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Ignition Delay Time and Chemical Kinetic Study of Methane and Nitrous Oxide Mixtures at High Temperatures Fuquan Deng, Feiyu Yang, Peng Zhang, Youshun Pan, Yingjia Zhang,* and Zuohua Huang State Key Laboratory of Multiphase Flows in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China S Supporting Information *

ABSTRACT: Ignition delay times of N2O/CH4/O2/Ar mixtures with varying N2O mixing ratios (N2O/CH4 mole blending ratio = 0:100, 30:70, 50:50, and 70:30) were measured behind reflected shock waves at pressures of 1.2−16 atm, equivalence ratios of 0.5−2.0, and temperatures of 1220−2336 K. At currently investigated conditions, the reactivity of methane is significantly promoted by N2O addition, resulting in an obvious reduction of the ignition delay time, and this effect becomes more pronounced at the fuel-rich condition and high pressure. However, N2O addition only results in a slight reduction in the global activation energy. To eliminate the effect of different hydrocarbon mechanisms, a widely accepted kinetic mechanism, Aramco Mech 1.3, is used to combine with three available NOx submodels (Gersen et al., Konnov, and Mathieu et al.) and simulate the measured ignition delay times. Generally, the three assembled models give a similar prediction performance and agree well with the experimental data for a low level of N2O addition, but they exhibit a discrepancy for a high level of N2O addition. Sensitivity analysis and radical pool analysis are conducted to interpret the kinetic effect of N2O addition on methane ignition chemistry. Results indicate that the promoting effect of N2O addition on methane ignition is mainly attributed to the contribution of the following three reactions: N2O + M = N2 + O• • + M, N2O + H• = N2 + •OH, and N2O + •CH3 = CH3O• + N2, which can significantly increase the concentration of the radical pool and accelerate the peak of the radical pool. tube at ϕ of 0.5−2.0 and pressures of 1−28 atm. Mathieu et al. proposed a NOx model used to simulate their measured data and compared to three common NOx models (Gersen et al.,7 Sivaramakrishnan et al.,22 and Mevel et al.12), and the results indicated that the differences among the four models were generally small. In addition, they found that their model could well reproduce the global activation energy for each condition but overpredicted the ignition delay times at 11.0 atm. Through the literature review above, it can be seen that the previous work mainly focused on N2O/CH4 with a low level of N2O addition. However, higher N2O concentrations exhibit a stronger effect on CH4 oxidation, and more work is still required to further evaluate the NOx model and understand the chemical interaction between NOx and hydrocarbons over wide ranges. The first objective of this study was, therefore, to measure ignition delay times for N2O/CH4 mixtures with a wider blending ratio (0:100, 30:70, 50:50, and 70:30) over ϕ of 0.5−2.0, p of 1.2−16 atm, and T of 1220−2336 K. In addition, through the comparisons of the NOx models proposed by both Mével and Shepherd18 and Mathieu et al.,5 we found that the hydrocarbon submodel exerted a significant effect on the reactivity of the NOx/CH4 mixtures. The second objective was thus to compare the measured data to literature NO x mechanisms coupling to the same hydrocarbon mechanism Aramco Mech 1.3,23 to eliminate the effect of hydrocarbon chemistry. Finally, the sensitivity analysis and radical pool

1. INTRODUCTION Coalbed methane (CBM) is methane trapped in coal seams underground, and it has attracted great attention as a clean alternative fuel over the past few decades1−3 for its importance in clean and efficient combustion of fossil fuels. However, some impurities, such as NO2 and N2O, exist in CBM, especially lowquality CBM, which can exert a great effect on the combustion of CBM.4−7 For internal combustion engines with exhaust gas recirculation (EGR), NOx (NO2 and N2O) formed during the combustion of the fuel can participate in the next combustion cycle; even a small number of NOx (50 ppm) can significantly influence the combustion and emission performances of fresh mixtures.4−11 Furthermore, as mentioned by Mével and Shepherd,12 N2O is a major intermediate and serves as an oxidizer, reacting with other small hydrocarbons formed in the combustion of many propellants. For the N2O/CH4 reaction system, only two studies are available thus far.5,12 Mével and Shepherd12 measured the ignition delay times of CH4, C2H6, CH4/C2H6, and C2H4/C2H2 mixtures with either N2O or N2O/O2 as the oxidizer at a pressure (p) of around 3.0 atm and at equivalence ratios (ϕ) of 0.78−1.8. They evaluated the performances of four NOx models (Konnov,13 GRI 3.0,14 Daugaut,15 and Blanquart15−21) ) by comparing the measured data (ignition delay times and OH* emission signals) and the model predictions and found that the reactions N2O (+M) = N2 + O• • (+M) and N2O + H• = N2 + •OH were important for the mixtures with only N2O, while for mixtures with both N2O and oxygen, the fuels were mostly driven by N2O + H• = N2 + •OH and H• + O2 = O• • + • OH. Recently, Mathieu et al.5 investigated the accelerating effect of N2O on methane ignition in a high-pressure shock © XXXX American Chemical Society

Received: November 2, 2015 Revised: January 8, 2016

A

DOI: 10.1021/acs.energyfuels.5b02581 Energy Fuels XXXX, XXX, XXX−XXX

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steepest slope of the OH* signal and the zero signal level, as shown in Figure 1. In this study, three baseline CH4/O2/Ar mixtures were diluted with roughly the same mole fraction of Ar (around 94.3%) for equivalence ratios from 0.5 to 2.0. N2O was added to the CH4/O2/Ar mixtures at mole fractions of N2O/CH4 = 0:100, 30:70, 50:50, and 70:30 at ϕ = 1.0 to investigate the effect of the N2O addition and equivalence ratios. The total mole fractions of Ar and N2O were equal to that in the neat CH4 mixtures (about 94.30%). Detailed compositions of the eight mixtures tested here are given in Table 1. All of the ignition delay data of eight mixtures measured in this study are provided in Table S1 of the Supporting Information.

analysis were also conducted using these models to interpret the effect of N2O addition.

2. EXPERIMENTAL SECTION All experiments were performed in a stainless-steel shock tube described in detail in our previous studies.24,25 Briefly, the shock tube

3. RESULTS AND DISCUSSION 3.1. Mechanism Selection and Numerical Procedure. In this study, three recently published NOx mechanisms (Gersen et al.,7 Konnov,30 and Mathieu et al.5) were individually merged into the same hydrocarbon model (Aramco Mech 1.323) to separate the effect of the NOx chemistry from that of the hydrocarbon chemistry. All of the simulations were performed in the CHEMKIN II program31 coupled with the SENKIN code.32 To consider the non-ideal facility effect (dp/ dt), the SENKIN-VITM approach suggested by Chaos and Dryer33 was used for all of the simulations. The simulated ignition delay times were defined as the steepest slope of the temperature, which was similar to that from the steepest slope of OH* used in the experiment. 3.1.1. Methane Mechanism. In this study, Aramco Mech 1.323 was chosen to investigate methane chemistry for its systematic modified and extensive validation against experimental data over a wide range of conditions and different experimental apparatuses, including shock tube, rapid compression machine (RCM), laminar flame, jet-stirred reactor (JSR), and flow reactor. Panels a−c of Figure 2 depict the comparisons between the simulations with Aramco Mech 1.3 and the ignition delay times of methane at pressures from 1.2 to 10 atm and at equivalence ratios of 0.5, 1.0, and 2.0. As expected, Aramco Mech 1.3 well reproduces the measured neat methane ignition delay times over all of the conditions studied herein. 3.1.2. NOx Mechanism. For the NOx chemistry, three models (Gersen et al.,7 Mathieu et al.,5 and Konnov30) were selected to test their performance against the data. A brief description of the three NOx submodels is given here. The NOx submodel by Gersen et al. consists of 66 species and 479 reactions and is developed on the basis of the study by Rasmussen et al.4 It has been extensively validated against RCM, shock tube, and plug-flow reactor data. The NOx submodel by Mathieu et al., involving 36 species and 305 reactions, is proposed on the basis of their previous work of H 2 /N 2 O mixtures. 34 In this model, the NH 3 /NO x /H 2

Figure 1. Typical end-wall pressure and OH* emission signal measurement for the N30/0.5 mixture at p = 10 atm and T = 1335 K. has a diameter of 11.5 cm, with a 4.8 m length driven section and a 4 m length driver section, employing a double-diaphragm system. Before each experiment, the entire tube was evacuated to below 1 Pa by a mechanical-roots vacuum pump system. The leak rate was typically less than 1 Pa/min. The N2O/CH4/O2/Ar mixtures were prepared in a 128 L stainless-steel tank according to Dalton’s low partial pressures and then rested for more than 12 h to ensure sufficient mixing. A highaccuracy pressure transmitter (Rosemount 3051) was applied to measure the partial pressure of each component (the uncertainty of each composition is below 1 Pa). High-purity He and N2 (purity up to 99.995 and 99.999% separately) were used as the driver gas. The purities of other gases used in this study were CH4 (purity up to 99.99%), N2O [diluted at 35% with Ar (purity up to 99.99%)], Ar (purity up to 99.99%), and O2 (purity up to 99.99%). Three time interval counters (Fluke PM6690), which were triggered by four pressure transducers (113B26), were installed along the final 1.3 m of the shock tube with the same interval of 300 mm between them and used to obtain the incident shock velocity. The reflected shock temperature was calculated using Geseq26 software. According to our previous publications,27,28 the typical uncertainties of the temperature and ignition delay times were within ±25 K and 20%, respectively. A photomultiplier (Hamamatsu, CR131) with a filter at 306 ± 10 nm and a piezoelectric pressure transducer (113B26) were installed at the end wall of the shock tube to monitor the OH* signal and pressure after the reflected waves. The typical pressure rise rate caused from the non-ideal facility effect was determined to be 4%/ ms.29 The ignition delay time was defined as the time interval between the arrival of the shock wave at the end wall and the intersection of the

Table 1. Detailed Constitutes of Four Mixtures Studied in the Current Experiment mixture

ϕ

1 2 3 4 5 6 7 8

1 0.5 2 1 1 1 0.5 2

addition ratio N0/1.0 N0/0.5 N0/2.0 N30/1.0 N50/1.0 N70/1.0 N30/0.5 N30/2.0

100% CH4 100% CH4 100% CH4 70% CH4/30% 50% CH4/50% 30% CH4/70% 70% CH4/30% 70% CH4/30%

CH4 (%) 1.904 1.137 2.875 1.902 1.896 2.013 1.153 2.881

N2O N2O N2O N2O N2O B

N2O (%)

O2 (%)

Ar (%)

0.823 1.890 4.872 0.495 1.233

3.830 4.549 2.872 3.826 3.792 4.081 4.614 2.881

94.266 94.314 94.253 93.448 92.422 89.035 93.738 93.005


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Figure 3. Comparison of the model predictions and measured data at pressures of 1.2−10 atm for the N30/0.5 mixture.

Figures 3−5 show comparisons between the measured ignition delay times of the N2O/CH4 mixtures and the predictions calculated by the three assembled models (Aramco−G denoting the combined model of Aramco Mech 1.3 and Gersen et al., Aramco−M denoting the combined model of Aramco Mech 1.3 and Mathieu et al., and Aramco−K denoting the combined model of Aramco Mech 1.3 and Konnov) for N30/1.0, N50/1.0, N70/1.0, N30/0.5, and N30/2.0. Generally, the three assembled models can well capture the trend of pressure and equivalence ratio dependences, especially for the N30/1.0, N30/0.5, and N30/2.0 mixtures. Although three assembled models give a similar prediction for N30/1.0, N30/0.5, and N30/2.0 mixtures, they still show a discrepancy at elevated pressure and high N2O mole fraction, and the discrepancy becomes more obvious at relatively low temperatures (Figure 4). For the N70/1.0 mixture, for instance, the three assembled models can capture well the pressure dependence and the measured data at high temperatures (above 1650 K) and at all pressures. However, the Aramco−M and Aramco−G models slightly overpredict at 1.2 atm, and the Aramco−K model underpredicts the ignition delay times at 16 atm and lower temperatures (below 1650 K). Particularly, the model prediction of Aramco−M is about 1.5 times longer than that of Aramco−K at 1250 K and 16 atm for N70/1.0. In the next section, a detailed interpretation will be given to state the reasons for the discrepancy among the three NOx submechanisms. 3.2. Results and Discussion. In this section, the effects of the pressure, equivalence ratio, and N2O concentration are analyzed carefully based on the measured and simulated results. As shown in Table 1, N30/1.0, N50/1.0, and N70/1.0 were measured over pressures of 1.2−16 atm and the other five mixtures were measured over 1.2−10 atm. Meanwhile, in consideration of the better predictive performance of Aramco−K for N0/1.0 and N30/1.0 mentioned above, the simulations of Aramco−K for N0/1.0 at 16 atm are used to compare to N30/1.0, N50/1.0, and N70/1.0 to investigate the effect of the N2O mole concentration at ϕ = 1.0 and the simulations of Aramco−K for N30/1.0 at 10 atm are used to compare to N30/0.5 and N30/2.0 to investigate the effect of the equivalence ratio. 3.2.1. Pressure Dependence of Methane with and without N2O Addition. Figures 2−5 depict the ignition delay times for eight tested mixtures at p = 1.2−16 atm and ϕ = 0.5−2.0. Previous publications have indicated that an increase of the

Figure 2. Comparison the methane ignition delay times measured in current experiment and the predictions simulated by Aramco Mech 1.3 at pressures of 1.2−10 atm: (a) ϕ = 0.5, (b) ϕ = 1.0, and (c) ϕ = 2.0.

chemistry is taken from Dagaut et al.35 the interaction chemistry of hydrocarbon/NOx is taken from Sivaramakrishnan et al.,22 and it was also widely validated against experimental shock tube and JSR. The NOx submodel by Konnov consists of 36 species and 453 reactions, is developed on the basis of the work of in-flame NOx formation and reburning, and was validated against numerous experimental targets. C


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Figure 4. Comparison of the model predictions and measured data at ϕ = 1.0 and pressures of 1.2−16 atm with different N2O mole fractions: (a) N30/1.0, (b) N50/1.0, and (c) N70/1.0.

equivalence ratio changes from 0.5 to 1.0 at p < 4.0 atm. At pressure p > 4.0 atm, however, this effect becomes significant with a change in equivalence ratios, even going from 0.5 to 1.0. This means that methane ignition has a stronger pressure dependence upon the equivalence ratio at the fuel-lean condition compared to fuel-stoichiometric and fuel-rich conditions. For CH4/N2O mixtures (Figure 7), the ignition delay times are very similar at T > 1650 K when changing the equivalence ratios, while they only exhibit a slight difference at T < 1650 K. With an increasing pressure, the change in the effect of equivalence ratios is small. In comparison to neat methane, the effect of the equivalence ratio on the ignition delay times is negligible when adding N2O. This means that the pressure-dependent equivalence ratio effect becomes less important for CH4/N2O mixtures. 3.2.3. Effect of the N2O Addition on Methane Ignition. Figure 8 shows the effect of the N2O addition on the ignition delay times of methane at ϕ = 0.5. As seen, the ignition delay times remarkably reduce with the N2O addition over the conditions tested herein. Particularly, at a lower temperature (about 1350 K), the reduction is around 39.6% at 1.2 atm and then the reduction increases to 41.9% at 4.0 atm and 44.5% at 10 atm, respectively. Meanwhile, N2O addition exhibits a weak effect on the change of activation energies (Ea) at all pressures (Ea = 38.9 and 36.3 kcal/mol for N0/0.5 and N30/0.5, respectively). Figure 9 shows the effect of the N2O addition on ignition delay times for methane at ϕ = 1.0. As mentioned above, the model predictions simulated by updated Aramco−K for N0/1.0 at 16 atm were used to compare to the data measured for N30/1.0, N50/1.0, and N70/1.0. It can be seen that the N2O addition reduces ignition delay times at all pressures, and the reduction increases with an increase in the N2O mole fraction. Specifically, the reductions for N30 are 28.2, 37.0, and 43.5% at 1350 K and 1.2, 4.0, and 16 atm, respectively. For N50/1.0, the results give reductions of about 35.2, 46.1, and 53.5% at 1350 K and 1.2, 4.0, and 16 atm, respectively. While for N70/1.0, the

Figure 5. Comparison of the model predictions and measured data at pressures of 1.2−10 atm for the N30/2.0 mixture.

pressure can significantly promote methane ignition.36,37 As shown in Figure 2, the ignition delay times of neat methane decrease with the pressure at equivalence ratios in the range of 0.5−2.0. For mixtures doped with N2O (Figures 3−5), the presence of N2O does not alter the pressure dependence phenomenon of the ignition delay. It is observed that the reduction in ignition delay times with pressure changing from 1.2 to 4 atm is more pronounced than that from 4.0 to 10 atm (Figures 3 and 5) but similar to that from 4 to 16 atm (Figure 4). 3.2.2. Effect of the Equivalence Ratio. Figures 6 and 7 give the effects of equivalence ratios at p = 1.2−10 atm and ϕ = 0.5−2.0 for neat CH4 and N2O/CH4 mixtures, respectively. As mentioned above, the ignition delay times of N30/1.0 at 10 atm simulated by updated Aramco−K were used to compare to the data of N30/0.5 and N30/2 measured at 10 atm (Figure 7c). For neat CH4 (Figure 6), only a limited effect is observed when the D


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Figure 6. Comparison of the equivalence ratio effect at pressures of 1.2−10 atm and ϕ of 0.5−2.0 for neat methane: (a) 1.2 atm, (b) 4.0 atm, and (c) 10.0 atm.

Figure 7. Comparison of the equivalence ratio effect at pressures of 1.2−10 atm and ϕ of 0.5−2.0 for the mole fraction of N2O/CH4 = 30:70 mixtures (namely, N30/0.5, N30/1.0, and N30/2.0): (a) 1.2 atm, (b) 4.0 atm, and (c) 10.0 atm. Note that the data of N30/1.0 at 10 atm is simulated by updated Aramco−K.

Figure 8. Evolution of the ignition delay times with the temperature for N0/0.5 and N30/0.5 at ϕ = 0.5 and pressures of 1.2−10 atm: (a) 1.2 atm, (b) 4.0 atm, and (c) 10.0 atm.

reductions are up to 39.8, 51.8, and 59.6% at 1350 K. Through the comparison above, it can be seen that the promoting effect of N2O is more obvious at high pressures. Furthermore, Ea only shows a slight decrease for the different N2O additions, with Ea

= 43.0, 39.6, 37.8, and 36.8 kcal/mol for N0/1.0, N30/1.0, N50/1.0, and N70/1.0, respectively. Figure 10 gives the effect of the N2O addition at ϕ = 2.0. The addition of N2O gives reductions of 39.6, 41.6, and 48.5% at E


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Figure 9. Evolution of the ignition delay times with the temperature for N0/1.0, N30/1.0, N50/1.0, and N70/1.0 at ϕ = 1.0 and pressures of 1.2−16 atm: (a) 1.2 atm, (b) 4.0 atm, and (c) 16.0 atm. Note that the data of N0/1.0 at 16 atm is a simulation using updated Aramco−K.

Figure 10. Evolution of the ignition delay times with the temperature for N0/2.0 and N30/2.0 at ϕ = 2.0 and pressures of 1.2−10 atm: (a) 1.2 atm, (b) 4.0 atm, and (c) 10.0 atm.

around 1350 K for p = 1.2, 4.0, and 10 atm, respectively. It is also observed that the presence of N2O exhibits a small effect on the activation energies (Ea = 43.1 and 40.8 kcal/mol for N0/2.0 and N30/2.0, respectively). As discussed above, the effect of N2O is closely related with the N2O mole concentration, pressure, and equivalence ratio. To quantitatively illustrate the pressure-dependent behavior in the N2O/CH4 mixtures, a reduction of ignition delay times (on the basis of neat methane) as a function of N2O/CH4 mole fractions calculated by updated Aramco−K at 1350 K and p = 1.2−16 atm is depicted in Figure 11. It is observed that the reduction increases with an increase in the pressure at all equivalence ratios. Note that, for the fuel-rich condition, the reduction at higher pressures (from 4.0 to 16 atm) is bigger compared to that at lower pressures (from 1.2 to 4.0 atm). For fuel-lean and fuel-stoichiometric conditions, however, the reduction become bigger at lower pressures relative to higher pressures. To quantitatively illustrate the equivalence ratio dependence behavior, Figure 12 shows the reduction ratios of ignition delay times (on the basis of neat methane) as a function of N2O/CH4 mole fractions simulated using updated Aramco−K at 1350 K and ϕ = 0.5−2.0. It is noted that the N2O addition gives the strongest effect at the fuel-rich

condition. For the lean condition, however, the presence of N2O exhibits the smallest effect. 3.3. Sensitivity Analysis. Sensitivity analyses were performed for the eight mixtures to identify the key reactions influencing ignition. The normalized sensitivity analyses were conducted by multiplying or dividing a pre-exponential factor by 2, to ascertain the sensitivity index by the effect of perturbation of the rate constant for each reaction on the ignition delay time. A negative coefficient indicates a promoting effect on the ignition and vice versa. This method have been widely used in the previous literature.38 In this section, the three assembled models were used to conduct sensitivity analysis at 1350 K and 10 atm (Figures 13−15). The hydrocarbon mechanisms in the three assembled models are the same; Aramco Mech 1.3 is used in all of these. Figure 13 shows the 11 most sensitive reactions for N0/0.5 and N30/0.5 at the fuel-lean condition. Generally, the three assembled models give similar sensitivity coefficients, and the top three inhibiting and promoting reactions for N0/0.5 and those of N30/0.5 are the same. This means that CH4 chemistry still dominates the ignition. Note that the top promoting reaction •CH3 + O2 = CH2O + •OH in N0/0.5 is replaced by the chain-branching reaction H• + O2 = O• • + •OH in N30/0.5. For Aramco−M and Aramco−G (panels b and d of Figure 13), a F


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Figure 12. Comparison of the equivalence ratios effect on the reduction ratio in ignition delay times based on neat methane with varying N2O mole fractions at pressures of 1.2−16 atm using updated Aramco−K: (a) 1.2 atm, (b) 4.0 atm, and (c) 16.0 atm.

Figure 11. Comparison of the pressure effect on the reduction ratio in ignition delay times compared to neat methane with varying N2O mole fractions over pressures of 1.2−16 atm using updated Aramco− K: (a) ϕ = 0.5, (b) ϕ = 1.0, and (c) ϕ = 2.0.

Aramco−K model, the reaction N2O + •CH3 = CH3O• + N2 becomes the most sensitive reaction of the new additional reactions, but its importance remains much less than that of the reaction H• + O2 = O• • + •OH. The increase in the reactivity is closely related with (1) the formation of the O• • atom via the reaction N2O (+M) = N2 + O• • (+M) and (2) the formation of CH3O• radicals via the reaction N2O + CH3 = N2

••

new additional sensitive reaction N2O (+M) = N2 + O (+M) can be recognized, but it just shows a relatively small sensitivity index. The increase in the reactivity can be attributed to the formation of the O• • atom via the reaction N2O (+M) = N2 + O• • (+M), which also explains why the reaction H• + O2 = O• • + •OH becomes the most sensitive reaction. For the G


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Figure 13. Normalized sensitivity analysis at ϕ = 0.5, p = 10.0 atm, and T = 1350 K: (a) N0/0.5 and Aramco Mech 1.3, (b) N30/0.5 and Aramco−M, (c) N30/0.5 and Aramco−K, and (d) N30/0.5 and Aramco−G.

Figure 14. Normalized sensitivity analysis at ϕ = 1.0, p = 10.0 atm, and T = 1350 K for N0/1.0, N30/1.0, N50/1.0, and N70/1.0 mixtures: (a) N0 and Aramco Mech 1.3, (b) Aramco−M for N30/1.0, N50/1.0, and N70/1.0, (c) Aramco−K for N30/1.0, N50/1.0, and N70/1.0, and (d) Aramco−G for N30/1.0, N50/1.0, and N70/1.0.

+ CH3O•, which is much more reactive than the •CH3 radical because the H• atom can be generated via subsequent decomposition of CH3O• radicals. For the fuel-stoichiometric condition (panels a−d of Figure 14), it can be seen that the top three inhibiting reactions of the four mixtures (N0/1.0, N30/1.0, N50/1.0, and N70/1.0) calculated by the three models are the same and give comparable sensitivity coefficients. However, the differences among the main promoting reactions are very obvious. For N0, the top two promoting reactions H• + O2 = O• • + •OH and •CH3 + O2 =

CH2O + •OH dominate the ignition and the other promoting reactions give small sensitive coefficients. For N2O addition, the top two promoting reactions calculated by Aramco−M and Aramco−G are the same as for neat methane and also show comparable sensitive coefficients. As shown in panels b and d of Figure 14, the sensitive coefficient of reaction N2O (+M) = N2 + O• • (+M) calculated by Aramco−M is bigger than that for Aramco−G. In general, the new added reaction N2O (+M) = N2 + O• • (+M) using the two combined models gives big sensitive indexes and becomes more predominant with more H


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Figure 15. Normalized sensitivity analysis at ϕ = 2.0, p = 10.0 atm, and T = 1350 K: (a) N0/2.0 and Aramco Mech 1.3, (b) N30/2.0 and Aramco−M, (c) N30/2.0 and Aramco−K, and (d) N30/2.0 and Aramco−G.

Table 2. Rate Coefficients of Reaction N2O (+M) = N2 + O• • (+M) in the Three Assembled Models reaction

reaction rate constant

N2O (+M) = N2 + O• • (+M) Aramco−M Aramco−G Aramco−K

high-pressure limit low-pressure limit high-pressure limit low-pressure limit high-pressure limit low-pressure limit

A 9.9 6.6 1.3 4.0 1.3 4.0

× × × × × ×

1010 1014 1012 1014 1012 1014

n

Ea

reference

0.00 0.00 0.00 0.00 0.00 0.00

57960 57500 62570 56600 62620 56640

5 7 30

atom and a bigger promoting effect on the ignition. For Aramco−K (Figure 14c), the reaction N2O + •CH3 = CH3O• + N2 becomes the most important promoting reaction and its sensitive coefficient significantly increases with more N2O addition, especially becoming the most important promoting reaction for N50/1.0 and N70/1.0. It explains the decrease in the normalized sensitivity of the reaction CH3 + O2 = CH2O + • OH, because the •CH3 radical is more easily consumed via the reaction N2O + •CH3 = CH3O• + N2. For the fuel-rich condition (Figure 15), the differences between the sensitive indexes calculated by Aramco−M and Aramco−G are quite slim. As shown in panels b and d of Figure 15, the 11 most sensitive reactions calculated by Aramco−M and Aramco−G are almost the same and most of them show a comparable sensitive coefficient. The top two inhibiting and promoting reactions of N30/2.0 calculated by Aramco−M and Aramco−G are the same with that of N0/2.0. The promoting reaction N2O (+M) = N2 + O• • (+M) becomes the third most promoting reaction. For Aramco−K, the top two inhibiting reactions are also identical with that for neat methane and the reaction N2O + •CH3 = CH3O• + N2 becomes the second largest promoting reaction.

Figure 16. Evolution of the rate coefficient of reaction N2O (+M) = N2 + O• • (+M) in the three assembled models with pressures at 1250 K.

N2O addition. It should be noted that the increase of N2O concentrations leads to a higher reaction rate of N2O (+M) = N2 + O• • (+M), which leads to a higher formation of the O• • I


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Figure 17. Comparison of the modified models on the performance of Aramco−G and Aramco−M at pressures of 1.2−16 atm for the N70/1.0 mixtures.

From the sensitivity analyses, it can be found that three NOxspecific reactions appear in the 11 most sensitive reactions: N2O ( +M) = N2 + O• • ( +M)

(1)

N2O + H• = N2 + •OH

(2)

N2O + •CH 3 = CH3O• + N2

(3)

That means the ignition-promoting effect of N2O can be attributed to the contribution of these three reactions. Note that reaction 3 only appears in Aramco−K. This may account for the overpredictions of Aramco−G and Aramco−M at the lower temperatures for N70/1.0, as shown in Figure 3. Moreover, Aramco−K gives relatively good predictions for N70/1.0 at 1.2 and 4.0 atm but slightly underpredicts at 16 atm. The pressuredependent reaction 1 may be responsible for the underpredictions of Aramco−K for N70/1.0 at 16 atm. Table 2 shows the rate coefficients of reaction N2O (+M) = N2 + O• • (+M) in the three assembled models. Figure 16 compares the rate coefficient of reaction 1 in the three models as a function of the pressure at 1250 K. It can be seen that the rate constants in Aramco−G and Aramco−K are bigger than that in Aramco−M. To improve the performance of the three models, reaction 3 is added to Aramco−G and Aramco−M. As shown in Figure 17, it can been seen that two modified models show similar calculations and agree well with the measured data for N70/1.0 at pressures of 1.2−16 atm. To improve the performance of Aramco−K at 16 atm for N70/1.0, the rate constant of reaction 1 in Aramco−K is replaced by the smaller rate constant used in Aramco−M. The modification improves the performance of Aramco−K at high pressures for N70/1.0 (Figure 18). For the

Figure 18. Comparison of the modified model on the performance of Aramco−K at 16 atm for the N70/1.0 mixtures.

It is noted that the concentration of N2O at ϕ = 2.0 (see Table 1) is bigger than that at ϕ = 1.0 (by a factor of about 1.5) and ϕ = 0.5 (by a factor of about 2.5). Therefore, the rates of N2O-specific reactions N2O + •CH3 = CH3O• + N2 and N2O (+M) = N2 + O• • (+M) at ϕ = 2.0 are faster than those at ϕ = 0.5 and 1.0, which means that more O• • atoms and CH3O• radicals are produced and have a bigger promoting effect on ignition. Thus, it is easy to see how the N2O-promoting effect is bigger at ϕ = 2.0 than that at ϕ = 0.5 and 1.0. J


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Figure 19. Evolution of the total concentration of the radical pool (H•, O• •, •OH, and HO•2) with the normalization time (on the basis of respective ignition delay times) using the updated Aramco−K at 16 atm and 1350 K for eight mixtures tested: (a) ϕ = 0.5, (b) ϕ = 1.0, and (c) ϕ = 2.0.

promoting effect at the fuel-rich and high-pressure conditions. Furthermore, the addition of N2O results in a moderate decrease in the global activation energy. (2) Three NOx submodels (namely, Mathieu et al., Gersen et al., and Konnov model) were merged with Aramco Mech 1.3 to simulate the measured data. Predictions simulated by three assembled models are very similar and agree with the measured data for N30/1.0, N30/0.5, and N30/2.0. For the N70/1.0 mixture, the discrepancy among simulations of the three assembled models is great. In more detail, for the N70/1.0 mixture, Aramco−M and Aramco−G overpredict at 1.2 atm and Aramco−K underpredicts at 16 atm. The overpredictions of Aramco−M and Aramco−G may be attributed to the absence of the reaction N2O + •CH3 = CH3O• + N2, and the underpredictions of Aramco−K may be attributed to the fast rate constant of reaction N2O (+M) = N2 + O• • (+M). (3) Sensitivity analysis shows that the N2O-promoting effect is closely related to the three N2O-specific reactions: N2O + •CH3 = CH3O• + N2, N2O (+M) = N2 + O• • (+M), and N2O + H• = N2 + •OH, especially for the N2O/CH4 mixtures with a high level of N2O addition at high pressures. (4) Radical pool analysis indicates that a big increase in the concentration of the radical pool can been observed in the presence of N2O. The concentration of the radical pool increases with N2O addition. Furthermore, the increase in the concentration at the fuel-rich condition is bigger than that at the fuel-lean condition.

low N2O mole fraction (N30/1.0 and N50/1.0) conditions, the modified models exhibit a negligible effect on predictions of the three models over the tested conditions; thus, the comparisons between the three modified models and the measured data of N30/1.0 and N50/1.0 are not plotted here. 3.4. Chemically Kinetic Interpretation of N2O/CH4 Mixture Ignition. To further interpret the effect of the N2O addition, the profiles of total radicals (H•, O• •, •OH, and HO•2) as a function of time simulated by the updated Aramco−K at 16 atm and 1350 K were plotted for the eight mixtures. Note that the three updated models show similar simulation results; only updated Aramco−K is thus considered to calculate the concentration of the free radical pool. Figure 19 shows the evolution of the total concentrations of the free radical pool with a normalization time. The results indicate that the presence of N2O results in a significant increase in the concentration of total radicals and the concentration of total radicals increases with an increase in the N2O addition. In comparison to the conditions at ϕ = 0.5 and 1.0, the total radical concentration increase is more obvious at ϕ = 2.0. This is consistent with the measured ignition delay times. When N2O is added, the reaction N2O + M = N2 + O• • + M produces a more reactive O• • atom, which is the significant source of the initial radical pool and dramatically drives the formation and development of the radical pool and, further, the consumption of methane.

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4. CONCLUSION To understand the effect of the N2O addition on methane ignition chemistry, ignition delay times of N2O/CH4/O2/Ar mixtures with different N2O blending ratios were measured in a high-temperature shock tube at pressures of 1.2−16.0 atm, equivalence ratios of 0.5−2.0, and temperatures of 1220−2336 K. Three assembled models were systematically evaluated against the experimental data. The sensitivity analysis and radical pool analysis were also conducted to interpret the effect of the N2O addition on CH4 ignition. The main conclusions obtained in this study are summarized as follows: (1) In comparison of the ignition delay times of neat CH4 to those of N2O/CH4 mixtures, a significant reduction in the ignition delay times can been seen and the reduction increases with an increasing N2O addition. Moreover, N2O exhibits a stronger

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b02581. Experimental conditions and ignition delay time data (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 86-29-82665075. Fax: 86-29-82668789. E-mail: [email protected]. Notes

The authors declare no competing financial interest. K


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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (51206132 and 91441203) and the National Basic Research Program (2013CB228406). The authors also appreciate the funding support from the Fundamental Research Funds for the Central Universities.

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