Study on the Ignition Characteristics of Ammonia Blended with C1–C4 Small-Molecule Alkanes

Abstract

With increasingly stringent greenhouse gas emission regulations, carbon emissions from marine engines have become a major concern, driving the shipping industry to actively explore efficient and clean alternative fuels. Among the various candidates, ammonia has attracted considerable attention in recent years due to its carbon-free nature and potential as a high-quality clean fuel. However, its practical application in marine engines is constrained by several inherent drawbacks, including a high auto-ignition temperature, low flame propagation speed, and low calorific value. Blending ammonia with natural gas has been demonstrated as an effective strategy to enhance its ignition performance. In this study, the ignition characteristics of NH₃/C1–C4 alkane mixed fuels were systematically investigated using numerical simulations. Rate of production (ROP) analysis, reaction pathway analysis, and other kinetic evaluation methods were employed to elucidate the underlying ignition mechanisms. The results reveal that blending NH₃ with C1–C4 alkanes significantly shortens the ignition delay time. When X_CH ≥ 30%, at high initial temperatures, the ignition-promoting effect is most pronounced for NH₃/C₂H₆ mixtures. In contrast, under low temperature conditions, ignition performance progressively improves with increasing carbon chain length of the blended alkane fuel. The ignition delay time across different operating conditions is primarily governed by highly reactive radicals, including O, H, and OH. Elevating the initial temperature, pressure, and blending ratio promotes the earlier formation of these key radicals and increases their production rates. ROP analysis of OH radicals indicates that reaction R10 (O₂ + H ⇌ OH + O) contributes most significantly to OH generation. Furthermore, reaction pathway analysis of NH₃ shows that at lower initial temperatures, NH₃ dehydrogenation is dominated by reactions with OH radicals. At higher temperatures, a greater fraction of NH₃ participates in NO reduction reactions, thereby decreasing the proportion of NH₃ involved in dehydrogenation pathways.

1. Introduction

The global energy crisis and environmental pollution have become pressing challenges worldwide. In the international shipping sector, approximately 90% of global trade is transported by sea, with the vast majority of vessels powered by internal combustion engines. In recent years, the rapid growth of international and cross-regional trade has exacerbated the environmental impacts associated with ship exhaust emissions. In response, the International Maritime Organization (IMO) has introduced increasingly stringent regulations on greenhouse gas emissions from ships, targeting a reduction of at least 50% in total emissions by 2050 relative to 2008 levels [1]. Under these tightening regulatory constraints, the adoption of low-carbon and zero-carbon fuels is widely regarded as the most promising pathway toward decarbonizing the shipping industry. Consequently, the development and application of novel alternative fuels have become a major research focus in recent years [2,3,4,5,6,7,8,9,10,11]. Among the various alternative fuels, ammonia has emerged as one of the most promising candidates for marine engine applications. Ammonia, recognized as an exceptional clean energy carrier capable of enabling zero-carbon emissions, possesses a high hydrogen content and can serve as a potential hydrogen source [12]. Moreover, ammonia combustion does not produce carbon-based emissions [13,14], further underscoring its potential as a clean fuel. Ammonia can be synthesized from a wide range of feedstocks, including fossil fuels (e.g., coal, natural gas, oil), industrial waste heat, or renewable energy sources, making it a viable transitional fuel during the energy transition [15,16,17]. However, its inherently poor ignition and combustion characteristics present significant challenges that limit its practical application in power machinery [18,19]. Compared with conventional fuels, pure ammonia exhibits a low flame propagation speed, a narrow flammability range, unstable combustion behavior, and a low heat release rate [20,21,22]. At present, blending ammonia with more reactive fuels in appropriate proportions is considered the most effective approach to improve its ignition characteristics and enhance overall combustion performance.

Peng et al. [23] investigated the ignition delay characteristics of ammonia using a shock tube at initial temperatures ranging from 1180 to 1941 K and pressures of 11–20 atm. Their results demonstrated that fuel concentration has a significant influence on ignition delay time. Nadiri et al. [24] measured the ignition delay times of NH₃/CH₃OH blended fuels in a shock tube at 10 bar and temperatures between 1050 and 1550 K and developed a systematic kinetic modeling framework. The results showed that the addition of a small amount of methanol can markedly shorten the ignition delay time. Compared with NH₃/alkane blends, NH₃/CH₃OH mixtures produce slightly higher NO emissions, while the formation of HCN is substantially suppressed. Xiao et al. [25] employed a shock tube to examine the ignition behavior of a mixture containing 60% ammonia and 40% methane. Ignition delay times of fuel–air mixtures were measured at equivalence ratios of 0.5, 1.0, and 2.0 over a temperature range of 1369–1804 K and pressures of 2 and 5 atm. The results indicated that, under otherwise identical conditions, increases in temperature, pressure, and methane content significantly enhance ignition performance, whereas variations in equivalence ratio exert a comparatively minor influence. Liao et al. [26] used a rapid compression machine (RCM) to measure ignition delay times of NH₃/CH₄ mixtures at temperatures of 910–1272 K and equivalence ratios of 0.5 and 1.0 under high pressures of 15 and 25 bar. Their findings revealed that methane exerts a strong promoting effect on ammonia ignition, with methane concentration identified as the dominant parameter governing ignition behavior. Similarly, Dai et al. [27] investigated ignition delay times using an RCM at equivalence ratios of 0.5, 1.0, and 2.0, pressures ranging from 20 to 70 bar, temperatures between 930 and 1140 K, and methane blending ratios of 0%, 5%, 10%, and 50%. The results showed that increasing the equivalence ratio from 0.5 to 2.0 had little effect on ignition delay time. Notably, at an equivalence ratio of 0.5, the addition of 5% methane reduced the ignition delay time by approximately a factor of five. Li et al. [28] conducted ignition experiments in an RCM at pressures of 20 and 40 bar, temperatures from 890 to 1110 K, equivalence ratios of 0.5, 1.0, and 2.0, and ethane blending ratios of 1%, 5%, and 10%. In addition, oxidation experiments of NH₃/C₂H₆ mixtures were performed in a jet-stirred reactor. The results demonstrated that ethane addition significantly enhances the auto-ignition reactivity and oxidation behavior of ammonia. Specifically, ethane provides additional OH radicals at relatively low temperatures and promotes interactions between hydrocarbon and ammonia chemistry. Jiang et al. [29] measured ignition delay times of NH₃/CH₄, NH₃/C₂H₆, and ammonia/natural gas mixtures at various ammonia blending ratios using a low-pressure shock tube over a temperature range of 1200–1800 K. The results indicated that ignition delay time increases with increasing ammonia fraction for all mixtures. Among them, the NH₃/C₂H₆ mixture exhibited significantly higher reactivity than NH₃/CH₄ and ammonia/natural gas mixtures. Zhang et al. [30] measured ignition delay times of NH₃/CH₃OH blended fuels with ammonia blending ratios of 20%, 40%, 80%, and 90% using an RCM at pressures of 15–25 bar and temperatures of 810–970 K. The experimental results showed that methanol addition effectively shortens the ignition delay time; however, when the methanol content exceeds 20%, further reductions in ignition delay time become negligible. Methanol significantly enhances OH radical formation, thereby increasing the overall reactivity of the mixture. Li et al. [31] investigated ignition delay times of NH₃/C₂H₅OH mixtures with ethanol blending ratios of 0%, 5%, 10%, and 30% in a shock tube at equivalence ratios of 0.5, 1.0, and 2.0, temperatures between 1250 and 1980 K, and pressures of 0.14 and 1.0 MPa. The results demonstrated that ethanol exhibits a strong and nonlinear ignition-promoting effect. For instance, compared with pure ammonia, the addition of 5% ethanol reduced the ignition delay time by more than 65%. This enhancement is attributed to the rapid consumption of ethanol during the early combustion stage, which generates abundant reactive radicals that accelerate ammonia oxidation.

Current research on ammonia–natural gas blended fuels has primarily focused on NH₃/CH₄ mixtures. However, natural gas is a multi-component fuel that also contains appreciable amounts of ethane, propane, and butane. The interaction mechanisms between ammonia and C2–C4 alkanes are not yet fully understood, and systematic investigations into their ignition delay behavior and underlying reaction chemistry remain limited. Moreover, comprehensive comparative studies examining the ignition characteristics of ammonia blended with C1–C4 alkanes are still scarce. To address these knowledge gaps, this study numerically simulates and analyzes the ignition characteristics of ammonia blended with C1–C4 alkane fuels and provides a systematic comparison among different alkane components. First, the effects of key operating parameters, including the equivalence ratio and alkane blending ratio, on the ignition delay times of NH₃/C1–C4 alkane mixtures are investigated. Subsequently, the kinetic mechanisms governing ignition delay are elucidated through analyses of key radical concentrations, rate of production (ROP), and reaction pathways. Finally, a comparative assessment of NH₃/C1–C4 alkane mixtures is conducted to clarify the distinct influences of different alkane additives on the ignition behavior of ammonia-based fuels.

2. Calculation Method

In this study, a zero-dimensional (0-D) homogeneous adiabatic reactor model implemented in the CHEMKIN-PRO 19.2 software was employed to simulate the ignition processes of NH₃/C1–C4 alkane mixtures. The model neglects heat exchange between the reactor and the surrounding environment and assumes uniform temperature and species concentration fields throughout the reactor. Owing to its computational efficiency, this approach enables rapid simulations and is well suited for large-scale calculations involving detailed chemical kinetic mechanisms. The fundamental assumptions of the 0-D homogeneous adiabatic reactor model are as follows: (1) the gas mixture within the reactor is perfectly homogeneous; (2) no heat transfer occurs between the reacting gas and the combustion chamber walls; (3) the reacting mixture behaves as an ideal gas; and (4) the reactor volume remains constant throughout the reaction process.

This study focuses on the ignition delay behavior, product yield characteristics, and reaction pathways of NH₃/C1–C4 alkane mixtures. To accurately capture the complex reaction processes involved, it is essential to employ a comprehensive chemical kinetic mechanism that ensures high simulation accuracy and reliability. Accordingly, five detailed chemical kinetic mechanisms, C3MechV4.0 [32], GalwayMech1.0 [33], NUIGMech1.3 [34], the Dong mechanism [35], and the Fang mechanism [36], were systematically evaluated and compared in this work. Experimental data reported in Refs. [29,37,38,39] were used to validate the predictive performance of these mechanisms. The comparative results are presented in Figure 1.

Comparison with the experimental data indicates that all five chemical kinetic mechanisms exhibit highly consistent trends in predicting the ignition delay times of the blended fuels. Although the more recent mechanisms, GalwayMech1.0 and C3MechV4.0, demonstrate higher accuracy than NUIGMech1.3 under certain operating conditions, NUIGMech1.3 performs more favorably in other regimes. Overall, the ignition delay times predicted by the five mechanisms are in close agreement. Based on these validation results, NUIGMech1.3 exhibits reliable predictive capability for the ignition behavior of NH₃/C1–C4 alkane blended fuels and is therefore well suited to support the objectives of this study. Consequently, NUIGMech1.3 was selected as the kinetic mechanism for all subsequent simulations presented in this work.

To investigate the ignition characteristics of blended fuels with different alkane fractions, the alkane blending ratio $X_{CH}$ is defined as follows:

$$X_{CH}={\displaystyle \frac{n_{CH}}{n_{CH}+n_{{NH}_3}}}$$

In this equation, $n_{CH}$ and $n_{{NH}_3}$ represent the mole fractions of the alkane and ammonia, respectively.

3. Results and Discussion

3.1. Study on the Ignition Characteristics of NH₃/CH₄ Mixtures

Figure 2 illustrates the effect of equivalence ratio on the ignition delay time of NH₃/CH₄ mixtures at a pressure of 20 bar, with methane blending ratios of 10% and 50%. As shown in Figure 2, increasing the methane content from 10% to 50% results in a pronounced reduction in ignition delay time, indicating that methane significantly promotes the ignition of ammonia. When the initial temperature exceeds 1000 K, the ignition delay time exhibits a slight increase with an increasing equivalence ratio. Over the equivalence ratio range of 0.5–2.0, the ignition delay time remains relatively unchanged, suggesting a weak dependence on equivalence ratio under these conditions. For both methane blending ratios (10% and 50%), the ignition delay time decreases markedly as the temperature increases from 1000 K to 1500 K, demonstrating that initial temperature plays a dominant role in governing mixture ignition. Consequently, the ignition delay time shows a high sensitivity to variations in initial temperature.

To elucidate the interaction mechanism between ammonia and methane during premixed ignition, the temporal evolution of key free-radical species was analyzed for mixtures with varying methane blending ratios under conditions of T_c = 1000 K, P_c = 20 bar, and φ = 1.0. Figure 3 presents the time histories of the molar fractions of OH, O, and H radicals for different methane blending ratios. As shown in Figure 3a, increasing the methane content progressively shortens the time required for these three radicals to reach their peak molar fractions, indicating an acceleration of radical formation. In contrast, the peak concentrations of OH, O, and H exhibit only a weak dependence on the methane blending ratio. Figure 3b shows that increasing the methane blending ratio not only advances the formation of H₂O₂ and HO₂ but also enhances their production rates. Furthermore, the peak molar fractions of both H₂O₂ and HO₂ increase markedly with higher methane content. Overall, for these five reaction-promoting radical species, methane blending significantly accelerates their formation and substantially increases their production rates, thereby facilitating the premixed ignition process.

To examine the influence of equivalence ratio on key radical species, Figure 4 presents the temporal variations in the molar fractions of important radicals under conditions of T_c = 1000 K, P_c = 20 bar, and a methane blending ratio of $X_{{\mathit{CH}}_4}=50\%$. As shown in Figure 4a, as the equivalence ratio increases from 0.5 to 2.0, the peak formation times of several radicals occur progressively earlier. This behavior indicates that higher equivalence ratios accelerate the primary and secondary reaction steps of ammonia oxidation, thereby shortening the ignition delay time. A similar trend is observed for the concentration evolution of H₂O₂ and HO₂ radicals. Overall, the variations in radical concentrations provide a mechanistic explanation for the ignition delay trends in the mixture with respect to equivalence ratio, as observed in the ignition delay characteristics.

3.2. Study on the Ignition Characteristics of NH₃/C₂H₆ Mixtures

Figure 5 illustrates the effect of temperature on the ignition delay time of NH₃/C₂H₆ mixtures under different equivalence ratios at P_c = 20 bar. As shown in Figure 5a, when the initial temperature T_c is below 1350 K, the ignition delay time decreases slightly as the equivalence ratio increases from 0.5 to 2.0. In contrast, when T_c exceeds 1350 K, this trend reverses, and the ignition delay time gradually increases with increasing equivalence ratio. Figure 5b shows that at a hydrocarbon blending ratio of 50%, the ignition delay time decreases with increasing equivalence ratio when T_c is below 1300 K. However, for T_c above 1300 K, the ignition delay time increases progressively as the equivalence ratio rises. Moreover, the sensitivity of ignition delay time to changes in equivalence ratio increases slightly with higher blending ratios.

To elucidate the chemical–kinetic mechanisms governing the ignition delay characteristics of the mixture, the temporal evolution of key free-radical species was analyzed for NH₃/C₂H₆ mixtures with different ethane blending ratios under conditions of P_c = 20 bar, T_c = 1000 K, and φ = 1.0. As shown in Figure 6a, increasing the ethane blending ratio from 0% to 70% leads to higher formation rates of the OH, O, and H radicals. Compared with pure ammonia ignition, ethane blending ratios of 10% and 30% markedly advance the formation times of these radicals and significantly enhance their production rates. However, this promoting effect gradually diminishes as the blending ratio increases further to 50% and 70%. Figure 6b further shows that increasing the ethane content advances the formation times of two additional important radical species, increases their generation rates, and substantially elevates the peak concentration of H₂O₂. Similarly to the trends observed for OH, O, and H radicals, this enhancement effect weakens once the ethane blending ratio exceeds 30%.

Figure 7 illustrates the main reaction pathways of NH₃ and C₂H₆ under conditions of T_c = 1000 K, P_c = 20 bar, and φ = 1.0. As shown in Figure 7a, for the initial dehydrogenation step of NH₃, the OH-induced dehydrogenation pathway plays a dominant role, accounting for 64.3–67.9% of the total reaction flux. As the ethane blending ratio increases, the sensitivity of the reaction to this pathway decreases, although it remains the primary contributor to the overall reaction rate. In the oxidation of NH₂ to form H₂NO, the contribution of HO₂ decreases from 9.5% to 8.1% as the ethane blending ratio increases from 10% to 50%. In contrast, during the subsequent dehydrogenation of H₂NO, the contribution of HO₂ increases from 15.9% to 22.9%. This behavior can be attributed to the fact that most NH₂ radicals participate in alternative dehydrogenation pathways leading to NH₃ formation, whereas only a small fraction of H₂NO reacts with CH₃, NO₂, and other intermediates to form NH₃ and HONO. Consequently, these pathways exert a relatively minor influence on the overall reaction rate. As shown in Figure 7b, for the first dehydrogenation reaction of C₂H₆, increasing the ethane blending ratio from 10% to 50% leads to a sharp decrease in the contribution of NH₂ from 52.1% to 12.9%, while the contribution of OH increases from 43.2% to 77.5%. Accordingly, the dominant dehydrogenation pathway shifts from being NH₂-driven to OH-driven. A similar trend is observed in the third dehydrogenation step of C₂H₆. Therefore, with increasing ethane blending ratio, the dehydrogenation reactions of ethane become progressively less dependent on NH₂, enabling NH₂ radicals to participate more actively in other elementary reactions (e.g., NO + NH₂ <=> N₂ + H₂O) and thereby promoting the overall oxidation rate of NH₃.

3.3. Study of the Ignition Characteristics of NH₃/C₃H₈ Mixtures

Figure 8 illustrates the effects of varying propane blending ratios on the ignition delay time of NH₃/C₃H₈ mixtures under initial pressures of 10 and 20 bar, initial temperatures ranging from 1000 to 1500 K, and φ = 1.0. As shown in the figure, the addition of propane markedly shortens the ignition delay time compared with pure ammonia ignition. As indicated in Figure 8a,b, increasing the initial pressure significantly reduces the ignition delay time. However, the effectiveness of propane blending in shortening the ignition delay diminishes progressively as the propane mixing ratio increases. When the propane blending ratio exceeds 50%, further reductions in ignition delay become negligible.

Under conditions of a 50% propane blending ratio and an initial pressure of 20 bar, the influence of equivalence ratio on the concentration evolution of key free-radical species was analyzed at initial temperatures of 1000 K and 1500 K. As shown in Figure 9a, when the equivalence ratio increases from 0.5 to 2.0 at an initial temperature of 1000 K, the peak concentrations of OH and HO₂ radicals occur at progressively earlier times. This behavior indicates that higher equivalence ratios promote the formation of these radicals, thereby shortening the ignition delay time. In contrast, as shown in Figure 9b, at a higher initial temperature of 1500 K, increasing the equivalence ratio from 0.5 to 2.0 leads to a gradual decrease in the generation rates of these radicals, accompanied by a delay in the timing of their peak concentrations. Consequently, the influence of equivalence ratio on ignition delay exhibits distinctly different trends under low- and high-temperature conditions.

Under conditions of T_c = 1000 K, P_c = 20 bar, and φ = 1.0, Figure 10 presents an analysis of key radical species during the reaction process of NH₃/C₃H₈ mixtures with different propane blending ratios. As shown in Figure 10a, increasing the propane blending ratio causes the peak formation times of OH and HO₂ radicals to occur progressively earlier. While the peak concentration of OH exhibits only weak sensitivity to the blending ratio, the peak concentration of HO₂ increases markedly with higher propane content, indicating that propane blending promotes subsequent reaction pathways involving NH₂. Figure 10b illustrates the concentration evolution of various radicals following the initial OH-induced dehydrogenation of the reactants. During the dehydrogenation of C₃H₈, two primary reactions occur: R728 (C₃H₈ + OH ⇌ n-C₃H₇ + H₂O) and R729 (C₃H₈ + OH ⇌ I-C₃H₇ + H₂O). The n-C₃H₇ radical preferentially undergoes further C-C bond scission to form C1 and C2 radicals, whereas I-C₃H₇ more readily participates in deoxygenation pathways leading to C₃H₆ formation. As shown in the figure, reaction R729 dominates during the early stages of the reaction; as the reaction progresses, the radical generation rates from R728 and R729 become comparable. With increasing propane blending ratio, the generation rate of NH₂ shows no significant change; however, the time at which its peak concentration occurs is noticeably advanced.

To elucidate the contributions of individual elementary reactions to OH radical formation during the reaction process, an ROP analysis of OH was performed under different propane blending ratios. Figure 11 presents the OH ROP profiles of the dominant elementary reactions governing OH concentration variations as temperature increases in the mixed-fuel system. The primary elementary reactions contributing to OH formation include R3 (O + H₂ <=> OH + H), R10 (O₂ + H ⇌ OH + O), R12 (H₂O + O <=> 2OH), and R27 (H₂O₂ (+M) <=> 2OH (+M)). In contrast, the major OH consumption pathways are R4 (H₂ + OH <=> H₂O + H) and R42 (CO + OH <=> CO₂ + H).

As shown in Figure 11a,b, increasing the propane blending ratio leads to higher peak OH ROP values for reactions R10, R12, R3, and R27. Moreover, the temperatures corresponding to these peak ROP values progressively shift from the high-temperature region toward lower temperatures, indicating enhanced low-temperature OH production. Figure 11c further shows that, on the OH consumption side, the dominant consumption pathway transitions from R4 to R42 as the reaction temperature increases. In addition, the overall OH consumption rate increases with increasing propane blending ratios. Overall, increasing the propane blending ratio enhances OH production rates in the temperature range of 1500–2000 K and promotes a downward shift in the peak OH ROP temperature from approximately 2500 K to 2300 K.

3.4. Study of the Ignition Characteristics of NH₃/C₄H₁₀ Mixtures

Figure 12 illustrates the effect of equivalence ratio on the ignition delay time of NH₃/C₄H₁₀ mixtures under low ($X_{{C_{4}H}_{10}}=10\%$) and high ($X_{{C_{4}H}_{10}}=50\%$) n-butane blending conditions. As shown in Figure 12a, under low blending conditions, when the initial temperature T_c is below 1250 K, the ignition delay time gradually decreases as the equivalence ratio increases from 0.5 to 2.0. In contrast, when T_c exceeds 1250 K, the ignition delay time increases with increasing equivalence ratio. Across the investigated temperature range, the sensitivity of ignition delay time to changes in equivalence ratio remains relatively weak under low blending conditions. Under high blending conditions, as shown in Figure 12b, a similar trend is observed: the ignition delay time decreases with increasing equivalence ratio when T_c < 1250 K, whereas for T_c > 1250 K, the ignition delay time increases as the equivalence ratio rises.

Figure 13 illustrates the effects of different n-butane blending ratios on the evolution of key radical species under conditions of T_c = 1000 K, P_c = 20 bar, and φ = 1.0. As shown in Figure 13a,b, compared with pure ammonia ignition, the addition of butane significantly enhances the formation rates of OH, O, H, H₂O₂, and HO₂ radicals. With increasing butane blending ratios, both the onset of radical formation and the times corresponding to their peak concentrations occur progressively earlier. In addition, the peak concentrations of H₂O₂ and HO₂ increase noticeably with higher butane content. When the butane blending ratio exceeds $X_{{C_{4}H}_{10}}$ > 30%, further increases in blending ratio result in a diminished enhancement of radical generation rates and a less pronounced advancement of the corresponding peak formation times. This behavior is consistent with the observed trends in ignition delay time at higher blending ratios. Overall, increasing the butane blending ratio promotes the formation of OH, O, H, H₂O₂, and HO₂ radicals, thereby accelerating the reaction rate and shortening the ignition delay time of the mixture.

Figure 14 illustrates the effect of different initial pressures on the concentration evolution of key radical species at a butane blending ratio of 10%. As shown in Figure 14a,b, under low blending conditions, increasing the initial pressure from 10 to 30 bar causes the peak formation times of important radicals to occur earlier. In contrast, the peak concentrations of these radicals exhibit relatively weak sensitivity to changes in initial pressure. This behavior is consistent with the corresponding ignition delay trends, indicating that higher initial pressure facilitates the formation of key radicals and thereby enhances the overall reaction rate. As further shown in Figure 14a, during the early stage of the reaction, the molar concentrations of the OH, O, and H radicals at higher initial pressures are lower than those observed under lower-pressure conditions, suggesting that elevated pressure suppresses radical formation in the initial reaction phase. However, as the reaction progresses from 0.1 to 0.7 ms, the concentrations of these radicals at higher pressures gradually exceed those at lower pressures, indicating an acceleration of radical generation at later stages of the reaction.

To clarify the influence of key elementary reactions on ignition delay time, a sensitivity analysis was performed for the NH₃/C₄H₁₀ mixture with respect to ignition delay, as shown in Figure 15. Among the reactions exhibiting positive sensitivity coefficients, under low butane blending conditions ($X_{{C_{4}H}_{10}}$ = 10% and 30%), the reaction NH₃ + OH <=> NH₂ + H₂O shows the largest positive sensitivity coefficient, indicating that it exerts the strongest suppressive effect on fuel ignition. Moreover, the sensitivity of this reaction decreases markedly, from a maximum value of 0.612 to 0.031, as the butane blending ratio increases, suggesting that increasing C₄H₁₀ content effectively weakens the inhibitory influence of ammonia dehydrogenation on ignition. At lower butane blending ratios, the suppressive effects of the reactions CH₃ + HO₂ ⇌ CH₄ + O₂ and 2HO₂ ⇌ H₂O₂ + O₂ are relatively minor, with sensitivity coefficients of 0.015 and 0.032, respectively. However, when the butane blending ratio increases to 70%, the sensitivity coefficients of these reactions rise significantly to 0.087 and 0.137, respectively, causing them to become the dominant reactions inhibiting ignition. Among the reactions with negative sensitivity coefficients, C₄H₁₀ + HO₂ <=> p-C₄H₉ + H₂O₂ and C₄H₁₀ + HO₂ <=> n-C₄H₉ + H₂O₂ exhibit the largest negative sensitivities, indicating the most pronounced ignition-promoting effects. Except for the reactions NH₂ + HO₂ <=> H₂NO + OH and C₄H₁₀ (+M) <=> n-C₃H₇ + CH₃ (+M), the absolute sensitivities of the remaining negative-sensitivity reactions generally increase with rising butane blending ratios, further enhancing their contribution to ignition promotion.

3.5. Comparison of Ignition Characteristics of NH₃/C1–C4 Alkane Mixtures

Figure 16 and Figure 17 compare the ignition delay times of NH₃/C1–C4 alkane mixtures under low (X_CH = 10%) and high (X_CH = 50%) alkane blending ratios, respectively. The simulations were performed at an initial pressure of P_c = 20 bar, over an initial temperature range of 900–1600 K, and across different equivalence ratios. As shown in Figure 16, under low blending conditions, the ignition delay time of all four mixtures decreases with increasing equivalence ratio at low initial temperatures, whereas a slight increase in ignition delay is observed at medium to high temperatures as the equivalence ratio rises. Across all conditions, the NH₃/CH₄ mixture consistently exhibits a significantly longer ignition delay time than the other three blends. In the low-to-medium temperature range (900 K < T_c < 1100 K), the ignition delay times decrease in the order NH₃/CH₄ > NH₃/C₂H₆ > NH₃/C₃H₈ > NH₃/C₄H₁₀, indicating that ignition performance improves progressively with increasing molecular weight of the blended alkane. As the initial temperature increases to the medium-temperature regime (1100 K < T_c < 1300 K), the ordering changes to NH₃/CH₄ > NH₃/C₃H₈ > NH₃/C₄H₁₀ > NH₃/C₂H₆, with the NH₃/C₂H₆ mixture exhibiting the strongest ignition-promoting effect. At higher initial temperatures (T_c > 1300 K), the ignition delay times follow the order NH₃/CH₄ > NH₃/C₂H₆ > NH₃/C₃H₈ > NH₃/C₄H₁₀. These results demonstrate that under low blending conditions, the ignition-promoting effect of C₂H₆ is most pronounced at intermediate temperatures, whereas at low and high temperatures, ignition performance improves gradually with the increasing carbon chain length of the blended alkane. Overall, the ignition delay time exhibits relatively weak sensitivity to changes in equivalence ratio compared with the effects of temperature and fuel composition.

Figure 17 illustrates the variation in ignition delay times for NH₃/C1–C4 alkane mixtures at an alkane blending ratio of X_CH = 50%. Compared with the case of X_CH = 10%, the sensitivity of the ignition delay time to the carbon chain length of the blended alkane is noticeably enhanced. Among the four mixtures, NH₃/CH₄ consistently exhibits a significantly longer ignition delay time than the other blends, and this difference becomes increasingly pronounced with rising initial temperature. As the equivalence ratio increases, the ignition delay times of all four mixtures show a slight increasing trend at medium to high initial temperatures, whereas a decreasing trend is observed at lower initial temperatures. In the low-temperature regime (900 K < T_c < 1100 K), the ignition delay times decrease in the following order: NH₃/CH₄ > NH₃/C₂H₆ > NH₃/C₃H₈ > NH₃/C₄H₁₀. When the initial temperature increases to the medium-to-high temperature range (1100 K < T_c < 1500 K), the ordering changes to NH₃/CH₄ > NH₃/C₃H₈ > NH₃/C₄H₁₀ > NH₃/C₂H₆, from the longest to shortest ignition delay time. These results indicate that, at X_CH = 50%, blending with C₂H₆ exerts a stronger ignition-promoting effect under high-temperature conditions. In contrast, at low and intermediate temperatures, ignition performance improves progressively with increasing carbon chain length of the blended alkane. Overall, the ignition delay time exhibits relatively weak sensitivity to changes in equivalence ratio across the conditions investigated.

Figure 18 compares the ignition delay times of NH₃/C1–C4 alkane mixtures under two initial pressures (Pc = 10 bar and 20 bar) and a range of alkane blending ratios (X_CH = 10%, 30%, 50%, and 70%). For all blending ratios and fuel combinations, increasing the initial pressure consistently shortens the ignition delay time, as indicated by the systematic downward shift of the dashed curves relative to the solid curves. However, the influence of pressure becomes less pronounced at higher initial temperatures. Further comparisons across different blending ratios reveal that, for NH₃/C2-C4 alkane mixtures, the reduction in ignition delay induced by increasing pressure is slightly enhanced as X_CH increases. In contrast, for the NH₃/CH₄ mixture, the pressure effect remains relatively insensitive to changes in blending ratio. Under all blending ratios, temperatures, and pressures examined, NH₃/CH₄ consistently exhibits the longest ignition delay time. When the alkane blending ratio exceeds 30%, the ignition delay times within the lower temperature range of 900–1200 K decrease in the following order: NH₃/CH₄ > NH₃/C₂H₆ > NH₃/C₃H₈ > NH₃/C₄H₁₀. Conversely, in the higher temperature range of 1200–1500 K, the ignition delay ordering changes to NH₃/CH₄ > NH₃/C₃H₈ > NH₃/C₄H₁₀ > NH₃/C₂H₆.

To elucidate the interactions between ammonia and different blended fuels at various initial temperatures, the temporal evolution of key free-radical concentrations during the ignition of NH₃/C1–C4 alkane mixtures was analyzed under conditions of P_c = 20 bar, φ = 1.0, and initial temperatures of T_c = 1000 K and 1500 K. As shown in Figure 19a,b, at T_c = 1000 K, the addition of C1–C4 alkanes to ammonia significantly advances the occurrence of the peak concentrations of the three major reactive radicals (OH, O, and H) compared with pure ammonia while simultaneously increasing their generation rates. The peak times of the molar concentrations of OH, O, H, H₂O₂, and HO₂ follow the order NH₃/C₄H₁₀ > NH₃/C₃H₈ > NH₃/C₂H₆ > NH₃/CH₄ > NH₃. Although the peak concentrations of OH, O, and H among the different blended fuels exhibit relatively small variations, the peak concentration of H₂O₂ in the NH₃/C2–C4 mixtures shows lower sensitivity to alkane type while remaining significantly higher than those in the NH₃/CH₄ mixture and pure ammonia. In addition, the peak HO₂ concentrations decrease in the order NH₃/C₄H₁₀ > NH₃/C₃H₈ > NH₃/C₂H₆ > NH₃/CH₄ > NH₃. As illustrated in Figure 19c,d, increasing the initial temperature to 1500 K leads to a substantial advancement in the peak occurrence times of all key radicals for the investigated fuel mixtures. Under these conditions, the peak molar concentrations of OH, O, H, H₂O₂, and HO₂ follow the order NH₃/C₂H₆ > NH₃/C₄H₁₀ > NH₃/C₃H₈ > NH₃/CH₄. Furthermore, during the early stage of the reaction, the concentrations of the major radicals are initially ranked as NH₃/C₄H₁₀ > NH₃/C₃H₈ > NH₃/C₂H₆ > NH₃/CH₄. As the reaction progresses, the radical concentrations in the NH₃/C₂H₆ mixture gradually surpass those of the NH₃/C₃H₈ and NH₃/C₄H₁₀ mixtures, indicating a stronger ignition-promoting effect of ethane under high-temperature conditions.

Figure 20 presents the ROP analysis of OH radicals during the ignition of NH₃/C1–C4 alkane mixtures under conditions of Pc = 20 bar, φ = 1.0, and initial temperatures of T_c = 1000 K and 1500 K. At T_c = 1000 K, comparison of the results indicates that the elementary reactions exerting the most significant influence on OH production include the following: R10: O₂ + H <=> O + OH; R12: 2OH <=> H₂O + O; R3: O + H₂ <=> OH + H; R34: HO₂ + H <=> 2OH; and R27: H₂O₂ (+M) <=> 2OH (+M). As shown in Figure 20a,b, R10 plays a dominant role in OH radical formation during the oxidation of NH₃/C1–C4 mixtures. Furthermore, the peak ROP of OH associated with these reactions increases with the increasing carbon number of the blended alkanes. When the initial temperature is raised to 1500 K, as illustrated in Figure 20c, R10 remains the primary pathway for OH generation, and its peak ROP is significantly enhanced compared with that at T_c = 1000 K. In contrast, the contribution of R12 decreases with increasing alkane carbon number at elevated temperatures. Overall, the ROP of OH radicals is mainly governed by the carbon number of the alkane component in the blended fuel. However, the ignition delay time is more strongly influenced by the timing of the peak OH generation rather than by the magnitude of the peak OH ROP itself.

Figure 21 illustrates the reaction pathways of NH₃ in NH₃/C1–C4 alkane mixtures at an alkane blending ratio of X_CH = 50% under conditions of P_c = 20 bar and initial temperatures of T_c = 1000 K and 1500 K. As shown in the figure, OH radicals play a dominant role in the dehydrogenation of NH₃. With increasing carbon number of the blended alkanes, the contribution of the reaction pathway OH + NH₃ <=> NH₂ + H₂O increases, whereas the participation of NH₂ in the initial dehydrogenation steps of alkanes gradually decreases. This behavior indicates that longer-chain alkanes enhance the efficiency of the initial NH₃ dehydrogenation process. At higher initial temperatures, the overall dehydrogenation fraction of NH₃ decreases, which can be attributed to the enhanced formation of NO, as a portion of NH₃ is consumed in NO reduction reactions. In addition, elevated temperatures lead to more complex reaction pathways, with increased involvement of NH₂ radicals in the formation of other ammonia-derived intermediates.

4. Conclusions

This study systematically investigated the effects of equivalence ratio, alkane blending ratio, initial pressure, and initial temperature on the ignition delay behavior of NH₃/C1–C4 alkane mixtures. The observed ignition delay characteristics were interpreted at the chemical kinetics level through comprehensive analyses of key radical concentrations, ROP, reaction pathways, and sensitivity coefficients. The main conclusions are summarized as follows:

• Under representative conditions (P_c = 10, 20 bar, φ = 1.0, and X_CH ≥ 30%), the ignition delay times of NH₃/C2–C4 alkane mixtures exhibit similar trends and are consistently shorter than those of the NH₃/CH₄ mixture. At low initial temperatures, the ignition delay time decreases in the following order: NH₃/CH₄ > NH₃/C₂H₆ > NH₃/C₃H₈ > NH₃/C₄H₁₀. In contrast, under high temperature conditions, the ignition delay time follows the order NH₃/CH₄ > NH₃/C₃H₈ > NH₃/C₄H₁₀ > NH₃/C₂H₆. These results indicate that, at low temperatures, ignition performance improves progressively with increasing carbon chain length of the blended alkane. In addition, variations in equivalence ratio exert only a minor influence on ignition delay time compared with temperature and fuel composition effects.
• The ignition delay under all investigated conditions is primarily governed by highly reactive radicals, particularly O, H, and OH. Increases in the initial temperature, initial pressure, and alkane blending ratio promote the earlier formation and higher production rates of these key radicals. At an initial temperature of 1000 K, the peak molar concentrations of radicals follow the order NH₃/C₄H₁₀ > NH₃/C₃H₈ > NH₃/C₂H₆ > NH₃/CH₄. When the initial temperature is increased to 1500 K, the corresponding order becomes NH₃/C₂H₆ > NH₃/C₄H₁₀ > NH₃/C₃H₈ > NH₃/CH₄. The generation rates of reactive radicals and the timing of their peak concentrations are consistent with the observed ignition delay trends under the same conditions. Among all reactions, reaction R10 (O₂ + H <=> OH + O) contributes the most significantly to OH radical production.
• Temperature-dependent reaction pathway characteristics of NH₃: Reaction pathway analysis reveals that at lower initial temperatures, NH₃ dehydrogenation is dominated by reactions involving OH radicals. As the carbon number of the blended alkane increases, the contribution of OH to NH₃ dehydrogenation increases, while the involvement of NH₂ radicals in alkane dehydrogenation decreases, resulting in an enhanced forward reaction rate in the initial dehydrogenation step. At higher initial temperatures, a larger fraction of NH₃ participates in NO reduction reactions, which reduces the proportion of NH₃ involved in dehydrogenation pathways.