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A chemical kinetic model of a multi-component gasoline surrogate with cross reactions Bo Li, and Yankun Jiang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01330 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018
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A chemical kinetic model of a multi-component gasoline surrogate with cross reactions Bo Li, Yankun Jiang * School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China * Corresponding author. Tel: +86 13886100246 E-mall address:
[email protected] Abstract A kinetic mechanism for multi-component gasoline surrogate consisting of isooctane, n-heptane, toluene and 1-hexene is developed including types of cross reactions. The mechanism comprises 1392 species and 6019 reactions. Validation results show good agreement with experimental observations for pure components and their various mixtures. Moreover, the multi-component surrogate and a TRF surrogate are validated for a research gasoline fuel using the present model. The multi-component surrogate perfomed better in predicting experimental data comparing with TRF surrogate because of the addition of 1-hexene. Effects of cross reactions on the ignition delays are discussed. It is found that three types of cross reactions, namely reactions between pure components, reactions between pure components and alkylperoxy radicals, and reactions between pure components and benzylperoxy radicals take effect for the acceleration of ignition delays in shock tube and HCCI engine. Accelerate effect is more pronounced at high pressures, low temperatures and fuel rich conditions. Model with cross reactions between pure components and alkylperoxy radicals ignites earliest, and model with reactions between pure components ignites latest. Cross reactions between decomposition intermediate species (ethylene, propene, iso-butene, phenyl and benzaldehyde) have no influence on ignition delay times.
Keywords Kinetic mechanism, Gasoline Surrogate, Ignition delay, toluene, 1-hexene, Cross Reactions
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1. Introduction Gasoline is an important industrial fuel that contains hundreds of hydrocarbons. Its composition varies with the fuel source and the refining process. Researchers usually used surrogates of limited components to represent gasoline when they investigated the oxidation behavior of a real gasoline. In the earlier studies, iso-octane was used as the simplest surrogate to represent gasoline 1-3. Later, PRF surrogates were accepted as gasoline surrogates4, 5. Recently, several studies showed that the ignition behavior of gasoline can be reproduced by TRF surrogates 6-8. However, Kukkadapu et al.9, 10 compared the ignition delay times of a research grade gasoline with two different surrogates formulations in a rapid compression machine (RCM). In their experiments, multi-component surrogate performed better than TRF in predicting RCM data under their operating conditions. Later, Mehl et al.11 found when 2-pentene was blended in TRF, more properties of a real gasoline such as H/C ratio and octane numbers could be well represented. In recent investigations, multi-component (more than three components) gasoline surrogates were proposed for different applications 6, 11-15. Naik et al.13 proposed a multi-component surrogate including TRF, 1-pentene and methylcyclohexane, and studied the oxidation of the surrogate in a HCCI engine. Vanhove et al.14 investigated the ignitions of a ternary mixtures of iso-octane, 1-hexene and toluene in a RCM. Andrae et al.16 17and Wang et al.15 used quaternary mixtures of TRF and diisobutylene (DIB) to describe the ignition behavior of gasoline in shock tubes. Based on the previous studies, the addition of olefins to TRF surrogate is encouraged. In the current study, 1-hexene is selected to represent the alkene class for the following reasons. Firstly, it displays more negative temperature coefficient (NTC) behavior than other candidates in literature such as DIB, 1-pentene and 2pentene, and NTC is one significant behavior of gasoline. Secondly, 1-hexene is known to produce aromatics and soot in non-premixed flames18, which is another characteristic of gasoline. Thirdly, 1-hexene was once used instead of n-heptane because it provided an insight into the low temperature chemistry of gasoline14. Numerous modeling studies on toluene have been conducted. The mechanism of Shen et al.19 was proposed to simulate the ignition data over the temperature between 1000 and 1400 K and pressure between 10 and 60 bar. Sivaramakrishnan et al.20 conducted a toluene modeling study at high-pressure conditions. Andrae et al.21 proposed a mechanism to predict shock tube data, and modified his model in later work 16. The high temperature mechanism proposed by Tian et al.22 was validated against the toluene premixed laminar flame data. Metcalfe et al.23 presented a detailed mechanism for toluene oxidation and validated it with ignition delay times, laminar flame speeds, and jet stirred reactor (JSR) data. Recently, Yuan et al.24, 25 developed a kinetic model to reproduce the pyrolysis and oxidation of toluene over a wide range conditions Yahyaoui et al.26, 27 constructed a detailed for 1-hexene oxidation. It was validated with shock tube data between 1270 and 1700 k with different equivalence ratios27. Later, they used the model to simulate the JSR data at 750-1200 K26. Touchard et al.28 developed a 1-hexene kinetic model to predict RCM data at low
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temperature regime. Mehl et al.29 constructed a substituted kinetic model for hexane isomers. They validated it through predictions of JSR, RCM and shock tube data. Recently, Kiefer et al.30 studied the decomposition channels of 1-hexene based on the quantum chemical calculation. All these datasets are helpful for our modeling study. The chemical interactions between the fuel components can be important in the combustion of multi-component mixtures. Andrae et al.31 thought that cross reactions may be important in the auto-ignition process for TRF surrogate. Sakai et al.7 demonstrated that chemical kinetic interactions between alkanes and aromatics improve the predictions of mixtures. The reactions between intermediates of aromatic radicals (benzyl radical, phenyl radicals and benzaldehyde radicals) and small alkene radicals have been considered in several modeling studies13, 32, 33. However, experimental and theoretical investigations on interaction reactions were very limited. In this study, we presented a mechanism for ignition of four-component gasoline surrogate. The new model not only improved the predictive capability, but also provided information for further study of interaction reactions. 2. Construction of the mechanism The mechanism was constructed using a semi-decoupling methodology 34. The sub mechanism of PRF extracted from Mehl et al.35 was employed as a starting model. The detailed low-temperature chemistry of toluene and 1-hexene taken from literature with some modifications were added, and cross reactions between alkanes, alkenes, aromatics and intermediate were further included to the new mechanism. 2.1 Toluene sub-mechanism We have compared predicted results using three detailed models23, 24, 35 with existed experimental measurements at different operating conditions to decide a best basic mechanism. Fig 1 shows the ignition delay times measured by Shen et al.19 and simulation results in a shock tube at 50 atm with equivalence ratios of 0.25, 0.5 and 1.0.
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Fig. 1 Comparison of experimental and calculated toluene ignition delay times at 50 atm with different equivalence ratios using three detailed models. Solid lines: LLNL model35, dash lines: Metcalfe’s model23, dash dot lines: Yuan’s model24.
It can be seen in Fig 1, both LLNL model and Metcalfe’s model over-predicted the ignition delays of toluene at all operating conditions, the largest discrepancies between measured and calculated ignition delays are even bigger than a factor of 2. The Yuan’s model can predict the ignition delays at Φ=0.5 and 0.25, but slight disagreements between the predictions and experimental data at Φ=1.0 were evident. Thus, we choose Yuan’s model as the basic mechanism. Some modifications were made to improve the prediction ability of the basic model, they are discussed below. The H-abstraction reaction of C6H5CH3 to produce C6H4CH3 (R1-R4) in the basic model was directly taken from Bounaceur et al.36. In the study of Metcalfe et al.23, they pointed out that H-abstraction may take place at any phenylic position and suggested to decrease the A factor by 1/6 to account for one less substitution site. Therefore, we updated the rate constants of R1-R4 on the recommendation of Metcalfe. C6H5CH3 + H ↔ C6H4CH3 + H2
R1
C6H5CH3 + O ↔ C6H4CH3 + OH
R2
C6H5CH3 + OH ↔ C6H4CH3 + H2O
R3
C6H5CH3 + HO2 ↔ C6H4CH3 + H2O2
R4
As discussed by Vanhove et al. 14, H atom at the benzylic site of toluene can be easily abstracted by the HO2 radicals to produce H2O2 radicals, H2O2 further
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decomposes to 2 OH radicals. It is an important pathway for OH accumulation and can accelerates the ignition of alkane/toluene mixture at NTC regime. Therefore, the rate constants of R5 was referred to Vahove’s work. C6H5CH3 + HO2 ↔ C6H5CH2 + H2O2
R5
The oxidation reaction of C6H5CH2 with O have been theoretical estimated by Sakai et al.7 and Wang et al.15. Two oxidation pathways were reported: one led to the formation of C6H5CHO+H (R6); and another produced C6H5+CH2O (R7). C6H5CH2 + O ↔ C6H5CHO+H
R6
C6H5CH2 + O ↔ C6H5+CH2O
R7
However, the calculated rate constants of two research groups were different. Thus, the geometries, vibrational frequencies and rotational constants were calculated at the B3LYP/6-311G(d) level of theory and energies were calculated at the CCSD(T)/6-31+G(d’) and MP4(SDQ)/6-31G+(d,p) levels of theory in this work. Our results are similar with those of Wang et al.15. Previous works, e.g., ref36, 37 have noted that reaction of C6H5CH2+HO2 (R8) shows importance for the oxidation of toluene. The rate constants of R8 taken from the model of da Silva et al.38 were used in the basic mechanism. However, Metcalfe et al.23 increased the rate constants by a factor of 2 and found the new model agreed better with shock tube measurement. The recommended rate constants of R8 by Metcalfe et al.23 was used in this model. C6H5CH2 + HO2 ↔ C6H5CH2O + OH
R8
Sakai et al.7 indicated that the reaction of C6H5CH3 with C6H5O radical (R9) had a high sensitivity to the ignition delay times at low temperatures. It is identified as the dominated pathway of toluene oxidation at low temperatures. C6H5CH3+C6H5O ↔ C6H5CH2 + C6H5OH
R9
The specific rate constants of R9 were calculated using the Canonical Transition State Theory (CTST) in Sakai’s work and was adopted in present work. The rate coeffecients of toluene unimolecular decomposition reactions R10 and R11 were updated according to the calculation results at B3LYP/6-31G(d) level by Wang et al.15, C6H5CH3 ↔ C6H5CH2 + H
R10
C6H5CH3 ↔ C6H5 + CH3
R11
The results were expressed in an Arrhenius expression as follows and adopted in the present model. KR10= 1.98×1015exp(-88037 [cal/mol]/RT) KR11= 2.50×1016exp(-97241 [cal/mol]/RT)
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2.2 1-hexene sub-mechanism We chose Yahyaoui's mechanism26 as the starting point for 1-hexene model development. The modifications to the 1-hexene sub-model were summarized as follows.
1-Hexene Dissociation: Yahyaoui’s mechanism included all possible dissociation pathways of 1-hexene via C-C fission (R12-R16). C6H12-1 ↔ C3H5-A + NC3H7
R12
C6H12-1 ↔ C3H6 + C3H6
R13
C6H12-1 ↔ CH3 + C5H9-3
R14
C6H12-1 ↔ C2H5 + C4H7-3
R15
C6H12-1 ↔ PC4H9+C2H3
R16
Kiefer et al.30 indicated that R12 was the dominated channel for 1-hexene dissociation and calculated its rate constants to fit their experimental results at 1200-1700 K. To ensure its low temperature applications, Fan et al.39 estimated those of R12 at T
