle \frac{S_L\left[N_2\right]-S_L\left[{FCO}_2\right]}{S_L\left[N_2\right]-S_L\left[{CO}_2\right]}}$$

(4)

$${CF}_{chemical, { CO}_2}={\displaystyle \frac{S_L\left[{FCO}_2\right]-S_L\left[{CO}_2\right]}{S_L\left[N_2\right]-S_L\left[{CO}_2\right]}}$$

(5)

It should be noted that, unlike CO₂, NO itself acts as an oxidizing agent. Whether FNO is treated as a reactive oxidant in the flame simulations significantly affects the resulting burning velocity. To account for this, two distinct inert reference cases are defined. When FNO is not considered an oxidizing agent, referred to as FNO-1 addition, the difference between FNO-1 addition and CO₂-diluted condition arises solely from the differences in thermodynamic and transport properties between FNO and CO₂. And the corresponding differences between simulated S_L can be attributed to the thermal-diffusion effect. In contrast, when FNO is treated as an oxidizing agent, referred to as FNO-2 addition, the nominal equivalence ratio is calculated based on the assumption that FNO contributes to oxidation, leading to a lower nominal ϕ than the actual value, which results in an apparent shift in the S_L—ϕ curve toward the fuel-lean side. Therefore, the difference in simulated S_L between the FNO-2 and FNO-1 conditions reflects the dilution effect associated with NO addition, specifically, its influence on the effective oxygen concentration in the premixed gas. Finally, the chemical effect is isolated by comparing the simulated S_L of real NO cases with that of the FNO-2 cases, ensuring that the chemical contribution is evaluated under consistent stoichiometric definitions. These three contributions are quantified using the following normalized contribution factors:

$${CF}_{thermal-diffusion, NO}={\displaystyle \frac{S_L\left[FNO-1\right]-S_L\left[{CO}_2\right]}{S_L\left[NO\right]-S_L\left[{CO}_2\right]}}$$

(6)

$${CF}_{dilution, NO}={\displaystyle \frac{S_L\left[FNO-2\right]-S_L\left[FNO-1\right]}{S_L\left[NO\right]-S_L\left[{CO}_2\right]}}$$

(7)

$${CF}_{chemical, NO}={\displaystyle \frac{S_L\left[NO\right]-S_L\left[FNO-2\right]}{S_L\left[NO\right]-S_L\left[{CO}_2\right]}}$$

(8)

As shown in Figure 6c,d, the thermal-diffusion effect dominates the reduction of S_L in CO₂-diluted atmosphere by contributing to over 60% of the total effect due to the low thermal conductivity and high specific heat of CO₂. The contribution factor (CF) of chemical effect increases significantly with rising O₂ concentration and reaches its maximum at an equivalence ratio of approximately 1.10. However, as illustrated in Figure 6e,f, the ${CF}_{thermal-diffusion, NO}$ is markedly reduced, accounting for less than 25% of the total effect under NO addition conditions, where NO constitutes only 2% of the oxidizer. Instead, the dilution effect plays a more prominent role because of the reduction in O₂ concentration and the corresponding shift in the nominal equivalence ratio caused by the inert dilution of FNO. The ${CF}_{dilution, NO}$ decreases monotonically with increasing ϕ, transitioning from a promoting effect on S_L under fuel-lean conditions to an inhibiting effect under fuel-rich conditions. This trend arises because the inclusion of FNO as an oxidizer overestimates the effective oxygen availability, resulting in the nominal ϕ being lower than the actual value. This assumption shifts the fuel-lean side closer to stoichiometric conditions while pushing the fuel-rich side further away, thereby inducing the opposite trend in the contribution factor. Moreover, the combined contribution of the dilution and thermal-diffusion effects also exhibits a monotonic decreasing trend with increasing ϕ and reaches zero in the range of ϕ = 1.05–1.15, except for a minor non-monotonic behavior is observed under 30% O₂ conditions in the range of ϕ = 0.6–0.7, attributed to the initial increase in ${CF}_{thermal-diffusion, NO}$.

Critically, this phenomenon indicates that the chemical effect becomes the dominant mechanism for flame propagation enhancement under NO addition. Moreover, all three positive contribution factors under fuel-lean conditions further explain the reason why the most significant enhancement in laminar burning velocity always occurs in the fuel-lean regime.

3.4. Sensitivity and Kinetic Analyses

To further illustrate the influence mechanism of CO₂ dilution and NO addition on laminar burning velocities of NH₃/CH₄ flames, a sensitivity analysis was conducted for stoichiometric conditions under three different atmospheres with comparable laminar burning velocities, as shown in Figure 7. The variation in reaction sensitivity coefficients from AIR to CO₂ conditions, particularly the significant increase in the sensitivity of (R8), is primarily attributed to the elevated oxygen mole fraction, which enhances chain-branching pathways and promotes radical generation. In contrast, the addition of 2% NO does not markedly alter the sensitivity weights of most reactions. However, it strongly promotes the forward direction of (R9), leading to accelerated OH radical accumulation. Concurrently, the substantial consumption of NH₂ radicals intensifies the reverse reaction of (R10), increasing the competition for H atoms. These effects suppress the forward rate of (R8), thereby modulating the overall flame propagation behavior.

H + O₂ = O + OH

(R8)

NH₂ + NO = NNH + OH

(R9)

NH₂ + O = HNO + H

(R10)

Crucially, the sensitivity analysis results in Figure 7 support the earlier discussion on the relative contributions of thermal-diffusion, dilution, and chemical effects: under CO₂ conditions, the chemical effect does not dominate, as no CO₂-involving reactions appear among the top-ranked sensitivity coefficients. In contrast, under NO condition, the chemical effect of NO addition becomes overwhelmingly dominant, which is clearly reflected in the sensitivity analysis, where the reaction (R9) exhibits a significantly enhanced sensitivity coefficient despite NO constituting only 2% of the oxidizer mixture. This highlights the exceptionally high reactivity of NO in nitrogen-containing fuel chemistry.

Figure 8 illustrates the reaction pathways for stoichiometric NH₃/CH₄ flames under CO₂-diluted conditions, both without and with NO addition. The overall reaction network remains largely similar before and after NO addition. However, the flux contributions of reactions involving NO as a reactant, such as NH₂ + NO = NNH + OH and NH + NO = N₂ + OH, show a noticeable increase, while the relative fluxes of competing reactions are reduced. Moreover, the fraction of NO converted to N₂ increases from 22.1% to 23.6%, indicating that NO addition promotes, to some extent, the conversion of fuel-Nitrogen toward N₂, thereby potentially reducing the formation of nitrogenous pollutants.

The pathway analysis underscores not only the transformation of nitrogen species but also the central role of key radicals in flame propagation. Specifically, active radicals such as H and OH drive chain-propagation reactions that sustain the combustion process, while NH₂ serves as a critical indicator for NH₃ oxidation. Additionally, NO is the critical emission species that warrants close attention in ammonia combustion research. Therefore, Figure 9 presents the temperature and mole fraction profiles of H, OH, and NH₂, along with NO formation, for stoichiometric NH₃/CH₄ flames under different atmosphere conditions, further elucidating the coupling between radical dynamics and flame propagation. Although the 21% O₂/(O₂ + N₂) condition exhibits the highest S_L and a higher mole fraction of H radicals, its OH mole fraction is significantly lower than those under the two CO₂-diluted conditions, primarily due to the substantially lower oxygen concentration. In contrast, the addition of NO in the oxidizer simultaneously increases the flame temperature and the concentrations of H, OH, and NH₂ radicals. And the NH₂ concentration exhibits a strong positive correlation with flame temperature, which can be attributed to the highly temperature-dependent NH₃ decomposition reactions (e.g., NH₃ + OH = NH₂ + H₂O).

Notably, the non-monotonic variation in NO mole fraction further indicates that NO is not merely a combustion product, but actively participates in reactions at the upstream edge of the flame front during the early reaction stage when the temperature rises rapidly. Although the addition of NO partially suppresses the H + O₂ = O + OH reaction, the primary source of OH radicals in conventional hydrocarbon combustion, it still promotes the generation and accumulation of H and OH radicals through alternative pathways. Specifically, the reaction NH₂ + NO = NNH + OH directly produces OH radicals, while the subsequent decomposition of NNH (e.g., NNH = N₂ + H) contributes to H radical formation. These NO-involved reactions collectively enhance the radical pool, thereby sustaining and even accelerating flame propagation in NH₃/CH₄ systems despite the inhibition of classical chain-branching routes.

However, it should not be overlooked that despite the generally good agreement between simulation results and experimental measurements, certain discrepancies still exist. The underlying mechanisms of CO₂ dilution and NO addition, particularly the dual role of NO as both a combustion intermediate and a reaction promoter, require further experimental investigation. This underscores the urgent need for accurate, spatially resolved measurements of key parameters beyond laminar burning velocity, including concentration profiles of radicals (e.g., H, OH, NH₂) and major species (e.g., NO), as well as flame temperature. Such comprehensive experimental data are essential for placing stronger constraints on kinetic mechanism development and for achieving a deeper understanding of the coupling between thermal and chemical effects in NH₃/CH₄ combustion systems under CO₂-rich and NO-doped conditions.

4. Conclusions

This work investigated the laminar burning velocities of NH₃/CH₄ flames under CO₂-diluted conditions both without and with NO addition using the heat flux method, providing a comprehensive dataset for validating chemical kinetic mechanisms. The influence mechanisms of CO₂ dilution and NO addition in the oxidizer were further explored through numerical simulations based on the CEU-2022 mechanism. The new findings are summarized below:

(1)

The addition of NO to the mixture stream significantly enhances the laminar burning velocity of NH₃/CH₄ premixed flames, with the promoting effect becoming increasingly pronounced as both the NO concentration and the ammonia blended ratio rise.

(2)

Thermal-diffusion effects dominate the reduction in laminar burning velocity (S_L) of NH₃/CH₄ premixed flames under CO₂ dilution. In contrast, the enhancement in S_L induced by NO addition is almost entirely attributed to the chemical effects and the dilution effects, which exhibit a transition from promoting to inhibiting as the equivalence ratio increases, and are nonnegligible.

(3)

Kinetic analysis indicates that NO deeply participates in NH_X chemistry during the early reaction stage and exerts a significant promoting effect on the generation and accumulation of H and OH radicals through key reaction pathways.