The goal of reducing carbon emissions had been pursued by various countries in recent years. However, the energy structure was still dominated by fossil fuels, whose incomplete combustion could inevitably generate the production of pollutants such as NO_x, carbon oxides, and harmful soot particles. The soot could reduce the efficiency of combustion, damage the environment, and endanger human health (Wang 2011; Dong et al. 2023). Therefore, it was important to explore technologies that could reduce dependence on fossil fuel sources, such as renewable energy sources and carbon-free fuel combustion.

Ammonia (NH₃) could be a viable clean fuel, with a 2018 report in the journal of Science lauding it as "liquid sunlight" that would provide a renewable, carbon-free energy source (Service 2018). NH₃ also had potential benefits and technical advantages as a sustainable fuel for power generation on vehicles. In particular, NH₃ was more effective than other fuels, had a longer driving range, and was more compact and cost-effective (Zamfirescu and Dincer 2009). Additionally, the octane rate of NH₃ was high, reaching about 110–130 (Sonker et al. 2022). The applications of NH₃ as fuel in gas turbines, pulverized coal co-combustion and industrial furnaces have been very successful in recent years (Kobayashi et al. 2019).

Given ammonia flame instability and low combustion intensity (Lhuillier et al. 2020; Zhou et al. 2021), researchers have been motivated to study its mixing with hydrocarbon fuels such as methane and ethylene (Chen and Liu 2023). Firstly, the combustion of the methane mixing ammonia (Grcar et al. 2004; Shu et al. 2021; Li et al. 2021) has been explored. It revealed that as the proportion of mixed NH₃ increased in the CH₄/NH₃ turbulent premixed flame, the maximum flame surface density decreased, leading to a decrease in the ratio of the turbulent combustion velocity of NH₃ to the unstretched laminar combustion velocity (Ichikawa et al. 2019). The primary way in which ammonia affected the velocity of the CH₄/NH₃ flame was by altering the concentration of H and OH radicals. The sensitivity of flame to stretch increased with equivalence ratio and NH₃ concentration (Okafor et al. 2018). Secondly, ethylene (C₂H₄), as the simplest alkene, has been established its detailed combustion reaction mechanism. For microgravity ethylene diffusion flames, researchers (Lecoustre et al. 2012) have evaluated experimentally and numerically that soot formation occurred in regions where the C/O atom ratio and temperature exceeded critical values of 0.53 and 1305 K, respectively. Moreover, the local C/O atomic ratio associated with soot incipience was lower in terrestrial gravity, falling within the range of 0.32 to 0.44 (Frolov et al. 2023). The carbon atom number and C/H ratio of the final soot particles in ethylene combustion were approximately twice that of methane combustion (Wang et al. 2021). Therefore, the soot production in ethylene turbulent flames after nitrogen was replaced by hydrogen or ammonia was studied (Boyette et al. 2021). It was found that hydrogen substitution increased soot production, while ammonia addition inhibited soot formation. Additionally, the replacement of some Ar by NH₃ in ethylene premixed flame was responsible for the decrease of the maximum mole fractions of C₄H₂ and C5 to C10 species (Renard et al. 2009). In the C₂H₄ diffusion flame, it was revealed that NH₃ delayed and suppressed the formation of polycyclic aromatic hydrocarbons (PAHs), and NH₃ had a greater reduction influence than Ar (Ren et al. 2022). By some kinetic studies, some researchers also found that doping ammonia decreased soot particle size, the volume fraction of soot particles and the mole fractions of soot precursors both in laminar premixed (Shao et al. 2022) and diffusion (Liu et al. 2021; Zhang et al. 2023) ammonia/ethylene flames. Then through the kinetic analyses of the mole fractions of typical soot precursors (Deng et al. 2022), it was found that NH₃ addition inhibited the production of important precursor polycyclic aromatic hydrocarbons, which was mainly due to the chemical effects of NH₃ in C₂H₄/NH₃ diffusion flame. Furthermore, the soot from ethylene/ammonia laminar flames was experimentally and numerically studied, and found that the reduction of soot formation was mainly due to the chemical effects of NH₃ (Bennett et al. 2020). However, it was not completely separate the chemical effect of NH₃ from its thermal and dilution effects (Liu et al. 2015a). It could be concluded that mixing ammonia with traditional hydrocarbon fuels improved the combustion properties of ammonia and inhibit the formation of soot precursors. However, it was not clear what factors will affect the chemical effects of NH₃ on C₂H₄ combustion chemistry, such as different diluents.

It had been found that different diluent gas could affect the reaction kinetics and change the thermophysical properties of the mixture such as specific heats, diffusion coefficients, etc. to change the combustion temperature or reduce the generation of pollutants (Vancoillie et al. 2013; Chen et al. 2022). Researchers (Yelverton and Roberts 2008) have measured the soot surface temperature in pure and diluted ethylene co-flow diffusion flames, using helium (He), argon (Ar), nitrogen (N₂), or carbon dioxide (CO₂) individually. It revealed that the addition of a diluent cooled the soot surface. The He-diluted flames were the warmest and the CO₂-diluted flames were the coolest. In addition, the sensitivity of the DME flame velocity to the H-atom production and consumption reactions also decreased with the dilution of CO₂. In the lean DME flame, both the inert third-body effect and the kinetic effect of CO₂ reduced the H-atom production. For the rich DME flame, the inert third-body effect increased the formation of H-atom by inhibiting the kinetic effect of CO₂ (Liu et al. 2013). Focusing on the soot formation in CO₂-diluted environments, researchers performed experiments on ethylene fuel pyrolysis with varying levels of CO₂ dilution and found that 25% CO₂ tended to increase soot production while a higher level of CO₂ reduced soot formation (Abián et al. 2012). In laminar co-flow C₂H₄/air diffusion flames, it concluded that the concentrations of the critical soot formation species, including H, C₂H₂, benzene, and pyrene were lowered in the CO₂-diluted flames due to the additional chemical effects of CO₂. CO₂ remained more effective than N₂ as a diluent to suppress soot formation at elevated pressures (Liu et al. 2015). The chemical effects of adding CO₂ to the fuel side or oxidant side were simulated in countercurrent ethylene/air diffusion flame, and it concluded that CO₂ could suppress soot formation by lowering both temperature and acetylene concentration and enhancing the concentration of OH radical (Liu et al. 2001).

It may be concluded that the combustion atmosphere had very significant influences on the combustion process. However, most of the available studies have focused on the effects of diluents on the combustion of hydrocarbon fuels. The studies of the effects of diluents on ethylene/ammonia counter-flow diffusion flames were limited. Therefore, the aim of this paper was to investigate the combustion chemistry of C₂H₄/NH₃ with various diluents via chemical kinetics analyses. The chemical effects of NH₃ addition on the flame temperature, typical radical species, and important intermediate species were analyzed, with particular attention to the coupled chemical effects of NH₃ and CO₂ on the combustion process. The present study had the potential to give a better understanding of the fundamental ammonia combustion with different diluents, and to provide a reference for the detailed alteration of soot precursors and the control of other pollutant emissions.

The counter-flow diffusion flame simulations were performed using the CHEMKIN/OPPDIF module. The detailed chemical mechanism employed here was KM2-G, which involved 202 species and 1351 reactions (Wang et al. 2013). The KM2-G mechanism coupled a comprehensive model for nitrogen chemistry with the KM2 hydrocarbon-PAH mechanism (Zhou et al. 2022). The nitrogen chemistry included the oxidation of NH₃ and the formation of nitric oxide species, which have been well validated against experimental data (Glarborg et al. 2018). The KM2 mechanism was also shown to perform excellently in terms of the prediction of the first aromatic ring as well as soot formation (Wang et al. 2020).

The separation distance between the two nozzles of the counter-flow diffusion flame burner was 8 mm. With the pressure condition of 1 atm, the inlet velocities of the fuel and oxidizer were set at 25 cm/s, and the initial temperature was 300 K. Considering that N₂ was unstable at high temperatures, and could produce thermal NO_x that affected the combustion chemistry of NH₃. Therefore, as the basic working condition, the fuel side consisted of 80% C₂H₄+ 20% Ar, and the oxidizer side was 21% O₂+ 79% Ar. Then He and CO₂ replaced the diluent gas Ar on the fuel side or the oxidizer side. Additionally, the coupled chemical effects of NH₃ and CO₂ were further analyzed by introducing the fictitious inert CO₂ and NH₃. The specific working conditions were shown in Tables 1 and 2. In Table 1, F1–F5 represented the conditions of the C₂H₄/NH₃ flames with Ar dilution and different NH₃ additions. F6–F10 and F11–F15 illustrated the conditions of the C₂H₄/NH₃ flames with He dilution on the fuel side and oxidizer side, respectively, and different NH₃ additions. In Table 2, F1–F9 outlined the conditions of the C₂H₄/NH₃ flames with CO₂ dilution on the fuel side and various NH₃ additions, F10–F18 delineated the conditions of the C₂H₄/NH₃ flames with CO₂ dilution on the oxidizer side and various NH₃ additions.

As the effects of additives on fuel combustion were divided into three types: (1) dilution effects resulting from the decrease of reactive species mole fractions and collision frequencies, (2) thermal effects as a result of flame temperature variation, and (3) chemical effects caused by the participation of additives in chemical reactions (Du et al. 1990).

These three effects were highly coupled which made it quite difficult to discuss them separately. Therefore, the fictitious species method, which was firstly proposed by Liu et al. (2001) and had been employed by many researchers to analyze the specific effects of additives in different flames (as shown in Table 3), could isolate the chemical effects of additives from the dilution and thermal effects. In the study, in order to identify the coupled chemical effects of NH₃ and CO₂, normal active CO₂ (denoted as CO₂) and fictitious inert CO₂ (denoted as FCO₂) were also introduced for comparisons (Liu et al. 2003, 2015; Gu et al. 2016; Naseri et al. 2017). Similarly, the normal active NH₃ was denoted as NH₃, and the fictitious inert NH₃ was denoted as FNH₃. Although the fictitious species could not be directly added to real flames, in the numerical strategy, FCO₂ and CO₂ had the same thermodynamic parameters, transport parameters, and third-body collision efficiency, and the only difference was that FCO₂ did not participate in any relevant chemical reactions. In this way, the dilution and thermal effects, which were similar and commonly both be classified as pure physical effects, were consistent. While the chemical effects of CO₂, which not only affected the main combustion products (CO, H₂) but also could play an important role in soot evolution, could be isolated (Zhao and Liu 2022). It could be considered that the variation of the flame temperatures and species concentration between the FCO₂ and the CO₂ flames was purely caused by the chemical effects. The similar method was also used for NH₃. The differences between the results from NH₃ and FNH₃ additions were attributed to the NH₃ chemical effects, and the coupled chemical effects of NH₃ and CO₂ could be suggested from the differences between the CO₂/NH₃ and FCO₂/FNH₃ flames (Liu 2014; Li et al. 2015; Ying and Liu 2015; Liu 2015a; Luo and Liu 2017; Pan and Liu 2017; Deng et al. 2022; Zhao and Liu 2022).

3.1 With Ar or He dilution

The effects of NH₃ addition on the flame temperature, major species, free radicals and important intermediate species with Ar or He dilution were investigated, with emphasis on distinguishing the detailed effects of dilution, thermal and chemical effects of NH₃.

3.1.1 Flame temperature profiles

The effects of NH₃ with Ar or He dilution on the flame temperatures were shown in Fig. 1. The dilution and thermal effects of NH₃ resulted in temperature differences between 0% NH₃ and 20% FNH₃ additions, and the differences between 20% NH₃ and 20% FNH₃ additions were due to the chemical effects of NH₃. The combined influences of dilution, thermal, and chemical effects led to the differences between 0% NH₃ and 20% NH₃ additions. For clarity of exposition, the specific effects were marked in Fig. 1.

Flame temperature with Ar or He dilution and different NH₃ additions. Notes: The 20% Ar/79% Ar represented the dilution gas was Ar both on the fuel side and the oxidizer side (F1-F5 conditions in Table 1). The 20% He/79% Ar and the 20% Ar/79% He represented dilution gas He replaced Ar on the fuel side (F6-F10 conditions in Table 1) and the oxidizer side (F11-F15 conditions in Table 1). The annotations for Figs. 2, 3, 4, 5, 6, 7, 8 and 9 have the same meanings, which will not be noted again in the following

In Fig. 1, the chemical effects of NH₃ increased the flame temperature, whereas the dilution and thermal effects of NH₃ caused the temperature to decrease. Consequently, the flame temperature was reduced due to the dilution and thermal effects of NH₃, which had a more pronounced influence on the flame temperature than the chemical effects. The data in Fig. 1c showed that when He replaced Ar on the oxidizer side, the peak flame temperature was slightly lower, which may be due to the higher thermal diffusivity of He (Yelverton and Roberts 2008).

3.1.2 Major species and radicals

The mole fraction profiles of the fuel C₂H₄ with different NH₃ additions were shown in Fig. 2. The differences between the NH₃ addition and FNH₃ addition revealed that the chemical effects of NH₃ could slightly accelerate fuel consumption. In Fig. 2c, as He replaced Ar on the oxidizer side (F11–F15 conditions in Table 1), the profiles shifted toward the oxidizer side. To analyze the detailed consumption of C₂H₄, the C₂H₄ rates of production were performed in Fig. 3, it could be found that when the diluent was Ar (F1–F5 conditions in Table 1), C₂H₄ was mainly consumed by radicals, including C₂H₄+ H = C₂H₃+ H₂ and C₂H₄+ OH = C₂H₃+ H₂O reactions. The rates of these reactions responsible for the depletion of C₂H₄ decreased with the addition of NH₃, which dominated the chemical effects. Therefore, it was necessary to analyze the mole fractions of these typical radicals, including H, O, and OH radicals.

Mole fraction profiles of C₂H₄ with Ar or He dilution and different NH₃ additions

Rate of production of C₂H₄ with different NH₃ additions

Figure 4 gave the mole fractions of the H radical with different NH₃ additions. When the dilution gas was Ar (F1–F5 conditions in Table 1) or He (F6–F15 conditions in Table 1), the mole fractions of H radical decreased with increasing concentrations of NH₃ additive, and with 20% NH₃ addition, the chemical effects of NH₃ further reduced H concentration. While the blending ratio was 40%, the chemical effects of NH₃ increased the mole fraction of H radical when the dilution gas was only Ar (as shown in Fig. 4a). The results were consistent with Deng et al. (2022) and Zhang et al. (2023).

Mole fraction profiles of H with Ar or He dilution and different NH₃ additions

Oxidation of PAH and soot particles may occur subsequent to formation in diffusion flames and the main oxidation reactants were OH, O, and O₂ (Richter and Howard 2000). Therefore, it was necessary to analyze the mole fraction profiles of the O radical, which were shown in Fig. 5. Comparing the O radical mole fractions with NH₃ and FNH₃ additions, it illustrated that the chemical effects of NH₃ could inhibit the formation of O radical regardless of the different dilution gas.

Mole fraction profiles of O with Ar or He dilution and different NH₃ additions

Furthermore, hydroxyl OH played a dominant role in the soot generation area of counter-flow diffusion flame, due to the relatively low concentration of O₂. The OH radical was mainly oxidized by reacting with the active sites on the surface of soot, which could inhibit the further generation of soot (Frenklach et al. 2018). The mole fraction profiles were performed in Fig. 6. The addition of NH₃ also suppressed the formation of OH radical, but different from the effects on O, H radicals, it was primarily because of the dilution and thermal effects of NH₃, which were more pronounced than its chemical effects. Notably, the mole distributions of OH radical with NH₃ addition were higher than those with FNH₃ addition, indicating that the chemical effect of NH₃ promoted the formation of OH radical indeed.

Mole fraction profiles of OH with Ar or He dilution and different NH₃ additions

3.1.3 Intermediate hydrocarbon species

The intermediate hydrocarbon species would be oxidized with radicals to inhibit the soot formation to some extent. Therefore, the mole fractions of acetylene (C₂H₂), propynyl (C₃H₃), and benzene (A1), which were important soot precursors, in the combustion process of C₂H₄/NH₃ diluted with Ar (F1–F5 conditions in Table 1) or He (F6–F15 conditions in Table 1) were shown in Figs. 7, 8 and 9. It could be found that the combined effects of the NH₃ additive reduced the mole fractions of these important intermediate hydrocarbon species from Figs. 7a, 8a and 9a. The similar results had been achieved by Li et al. (2021), Shao et al. (2022) and Ren et al. (2022). In Fig. 7a, the differences between the mole distributions of C₂H₂ with NH₃ addition and FNH₃ addition demonstrated that the chemical effects of NH₃ promoted the formation of C₂H₂. Similarly, the chemical effects of NH₃ were also observed to increase the mole fractions of C₃H₃ and A1, as shown in subsequent Figs. 8a and 9a. Therefore, regardless of the dilution environment, the productions of C₂H₂, C₃H₃, and A1 were inhibited due to the dilution and thermal effects of NH₃, instead of the chemical effects of NH₃. It was worth pointing out that, as shown in Figs. 8a and 9a, the mole fractions of C₃H₃ and A1 were much lower when He replaced Ar on the fuel side or oxidizer side (F6–F15 conditions in Table 1). It revealed that the pathway of A1 generated by small molecule C₃H₃ in the C₂H₄/NH₃ counter-flow diffusion flame was more easily susceptible to He.

Mole fractions and Rates of production of C₂H₂ with Ar or He dilution and different NH₃ additions. a Mole fractions; b Rates of production

Mole fractions and Rates of production of C₃H₃ with Ar or He dilution and different NH₃ additions. a Mole fractions; b Rates of production

Mole fractions and Rates of production of A1 with Ar or He dilution and different NH₃ additions. a Mole fractions; b Rates of production

To analyze the detailed formation and consumption of these important intermediate hydrocarbon species, the rates of production were analyzed in Figs. 7b, 8b and 9b. It could be found that the main reaction concerning C₂H₂ formation was C₂H₃(+M) = C₂H₂+ H(+M), although the dilution environment was different, and the reaction rates were greatly reduced with the chemical effects of NH₃, as shown in Fig. 7b. Whereas the rate of the main reaction C₂H₂+ CH₂= C₃H₃+ H for C₃H₃ formation was reduced mainly due to the dilution and thermal effects of NH₃, as shown in Fig. 8b. Figure 9b illustrated that the reaction A1- + C₂H₄= A1 + C₂H₃ was mainly responsible for A1 formation, and the oxidation reaction A1- + H(+M) = A1(+M) was added when He diluted on the fuel side. Comparing the NH₃ and FNH₃ additions, it could be found that the chemical effects of NH₃ promoted the reaction A1- + C₂H₄= A1 + C₂H₃, whereas it suppressed other reactions.

3.2 With CO₂ dilution

When CO₂ was used as the diluent, the chemical effects of NH₃ may be influenced by the chemical effects of CO₂. Therefore, this section would put a special emphasis on the analysis of the coupled chemical effects of NH₃ and CO₂ on flame temperature, major species, free radicals, important intermediate hydrocarbon species, and oxygenated species.

3.2.1 Flame temperature profiles

As shown in Fig. 10, the addition of NH₃ reduced the flame temperature. In Fig. 10a, the differences between CO₂/NH₃ and CO₂/FNH₃ revealed that the chemical effects of NH₃ increased the temperature. The differences between CO₂/NH₃ and FCO₂/FNH₃ additions suggested that the coupled chemical effects of NH₃ and CO₂ also led to an increase in the flame temperature. However, with CO₂ addition on the oxidizer side, as shown in Fig. 10b, the coupled chemical effects decreased the temperature. This was possibly because the chemical effects of CO₂ lowered the temperature (Liu et al. 2001) and were dominant with oxidizer-side addition. Meanwhile, it was worth noting that the temperature was greatly lower with the oxidizer-side CO₂ addition. Even when the condition was 40% NH₃/79% CO₂ (F15 condition in Table 2), the fuel could not be ignited. Therefore, the working condition would not be discussed in the following. As the known literature data showed that the threshold local temperature for the onset of soot formation in diffusion flames, T_c, satisfied the condition T_c> 1300–1500 K (Frolov et al. 2023), the analysis of soot precursors concentrations in flame conditions were necessary.

Flame temperature with CO₂ dilution and different NH₃ additions. Notes: The 20% (F)CO₂/79% Ar and the 20% Ar/79% (F)CO₂ respectively represented dilution gas CO₂ replaced Ar on the fuel side (F1-F9 conditions in Table 2) and the oxidizer side (F10-F18 conditions in Table 2). The (F)CO₂ represented the normal CO₂ (CO₂) or fiction CO₂ (FCO₂) addition. The annotations for Figs. 11, 12, 13, 14, 15, 16 and 18 have the same meanings, which will not be noted again in the following

3.2.2 Major species and radicals

Figure 11 gave the mole fraction profiles of C₂H₄. The comparison with the addition of 40% NH₃ and 40% FNH₃ illustrated that the chemical effects of NH₃ reduced the C₂H₄ mole fraction slightly. However, the coupled effects of NH₃ and CO₂ on C₂H₄ were less pronounced when CO₂ is added to either the fuel side or the oxidizer side.

Mole fraction profiles of C₂H₄ with CO₂ dilution and different NH₃ additions

The mole fraction profiles of H radical were displayed in Fig. 12. It illustrated that the addition of NH₃ decreased the H concentration. The fuel-side CO₂ dilution (F1–F9 conditions in Table 2) was similar to the condition of Ar-diluted, the chemical effects of NH₃ reduced H concentration with 20% NH₃ addition, while increased the mole fraction of H when the blending ratio was 40%, as shown in Fig. 12a. Whereas, Fig. 12b illustrated that the chemical effects of NH₃ inhibit H formation with oxidizer-side CO₂ dilution (F10–F18 conditions in Table 2). The differences between the H radical of 20% CO₂/NH₃ and 20% FCO₂/NH₃ were owing to the chemical effects of CO₂, which could decrease the mole fraction of H radical This was the same result as the known literature (Liu 2015a). Moreover, the differences between NH₃/CO₂ and FNH₃/FCO₂ additions revealed that the reductions in H concentration were due to the coupled chemical effects of NH₃ and CO₂, especially with the oxidizer-side CO₂ addition. The existing literature (Liu et al. 2001; Mahmoud et al. 2019) indicated that in ethylene counter-flow diffusion flames, the flame sheet front was situated on the oxidizer side of the stagnation plane. Consequently, the addition of oxidizer may result in a more pronounced increase in the CO₂ concentrations at the flame sheet. The promotion of the CO₂+ H = CO + OH reaction was also more significant for the oxidizer side addition.

Mole fraction profiles of H with CO₂ dilution and different NH₃ additions

Figure 13 illustrated the mole fraction profiles of O radical. Similar to the reduction of the mole fractions of H radical, the coupled chemical effects of NH₃ and CO₂ reduced O radical concentration, especially with the addition of CO₂ on the oxidizing side.

Mole fraction profiles of O with CO₂ dilution and different NH₃ additions

However, the coupled chemical effects of NH₃ and CO₂ on the mole fraction of OH radical were different. As shown in Fig. 14a, the differences between NH₃/CO₂ and FNH₃/FCO₂ additions revealed that the coupled chemical effects of NH₃ and CO₂ promoted the formation of OH with fuel-side CO₂ addition. Nevertheless, the coupled chemical effects had obvious inhibitory effects when CO₂ addition on the oxidizer side, which were shown in Fig. 14b. It was possibly because the chemical effects of CO₂ also decreased the OH radical mole fractions, which could be found from the differences between NH₃/CO₂ and NH₃/FCO₂ additions. And with the oxidizer-side CO₂ dilution, the chemical effects of CO₂ were more significant.

Mole fraction profiles of OH with CO₂ dilution and different NH₃ additions

3.2.3 Intermediate hydrocarbon species

C₂H₂ was considered to be an important small molecule precursor of soot formation (Ruiz et al. 2007; Chernov et al. 2014; Wang and Chung 2019). C₃H₃ and A1 also were very vital for the soot production.

Firstly, the chemical effects of CO₂ and NH₃ on C₂H₂ concentration were identified in Fig. 15. The differences between CO₂/NH₃ and FCO₂/FNH₃ additions represented the coupled chemical effects of NH₃ and CO₂. Compared with Fig. 15a, b, it could be found that the coupled chemical effects of NH₃ and CO₂ were weak when CO₂ was from the fuel side (F1–F9 conditions in Table 2), while it led to a significant reduction in the mole fractions of C₂H₂ with CO₂ addition on the oxidizer side (F10–F18 conditions in Table 2). To discover C₂H₂ variation trends with increasing NH₃ additions, Fig. 15b gave the C₂H₂ peak mole fractions. It revealed explicitly that the coupled chemical effects of NH₃ and CO₂ increased the mole fractions of C₂H₂ more significantly with the concentration of NH₃ increasing, when CO₂ was added to the fuel side. Fig. 15c illustrated the rates of production analysis of C₂H₂, it could be found that the coupled chemical effects of NH₃ and CO₂ decreased the rate of the reaction C₂H₃(+M) = C₂H₂+ H(+M), which was the main reaction for C₂H₂ formation. Therefore, the participation of C₂H₂ in the growth reaction of polycyclic aromatic hydrocarbons (PAHs) through the HACA mechanism would be affected (Frenklach 2002), then the surface growth of soot could be inhibited to some extent.

Mole fractions and Rates of production of C₂H₂ with CO₂ dilution and different NH₃ additions. a Mole fractions; b Peak mole fractions; c Rates of production

According to the peak mole fractions of C₂H₂ shown in Fig. 15b and the calculation method shown in Table 4, the chemical effects of NH₃ and CO₂ could be demonstrated quantitatively (Pan and Liu 2017). In Table 4, Ai (i = 1, 2, 3) represented the chemical effects of NH₃ in the combined effects of NH₃ and CO₂. Similarly, Bi (i = 1, 2, 3) denoted the chemical effects of CO₂, Ai + Bi (i = 1, 2, 3) represented the sum of the separate chemical effects of NH₃ and CO₂, Ci (i = 1, 2, 3) was equal to the coupled chemical effects of NH₃ and CO₂. In the results of the calculation, a positive value indicated an increase and a negative value represented a decrease. When the condition was 40% NH₃/79% CO₂ (F15 condition in Table 2), the fuel could not be ignited, so only the coupled chemical effects of adding 20% NH₃ were considered with CO₂ addition on the oxidizer side. The B1, B2, B3 revealed that the chemical effects of CO₂ suppressed the formation of C₂H₂, which was consistent with the known literature (Liu et al. 2001; Liu 2015a), whereas the chemical effects of NH₃ increased the mole fractions of C₂H₂. The former dominated the latter and the net effects were to reduce the mole fractions of C₂H₂ with the oxidizer-side CO₂ addition, different from CO₂ addition on the fuel side. Compared with the (Ai + Bi) and Ci, it could be found that the changes of C₂H₂ mole fractions in the simultaneous presence of chemical effects of NH₃ and CO₂ were slightly smaller than the sum of the separate chemical effects of NH₃ and CO₂. It suggested that chemical interactions between the dopants were negligible, as the results of Mahmoud et al. (2019).

Figure 16 illustrated the mole fraction profiles and the rates of production of C₃H₃. From Fig. 16a, it could be concluded that the addition of NH₃ reduced the mole fractions of C₃H₃. The differences between NH₃/CO₂ and FNH₃/FCO₂ revealed that the coupled chemical effects of NH₃ and CO₂ increased the concentration of C₃H₃ with CO₂ addition on the fuel side (F1–F9 conditions in Table 2), contrary to the results of the oxidizer-side CO₂ addition (F10–F18 conditions in Table 2). Figure 16b illustrated that C₃H₃ was generated in large quantities by the complex reactions of C₂H₂+ CH₂, which was one of the reasons why C₃H₃ was considered as the main precursor of A1 generated by small molecules (Jin et al. 2015), then PAHs were easy to form soot after physical or chemical coalesce. When CO₂ was added to the oxidizer side, the coupled chemical effects of NH₃ and CO₂ decreased the rates of all reactions, whereas CO₂ was from the fuel side, the rates of reactions PC₃H₄= C₃H₃+ H and 2C₃H₃=>A1- + H were decreased mainly due to the coupled dilution and thermal effects of NH₃ and CO₂. The bar plot of the chemical effects of NH₃ and CO₂ on the peak mole fractions of C₃H₃ was shown in Fig. 17. The formation of C₃H₃ was inhibited mainly because of the chemical effects of CO₂, Zhang et al. (2018) also gave the similar results, while the chemical effects of NH₃ promoted the formation of C₃H₃. When CO₂ was added to the oxidizer side, the coupled chemical effects of NH₃ and CO₂ could inhibit the generation of C₃H₃ due to the dominance of the chemical effects of CO₂, different from the fuel-side CO₂ addition.

Mole fractions and Rates of production of C₃H₃ with CO₂ dilution and different NH₃ additions. a Mole fractions; b Rates of production

The Difference of peak mole fractions of C₃H₃. Notes: A represented the fuel side was 60% C₂H₄ + 20% CO₂ + 20% NH₃, the oxidizer side was 79% Ar + 21% O₂; B represented the fuel side was 40% C₂H₄ + 20% CO₂ + 40% NH₃, the oxidizer side was 79% Ar + 21% O₂; C represented the fuel side was 60% C₂H₄ + 20% Ar + 20% NH₃, the oxidizer side was 79% CO₂ + 21% O₂. The positive results represented an increase and the negative results represented a decrease. The letters in Figs. 19 and 20 have the same meanings, which will not be noted again in the following. 

It was generally agreed that PAHs were the precursors of soot formation, and the unique chemical structure of benzene ring (A1) was a prominent factor (Dobbins 2007). Therefore, it was necessary to analyze the variations of A1 concentration. The mole fraction profiles of A1 performed in Fig. 18a revealed that the normal NH₃ addition could cause the decrease of A1 mole fractions. However, the coupled chemical effects of NH₃ and CO₂ on the mole fraction of A1 were different due to the diverse sides and concentrations of CO₂ addition. With the fuel-side CO₂ addition (F1–F9 conditions in Table 2), the differences between CO₂/NH₃ and FCO₂/FNH₃ additions suggested that the coupled chemical effects of NH₃ and CO₂ increased the A1 mole fractions, in contradiction with adding CO₂ to the oxidizer side. Meanwhile, comparing the two situations, it could be found that the concentration of A1 was much lower when CO₂ was added to the oxidizer side (F10–F18 conditions in Table 2).

Mole fractions and Rates of production of A1 with CO₂ dilution and different NH₃ additions. a Mole fractions; b Rates of production

To analyze the detailed formation and consumption of A1, Fig. 18b gave the rates of production of A1. It could be found that the main reactions responsible for A1 formation were the closed-loop reactions 2C₃H₃=>A1 and C₂H₂+ C₄H₅-2 = A1 + H. The main reactions responsible for A1 consumption were A1 + H = A1- + H₂, A1 + H = C₄H₅-2 + C₂H₂, and A1 + OH = A1- + H₂O. The pyrolysis consumption paths were mainly through hydrogen extraction reaction attacked by H radical (Yang et al. 2015). Comparing with CO₂/NH₃ and FCO₂/FNH₃ additions, it could be concluded that the reduced rates of all reactions were due to the coupled chemical effects of NH₃ and CO₂ with the oxidizer-side CO₂ addition. While adding CO₂ to the fuel side, the rate of reactions 2C₃H₃=>A1 and A1 + H = C₄H₅-2 + C₂H₂ were decreased mainly because of the coupled dilution and thermal effects of NH₃ and CO₂.

By distinguishing the chemical effects of NH₃ and CO₂, as shown in Fig. 19, it could be found that the chemical effects of NH₃ promoted the generation of A1, while the chemical effects of CO₂ suppressed it. The results tied well with previous study (Naseri et al. 2017). Therefore, the coupled chemical effects of NH₃ and CO₂ on A1 mole fractions depended on the dominant effects. With the oxidizer-side CO₂ addition, the chemical effects of CO₂ dominated the chemical effects of NH₃, then the coupled chemical effects of NH₃ and CO₂ decreased the mole fraction of A1. For fuel-side CO₂ addition, the chemical effects of NH₃ on the A1 mole fraction were more significant and then the coupled chemical effects of NH₃ and CO₂ increased A1 concentration.

The Difference of peak mole fractions of A1

As the known literature (Liu et al. 2015), the chemical effect of CO₂ on soot loading reduction was primarily through reducing the rates of soot formation steps, rather than prompting soot oxidation. Therefore, with the addition of CO₂ on the oxidizer side, the coupled chemical effects of NH₃ and CO₂, which were mainly influenced by the chemical effect of CO₂, reduced both the mole fractions of O, H, OH radicals, and C₂H₂, C₃H₃, A1 soot precursors.

3.2.4 Intermediate oxygenated species

In addition to intermediate hydrocarbon species, intermediate oxygenated species such as formaldehyde (CH₂O) and acetaldehyde (CH₃CHO) were also important aldehyde contaminants generated in the combustion of hydrocarbon fuels. Formaldehyde was particularly irritant to the respiratory system and eyes, toxic and carcinogenic and could accelerate the production of the harmful substance ozone (Konnov et al. 2021). Therefore, the mole fraction profiles of CH₂O and CH₃CHO in C₂H₄/NH₃ flames with diluents were necessary to be revealed in Fig. 20.

The Difference of peak mole fractions of CH₂O and CH₃CHO. a CH₂O; b CH₃CHO

Due to the chemical effects of CO₂, the chemical effects of NH₃ performed diverse impacts on CH₂O and CH₃CHO concentrations with different side CO₂ additions. Figure 20a illustrated that the coupled chemical effects of NH₃ and CO₂ decreased the mole fractions of CH₂O with the fuel-side CO₂ addition slightly (F1–F9 conditions in Table 2), whereas for the oxidizer side (F10–F18 conditions in Table 2), the coupled chemical effects promoted CH₂O formation. It could be explained that the chemical effects of CO₂ promoted the formation of CH₂O, and the coupled chemical effects of NH₃ and CO₂ were mainly affected by the chemical effects of CO₂, leading to an increase of the mole fraction of CH₂O with the oxidizer-side CO₂ addition.

Additionally, the chemical effects of NH₃ and CO₂ on the mole fraction of CH₃CHO were different from those of CH₂O. According to Fig. 20b, it could be found that the chemical effects of NH₃ promoted the formation of CH₃CHO, meanwhile, the coupled chemical effects of NH₃ and CO₂ further promoted the formation of CH₃CHO, especially when CO₂ was added to the oxidizer side.

The chemical kinetic analyses for the chemical effects of NH₃ with different diluents, in particular the coupled chemical effects of NH₃ and CO₂ in ethylene counter-flow diffusion flames, were investigated in this work. The analyses encompassed several critical aspects, such as flame temperature, major species, typical free radicals, intermediate hydrocarbon products, and oxygenated species. The outcomes of the study led to the following conclusions:

1. (1)

   Regardless of the diluents utilized, the flame temperature and the mole fractions of O, H, OH, C₂H₂, C₃H₃, A1 were decreased with NH₃ addition in C₂H₄/NH₃ counter-flow diffusion flames.

2. (2)

   With Ar or He dilution, the chemical effects of NH₃ promoted the formation of OH, C₂H₂, C₃H₃, and A1. With replacing Ar with He on the oxidizer side, the high thermal diffusivity of He reduced the flame temperature, and the pathway of A1 generated by small molecules C₃H₃ was more easily susceptible to He, leading to the lower concentrations of C₃H₃ and A1.

3. (3)

   With fuel-side CO₂ addition, the coupled chemical effects of NH₃ and CO₂ increased the flame temperature and the mole fractions of OH, C₂H₂, C₃H₃, A1. However, with oxidizer-side CO₂ addition, the coupled chemical effects, which were affected by the chemical effects of CO₂ significantly, inhibited the flame temperature and the formations of O, H, OH, C₂H₂, C₃H₃, A1.

4. (4)

   The chemical effects of NH₃ resulted in a decrease in the mole fraction of CH₂O and an increase in the concentration of CH₃CHO. The coupled chemical effects of NH₃ and CO₂ increased the CH₂O concentration with the oxidizer-side CO₂ addition.