Exploring NH3 and NOx Interaction Chemistry With CH4 and C2H4 at Moderate Temperatures and Various Pressures

The oxidation of CH₄/C₂H₄/NH₃/NO/NO₂ gas mixtures was studied aiming to explore the homogenous chemistry of exhaust gas from lean-operated natural gas engine. With respect to this goal, experiments were carried out with a laminar flow reactor under engine-relevant (diluted and lean) conditions over the temperature range of 600–1400 K. Four gas mixtures were designed to evaluate the effects of NO/NO₂ ratio (1, 4) and pressure (0.04 and 1.0 atm) on the interaction chemistry of NH₃/NO_x with CH₄ and C₂H₄. By using synchrotron vacuum ultraviolet photoionization mass spectrometry, fingerprint products for revealing interaction chemistry were identified and quantified, e.g., nitrogenous and oxygenated intermediates. The experimental results show that the NO concentrations are significantly affected by adding CH₄/C₂H₄, changing NO/NO₂ ratio and pressure. Besides, the promotion of DeNO_x reactions and narrower temperature windows of NO reduction are unexpectedly observed in the presence of CH₄/C₂H₄. To interpret the experimental observations, a detailed kinetic model was developed by integrating hydrocarbons/NH₃/NO_x interaction chemistry. Rate of production and sensitivity analyses indicate that the active radical pool is enriched and additional chain-branching pathways regarding NO/NO₂ interconversion are activated with the addition of hydrocarbons. In the presence of both CH₄ and C₂H₄, reaction C₂H₃ + O₂ = CH₂CHO + O was demonstrated as a crucial reaction that drives the reactivity of CH₄/C₂H₄/NH₃/NO/NO₂ mixture. This is explained by the production of CH₂CHO, whose dissociation generates CH₂O and ultimately leads to the abundant production of active OH via the reaction sequence CH₂O → HCO → HO₂ → OH. The conversion kinetics of hydrocarbons, NO and NH₃ under different NO/NO₂ ratios and pressure, as well as the formation kinetics of oxygenated and nitrogenous intermediates was also analyzed in this work.

1 Introduction

Modern combustion engine technologies have increasingly focused on the reduction of carbon dioxide emissions owing to the globally tightening legislation (Johnson, 2015; Börnhorst and Deutschmann, 2021), whereas the highly optimized engine structures and emission control measures render it extremely difficult to further make a breakthrough on decreasing raw emissions. Meanwhile, lean-operated natural gas engines are considered as a promising technology because of their high thermo-efficiency and comparably low pollutant emissions (Lott and Deutschmann, 2021). Alternative concepts like biomethane and power-to-gas technologies will endow gas engines with outstanding potential on the way towards carbon neutrality (Schmitt et al., 2021). Nevertheless, the excessive engine-out emissions of methane, known with potent greenhouse potential and liable to produce carcinogenic formaldehyde by partial oxidation, bring new challenges to natural gas engines (Hutter et al., 2018). Also, the presence of nitrogen oxides (NO_x) in exhaust gas deserves special attention given its particular environmental hazard in engendering photochemical smog and acid rain (Gasnot et al., 2012; Piumetti et al., 2016). Hence, a highly efficient and durable exhaust gas abatement system, commonly containing both catalytic oxidation of unburned hydrocarbons (UHC) and NO_x reduction sections, was considered imperative (Lott and Deutschmann, 2021). The ammonia-based selective catalytic reduction (SCR) technology is currently widely applied for NO_x removal from gas engines benefiting from its higher DeNO_x efficiency and relatively moderate cost. However, it must be pointed out that despite the whole DeNO_x process integrating simultaneously complex gas-phase and surface chemistry (Vassallo et al., 1995; Tamm et al., 2009), the probability of the gas-phase reactions between hydrocarbons, NO_x and NH₃ in SCR was exclusively ignored among most of the studies regarding exhaust gas abatement (Mejía-Centeno et al., 2013; Wang et al., 2019; Lee et al., 2020; Jia et al., 2021; Savva et al., 2021), especially given the recently proposed post-treatment measures allowing the catalytic converters to be positioned closer to the engine thus featuring much higher pressure and temperatures (Lott and Deutschmann, 2021).

To the best of our knowledge, only limited studies have been reported to date in terms of the homogeneous reactivities of NH₃ and NO_x in the presence of multiple other typical exhaust gas constituents, mainly C₁-C₂ hydrocarbons like methane and ethylene, etc. Hemberger et al. (Hemberger et al., 1994) investigated the selective non-catalytic reduction (SNCR) of NO by ammonia with the addition of methane and ethane in a flow reactor over the temperature range 800–1300 K. Gasnot et al. (Gasnot et al., 2012) performed an experimental and kinetic study of the effect of several additives such as CH₄, C₂H₄, C₂H₆, C₂H₂, CH₃OH, C₂H₅OH and CO on ammonia-based SNCR process at temperatures of 900–1200 K. Torkashvand et al. (Torkashvand et al., 2019) investigated hydrocarbon abatement from the exhaust of lean-burn gas engines under ambient pressure and pre-turbine conditions. Most recently, Schmitt et al. (Schmitt et al., 2021) conducted flow reactor experiments in the temperature range of 700–1200 K at atmospheric pressure to reveal influences of individual components by adding NO₂, CH₄, CO, and C₂H₄ sequentially to a highly argon-diluted NO/NH₃ base mixture.

The aim of the present work is to explore the impact of methane and ethylene on the homogeneous conversion chemistry of NO_x and NH₃ under near-real exhaust conditions. With respect to this goal, the oxidation experiments of CH₄/C₂H₄/NH₃/NO/NO₂ gas mixtures were firstly carried out in a laminar flow reactor at lean conditions, at temperatures of 600–1400 K and pressures of 0.04–1.0 atm. Different gas mixtures were designed to investigate the impact of NO/NO₂ ratio and pressure on the conversion kinetics of NH₃, NO and hydrocarbons. Mole fraction profiles of reactants, products, hydrocarbon, nitrogenous and oxygenated intermediates were evaluated by using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). On the basis of these experiments, a detailed kinetic model was developed to interpret the experimental results and reveal the conversion kinetics of NH₃ and NO_x in the presence of CH₄ and C₂H₄ under different conditions.

2 Experimental Methods

The experimental work was conducted at the National Synchrotron Radiation Laboratory, China. Details about the experimental setup have been introduced in our previous work (Qi, 2013; Zhou et al., 2016). Four experimental conditions were selected to cover the pressure ranges from 0.04 to 1 atm, the temperature ranges from 600 to 1400 K, and different NO/NO₂ ratios. Detailed experimental conditions are listed in Table 1. Furthermore, two gas mixture conditions in the absence and presence of methane (Sun et al., 2021), which were reported previously, are also summarized in Table 1 and will be discussed in the following sections to clarify the impact of individual gas additives on the conversion chemistry of NH₃/NO_x. The gas mixtures were highly diluted in carrier gas argon with a total flow rate of 250 standard cubic centimeters per minute (sccm) and an average residence time of 0.6–1.4 s. Gas mixtures were fed into a quartz flow reactor with 0.7 cm inner diameter and 40 cm heating length. The identification and quantification of intermediate species were achieved by using SVUV-PIMS. The calculated uncertainties of mole fractions for species with known photoionization cross sections (PICSs) were estimated to be ±25% and a factor of 2 for those with estimated PICSs.

TABLE 1

TABLE 1. Experimental conditions.

3 Kinetic Modeling

The kinetic model used for the simulation was developed based on AramcoMech 3.0 (Zhou et al., 2018), which contains a well-established C₀-C₂ base mechanism. The sub-mechanism describing NH₃ oxidation as well as interaction kinetics between C₀-C₂ species and NO_x was taken from the work of Glarborg et al. (Glarborg et al., 2018). In addition, rate constants of several key reactions in the sub-mechanism of NH₃, which are mainly related to the production of NH₂ and NO, have been updated from the theoretical studies of Stagni et al. (Stagni et al., 2020). These reactions include the dissociation of HNO and H-abstraction reactions of NH₃ by H, O, OH and HO₂ radicals. The newly proposed reactions pathways of NH radicals by Duynslaegher et al. (Duynslaegher et al., 2012), namely the reactions of NH with O or N₂O to produce N + OH or N₂ + HNO, and the reaction of NH₂ with NO yielding N₂O + H₂, etc., were also implemented in the present kinetic model. The rate constant of NH₂ + O = HNO + H calculated by Sumathi et al. (Sumathi et al., 1998) was also adopted to improve the prediction results of NO_x. The detailed updating list of the rate constants involved in the C₁-C₂/NO_x/NH₃ sub-mechanism has been summarized in our recent work on CH₄/NH₃/NO_x (Sun et al., 2021), thus not outlined here.

Meanwhile, the C₁-C₂/NO_x/NH₃ sub-mechanism has also been further improved in the present work to describe the interaction kinetics among hydrocarbon species, NH species and NO_x. The reaction pathways of carbonyl species such as acrolein, acetaldehyde and ketene with NO_x, and the C₂H₄ + NO₂ channel laid out by Deng et al. (Deng et al., 2019) were included in the present model. Also, the recombination reactions of CH₃ and NH₂, together with C₂H₃ and NO/NO₂/CN, are implemented from the calculation results of Deng et al. (Deng et al., 2019) or by analogy with similar reactions when available. The reaction pathways describing the interaction chemistry between C₂H₂ and NO_x were taken from the recent studies of Marshall et al. (Marshall et al., 2019). Thermodynamic parameters of newly added species such as CH₃NH2CH₃NO₂, C₂H₃NO₂ and C₂H₃CN, etc. in the present model were taken from Glarborg et al. (Glarborg et al., 2018) and the theoretical calculations of Deng et al. (Deng et al., 2019). The simulation of flow reactor was carried out by using the Plug Flow Reactor module in Chemkin-Pro software (ReactionDesign, 2009) with the measured centerline temperatures of flow reactor as input parameters.

4 Results and Discussion

Dozens of species were detected in the oxidation of CH₄/C₂H₄/NH₃/NO/NO₂ gas mixtures and their mole fractions were evaluated as functions of temperatures. Specifically, major nitrogenous intermediates detected and identified in this work include nitromethane (CH₃NO₂), nitroethylene (C₂H₃NO₂), methylamine (CH₃NH₂), nitrous acid (HONO) and cyanides (HCNO). Major oxygenated intermediates detected include formaldehyde (CH₂O), methanol (CH₃OH) and acetaldehyde (CH₃CHO). In the following sections, effect of CH₄/C₂H₄ on the NO_x/NH₃ homogeneous conversion chemistry will be discussed first. Subsequently, the impacts of pressure and NO/NO₂ ratio on the kinetics of HCs/NO_x/NH₃ mixture will be analyzed. Finally, the formation kinetics of nitrogenous and oxygenated intermediates will be analyzed to reveal the unique interactive reactions among hydrocarbons, NH₃ and NO_x.

4.1 Effect of CH₄ and C₂H₄ Addition on NO_x/NH₃ Conversion

Figure 1 shows the experimental and predicted mole fraction profiles of reactants (NH₃, NO, NO₂ and O₂) in the oxidation of gas mixtures 1–6 (GM1-6), Figure 2 shows the experimental and simulated mole fraction profiles of major products (N₂O, N₂, H₂O, CO and CO₂) in the oxidation of GM1-3 and GM5-6. For all the investigated gas mixtures, the model is capable of predicting the temperature-dependent decomposition profiles of reactants except for the discrepancy observed for NO in GM1 and GM4. Regarding the major products, the model is able to reproduce the formation of most products except that the under-estimation of equilibrium concentration of N₂ at 0.04 atm. Rate of production (ROP) analyses have been carried out to reveal the conversion kinetics of NO_x and NH₃ under different conditions. The selected temperatures for the ROP analyses, i.e., 1239 K (GM1), 886 K (GM2), 911 K (GM3), 1060 K (GM4) and 1239 K (GM5), correspond to the temperature at which the conversion rate of ammonia reaches about 50%.

FIGURE 1

FIGURE 1. Experimental (symbols) and simulated (lines) mole fraction profiles of ammonia (NH₃), nitric oxide (NO), nitrogen dioxide (NO₂) and oxygen (O₂) from the oxidation of GM1-6. Experimental data of GM3-4 are taken from the work of Sun et al. (Sun et al., 2021).

FIGURE 2

FIGURE 2. Experimental (symbols) and simulated (lines) mole fraction profiles of nitrous oxide (N₂O), nitrogen (N₂), water (H₂O), carbon monoxide (CO) and carbon dioxide (CO₂) from the oxidation of GM1-3, GM5 and GM6. Experimental data of GM3 are taken from the work of Sun et al. (Sun et al., 2021).

Numerous previous studies (Miller and Bowman, 1989; Glarborg et al., 2018; Okafor et al., 2018; Schmitt et al., 2021) have concluded that reactions R1-R5 are the major reactions in the oxidation of NH₃/NO_x. Figure 3 summarizes the reaction pathways of NH₃ in the oxidation of GM1-5. The results show that regardless of whether there is the addition of hydrocarbons, over 90% of NH₃ is converted to NH₂ via H-abstraction reaction by OH. In the oxidation of NH₃/NO_x (GM4), reactions R2 and R4 contribute to a major part of NH₂ consumption, producing N₂ and nitroxide (H₂NO), respectively. The remaining two reactions R1 and R5 produce NNH and N₂O, respectively. NNH and N₂O are eventually converted to N₂ through single-step collision reactions. H₂NO produced from R4 is eventually converted to NO via nitroxyl (HNO) through stepwise dehydrogenation reactions.

$N{H}_2+NO=NNH+OH(\mathrm{R1})$

$N{H}_2+NO=N_2+H_{2}O(\mathrm{R2})$

$NO+H{O}_2=N{O}_2+OH(\mathrm{R3})$

$N{H}_2+N{O}_2=H_{2}NO+NO(\mathrm{R4})$

$N{H}_2+N{O}_2=N_{2}O+H_{2}O(\mathrm{R5})$

FIGURE 3

FIGURE 3. Major reaction paths of NH₃ in the oxidation of GM1, GM2, GM3, GM4, GM5 and GM6.

By examining Figure 3 one can see that the profile of NO with temperature in GM4 has a distinct difference from that in other mixtures, and is also different from the result observed in the work of Schmitt et al. (Schmitt et al., 2021). To clarify the kinetics behind it, ROP analyses were performed at low- to high-temperatures (860, 1,162 and 1316 K). The results indicate that the initial increase of NO at the temperature of 863 K is mainly caused by the decomposition of nitrous acid (HONO), followed by the interconversion reaction R4. HONO is produced through the reaction sequence NH₂ → H₂NO → HNO (+NO₂) → HONO. Over the temperature range of 900–1100 K, the concentration of NO has only a slight change, generally because the thermal DeNO_x reactions R1 and R2 proceed rather slow over this temperature range. When the temperature increases above 1100 K, NO is quickly consumed via the thermal DeNO_x reactions R1 and R2. As the temperature increases up to 1300 K, a second rise of NO concentration with temperature is observed. ROP analysis results show that reaction R6 is the dominant reaction that contributes to the formation of NO at temperatures above 1300 K.

$N{O}_2+H=NO+OH(\mathrm{R6})$

In comparison with the oxidation of NH₃/NO_x (GM4), no noticeable change was observed for the initial conversion temperatures of NH₃ and NO_x in the oxidation of CH₄/NH₃/NO_x (GM3). However, in the presence of both CH₄ and C₂H₄ (GM2), the initial conversion temperatures of NH₃ and NO_x are found around 50 K lower than those in the oxidation of NH₃/NO_x, as seen in GM2, 3, 4 in Figure 1. By comparing GM2, GM3, GM4 and GM6, it can be also concluded that the addition of hydrocarbons promotes the consumption rate of NH₃, NO₂ and O₂, i.e., for GM2, GM3 and GM6, NH₃, NO₂ and O₂ are quickly consumed over a narrow temperature range, while for GM4, the temperature regime is much wider.

The comparison of experimental and simulated mole fraction profiles of CH₄ and C₂H₄ in GM1-3 and GM5-6 is illustrated in Figure 4. As expected, the initial conversion temperatures, as well as reaction temperature regime of CH₄ and C₂H₄ are noticeably affected by the NO/NO₂ ratio and the pressure. To elucidate the effect of CH₄/C₂H₄ addition on NO reduction, sensitivity analysis on NO was also performed for GM1-4 and the results are summarized in Figure 5, a negative sensitivity coefficient indicates the promoting effect on NO consumption. As seen in Figure 5D, the competing reactions R1 and R2 show considerable sensitivity to NO consumption in GM4. Other sensitive reactions include chain-branching reactions promoting the formation of active radical pool along with those related to NO/NO₂ interconversion.

FIGURE 4

FIGURE 4. Experimental (symbols) and simulated (lines) mole fraction profiles of methane (CH₄) and ethylene (C₂H₄) in the oxidation GM1, GM2, GM5 and GM6.

FIGURE 5

FIGURE 5. Sensitivity analysis of NO during the flow reactor oxidation of GM1, GM2, GM3 and GM4 at 1213, 860, 911, and 1188 K, respectively. Negative sensitivity coefficients indicate promotion to NO consumption.

In the oxidation of CH₄/NH₃/NO_x mixture (GM3), the concentration of NO has a first rise at a lower temperature ( ∼ 860 K), and then it quickly decreases at the temperature around 911 K (Figure 1J). As the temperature increases to 1080 K, the concentration of NO has a second rise. According to the ROP analyses, the first rise of NO concentration is attributed by the reaction of CH₃ and NO₂ (R7). The reduction of NO between 911–1080 K is attributed by the thermal DeNO_x reactions (R1 and R2), the interaction between NO and HO₂ (R3), and the reaction of CH₃O with NO (R8). The second rise of NO at higher temperatures is attributed by the reaction R6, which is the same as that in the oxidation of GM4. The sensitivity analyses shown in Figure 5C again illustrate the crucial role of reactions R7 and R8, i.e., converting the relatively inactive CH₃ and CH₃O₂ into highly active and unstable CH₃O radicals. The decomposition of CH₃O radical initiates the chain-branching reaction by replenishing the active radical pool to a major extent via the reactions CH₃O(+M) = CH₂O + H(+M) and H + O₂ = OH + O. In addition, another consumption pathway of CH₃O, i.e., CH₃O + O₂ = CH₂O + HO₂, produces HO₂ of lower reactivity and competes with the above-mentioned direct dehydrogenation channel, thus exhibiting negative sensitivity to the reduction of NO.

$C{H}_3+N{O}_2=C{H}_{3}O+NO(\mathrm{R7})$

$C{H}_3{O}_2+NO=C{H}_{3}O+N{O}_2(\mathrm{R8})$

In the presence of both CH₄ and C₂H₄ (GM1-2, GM5-6), the reaction network becomes more complex. Base on the ROP analysis, the reaction scheme is summarized in Figure 6. It should be noted that for all the four gas mixtures the major reaction pathways are similar, while the reaction flux is different. In the case of CH₄/C₂H₄ addition, the mole fraction profiles of NO in GM2 also show multiple extreme points. ROP analyses at 835, 860, 911, and 985 K suggests that the occurrence of the first maximum and minimum points of NO mole fraction is the same as GM3 in terms of the kinetic interpretation, while the following attenuation of NO is dominated by the oxidation reaction with HO₂ (R3), while other reactions have little impacts on it. Additionally, for the low pressure case of GM1 (0.04 atm), the reduction of NO is not sensitive to the recombination of methyl or vinyl group with NO₂, which is in clear contrast to GM2.

FIGURE 6

FIGURE 6. Major reaction paths of CH₄ and C₂H₄ in the oxidation of GM1, GM2, GM5 and GM6 at 1239, 886, 1239, and 886 K, respectively.

According to the ROP analyses, CH₄ mainly proceeds the H-abstraction reaction initiated by OH to produce CH₃, which is further oxidized by NO₂ or HO₂ producing methoxyl, or undergoes the recombination reactions with NO₂, O₂ and CH₃. Particularly, the recombination reaction of CH₃ and NO₂ (R9) is a major reaction that inhibits the reduction of NO, as can be seen in Figure 5B, while the competing oxidizing reaction R7 is the dominant reaction that promotes the consumption of NO. CH₃O subsequently decomposes to CO through CH₂O and HCO via the stepwise H-elimination process, CO is mainly converted to CO₂ via the reaction CO + OH = CO₂+H.

$C{H}_{3}N{O}_2\left(+M\right)=C{H}_3+N{O}_2\left(+M\right)(\mathrm{R9})$

At low pressure, the consumption of C₂H₄ mainly undergoes the H-abstraction reaction by OH, giving vinyl radical (C₂H₃). At atmospheric pressure, OH-addition reaction producing 2-hydroxyethyl (PC₂H₄OH) also becomes important. PC₂H₄OH prefers to add O₂ to form O₂C₂H₄OH, which further dissociates into two formaldehyde molecules via the C-C bond fission reaction. The consumption pathways of C₂H₃ are more complex than those of PC₂H₄OH. On one hand, C₂H₃ can add O₂ or NO, followed by β-C-C scission reaction and finally produce formaldehyde. On the other hand, it can take an O atom from O₂ (R10) or NO₂ (R11) to form vinoxy (CH₂CHO). Besides, part of C₂H₃ undergoes recombination reaction with NO₂ (R12) and this reaction is the most sensitive reaction that promotes the reduction of NO at 1 atm, as shown in Figure 5B. Reaction R12 is a typical competitive reaction of R7 (CH₃ + NO₂ = CH₃O + NO), which is the most sensitive reaction that inhibits the reduction of NO. Therefore, in the presence of both CH₄ and C₂H₄, the role of CH₄ and C₂H₄ on the reduction of NO is opposite. This can be also proved by the extremely slight change of NO reduction ratios as well as the DeNO_x temperature windows of GM2 and GM3 shown in Figure 1. Although the subsequent reaction of ethylene also involves interaction reactions with NO_x, the proportion of this interaction is not appreciable (mostly less than 5%).

$C_2{H}_3+O_2=C{H}_{2}CHO+O(\mathrm{R10})$

$C_2{H}_3+N{O}_2=C{H}_{2}CHO+NO(\mathrm{R11})$

$C_2{H}_{3}N{O}_2=C_2{H}_3+N{O}_2(\mathrm{R12})$

As a key intermediate produced from the interaction between C₂H₄ and NO_x, CH₂CHO can either react with O₂ to produce CH₂O, or produce ketene (CH₂CO) via H-abstraction reaction by NO₂. It is noteworthy that CH₂O is also a major decomposition product of CH₃O. As a result, CH₂O is an abundant product in the oxidation of CH₄/C₂H₄/NO_x/NH₃ mixtures at atmospheric pressure The decomposition of CH₂O eventually produces active OH via the reaction sequence CH₂O _(+OH/H/O) → HCO _(+O2) → HO_{2 (+NO)} → OH and thereby promote the reactivity. The ketenyl (HCCO) radical produced by CH₂CO can either be directly converted to CO and CO₂, or proceed the reactions with NO/NO₂ leading to the formation of HCNO. A majority of HCNO undergoes the reaction with OH yielding CO, the remaining part decomposes to CO via the HCO as intermediate.

In general, the addition of CH₄/C₂H₄ at lean conditions promotes the formation of reactive radicals, thereby enhancing the global reaction reactivity. In addition, additional chain-branching pathways that consume NO are introduced due to the formation of CH- or CHO- type radicals such as CH₃O. As a result, in the presence of CH₄/C₂H₄, the NO conversion temperature regime is narrower than that in the oxidation of NH₃/NO_x, which is in agreement with the conclusions of Glarborg et al. (Glarborg et al., 2018).

4.2 Effects of NO/NO₂ Ratio and Pressure on HCs/NO_x/NH₃ Kinetics

4.2.1 CH₄ and C₂H₄

As can be observed in Figure 4, when NO/NO₂ ratio increases from 1 to 4, the initial decomposition temperature of C₂H₄ is only slightly changed, while the initial decomposition temperature of CH₄ is decreased by around 50 K. As pressure increases from 0.04 to 1 atm, the initial decomposition temperatures of CH₄ and C₂H₄ are decreased by around 150–400 K. ROP and sensitivity analyses have been performed to clarify the influence of NO/NO₂ ratio and pressure on the conversion kinetics of CH₄ and C₂H₄. Figure 7 displays the sensitivity analysis results of CH₄ and C₂H₄ in the flow reactor oxidation of GM1, GM2, GM5 and GM6 at 1239, 886, 1239, and 886 K, respectively. It can be seen that the dominant reactions have not changed significantly as the NO/NO₂ ratio increases from 1 to 4, indicating that the NO/NO₂ ratio has little effect on the conversion kinetics of CH₄ and C₂H₄.

FIGURE 7

FIGURE 7. Sensitivity analysis on CH₄ and C₂H₄ consumption during the flow reactor oxidation of GM1, GM2, GM5 and GM6 at 1239, 886, 1239, and 886 K, respectively.

In contrast to NO/NO₂ ratio, the change of pressure has significant effect on the dominant reactions of CH₄ and C₂H₄ consumption. As seen in Figure 7, at low pressure, the typical chain-branching reaction H + O₂ = OH + O is the most sensitive reaction promoting the consumption of CH₄ and C₂H₄. The reaction CH₃+NO₂ = CH₃O + NO is also identified as a sensitive promoting reaction. This does not seem surprising since this reaction and the following decomposition reactions of CH₃O have been already identified as one of the most important reactions that initiate the production of radical pool. At atmospheric pressure, H-abstraction reaction of C₂H₄ by OH is identified as the dominant reaction responsible for the consumption of both CH₄ and C₂H₄. H-abstraction reaction of CH₄ by OH is observed as a promoting reaction for CH₄ consumption but an inhibiting reaction for C₂H₄ consumption. This indicates that OH plays a key role in controlling the global reactivity. The O₂-addition reaction of C₂H₃ is also a sensitive promoting reaction for CH₄ and C₂H₄ consumption. As discussed above, the decomposition of CH₂CHO produced from this reaction readily generates CH₂O, whose further decomposition reaction produces HCO as major products.

4.2.2 NH₃ and NO

Figure 8 depicts the sensitivity analysis results of NH₃ during the flow reactor oxidation of GM1, GM2, GM5, and GM6 at 1239, 886, 1239, and 886 K, respectively. Similar to CH₄ and C₂H₄, the dominant reactions for NH₃ consumption are almost identical at different NO/NO₂ ratios. At low pressure (0.04 atm), the most sensitive reaction for NH₃ conversion is H + O₂ = O + OH owing to the relatively higher temperatures. At atmospheric pressure, H-abstraction reaction by OH controls the decomposition of NH₃. Reaction C₂H₄ + OH = C₂H₃ + H₂O and reaction C₂H₃ + O₂ = CH₂CHO also play a key role in the conversion of NH₃ regardless of low and atmospheric pressure, the latter reaction is closely related to the abundant production of CH₂O which ultimately dominates the yield of OH. Under both low and atmospheric pressure, the H-abstraction reaction of CH₄ by OH always plays the role that inhibits the conversion of NH₃, since CH₄ and its decomposition products strongly compete for the active radical pool (OH/O/H).

FIGURE 8

FIGURE 8. Sensitivity analysis on NH₃ consumption during the flow reactor oxidation of GM1, GM2, GM5and GM6 at 1239, 886, 1239, and 886 K, respectively.

As can be observed in Figure 1, the maximum conversion ratio of NO increases as NO/NO₂ ratio increases. Besides, the impact of pressure on the conversion of NO is also significant. At atmospheric pressure the conversion ratio of NO increases (from 9% in GM2 increased to 45% in GM6) as NO/NO₂ ratio increases from 1 to 4. However, at low pressure (0.04 atm) the concentration of NO is found to go up with increasing temperature, i.e., at the same NO/NO₂ ratio, the fall of pressure directly inhibits the reduction of NO. In the oxidation experiment on CH₄/NO/NO₂ carried out by Sahu et al. (Sahu et al., 2021), it is also found that the initial NO fraction needs to approach a certain level to enhance the reactivity of the mixture, or else the low level of NO would act as an inhibitor. This non-monotonous sensitization impact of NO is attributed to the antagonism between chain-terminating reaction CH₃ + NO₂ (+M) = CH₃NO₂ (+M) and chain-branching process CH₂O + HO₂ = HCO + H₂O₂, H₂O₂(+M) = OH + OH(+M). Sahu et al. pointed out that when the initial concentration of NO is high, more CH₂O and HO₂ are produced, thus leading to the transition to chain-branching process.

4.3 Formation of Major Products and Intermediates

N₂ and N₂O are two major nitrogenous products observed in the present work. As seen in Figure 2, the present model can capture the profiles of N₂ at atmospheric pressure, while highly under-predicts its formation at low pressure. Base on the sensitivity analysis, the formation of N₂ is controlled by reaction R7 (CH₃ + NO₂ = CH₃O + NO). However, the kinetics of CH₃ + NO₂ under low pressure remains controversial despite certain studies available in the literature (Yamaguchi et al., 1999; Srinivasan et al., 2005; Glarborg et al., 2018). Therefore, the under-estimation of N₂ at low-pressure may be attributed to the under-estimation of the rate constant of R7 at higher temperatures.

Glarborg et al. (Glarborg et al., 2018) mentioned that one of the disadvantages of SNCR with urea lies in the formation and possible emission of N₂O, which is a harmful gas destroying ozone layer. In the present experiments, N₂O has been observed to form in the order of tens of ppm on average over a wide temperature range. ROP analysis shows that almost all of N₂O is produced from reaction R5 (NH₂ + NO₂ = N₂O + H₂O) at atmospheric pressure, while at low pressure the reaction NH + NO = N₂O + H controls the formation of N₂O. In the work of Alzueta et al. (Alzueta et al., 2021), few ppm of N₂O formation was also detected in the flow reactor oxidation experiment of NH₃/NO, and the reaction N₂H₂ + NO = NH₂ + N₂O was considered as dominant reaction for N₂O formation. However, in the present work the contribution of this pathway is found almost negligible (<1%) for N₂O formation.

Figure 9 illustrates the experimental and simulated mole fraction profiles of CH₃OH, CH₂O and CH₃CHO in the oxidation of GM1, GM2, GM5 and GM6. Among them, CH₂O is the most abundantly produced. ROP analysis shows that the reaction CH₃O(+M) = CH₂O + H(+M) dominates CH₂O formation independent of pressure and NO/NO₂ ratio. The production of CH₂O is derived from either methane- or ethylene-related oxidation steps. At atmospheric pressure, these two parts account for almost half of each, while at low pressure due to the activation of the additional formaldehyde generation path, CH₃ + O = CH₂O + H, the part derived from methane occupies about 70%. The concentration of CH₂O at low pressure peaks at a relatively high temperature of 1200 K, demonstrating that at this temperature it is sufficient to overcome the energy barrier of the reaction between CH₃ and O.

FIGURE 9

FIGURE 9. Experimental (symbols) and simulated (lines) mole fraction profiles of methanol (CH₃OH), formaldehyde (CH₂O) and acetaldehyde (CH₃CHO) in the oxidation of GM1, GM2, GM5 and GM6.

In regard to acetaldehyde (CH₃CHO), its production is closely related to ethenol (C₂H₃OH). At atmospheric pressure a majority of CH₃CHO is produced via the reaction C₂H₃OH + NO/NO₂ = CH₃CHO + NO/NO₂. At low pressure, most of CH₃CHO is formed from the isomerization of ethenol. As an unstable reactive intermediates, C₂H₃OH is primarily generated from the reaction C₂H₄ + OH = C₂H₃OH + H, and partly from the reaction C₂H₄ + OH = CH₃CHO + H. As for methanol, the yield observed in the experiment is not notable, mainly deriving from the H-abstraction of CH₃O from HO₂ or CH₂O, or through the recombination of CH₃ and OH.

Figure 10 displays the experimental and simulated mole fraction profiles of HCNO and HONO in gas mixtures of GM1, GM2, GM5 and GM6. HCNO is a key nitrogenous intermediate produced from the interaction of hydrocarbons and NO_x under high temperature and reducing conditions in reburning chemistry, where nitric oxides are converted into cyanides and isocyanides by reactions with multiple hydrocarbon-derived radicals such as CH₃, ³CH₂, and HCCO. Under the excess oxygen condition investigated in this work, part of the contribution of hydrocarbon derivatives to NO reduction is found mainly through the reactions R13-R15.

$C_2{H}_3+NO=HCN+C{H}_{2}O(\mathrm{R13})$

$CHCHO+NO=HCNO+CO+H(\mathrm{R14})$

$HCCO+NO=HCNO+CO(\mathrm{R15})$

The above reactions contribute less than 1% to NO reduction at atmospheric pressure, but at low pressure, it approaches around 9%. Consequently, although the addition of CH₄/C₂H₄ enriches the free radical pool and promotes the reactivity of the NH₃/NO/NO₂ mixture, its direct contribution to NO reduction is still negligible compared to the thermal DeNO_x reactions (R1-R5). Furthermore, R13-R15 mostly feed into the cyanide pool (HCN, HCNO), while the subsequent consumption of HCNO through R16 and R17 recycle NO with the dissociation of H₂NO/HNO, which complicates the process of HCNO kinetics on NO reduction.

$HCNO+OH=H_{2}NO+CO(\mathrm{R16})$

$HCNO+OH=HNO+HCO(\mathrm{R17})$

FIGURE 10

FIGURE 10. Experimental (symbols) and simulated (lines) mole fraction profiles of HCNO and nitrous acid (HONO) in the oxidation of GM1, GM2, GM5 and GM6.

Apart from HCNO, nitrous acid (HONO) was also detected in the experiment, as seen in Figure 10. HONO is mainly produced from the H-abstraction reactions of CH₂CHO and H₂NO by NO₂. HONO majorly undergoes N-O bond fission to yield NO and OH, thus the HONO participated reactions belong to sensitivity reactions for the consumption of NH₃, NO, CH₄ and C₂H₄.

Figure 11 shows the measured mole fractions of CH₃NH₂, CH₃NO₂, C₂H₃CN, C₂H₃NO and C₂H₃NO₂ in the oxidation of GM1, GM2, GM5 and GM6. The identification of these species provides key evidence for the direct recombination reactions of CH₃, C₂H₃ with NH₂, NO_x, CN, i.e., CH₃NO₂ is the product from the reaction of CH₃ with NO₂, while C₂H₃CN is produced from the reaction of C₂H₃ with CN. Nevertheless, these species are beyond the focus of the present study, thus only a brief description is given herein.

FIGURE 11

FIGURE 11. Experimental mole fraction profiles of methylamine (CH₃NH₂), nitromethane (CH₃NO₂), vinyl cyanide (C₂H₃CN)nitrosoethylene (C₂H₃NO) and nitroethylene (C₂H₃NO₂) in the oxidation of GM1, GM2, GM5 and GM6.

As can be observed in Figure 11B, CH₃NO₂ is the most abundantly produced nitrogenous intermediates at 1 atm. CH₃NO₂ mainly decomposes to CH₂O and NO via CH₂NO₂ as intermediate. CH₃NH₂ can be observed at both 0.04 and 1 atm and it is mainly formed from the recombination of CH₃ with NH₂. The decomposition of CH₃NH₂ finally gives HCN via sequential dehydrogenation reactions CH₃NH₂ →CH₂NH₂ → CH₂NH → HCNH/H₂CN → HCN. The oxidation of HCN produces NO, N₂ and CO as major products under fuel-lean conditions. The decomposition of species produced from the reactions of C₂H₃, such as C₂H₃CN, C₂H₃NO and C₂H₃NO₂, also proceed through the stepwise H-elimination reaction, leading to the production of C₂H₂, CN and NO. C₂H₂ and CN are finally converted into CO and NO through sequential oxidation reactions. Because the consumption pathways of these nitrogenous intermediates are still not well understood, the present model is not able to reproduce their mole fraction profiles. More experimental and theoretical studies are deserved to achieve a better interpretation of the interaction kinetics between hydrocarbon radicals with NH, CN radicals and NO_x.

5 Conclusion

The present work has focused on two aspects with the aim to reveal the NH₃ and NO_x interaction chemistry with CH₄ and C₂H₄ at moderate temperatures and various pressures. First, speciation profiles of reactants, products, nitrogenous and oxygenated intermediates were obtained by using synchrotron vacuum ultraviolet photoionization mass spectrometry. Second, a detailed kinetic model integrating HCs/NH₃/NO_x interaction chemistry was developed and applied to interpret the experimental observations. Rate of production and sensitivity analyses were performed to analyze the conversion chemistry of NO_x and NH₃ in the presence of hydrocarbons.

The experimental results show that the addition of CH₄ and C₂H₄ promotes the conversion of NO and NH₃ at atmospheric pressure in terms of decreasing the initial conversion temperature and narrowing the reaction temperature range. The analysis results indicate that CH₄/C₂H₄ addition at atmospheric pressure promotes DeNO_x-related reactions mainly by enriching the radical pool. Besides, additional chain-branching pathways that convert NO₂ to NO are introduced due to the production of CH₃O radical. Reaction C₂H₃ + O₂ = CH₂CHO + O is found to play a key role in driving the reactivity of CH₄/C₂H₄/NH₃/NO/NO₂ mixture at all the investigated conditions, generally because the further decomposition of CH₂CHO generates CH₂O, which eventually produce OH abundantly via the reaction sequence CH₂O → HCO → HO₂ → OH.

The NO/NO₂ ratio is found to have only a slight impact on the conversion of CH₄, C₂H₄, NH₃ and NO from the experimental observations, while the change of pressure has significant impacts. Regarding the oxygenated and nitrogenous intermediates, formaldehyde and nitromethane are observed as the most abundantly produced oxygenated and nitrogenous intermediates, respectively. The identification of nitrogenous intermediates such as methylamine, nitromethane, vinyl cyanide, nitrosoethylene and nitroethylene provides key evidence for the direct recombination reactions of hydrocarbon or hydrocarbon radicals with NO_x, NH₂ or CN. The new data and corresponding analyses are expected to provide valuable information for understanding the complex gas phase reactions in the exhaust gas as well as an extension of the kinetic model of HCs/NH₃/NO_x reaction systems.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author Contributions

YD: Methodology, Investigation, Data curation, Writing- original draft. ZS: Methodology, Investigation, Data curation. WY: Conceptualization, Methodology, Writing -review and editing, Funding acquisition. JY: Methodology, investigation. ZZ: Methodology, Writing - review and editing. FQ: Funding acquisition, Writing - review and editing.

Funding

The authors are grateful for the funding support from the National Natural Science Foundation of China (51706137, 51761135111) and the National Key R&D Program of China (2019YFA0405602).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, orclaim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

The authors wish to acknowledge the financial support provided by the National Natural Science Foundation of China.

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