Experimental and Modeling Study of the Kinetics of Oxidation of

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Energy & Fuels 2008, 22, 3499–3505

3499

Experimental and Modeling Study of the Kinetics of Oxidation of Ethanol-Gasoline Surrogate Mixtures (E85 Surrogate) in a Jet-Stirred Reactor Philippe Dagaut* and Casimir Togbe´ CNRS, 1c, AVenue de la Recherche Scientifique, 45071 Orle´ans cedex 2, France ReceiVed March 25, 2008. ReVised Manuscript ReceiVed May 16, 2008

The kinetics of oxidation of ethanol-gasoline surrogate mixtures (85-15 vol %) was studied using a fused silica jet-stirred reactor. One representative of each class constituting E85 gasoline was selected. These constituents were iso-octane, toluene, 1-hexene, and ethanol. The experiments were performed in the temperature range of 770-1220 K, at 10 atm, at four equivalence ratios (0.3, 0.6, 1, and 2), and with an initial fuel concentration of 0.2 mol %. A detailed kinetic scheme resulting from the merging of validated kinetic schemes for the oxidation of the components of the present E85 surrogate (gasoline surrogate and ethanol) was used. Good agreement between the experimental results and the computations was observed under the present JSR conditions.

1. Introduction The increasing number of ground transportation vehicles using fossil fuels is responsible for a dramatic increase of atmospheric pollution and fossil CO2 emission.1 During the last 2 decades, two major trends have been evident (i) improvements in efficiency obtained by engine redesign and (ii) the incorporation of nonfossil compounds in automotive fuels. In this contest, mixtures of conventional gasoline with ethanol is used worldwide2 and most of the engine manufacturers propose “flex-fuel” engines that can run with E85 (85/15 vol % ethanol/petrolgasoline), whereas the extensive use of ethanol in fuels is a source of concerns.3–9 Therefore, kinetic data and kinetic models are necessary for modeling the combustion of such oxygenated fuels. Unfortunately, the oxidation of pure hydrocarbons has * To whom correspondence should be addressed: CNRS, 1c, Avenue de la Recherche Scientifique, 45071 Orle´ans Cedex 2, France. Telephone: +(33) 238-25-54-66. Fax: +(33) 238-69-60-04. E-mail: [email protected]. (1) Barker, T.; Bashmakov, I.; Bernstein, L.; Bogner, J. E.; Bosch, P. R.; Dave, R.; Davidson, O. R.; Fisher, B. S.; Gupta, S.; Halsnæs, K.; Heij, G. J.; Kahn Ribeiro, S.; Kobayashi, S., Levine, M. D.; Martino, D. L.; Masera, O.; Metz, B.; Meyer, L. A.; Nabuurs, G.-J.; Najam, A.; Nakicenovic, N.; Rogner, H.-H.; Roy, J.; Sathaye, J.; Schock, R.; Shukla, P.; Sims, R. E. H.; Smith, P.; Tirpak, D. A.; Urge-Vorsatz, D.; Zhou, D. Technical summary. In Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the IntergoVernmental Panel on Climate Change; Metz, B., Davidson, O. R., Bosch, P. R., Dave, R., Meyer, L. A., Eds.; Cambridge University Press: Cambridge, U.K., 2007. (2) Jeuland, N.; Montagne, X.; Gautrot, X. Potentiality of ethanol as a fuel for dedicated engine. Oil Gas Sci. Technol. 2004, 59 (6), 559–570. (3) Winebrake, J. J.; Wang, M. Q.; He, D. Q. Toxic emissions from mobile sources: A total fuel-cycle analysis for conventional and alternative fuel vehicles. J. Air Waste Manage. Assoc. 2001, 51 (7), 1073–1086. (4) Jacobson, M. Z. Effects of ethanol (E85) versus gasoline vehicles on cancer and mortality in the United States. EnViron. Sci. Technol. 2007, 41 (11), 4150–4157. (5) Kim, S.; Dale, B. E. Ethanol fuels: E10 or E85sLife cycle perspectives. Int. J. Life Cycle Assess. 2006, 11 (2), 117–121. (6) Farrell, A. E.; Plevin, R. J.; Turner, B. T.; Jones, A. D.; O’Hare, M.; Kammen, D. M. Ethanol can contribute to energy and environmental goals. Science 2006, 311 (5760), 506–508. (7) Magnusson, R.; Nilsson, C.; Andersson, B. Emissions of aldehydes and ketones from a two-stroke engine using ethanol and ethanol-blended gasoline as fuel. EnViron. Sci. Technol. 2002, 36 (8), 1656–1664.

been the subject of numerous studies,10 whereas only few studies concern the oxidation of hydrocarbon mixtures or hydrocarbonoxygenate mixtures.11–16 Also, the oxidation of ethanol was the subject of many studies as recently reported,17,18 whereas that of it co-oxidation with hydrocarbons was not. Therefore, experimental databases are needed to propose and validate detailed kinetic models for the combustion of such oxygenated fuel mixtures. Because gasoline is a complex mixture of several hundred hydrocarbons, it is necessary to use surrogate model fuels to describe gasoline combustion chemistry. (8) Niven, R. K. Ethanol in gasoline: Environmental impacts and sustainability review article. Renewable Sustainable Energy ReV. 2005, 9 (6), 535–555. (9) von Blottnitz, H.; Curran, M. A. A review of assessments conducted on bio-ethanol as a transportation fuel from a net energy, greenhouse gas, and environmental life cycle perspective. J. Cleaner Prod. 2007, 15 (7), 607–619. (10) Simmie, J. M. Detailed chemical kinetic models for the combustion of hydrocarbon fuels. Prog. Energy Combust. Sci. 2003, 29 (6), 599–634. (11) Dagaut, P.; Koch, R.; Cathonnet, M. The oxidation of n-heptane in the presence of oxygenated octane improvers: MTBE and ETBE. Combust. Sci. Technol. 1997, 122 (1-6), 345–361. (12) Dagaut, P.; Reuillon, M.; Cathonnet, M. High-pressure oxidation of liquid fuels from low to high-temperature. 2. Mixtures of n-heptane and isooctane. Combust. Sci. Technol. 1994, 103 (1-6), 315–336. (13) Yahyaoui, M.; Djebaili-Chaumeix, N.; Dagaut, P.; Paillard, C. E.; Gail, S. Experimental and modelling study of gasoline surrogate mixtures oxidation in jet stirred reactor and shock tube. Proc. Combust. Inst. 2007, 31 (1), 385–391. (14) Yahyaoui, M.; Djebaili-Chaumeix, N.; Dagaut, P.; Paillard, C. E.; Heyberger, B.; Pengloan, G. Ignition and oxidation of 1-hexene/toluene mixtures in a shock tube and a jet-stirred reactor: Experimental and kinetic modeling study. Int. J. Chem. Kinet. 2007, 39 (9), 518–538. (15) Tan, Y.; Dagaut, P.; Cathonnet, M.; Boettner, J. C. Oxidation and ignition of methane-propane and methane-ethane-propane mixturess Experiments and modeling. Combust. Sci. Technol. 1994, 103 (1-6), 133– 151. (16) Fikri, M.; Herzler, J.; Starke, R.; Schulz, C.; Roth, P.; Kalghatgi, G. T. Autoignition of gasoline surrogates mixtures at intermediate temperatures and high pressures. Combust. Flame 2008, 152 (1-2), 276–281. (17) Leplat, N.; Seydi, A.; Vandooren, J. An experimental study of the structure of a stoichiometric ethanol/oxygen/argon flame. Combust. Sci. Technol. 2008, 180 (3), 519–532. (18) Saxena, P.; Williams, F. A. Numerical and experimental studies of ethanol flames. Proc. Combust. Inst. 2007, 31 (1), 1149–1156.

10.1021/ef800214a CCC: $40.75  2008 American Chemical Society Published on Web 07/01/2008

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In this study, we intended to provide the needed inputs by (i) performing experiments on the oxidation of an E85 surrogate mixture in a jet-stirred reactor (JSR) and (ii) proposing a kinetic model representing the data. The three-component hydrocarbon mixture used before as a gasoline surrogate13 was used with ethanol to represent E85: iso-octane (iso-paraffin representative), toluene (aromatic representative), 1-hexene (olefin representative), and ethanol. This approach follows that previously used for representing gasoline,13 a diesel fuel,19 a jet fuel,20–22 and biodiesel23 with a limited number of constituents. The experimental and modeling results obtained in the present study are reported in the next sections. 2. Experimental Section A spherical fused silica jet-stirred reactor (JSR) similar to that used previously24 was used. It was located inside a regulated electrical resistance oven of ≈1.5 kW, surrounded by insulating material and a pressure-resistant stainless-steel jacket, allowing operation up to 10 atm. Ethanol absolute grade (99.9% pure, Riedel, de Hae¨n), toluene (99.9% pure, Aldrich), iso-octane (>99.8% pure, Aldrich), and 1-hexene (>99% pure, Aldrich) were mixed after ultrasonic degassing. The fuel mixture was pumped using a micro piston high-performance liquid chromatography (HPLC) pump (Shimadzu LC-120 ADvp) and an online degasser (Shimadzu DGU20 A3) and sent to an atomizer-vaporizer assembly maintained at 175 °C. A flow of nitrogen (50 L/h) was used for the atomization. Oxygen (99.995% pure) flow rates were measured and regulated by a thermal mass-flow controller and diluted by a flow of nitrogen (>50 ppm of O2, >1000 ppm of Ar, and >5 ppm of H2), also regulated by a thermal mass-flow controller. This flow was mixed with the fuel-nitrogen flow just before the entrance of the injectors, after preheating. Residence time distribution studies showed that the reactor is operating under macro-mixing conditions.24 As described previously,24,25 a good thermal homogeneity was recorded along the vertical axis of the reactor by thermocouple measurements (Pt/Pt-Rh 10%, 0.1 mm diameter located inside a thin-wall fusedsilica tube to prevent catalytic reactions on the wires). Typical temperature gradients of >2 K/cm were measured. Because we operated under a high degree of dilution, the temperature rise because of the reaction was generally >30 K. Low-pressure samples of the reacting mixtures were taken by sonic probe sampling and collected in 1 L Pyrex bulbs at ca. 40 mBar for immediate gas chromatography (GC) analyses as in refs 25 and 26. Capillary columns of 0.32 mm inner diameter (DB-624, 50 m; and Al2O3/KCl, 50 m) were used with a flame ionization detector (19) Mati, K.; Ristori, A.; Gail, S.; Pengloan, G.; Dagaut, P. The oxidation of a diesel fuel at 1-10 atm: Experimental study in a JSR and detailed chemical kinetic modeling. Proc. Combust. Inst. 2007, 31 (2), 2939– 2946. (20) Dagaut, P.; El Bakali, A.; Ristori, A. The combustion of kerosene: Experimental results and kinetic modelling using 1- to 3-component surrogate model fuels. Fuel 2006, 85 (7-8), 944–956. (21) Dagaut, P.; Cathonnet, M. The ignition, oxidation, and combustion of kerosene: A review of experimental and kinetic modeling. Prog. Energy Combust. Sci. 2006, 32 (1), 48–92. (22) Dagaut, P.; Gail, S. Chemical kinetic study of the effect of a biofuel additive on Jet-A1 combustion. J. Phys. Chem. A 2007, 111 (19), 3992– 4000. (23) Dagaut, P.; Gail, S.; Sahasrabudhe, M. Rapeseed oil methyl ester oxidation over extended ranges of pressure, temperature, and equivalence ratio: Experimental and modeling kinetic study. Proc. Combust. Inst. 2007, 31 (2), 2955–2961. (24) Dagaut, P.; Cathonnet, M.; Rouan, J. P.; Foulatier, R.; Quilgars, A.; Boettner, J. C.; Gaillard, F.; James, H. A jet-stirred reactor for kinetic studies of homogeneous gas-phase reactions at pressures up to 10 atm (∼1 MPa). J. Phys. E: Sci. Instrum. 1986, 19 (3), 207–209. (25) Dayma, G.; Ali, K. H.; Dagaut, P. Experimental and detailed kinetic modeling study of the high pressure oxidation of methanol sensitized by nitric oxide and nitrogen dioxide. Proc. Combust. Inst. 2007, 31 (1), 411– 418. (26) Dubreuil, A.; Foucher, F.; Mounaim-Rousselle, C.; Dayma, G.; Dagaut, P. HCCI combustion: Effect of NO in EGR. Proc. Combust. Inst. 2007, 31 (2), 2879–2886.

Dagaut and Togbe´ Table 1. Composition (in Mole Fraction) of the E85 Surrogate Mixtures (85 vol % of Ethanol) equivalence ratio

ethanol

iso-octane

toluene

1-hexene

oxygen

0.3 0.6 1 2

0.001 862 0.001 862 0.001 862 0.001 862

0.000 069 0.000 069 0.000 069 0.000 069

0.000 048 0.000 048 0.000 048 0.000 048

0.000 021 0.000 021 0.000 021 0.000 021

0.023 56 0.011 78 0.007 07 0.003 534

(FID) for the measurements of hydrocarbons and oxygenates. Helium was used as a carrier gas. Hydrogen and oxygen were measured using a 0.53 mm inner diameter capillary column (Carboplot, 25 m) fitted to a thermal conductivity detector (TCD). Nitrogen was used as carrier gas. Online Fourier transform infrared (FTIR) analyses of the reacting gases were also performed by connecting the sampling probe to a temperature-controlled (140 °C) gas cell (10 m path length) via a Teflon heated line (175 °C). The sample pressure in the cell was 0.2 bar. This analytical equipment allowed for the measurements of ethanol, ethanal, toluene, iso-octane, methane, ethane, ethylene, acetylene, propene, H2O, CH2O, CO, and CO2. As described previously,25 very good agreement between the GC and FTIR analyses was found for the compounds measured by both techniques. Carbon balance was checked for every sample and found good (100 ( 8%).

3. Kinetic Modeling We used the PSR computer code27 for the present modeling. The proposed kinetic reaction mechanism (235 species and 1866 reversible reactions) is based on that proposed previously for the oxidation of various fuels28 and a surrogate gasoline mixture.13 The ethanol submechanism was updated on the basis of recent rate constant determinations obtained by combining sophisticated measurements and computations29–32(Table 2). The rate constants for the reverse reactions were computed from the forward rate constants and the appropriate equilibrium constants calculated using thermochemical data.13,33 The pressure dependencies of unimolecular reactions and some pressuredependent bimolecular reactions were taken into account [i.e., k(P,T)]. The full mechanism, including thermochemical data, is available from the authors ([email protected]) and as Supporting Information. To rationalize the results, first-order local sensitivity analyses and reaction rate analyses were performed. The rates of consumption (R with a negative sign) and production (R with a positive sign) were also computed for every species. 4. Results and Discussion In this work, the oxidation of an E85 surrogate fuel was studied. The composition of the reacting mixtures is given in (27) Glarborg, P.; Kee, R. J.; Grcar, J. F.; Miller, J. A. PSR: A FORTRAN Program for Modeling Well-Stirred Reactors; Sandia National Laboratories: Livermore, CA, 1986; SAND86-8209. (28) Dagaut, P. On the kinetics of hydrocarbons oxidation from natural gas to kerosene and diesel fuel. Phys. Chem. Chem. Phys. 2002, 4 (11), 2079–2094. (29) Park, J.; Xu, Z. F.; Lin, M. C. Thermal decomposition of ethanol. II. A computational study of the kinetics and mechanism for the H + C2H5OH reaction. J. Chem. Phys. 2003, 118 (22), 9990–9996. (30) Xu, S.; Lin, M. C. Theoretical study on the kinetics for OH reactions with CH3OH and C2H5OH. Proc. Combust. Inst. 2007, 31 (1), 159–166. (31) Xu, Z. F.; Park, J.; Lin, M. C. Thermal decomposition of ethanol. III. A computational study of the kinetics and mechanism for the CH3 + C2H5OH reaction. J. Chem. Phys. 2004, 120 (14), 6593–6599. (32) Wu, C. W.; Lee, Y. P.; Xu, S. C.; Lin, M. C. Experimental and theoretical studies of rate coefficients for the reaction O(P-3) plus C2H5OH at high temperatures. J. Phys. Chem. A 2007, 111 (29), 6693–6703. (33) Tan, Y. W.; Dagaut, P.; Cathonnet, M.; Boettner, J. C. Acetylene oxidation in a JSR from 1-atm to 10-atm and comprehensive kinetic modeling. Combust. Sci. Technol. 1994, 102 (1-6), 21–55.

Oxidation of E85 Surrogate

Energy & Fuels, Vol. 22, No. 5, 2008 3501 Table 2. Updated Reaction Kinetics for Ethanol

reaction number 1796 1797 1798 1799 1800 1801 1802 1803 1804 1805 1806 1807 a

reaction C2H5OH C2H5OH C2H5OH C2H5OH C2H5OH C2H5OH C2H5OH C2H5OH C2H5OH C2H5OH C2H5OH C2H5OH

+ + + + + + + + + + + +

OH S C2H4OH + H2O OH S CH3CHOH + H2O OH S CH3CH2O + H2O H S C2H4OH + H2 H S CH3CHOH + H2 H S CH3CH2O + H2 O S C2H4OH + OH O S CH3CHOH + OH O S CH3CH2O + OH CH3 S C2H4OH+ CH4 CH3 S CH3CHOH + CH4 CH3 S CH3CH2O + CH4

A 6.20 × 103 1.31 × 105 2.81 × 102 1.88 × 103 1.79 × 105 5.55 × 10-23 9.69 × 102 1.45 × 105 1.46 × 10-3 3.30 × 102 1.99 × 101 2.04

n

E

2.7 2.4 3.0 3.2 2.5 10.6 3.2 2.4 4.7 3.3 3.4 3.6

-576 -1457 -580 7150 3420 -4459 4658 876 1727 12 291 7635 7722

reference 30 30 30 29 29 29 32 32 32 31 31 31

k ) ATn exp-E/(1.9872T), in mol, cm3, s, and cal units.

Figure 1. Oxidation of a E85 surrogate fuel mixture in a JSR at 10 atm and φ ) 0.3. Comparison between experimental results (symbols) and modeling (lines and small symbols).

Figure 2. Oxidation of a E85 surrogate fuel mixture in a JSR at 10 atm and φ ) 0.6. Comparison between experimental results (symbols) and modeling (lines and small symbols).

Figure 3. Oxidation of a E85 surrogate fuel mixture in a JSR at 10 atm and φ ) 1. Comparison between experimental results (symbols) and modeling (lines and small symbols).

Table 1. The oxidation of these mixtures was performed in a JSR at a fixed residence time of 0.7 s and at 10 atm. During the JSR experiments, ca. 20 species were identified and measured by gas chromatography/mass spectrometry (GC/MS), FID, and TCD. Experimental mole fractions as a function of temperature were obtained for H2, H2O, O2, CO, CO2, CH2O, CH4, C2H6, C2H4, C2H2, ethanol, acetaldehyde (ethanal), C3H6, 1-C4H8, i-C4H8, 1,3-C4H6, 1-C6H12, C6H5CH3, and i-C8H18.

Figures 1–4 show examples of the present results. It is clear from Figure 3 that acetaldehyde is an important intermediate formed during the oxidation of this E85 surrogate fuel. This must be interpreted as a result of the oxidation of the main component in this E85 surrogate fuel, namely, ethanol. A detailed chemical kinetic reaction mechanism was used to represent the oxidation of the present E85 surrogate. The mechanism used previously for modeling the oxidation of a

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Dagaut and Togbe´

Figure 4. Oxidation of a E85 surrogate fuel mixture in a JSR at 10 atm and φ ) 2. Comparison between experimental results (symbols) and modeling (lines and small symbols).

Figure 5. Oxidation of ethanol in a JSR at 10 atm and φ ) 0.6. Comparison between experimental results (symbols) and modeling (lines and small symbols).

similar gasoline surrogate13 was used as a starting point. This mechanism derives from one proposed earlier28 and already includes the reaction subschemes for the oxidation of iso-octane, toluene, 1-hexene, and ethanol. Ethanol reaction kinetics, initially taken from Marinov,34 were updated (Table 2) thanks to new experimental and computational results. The kinetics of ethanol with OH was taken from ref 30; that for the reactions of H with ethanol were determined by ref 29; that for the reaction of ethanol with O atoms were taken from ref 32; and that for the reaction of ethanol with CH3 were taken from ref 31. In previous studies,13,14 we showed that interactions between hydrocarbons are not limited to small labile radicals, but other interactions involving large radicals produced during fuel oxidation are possible. Thus, the kinetic mechanism built to model the oxidation of surrogate gasoline mixtures contains submechanisms for the oxidation of the model fuel components and interactions between these subschemes. These interactions mainly consist of hydrogen abstraction reactions.13,14 As can be seen from Figures 1–4, the proposed model represents fairly well the data, although ethane is overpredicted in fuel lean conditions. The proposed model was also successfully used to simulate the oxidation of the gasoline surrogate, with results undistinguishable from those previously published.13 It could also be used to simulate ethanol oxidation under JSR conditions,35 as depicted in Figures 5–7. Sensitivity analyses and reaction paths analyses were used to delineate the main reaction paths influencing the kinetics of oxidation of the fuel. Because the main component, in mole fraction, in the E85 surrogate is by far ethanol (93.1 mol %), the kinetics of oxidation of the fuel is driven by that of ethanol. This is clearly shown by comparing ethanol and E85 intermediates concentration (34) Marinov, N. M. A detailed chemical kinetic model for high temperature ethanol oxidation. Int. J. Chem. Kinet. 1999, 31 (3), 183–220. (35) Togbe´, C. Combustion de bio-ke´rose`ne: Etude expe´rimentale. University of Orleans, France, 2007.

Figure 6. Oxidation of ethanol in a JSR at 10 atm and φ ) 1. Comparison between experimental results (symbols) and modeling (lines and small symbols).

Figure 7. Oxidation of ethanol in a JSR at 10 atm and φ ) 2. Comparison between experimental results (symbols) and modeling (lines and small symbols).

profiles (Figure 8) obtained here. The presence of ethanol in E85 yields an increased formation of aldehydes during the fuel oxidation. The model and the data agree on this. Simulations performed for the oxidation of the E85 surrogate and the gasoline surrogate, keeping all other parameters constant (residence time, pressure, equivalence ratio, and initial carbon concentration), indicated an increase of intermediate concentrations of acetaldehyde (by a factor of >100) and of formaldehyde

Oxidation of E85 Surrogate

Energy & Fuels, Vol. 22, No. 5, 2008 3503

C2H5OH + HO2 S C2H4OH + H2O2;

R(C2H5OH) ) -0.032 (1810)

producing CH3CHOH that essentially reacts with oxygen to yield acetaldehyde CH3CHOH + O2 S CH3HCO + HO2;

R(CH3CHOH) ) 0.999(1819) CH3HCO predominantly reacts with HO2, yielding CH3CO

Figure 8. Oxidation of 2000 ppm ethanol and 2000 ppm E85 surrogate fuel mixture in a JSR at 10 atm, 0.7 s, and φ ) 1. Closed symbols represent the ethanol data, and the open symbols represent the E85 data. The carbon content is 1.185 times higher for the E85, which does not explain the higher intermediate concentrations obtained with this fuel.

CH3HCO + HO2 S CH3CO + H2O2;

R(CH3HCO) ) -0.840 (248)

CH3HCO + OH S CH3CO + H2O;

R(CH3HCO) ) -0.090 (249)

CH3HCO + O2 S CH3CO + HO2;

R(CH3HCO) ) -0.056 (252)

CH3HCO + CH3 S CH3CO + CH4;

R(CH3HCO) ) -0.012 (253)

CH3CO decomposes to produce CO and CH3 CH3CO + M S CH3 + CO + M;

R(CH3CO) ) -1.0 (255)

that reacts via Figure 9. Sensitivity spectrum from CO2 during the oxidation of 2000 ppm ethanol and 2000 ppm E85 surrogate fuel mixture in a JSR at 10 atm, 0.7 s, and φ ) 1. Reactions: 6, H + O2 S OH + O; 7, H + O2 + M S HO2 + M; 16, HO2 + HO2 S H2O2 + O2; 17, HO2 + HO2 S H2O2 + O2; 22, CO + HO2 S CO2 + OH; 23, CO + OH S CO2 + H; 42, CH3 + HO2 S CH4 + O2; 45, CH3 + HO2 S CH3O + OH; 112, CH2O + HO2 S HCO + H2O2; 248, CH3HCO + HO2 S CH3CO + H2O2; 249, CH3HCO + OH S CH3CO + H2O; 1066, 1-C6H12 + OH S 1,4-C6H11 + H2O; 1798, C2H5OH + OH S CH3CHOH + H2O; 1809, C2H5OH + HO2 S CH3CHOH + H2O2; 1819, CH3CHOH + O2 S CH3HCO + HO2; 1830, OH + OH(+M) S H2O2(+M); and 1832, CH3 + CH3(+M) S C2H6(+M).

(by ca. 25%), in line with light-duty vehicles emissions data.36–38 E85 produces more methyl radicals via the oxidation of isooctane, which results in higher concentrations of CH4, C2H6, and C3H6. Therefore, the kinetic analysis presented here concentrates on ethanol reaction paths. In fuel lean conditions and low temperature (φ ) 0.3 and 740 K), ethanol mostly reacts with HO2 C2H5OH + OH S C2H4OH + H2O;

R(C2H5OH) ) -0.037 (1797)

C2H5OH + OH S CH3CHOH + H2O;

R(C2H5OH) ) -0.272 (1798)

C2H5OH + OH S CH3CH2O + H2O;

R(C2H5OH) ) -0.011 (1799)

C2H5OH + HO2 S CH3CHOH + H2O2; R(C2H5OH) ) -0.630 (1809) (36) MacLean, H. L.; Lave, L. B. Environmental implications of alternative-fueled automobiles: Air quality and greenhouse gas tradeoffs. EnViron. Sci. Technol. 2000, 34 (2), 225–231. (37) Benson, J. D.; Koehl, W. J.; Burns, W. R.; Hochhauser, A. M.; Knepper, J. C.; Leppard, W. R.; Painter, L. J.; Rapp, L. A.; Reuter, R. M.; Rippon, B.; Rutherford, J. A. Emissions with E85 and gasolines in flexible/ variable fuel vehiclessThe auto/oil air quality improvement research program. SAE 1995, 952508. (38) Magnusson, R.; Nilsson, C.; Andersson, B. Emissions of aldehydes and ketones from a two-stroke engine using ethanol and ethanol-blended gasoline as fuel. EnViron. Sci. Technol. 2002, 36 (8), 1656–1664.

CH3 + HO2 S CH3O + OH; CH3 + O2 S CH2O + OH;

R(CH3) ) -0.191 R(CH3) ) -0.150

(45) (49)

CH3O decomposes and reacts with oxygen, yielding formaldehyde CH3O + M S CH2O + H + M;

R(CH3O) ) -0.846 (99)

CH3O + O2 S CH2O + HO2;

R(CH3O) ) -0.151 (104)

CH2O predominantly reacts with HO2, yielding HCO CH2O + HO2 S HCO + H2O2;

R(CH2O) ) -0.915 (112)

CH2O + OH S HCO + H2O;

R(CH2O) ) -0.053 (113)

that in these conditions mostly reacts with oxygen HCO + M S H + CO + M; HCO + O2 S CO + HO2;

R(HCO) ) -0.019 R(HCO) ) -0.981

(26) (31)

Reactions of HO2 radicals are particularly important here at low fuel conversion, low temperature, and elevated pressure. They are mostly produced by reaction of CH3CHOH with oxygen CH3CHOH + O2 S CH3HCO + HO2;

R(HO2) ) 0.892 (1819)

Minor contribution of several other reactions to its production include H + O2 + M S HO2 + M;

R(HO2) ) 0.027

(7)

BDDTMPE2 + O2 S I2C8H16 + HO2;

R(HO2) ) 0.026 (1468)

BDDTMPE2 + O2 S I1C8H16 + HO2;

R(HO2) ) 0.026 (1469)

whereas they mostly react through self-reaction and with ethanol 2HO2 S H2O2 + O2;

R(HO2) ) -0.238 (16and17)

3504 Energy & Fuels, Vol. 22, No. 5, 2008

C2H5OH + HO2 S CH3CHOH + H2O2;

Dagaut and Togbe´

R(HO2) ) -0.633 (1809)

At higher temperature (820 K), where ca. 50% of the fuel is consumed, the reaction paths change significantly. Ethanol reacts predominantly with OH yielding mostly CH3CHOH. C2H5OH + OH S C2H4OH + H2O;

R(C2H5OH) ) -0.109 (1797)

C2H5OH + OH S CH3CHOH + H2O;

R(C2H5OH) ) -0.737 (1798)

The decomposition of CH3CHOH yields acetaldehyde that is still oxidized into CH3CO. However, the reaction with OH is also more important at this temperature CH3HCO + HO2 S CH3CO + H2O2;

R(CH3HCO) ) -0.273 (248)

CH3HCO + OH S CH3CO + H2O;

R(CH3HCO) ) -0.616 (249)

H + O2 S OH + O; H2O + O S 2OH;

R(OH) ) 0.216 R(OH) ) 0.231

R(C2H4OH) ) -0.910 (1827)

that reacts via reaction with OH and H, mostly C2H4 + OH S C2H3 + H2O;

R(C2H4) ) -0.375 (137)

C2H4 + H(+M) S C2H5(+M);

R(C2H4) ) -0.452 (1834)

The ethyl radical reacts with oxygen, reforming ethylene, either directly or indirectly: C2H5 + O2 S C2H4 + HO2;

R(C2H5) ) -0.431 (1857)

C2H5 + O2 S C2H4O2H;

R(C2H5) ) -0.547 (1858)

C2H4 + HO2 S C2H4O2H;

R(C2H5) ) -0.982

(136)

HO2 radicals are still mainly formed via reaction 1819 (51%), whereas two other reactions contribute to its formation

(10)

It is well-known that the reaction paths change with equivalence ration. In the condition of the fuel-rich mixture (φ ) 2), the reaction paths do not change at 740 K, where the reaction only starts. However, at higher fuel conversion, significant differences from the fuel-lean case appear. At 840 K, where ca. 50% of the fuel has reacted, ethanol still reacts with OH. However, the depletion of acetaldehyde via the reaction with OH is less important, whereas that by reaction with H increases C2H5OH + OH S C2H4OH + H2O;

R(C2H5OH) ) -0.107 (1797)

C2H5OH + OH S CH3CHOH + H2O;

R(C2H5OH) ) -0.707 (1798)

C2H5OH + H S CH3CHOH + H2;

R(C2H5OH) ) -0.057 (1801)

C2H5OH + CH3 S C2H4OH + CH4;

R(C2H5OH) ) -0.018 (1806)

C2H4OH mostly decomposes, yielding ethylene C2H4OH S C2H4 + OH;

(6)

C2H5OH + CH3 S CH3CHOH + CH4; R(C2H5OH) ) -0.028 (1807) C2H5OH + HO2 S CH3CHOH + H2O2; R(C2H5OH) ) -0.017 (1809) Also, the decomposition of HCO becomes important; conversely, the importance of the reaction with O2 decreases HCO + M S H + CO + M; HCO + O2 S CO + HO2;

R(HCO) ) -0.344 R(HCO) ) -0.645

(26) (31)

At 1200 K, the reaction paths differ more significantly when comparing the fuel-lean and fuel-rich conditions. In the fuelrich conditions, the depletion of ethanol now involves several pyrolysis paths, at the expense of oxidation routes C2H5OH(+M) S CH3 + CH2OH(+M); R(C2H5OH) ) -0.233 (1793)

H + O2 + M S HO2 + M;

R(HO2) ) 0.176

(7)

HCO + O2 S CO + HO2;

R(HO2) ) 0.173

(31)

C2H5OH(+M) S C2H4 + H2O(+M);

The formation of OH radicals is mostly due to the decomposition of H2O2 and, to some extent, the reaction of methyl with HO2

R(C2H5OH) ) -0.306 (1795)

C2H5OH + OH S CH3CHOH + H2O;

R(C2H5OH) ) -0.053 (1798)

CH3 + HO2 S CH3O + OH; 2OH(+M) S H2O2(+M);

R(OH) ) 0.177 R(OH) ) 0.644

(45) (1830)

At higher temperature still (1200 K), the consumption of ethanol is mostly due to reactions 1797 and 1798. Acetaldehyde reacts with OH (72%) and decomposes (9%). Ethylene produced in C2H5OH(+M) S C2H4 + H2O(+M); C2H4 + OH S C2H4OH;

R(C2H5OH) ) 0.271 (1795)

R(C2H5OH) ) 0.403 (1827)

C2H5(+M) S C2H4 + H(+M);

R(C2H5OH) ) 0.254 (1834)

is consumed by reactions with OH (50%) and O (50%). The production of OH still occurs via reaction 1830 2OH(+M) S H2O2(+M); but also through

R(OH) ) 0.246

(1830)

C2H5OH + H S C2H4OH + H2;

R(C2H5OH) ) -0.073 (1800)

C2H5OH + H S CH3CHOH + H2;

R(C2H5OH) ) -0.231 (1801)

The same trend is observed for acetaldehyde that decomposes and reacts with H CH3HCO S CH3 + HCO;

R(CH3HCO) ) -0.227 (247)

CH3HCO + OH S CH3CO + H2O; CH3HCO + H S CH3CO + H2; CH3HCO + CH3 S CH3CO + CH4;

R(CH3HCO) ) -0.055 (249) R(CH3HCO) ) -0.662 (251) R(CH3HCO) ) -0.036 (253)

Oxidation of E85 Surrogate

Energy & Fuels, Vol. 22, No. 5, 2008 3505

CH3HCO + HO2 S CH3CO + H2O2

Also, methyl radicals are mostly involved in pyrolytic processes C2H4 + CH3 S C2H3 + CH4;

R(CH3) ) -0.130 (143)

CH4(+M) S CH3 + H(+M);

R(CH3) ) -0.133 (1831)

2CH3(+M) S C2H6(+M);

R(CH3) ) -0.472 (1832)

Ethane mostly reacts with H C2H6 + OH S C2H5 + H2O;

R(C2H6) ) -0.757

C2H6 + CH3 S C2H5 + CH4;

(121)

R(C2H6) ) -0.068 (124)

yielding ethyl radicals that decompose to yield ethylene C2H5(+M) S C2H4 + H(+M);

R(C2H5) ) -0.921 (1834)

The formation of acetylene increases strongly when the initial concentration of oxygen is reduced (Figures 3 and 4). It is mostly formed via the decomposition of vinyl radicals, produced by H-atom abstraction by H and CH3 on ethylene C2H3(+M) S C2H2 + H(+M);

R(C2H2) ) 0.717

(1835)

Sensitivity analyses (Figure 9) indicated that the kinetics of oxidation of the E85 surrogate is mainly sensitive to the rate constants of HO2 and H2O2 reactions at low temperature [16 and 17, HO2 + HO2 S H2O2 + O2; and 1830, OH + OH(+M) S H2O2(+M)] because the fuel consumption mainly occurs via reaction with HO2. Consequently, reaction (1809, C2H5OH + HO2 S CH3CHOH + H2O2) is also sensitive. At higher temperatures, two other reactions involving HO2 influence the kinetics of oxidation of the fuel by consuming acetaldehyde, an important intermediate of ethanol oxidation, and CH3 CH3 + HO2 S CH3O + OH and

The fuel oxidation is also sensitive to reactions involving the hydrocarbon constituents of the fuels; this is the case for reaction 1066 (1-C6H12 + OH S 1,4-C6H11 + H2O). At higher temperature still, the kinetics of the main branching reaction (6, H + O2 S OH + O) is increasingly influent.

R(C2H6) ) -0.139 (119)

C2H6 + H S C2H5 + H2;

(45)

(248)

5. Conclusion The two main objectives of this study were achieved: (i) New data consisting of concentration profiles of reactants, stable intermediates, and products were obtained as a function of the reaction temperature for the oxidation of an E85 surrogate in a JSR at 10 atm and 700 ms for fuel-lean to fuel-rich mixtures. (ii) A detailed chemical kinetic modeling of these experiments was performed yielding good agreement between the data and the computations using the presently updated kinetic scheme. The present work clearly shows that the high-temperature oxidation of this E85 surrogate is similar to that of ethanol, as a result of the high initial concentration of ethanol in the fuel. Acetaldehyde was found to be an important intermediate formed during the oxidation of this E85 surrogate fuel, as a result of the oxidation of ethanol, the main component in this E85 surrogate fuel; its intermediate concentration is much higher than that yielded from the oxidation of the ethanol-free gasoline surrogate, in line with SI engine data. Finally, flame speeds and ignition delays for E85 or surrogates are needed for further model validations. Supporting Information Available: Chemical kinetic reaction mechanism and thermochemistry. This material is available free of charge via the Internet at http://pubs.acs.org. EF800214A