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Shock-induced ignition of methane, ethane and methane/ethane mixtures sensitized by NO

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Xia Zhang, Wei Ye, Jinchun Shi, Xiangji Wu, Runtong Zhang, and Sheng-Nian Luo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01632 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 4, 2017

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Shock-induced ignition of methane, ethane and methane/ethane mixtures sensitized by NO2 X. Zhang,†,‡ W. Ye,¶,‡ J. C. Shi,‡ X. J. Wu,‡ R. T. Zhang,∗,§ and S. N. Luo∗,‡,† Key Laboratory of Advanced Technologies of Materials, Ministry of Education, Southwest Jiaotong University, Chengdu, Sichuan 610031, People’s Republic of China, The Peac Institute of Multiscale Sciences, Chengdu, Sichuan 610031, People’s Republic of China, School of Science, Wuhan University of Technology, Wuhan, Hubei 430070, People’s Republic of China, and Department of Chemistry, Sourthern University of Science and Technology, Shenzhen, Guangdong 518055, People’s Republic of China E-mail: [email protected]; [email protected]

Abstract Ignition delay times of stoichiometric CH4 /NO2 /O2 /Ar, C2 H6 /NO2 /O2 /Ar, and CH4 /C2 H6 /NO2 /O2 /Ar mixtures are measured with a shock tube, to investigate the role of NO2 in ignition of methane, ethane, and natural gas (CH4 :C2 H6 = 9:1). All measurements are carried out at 1016−1981 K and 5−16 atm. Different NO2 concentrations (0%, 25%, 50% and 75% of fuel molar concentrations) are explored. The addition of NO2 promotes considerably the ignition of methane and natural gas, but moderately for ethane. In addition, the promoting effect is temperature-dependent. ∗

To whom correspondence should be addressed SWJTU ‡ PIMS ¶ WHUT § SUSTech †

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Four assembled models are examined against the present measurements, and an updated kinetic model is proposed and validated in comparison with the experimental data. The current model is chosen for sensitivity and reaction pathway analyses, which reveal the role of NO2 in ignition of methane, ethane, and natural gas.

1

Introduction

Understanding the influence of NOx on combustion kinetics of hydrocarbon fuels is important to modeling properly combustion in internal combustion engines and gas turbines with exhaust gas recirculation, partially due to the presence of NOx in recirculated exhaust gas. Oxidation of hydrocarbon fuels with NOx addition has long been investigated, including H2 , 1–3 CH4 , 4–18 C2 hydrocarbons, 19–21 natural gas 22–27 and higher hydrocarbons. 28,29 It has been shown that even a small amount of nitrogen oxides can have a significant effect on a combustion process. In particular, several studies focused on the sensitization effect of NO2 on hydrocarbon ignition. 4,15–17,26 For example, Slack and Grillo 4 explored methane ignition with NO2 addition under fuel lean condition (equivalence ratio = 0.5) over a temperature range of 1310−1790 K. Herzler and Naumann 26 studied natural gas in a high pressure shock tube in the presence of NOx (20−250 ppm) at 16 bar, 1000−1700 K and equivalence ratios of 0.25, 0.5 and 1.0, and found a pronounced effect of NO2 on ignition delay time. Mathieu et al. 16 examined ignition of methane in a shock tube with the addition of NO2 and N2 O at 1−28 atm and equivalence ratios of 0.5−2.0, and proposed and validated a model against their experimental data. Deng et al. 18 investigated the promoting effect of NO2 on methane ignition at 1.2−10 atm, 933−1961 K and equivalence ratio of 1.0, the molar fraction ratios of NO2 :CH4 were 30:70, 50:50 and 70:30. Their shock-tube study showed that NO2 addition promotes the reactivity of methane and reduces the global activation energy at all pressures explored, and the promoting effect is more pronounced at higher NO2 concentrations. With a rapid compression machine, Gersen et al. 15 measured ignition delay times of stoichiometric 2 ACS Paragon Plus Environment

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methane, ethane and methane/ethane mixtures doped with 100 and 270 ppm of NO2 in a temperature range of 900−1050 K and a pressure range of 25–50 bar, and they found that sensitization effect of NO2 was different for methane and ethane, but no explanation was offered. Despite previous efforts on methane and ethane ignition with the addition of NOx , the data from rapid combustion machine or shock tube experiments is sparse, and some results, to be confirmed. In particular, the cause for the difference in NOx sensitization between methane and ethane ignition is still unclear, and chemical kinetics modeling of methaneethane-NOx system is highly desirable to resolve this issue. In this work, a comparative study of NO2 addition effects on ignition of natural gas, and its main constituents, CH4 and C2 H6 , is performed with a shock tube. Ignition delay times for stoichiometric CH4 /NO2 /O2 /Ar, C2 H6 /NO2 /O2 /Ar, and CH4 /C2 H6 /NO2 /O2 /Ar mixtures, are measured for a wide range of temperature (1016−1981 K) and pressure (5−16 atm). Four chemical kinetics models are examined against experimental results, and a kinetic model is proposed with updated reaction rate constants. Sensitivity and reaction pathway analyses are performed to evaluate chemical kinetics of ignition of methane, ethane, and natural gas with NO2 addition, and reveal the cause for the difference in sensitization effect between methane and ethane ignition.

2

Experimental apparatus

All experiments are performed with a stainless steel shock tube. A detailed description of this shock tube was presented previously, 30 and we briefly introduce it here. Before each experiment, the shock tube is evacuated to below 10 Pa by a mechanical vacuum pump, and then He and the test gas mixture are injected into the driver section and the driven section, respectively. Four piezoelectric pressure transducers, located at 971, 571, 271 and 11 mm away from the shock tube endwall, are applied to measure the incident shock wave speeds.


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The incident shock wave speed at the endwall is determined by linearly extrapolating the measured shock wave speeds at other locations to the endwall. Without considering chemical reactions, we use the chemical equilibrium program Gaseq 31 to calculate the temperature T5 and pressure P5 behind the reflected shock wave, with the experimental conditions including molar fractions in a mixture, initial temperature T0 and pressure P0 , and the incident shock wave speed at the endwall as input parameters. Moreover, the thermodynamic properties of the relevant species, selected from the default thermodynamic database of Gaseq, are also provided as input in calculation. The gas mixture is prepared in advance in a 15 L stainless steel tank, and the fractions of the constituent gases are determined by Dalton’s law of partial pressures. The prepared gas mixture is allowed to mix and diffuse for more than 24 h to become fully homogenized. The effect of N2 O4 is not considered here given the low initial pressure in the driven section, as discussed previously. 32

Figure 1: Typical pressure and OH* emission histories at 5.2 atm and 1126 K. Ignition delay time τ is indicated. A photomultiplier, installed at 11 mm from the shock tube endwall, is used to obtain the OH* emission through a narrowband filter centered at 307 ± 10 nm. The pressure and OH* emission signals are used to determine ignition characteristics. The ignition delay time τ is defined as the time interval between the arrival of the incident shock wave at the endwall 4 ACS Paragon Plus Environment

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and the extrapolation of the maximum slope of OH* emission to the baseline, as shown in Figure 1. There is a pressure rise (dP /dt = 8%/ms) before the main ignition event, caused by interaction between the reflected shock wave and the boundary layer. 33 The uncertainty in ignition delay time is estimated through the standard root-sum-squares method, and the overall uncertainty is approximately 20%.

1.87%CH /1.87%NO /3.71%O /92.55%Ar 4

2

2

10 atm

s)

1000

(

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100

Deng et al. (2015) This study

0.7

0.8

0.9 -1

1000/T (K )

Figure 2: Comparison of the current and literature ignition data 18 for NO2 -sensitized methane. To confirm the reliability of the present shock tube facility, a comparison experiment is carried out. Figure 2 shows a comparison between our measurements and literature ignition delay times for 1.87%CH4 /1.87%NO2 /3.71%O2 /92.55%Ar, and both the datasets agree well with each other. The composition of the test gas in this study is listed in Table 1. The purities of the gases used in this study are >99.999% for He, >99.9% for NO2 , >99.99% for O2 , >99.99% for Ar, >99.9% for CH4 and >99.9% for C2 H6 .

3 3.1

Results and Discussions Ignition delay time measurements

Ignition delay times are obtained at different NO2 concentrations and different pressures for different mixtures (Figs. 3–5). Figure 3 shows that for CH4 ignition at 5, 10 and 16 atm, 5 ACS Paragon Plus Environment

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Table 1: Mixture compositions investigated in the present study. The percentages refer to molar ratios. N0 , N25 , N50 , and N75 denote the cases where the ratios between NO2 and fuels (in percentage) are approximately 0, 25%, 50%, and 75%, respectively. Mixture number Mix1 (N0 ) Mix2 (N25 ) Mix3 (N50 ) Mix4 (N75 ) Mix5 (N0 ) Mix6 (N25 ) Mix7 (N50 ) Mix8 (N75 ) Mix9 (N0 ) Mix10 (N25 ) Mix11 (N50 ) Mix12 (N75 ) Mix13 Mix14

CH4 (%) 1.982 1.982 1.982 1.982 0.000 0.000 0.000 0.000 1.659 1.659 1.659 1.659 1.982 1.982

C2 H6 (%) 0.000 0.000 0.000 0.000 1.133 1.133 1.133 1.133 0.184 0.184 0.184 0.184 0.000 0.000

O2 (%) 3.964 3.964 3.964 3.964 3.964 3.964 3.964 3.964 3.964 3.964 3.964 3.964 3.964 3.964

Ar (%) 94.054 93.559 93.063 92.568 94.904 94.620 94.337 94.053 94.192 93.731 93.271 92.810 93.954 93.856

NO2 (%) 0.000 0.496 0.991 1.487 0.000 0.283 0.567 0.850 0.000 0.461 0.922 1.383 0.100 0.198

NO2 addition leads to a significant reduction in τ . The reduction increases with increasing NO2 concentration. For example, the reductions in τ for N25 , N50 and N75 are 72.6%, 87.0% and 91.8% at 5 atm and 1500 K, respectively (Fig. 3a). At 10 atm and 1500 K (Fig. 3b), reductions of 74.4%, 87.2%, 92.2% are observed for N25 , N50 and N75 , respectively. At 16 atm and 1500 K (Fig. 3c), τ is reduced by 76.8% (N25 ), 87.9% (N50 ) and 92.5% (N75 ). The promoting effect of NO2 shows a strong temperature dependence, and it is more pronounced at low temperatures (below 1400 K) than at high temperatures (above 1400 K).

Figure 3: Effect of NO2 concentration on CH4 ignition at different pressures. Solid lines: predictions from current model. The effect of NO2 addition on natural gas ignition is shown in Fig. 4 for three different 6 ACS Paragon Plus Environment

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Figure 4: Effect of NO2 concentration on CH4 /C2 H6 ignition at different pressures. Solid lines: predictions from current model. pressures (5, 10 and 16 atm). The addition of NO2 in the CH4 /C2 H6 /NO2 /O2 /Ar mixture also reduces markedly ignition delay time, and similar observation was made by Herzler and Naumann. 26 For instance, the reductions in τ are 67.3%, 78.5% and 85.6% at 5 atm and 1300 K for N25 , N50 and N75 , respectively (Fig. 4a). At 10 atm and 1300 K (Fig. 4b), the corresponding reductions are 61.8%, 76.3%, 84.2%. At around 16 atm and 1300 K (Fig. 4c), the reductions are 67.3% (N25 ), 79.0% (N50 ) and 85.9% (N75 ). The NO2 induced reduction in ignition delay time for the natural gas mixture is also temperature dependent: the promoting effect is more pronounced at low temperatures (below 1300 K).

Figure 5: Effect of NO2 concentration on C2 H6 ignition at different pressures. Solid lines: predictions from current model. However, drastically different behavior is observed for ethane mixtures (Mix 5–8, Table 1), as illustrated by Fig. 5. At P5 = 5, 10 and 16 atm, addition of NO2 to ethane results in only a modest reduction in τ . Specifically, at 5 atm and 1140 K (Fig. 5a), the reductions in τ are 44.2%, 58.1% and 64.9%, respectively, for N25 , N50 and N75 . At 10 atm and 1140 K (Fig. 5b), the corresponding reductions are 52.8%, 64.1%, and 71.4%. At around 16 atm


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and 1140 K (Fig. 5c), the reductions are 56.5% (N25 ), 67.4% (N50 ) and 73.5% (N75 ). The addition of NO2 to C2 H6 has more pronounced effects at low temperatures (below 1200 K) than at high temperatures (over 1200 K), and the difference in reductions between low and high temperatures increases with increasing pressure. However, compared with methane, addition of NO2 to ethane has only a modest effect on ignition delay time. The influence of pressure on ignition delay time reduction is weak (see Figure S1–3 in Supporting Material). （a）

(b)

Figure 6: (a) Effect of NO2 concentration on CH4 ignition at 10 atm. Solid lines: predictions from the current model; (b) Simulated ignition delay times as a function of NO2 concentration at 10 atm and different temperatures. Dashed lines: linear fitting. As motivated by a recent work, 34 we also examine the effect of NO2 concentrations (0– 14865 ppm) on CH4 ignition at 10 atm, and the result is shown in Fig. 6a. The NO2 addition leads to a significant reduction in ignition delay time, consistent with a previous study. 26 Since the current updated model (see details in Section 3.3) reproduces the experimental results, we present a plot of simulated ignition delay times (on the logarithmic scale, log10 τ ) as a function of NO2 concentration at 10 atm and different temperatures (Fig. 6b). In the log10 τ versus concentration plot, there are two linear segments can be identified, separated at 2500 ppm. At lower concentrations (below 2500 ppm), increasing NO2 concentration results in a rapid reduction in ignition delay times, but this effect becomes weakened at higher concentrations (2500−14865 ppm).


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3.2

Kinetic model evaluation

The chemical reaction kinetics in the post-reflected-shock region are simulated with Senkin 35 in the Chemkin II package. 36 Given the influence of boundary layers, the VTIM method (i.e., volume as a function of time) 33 is applied to the cases where ignition delay time is longer than 0.7 ms, and a pressure rise rate of 8%/ms is used. A calculated ignition delay time is the interval between time zero and the instant defined at the temperature inflection point on an OH production curve, consistent with the experimental definition. The data presented in this work are compared to the predictions by Aramco Mech 1.3 37 with four NOx sub-models, denoted as Aramco-K by Konnov et al., 12 Aramco-G by Gersen et al., 15 Aramco-M by Mathieu et al., 16 and Aramco-Y by Ye et al.,. 32 Aramco-K’s NOx sub-model consists of 36 species and 453 reactions, which was developed on the the basis of in-flame NOx formation and reburning, and validated against experiments on different substances. Aramco-G’s NOx sub-model includes 66 species and 479 reactions, and is based on Rasmussen et al. 38 Aramco-M’s NOx sub-model involves 36 species and 305 reactions. Aramco-Y’s NOx sub-model is based on Aramco-M, with the following amendment: the rate constant of reaction CH3 + NO2 ⇔ CH3 O + NO is a factor of 1.5 smaller than that by Glaborg et al. 39

Current model

Figure 7: Comparison between experiments and predictions (current updated model and four literature models) for CH4 ignition at 10 atm and different NO2 concentrations. The predicted ignition delay times are compared with our measurements in Figure 7– 9. For CH4 with different NO2 additions (Fig. 7), the measured ignition delay times at 10 atm are compared with the predictions of the four assembled models. The differences 9 ACS Paragon Plus Environment

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Current model

Figure 8: Comparison between experiments and predictions (current updated model and four literature models) for CH4 /C2 H6 ignition at 10 atm and different NO2 concentrations.

Current model

Figure 9: Comparison between experiments and predictions (current updated model and four literature models) for C2 H6 ignition at 10 atm and different NO2 concentrations. between the model predictions are small, and these four models under-predict ignition delay time at temperatures over 1250 K. For N50 , Aramco-M model shows the best prediction at temperatures below 1250 K. For CH4 /C2 H6 with different NO2 additions (Fig. 8), the four models under-estimate ignition delay time at over 1350 K. However, the Aramco-G, Aramco-M and Aramco-Y models show better predictions at temperatures below 1350 K. For C2 H6 with different NO2 additions (Fig. 9), the differences between the predictions by these models are pronounced, in contrast with the cases of CH4 and CH4 /C2 H6 . Aramco-M and Aramco-Y under-predict ignition delay time considerably, while Aramco-K and AramcoG show appreciable over-estimate. To better predict ignition delay time for CH4 /NO2 , C2 H6 /NO2 , and CH4 /C2 H6 /NO2 mixtures, we choose Aramco-M as the base model for further development. The rate constants of several key reactions are updated to achieve better agreement with our measurements. Table 2 lists the modified rate constants for five selected reactions. As can be seen from Figures 3–9, the current updated model reproduces well our experimental data. The com10 ACS Paragon Plus Environment

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parisons for pressures of 5 and 16 atm are provided in Supporting Material.

3.3

Kinetic analysis

Based on the current updated model, sensitivity analyses are conducted for the mixtures in terms of sensitivity coefficient 43

Si =

τ (2ki ) − τ (0.5ki ) 1.5τ (ki )

(1)

where ki is the pre-exponential factor of the ith elementary reaction, τ denotes the ignition delay time. A negative sensitivity coefficient indicates that the corresponding elementary reaction promotes ignition, while a positive value corresponds to an inhibiting effect. Figure 10 presents the sensitivity analysis for neat methane and NO2 -added methane mixtures at 10 atm, and two temperatures of 1200 K and 1700 K are considered. At high temperatures (1700 K, Fig. 10b and 10d), the differences in sensitivity coefficient between these two tested mixtures are relatively small, except for the NOx -related reactions, R1717 (NO2 + H ⇔ NO + OH) and R1773 (CH3 + NO2 ⇔ CH3 O + NO). Reactions R1 (H + O2 ⇔ O+OH) and R148 (CH3 +O2 ⇔ CH2 O+OH) have the highest sensitivity coefficients for both mixtures, which produce reactive H atoms and OH radicals. Addition of NO2 has opposite effects on R1 and R148. The increase in sensitivity coefficient for R1 and decrease for R148 are attributed to the following reasons: (i) CH3 radicals react with NO2 preferentially via R1773, downgrading the role of CH3 oxidation via R148 and its self-recombination via R189 Table 2: List of the updated reaction rate constants. Reactions HONO + OH = NO2 + H2 O CH3 + NO2 = CH3 O + NO CH2 O + NO2 = HCO + HONO NO3 + NO2 = NO2 + NO + O2 NO2 + O = NO + O2

A n Ea (cm3 mol−1 s−1 ) (cal mol−1 ) 1.55 × 1012 0.00 0.0 12 2.50 × 10 0.00 0.0 1.40 × 10−7 5.64 9220.0 2.35 × 1010 0.00 2960.0 14 1.00 × 10 −0.52 0.0


References 40 41 42 25 15

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(a) Mix 1, 10 atm, 1200 K, Aramco-D model

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(b) Mix 1, 10 atm, 1700 K, Aramco-D model

R189: CH3+CH3(+M) C2H6(+M)

R128: CH4+H CH3+H2

R128: CH4+H CH3+H2

R189: CH3+CH3(+M) C2H6(+M) R129: CH4+OH CH3+H2O

R129: CH4+OH CH3+H2O R75: CH2O+CH3 HCO+CH4

R39: CH2O+O2 HCO+HO2

R147: CH3+O2 CH3O+O

R145: CH3+HO2 CH4+O2


R144: CH3+HO2 CH3O+OH

R131: CH4+HO2 CH3+H2O2

R206: CH3+CH3 H+C2H5


R147: CH3+O2 CH3O+O

R1: H+O2 O+OH

R148: CH3+O2 CH2O+OH


R1: H+O2 O+OH

Sensitivity coefficient


(c) Mix 2, 10 atm, 1200 K, Aramco-D model

(d) Mix 2, 10 atm, 1700 K, Aramco-D model R1717: NO2+H NO+OH

R1717: NO2+H NO+OH R189: CH3+CH3 (+M) C2H6(+M)

R128: CH4+H CH3+H2

R128: CH4+H CH3+H2

R127: CH3+H(+M) CH4(+M) R39: CH2O+O2 HCO+HO2

R147: CH3+O2 CH3O+O

R92: CH3O+O2 CH2O+HO2




R1758: NO+CH3O2 NO2+CH3O


R31: HCO+O2 CO+HO2


R1773: NO2+CH3 CH3O+NO


R1: H+O2 O+OH

R1: H+O2 O+OH



Figure 10: Sensitivity analysis for neat CH4 mixture at (a) 1200 K and (b) 1700 K, and for the N25 mixture at (c) 1200 K and (d) 1700 K, from current model. (CH3 + CH3 (+M) ⇔ C2 H6 (+M)); and (ii) CH3 radicals are largely converted into reactive CH3 O radicals, which can decompose to H atoms via R91 (CH3 O(+M) ⇔ CH2 O + H(+M)), accelerating R1. CH3 radicals can be also oxidized via R144 (CH3 + HO2 ⇔ CH3 O + OH) and R147 (CH3 + O2 ⇔ CH3 O + O), so more reactive radicals are formed. CH3 radicals play an important role in the oxidation of methane-based mixtures, and reaction R1773 decreases the contribution of R144, R147, R148 and R189 in CH3 radicals consumption when NO2 is present, assisting faster formation of reactive radicals. At lower temperatures (1200 K, Fig. 10a and 10c), sensitivity coefficients differ considerably for neat and NO2 -added mixtures. With NO2 addition, R1 becomes the most promoting reaction instead of R148, and this switch is probably due to the competition between R148 and R1773, as described above. The sensitivity coefficient of R31 (HCO + O2 ⇔ CO + HO2 )


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increases markedly for the mixture containing NO2 and becomes the third most sensitive reaction as a result of abundant HCO radicals in NO2 -added methane. Furthermore, reaction R1717 (NO2 + H ⇔ NO + OH) is the most important reaction to inhibit fuel ignition for the mixture containing NO2 , with its sensitivity greater than that of reaction R189. It is also interesting to note that the role of peroxide radicals is more important in methane oxidation at low temperatures (1200 K), similar to the study by Mathieu et al. 16 As can be seen in Fig. 9a, R144 (CH3 + HO2 ⇔ CH3 O + OH) is the third most-sensitive reaction to promote methane ignition, whereas this reaction plays a lesser role at 1700 K. At this low temperature, R131 (CH4 + HO2 ⇔ CH3 + H2 O2 ) and R1758 (NO + CH3 O2 ⇔ NO2 + CH3 O) also exhibit high sensitivity, indicating abundant peroxide radicals under this condition. With the increased abundance and importance of peroxide radicals, the reaction cycle involving NO/NO2 is affected greatly, so NO2 additive has a stronger effect on ignition delay time at low temperatures (1200 K, Fig. 3). Similarly, the CH3 radicals can be consumed at high temperatures via the following four reaction pathways (Fig. 14a): (i) most of the CH3 radicals react with HO2 (2.4%) and NO2 (57.5%) via R144 and R1773 to generate CH3 O radicals which can quickly decompose into formaldehyde and H atoms; (ii) R75 (CH2 O + CH3 ⇔ HCO + CH4 ) consumes the CH3 radicals (7.9%) to form stable molecules HCO and CH4 ; (iii) the CH3 radicals (2.8%) react with O2 via R148 to form formaldehyde and OH radicals which actually promote reactivity; (iv) the CH3 radicals undergo a self-recombination termination reaction to form stable alkane C2 H6 . However, more CH3 radicals react with NO2 (70.1%) via R1773 at low temperatures to generate CH3 O radicals which promote reactivity, and certain CH3 radicals (9.8%) undergo the self-recombination termination reaction. The sensitivity analysis for the natural gas with and without a NO2 addition is shown in Figure 11. The ten most sensitive reactions at specific pressure (10 atm) and temperature conditions (1200 K and 1600 K) are presented. For the high temperature cases (e.g., 1600 K, Fig. 11b and 11d), the results are similar for sensitive reactions with and without NO2 .


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(a) Mix 9, 10 atm, 1200 K, Aramco-D model

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(b) Mix 9, 10 atm, 1600 K, Aramco-D model

R189: CH3+CH3(+M) C2H6(+M)

R128: CH4+H CH3+H2

R128: CH4+H CH3+H2

R129: CH4+OH CH3+H2O R127: CH3+H(+M) CH4(+M)

R129: CH4+OH CH3+H2O R302: C2H4+OH C2H3+H2O

R330: C2H3+O2 CH2CHO+O

R317: C2H4+CH3 C2H3+CH4

R147: CH3+O2 CH3O+O

R193: C2H6+OH C2H5+H2O

R195: C2H6+CH3 C2H5+CH4


R317: C2H4+CH3 C2H3+CH4

R195: C2H6+CH3 C2H5+CH4



R148: CH3+O2 CH2O+OH R1: H+O2 O+OH

R1: H+O2 O+OH




(d) Mix 10, 10 atm, 1600 K, Aramco-D model

R1717: NO2+H NO+OH

R128: CH4+H CH3+H2

R189: CH3+CH3(+M) C2H6(+M)

R1717: NO2+H NO+OH

R128: CH4+H CH3+H2

R127: CH3+H(+M) CH4(+M) R330: C2H3+O2 CH2CHO+O

R135: CH3+OH CH2OH+H




R3: OH+H2 H+H2O

R1758: NO+CH3O2 NO2+CH3O


R31: HCO+O2 CO+HO2




R1: H+O2 O+OH

R1: H+O2 O+OH Sensitivity coefficient


Figure 11: Sensitivity analysis for neatCH4 /C2 H6 mixture at (a) 1200 K and (b) 1600 K, and for the N25 mixture at (c) 1200 K and (d) 1600 K, based on the current model. Reaction R1 is the sensitive in the two mixtures, and the corresponding sensitivity coefficients are similar. The different reaction is R1773 (CH3 + NO2 ⇔ CH3 O + NO) which promotes the reactivity with an overall small sensitivity coefficient. Hence, the influence of addition NO2 to natural gas is weak at high temperatures. At low temperatures (e.g., 1200 K, Fig. 11a and 11c), the addition of NO2 has an significant effect on ignition delay time. Only half of the reactions remain the same between the two mixtures (with and without NO2 ). Reaction R1 is still the most sensitivity reaction, However, its sensitivity coefficient is higher in the mixture with NO2 addition (Fig. 11c) than that in the mixture without NO2 addition (Fig. 11a). Reaction R1773 becomes the second most important promoting reaction, which has a high sensitivity coefficient. Reaction R1717 is among the most inhibiting reactions due to its competition for H atom with R1. Hence, 14 ACS Paragon Plus Environment

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the promoting effect of NO2 addition is more pronounced at low temperatures. (b) Mix 5, 10 atm, 1450 K, Aramco-D model

(a) Mix 5, 10 atm, 1100 K, Aramco-D model R191: C2H6 +H C2H5+H2


R15: HO2+OH H2O+O2

R191: C2H6+H C2H5+H2

R222: C2H5+O2 C2H4+HO2

R127: CH3+H(+M) CH4(+M) R330: C2H3+O2 CH2CHO+O

R3: OH+H2 H+H2O




R12: HO2+H OH+OH

R201: C2H4+H(+M) C2H5(+M)


R19: H2O2(+M) OH+OH(+M)


R1: H+O2 O+OH

R201: C2H4+H(+M) C2H5(+M)

R196: C2H6+HO2 C2H5+H2O2

R1: H+O2 O+OH



(d) Mix 6, 10 atm, 1450 K, Aramco-D model



R491: C3H8(+M) CH3+C2H5(+M) R191: C2H6+H C2H5 +H2

R191: C2H6+H C2H5+H2

R1784: NO2+C2H5 NO+C2H5O

R127: CH3+H(+M) CH4(+M)


R3: OH+H2 H+H2O


R189: CH3+CH3(+M) C2H6(+M)

R201: C2H4+H(+M) C2H5(+M)


R222: C2H5+O2 C2H4+HO2


R1785: C2H5+HONO C2H6+NO2



R201: C2H4+H(+M) C2H5(+M)

R1: H+O2 O+OH

R1: H+O2 O+OH



Figure 12: Sensitivity analysis for neat C2 H6 mixture at (a) 1100 K and (b) 1450 K, and for the N25 mixture at (c) 1100 K and (d) 1450 K, based on the current model. For ethane with and without NO2 added, the ten most-sensitive reactions at specific pressure (10 atm) and temperature conditions (1100 K and 1450 K) are shown in Fig. 12. At high temperatures (1450 K, Fig. 12b and 12d), the differences in sensitivity coefficients between these two tested mixtures are relatively small. Reactions R1 (H+O2 ⇔ O+OH) and R201 (C2 H4 + H(+M) ⇔ C2 H5 ) have the highest sensitivity coefficients for both mixtures, which produce reactive H atoms and OH radicals. R1, R201, R302 (C2 H4 + OH ⇔ C2 H3 + H2 O) and R330 (C2 H3 + O2 ⇔ CH2 CHO + O) are the four most sensitive reactions for both neat and NO2 -added ethane mixtures. The difference between these two tested mixtures is reaction R1773, which can consume CH3 radicals to form CH3 O radicals. However, the sensitivity coefficient of R1773 is small in the mixture with NO2 added. Hence, the effect 15 ACS Paragon Plus Environment

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of NO2 addition on ignition delay time is mild at high temperature, similar to the methane case. At lower temperatures (1100 K, Fig. 12a and 12c), sensitivity coefficients differ considerably for neat and NO2 -added mixtures. For the neat ethane, the most important reaction at 1100 K is H atom abstraction reaction R196 (C2 H6 + HO2 ⇔ C2 H5 + H2 O2 ), and H2 O2 can decompose into OH radicals via R19 (H2 O2 (+M) ⇔ OH + OH(+M)) (Fig. 12a). However, R196 becomes insignificant (it does not appear in Fig. 12b) at high temperatures. When NO2 is added (Fig. 12c), R1785 and R1773 show a considerable promoting effect on ethane ignition, leading to a reduced importance of R196. Mathieu et al. 16 also observed that H atom abstraction from fuel by NO2 plays an important role in low-temperature reactions (fuel oxidation). Therefore, NO2 addition has a modest effect on the reduction of ignition delay time for ethane, however, this effect is relatively more pronounced at low temperatures than at high temperatures (Fig. 5). At high temperatures, C2 H6 is mainly consumed via H atom abstraction by OH (60.2%), H (25.4%), O (6.2%), CH3 (2.4%) and NO2 (1.2%), yielding C2 H5 radicals (Fig. 14b). Another initial reaction channel is the thermal dissociation of ethane (4.5%), generating CH3 radicals, and these can further react with NO2 (66.6%) via R1773 to yield CH3 O radicals which can be oxidized to form CH2 O radicals. Subsequently, the C2 H5 radicals can be consumed via the following three reaction pathways: (i) the C2 H5 radicals decompose into C2 H4 (81.1%) radicals or react with O2 (2.6%) to form C2 H4 radicals; (ii) the C2 H5 radicals undergo a termination reaction to form stable alkane C3 H8 (0.8%); and (iii) the C2 H5 radicals react with NO2 (12.2%) via R1784 to yield C2 H5 O which can decompose into CH3 and CH2 O, promoting reactivity. However, at low temperatures, more ethane reacts with NO2 (3.1%) to form C2 H5 radicals, and more C2 H5 radicals react with NO2 (55.7%) to yield C2 H5 O radicals. Hence, the effect of NO2 addition on ignition delay time is more pronounced at low temperatures than at high temperatures. The most sensitive reactions of neat natural gas and CH4 /C2 H6 /NO2 mixtures at 10 atm


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R1717: NO2+H NO+OH

R128: CH4+H CH3+H2 R129: CH4+OH CH3+H2O

R128: CH4+H CH3+H2

R189: CH3+CH3 (+M) C2H6(+M)

R127: CH3+H(+M) CH4(+M)

R127: CH3+H(+M) CH4(+M)

R189: CH3+CH3(+M) C2H6(+M) R193: C2H6+OH C2H5+H2O

R92: CH3O+O2 CH2O+HO2

R147: CH3+O2 CH3O+O

R302: C2H4+OH C2H3+H2O R129: CH4+OH CH3+H2O

R330: C2H3+O2 CH2CHO+O (a)

(b)

R317: C2H4+CH3 C2H3+CH4

R135: CH3+OH CH2OH+H


R330: C2H3+O2 CH2CHO +O

R195: C2H6+CH3 C2H5+CH4




R1: H+O2 O+OH

R1: H+O2 O+OH

Figure 13: Sensitivity analysis for CH4 /C2 H6 mixtures at 1500 K and 10 atm based on the updated model. (a) N0 mixtures. (b) N25 , N50 and N75 mixtures. and 1500 K are presented in Fig. 13. The most important reaction for neat natural gas is the chain branching reaction R1, which can generate reactive O atoms and OH radicals (Fig. 13a), seconded by R148, R195 (C2 H6 + CH3 ⇔ C2 H5 + CH4 ), R144 and R317 (C2 H4 + CH3 ⇔ C2 H3 + CH4 ). CH3 radicals play an essential role for neat natural gas mixture, indicating that the production and consumption of CH3 radicals are critical in natural gas ignition. Reaction R128 shows the most inhibiting effect due to the consumption of H atoms to form a relatively unreactive CH3 radicals, competing with reaction R1. For the mixtures with added NO2 (Fig. 13b), several NOx -related reactions become the most important reactions. R1 still the most sensitive one when NO2 is seeded, and the sensitivity coefficient of this reaction is maximum for N25 but then decreases with increasing NO2 concentration (from N25 to N75 ). The reason is that the added NO2 is consumed rapidly via R1773 and 1717 at low NO2 concentrations, and the production of H atoms and OH radicals can enhance reaction R1. However, For high NO2 concentrations, the added NO2 persists for a long time before ignition, and R1717 competes with R1 for H atoms, so the sensitivity coefficient of R1 decreases. Similar observation was made in a previous study. 18 R1773 becomes the second most promoting reaction for the mixtures, and the sensitivity coefficient increases with increasing NO2 concentration. Essentially, when NO2 is added, the CH3 radicals react with NO2 via R1773, downgrading the importance of R148, R195, R144, R317 and R189. Hence, the CH3 radicals can form CH3 O radicals via R1773, assisting faster 17 ACS Paragon Plus Environment

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CH4 6.4% +CH2O 7.9%

17.4% 9.8% +HO2 2.4% 0.7% +NO2 57.5% 70.1% +O2 2.8% 1.6%

CH3

+OH 78.6% 88.9% + H 12.4% 7.1% +O 6.9% 3.3% +HO2 0.6% 0.1% +NO2 1.5% 0.7%

CH3

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+M 4.5% ......

+OH 60.2% 81.2% +H 25.4% 11.1% +O 6.2% 1.9% +CH3 2.4% 2.5% +NO2 1.2% 3.1%

20 +C .3 2 H % 6 +NO2 66.6% ... 92.7% ...

C2H6

+NO2

CH3O

C2H4 71.5% 7.0% 1.9%

+H 39.8% 22.3% C2H3 +OH 50.1% 60.6% +O 2.4% 1.2% +O2 36.2% +NO2 ...... ...... +NO2 10.2%

46.0% 9.7%

HCO

C3H8

16.7% 17.6%

+OH 69.6% +CH3 4.7% +H 6.1%

CH2O

+CH3 19.4% 1 3.0% +H 25.9% 24.9% +OH 38.4% 58.1% +O 4.2% 3.3% +NO2 4.7% ......

+CH3 0.8% 1.0%

+M 81.1% +O2 2.6%

+M 85.8% 85.7% +M 71.9% 89.0%

CH2O

C2H5

12.2% 55.7%

C2H5O

CH3O +M 84.8% 86.3% +O2 13.7% 9.9% +NO 0.7% 2.3% +NO2 0.5% 1.4%

C2H6

CH2CHO

HCO

(b)

(a)

Figure 14: Reaction flux analysis for the N25 mixtures of CH4 and C2 H6 at 10 atm using the current model. (a) Black fonts: 1500 K; red fonts: 1200 K and (b) Black fonts: 1500 K; red fonts: 1100 K. formation of H atoms through reaction R91. In addition, reaction R1717 is among the most inhibiting reactions due to its competition for H atoms with R1. To further understand the kinetic features relevant to NO2 -perturbed mixtures, reaction flux analysis for the case of 20% fuel consumption is conducted with the current model at 1500 K and 10 atm, and the results are different for different gas components (Fig. 15). Figure 15a presents the reaction pathways of methane-based mixtures. For neat methane (N0 ), the fuel is mainly consumed via H atom reaction by small radicals, such as OH (60.5%), H (21.4%), O (14.8%) and HO2 (3.0%), yielding CH3 radicals. Subsequently, the CH3 radicals can consume via four reaction pathways. First, most of the CH3 radicals (47.5%) undergo a self-recombination termination reaction to form stable alkane C2 H6 . Second, the CH3 radicals (11.3%) via R75 (CH2 O + CH3 ⇔ HCO + CH4 ) to form stable molecules HCO and CH4 . Third, the CH3 radicals (19.1%) react with O2 via R148 (CH3 + O2 ⇔ CH2 O + OH) to form formaldehyde and OH radicals which actually promoting reactivity. The fourth is that CH3 radicals (7.2%) combine with HO2 to generate CH3 O radicals which can quickly decompose 18 ACS Paragon Plus Environment

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to formaldehyde and H atoms. In the presence of NO2 , unlike neat methane mixture, more CH3 radicals (57.5%, 75.9% and 80.7% for N25 , N50 and N75 mixtures, respectively) are consumed via the reaction R1773 to form CH3 O radicals, which can decompose thermally to create abundant H atoms promoting reactivity. With increasing NO2 concentration, the consumption of CH3 radicals via the termination reactions (products of CH4 and C2 H6 ) decreases to 25.3%, 9.7% and 5.1% for N25 , N50 and N75 , respectively. Therefore, more reactive radicals can be generated instead of stable intermediates, increasing the reactivity of fuel mixtures. Moreover, other oxidation pathways of CH3 radicals also become less important because of the great contribution of reaction R1773. CH4 1.7% 3.2% +CH2O 7.85% 11.3%

47.5% 17.4% 6.5% 3.4%

CH3

+HO2 7.2% 2.4% 2.1% 1.9% +NO2 ...... 57.5% 75.9% 80.7% +O2

19.1% 2.8% 0.9% ......

+M CH 3.65% 4.47% 4.63% 4.44% 2 6 +OH 24.8% 60.2% 71.5% 76.0% 4 3 ..... .08 + +H 56.6% 25.4% 14.9% 10.3% % C2 H 66.6% 20 .3% 6 +NO2 +O 11.9% 6.2% 4.2% 3.5% 75.3% 10 +CH3 2.9% 2.4% 2.1% 1.8% . 5% 79.7% 7.1 +NO2 ...... 1.2% 2.3% 3.3% % +CH3 +NO2

CH3

+OH 60.5% 78.6% 88.1% 90.3% + H 21.4% 12.4% 5.2% 3.1% + O 14.8% 6.9% 4.1% 3.8% +HO2 3.0% 0.6% 0.5% 0.5% +NO2 ...... 1.5% 0.9% 2.2%

C2H6

C2H5

1.4% 0.8% 0.6% 0.5% +M 93.2% 81.1% 71.2% 62.5% +O2 3.2% 2.6% 2.2% 1.9%

...... 12.2% 23.6% 32.6%

CH3O

CH3O +M 86.2% 84.8% 83.7% 82.8% +O2 13.7% 13.7% 13.7% 13.6% +NO ...... 0.7% 1.0% 1.0% +NO2 ...... 0.5% 1.3% 2.3%

CH2O

C3H8

CHO

86.6% 2 5 C2H4 +M 85.8% +M 73.4% 71.9% 71.7% 71.6% +OH 44.0% 69.6% 76.3% 79.0% 85.1% +CH3 6.8% 4.7% 4.0% 3.5% 84.5% +H 18.0% 6.1% 3.2% 2.1% CH2O +H 73.9% 39.8% 24.0% 17.4% C2H3 +OH 19.4% 50.1% 62.1% 69.9% +O 4.0% 2.4% 1.6% 1.3% +O2 40.1% 36.2% 31.2% 26.8% +NO2 ...... ...... 1.7% 2.8% +NO2 ...... 10.2% 20.8% 29.2%

+CH3 46.6% 19.4% 8.5% 5.1% +H 21.1% 25.9% 14.2% 8.7% +OH 13.1% 38.4% 59.0% 66.0% +O 3.9% 4.2% 2.7% 2.2% +NO2 ...... 4.7% 8.3% 10.6%

CH2CHO

HCO

HCO

CO

CO2

CO

CO2

CH2CO

HCCO

CO2

(b)

(a)

Figure 15: Reaction flux analysis for the CH4 and C2 H6 mixtures at 1500 K and 10 atm using the current model. Black fonts: N0 ; red fonts: N25 ; purple fonts: N50 ; blue fonts: N75 . Reaction pathways analyses of ethane-based mixtures are shown in Fig. 15b. For neat ethane (N0 ), ethane is mainly consumed via H atom abstraction by small radicals, such as OH (24.8%), H (56.6%), O (11.9%) and CH3 (2.9%), yielding C2 H5 radicals, which can further decompose to C2 H4 (93.2%). The C2 H5 radicals can also react with CH3 radicals to form propane (C3 H8 ), inhibiting ethane ignition. Another initial channel is the thermal dissociation of ethane (3.7%), generating CH3 radicals, and these can be further oxidized to yield CH3 O radicals or CH2 O. For the NO2 -added mixtures, only a very few of C2 H6 (1.2%, 19 ACS Paragon Plus Environment

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2.3%, 3.3% for N25 , N50 and N75 , respectively) react with NO2 to product C2 H5 radicals. The reaction flux of C2 H5 consumption is only slightly affected by NO2 addition. Ethyl radicals react with NO2 via R1784 to form C2 H5 O radicals (12.2%, 23.6% and 32.6% for N25 , N50 and N75 mixtures, respectively), which can decompose to create CH2 O and CH3 redicals. With increasing NO2 concentration, thermal decomposition of C2 H5 radicals (R201) decreases to 81.1%, 71.2% and 62.5% for N25 , N50 and N75 , respectively. Therefore, reaction R1784 may restrain the promoting effect of R201, inhibiting the formation of H atoms and C2 H4 . Furthermore, the CH3 radicals (66.6%, 75.3% and 79.7% for N25 , N50 and N75 mixtures, respectively) are consumed via the reaction R1773 to form CH3 O radicals, which can decompose thermally to create abundant H atoms promoting reactivity. However, the concentration of CH3 radicals is modicum. This may be the reason why NO2 additive only results in a modest promoting effect on ethane ignition (Figure 5), in contrast to its effect on methane ignition.

4

Conclusion

Ignition delay times of stoichiometric methane, ethane and natural gas (CH4 :C2 H6 =9:1) mixed with NO2 at 0%, 25%, 50% and 75% of the fuel concentrations, are measured in a shock tube at 1016–1984 K and 5–16 atm. Addition of NO2 to CH4 /O2 /Ar and CH4 /C2 H6 /O2 /Ar leads to a significant reduction in ignition delay times, and the reduction increases with increasing NO2 concentration. However, such a reduction is minor for C2 H6 /O2 /Ar mixtures. The NO2 promoting effect also shows a strong temperature-dependence, and the effect of pressure on ignition delay time reduction due to NO2 addition is weak. An updated mechanism is proposed, and reproduces accurately the ignition delay times measured in this study. Based on the updated mechanism, kinetic analyses are conducted. For CH4 , reactions R1773 (NO2 + CH3 ⇔ CH3 O + NO), R1717 (NO2 + H ⇔ NO + OH) and R1758 (NO +


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CH3 O2 ⇔ NO2 + CH3 O) exhibit higher sensitivity, and the role of peroxide radicals is more important at low temperatures than high temperatures. For C2 H6 , reactions R1773 and R1785 (C2 H5 + HONO ⇔ C2 H6 + NO) show higher sensitivity at low temperatures than high temperatures. The significant difference in NO2 sensitization between methane and ethane is explained by means of the reaction pathway analysis. For CH4 , the vast majority of CH3 radicals are combined with NO2 to form CH3 O radicals, and the self-recombination reaction of CH3 radicals (forming C2 H6 ) is restrained. Those two factors enhance the ignition of CH4 . However, for C2 H6 , only few C2 H5 radiacals react with NO2 to form C2 H5 O radiacals which can decompose to create CH2 O and CH3 redicals. Hence, the addition of NO2 has only a modest promoting effect on ethane ignition.

Acknowledgment This work was supported in part by the 973 Project of China (No. 2014CB845904) and NSFC (No. 11627901).

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Energy & Fuels

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Ethane

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