Energy & Fuels 2009, 23, 2389–2395
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Chemical Kinetic Study of the Oxidation of Isocetane (2,2,4,4,6,8,8-Heptamethylnonane) in a Jet-stirred Reactor: Experimental and Modeling P. Dagaut* and K. Hadj-Ali Centre National de la Recherche Scientifique, I.C.A.R.E.; 1C, AVenue de la Recherche Scientifique; 45071 Orle´ans Cedex 2, France ReceiVed February 2, 2009. ReVised Manuscript ReceiVed March 20, 2009
The kinetics of oxidation of isocetane (2,2,4,4,6,8,8-heptamethylnonane), a cetane number reference fuel, was studied experimentally in a jet-stirred reactor (JSR) at 10 atm and a constant residence time of 1 s, over the temperature range 770-1070 K, and for variable equivalence ratios (0.5, 1, and 2). Concentration profiles of reactants, stable intermediates, and final products were obtained by probe sampling followed by online and offline gas chromatography analyses and online Fourier transformed infrared spectrometry. The oxidation of this fuel in these conditions was modeled using computer-generated semidetailed kinetic reaction mechanisms. A reasonable representation of the kinetics of oxidation of isocetane and the formation of the main products under the present JSR conditions could be obtained. However, the prediction of most of the intermediates was poor as a result of the use of too simplified lumped chemistry in these schemes. This indicates needs for additional model development that could benefit from the present experimental database.
1. Introduction Diesel fuels have cetane numbers (CN) in the range 40-60, depending on the country where it is distributed.1 In Europe, the EN 590 standard requires a minimum of cetane rating of 49 (only 46 in Scandinavian countries) whereas in the US it is required a minimum of 40 ASTM D 975 (American Society for Testing and Materials). The CN was initially defined as the n-hexadecane % (in volume) in a blend of n-hexadecane (cetane) and 1-methylnaphthalene (R-methylnaphthalene) giving the same ignition delay as the test fuel in a standard single-cylinder diesel CFR test engine. 1-Methylnaphthalene, which is particularly resistant to autoignition, was assigned a CN of 0. n-Hexadecane, which ignites very readily, was assigned a CN of 100. Since 1962, due to difficulties in getting large supply of high-purity 1-methylnapthalene and its expense, the ASTM replaced it with a secondary reference fuel, 2,2,4,4,6,8,8heptamethylnonane (isocetane, CAS 4390-04-9) that has a CN ) 15. Therefore, isocetane was included in recently proposed surrogate fuels.2,3 However, whereas several experimental and kinetic modeling studies of the oxidation of the original CN * To whom correspondence should be addressed. E-mail: dagaut@ cnrs-orleans.fr; phone: (33) 238 25 54 66; fax: (33) 238 69 60 04. (1) Guibet, J. C., Fuels and Engines. Technology - Energy - EnVironment;Technip: Paris, 1999. (2) Agosta, A.; Cernansky, N. P.; Miller, D. L.; Faravelli, T.; Ranzi, E. Exp. Thermal Fluid Sci. 2004, 28 (7), 701–708. (3) Mueller, C. J.; Pitz, W. J.; Pickett, L. M.; Martin, G. C.; Siebers, D. L.; Westbrook, C. K., Effects of Oxygenates on Soot Processes in DI Diesel Engines: Experiments and Numerical Simulations; SAE: 2003; 2003-01-1791. (4) Dagaut, P. Phys. Chem. Chem. Phys. 2002, 4 (11), 2079–2094. (5) Ristori, A.; Dagaut, P.; Cathonnet, M. Combust. Flame 2001, 125 (3), 1128–1137. (6) Fournet, R.; Battin-Leclerc, F.; Glaude, P. A.; Judenherc, B.; Warth, V.; Come, G. M.; Scacchi, G.; Ristori, A.; Pengloan, G.; Dagaut, P.; Cathonnet, M. Int. J. Chem. Kinet. 2001, 33 (10), 574–586. (7) Mati, K.; Ristori, A.; Pengloan, G.; Dagaut, P. Combust. Sci. Technol. 2007, 179 (7), 1261–1285.
primary reference fuels (n-hexadecane and 1-methylnaphtalene) have been published,4-13 no comparable experimental kinetic study of isocetane oxidation was previously published to allow kinetic model validation. The study of Agosta et al.2 only showed that isocetane does not react under their plug flow reactor conditions over the temperature range 600-900 K. We therefore have included such investigation in our laboratory program aimed at elucidating the kinetic reaction mechanisms of hydrocarbons oxidation. To provide the needed experimental database, a series of experiments was performed on the kinetics of oxidation of isocetane. We present here our new experimental results obtained in a JSR for the oxidation of isocetane at 10 atm, over a wide range of equivalence ratios and temperatures. The oxidation of isocetane in these conditions was modeled using semidetailed computer-generated kinetic schemes. 2. Experimental Setup The experiments (Table 1) were performed in the JSR experimental setup used and described earlier.14,15 The JSR consisted of a small sphere of 4 cm diameter (39 cm3) made of fused-silica (to minimize wall catalytic reactions), equipped with four nozzles of (8) Mati, K.; Ristori, A.; Gail, S.; Pengloan, G.; Dagaut, P. Proc. Combust. Inst. 2007, 31 (2), 2939–2946. (9) Pfahl, U.; Fieweger, K.; Adomeit, G. Symp. (Intl.) Combust. 1996, 26 (1), 781–789. (10) Pitsch, H. Symp. (Intl.) Combust. 1996, 26 (1), 721–728. (11) Shaddix, C. R.; Brezinsky, K.; Glassman, I. Symp. (Intl.) Combust. 1992, 24 (1), 683–690. (12) Shaddix, C. R.; Brezinsky, K.; Glassman, I. Combust. Flame 1997, 108 (1-2), 139–157. (13) Westbrook, C. K.; Pitz, W. J.; Herbinet, O.; Curran, H. J.; Silke, E. J. Combust. Flame 2009, 156 (1), 181–199. (14) Dagaut, P.; Cathonnet, M.; Rouan, J. P.; Foulatier, R.; Quilgars, A.; Boettner, J. C.; Gaillard, F.; James, H. J. Phys. E 1986, 19 (3), 207– 209. (15) Dagaut, P. J. Eng. Gas Turbines Power-Transact. ASME 2007, 129 (2), 394–403.
10.1021/ef900089n CCC: $40.75 2009 American Chemical Society Published on Web 04/07/2009
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Energy & Fuels, Vol. 23, 2009
Dagaut and Hadj-Ali
Table 1. Experimental Conditions for the Oxidation of Isocetane in a JSR at a Pressure of 10 atm and a Mean Residence Time of 1 s initial mole fractions in ppm equivalence ratio, φ
isocetane
O2
N2
0.5 1 2
700 700 700
34 300 17 150 8575
965 000 982 150 990 725
1 mm I.D. for the admission of the gases that are achieving the stirring. A nitrogen flow of 100 L/h was used to dilute the fuel. As before,8,16 all gases were preheated before injection at a temperature close to that in the JSR to minimize temperature gradients inside the reactor. A regulated heating wire of ca. 1.5 kW maintained the temperature of the reactor at the desired working temperature. The reactants were diluted by nitrogen (
