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. 2026 Jan 8;11(2):3150–3161. doi: 10.1021/acsomega.5c09757

An Experimental and Kinetic Modeling Study of NO2 Sensitization on the Autoignition of Ammonia

Fangjie Liu 1, Junjie Cheng 1, Xin Wang 1,*, Zhixie Chen 1, Jiaqi Tian 1, Runsheng Cao 1
PMCID: PMC12824713  PMID: 41585670

Abstract

An experimental and kinetic modeling study was performed to investigate the influence of NO2 addition on the autoignition characteristics of NH3/O2/Ar mixtures. Ignition delay times were measured using a shock tube at temperatures of 1450–2200 K, pressures of 0.14 and 1 MPa, equivalence ratios of 0.5, 1.0, and 1.5, and NO2 concentrations from 0 to 2000 ppm. The results show that NO2 addition significantly promotes NH3 autoignition, with the enhancement strongly dependent on temperature, pressure, equivalence ratio, and NO2 concentration. The effect is particularly pronounced at intermediate temperatures (1450–1700 K) and higher pressure, and exhibits a nonlinear relationship with NO2 concentration. The updated Shrestha and Glarborg mechanisms accurately reproduced the experimental IDT data. The evolution of key species indicates that NO2 addition significantly promotes NH3 consumption, along with the formation of NO and the radicals OH and NH2. Flux analysis reveals that NO2 addition introduces a novel ammonia consumption pathway via NH3 + NO2 = NH2 + HONO, facilitating rapid NH2 radical production. HONO decomposition further accelerates OH radical formation. Reaction rate analysis confirms that NO2 promotes OH generation through NO2 + H = NO + OH, especially in the early stages of NH3 oxidation. Moreover, NO2 addition significantly alters NH2 consumption pathways, with both NO2 and NO participating in reactions with NH2, not only accelerating its consumption but also suppressing NH2 self-recombination and other radical-consuming channels.

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1. Introduction

Ammonia has recently emerged as a promising carbon-free fuel, with significant potential for application in marine engines and gas turbines. , The industrial synthesis of ammonia is mature and widely employed, supported by a robust production infrastructure and supply chain that facilitate its large-scale deployment as an alternative fuel. Ammonia has a higher energy density than hydrogen and enables more efficient storage and transport, , which demonstrates greater potential for practical applications. It can be used directly as a fuel in spark-ignition engines or blended with other fuels for combustion, including hydrogen, − methane, natural gas, diesel, , biodiesel, and dimethyl ether.

Exhaust gas recirculation (EGR) has been employed in engines using ammonia fuel blends to control the combustion process and mitigate harmful emissions. − Nie et al. investigated the combustion and emission performance of an engine fueled with ammonia-diesel dual-fuel under varying EGR rates, demonstrating that higher EGR rates decreased in-cylinder pressure and heat release rate, prolonged the combustion duration, and significantly reduced NO x and NH3 emissions. The impact of EGR on engine combustion can be attributed to three primary effects: dilution (caused by inert gases), thermal effects (due to increased heat capacity), and chemical interactions. − Early investigations mainly focused on the dilution and thermal effects. , More recently, the chemical effects of EGR have attracted increasing attention as researchers deepen their understanding of combustion chemical kinetics. The composition of engine exhaust gases, measured using gas chromatography and Fourier transform infrared (FTIR) spectroscopy, primarily consists of O2, H2O, CO2, NO, NO2, CO, CH2O, and other trace species. In ammonia-fueled engines, NO x is the predominant reactive species in the EGR stream. When mixed with fresh intake air, the temperature of the EGR gas decreases significantly, promoting the further conversion of NO to NO2 by dilution and oxidation. Multiple studies have demonstrated that NO2 has a significant impact on fuel autoignition characteristics, as summarized in Table S1 of the Supporting Information.

NO2 addition markedly affects the ignition delay time (IDT) of fuels, with its sensitization effect depending on fuel type, temperature, pressure, and blending concentration. NO2 can significantly reduce the IDT of light hydrocarbon fuels (including methane, ethane, ethylene, propylene, butane, and n-pentane), − particularly under conditions of high pressure, low temperature, and fuel-rich mixtures. The reduction in IDT is primarily attributed to NO2 and NO (formed through the conversion of NO2), both of which accelerate the consumption of stable intermediates such as CH3 radicals and facilitates the conversion of HO2 radicals into key chain-initiating OH radicals. The effect of NO2 on the oxidation of medium- and long-chain n-alkanes, such as n-heptane and n-dodecane, is strongly temperature dependent. At elevated temperatures (T > 860 K), NO2 promotes chain-branching through reactions with free radicals like CH3 and HO2, enhancing fuel oxidation. At temperatures below 700 K, NO2 readily reacts with intermediate species like heptyl radicals (C7H15 + NO2 = C7H15O + NO), competing with the conventional low-temperature oxidation pathway (C7H15 → C7H15O2 → C7H14OOH), thereby reducing OH radical generation. For short-chain alcohol fuels such as methanol and ethanol, the addition of NO2 also results in a notable reduction in IDT. For methanol, the addition of NO2 can generate the intermediate HONO via the reaction CH3OH + NO2 = HONO + CH2OH, which rapidly decomposes to produce OH radicals and NO, increasing the concentration of free radicals. Additionally, the NO + HO2 (NO + HO2 = NO2 + OH) and NO2 + H (NO2 + H = NO + OH) reactions form a cyclic pathway, continuously regenerating OH radicals and enhancing system reactivity.

To the best of our knowledge, a lack of shock tube experimental data and thorough kinetic studies has limited understanding of ammonia autoignition in the presence of NO2. Accordingly, IDTs of NH3/O2/Ar mixtures with NO2 concentrations ranging from 0 to 2000 ppm were measured in a shock tube. In addition, the reaction mechanism describing the interactions between NO2 and NH3 was updated, and detailed chemical kinetic analyses, including reaction rate, sensitivity, and reaction pathway analyses, were performed to elucidate the mechanism by which NO2 affects the autoignition characteristics of ammonia.

2. Experimental Method and Computational Model

2.1. Experimental Method

The IDTs of NH3/O2/Ar mixtures with varying NO2 concentrations were investigated using a stainless steel shock tube, as schematically illustrated in Figure . The shock tube is fabricated from cold-drawn stainless steel tubing with an internal diameter of 100 mm and a wall thickness of 17.5 mm. The high-pressure driver section is 4 m in length, whereas the low-pressure driven section extends 5.5 m. Detailed descriptions of the shock tube design and operation can be found in our previous publications. − Due to the strong adsorption tendency of NH3 on metallic surfaces, both the mixing system and the low-pressure driven section were passivated before each experiment, following the method described by Mathieu et al. In particular, 5 kPa of NH3 was introduced into the system and held for 15 min to ensure complete surface adsorption, after which the system was evacuated to 10 Pa. The test gas mixtures were prepared in a 295 L stainless steel mixing tank. The shock tube was evacuated to a pressure lower than 5 Pa using a vacuum pump before each experiment. Gas mixtures were formulated according to Dalton’s law of partial pressures, and the detailed compositions are provided in Table . The purities of the gases used were 99.99% for NH3 and NO2, and 99.999% for Ar and O2. The prepared gas mixtures were allowed to equilibrate for 12 h to ensure complete homogeneity before being introduced into the shock tube.

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Simplified schematic of the shock tube apparatus.

1. Mixture Compositions and Experimental Conditions Investigated in This Study.

no. φ NH3 (%) O2 (%) Ar (%) NO2 (%) p (MPa)
1 0.5 2 3 95 0 0.14, 1
2 0.5 2 3 94.95 0.05 0.14, 1
3 0.5 2 3 94.9 0.1 0.14, 1
4 0.5 2 3 94.8 0.2 0.14, 1
5 1 2 1.5 96.5 0 0.14, 1
6 1 2 1.5 96.45 0.05 0.14, 1
7 1 2 1.5 96.4 0.1 0.14, 1
8 1 2 1.5 96.3 0.2 0.14, 1
9 1.5 2 1 97 0 0.14, 1
10 1.5 2 1 96.95 0.05 0.14, 1
11 1.5 2 1 96.9 0.1 0.14, 1
12 1.5 2 1 96.8 0.2 0.14, 1
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Three high-frequency response pressure transducers (PCB 113B24) were installed in the low-pressure driven section, 20 cm from the end wall and equally spaced along the axial direction. The transit times were recorded as the incident shock wave passed the three sensors in sequence, and the incident shock speed at the end wall (Vs) was then determined by linear extrapolation. The initial temperature (T5) and pressure (p5) behind the reflected shock wave were calculated using the Gaseq software in conjunction with the ideal shock relations. , A photomultiplier tube (PMT, Hamamatsu CR131) fitted with a narrow-band filter centered at 310 ± 2 nm was mounted at the same axial location as PCB3 to detect the OH* chemiluminescence signal. All signals were recorded simultaneously using a four-channel high-speed oscilloscope, with PCB1 serving as the trigger, to capture the shock wave passage and the OH* chemiluminescence. The IDT was defined as the interval between the arrival of the reflected shock wave at the measurement location and the intersection of the baseline with the tangent at the maximum slope of the rising OH* chemiluminescence, as illustrated in Figure . The arrival time of the reflected shock wave was determined by the onset of the second pressure rise recorded by PCB3, whereas the ignition point was determined from the OH* chemiluminescence recorded by the PMT. The primary source of uncertainty in the IDT measurement was the estimation of T5. The maximum uncertainty in the IDT, evaluated using the standard root-sum-square (RSS) method, , was estimated to be less than 15%. Considering additional contributions from uncertainties in mixture composition control and the measurement system response, the overall uncertainty of the IDT measurement in this study was estimated to be within 20%.

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Definition of IDT based on pressure trace and OH* signal.

To verify the reliability and measurement precision of the present shock tube facility, a benchmark test was conducted under the same conditions as reported by Mathieu et al. Specifically, the IDTs of the NH3/O2/Ar mixture at a pressure of 0.14 MPa were measured and compared with the corresponding literature data. As shown in Figure , the present results exhibit good agreement with the published data, with deviations well within the overall experimental uncertainty.

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Comparison of IDTs obtained in this study with those of Mathieu et al.

2.2. Chemical Kinetic Model

Five representative kinetic mechanisms, developed by Shrestha et al., Glarborg et al., Gotama et al., Zhang et al., and Sun et al., were employed to simulate the oxidation and autoignition of NH3/O2/Ar mixtures in the presence of NO2. The forward reaction rate parameters for the reactions of NH3 with NO2 and NO were compiled and are presented in Table . The rate constants were obtained from the theoretical calculations of Xu et al. , and Mebel et al. The five mechanisms include some or all of the reverse reactions associated with these three reactions. The specific details are as follows:

NH3+NO2=NH2+HONO R1

2. Rate Parameters for NH3 Reactions with NO2 and NO.

  reaction rate (cm, mol, s, cal)
  
reaction A n E source
R1: NH3 + NO2 = NH2 + HONO 4.818 × 10–2 4.25 22,972
R2: NH3 + NO2 = NH2 + HNO2 2.451 3.41 29,880
R3: NH3 + NO = NH2 + HNO 1.463 × 103 3.12 54,410
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The latest ab initio chemical kinetics data for this reaction was provided by Xu et al., with both the forward and reverse rate constants for the reaction being reported. The mechanism developed by Glarborg et al. uses the reverse reaction and reduces the A-factor by 30%, while Shrestha et al. employed the rate constants from Glarborg mechanism for this reaction.

NH3+NO2=NH2+HNO2 R2

The reverse of this reaction is included only in the mechanisms by Shrestha et al. and Glarborg et al., with the corresponding rate constants derived from the work of Thaxton et al.

NH3+NO=NH2+HNO R3

Xu et al. provided the latest ab initio chemical kinetics data for this reaction, including both the forward and reverse rate constants. The reverse reaction is included in the mechanisms of Shrestha et al., Glarborg et al., Zhang et al., and Sun et al.

Given that this study primarily focuses on the forward reactions between NH3 and NO2, the three forward reactions from Table were incorporated into each of the kinetic mechanisms, while the corresponding reverse reactions in the five mechanisms were deleted for the simulations in this study. An assessment of the impact of the updated forward reaction rate constants on IDTs, speciation, and emissions was performed using the Shrestha mechanism, as detailed in Section S2 of the Supporting Information.

3. Results and Discussion

3.1. Experimental Results

The effect of NO2 addition on the ignition characteristics of ammonia was experimentally measured using the shock tube at two pressures (0.14 and 1 MPa) under various equivalence ratios (φ = 0.5, 1.0, and 1.5) and NO2 concentrations (500, 1000, and 2000 ppm), with the detailed mixture compositions and experimental conditions listed in Table . The updated Shrestha, Glarborg, Gotama, Zhang, and Sun mechanisms were used to calculate the IDTs. Figures and S4 compare the experimental data with the simulation results from these mechanisms. Overall, NO2 addition generally promotes the ignition of NH3,but the extent of this promotion varies significantly with temperature, equivalence ratio, and NO2 concentration.

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Measured (symbols) and predicted (lines) IDTs of NH3/O2/Ar mixtures with varying NO2 addition. (a) p = 0.14 MPa, φ = 0.5; (b) p = 0.14 MPa, φ = 1; (c) p = 0.14 MPa, φ = 1.5; (d) p = 1 MPa, φ = 0.5; (e) p = 1 MPa, φ = 1; (f) p = 1 MPa, φ = 1.5. Solid lines: Shrestha mechanism; Dashed lines: Glarborg mechanism.

Figures a-f illustrate the effect of NO2 on the IDTs of NH3, with measured data and predictions using the updated Shrestha and Glarborg mechanisms. The predictions from both mechanisms show good agreement with the experimental data. The effect of NO2 addition on the IDT is strongly dependent on the reaction temperature, particularly within the intermediate temperature range of 1450–1700 K, where its sensitizing effect is especially pronounced. For example, according to simulations based on the Shrestha mechanism shown in Figure e (p = 1 MPa, φ = 1), the IDT at T = 1500 K is approximately 1807 μs without NO2, but is significantly reduced to about 1098 μs when 500 ppm of NO2 is added, corresponding to a reduction of about 39%. At higher temperatures (T > 1800 K), the sensitizing effect of NO2 gradually diminishes, indicating that the NH3–O2 system can independently generate sufficient free radicals to sustain fuel consumption. Such behavior has also been reported in NO2-containing fuel-air mixtures of C2H4 and DME. Additionally, the sensitizing effect of NO2 does not increase linearly with increasing concentration. Under the conditions illustrated in Figure a (p = 0.14 MPa, φ = 0.5), simulations based on the Shrestha mechanism indicate that at T = 1700 K, the IDT decreases from 1228 μs (without NO2) to 727 μs (with 500 ppm of NO2), and further declines to 620 μs (with 1000 ppm of NO2) and 495 μs (with 2000 ppm of NO2), corresponding to reductions of 41%, 50%, and 60%, respectively. Similar behavior has been reported for the n-C5H12/NO2 system, where the effect of NO2 on IDT reduction is attenuated when concentrations exceed 1000 ppm due to competition between NO2 and O2 for reactions with key intermediates, which suppresses the growth of the radical pool. Furthermore, as the equivalence ratio increases, the promoting effect of NO2 on NH3 ignition becomes more evident. For example, simulations using the Shrestha mechanism with 2000 ppm of NO2 addition show that at 0.14 MPa and T = 1500 K, the IDT reduction rates for equivalence ratios (φ = 0.5, 1.0, and 1.5) are 64.3%, 71.2%, and 73.1%, whereas at T = 2000 K, the corresponding IDT reductions for φ = 0.5, 1.0, and 1.5 are 57.4%, 61.8%, and 64.1%. Under fuel-rich conditions, oxygen deficiency limits chain-branching reactions, while the addition of NO2 can effectively facilitate oxidative pathways, thereby enhancing the reactivity of NH3. Under fuel-lean conditions, sufficient oxygen promotes the generation of chain-branching species such as OH radicals, so the promoting effect of NO2 is less significant than under rich conditions. This dependence on equivalence ratio has also been observed for other fuels such as ethylene, CH4, and DME. In addition, pressure significantly affects the sensitizing effect of NO2. At pressures around 1 MPa, the reduction in IDT is generally more pronounced than at 0.14 MPa, mainly due to the increased collision frequency, which accelerates free radical formation. Consistent with this trend, previous studies on the C5H12/NO2 system demonstrated that increasing the pressure from 10 to 30 bar led to an approximately 50% decrease in IDT at a constant temperature. Good overall agreement with the experimental data is observed for both mechanisms. The high predictive accuracy can be attributed to the more comprehensive representation of NO2-induced chain reactions, radical propagation pathways, and the NO/NO2 cycle, which confers enhanced adaptability.

3.2. Kinetic Analysis

To clarify the chemical kinetic mechanisms behind the substantial impact of NO2 addition on NH3 ignition behavior, comprehensive kinetic calculations were performed under two representative conditions  condition 1 (T = 1500 K, p = 1 MPa, φ = 1.0) and condition 2 (T = 2000 K, p = 0.14 MPa, φ = 0.5). These cases were simulated with NO2 concentrations of 0 and 1000 ppm using the Shrestha and Glarborg mechanisms. Detailed kinetic analyses were subsequently conducted, including species concentration profiles, key reaction rate assessments, sensitivity analyses, and reaction pathway flux analyses.

3.2.1. Evolution of Key Species

The time histories of the reactant NH3 and the key species NO2, NO, NH2, and OH under different conditions are shown in Figure . Figure a shows the simulated species profiles predicted by the Shrestha mechanism at condition 1. Without NO2 addition (dashed lines), the mole fraction of NH3 decreased slowly at first, and significant consumption was observed starting at around 1760 μs. The appearance of NO and OH occurred later. The OH radical concentration increased rapidly after 1760 μs, which aligns with the sharp consumption of NH3. When 1000 ppm of NO2 was added (solid lines), the NO2 concentration dropped rapidly at the beginning of the reaction. NO2 mainly promotes NH3 ignition through two dominant reaction pathways. First, NH3 reacts with NO2 via R1, forming NH2 and HONO, which initiates chain propagation by providing NH2 radicals while consuming NH3. The subsequent reaction NH2 + NO2 = H2NO + NO (R110) leads to the formation of H2NO and NO, resulting in increased NO formation rates and concentrations. The NO produced then participates in subsequent reactions such as NH2 + NO = N2 + H2O (R107) and NH2 + NO = NNH + OH (R108), which further accelerate NH2 consumption and enhance OH radical formation, thus enhancing the rapid oxidation of NH3.

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Time histories of NH3, NO2, NO, OH, and NH2 calculated using the Shrestha mechanism. (a) Condition 1. (b) Condition 2. Solid lines: with 1000 ppm of NO2; dashed lines: without NO2.

The simulation results obtained using the Glarborg mechanism (Figure S5) show an overall trend similar to that of the Shrestha mechanism, confirming that NO2 addition significantly promotes NH3 consumption and radical formation. Meanwhile, the onset of rapid NH3 depletion is predicted slightly earlier with the Glarborg mechanism than with the Shrestha mechanism, and the NH2 peak concentration is correspondingly higher. Figures b and S5b respectively present the simulation results of the Shrestha and Glarborg mechanisms under condition 2. At 2000 K, the amount of OH produced is significantly greater than under condition 1. This increase is mainly due to enhanced OH radical generation through reactions such as H + O2 = O + OH (R4) and H2 + O = H + OH (R5), which amplifies the role of OH in the overall system. Although the addition of NO2 still facilitates NH3 consumption and increases radical concentrations under these conditions, its effect on advancing the ignition timing is less pronounced than at lower temperatures.

3.2.2. Reaction Rate Analyses

The influence of NO2 addition on the reaction rates of key species under condition 1 is illustrated in Figure , calculated using the Shrestha mechanism. The consumption of NO2 mainly occurs through reactions with NH3, NH2, and H, as shown in Figure a. Specifically, NO2 is primarily consumed via three dominant pathways: (1) reaction with NH3 to produce NH2 and HONO (R1); (2) reaction with NH2 to form H2NO and NO (R110); and (3) reaction with H (R244: NO2 + H = NO + OH) to generate NO and OH. The HONO formed subsequently decomposes (R242: HONO = NO + OH), supplying additional NO and OH radicals, which increase the OH concentration and promote further NH3 oxidation. The NO generated from the reaction with NH2 not only participates in subsequent reactions but also reacts with HO2 (R243: NO + HO2 = NO2 + OH) to regenerate NO2, forming a NO-NO2 cycle that further enhances the reactivity of the system.

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Reaction rate analysis of key species under condition 1 using the Shrestha mechanism. (a) NO2 consumption, (b) NH3 consumption, (c) NH2 consumption, (d) NO production, (e) NO consumption, and (f) OH production. Solid lines: with 1000 ppm of NO2; dashed lines: without NO2.

A comparison of NH3 consumption rates with and without NO2 addition is provided in Figure b. In the absence of NO2, NH3 consumption mainly relies on reactions with active radicals such as OH, H, and O, with dominant pathways including NH3 + OH = NH2 + H2O (R116), NH3 + H = NH2 + H2 (R114), and NH3 + O = NH2 + OH (R115). With NO2 addition, an extra and faster reaction pathway (R1) is introduced. Due to its relatively high rate constant, R1 rapidly becomes dominant in the early reaction stage, quickly producing NH2 radicals. It is also evident that with NO2 addition, the reaction between NH3 and NO2 increases significantly, and the rates of NH3 reacting with OH, H, and O radicals likewise rise, indicating an overall increase in the radical pool concentration. This indicates that NO2 acts not only as an oxidant directly consuming NH3 but also as an initiator for radical chain propagation.

The NH2 radical is a key intermediate species in the NH3 oxidation process, and its consumption rate (Figure c) strongly influences the overall reaction rate, the formation of intermediate species, and ignition behavior. Without NO2 addition, NH2 is mainly consumed through reactions with NO, H, OH, and by self-recombination, with dominant pathways including NH2 + NO = NNH + OH (R108), NH2 + H = NH + H2 (R91), NH2 + OH = NH + H2O (R97), and NH2 + NH2 = NH3 + NH (R104). With NO2 addition, the NH2 consumption pathways shift. NH2 reacts with NO2 to produce NO and H2NO (R110). The substantial formation of NO also increases the reaction rates of NH2 with NO (R107 and R108), promoting further NH2 consumption.

As shown in Figure d, without NO2 addition, NO formation mainly depends on the decomposition of HNO. With NO2 addition, NO production is strongly enhanced. NO2 directly contributes to NO formation through its reaction with NH2 (R110), which significantly accelerates NO generation during the early reaction stage. This reaction becomes one of the main pathways for NO production. Due to the high rate of the reaction between NH2 and NO2, the NO concentration rises sharply in the initial stage. The NO produced further accelerates reactions such as R107 and R108 and participates in the NO-NO2 cycle through reactions like R243, which promotes OH formation and sustains chain propagation.

Figure e reveals that, NO consumption mainly occurs through pathways such as R107, R108, NH + NO = N2O + H (R85), and N + NO = N2 + O (R73). With NO2 addition, the NO consumption rate is significantly increased, especially for reactions with NH2, which show higher rates than those without NO2 addition. This enhancement is due to the elevated NO and NH2 radical concentrations resulting from NO2 addition, which promote the rates of R107 and R108. The rapid accumulation of NO also promotes its subsequent reactions with NH (R85) and N (R73). More importantly, NO2 reacts with NH3 in the early stage to form NH2 and HONO, and the HONO rapidly decomposes into NO and OH (R242). The increased OH promotes further NH3 oxidation and sustains high NH2 levels, which continuously enhances NO consumption through the NH2 + NO pathways (R107 and R108).

According to Figure f, the OH radical is a key intermediate that sustains the chain propagation of NH3 oxidation and plays a crucial role in determining the IDT. Without NO2 addition, OH formation mainly relies on H–O reactions, such as R4 and R5. This limits the initial radical concentration and results in a lower OH production rate, leading to a longer ignition delay. With NO2 addition, the OH production pathways change significantly. Reactions such as R243 and R244 show higher reaction rates in the early stage and become important OH sources, enabling rapid OH accumulation. Compared with the conventional pathways R4 and R5, the reaction rates of R243 and R244 are significantly higher during the initial stage, indicating that NO2 addition effectively enhances OH formation. This reaction pathway mitigates the limitation of relying solely on conventional OH formation routes (R4 and R5) and significantly increases the OH radical concentration, thereby shortening the ignition delay of NH3.

The reaction rate analysis of key species under Condition 2 using the Shrestha mechanism is shown in Figure . Compared to Condition 1, the higher temperature and oxygen-rich environment of Condition 2 enhances the contribution of conventional H–O reactions, such as R4 and R5, to OH radical production. Although NO2 addition still promotes OH formation via R243 and R244 during the initial reaction phase, the relative contribution of these NO2-assisted pathways is lower than under Condition 1. Additionally, the direct reaction between NH3 and NO2 via R1 shows a significantly lower reaction rate, shifting NH3 consumption toward hydrogen abstraction by OH, H, and O radicals. Moreover, the rate of NH2 self-recombination via R104 increases markedly during the initial reaction phase under Condition 2.

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Reaction rate analysis of key species under condition 2 using the Shrestha mechanism. (a) NO2 consumption, (b) NH3 consumption, (c) NH2 consumption, (d) NO production, (e) NO consumption, and (f) OH production. Solid lines: with 1000 ppm of NO2; dashed lines: without NO2.

Reaction rate analyses of key species under Condition 1 and Condition 2 using the Glarborg mechanism are presented in Figure S6 and Figure S7, respectively. Compared to the Shrestha mechanism, the same key reaction pathways are identified for each species in the Glarborg mechanism. However, minor differences in reaction rates at corresponding reaction times are observed, contributing to the slight discrepancies in the predicted IDTs between the two mechanisms.

3.2.3. Sensitivity Analyses of IDTs

To identify the key elementary reactions governing the ignition behavior of NH3 with NO2 addition, sensitivity analyses of IDTs (τ) were conducted. In each case, the rate constant of every reaction was individually doubled and halved to evaluate its effect on the corresponding IDT. The sensitivity coefficient is defined as ,

Si=ln(τ+τ−)ln(k+k−)=ln[(τ(2.0ki)τ(0.5ki)]ln(2.0ki0.5ki)≈τ(2.0ki)‐τ(0.5ki)1.5τ(ki)

Here, τ+ and τ– represent the IDTs calculated with the increased and decreased rate constants, respectively. A negative sensitivity coefficient indicates that the reaction promotes ignition and reduces the IDT, whereas a positive value corresponds to a delay in ignition.

Figure shows the normalized sensitivity coefficients of key reactions under various conditions. Across all cases, the reaction R4 consistently exhibits the most strongly negative sensitivity coefficient, indicating that it is the most critical pathway for radical generation during ignition. This reaction rapidly produces OH radicals, thereby accelerating chain-branching processes. When NO2 is added under Condition 2, the negative sensitivity of this reaction is slightly amplified, suggesting that NO2 promotes OH radical formation and marginally enhances the overall reactivity of the system. This observation is consistent with the IDT sensitivity analysis, which shows similar effects with and without NO2 addition to CH4. Moreover, NO2 addition significantly increases the sensitivity of several other key reaction pathways. Reaction R108 exhibits a notably negative sensitivity coefficient without NO2, consistent with the IDT sensitivity analysis reported by Glarborg et al., while a stronger negative sensitivity is observed with NO2 addition, highlighting its growing role in promoting ignition. This reaction consumes NH2 radicals, thereby suppressing pathways that reduce radical concentrations, such as R99 (NH2 + HO2 = NH3 + O2). Moreover, it generates NNH, which further reacts with O atoms via R137 (NNH + O = N2 + OH), contributing to OH radical accumulation and sustaining the chain reaction. Additionally, reaction R1 becomes considerably more sensitive with NO2 addition, serving as an important radical source in the early stages of reaction. This reaction promotes NH3 consumption and facilitates the formation of NH2 and HONO, the latter of which decomposes via R242 to produce OH radicals, thereby increasing their concentration. Without NO2, the sensitivity of this pathway is nearly negligible. Notably, NO2 addition also introduces a competing inhibitory pathway, NH2 + NO2 = N2O + H2O (R109), which consumes NH2 radicals and generates H2O and the less reactive species N2O, resulting in an overall suppressive effect on ignition. Furthermore, reaction R114 generally exhibits a positive sensitivity coefficient across different conditions.

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Normalized sensitivity analysis of IDTs using the Shrestha mechanism. (a) Condition 1. (b) Condition 2.

A comparison between Condition 1 and Condition 2 shows that under higher temperature and oxygen-rich conditions (Condition 2), the positive sensitivity coefficient of reaction R109 is significantly reduced. This suggests that such conditions are more favorable for radical-generating reactions. Additionally, the negative sensitivity coefficient of reaction R108 is also weakened under Condition 2, indicating that the promoting effect of NO2 addition on ignition is less pronounced at higher temperatures and oxygen concentrations. This trend aligns with the experimental and simulation results in Figure , which show a diminished effect of NO2 on IDTs at elevated temperature. Furthermore, comparison of sensitivity analyses between the Shrestha (Figure ) and Glarborg (Figure S8) mechanisms shows high consistency in the key reactions identified.

3.2.4. Flux analyses

The effect of NO2 addition on NH3 oxidation was investigated through reaction flux analyses with and without 1000 ppm of NO2. Flux calculations were conducted at 5% NH3 consumption using the Shrestha mechanism, as illustrated in Figure . Notably, NO2 addition introduces an additional NH3 consumption pathway. With 1000 ppm of NO2 added, reaction R1 accounts for 1.8% and 3.5% of NH3 consumption under Conditions 1 and 2. This pathway promotes the conversion of NH3 to NH2, along with the formation of the intermediate species HONO. As noted earlier, at high temperatures, HONO thermally decomposes to form OH and NO, thereby initiating further OH radical production. Consistent contributions from R1 are observed for the Glarborg mechanism (Figure S9), with 1.2% and 2.6% NH3 consumption under Conditions 1 and 2. Another significant effect of NO2 addition is the increased contribution of OH radical-mediated hydrogen abstraction to total NH3 consumption. Under Condition 1, NO2 addition increases NH3 consumption via reaction R116 from 57.2% to 73.0% for the Shrestha mechanism and from 55.8% to 70.4% for the Glarborg mechanism. A comparable increase is also observed under Condition 2, confirming the sustained enhancement of OH-mediated NH3 consumption. Similar mechanisms have also been reported in NO2-assisted oxidation of other fuels, including methane and n-pentane.

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Flux analysis of NH3 oxidation at 5% fuel consumption using the Shrestha mechanism. Black/red roman: without/with NO2 under condition 1; black/red italic: without/with NO2 under condition 2.

The addition of NO2 also markedly alters the consumption pathways of NH2. For the Shrestha mechanism, the fraction of NH2 consumed via R110 rises from 0.1% to 11.8% under Condition 1 and reaches 11.6% under Condition 2 with 1000 ppm of NO2 addition. A similar increase is observed for the Glarborg mechanism, where the corresponding values are 14.3% and 19.7% under Conditions 1 and 2. The generated H2NO, characterized by a highly polar N–O bond, exhibits high reactivity and can further decompose to form HNO, NO, and OH radicals, thereby promoting chain propagation. Without NO2, NH2 self-reactions constitute a larger fraction of NH2 consumption. For example, under Condition 1 for the Shrestha mechanism, NH2 consumption via R104, NH2 + NH2(+M) = N2H4(+M) (R183), and NH2 + NH2 = N2H2 + H2 (R146) accounts for 27.0%, 9.8%, and 0.2%, respectively. For the Glarborg mechanism, the corresponding proportions are 35.4%, 11.2%, and 0.3%. These reactions consume NH2 radicals and yield relatively stable species such as NH3, N2H2, and N2H4, leading to a reduction in the radical pool and inhibiting chain propagation. With NO2 addition, NH2 consumption via these radical-reducing pathways decreases substantially. These changes contribute to enhanced system reactivity by altered NH2 consumption pathways.

It is noteworthy that the generated NO participates in NH2 consumption via reactions R107 and R108. For instance, under Condition 1 for the Shrestha mechanism, NH2 consumption via R107 increases from 2.9% to 28.0%, while consumption via R108 increases from 2.5% to 24.2%. For the Glarborg mechanism, the corresponding values rise to 29.8% and 25.8%. These changes indicate that NO2 addition promotes NH2 consumption by increasing NO concentrations, thereby accelerating reactions between NO and NH2. From the perspective of chain reaction initiation, NO2 rapidly generates OH through reaction R244 at the onset of the reaction. In contrast to fuels like n-heptane and n-dodecane, which generate OH radicals through low-temperature reactions that promote the oxidation process, NH3 fuel has a limited capacity to generate OH radicals, restricting the chain initiation pathway. The addition of NO2 compensates for this limitation by promoting OH radical formation, thereby enhancing the oxidation process of NH3.

4. Conclusions

This study systematically investigates the effect of NO2 addition on the ignition characteristics of NH3 through shock tube experiments combined with detailed kinetic modeling. IDTs of NH3/O2/Ar mixtures were measured in a high-pressure shock tube over a wide temperature range, at pressures of 0.14 and 1 MPa, equivalence ratios of 0.5, 1.0, and 1.5, and NO2 concentrations from 0 to 2000 ppm. Complementary kinetic analyses were performed using the Shrestha and Glarborg mechanisms, including key species evolution, reaction rate analysis, sensitivity analysis, and reaction pathway flux analysis, to clarify the kinetic role of NO2 in NH3 oxidation.

The experimental results showed that NO2 addition significantly promotes NH3 autoignition, with the effect strongly dependent on temperature, equivalence ratio, pressure, and NO2 concentration. The promoting effect is most pronounced at intermediate temperatures (1450–1700 K) and higher pressure (1 MPa), but weakens at higher temperatures as the NH3–O2 system becomes more capable of sustaining radical production. The enhancement also exhibits a nonlinear relationship with NO2 concentration, as further increases beyond 1000 ppm lead to a progressively weaker promoting effect. Under fuel-rich conditions, the role of NO2 as a promoter becomes more significant, while under fuel-lean conditions, its effect is slightly weakened due to the sufficient radical pool generated by NH3 oxidation. Elevated pressures further amplify the promoting effect by accelerating radical formation. Both the Shrestha and Glarborg mechanisms captured the trend of NO2 promoting NH3 ignition.

The time histories of key species indicate that, without NO2, NH3 consumption proceeds slowly. In contrast, with the addition of 1000 ppm of NO2, NH3 consumption is significantly accelerated, accompanied by an earlier formation of NO, as well as NH2 and OH radicals. Reaction rate and flux analyses indicate that NO2 addition profoundly affects the reaction rates of key species and crucial reaction pathways. The addition of NO2 introduces new reaction pathways, such as R1 and R110, which not only directly consume NH3 and NH2, but also produce important intermediates like HONO and H2NO. HONO then decomposes rapidly to generate OH radicals and NO, both of which are essential for sustaining radical chain reactions. NO2 alters NH2 consumption pathways by reducing self-reactions that consume NH2 and produce stable species like NH3 and N2H4. With NO2 addition, these pathways decrease, enhancing system reactivity by promoting more efficient NH2 consumption. The NO produced in this process significantly influences the reaction pathways. It not only reacts with NH2 through pathways R107 and R108, but also participates in the NO-NO2 cycle by reacting with HO2 (via R243) to regenerate NO2. This continuous regeneration of NO2 enhances OH radical production, further accelerating the reaction. Sensitivity analyses of IDTs identified key reactions affecting NH3 ignition with and without NO2. Reaction R4, producing OH radicals, had the most negative sensitivity coefficient, highlighting its role in ignition promotion. NO2 addition enhanced this effect, enhancing OH formation. Reactions such as R108 became more sensitive with NO2, promoting ignition by consuming NH2 radicals and generating OH. Reaction R1 also became more sensitive with NO2, increasing OH radicals. Overall, NO2 addition enhances the oxidation process by promoting OH radical formation and altering key consumption pathways, leading to more efficient NH3 oxidation.

Understanding the role of NO2 in ammonia ignition and oxidation can aid in optimizing EGR strategies for ammonia engines, contributing to improved combustion stability and control.

Supplementary Material

ao5c09757_si_001.pdf (1.8MB, pdf)
ao5c09757_si_002.xlsx (11.5KB, xlsx)
ao5c09757_si_003.zip (55.5KB, zip)

Acknowledgments

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51906061) and the Department of Science and Technology of Henan Province (Grant No. 232102320212).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c09757.

  • Summary of studies on NO2 effects on fuel oxidation; assessment of the updated forward reaction rate constants; comparison of experimental data and simulation results from the Gotama, Zhang, and Sun mechanisms; and kinetic analysis results using the Glarborg mechanism (PDF)

  • Experimental conditions and measured ignition delay times (XLSX)

  • Updated kinetic mechanisms (ZIP)

F.L.: Investigation, original draft preparation, kinetic analysis. J.C.: Experimental measurements, writing–review and editing. X.W.: Conceptualization, project administration, funding acquisition, writing–review and editing. Z.C.: Experimental measurements, data analysis. J.T.: Investigation, kinetic analysis. R.C.: Experimental measurements, data analysis.

The authors declare no competing financial interest.

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Supplementary Materials

ao5c09757_si_001.pdf (1.8MB, pdf)
ao5c09757_si_002.xlsx (11.5KB, xlsx)
ao5c09757_si_003.zip (55.5KB, zip)

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