Experimental and Kinetic Modeling Study on trans-3-Hexene Ignition

Dec 23, 2015 - trans-3-Hexene ignition delay times were measured behind reflected shock waves for fuel-lean (Φ = 0.5), stoichiometric (Φ = 1.0), and...
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Experimental and Kinetic Modeling Study on trans-3-Hexene Ignition behind Reflected Shock Waves Feiyu Yang, Fuquan Deng, Peng Zhang, Zemin Tian, Chenglong Tang,* and Zuohua Huang State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China S Supporting Information *

ABSTRACT: trans-3-Hexene ignition delay times were measured behind reflected shock waves for fuel-lean (Φ = 0.5), stoichiometric (Φ = 1.0), and fuel-rich (Φ = 1.5) mixtures between 1080 and 1640 K, at pressures between 1.2 and 10 atm. Two fuel concentrations (1000 and 5000 ppm trans-3-hexene) diluted in argon were examined, and the ignition delay times were obtained by following OH* radical chemiluminescence emission. The experimental results satisfied the Arrhenius equation, and the influences of pressure, equivalence ratio, fuel concentration, and dilution gas on trans-3-hexene ignition behavior were discussed. The Lawrence Livermore National Laboratory (LLNL) model overestimates the low-temperature reactivity and underestimates the pressure-dependence at high-temperature. Improvements have been made to the LLNL model, and the modified mechanism offers better predictions for the ignition delay times of this work as well as the shock tube, rapid compression machine, and jet stirred reactor experimental data from the literature. Reaction pathway and sensitivity analysis were performed to gain insight into the trans-3-hexene oxidation chemistry. channels the simplest among the 3 n-hexene isomers. The single CC double-bond structure permits its ignition and oxidation behaviors to be comparable to that of both an olefin and a linear paraffin.6 Unlike saturated alkanes,7,8 the characteristics of alkene oxidation have not been sufficiently investigated, especially for alkenes with a longer chain. Westbrook et al.5 measured the ignition delay times and species concentration profiles of 2methyl-2-butene (2M2B) in a shock tube and a jet stirred reactor (JSR), respectively, proposing a kinetic model. Ribaucour et al.6 measured the ignition delay times of npentane and 1-pentene in a rapid compression machine (RCM) at 600−900 K and proposed a kinetic model. Touchard et al.9 examined the ignition delay times of 1-pentene in a shock tube and developed a model with more comprehensive scheme. Prabhu et al.10 explored the oxidation behaviors of 1-pentene in a flow reactor. Tanaka et al.11 have reported the pressure profiles of linear heptene isomer combustion in a RCM. Mehl et al.12 measured the ignition delay time of two n-pentene isomers in a shock tube and proposed a kinetic model. As far as hexenes are concerned, Yahyaoui et al.13,14 studied the oxidation behaviors of 1-hexene in a JSR at 750−1200 K and 10 atm and in a shock tube at 1270−1700 K and 0.2−1 MPa. The developed model overestimates the ignition delay times in the low-temperature region. Using the Exgas15 software, Touchard et al.9 proposed a mechanism for 1pentene and 1-hexene, and modifications have been made to achieve better predictions. Vanhove et al.16,17 investigated the autoignition features of 1-, 2-, and 3-hexene after rapid compression at 630−850 K for stoichiometric mixtures. Mehl et al.12,18 reported the ignition delay times in shock tube of

1. INTRODUCTION Since the beginning of the Industrial Revolution in the 18th century, population growth and industrialization throughout the world, especially in developing countries, have driven a steady rise of fossil fuels consumption (in terms of petroleum, coal, and natural gas consumption). Combustion of petroleum for transportation accounts for 13% of greenhouse gas emissions globally,1 while this value in the United States is 28%. Global concerns over increasing carbon emissions and fossil fuel consumption have motivated the research on developing environment friendly combustion techniques with high efficiency. Chemical kinetics of a specific fuel describe its oxidation pathways, rate of each path, and the downstream products. Accurate evaluation of the fuel oxidation kinetics, which is important for combustion simulation in engines, burners, and other combustors, holds the potential to increase combustion efficiency, reduce pollutant emission, and optimize combustor design. The powerful calculation capabilities of modern computers can afford high fidelity simulation using detailed kinetic mechanisms of a specific fuel. However, because practical fuels consist of hundreds of hydrocarbons with different functional groups and concentrations, numerical simulations using practical fuels is so challenging that even modern computer clusters have difficulty in time cost and convergence of solution if the compositions are not simplified. Thus, kinetic studies of gasoline or diesel surrogates2,3 have received significant research attention. Our target fuel in this work is trans-3-hexene because it has a high octane number (research octane number 94 and motor octane number 804) and high sensitivity value (RON-MON5) and it is an important gasoline component or additive. The CC double bond separates the long paraffin chain from the midpoint; therefore, the trans-3-hexene molecule possesses a geometric symmetry which makes its chemical reaction © 2015 American Chemical Society

Received: November 13, 2015 Revised: December 21, 2015 Published: December 23, 2015 706

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Energy & Fuels three linear hexene isomers and a kinetic mechanism (Lawrence Livermore National Laboratory (LLNL) model) was proposed as well. Battin-Leclerc et al.4 observed the products from the oxidation linear hexene isomers in two JSRs. To our knowledge, the kinetic study on trans-3-hexene chemistry has not been adequately conducted, in terms of experimental data reporting and kinetic modeling. Only the stoichiometric 3-hexene mixture was measured in previous works. The first objective of this work is to measure the ignition delay times of trans-3-hexene at different equivalence ratios using a shock tube. In addition, a better model was obtained on the basis of previous investigations, and almost all the associated experimental databases (including shock tube, RCM, and JSR) from the literature were used to validate the trans-3-hexene mechanism; further analysis was performed to provide clearer insight into trans-3-hexene oxidation chemistry. Figure 1. Definition of ignition delay time in this work.

2. EXPERIMENTAL SETUP AND PROCEDURE The shock tube employed in this work has been introduced in previous contributions.19−22 Only a brief introduction is presented here. The high-temperature shock tube consists of a 2 m driver section and a 7.3 m driven section. There is a two PET (polyester terephthalate) diaphragms section (flange section, 0.06 m long) between them. The three sections have the same inner diameter of 11.5 cm. Before each test, the shock tube is evacuated to a pressure below 10−4 Torr. Then, high-purity helium (99.999%) and nitrogen (99.999%) are charged into the driver and the double diaphragm section. The driven section is filled with the test mixture, the preparation of which is described later. The shock wave is triggered by rupture of the double diaphragm once the flange section is evacuated. Four pressure transducers (PCB 113B26) are located along the driven section with an identical interval of 30 cm. The distance between the last pressure transducer and the end wall is 2.0 cm. The time instant of the shock wave arrival at each pressure transducer location is sent to three time counter (Fluke PM6690) with an accuracy of 10 ps, so that the shock wave velocity profile along the shock tube direction is determined. A pressure transducer (PCB 113B03) is also mounted at the end wall flange to measure the end wall pressure. In addition, a quartz optical window is also mounted at the end wall flange, through which the OH* emission signal can be detected by a 306 nm narrow band-pass filter and a photomultiplier (Hamamatsu CR 131). All the pressure and OH* emission signals are recorded by a digital oscilloscope (Yokogawa, ScopeCorder DL750). The temperature behind the reflected shock wave is calculated according to the shock wave velocity at the end wall (extrapolated by its velocity profile along the shock tube), the reactant gas property, and the initial temperature (T1) and pressure (p1) using the normal shock wave module in the Gaseq software.23 Figure 1 shows the typical pressure and OH* emission signal profiles captured by the end wall pressure transducer and the narrow pass filter, respectively. At time t0 = 1269.2 μs, the shock wave arrives at the end wall. At around t1 = 3737.1 μs, there is a sharp increase in the pressure profile, which demonstrates the onset of ignition. The ignition at t1 is also manifested by the steep increase in the OH* emission signal. Thus, the ignition delay time is defined as (t1 − t0) = 2467.9 μs, which is essentially the time duration between the arrival of the shock wave at the end wall and the onset of ignition. This definition is consistent with most of the previous work.20,22 The molecular structure of the trans-3-hexene molecule is depicted in Figure 2. It is noted that many unsaturated hydrocarbons have cis− trans isomers, and 3-hexene is no exception. In general, the transisomer exhibits a heat of combustion lower than that of its corresponding cis-isomer because of its more stabilized structure. In addition, many trans-isomers can undergo isomerization reaction to form the corresponding cis-isomers under high-temperature circumstances. Generally, the difference between cis−trans hexenes oxidation are negligible, that is to say if one mechanism can readily predict the oxidation behavior of one isomer, usually it is also applicable to both

Figure 2. Real geometry of trans-3-hexene molecule. cis-, trans-isomers and their blends. In this study, trans-3-hexene are used. The trans-3-hexene/oxygen/argon mixtures were prepared in a 128 L stainless steel tank at 298 K (using air conditioner to maintain the temperarure) before each run. The reactant fractions are determined by the partial pressure of each component. Because the fuel concentration in this study is very low, it is important to guarantee the accuracy of the partial pressure of each component, especially the fuel. A vacuum meter (Rosemount 3051C) with an accuracy of 1 Pa is used to measure the partial pressure of the fuel. The tank pressure is less than 300 kPa so that the condensation of the fuel can be avoided. The purities of trans-3-hexene, oxygen, and argon are 99%, 99.999%, and 99.999%, respectively. Once the mixtures are prepared, the tank is stirred and then settled for at least 12 h before experiments. The measurements were carried out at 1.2, 4.0, and 10 atm for two fuel concentrations (5000 and 1000 ppm trans-3-hexene) and at equivalence ratios of 0.5, 1.0, and 1.5. The compositions of the mixtures are shown in Table 1.

Table 1. Mixture Compositions in This Study trans-3-hexene (ppm)

O2 (%)

Ar (%)

Φ

P (atm)

5000 5000 5000 1000

9.0 4.5 3.0 1.8

90.5 95.0 96.5 98.1

0.5 1.0 1.5 0.5

1.2/4/10 1.2/4/10 1.2/4/10 1.2/4/10

3. KINETIC MODELING Parallel to the experimental research, kinetic modeling has been performed as well. To the best knowledge of the authors, there are two detailed mechanisms available: the n-hexane mechanism proposed by the National University of Ireland Galway (NUIG model8) and the gasoline surrogate mechanism proposed by the Lawrence Livermore National Laboratory (LLNL model24). Considering the better behavior of the LLNL 707

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Energy & Fuels Table 2. Modifications Made to the LLNL Mechanism no. R1 R16

R47 R92 R184

R185 R190

R230

R264 R265 R266 R648 R777

R931 R3762 R3763 R3764 R3768 R3769 R3770 R4087 R4207 R4208 R4354 R4428 R4439 R4430 R4431 R4465

reactions

A

H + O2 ⇔ O + OH H2O2(+M) ⇔ OH + OH(+M) LOW/ 2.49 × 1024 −2.3 48700 / TROE/ 4.300 × 10−1 1.000 × 10−30 1.000 × 1030 / H2O/0.00/CO2/1.60/N2/1.50/O2/1.20/HE/0.65/H2O2/7.70/ CH2O + HO2 ⇔ HCO + H2O2 CH3OH + OH ⇔ CH2OH + H2O C2H5 + O2 ⇔ C2H4 + HO2 DUP PLOG/ 0.0400 2.094 × 109 0.490 −391.4 / PLOG/ 1.0000 1.843 × 107 1.130 −720.6/ PLOG/ 10.0000 7.561 × 1014 −1.010 4749.0/ C2H5 + O2 ⇔ C2H4 + HO2 DUP C2H5O2 ⇔ C2H4 + HO2 PLOG/ 0.0400 1.782 × 1032 −7.100 32840.0/ PLOG/ 1.0000 2.701 × 1037 −8.470 35840.0 / PLOG/ 10.0000 1.980 × 1038 −8.460 37900.0 / CH2CHO + O2 ⇒ CH2O + CO + OH PLOG / 0.0100 2.680 × 1017 −1.840 6530.0/ PLOG/ 0.1000 1.520 × 1020 −2.580 8980.0/ PLOG/ 1.0000 1.650 × 1019 −2.220 10340.0/ PLOG/ 10.0000 8.953 × 1013 −0.600 10120.0/ C2H3 + O2 ⇔ CH2O + HCO C2H3 + O2 ⇔ CH2CHO + O C2H3 + O2 ⇒ H + CO + CH2O C3H5O ⇔ C2H3CHO + H C4H71-4 ⇔ C4H6 + H PLOG/ 1.0000 2.480 × 1043 −12.300 52000.0/ PLOG/ 10.0000 1.850 × 1038 −10.500 51770.0/ NC4KET13 ⇔ CH3CHO + CH2CHO + OH C5H81-3 + H ⇔ C5H91-3 C5H81-3 + H ⇔ C5H91-4 C5H81-3 + H ⇔ C5H92-4 C5H81-3 + OH ⇔ CH2O + C4H71-3 C5H81-3 + OH ⇔ C2H3CHO + C2H5 C5H81-3 + OH ⇔ CH3CHO + C3H5-S C6H12-3 + OH ⇒ PC4H9 + CH3CHO C6H112O2-4 ⇒ C6H102-4 + HO2 C6H112O2−5 ⇒ C6H102-4 + HO2 C6H12-3 + O ⇔ NC3H7 + C2H5CO C6H101-3 + H ⇔ C6H111-3 C6H101-3 + H ⇔ C6H111-4 C6H101-3 + H ⇔ C6H113-1 C6H101-3 + H ⇔ C6H112-4 C6H11O2-4 ⇔ SC3H5CHO + C2H5

1.04 × 1014 2.00 × 1012

0 0.9

n

Ea (cal/mol) 1.53 × 104 4.87 × 104

35 36

1.88 × 104 3.08 × 104 2.09 × 109

2.7 2.65 0.49

1.15 × 104 −8.07 × 102 −3.91 × 102

37 38 39

6.61 × 100

3.51

1.42 × 104

39

1.78 × 1032

−7.1

3.28 × 104

39

2.68 × 1017

−1.84

6.53 × 103

40

2.38 5.34 3.38 1.00 2.48

× × × × ×

1027 1012 1014 109 1043

−4.86 −0.01 −0.89 0 −12.3

4.91 3.32 2.18 2.91 5.20

1.05 2.50 2.50 2.50 3.00 3.00 3.00 6.00 1.00 5.04 2.00 2.50 2.50 4.25 2.50 1.09

× × × × × × × × × × × × × × × ×

1015 1012 1012 1012 1012 1012 1012 1010 1038 1037 1013 1013 1013 1013 1013 1015

0 0.5 0.5 0.5 0 0 0 0 −8.11 −8.11 0 0.5 0.5 0.5 0.5 −1.92

4.16 × 104 2.62 × 103 2.62 × 103 2.62 × 103 0 0 0 −4.00 × 103 4.05 × 104 3.75 × 104 −1.05 × 103 2.62 × 103 2.62 × 103 1.23 × 103 2.62 × 103 1.08 × 104

× × × × ×

103 103 103 104 104

ref.

41 41 41 42 43, 44

× × × × × × × × × × × × × × × ×

0.1 10 10 10 3 3 3 0.6 0.1 0.1 10 10 10 10 10 10

elimination reaction will produce a diene instead of an alkene. To our knowledge, no investigations on HO2 elimination for this case are reported. However, in the case of alkenylperoxy radicals, compared with secondary C−H bond, the weaker allyl C−H bond can enhance the elimination reaction. For trans-3hexene molecule, the short alkyl chains avoid isomerization reaction, and as a result, this elimination reaction rules the reactivity of the system. That is why large quantities of 2,4hexadiene and 1,3-hexadiene are probed during the lowtemperature oxidation. The pre-exponent factor is divided by a factor of 10 to meet better predictions. The no-barrier reactions of H addition were emphasized in the pathway analysis of Battin-Leclerc et al.4 The H radical can add not only to trans-3-

model at low temperature (the JSR and RCM simulations of the NUIG and LLNL models are attached as Supporting Information), the LLNL model was adapted in this investigation. Containing 1389 species and 5935 reactions, the LLNL model was developed aiming at offering predictions of gasoline surrogate oxidation. However, when it comes to the 3-hexene submechanism, it has been validated against only a small number of RCM and shock tube experimental data of 3hexene under only stoichiometric conditions; further validations and necessary modifications are still required. It is well-known that an alkylperoxy radical (ROO•) can dissociate directly yielding alkene and hydroperoxy (HO2) radical.25 In the case of an alkenylperoxy radical, this direct 708

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Energy & Fuels hexene but also to dienes, say, hexadienes and pentadienes. Considering the large concentration of dienes, this H addition channel exerts significant impact on the oxidation process. To attach more importance to H addition, the rate constant is increased by a factor of 10. At low temperature, the trans-3hexene molecule can readily be attacked by OH radical producing n-butyl (PC4H9) and acetaldehyde (CH3CHO). This pathway will directly compete the OH radical with the addition and H-abstraction reactions and can overwhelmingly determine the species profiles at around 650 K. The produced PC4H9 radical will proceed the well-known low-temperature chain-branching channel producing two active OH radicals. In this study the rate constant is multiplied by 0.6 to correct the over prediction of low-temperature reactivity. In addition, the famous chain-branching reaction and the dissociation of H2O2 are updated. The pressure dependence of C2H4 reactions are introduced. Note that all the modifications are within their claimed uncertainty, and the modified reactions along with their references are listed in Table 2.

4. RESULTS AND DISCUSSION 4.1. Ignition Delay Time Data. The measured ignition delay times of trans-3-hexene are presented in Figure 3 and also are available in Table S1. Note that the ignition characteristics of trans-3-hexene obey the Arrhenius rule that the logarithm of ignition delay times are proportional to the reciprocal of temperatures. Westbrook et al.5 suggested eq 1 to obtain the quantitative relationship between the ignition delay times and the pressures and equivalence ratios. In eq 1, τign, Ea, R, A, α, and β denote the ignition delay time (μs), activation energy (kcal/mol), ideal gas constant (R = 1.968 kcal/(mole·K)), pre-exponential factor, exponent factor for equivalence ratio, and exponent factor for pressure, respectively. τign = A ·ϕα ·p β ·exp(Ea /RT )

(1)

Fitted from the entire experimental data at 5000 ppm concentration, the following correlation (eq 2, R2 = 0.97) can be derived: τign (μs) = 3.0 × 10−3· ϕ1.064 ·p−0.421 · exp(33.0 (kcal/mol)/RT ) (2)

This correlation offers a positive equivalence ratio exponent and a negative pressure exponent, which indicates the inhibition and promotion effect of equivalence ratio and pressure, respectively, on trans-3-hexene ignition. It is noted that the absolute pressure exponent (−0.421) is small compared to that of real gasoline fuels (−1.0626) and that of n-heptane (−1.6426), which indicates that pressure exerts only slight effects on ignition delay times of trans-3-hexene. This feature makes it a promising fuel or additive to inhibit the knock in supercharged gasoline engines, in which the in-cylinder pressure is high compared to that in naturally aspirated engines. To some extent, this high knock resistance property can as well be speculated from the high octane number (research octane number 94) mentioned before. As reported by Westbrook et al.,5 the pressure exponent of 2M2B (RON = 97.3) is −0.40, which is quite close to that of trans-3-hexene. Note that both 2M2B and trans-3-hexene have a CC double bond located at the center of the molecule. Yahyaoui et al.14,27,28 studied the ignition of 1-hexene in a shock tube, and although no pressure exponent was given

Figure 3. Experimental data and correlations of trans-3-hexene at different equivalence ratios. 709

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4.2. Model Validation. To evaluate the performance of the modified mechanism, in this section the modified model was widely validated against not only the ignition delay times of this work but also the shock tube, RCM, and JSR data from the literature. 4.2.1. Shock Tube Ignition Delay Times. 4.2.1.1. Shock Tube Ignition Delay Times of This Work. The measured ignition delay times of this work were used to validate the modified model. The simulation has been conducted using the zero dimensional closed homogeneous model of the SENKIN code in the CHEMKIN II32 package. The problem type was set as a constrained volume and solved energy equation. However, in practice, under the influence of the boundary layer, the volume shrinks and the pressure rises. The pressure rising rate (dp/dt) is about 4% per ms, which is an average value obtained from the pressure time history curves. The dp/dt can obviously decrease ignition delay time at temperatures below 1300 K. Figure 4b illustrates the ignition delay time with and without considering dp/dt. As a result, the dp/dt is considered in all the simulations of ignition delay time obtained in our shock tube. A maximum uncertainty of 18%33 is taken into account. Figure 4 compares the simulations before and after modification. For 5000 ppm trans-3-hexene mixtures, the LLNL mechanism underestimates the reactivity at high pressure and overestimates at low pressure. As for the modified model, better agreement was achieved for all the measurements. When the fuel concentration decreases to 1000 ppm, the simulation of the LLNL model presents a weaker pressure dependence which is not in agreement with the measurements; however, the modified model captures this feature readily. Overall, the predicted ignition delay times of the modified model are in reasonable agreement with the experimental data. 4.2.1.2. Shock Tube Ignition Delay Times from the Literature. The shock tube ignition delay times from the literature were also adopted to validate the modified model. Mehl et al. 12 explored the ignition characteristics of stoichiometric 3-hexene/oxygen/nitrogen mixture at 993− 1326 K, 8.9−11.8 atm in a shock tube. In this mixture, 3hexene concentration is maintained at 1.92%, which is approximately 4 times that in the current research (trans-3hexene % = 0.5%). As depicted in Figure 5, the modified model provides a better simulation. Note that the ignition delay times at 10 atm (Φ = 1) in this study are about 4 times longer than those of Mehl et al.,12 which is likely to result from the difference in dilution ratio. In addition, the activation energy of Mehl’s tests is smaller than of this study, which is possibly due to different diluents. Mathieu et al.34 investigated methane ignition in NOx environment and reported that nitrous oxide and nitrogen dioxide can readily decrease the activation energy of the mixture and facilitate ignition. This theory can be applied to the exhaust gas recirculation (EGR) system, which has been widely used in vehicles to reduce NOx emission and raise combustion efficiency. 4.2.2. RCM Ignition Delay Times. The RCM ignition delay times reflect the low and intermediate temperature chemistry. Vanhove et al.17 measured the ignition delay times of stoichiometric trans-3-hexene/air mixture at 6.8−8.5 bar, 630−850 K with RCM. As shown in Figure 6, the LLNL model exhibits longer ignition delay times and a slight negative temperature coefficient (NTC) phenomenon occurs. As reported by Vanhove et al.,17 no cool flame and NTC occur for trans-3-hexene ignition under this condition. This

directly from their experimental data, a pressure exponent of −0.30 was derived by Westbrook et al.5 Mehl et al.18 compared the octane numbers of 1-, 2-, and 3-alkene and those of their corresponding alkanes. They found the octane numbers of alkenes are larger than that of their corresponding alkanes, especially when CC double bond locates at the center of the molecule. More recently, the conjecture that the weak pressure dependence of some unsaturated hydrocarbons result from their high octane number are proposed to be supported by some experiments. However, this point of view is challenged when the pressure exponent of 1-hexene and 3-hexene are concerned. This discrepancy may perhaps result from the different octane sensitivity,5 and further studies are required. Compared with methane (Ea = 47.2 kcal/mol29) and ethylene (Ea = 47.4 kcal/mol30), trans-3-hexene presents a rather smaller activation energy of 33.0 kcal/mol, being equal to that of ethane (Ea = 33.0 kcal/mol31), which indicates that the paraffin chain at each end of the CC double bond facilitates the reactivity. To obtain the effect of equivalence ratio and fuel concentration on trans-3-hexene ignition, multiple linear regression was conducted for each set of operating conditions. The ignition data are correlated as τign (μs) = 5.7 × 10−4 ·p−0.436 · exp(35.4 (kcal/mol)/RT ), ϕ = 0.5, 5000 ppm

(3)

τign (μs) = 1.8 × 10−3·p−0.452 · exp(34.2 (kcal/mol)/RT ), ϕ = 1.0, 5000 ppm

(4)

τign (μs) = 1.1 × 10−2 ·p−0.459 · exp(30.9 (kcal/mol)/RT ), ϕ = 1.5, 5000 ppm

(5)

τign (μs) = 3.3 × 10−4 ·p−0.434 · exp(39.3 (kcal/mol)/RT ), ϕ = 0.5, 1000 ppm

(6)

With the increase in equivalence ratio, the concentration of oxygen decreases and, consequently, the dominating Habstraction reactions of O and OH radicals (C6H12-3 + O ⇔ C6H112-4 + OH and C6H12-3 + OH ⇔ C6H112-4 + H2O) are replaced by the addition and H-abstraction reactions of H radical (C6H13-3 ⇔ C6H12-3 + H, C6H12-3 + H ⇔ C6H112-4 + H2 and C6H12-3 + H ⇔ C6H113-1 + H2). It is obvious that the C6H112-4 radical with a resonance-stabilized allylic structure is relatively more stable than the C6H111-3 and C6H13-3 radical. What is more, the preferred H addition reaction exhibits significant pressure dependence. As a consequence, when it comes to a fuel-rich mixture, the absolute pressure exponent increases (0.436 < 0.452 < 0.459) and activation energy declines (35.4 > 34.2 > 30.9). When eqs 3 and 6 are compared, the increase in dilution ratio gives rise to a smaller absolute pressure exponent (0.436 > 0.434) and a larger activation energy (35.4 < 39.3). Under highly diluted conditions, more argon atoms are contained; however, argon has almost the smallest enhanced three-body coefficient (about 0.7024), which may lead to weaker pressure dependence. In addition, high argon concentration reduces the probability of collision between reactive molecules, which inhibits the reaction processes. 710

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Figure 5. Validation against the shock tube data of Mehl et al.12 and this work at 10 atm.

Figure 6. Validation against the RCM ignition delay times of Vanhove et al.17

discrepancy can be readily attributed to the overestimate of low-temperature activity in the LLNL mechanism. 4.2.3. JSR Data. To further clarify the reaction pathway and intermediate speciation, validation against JSR data is required. Battin-Leclerc et al.4 reported the concentration profile during the oxidation of stoichiometric 3-hexene/oxygen/argon mixture at 500−1100 K, 800 Torr with a residence time of 2 s. The comparison between the predictions and experimental data are depicted in Figures 7 and 8. From 600 to 775 K, the trans-3hexene consumes slowly and many products, say 1-3-hexanene, formaldehyde, and methanol, reach the first peak. At this temperature range, the hydroxyl addition and H-abstraction reactions dominate and reaction R4087 C6H12-3 + OH ⇒ PC4H9 + CH3CHO is of significant importance as well. The produced PC4H9 radical proceeds through the following chainbranching channel: PC4H9 ⇒ PC4H9O2 ⇒ PC4H8OOH1-3 ⇒ PC4H8OOH13O2 ⇒ NC4KET13 + OH ⇒ CH3CHO + CH2CHO + OH + OH. This degeneration pathway can overwhelmingly improve the OH radical and CH3CHO concentration and therefore facilitates the reactivity. This pathway accounts for 14% in the LLNL mechanism, which leads to the overestimate of many products at about 650 K. Reasonable agreements are obtained when the rate constant of R4087 decreases by 40% and the corresponding pathway

Figure 4. Validation against the LLNL and modified mechanism with current data at different equivalence ratios. Dashed line, LLNL model; solid line, modified model; dash−dotted line, LLNL model without dp/dt; symbols, measurements. 711

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Figure 7. Validation of the LLNL and the modified mechanism against the concentration profile data of 3-hexene, oxygen, carbon monoxide, carbon dioxide, methane, ethylene, propene, 1-butene, and 1,3-butadiene from JSR. Symbols, experimental data; blue line, LLNL model; black line, modified model.

Figure 8. Validation of the LLNL model and the modified mechanism against the concentration profile data of 1,3-pentadiene, 2,4-hexadiene, 1,3hexadiene, acrolein, 2-butenal, formaldehyde, acetaldehyde, propanal, and n-butanal from JSR. Symbols, experimental data; blue line, LLNL model; black line, modified model.

decreases to 6%. From 775 to 1100 K, almost all products peak and the reactants are consumed sharply because of the higher concentration of radical pool. At intermediate temperature, the alkene are chiefly consumed through O atom collision (R4354 C6H12-3 + O ⇔ NC3H7 + C2H5CO) and H atom addition (R4066 C6H13-3 ⇔ C6H12-3 + H) channel, and the hexadienes are produced through OH H-abstraction channel. The LLNL mechanism attaches less importance to the O atom collision and H atom addition channels, and these two channels will

directly compete with the OH H-abstraction channel. As a consequence, the LLNL model predicts more dienes and less alkenes. 4.3. Kinetic Analysis. 4.3.1. Pathway Analysis. Because initiation reactions involve only the reactant species, there are only four possible reactions in the present trans-3-hexene/ oxygen/argon system, namely, the dissociation of C6H12-3, the dissociation of O2, and the reaction between C6H12-3 and O2, as in 712

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Energy & Fuels C6H12‐3 ⇔ CH3 + C5H 91‐3 O2 + M ⇔ O + O + M

(R4073, Ea = 72 kcal/mol)

(R6, Ea = 119 kcal/mol)

C6H12‐3 + O2 ⇔ C6H112‐4 + HO2

(R4175, Ea = 37 kcal/mol)

C6H12‐3 + O2 ⇔ C6H111‐3 + HO2

(R4176, Ea = 53 kcal/mol)

The activation energies of those four reactions are 72, 37, 53, and 119 kcal/mol, respectively. Note that R4175 and R4176 are the most important initiation reactions under almost all conditions. R4037 may also contribute to initiation only at high temperature, and R6 is usually not preferred because of the large dissociation energy of oxygen. When an oxygen molecule abstracts a secondary H atom from a carbon atom in α position, the obtained 3-hexen-2-yl radical (C6H113-2) isomerizes to a resonance-stabilized 2-hexen-4-yl radical (C6H112-4). However, when H-abstraction reaction takes place at β-carbon atom, the produced 3-hexen-1-yl (C6H1131) radical is not a resonance-stabilized allylic radical, so the R4175 channel with a smaller activation energy is more preferred than R4176. As shown in Figure 9, the dominant reactions shift at different temperature regions. At low temperature, OH radical

Figure 9. Dominant reactions at different temperature regions.

addition reaction dominates; the product C6H12OH-3 will easily add to O2, forming O2C6H12OH-3. Then two propanal molecules are generated through O2C6H12OH-3 ⇔ C2H5CHO + C2H5CHO + OH, which results in the first peak of propanal at 650 K in Figure 8h. The OH radical H-abstraction reactions are of secondary importance. For long chain alkanes or alkenes with long saturate carbon chain, the H-abstraction products can easily transform to ketohydroperoxides and OH radical at low temperature. The ketohydroperoxides decompose to generate OH radical. The rapid production of OH radical significantly facilitates the reactivity and results in the NTC phenomenon. This is the well-known degeneration channel of ketohydroperoxides. Nevertheless, a transition state with 5-member-cycle or 6-member-cycle is required for alkanylperoxide to isomerize forming alkanylhydroperoxide radical. This is due to the lower ring strain of the 5-member cycle or 6-menber cycle. In the trans-3-hexene molecule, the four atoms connected to the double bond locate in one plane. The location of the double bond and the short saturated carbon chain fail to form a transition state with 5-member-cycle or 6-member-cycle. As a consequence, instead of forming ketohydroperoxides, the Habstraction products of trans-3-hexene react with hydroperoxygen radical and decompose, forming small aldehydes and alkenes (Figure 10a). At intermediate temperature region, H-abstraction of OH radical and addition of H radical dominate in turn. The conversion of reactants accelerates, and many intermediates reach the maximum at about 850 K because of the booming of the radical pool. Only a few derivatives of oxirane are produced. At temperatures above 1275 K, initiation reaction dominates and the radicals decompose rapidly through beta-scission channel. 4.3.2. Sensitivity Analysis. To have a further look into the mechanism and find the most sensitive reactions at different

Figure 10. Pathways analysis of trans-3-hexene oxidation at low (a, 650 K), intermediate (b, 850 K), and high (c, 1350 K) temperature.

temperature regions, sensitivity analysis was conducted for stoichiometric mixture at 4 atm for 650, 850, and 1350 K. The sensitivity coefficient is defined as S=

τ(2ki) − τ(0.5ki) 1.5τ(ki)

(7)

where ki is the rate constant of the ith reaction and τ is the ignition delay time. A negative sensitivity coefficient indicates the promotion effect of the reaction, and a positive value indicates the inhibition effect. The results are illustrated in Figure 11. 713

DOI: 10.1021/acs.energyfuels.5b02682 Energy Fuels 2016, 30, 706−716

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Energy & Fuels

consume the active OH radical and yield relatively inactive products; therefore, R4142 and R4142 are the chief inhibition reactions. When considering 850 K, the major channel is Habstraction and H addition. Reaction R4147 C6H12-3 + HO2 ⇔ C6H112-4 + H2O2 converts the slow reacting radical HO2 to the H2O2 molecule, which can sequentially produce two OH radicals and promote ignition. Reactions R110 CH3 + HO2 ⇔ CH4 + O2 and R4183 CH3 + C5H81-3 ⇔ C6H112-4 are typical chain termination reactions and can overwhelmingly reduce the system activity. In addition, for the high-temperature region, say 1350 K, the main promoting reactions are those involving small radicals and small molecules. The well-known chain-branching reaction R1 H + O2 ⇔ O + OH offers the most prominent promoting effect. Chain termination reactions such as R110 CH3 + HO2 ⇔ CH4 + O2, R4141 C6H12-3 + H ⇔ C6H112-4 + H2, and R239 HCCO + OH ⇒ H2 + 2CO are the chief inhibitive reactions.

5. CONCLUSIONS In the present work, the ignition delay times of trans-3-hexene/ oxygen/argon mixtures have been measured at 1.2, 4, and 10 atm for equivalence ratios ranging from 0.5 to 1.5. To the best knowledge of the authors, these are the first experimental data reported for 3-hexene under nonstoichiometric conditions. The LLNL model overestimates the low-temperature reactivity and fails to capture the pressure dependence at high temperature. Improvements have been made to the LLNL model, and the modified model can successfully predict the experimental data of the shock tube, RCM, and JSR. The reaction pathway analysis shows that at low-, intermediate-, and high-temperature regions the trans-3-hexene system is dominated by OH addition, OH H-abstraction, and initiation reactions, respectively. The short alkyl chain inhibits the formation of cyclic compounds, and no cool flame or NTC phenomena occur.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b02682. Measured ignition delay times for trans-3-hexene in shock tube and JSR and RCM simulations of the NUIG and LLNL models (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

Figure 11. Ignition delay time sensitivity of trans-3-hexene at low (a, 650 K), intermediate (b, 850 K), and high (c, 1350 K) temperature.

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (91541107, 51206131, and 91441203) and the National Basic Research Program (2013CB228406). The support of the State Key Laboratory of Engines, Tianjin University (K2015-01) is also acknowledged.

As mentioned above, at 650 K the major consumption channel of trans-3-hexene is OH addition (R4459) and its subsequent low-temperature O2 addition pathway. The produced PC4H9 from reaction R4087 C6H12-3 + OH ⇒ PC4H9 + CH3CHO undergoes the well-known low-temperature degeneration pathway producing OH radical and promoting ignition. As a result, R4087 offers the strongest promoting effect. Reaction R4142 C6H12-3 + OH ⇔ C6H113-1 + H2O and R341 C2H5CHO + OH ⇔ C2H5CO + H2O

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