Article pubs.acs.org/EF
Experimental and Kinetic Modeling Study of 3‑Methylheptane in a Jet-Stirred Reactor F. Karsenty,† S. M. Sarathy,*,‡ C. Togbé,† C. K. Westbrook,§ G. Dayma,† P. Dagaut,† M. Mehl,§ and W. J. Pitz§ †
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, France ‡ Clean Combustion Research Center, King Abdullah University of Science and Technology, Thuwal, Makkah 23955-6900, Kingdom of Saudi Arabia § Lawrence Livermore National Laboratory (LLNL), 7000 East Avenue, Livermore, California 94550, United States S Supporting Information *
ABSTRACT: Improving the combustion of conventional and alternative fuels in practical applications requires the fundamental understanding of large hydrocarbon combustion chemistry. The focus of the present study is on a high-molecular-weight branched alkane, namely, 3-methylheptane, oxidized in a jet-stirred reactor. This fuel, along with 2-methylheptane, 2,5-dimethylhexane, and n-octane, are candidate surrogate components for conventional diesel fuels derived from petroleum, synthetic Fischer−Tropsch diesel and jet fuels derived from coal, natural gas, and/or biomass, and renewable diesel and jet fuels derived from the thermochemical treatment of bioderived fats and oils. This study presents new experimental results along with a low- and hightemperature chemical kinetic model for the oxidation of 3-methylheptane. The proposed model is validated against these new experimental data from a jet-stirred reactor operated at 10 atm, over the temperature range of 530−1220 K, and for equivalence ratios of 0.5, 1, and 2. Significant effort is placed on the understanding of the effects of methyl substitution on important combustion properties, such as fuel reactivity and species formation. It was found that 3-methylheptane reacts more slowly than 2-methylheptane at both low and high temperatures in the jet-stirred reactor.
1. INTRODUCTION Detailed chemical kinetic combustion models of real fuels (e.g., gasoline, diesel, and jet fuels) are important tools for improving the design, efficiency, and environmental performance of combustion technologies. A recent review paper by Pitz and Mueller1 describing the development of diesel surrogate fuel models concluded that major research gaps remain in modeling high-molecular-weight (i.e., C8 and greater) aromatics, alkyl aromatics, cyclo-alkanes, and lightly branched iso-alkanes. The focus of the present research study is on a high-molecularweight monomethylated alkane, namely, 3-methylheptane. Monomethylated alkanes are important components of conventional diesel and jet fuels derived from petroleum,1,2 synthetic Fischer−Tropsch diesel and jet fuels derived from coal, natural gas, and/or biomass,3,4 and renewable diesel and jet fuels derived from thermochemical treatment of bioderived fats and oils [e.g., hydrotreated renewable jet (HRJ) fuels].5,6 For example, a recent detailed composition reported by Smith and Bruno3 for conventional Jet A and a synthetic Fischer−Tropsch paraffinic jet fuel indicate that monomethylated alkanes make up the largest fraction of all alkanes found in Jet A and the largest fraction of any organic structure found in purely paraffinic S-8. Huber et al.7 have also proposed 3-methyldecane as a surrogate component for synthetic aviation fuel S-8. The feasibility of using monomethylated alkanes (e.g., 3-methylalkanes, 2-methylalkanes, etc.) as surrogates for real fuels is uncertain. One constraining factor is their high cost, © 2012 American Chemical Society
which makes it expensive to run large-scale combustion experiments. Dooley et al.8 showed that a less expensive mixture of n-decane and iso-octane (2,2,4-trimethylpentane) is an appropriate surrogate for a synthetic paraffinic jet fuel (i.e., S-8) rich in monomethylated alkanes because the “distinct chemical functionalities” of the real fuel8 are captured in the surrogate. They also showed that a combination of a n-alkane and a highly branched iso-alkane could match the global reactivity of 2-methylheptane in a flow reactor, “further validating the distinct functionality concept”.8 Mueller et al.9 also proposed the use of iso-cetane (2,2,4,4-heptamethylnonane) as a surrogate for the lightly branched iso-alkanes present in petroleum diesel fuels; however, they also concluded that “it would be interesting to investigate the use of iso-alkanes more representative of the constituents of these hydrocarbon classes in the target fuels”.9 We encourage interested readers to further assess the feasibility of monomethylated alkanes as surrogate fuel components. The purpose of this study is to provide fundamental insights into the effects of fuel molecular structure on important combustion properties, which will in turn offer flexibility in selecting surrogate fuel components. Recent comprehensive experimental and modeling studies on 2-methylalkanes10,11 have shown that mono methylated alkanes (e.g., 2-methylheptane) exhibit notably different combustion Received: May 17, 2012 Revised: July 17, 2012 Published: July 17, 2012 4680
dx.doi.org/10.1021/ef300852w | Energy Fuels 2012, 26, 4680−4689
Energy & Fuels
Article
3-methylheptene species, the location of a double bond is identified by another hyphen identifying the location of the double bond (e.g., 3-methyl-3-heptene is C8H16-3-3). Additional notations are provided to denote radical sites in the molecule, wherein the carbon sites are labeled alphabetically (i.e., a, b, c, etc.) such that the location of the first methyl branch is minimized (Figure 1). In this way, the 3-methyl3-heptyl radical is denoted as C8H17-3c, while the 3-methyl-1-heptyl radical is written as C8H17-3a. The carbon on the methyl branch is labeled with the letter “z” to differentiate it from the other carbons, such that the 3-methylheptyl radical with a radical site on the methyl branch is denoted as C8H17-3z. 2.2. Classes of Reactions and Thermochemical Data. The major classes of elementary reactions considered for the oxidation of 3-methylheptane include 10 high-temperature reaction classes and 20 low-temperature reaction classes. These reaction classes and the selected reaction rate rules for branched alkanes have been described in detail previously.11 The thermodynamic parameters for the species are very important because they are used to determine reverse rate constants. The THERM16 software was used to compute the thermochemical properties of species not present in the 2-methylalkanes model. The THERM group values are from Benson9 and Bozzelli.17 It is noted that the combustion model for 3-methylheptane has been validated under high-temperature conditions against premixed laminar flame speeds15 and counterflow flame data18 and under lowtemperature conditions against shock tube and rapid compression machine ignition delay.19 The proposed model well predicts lowtemperature and negative-temperature coefficient (NTC) auto-ignition behavior of 3-methylheptane, as well as high-temperature premixed flame propagation rates and diffusion flame extinction, ignition, and species profiles. The present study extends the model validation to low-temperature oxidation and speciation targets under premixed stirred reactor conditions. These experiments allow for a more comprehensive validation of the low-temperature oxidation pathways, including concerted elimination reactions leading to alkene formation and reactions producing cyclic ethers.
properties than their linear alkane counterparts (e.g., n-octane), including lower laminar flame speeds and lower low-temperature reactivity. Prior fundamental combustion studies on 3-methylalkanes are limited. In a rapid compression machine ignition study of the heptane isomers (φ = 1, P = 15 atm, and 650−950 K), Silke et al.12 showed that 3-methylhexane exhibits similar reactivity as 2-methylhexane, both of which are less reactive than n-heptane. In a series of comprehensive kinetic modeling studies on the autoignition of heptane isomers, Westbrook et al.13,14 also showed that the reactivity of 3-methylhexane closely mimicked that of 2-methylhexane under specific rapid compression machine conditions; however, potential differences in reactivity may not have been observed because of the narrow range of experimental conditions studied. Recently, Ji et al.15 conducted experimental laminar flame speed measurements of five octane isomers and compared them to chemical kinetic modeling predictions; both experimental and modeling results indicated that 3-methylheptane exhibits indistinguishable laminar flame propagation rates when compared to 2-methylheptane. High-temperature combustion in flames is dominated by the ability of a fuel to populate the hydrogen radical pool,15 and both 3-methylheptane and 2-methylheptane provide similar hydrogen radical populations following secondary chain branching. As will be shown in this study, the chemistry is different in low- and intermediate-temperature combustion systems, wherein the competition between primary OH branching and OH propagation pathways control fuel reactivity. The objective of the current study is to present new experimental data for 3-methylheptane oxidation in a jet-stirred reactor (JSR) at conditions identical to a previous study on 2-methylheptane oxidation.11 The experimental data is used to compare the reactivity of these two fuels, as well as the concentrations of stable intermediates and final products. Furthermore, a recently developed comprehensive detailed chemical kinetic combustion model is developed for 3-methylheptane and validated against the experimental data. The model is then used to understand the effect of the methyl branch location on important combustion phenomenon in the JSR. This 3-methylheptane exercise also aids in testing the selected reaction pathways and rate rules, thus providing a methodology for which similar models could be built for larger 3-methylakanes.
3. EXPERIMENTAL VALIDATION STUDIES The proposed model is validated against a JSR for 3-methylheptane. We used the JSR experimental setup used in previous studies.20,21 It consisted of a small spherical fused-silica reactor (4 cm outside diameter) equipped with four nozzles of 1 mm inner diameter each. High-purity reactants were used: oxygen (99.995% pure) and 3-methylheptane (>98.7% pure from ChemSampCo, CAS 589-81-1). The reactants were diluted with nitrogen (
