Modeling of the Oxidation of Primary Reference Fuel in the

M. Yahyaoui , N. Djebaili-Chaumeix , P. Dagaut and C.-E. Paillard .... Teppei Ogura , John P. Angelos , William H. Green , Wai K. Cheng , Thomas E. Ke...
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Energy & Fuels 2007, 21, 3233–3239

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Modeling of the Oxidation of Primary Reference Fuel in the Presence of Oxygenated Octane Improvers: Ethyl Tert-Butyl Ether and Ethanol Teppei Ogura,† Yasuyuki Sakai,† Akira Miyoshi,† Mitsuo Koshi,*,† and Philippe Dagaut‡ Department of Chemical System Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and C.N.R.S., Institut de Combustion, Aérothermique, RéactiVité et EnVironnment, 1C, AVenue de la Recherche Scientifique 45071 Orléans Cedex 2, France ReceiVed June 3, 2007. ReVised Manuscript ReceiVed July 31, 2007

A detailed chemical kinetic mechanism has been developed for the oxidation of primary reference fuel (PRF, mixture of n-heptane and iso-octane) in the presence of ethyl tert-butyl ether (ETBE) or ethanol. The mechanism was validated by comparison with the existing experimental data from shock tubes, a jet-stirred reactor, and a flow reactor. ETBE and ethanol are known as octane number improvers. Enhancement of research octane number (RON) by the addition of ETBE and ethanol to PRF has been measured using a cooperative fuel research (CFR) engine. Increase in RON was simulated with the present detailed kinetic mechanism by estimating the critical compression ratio (CCR) for autoignition in a motored engine. The correlation curve between CCR and RON was derived by calculating the CCR for PRF whose composition defines the RON. The kinetic model reproduces observed variations in RON by the addition of ETBE and ethanol to PRF. Those additives showed a very similar effect on RON.

1. Introduction In recent years, the use of oxygenated fuels such as ethanol or branched ethers for the reduction of carbon dioxide emission from vehicles has attracted considerable attention. The advantage of ethanol is that it can be produced from renewable fuels like biomass. It is considered as a “carbon neutral” fuel. Branched ethers such as ethyl tertiary-butyl ether (ETBE) are also considered as “partly carbon neutral”, since they are synthesized from ethanol and unsaturated hydrocarbons, such as iso-butene. Those oxygenated fuels as components of gasoline for sparkignited engines have another advantage as an octane number enhancer and a fuel extender. Attempts have been made to replace alkyl lead compounds, which were used as antiknock additives. This has led to an increased use of ethanol and ETBE as components of gasoline. The modeling of the combustion of these species would provide an improved understanding of the autoignition process in engines and would allow us to predict pollutant emission. The gas-phase oxidation of pure ethanol and ETBE has been investigated widely during the early 1990s using several techniques, such as static reactors,1–4 flow reactors,5–8 jet-stirred * To whom correspondence should be addressed. E-mail: koshi@ chemsys.t.u-tokyo.ac.jp. Phone: +81-3-5841-7295. Fax: +81-3-5841-7488. † The University of Tokyo. ‡ C.N.R.S. (1) Brocard, J. C.; Hughes, H. W. D. Trans. Faraday Soc. 1960, 56, 55. (2) Borisov, A. A.; Zamanskii, V. M.; Konnov, A. A.; Lisyanskii, V. V.; Rusakov, S. A.; Skachov, G. I. SoV. J. Chem. Phys. 1991, 8, 121–141. Borisov, A. A.; Zamanskii, V. M.; Konnov, A. A.; Lisyanskii, V. V.; Rusakov, S. A.; Skachov, G. I. SoV. J. Chem. Phys. 1992, 9, 2527–2537. (3) Böhm, H.; El Kadi, B.; Baronnet, F. Oxid. Commun. 1996, 19, 25–32. (4) Böhm, H.; Baronnet, F.; El Kadi, B. Phys. Chem. Chem. Phys. 2000, 2, 929–1933. (5) Egolfopoulos, F. N.; Du, D. X.; Law, C. K. Proc. Combust. Inst. 1992, 24, 833–841. (6) Norton, T. S.; Dryer, F. L. Int. J. Chem. Kinet. 1992, 24, 319–344.

reactors,9–12 shock tubes,5,9,13–16 and rapid compression machines.17 Measurements of ignition delay times for a series of oxygenated hydrocarbons have been performed by Dunphy et al.14–16,18 using a shock tube. Most of these experimental works were on ethanol oxidation, and these studies were reviewed by Norton and Dryer6 and Marinov.19 Combustion of ETBE was studied by Böhm et al.3,4 and Dagaut et al.10,11 In addition to these experimental studies, modeling studies for ethanol oxidation have also been performed widely.2,5,6,9,13,16,19,20 Among these, Marinov published a comprehensive kinetic mechanism of ethanol combustion.19 Less modeling efforts concern the oxidation of ETBE.3,4,11,12 Glaude et al.12 constructed a kinetic model for methyl tert-butyl ether (MTBE) and ETBE on the basis of the program EXGAS for automatic generation (7) Norton, T. S.; Dryer, F. L. Proc. Combust. Inst. 1990, 23, 179–185. (8) Taylor, P. H.; Shanbhag, S.; Dellinger, B. SAE Paper 1994, 941904. (9) Dagaut, P.; Cathonnet, M.; Boettner, J. C. J. Chem. Phys. 1992, 89, 867–884. (10) Dagaut, P.; Koch, R.; Cathonnet, M. Combust. Sci. Technol. 1997, 122, 345–361. (11) Goldaniga, A.; Faravelli, T.; Ranzi, E.; Dagaut, P.; Cathonnet, M. Proc. Combust. Inst. 1998, 27, 353–360. (12) Glaude, P. A.; Battin-Leclerc, F.; Judenherc, B.; Warth, V.; Fournet, R.; Côme, G. M.; Scacchi, G.; Dagaut, P.; Cathonnet, M. Combust. Flame 2000, 121, 345–355. (13) Natarajan, K.; Bhaskaran, K. A. Int. Shock Tube Symp. 1981, 13, 834. (14) Dunphy, M. P.; Simmie, J. M. J. Chem. Soc., Faraday Trans. 1991, 87, 1691–1696, 2549–2559. (15) Dunphy, M. P.; Simmie, J. M. Int. J. Chem. Kinet. 1991, 25, 553– 558. (16) Curran, H. J.; Dunphy, M. P.; Simmie, J. M.; Westbrook, C. K.; Pitz, W. J. Proc. Combust. Inst. 1992, 24, 769–776. (17) Lee, D.; Hochgreb, S.; Keck, J. C. SAE Paper 1993, 932755. (18) Dunphy, M. P.; Simmie, J. M. Combust. Flame 1991, 85, 489–98. (19) Marinov, N. M. Int. J. Chem. Kinet. 1999, 31, 183–220. (20) Lawrence Livermore National Laboratory (LLNL) Homepage. http://www-cmls.llnl.gov/?url)science_and_technology-chemistry-combustion-ethanol/ (accessed June 3, 2007).

10.1021/ef700321e CCC: $37.00  2007 American Chemical Society Published on Web 09/29/2007

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of detailed kinetic models for gas-phase reactions of normal and iso-alkanes. They compared their simulations with the results of experiments obtained by using a jet-stirred reactor (JSR) for pure ETBE and mixtures of ETBE and n-heptane.10 Although ethanol and ETBE are considered as “additives” to gasoline, most of the previous studies are for the oxidation of pure ethanol or ETBE. There exist only two experimental studies10,21 and one detailed kinetic modeling study12 for mixtures of primary reference fuel (PRF) with ETBE or EtOH. It is necessary to understand the characteristics of ignition and combustion for the mixtures of these oxygenates and base fuels. In the present study, a chemical kinetic model has been developed for the mixtures of ethanol or ETBE and base fuels. Primary reference fuels (PRFs, mixtures of n-heptane and isooctane) are considered as a representative base fuel. The kinetic mechanism developed in the present study was validated with various experimental data in the literature. Those are ignition delay times obtained by shock tube experiments and concentration profiles in a JSR and a flow tube reactor. Sakai et al.22 measured the research octane number (RON) of PRF in the presence of ETBE or ethanol by using a cooperative fuel research (CFR) engine. They found linear increases in RON with the amount of additives up to 15 volume % (in liquid). An attempt has been made in the present study to estimate RON by calculating the critical compression ratio (CCR) for the autoignition in a homogeneous engine reactor. 2. Kinetic Modeling A chemical kinetic reaction mechanism for the oxidation of mixtures of n-heptane and iso-octane was generated by the KUCRS program developed by Miyoshi.23,24 KUCRS (knowledge-basing utilities for complex reaction system) is a utility software library for the development of gas-phase chemical kinetic models of hydrocarbon oxidation or combustion systems. The current version of KUCRS can generate the detailed chemical kinetic mechanism for a single component or mixtures of any noncyclic alkanes and single-ring (five- to sevenmembered) cyclic alkanes as fuels. In KUCRS, all the isomers of fuels generated by the hydrogen abstraction reactions are distinguished. The mechanism generated by KUCRS generally shows good agreement with the experimental ignition delay times in the negative temperature coefficient (NTC) region. Several modifications were made in the present study to improve the performance at high temperatures. It is also found that further modifications are necessary for the iso-octane mechanism to get better agreement with the experimental ignition delay times. Those modifications and validation of the mechanism are described in the next section. A novel chemical kinetic mechanism for ETBE oxidation is developed in the present study. At first, we generated a mechanism for 2,2-dimethyl-pentane by using KUCRS. ETBE is derived from 2,2-dimethyl-pentane by replacing the CH2 group with an O atom. Elementary chemical reactions necessary for ETBE oxidation can be generated from reactions of 2,2dimethyl-pentane and of its derivatives by the substitution of the CH2 group for an O atom in those species generated by KUCRS. Next, the rate constants of those reactions for ETBE (21) McEnally, C. S.; Pfefferle, L. D. Int. J. Chem. Kinet. 2004, 36, 345–358. (22) Sakai, Y.; Ogura, T.; Koshi, M.; Arai, M.; Kaneko, T. Jpn. Soc. Autom. Eng. 2006, 37, 103–108 (in Japanese). (23) Miyoshi, A. Jpn. Soc. Autom. Eng. 2005, 36, 35–40 (in Japanese). (24) Miyoshi, A. KUCRS-What is KUCRS? http://www.frad.t. u-tokyo.ac.jp/∼miyoshi/KUCRS/ (accessed June 3, 2007).

Ogura et al.

oxidation are evaluated. The rate constants of the reactions which are considered to be affected by an O atom in an ether bond have to be modified. H atom abstraction from ETBE by free radicals and intermolecular isomerization of alkylperoxy radical the important reactions in the low-temperature region, and the rate constants of those reactions are different from the rate constants for corresponding alkanes evaluated by KUCRS. The rate constants of these reactions for ETBE were calculated by using density functional theory, transitional state theory, Rice–Ramsperger–Kassel–Marcus (RRKM) theory, and master equation analysis. Details of these calculations were presented elsewhere.25 These reactions and their rate constants were summarized in Table 1.The rate constants for unimolecular decomposition, molecular four-center elimination, and β-scissions involving the breaking of a C–O bond were taken from the literature.12,26,27 The present kinetic model for ethanol oxidation is based on the mechanism proposed by Marinov.19,20 The final mechanism for the combustion of fuel mixtures of PRF/ETBE/ethanol, that includes further modifications described in the next section, consists of 634 chemical species and 2390 reversible reactions. The mechanism with thermochemical data is available from the author on request. 3. Mechanism Validation The mechanism developed in the present study was validated by modeling a large set of experiments. All of the present simulations were performed by using the CHEMKIN 4.1 program package.28 3.1. Primary Reference Fuels and Their Mixtures. The present detailed chemical kinetic mechanism has been used to simulate the autoignition of primary reference fuel mixtures assuming constant-volume, homogeneous, adiabatic conditions behind the reflected shock wave at low temperatures, as studied by Fieweger et al.29 They studied the ignition of PRF/air mixtures at an equivalence ratio of 1.0 in the temperature range 700–1200 K and reflected shock pressures of 40 atm. The original mechanism for PRF mixtures generated by KUCRS well reproduced the NTC behavior of the n-heptane ignition delay. However, the KUCRS mechanism gave slightly (about 1.5–2 times) faster ignition delay times of iso-octane in the NTC region. Since it has been found that the precise evaluation of iso-octane ignition delay in the NTC region is very crucial to evaluate RON, improvement of the iso-octane chemical kinetic model is required. A comprehensive chemical kinetic analysis for iso-octane combustion at low temperatures has been performed by Curran et al.30 At temperatures below 900 K, the predominant reactions of alkyl radicals, R, are their addition to molecular oxygen, the reaction of R + O2 ) RO2, followed by internal hydrogen (25) Ogura, T.; Miyoshi, A.; Koshi, M. Phys. Chem. Chem. Phys., submitted for publication. (26) Fischer, S. L.; Dryer, F. L.; Curran, H. J. Int. J. Chem. Kinet. 2000, 32, 713–740. (27) Daly, N. J.; Wentrup, C. Aust. J. Chem. 1968, 21, 1535. (28) Kee, R. J.; Rupley, F. M.; Miller, J. A.; Coltrin, M. E.; Grcar, J. F.; Meeks, E.; Moffat, H. K.; Lutz, A. E.; Dixon-Lewis, G.; Smooke, M. D.; Warnatz, J.; Evans, G. H.; Larson, R. S.; Mitchell, R. E.; Petzold, L. R.; Reynolds, W. C.; Caracotsios, M.; Stewart, W. E.; Glarborg, P.; Wang, C.; McLellan, C. L.; Adigun, O.; Houf, W. G.; Chou, C. P.; Miller, S. F.; Ho, P.; Young, P. D.; Young, D. J.; Hodgson, D. W.; Petrova, M. V.; PuduppakkamK. V. CHEMKIN, release 4.1; Reaction Design: San Diego, CA, 2006. (29) Fieweger, K.; Blumenthal, B.; Adomeit, G. Combust. Flame 1997, 109, 599–619. (30) Curran, H. J.; Gaffuri, P.; Pitz, W. J.; Westbrook, C. K. Combust. Flame 2002, 129, 253.

Modeling of Oxidation of PRF/ETBE/EtOH Mixtures

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Table 1. Important Reactions and Rate Parameters for the ETBE Submodela reaction

A

n

Ea

C(CH3)3 + OC2H5 ) C(CH3)3OCH2CH3 OC(CH3)3 + C2H5 ) C(CH3)3OCH2CH3 C(CH3)3OCH2CH3 ) i-C4H8 + C2H5OH C(CH3)3OCH2CH3 ) i-C4H8 + C2H5OH C(CH3)3OCH2CH3 + H ) C(CH3)3OCHCH3 + H2 C(CH3)3OCH2CH3 + OH ) C(CH3)3OCHCH3 + H2O C(CH3)3OCH2CH3 + O ) C(CH3)3OCHCH3 + OH C(CH3)3OCH2CH3 + CH3 ) C(CH3)3OCHCH3 + CH4 C(CH3)3OCH2CH3 + HO2 ) C(CH3)3OCHCH3 + H2O2 C(CH3)3OCH2CH3 + O2 ) C(CH3)3OCHCH3 + HO2 H2CC(CH3)2OCH2CH3 ) i-C4H8 + C2H5O C(CH3)3OCHCH3 ) C(CH3)3 + CH3CHO C(CH3)3OCHCH3 + O2 ⇒ C(CH3)3OCH(OO)CH3 C(CH3)3OCH(OO)CH3 ⇒ C(CH3)3OCHCH3 + O2 C(CH3)3OCH2CH2 ) OC(CH3)3 + C2H4 C(CH3)2OCH2CH3 ) CH3COCH3 + C2H5 C(CH3)3OCH2 ) C(CH3)3 + CH2O OOCH2C(CH3)2OCH2CH3 ) HOOCH2C(CH3)2OCHCH3 C(CH3)3OCH(OO)CH3 ) H2CC(CH3)2OCH(OOH)CH3 C(CH3)3OCH(OO)CH3 ) C(CH3)3OCH(OOH)CH2 C(CH3)3OCH(OO)CH3 ⇒ C(CH3)3OCHCH2 + HO2 C(CH3)3OCH2CH2OO ) C(CH3)3OCHCH2OOH OOC(CH3)2OCH2CH3 ) HOOC(CH3)2OCHCH3 CH3CHOCH2CH3 ⇒ CH3CHO + C2H5 C(CH3)2OCOCH3 ⇒ CH3COCH3 + CH3CO C(CH3)3OCO ⇒ C(CH3)3 + CO2 HCO + OC(CH3)3 ) C(CH3)3OCHO C(CH3)2OCHO ⇒ CH3COCH3 + HCO

1.50 × 1013 1.50 × 1013 1.14 × 1012 3.30 × 102 5.74 × 105 3.60 × 105 4.77 × 104 2.71 × 104 5.60 × 1012 1.40 × 1013 2.00 × 1013 2.00 × 1013 3.79 × 109 8.91 × 1011 2.00 × 1013 2.00 × 1013 2.00 × 1013 3.12 × 109 3.42 × 108 3.63 × 107 1.63 × 106 2.00 × 1011 2.50 × 1010 2.00 × 1013 2.00 × 1013 2.00 × 1013 1.50 × 1013 2.00 × 1013

0.0 0.0 0.0 0.0 2.49 2.30 2.71 2.26 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.0 0.0 0.0 0.0 0.0 0.00 0.00 0.00 0.0 0.00

0 0 59990 10000 1500 -1800 -500 4700 15100 45600 26050 23090 -10220 20140 26050 23090 23090 17300 16950 13870 18920 25100 18000 23090 23090 23090 0 23090

b b c d,e f,g f,g f,g f,g f,g d d d f,h f,h d d d f,i f,h f,h f,h f,i f,i d d d b d

a The term k ) ATn exp(–E /RT). Units are mol, cm3, s, K, and cal/mol. b Reference 26. Half value was used according to the number of C–O bonds. a Reference 27. d Reference 12. e Catalytic molecular reaction. f Reference 25. g General expression of the rate constants of H atom abstraction from the R-C atom in ethers (see ref 25). h RRKM results for ETBE (see ref 25). i General expression of the rate constants of isomerization of alkoxyalkylperoxy radical (see ref 25). c

isomerizaion, a second addition to O2, and subsequent decompositions to form two reactive hydroxyl radicals and a carbonyl radical. This sequence is responsible for the low-temperature chain branching processes, and the first addition of O2 to R is the most important reaction. Reactions of RO2 have been well examined on the basis of the quantum chemical calculations of energy barrier height and RRKM-master equation analysis. Their rate constants used in KUCRS are based on these theoretical works. On the other hand, the rate constants for the reactions of second additions of O2 and their consecutive isomerization and decomposition reactions are not well known. Curran et al.30 claimed that, in order to reproduce the very low reactivity of iso-octane at low temperatures (600–770 K) observed experimentally, the rates of RO2 isomerization (RO2 S QOOH) and peroxy-alkylhydroperoxyl radical (O2QOOH) isomerization had to be reduced by a factor of 3 relative to the value used in their n-heptane model. The use of those rate constants for iso-octane will give better agreement with iso-octane ignition delay. However, the adjustment of these rate constants only for isooctane is unreasonable. Moreover, these rate constants could not be used in the present work, since automatic generation of the kinetic mechanism requires systematic definition of the rate parameters. Instead, we slightly adjusted all of the activation energies for the reactions of intramolecular isomerization of RO2 and O2QOOH, and carbonyl-hydroperoxide decomposition, since the rate constants of these reactions have a large uncertainty. The resulting activation energies of RO2 and O2QOOH isomerization are changed by -1 and +1 kcal/mol from the original values used in KUCRS. The activation energies of carbonyl-hydroperoxide decomposition are changed by -2 kcal/mol. The rate constant of H atom abstraction from a tertiary carbon site by OH radical is also modified so that the experimental measurements for the total rate constant of H atom abstraction

from iso-butane by an OH radical31–34 can be reproduced. In addition, the reactions of alkene and alkenyl radicals are added or modified to the values proposed by Curran et al.30 in order to improve the high-temperature performance of the model. Inclusion of reactions related to i-C4H8 and C3H6 is necessary for the proper description of high-temperature ignition of isooctane. Similarly, reactions related to 1-C4H8 are important for n-heptane oxidation at high temperatures. Figure 1 shows comparisons between experimental ignition delay data of Fieweger et al.29 and model predictions for stoichiometric PRF/air mixtures at P ) 40 bar. In these model calculations, the ignition point is determined by the inflection point in temperature time profile. The NTC behavior is properly captured by the model for neat PRFs (i.e., n-heptane and isooctane) as well as for their blends. In the high-temperature (>1000 K) region, the model tends to overpredict the ignition delay times. Sakai et al.35 measured ignition delay times of stoichiometric PRF mixtures of 0.4% fuels diluted in Ar in the temperature range from 1300 to 1650 K and at a pressure of 2 atm. These ignition delay times are also compared with the simulation results using the present mechanism. In high-temperature ranges, ignition can not be defined on the basis of temperature profile because the temperature rise is not fast enough to define the ignition. Therefore, the ignition is defined by the peak position of the CH concentration profile. This choice corresponds to the experimental observation of UV emission in hydrocarbon (31) Tully, F. P.; Goldsmith, J. E. M.; Droege, A. T. J. Phys. Chem. 1986, 90, 5932. (32) Bott, J. F.; Cohen, N. Int. J. Chem. Kinet. 1989, 21, 485–98. (33) Talukdar, R. K.; Mellouki, A.; Gierczak, T.; Barone, S.; Chiang, S.-Y.; Ravishankara, A. R. Int. J. Chem. Kinet. 1994, 26, 973. (34) Donahue, N. M.; Anderson, J. G.; Demerjian, K. L. J. Phys. Chem. A 1998, 102, 3121. (35) Sakai, Y.; Ozawa, H.; Ogura, T.; Miyoshi, A.; Koshi, M. SAE, submitted for publication.

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Figure 1. Experimental (symbols) and simulated (curves) ignition delay times of the PRF/air mixtures behind reflected shock waves at 40 atm and φ ) 1. Experimental results were taken from Fieweger et al.29 Squares and solid curve, i-C8H18/O2/N2 ) 1.66/20.80/77.54 (RON ) 100); circles and dashed curve, n-C7H16/i-C8H18/O2/N2 ) 0.70/1.05/ 20.78/77.47 (RON ) 60); triangles and dotted curve, n-C7H16/O2/N2 ) 1.89/20.75/77.36 (RON ) 0).

Figure 3. Experimental (symbols) and simulated (curves) mole fractions of main species for ethanol oxidation in a flow reactor at 1 atm, 1100 K, and φ ) 0.61 (C2H5OH/O2/N2 ) 0.565/2.786/96.649). Experimental results were taken from Norton and Dryer.6 Solid and dashed curves correspond to closed and open symbols.

Figure 2. Experimental (symbols) and simulated (curves) ignition delay times of the PRF/O2/Ar mixtures behind reflected shock waves at 2 atm and φ ) 1. Experimental results were taken from Sakai et al.35 Squares and dotted curve, i-C8H18/O2/Ar ) 0.4/5.0/94.6 (RON ) 100); circles and solid curve, i-C8H18/n-C7H16/O2/Ar ) 0.2/0.2/4.7/94.9 (RON ) 50); triangles and dashed curve, n-C7H16/O2/Ar ) 0.4/5.0/94.6 (RON ) 0).

oxidation which is mainly caused by the CH + O2 ⇒ OH* + CO reaction.36 A comparison between experimental data and model predictions is shown in Figure 2. The agreement between the computations and iso-octane ignition delay times is quite satisfactory. On the other hand, the model predictions of n-heptane ignition delay times are shorter than the experimental values, while the model predictions at high temperatures (T > 1000 K) in Figure 1 are longer. The reason of this discrepancy is not known, and no further attempt to improve the agreement of n-heptane ignition delay at high temperatures has been made. 3.2. Ethanol. A submechanism for ethanol oxidation is taken from a detailed chemical kinetic model of Marinov.19,20 The original model has been validated against a variety of experimental data sets, including laminar flame speed data, ignition delay data behind reflected shock wave, and product profiles of ethanol oxidation from jet-stirred and turbulent flow reactors. Although the ethanol subset of the model is merged into the present kinetic mechanism without any changes, consecutive and side reactions may affect the concentration profiles in (36) Hall, J. M.; Petersen, E. L. Int. J. Chem. Kinet. 2006, 38, 714.

ethanol oxidation. Therefore, validation of the current mechanism for ethanol oxidation is required. The flow tube data of Norton and Dryer6 were used to validate the current chemical kinetic model by comparing the predicted stable species profiles to those measured during the ethanol oxidation. In the upper panel of Figure 3, a comparison of the experimental data at an equivalence ratio of φ ) 0.6 against the numerically computed species concentrations is shown for the major species of C2H5OH, O2, CO, CO2, H2, and H2O. Due to the uncertainty in the experimental induction time, the numerical results were shifted by 10 ms to get the overall agreement with the experimental data for major species, as was done in the original paper.19 The results show good agreement for those major species, as can be seen in the upper panel of Figure 3. In fact, the decay profile of C2H5OH agrees better than the original model. On the other hand, modeling agreement with the concentration profiles for minor species, C2H4, CH4, and CH3CHO, is worse than the original model. However, the present mechanism still captured the qualitative trends of the concentration profiles for those minor species, as shown in the lower panel of Figure 3. The numerical computations were also performed at φ ) 1.24 and compared with the experimental data. The agreement is similar to the case of φ ) 0.6. 3.3. ETBE. The gas-phase oxidation of ETBE has been experimentally studied using a JSR reactor between 750 and 1150 K at a pressure of 10 atm, equivalence ratios from 0.5 to 2 by Dagaut et al.11,12 Glaude et al.12 performed the validation of their detailed chemical kinetic model for ETBE oxidation generated by EXGAS. They suggested that the occurrence of a catalytic decomposition should be taken into account in order to correctly explain the experimental results at temperatures below 850 K. In the present validation study for our ETBE model, we also include the catalytic molecular reaction proposed

Modeling of Oxidation of PRF/ETBE/EtOH Mixtures

Energy & Fuels, Vol. 21, No. 6, 2007 3237

Figure 4. Experimental (symbols) and simulated (curves) outlet mole fractions of ETBE in a jet-stirred reactor at 10 atm and τ ) 0.5 s. Experimental results were taken from Glaude et al.12 Squares and solid curve, φ ) 2.0 (ETBE/O2/N2 ) 0.1/0.45/99.45); circles and dashed curve, φ ) 1.0 (ETBE/O2/N2 ) 0.1/0.9/99.0); triangles and dotted curve, φ ) 0.5 (ETBE/O2/N2 ) 0.1/0.018/98.1).

by Glaude et al. Figure 4 shows the temperature dependence of the outlet mole fraction of ETBE at φ ) 0.5, 1.0, and 2.0. The mole fractions of those mixtures are ETBE/O2/N2 ) 0.1/0.018/ 98.1 for φ ) 0.5, ETBE/O2/N2 ) 0.1/0.9/99.0 for φ ) 1, and ETBE/O2/N2 ) 0.1/0.45/99.45 for φ ) 2. The agreement between the computational results and experiments is quite satisfactory. It is worth noting that no fitting or adjustment was performed on the rate constants for the construction of the present ETBE submechanism. A comparison of major combustion products at the outlet of the JSR is presented in Figure 5 for the case of φ ) 1. The temperature dependences of O2, CO, CO2, C2H5OH, and iso-C4H8 are well reproduced by the simulation. On the other hand, H2 concentration is overestimated by about 2 times, and C2H4 concentration is underestimated. The overestimation of H2 suggests that the H atom concentrations in the model are too high, since most of the H2 is generated by the abstraction reaction, RH + H ) R + H2. The concentration of CH4 is also overestimated at temperatures above 950 K. Although the prediction of present ETBE oxidation is not perfect, the overall feature of the ETBE combustion is captured reasonably well. 3.4. ETBE/n-Heptane Mixture. Dagaut et al.10 have studied the oxidation of ETBE with n-heptane in a JSR in the temperature range 600–1050 K, at a pressure of 10 atm, and at φ ) 1. Glaude et al.12 compared their simulation with these experimental results. They found the retarding effect of nheptane oxidation due to the addition of ETBE. Above 800 K, their model could well reproduce the conversion of n-heptane and ETBE and production of iso-C4H8 at the outlet of the JSR. However, the model underestimated the conversions of ETBE and n-heptane at temperatures below 780 K. In fact, the model predicted almost no reaction below 780 K within the space time of 0.5 s, whereas a significant amount of conversion of reactants was observed experimentally. We compared the prediction of the present model with the JSR results of Dagaut et al.10 Figure 6 shows the experimental and computational conversion of n-heptane, ETBE, and i-C4H8. The agreement of the conversion is reasonably good and is greatly improved at temperatures below 800 K compared with the previous simulation by Glaude et al..12 Although there is no essential difference in the chemical kinetic mechanism between the present and their mechanism, values of rate constants are different for the isomerization reactions of RO2 produced by the internal H atom abstraction from a primary

Figure 5. Experimental (symbols) and simulated (curves) outlet mole fractions of main species for ETBE oxidation in a jet-stirred reactor at 10 atm, φ ) 1.0 (ETBE/O2/N2 ) 0.1/0.9/99.0) and τ ) 0.5 s. Experimental results were taken from Goldaniga et al.11 and Glaude et al.12 Solid and dashed curves correspond to closed and open symbols.

Figure 6. Experimental (symbols) and simulated (curves) fuel conversions and outlet mole fractions of i-butene for the oxidation of ETBE/ n-heptane mixtures in a jet-stirred reactor at 10 atm, φ ) 0.5 (ETBE/ n-C7H16/O2/N2 ) 0.05/0.05/1.0/98.9), and τ ) 0.5 s. Experimental results were taken from Dagaut et al.10 Squares and solid curve, ETBE; circles and dashed curve, n-heptane; open triangles and dotted curve, i-butene.

site via a transition state (TS) with a six-membered ring structure. There is an order of magnitude difference in the values of the rate constant for the following reaction: O–O–CH2– C(–CH3)2–O–CH2CH3 S HO–O–CH2–C(–CH3)(–CH2)–O– CH2CH3. This reaction causes the generation of two OH radicals, and ignition delay times of ETBE at temperatures of 600–700 K are very largely affected by this reaction. The estimation of the value of this rate constant in the present work (which is based on KUCRS) is based on the quantum mechanical and RRKM

3238 Energy & Fuels, Vol. 21, No. 6, 2007

Ogura et al.

Table 2. Mole Fractions of Fuel Mixtures Used for CCR Calculations and Their RON Measured in the CFR Engine22 test gasesa PRF0 PRF10 PRF20 PRF30 PRF40 PRF50 PRF60 PRF70 PRF80 PRF90 PRF100 PRF80ETBE5% PRF80ETBE10% PRF80ETBE15% PRF80EtOH5% PRF80EtOH10% PRF80EtOH15% PRF90ETBE5% PRF90ETBE10% PRF90ETBE15% PRF90EtOH5% PRF90EtOH10% PRF90EtOH15% PRF100ETBE5% PRF100ETBE10% PRF100ETBE15% PRF100EtOH5% PRF100EtOH10% PRF100EtOH15%

n-heptane i-octane ethanol ETBE 100.0 91.1 82.0 72.6 63.0 53.2 43.1 32.8 22.1 11.2 0.0 20.8 19.6 18.3 19.3 16.9 14.9 10.6 9.9 9.3 9.8 8.6 7.5 0.0 0.0 0.0 0.0 0.0 0.0

0.0 8.9 18.0 27.4 37.0 46.8 56.9 67.2 77.9 88.8 100.0 73.4 68.9 64.6 68.0 59.6 52.4 83.6 78.5 73.5 77.4 67.8 59.5 94.1 88.2 82.5 87.0 76.1 66.7

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 12.7 23.4 32.7 0.0 0.0 0.0 12.8 23.7 33.0 0.0 0.0 0.0 13.0 23.9 33.3

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 5.8 11.5 17.1 0.0 0.0 0.0 5.9 11.6 17.3 0.0 0.0 0.0 5.9 11.8 17.5 0.0 0.0 0.0

RON 0 10 20 30 40 50 60 70 80 90 100 83.1 86.3 89.1 84.9 89.7 93.7 92.5 94.9 97.1 94.0 97.8 100.8 102.1 104.2 106.0 103.9 N/A N/A

b b b b b b b b b b b

a The naming convention indicates the volume % (in the liquid phase) of the mixtures. For example, “PRF80ETBE5%” means the test gas which is prepared by the addition of 5 volume % (in liquid) ETBE to PRF80. b RON of PRF by definition.

calculations for the similar reactions, and those values may be more reliable than the values used in previous models. 4. RON Enhancement by the Addition of ETBE/Ethanol to PRF ETBE and ethanol are well known as octane number enhancers. Sakai et al.22 measured the RON of the PFR mixtures with ETBE or ethanol by using a CFR engine operated under conditions defined by JIS-K2280 (which is the same as the ASTM definition37). They observed a linear increase in RON with the addition of ETBE or ethanol up to 15 volume % to the PRF mixtures having RONs of 100, 90, and 80. Gas-phase mole fractions of each component used to measure the RON are summarized in Table 2 with their experimental results. Such enhancement of RON should be predicted by the detailed chemical kinetic model for the fuel mixtures of PRF, ETBE, and ethanol developed in the present work. Experimental measurements of the critical compression ratio (CCR) have been correlated to RON in spark ignition engines. Curran et al.38 indicated that the RON measured by a CFR engine could be correlated to the CCR for the autoignition in a motored engine. They also calculated the CCRs of the PRF mixtures with their detailed kinetic model as a function of RON, and compared them with the experimental results. Similar simulations have been performed by Callahan et al.39 to confirm the relation between CCR and RON. We employed the same (37) Standard test method for research octane number of spark-ignition engine fuel, ASTM designation D2699-07, 2007. (38) Curran, H. J.; Gaffuri, P.; Pitz, W. J.; Westbrook, C. K.; Leppard, W. R. Proc. Combust. Inst. 1996, 26, 2669. (39) Callahan, C. V.; Held, T. J.; Dryer, F. L.; Minett, R.; Ribaucour, M.; Schet, L. R.; Faravelli, T.; Gaffuri, P.; Ranzi, E. Proc. Combust. Inst. 1996, 26, 739.

Figure 7. Experimental (symbols) and simulated (curve) RON for PRF mixtures as a function of CCR. Experimental results were taken from Curran et al.38 The dashed curve between RON ) 100 and 110 is the extrapolation from the simulation results below RON ) 100.

strategy as Curran et al. to evaluate the RON of PRF mixtures with the addition of ETBE and ethanol. In the present evaluation of CCR, the engine is treated as a homogeneous reactor, ignoring spatial variations in temperature and species concentrations. Calculations were performed by using the internal combustion engine module of Chemkin 4.1. In this program module, the time history of the combustion chamber volume is determined by a slider-crank formula described by Heywood.40 Engine parameters in this formula are set to the same as the CFR engine used for the measurement of RON: a bore radius of 8.26 cm and a ratio of the connecting rod length to the crank arm of 4.5. Operating conditions are also set equal to those used by Curran et al.:38 stoichiometric fuel/air mixtures, engine speed of 600 RPM, intake temperature of 403 K, engine wall temperature of 430 K, and intake manifold pressure of 0.8 atm. It was found that heat losses at the engine chamber walls had large effects on the value of the CCR. Heat loss was included in the calculation by employing the Woschni formula40 which was implemented in the IC engine module. The heat transfer coefficient is obtained from the Nusselt number, Nu, given by the following equation: Nu ) aRebPrc Here, a, b, and c are parameters, Re is the Reynolds number, and Pr is the Prandtl number. We assumed values of b ) 0.8 and c ) 0. The value of a ) 0.07 was determined so that the CCR at RON ) 100 agreed with the experimental value. Other parameters included in the Woschni formula were taken from Heywood.40 Curran et al.38 indicated that the residual gas composition on successive engine cycles had crucial effects on the evaluation of CCR. The residual gas for the nonfired cycle has a complex composition because of partial decomposition of fuels. This partially reacted residual gas can affect the ignition at the next cycle. However, preliminary calculations with the addition of 10% residual fuel indicated that the effect was insignificant in the present case. Therefore, this effect was not included in the present calculation. Calculated values of CCR for the PFR mixtures in the range from RON ) 0 to 100 are depicted in Figure 7 with the experimental values of Curran et al.38 Overall agreement (40) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw-Hill: 1988.

Modeling of Oxidation of PRF/ETBE/EtOH Mixtures

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Values of RON in the range from 100 to 110 are required for blended fuels, and those values are obtained by the extrapolation using a polynomial function fitted to the calculated curve, as shown by a dotted line in Figure 7. CCR values have been calculated for the mixtures of PRF and ETBE/ethanol with the compositions given in Table 2. The resulting enhancements of RON by the addition of ETBE or ethanol to the PRF mixtures are plotted in Figure 8. RON enhancement by ETBE addition to PRF90 mixtures is overestimated by the simulations, but agreement for other cases is satisfactory. Additions of ETBE and ethanol to PRF mixtures have a similar effect on RON. 5. Concluding Remarks

Figure 8. Experimental (symbols) and simulated (curves) RONs for PRF/oxygenate/air mixtures in a CFR engine: (a) PRF/ETBE/air mixtures; (b) PRF/ethanol/air mixtures. The compositions of the fuels are given in Table 2.

between calculated and experimental values is satisfactory, but the slope of the RON–CCR curve is somewhat different. The slope is found to be sensitive to the chemical kinetics and could not fit to the experimental values by adjusting the heat loss parameters. The difference may be caused by other fluid dynamics factors such as effects of piston motion, fuel trapping in crevice volumes, and nonuniformity of the fuel concentration. No further adjustment of the RON–CCR curve seems to be possible in the present zero-dimensional engine model. The calculated CCR–RON curve in Figure 7 is used to evaluate the RON of PRF mixtures in the presence of ETBE or ethanol.

A detailed chemical kinetic model was developed in the present study for PRF mixtures with the addition of ETBE and ethanol. The model was validated against experimental data. The basis of the present model is KUCRS, which is the utility program for automatic generation of the chemical kinetic mechanism. A submodel for ETBE oxidation was newly constructed by using generic rules for ether oxidation. The present model could capture the main features of oxidation of mixtures of PRF/ETBE and PRF/ethanol, though further improvement is still required for the proper descriptions of the behavior of minor species. Enhancement of RON by the addition of ETBE and ethanol to PRF was also predicted with the present detailed chemical kinetic model for blended fuels. The method of RON calculation is based on the estimation of CCR. The relationship between CCR and RON was calculated with the present model assuming homogeneous compression in a motored engine. Although enhancement of RON by the addition of ETBE or ethanol to PRF is well predicted by the simulations, there is a small discrepancy between the experimental and calculated calibration curve for the CCR–RON relation. This discrepancy may be responsible for the nonuniformity in the real engine or the insufficient accuracy of the chemical kinetic model prediction. Solution of this discrepancy is open for future studies. Acknowledgment. This research was partially supported by the Advanced Technology and Research Institute, Petroleum Energy Center, Japan. EF700321E