Article pubs.acs.org/EF
Experimental and Modeling Study of the Oxidation Kinetics of n-Undecane and n-Dodecane in a Jet-Stirred Reactor Amir Mzé-Ahmed,†,‡ Kamal Hadj-Ali,† Philippe Dagaut,*,† and Guillaume Dayma†,‡ †
Institut des Sciences de l’Ingénierie et des Systèmes (INSIS), Centre National de la Recherche Scientifique (CNRS), 1C, Avenue de la Recherche Scientifique, 45071 Orléans cedex 2, France ‡ Faculté des Sciences, 1 Rue de Chartres, Université d’Orléans, BP 6759, 45067 Orléans cedex 2, France S Supporting Information *
ABSTRACT: The kinetics of oxidation of two large n-alkanes (n-undecane and n-dodecane) was studied experimentally in a jetstirred reactor (JSR) at high pressure (P = 10 bar), at temperatures ranging from 550 to 1150 K, at a constant residence time (τ) of 1 s, and for three equivalence ratios (ϕ = 0.5, 1.0, and 2.0). Chemical analyses by Fourier transform infrared (FTIR) spectrometry and gas chromatography allowed for the measurement of the mole fraction of reactants, stable intermediates (including substituted tetrahydrofurans), and final products as a function of the temperature. A similar behavior was observed for the oxidation of n-undecane, n-dodecane, and Jet A-1 in a JSR. However, it was shown that the pure n-alkanes oxidized faster than Jet A-1 under cool-flame conditions and intermediately yielded more ethylene. A kinetic reaction mechanism based on previous studies1,2 was developed and validated by a comparison to the present experimental results. The proposed reaction mechanism consisted of 5864 reversible reactions involving 1377 species. Experimental data and simulation results obtained in the current work were compared to simulations performed with a literature model.3 Our model was also applied successfully to the modeling of the oxidation of n-dodecane under shock-tube conditions.4,5 Species time histories and ignition delay times representing valuable complementary tests were simulated.
1. INTRODUCTION Aviation fuels are very complex mixtures of hydrocarbons.6,7 To simulate the oxidation of these fuels, we have to use simple model fuels (surrogate) consisting of a few representative hydrocarbons. The compounds identified in jet fuels (Jet A-1, Jet A, and JP-8) at the highest levels of concentration are n-alkanes.7 Previous studies showed a strong similarity between the oxidation of kerosene and n-decane under the same initial experimental conditions (P = 10 bar; T = 550−1150 K; τ = 1 s; and ϕ = 0.5, 1.0, and 2.0) in a jet-stirred reactor (JSR).6−10 However, whereas the chemical formula of Jet A-1 is close to C11H22,7 the n-decane formula is C10H22. Therefore, n-alkanes larger than C10 could be preferred to represent Jet A-1. Furthermore, synthetic paraffinic kerosene (SPK) has high concentrations of n-alkanes (∼20 vol %),1 which further increases the interest for studying the kinetics of oxidation of long-chain n-alkanes. The purpose of this study is to expand the work performed previously on the kinetics of oxidation of n-alkanes using a pressurized JSR. To date, no data are available for the kinetics of oxidation of C11, C12, and C13−C15 n-alkanes under JSR conditions, whereas data are available for the oxidation of C10 and C16 n-alkanes and for the pyrolysis of n-dodecane.11 This work intends to provide new data to fill this gap and also to propose a validated kinetic scheme. Therefore, new experiments were performed for the oxidation of two large n-alkanes (n-undecane and n-dodecane). A detailed kinetic reaction mechanism based on the work by Diévart2 was developed for modeling the oxidation of these compounds. It was validated by comparison to the present experimental results and complementary data taken from the literature.4,5 © 2012 American Chemical Society
2. EXPERIMENTAL SECTION The experiments were performed using a JSR presented and used earlier.1,12 The reactor is a small sphere of 33 cm3 in volume made of fused silica to minimize wall catalytic reactions. The gas mixture is introduced into the reactor through four nozzles (1 mm inner diameter). The nozzles are opposite in pairs to make the gas mixture more homogeneous. Two insulated heating elements surrounding the reactor allow for heating of the reaction zone to the desired temperature. A nitrogen flow of 100 L/h was used to dilute the fuel before admission in the reactor. All gases were preheated before injection to minimize temperature gradients inside the JSR. A high-performance liquid chromatography (HPLC) pump (Shimadzu LC10 ADVP) was used to deliver the liquid fuel to an atomizer−vaporizer assembly maintained at ca. 550 K. Therefore, the fuel was atomized and vaporized before injection into the reactor. The reactants were diluted by a flow of nitrogen ( ∼740 K). For the kinetic modeling of n-dodecane oxidation, the same trends as for the oxidation of n-undecane were observed. Also, whereas the kinetic model agrees with the experimental trends, at ϕ = 1 and 2, the computed rates of consumption of n-dodecane were overestimated by up to a factor of 2. Measured and simulated concentration profiles showed that the fuel conversion at the end of the NTC region decreases as the initial concentration of oxygen decreases. Good agreement Table 2. Minor Species Identified by GC−MS oxygenated species
large alkenes
propanal butanal pentanal hexanal acetone butanone pentanone
1-hexene 1-heptene 1-octene 1-nonene
Figure 8. Main reaction paths for the oxidation of n-undecane in a JSR at ϕ = 1 and T = 650 K (P = 10 bar and τ = 1 s). Species identified by GC−MS appear in red. The thickness of arrows indicates the importance of the reactions. 4261
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Figure 9. Main reaction paths for the oxidation of n-undecane in a JSR at ϕ = 1 and T = 1030 K (P = 10 bar and τ = 1 s). Species identified by GC−MS appear in red. The thickness of arrows indicates the importance of the reactions.
Figure 10. Sensitivity spectrum for n-undecane during the oxidation of undecane in a JSR at ϕ = 1 and T = 650 K (P = 10 bar and τ = 1 s).
Figure 11. Sensitivity spectrum for n-undecane during the oxidation of undecane in a JSR at ϕ = 1 and T = 1030 K (P = 10 bar and τ = 1 s).
produce 1-olefins and 1-alkyl radicals. In summary, at a low temperature, the oxidation mechanism takes places via peroxidation−isomerization processes, and at a high temperature, bond breaking reactions predominate. The present sensitivity analyses showed (Figure 10) that the metathesis reaction CH2O + OH ⇌ HCO + H2O (S = 0.277; the rate constant taken from Tsang and Hampson18 has an uncertainty of A ∼ E. This is true at low and high temperatures. However, at a high temperature, the system is mostly sensitive to H + O2 ⇌ OH + O, whereas metathesis reactions become less influential, as observed in Figure 11. The reactions HO2 + OH ⇌ H2O + O2 (S = 0.193; the rate constant taken from Keyser20 has an uncertainty of 750 K). Overall, results show a similar oxidation behavior for the three fuels under the same JSR conditions. Then, we compared the concentrations of several intermediate species (CH4, C2H4, 1,3-C4H6, and CH3CHO) measured during the oxidations of the three fuels. These species are considered as potential pollutants emitted by incomplete combustion of fuels. In Table 3, we report the maximum mole fractions of these pollutants for three equivalence ratios (ϕ = 0.5, 1.0, and 2.0). Similar maximum mole fractions were observed for most of these pollutants. However, the maximum concentration of ethylene was ∼40% less in the case of Jet A-1, because of the presence of hydrocarbons yielding less ethylene via β-scissions than n-alkanes.
5. CONCLUSION The kinetics of oxidation of n-undecane and n-dodecane was experimentally studied in a JSR under the same initial conditions (T = 550−1150 K; P = 10 bar; equivalence ratios of 0.5−2; 1000 ppm of fuel; and at a constant mean residence time τ = 1 s). A similar behavior was observed for the oxidation of n-undecane, n-dodecane, and a conventional Jet A-1 in a JSR over the high-temperature oxidation regime. However, the pure n-alkanes were more reactive than Jet A-1 under cool-flame conditions. A chemical kinetic reaction mechanism involving 1377 species and 5865 reversible reactions was proposed on the basis of previous work on the kinetics of oxidation of n-alkanes. Computed results were compared to the experimental data obtained here for the oxidation of n-undecane and n-dodecane in a JSR and for the ignition of these fuels in shock tubes.12,13 Computations showed that the detailed kinetic mechanism developed here allows for correctly simulating the present JSR experiments, whereas the model overestimates the ignition delays of n-dodecane under cool-flame conditions. Overall, the proposed model seems to perform better than previously proposed models. Sensitivity and reaction path analyses were used to interpret the results. At low temperatures, the oxidation of the fuels proceeds via peroxidation−isomerization routes, and at high temperatures, bond breaking reactions predominate.
Volumetric composition: 13.5% n-alkanes, 28.4% isoalkanes, 24% cycloalkanes, and 30% aromatics.
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ASSOCIATED CONTENT
S Supporting Information *
Kinetic model used here in CHEMKIN format. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +33-238-255-466. Fax: +33-238-696-004. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
a
852 (1030 K) 761 (945 K) 22 (870 K) 60 (800 K) 65 (800 K) 321 (870 K) 530 (870 K) 12 (870 K) 64 (620 K) 83 (800 K) CH4 C2H4 1,3-C4H6 CH3CHO
194 (870 K) 323 (870 K) 8.4 (800 K) 89 (650 K) 80 (800 K)
205 (850 K) 595.5 (850 K) 12 (810 K) 142 (610 K) 170 (700 K)
ϕ = 1.0
321 (900 K) 918.8 (850 K) 18 (810 K) 134 (670 K) 96 (810 K)
1214 (1075 K) 1209 (1075 K) 31.3 (850 K) 118 (640 K) 99 (810 K)
ϕ = 1.0 ϕ = 0.5 ϕ = 2.0
n-undecane
ϕ = 0.5 ϕ = 2.0 ϕ = 1.0 ϕ = 0.5 unburned HC
Xmax (Tmax)
Jet A-1a
Table 3. Maximum Mole Fractions (X, ppm) of the Main Pollutants and Unburned, Formed at ϕ = 0.5, 1.0, and 2.0 in the JSR
n-dodecane
ϕ = 2.0
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ACKNOWLEDGMENTS Amir Mzé-Ahmed is grateful to the French Ministry of Education and Research for a doctoral grant and for an ATER position at the University of Orléans (2011−2012). The authors thank Dr. Pascal Diévart for his help.
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