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Energy & Fuels 2001, 15, 372-376
Kinetic Effect of Dimethoxymethane on the Oxidation of Indane P. Dagaut,* A. Ristori, G. Pengloan, and M. Cathonnet C.N.R.S., Laboratoire de Combustion et Syste` mes Re´ actifs, 1C, Avenue de la Recherche Scientifique, 45071 Orle´ ans Cedex 2, France Received July 6, 2000. Revised Manuscript Received November 7, 2000
The oxidation of 0.1% vol indane has been studied in a jet-stirred reactor at high temperature (950-1350 K), a fixed residence time τ of 70 ms, and variable equivalence ratio (0.5 e φ e 1). Indane-dimethoxymethane (0.095%-0.005% vol) mixtures were also oxidized in stoichiometric conditions. Molecular species concentration profiles of reactants, stable intermediates, and final products were obtained by probe sampling followed by on-line and off-line GC analyses. Dimethoxymethane (DMM) appears to accelerate the oxidation of the aromatic fuel, shifting the reactivity of indane by 40 K. This work provided some kinetic insights into the processes involved in indane oxidation and reactivity enhancement by DMM. A reaction sequence based on the experimental results obtained for oxidation of indane and reactivity enhancement by DMM is proposed to rationalize the present results. The present experiments showed that DMM addition tends to reduce the maximum mole fraction of higher PAHs.
1. Introduction One of the major concerns regarding diesel engine emissions is particulate matter (PM). PM formation is generally considered to be favored by higher concentrations of polycyclic unsaturated compounds in the fuel.1 Among these, naphthenoaromatics are important constituents of diesel fuels (6.9% wt according to Guibet1). Indane is the simplest naphtheno-aromatic hydrocarbon. No previous study regarding its oxidation kinetics has been reported before. This is a major lack since indane oxidation yields large amounts of indene that, by oxidation/combustion, can easily form higher molecular weight polyaromatic hydrocrabons (PAHs). Also, indene shows a strong sooting tendency, as reported by Ladommatos et al.2 This study intends to provide the missing kinetic data to build a kinetic reaction mechanism to be included in diesel fuel combustion kinetic models. Recent studies3,4 performed in engines have demonstrated the efficiency of dimethoxymethane (DMM), also called methylal, for reducing PM. This acetal was mixed with diesel fuel in various proportions (up to 30%) and Maricq et al.3 reported (36 ( 8)% PM mass reduction when 16.6% (vol) DMM was added to diesel fuel. However, no kinetic interpretation of these engine results could be offered in these studies. Many other oxygenated chemicals such as ethers, esters, hydroxy* Corresponding author. Tel: (33) 238 25 54 66. Fax: (33) 238 69 60 04. E-mail:
[email protected]. (1) Guibet, J.-C. Fuels and Engines; Editions Technip: Paris, 1999. (2) Ladommatos, N.; Rubenstein, P.; Bennett, P. Fuel 1996, 75, 114. (3) Maricq, M. M.; Chase, R. E.; Podsiadlik, D. H.; Siegl, W. O.; Kaiser, E. W. SAE Technical Paper 1998, 982572. (4) Vertin, K. D.; Ohi, J. M.; Naegeli, D. W.; Childress, K. H.; Hagen, G. P.; McCarthy, C. I.; Cheng, A. S.; Dibble, R. W. SAE Technical Paper 1999, SP-1458, 1999-01-1508.
ethers, polyethers, have been tested as additives to reduce soot emissions from diesel engines. Recently, Miyamoto et al.5 showed that exhaust PM emissions disappeared for g30% in mass of oxygen content in the diesel fuel. Using kinetic modeling, Flynn et al.6 interpreted this finding as a result of the strong reduction of the concentration of soot precursors, considered in their work to be C2H2, C2H4, and C3H3, by the addition of oxygenates. According to a recent kinetic study from this laboratory,7 the oxidation of DMM readily yields methoxy radicals (CH3O) via thermal dissociation and decomposition of the parent radicals formed by H-atom abstraction on DMM. Methoxy radicals are very reactive and yield important chain-branching agents through thermal decomposition and reaction with molecular oxygen (CH3O f CH2O + H, CH3O + O2 f CH2O + HO2). Thus, a series of experiments was performed to study the effect of DMM on indane oxidation and to understand the mechanism involved in the oxidation of indane-DMM mixtures. The results of this study are reported here. 2. Experimental Section The JSR setup used in this work is similar to that described earlier.8,9 The reactor consists of a small sphere of 40 mm (5) Miyamoto, N.; Ogawa, H.; Nurun, N. Md.; Obata, K.; Arima, T. SAE Technical Paper 1998, 980506. (6) Flynn, P. F.; Durrett, R. P.; Hunter, G. L.; zur Loye, A. O.; Akinyemi, O. C.; Dec, J. E.; Westbrook, C. K. SAE Technical Paper 1999, SP-1469, 1999-01-0509. (7) Daly, C.; Simmie, J. M.; Dagaut, P.; Cathonnet, M. Combust. Flame, submitted. (8) Dagaut, P.; Cathonnet, M.; Rouan, J. P.; Foulatier, A.; Quilgars, A.; Boettner, J.-C.; Gaillard, F.; James; H. J. Phys. E: Sci. Instrum. 1986, 19, 207. (9) Dagaut, P.; Reuillon, M.; Cathonnet, M. Combust. Sci. Technol. 1994, 95, 233.
10.1021/ef000146j CCC: $20.00 © 2001 American Chemical Society Published on Web 01/18/2001
Dimethoxymethane Effect on the Oxidation of Indane
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Figure 1. Indane oxidation in a JSR at 1 atm. The initial conditions were indane, 0.1%; O2, 2.3%, N2, 97.6%; τ ) 0.07 s; φ ) 0.5.
Figure 2. Indane oxidation in a JSR at 1 atm. The initial conditions were indane, 0.1%; O2, 1.15%, N2, 98.75%; τ ) 0.07 s; φ ) 1.
diameter (30.5 cm3) made of fused silica (to prevent wall catalytic reactions), equipped with 4 nozzles of 1 mm i.d. for the admission of the gases which are achieving the stirring. A nitrogen flow of 100 L/h is used to dilute the fuel. As in our previous JSR studies, all the gases are preheated before injection in order to minimize temperature gradients inside the JSR. A regulated heating wires of ∼1.5 kW is used to maintain the temperature of the reactor at the desired working temperature. The reactants are diluted by nitrogen (
