Methyl-3-hexenoate combustion chemistry: Experimental study and numerical kinetic simulation
Introduction
Biodiesel, which is a blend of methyl esters of fatty acids, is a clean-burning renewable fuel as compared to petroleum fuels [1,2]. Derived mostly from the process of transesterification of vegetable oils with methanol, biodiesel has been extensively studied experimentally and numerically during the last decade. This allowed significant advances in understanding the processes occurring during its oxidation and in the development of several detailed kinetic combustion mechanisms [3]. Composition of biodiesel includes unsaturated esters with varying degree of unsaturation, for example methyl oleate, methyl linoleate, methyl linolenate. Moreover, unsaturated esters are known to be important intermediates in combustion and oxidation of saturated esters, as well as biodiesel and its surrogates [4,5]. The unsaturated esters have several additional classes of reactions, comparatively to saturated esters, and different investigations have shown that the presence of one or more double bonds in their alkyl chain considerably affect their chemical and physical properties [2]. Thereby, development of kinetic mechanisms suitable for representing real biodiesel fuels requires a good understanding of the combustion chemistry of unsaturated esters.
In this regard, combustion chemistry of the simplest esters with unsaturated alkyl chain, like methyl-2-butenoate (methyl crotonate) and methyl methacrylate, has been investigated experimentally and numerically [6], [7], [8], [9]. However, these small esters have too short alkyl chains to be regarded as surrogate biodiesel fuels [6,10]. Therefore, combustion chemistry of esters with long alkyl chain and with different degree of unsaturation has been of particular interest over recent years to reveal the impact of C=С double bond on combustion characteristics of biodiesel and its surrogates.
Investigation of methyl oleate (methyl-9-octadecenoate) oxidation in a jet-stirred reactor (JSR) by Bax et al. [11] has shown that unsaturated compounds can produce a specific set of intermediate species as a result of the addition reactions of H, OH or HO2 to the double bond. Several research teams studied autoignition of different fuel blends with large unsaturated esters and all have demonstrated that the position of the double bond in ester alkyl chain has a substantial influence on its oxidation rate [4,12,13]. In particular, the presence of the double bond reduces the rates of radical isomerization reactions if the double bond is embedded in the transition state ring. These reactions normally accelerate the overall rate of low temperature combustion, and thus double bond has a lesser effect if situated near the free end of the hydrocarbon chain. Similar results were obtained by Zhang et al. [14]: experiments on the autoignition of n-heptane/methyl hexanoate and n-heptane/methyl-3-hexenoate fuel blends have shown that unsaturated esters have smaller low-temperature reactivity compared to alkanes and their decomposition pathways include reactions of addition to the double bond.
In further studies, a detailed combustion mechanism for methyl stearate and methyl oleate [15], as well as other components of soy and rapeseed biodiesel fuels [5] (including unsaturated esters) was proposed. This mechanism was validated on experimental data on oxidation of the rapeseed oil methyl esters in a JSR presented earlier by Dagaut et al. [16], and a good agreement was achieved. Additional refinement of this model was performed by Campbell et al. [17] using the data on ignition delay times of methyl oleate and methyl linoleate in an aerosol shock tube. Authors achieved a good agreement with available experimental data by updating thermochemical parameters of the initial fuel molecules in the mechanism.
Rodriguez et al. [18] studied oxidation in a JSR at 1 atm of three components of biodiesel fuels: methyl stearate (saturated alkyl chain), methyl oleate (one C=C double bond in alkyl chain) and methyl linoleate (two C=C double bonds in alkyl chain) to address the effect of unsaturation on chemical kinetics. Authors used their experimental results for the refinement of their lumped kinetic model for biodiesel oxidation published earlier [19] in order to provide a more efficient approach to the simulation of biodiesel fuels. One of the main results of this work is that the mechanism is complemented with the reactions of OH radical addition to the double bond with subsequent decomposition of initial fuel molecules also called Waddington mechanism [20]. This class of reactions has a considerable impact on a low-temperature oxidation of unsaturated species and therefore it was necessary to address difference in low-temperature reactivity between these three esters.
Despite the achieved progress in development of detailed chemical kinetic models for combustion of major components of biodiesel fuels, significant difficulties still remain. Particularly, due to a huge number of species and reactions in the proposed kinetic mechanisms, most reaction rate constants, including those specific for unsaturated esters, are roughly estimated and require revisiting. In this regard, the combustion kinetics of esters with unsaturated aliphatic chain of moderate length, may appear as more simple, however still represent adequate surrogates of biodiesel components. Methyl-3-hexenoate (mhx3d) is one of such representatives (see Fig. 1). However, the studies of its combustion chemistry are fairly scarce [21,22].
Zhang et al. [21] studied the oxidation of methyl-3-hexenoate (mhx3d) in a JSR at 10 atm for 3 different equivalence ratios and used these data to validate a new chemical kinetic mechanism based on their previous studies of methyl hexanoate. A careful refinement of the mechanism provided a very good agreement between experiments and simulations for many intermediate species measured in these conditions. Autoignition of mhx3d at fuel-lean conditions and at 10.5 atm was studied by Wagnon et al. [22] using a rapid compression machine (RCM). Simulations were performed with a new mechanism based on Herbinet et al. [12] and the mechanism of Zhang et al. [21]. This work identified several reaction pathways which require a more thorough investigation due to significant differences in reaction rate constants compared to saturated esters.
This work represents a continuation in the development of a detailed kinetic mechanism for combustion and oxidation of methyl-3-hexenoate (mhx3d) to achieve a better understanding of combustion chemistry of unsaturated esters. As seen from the literature survey provided above, only experimental data on mhx3d oxidation and autoignition at about 10 atm [21,22] are currently available. Further refinement of the chemical kinetic model for mhx3d combustion needs a more extended experimental database.
In this work, we present a combined experimental investigation of the oxidation and combustion characteristics of mhx3d using the facilities at ICARE-CNRS (speciation measurements in JSR at 1 atm), Université de Bourgogne (ignition delay time measurements at 20 and 40 bar), Lund University (laminar burning velocity measurements at 1 atm), and ICKC (speciation measurements in burner-stabilized laminar premixed flame at 1 atm). Our original aim was to extend the available experimental database with the new measurement results and to validate the chemical kinetic mechanism previously developed for oxidation of mhx3d [21] against the novel data. However, disagreements between the predictions and measurements motivated us to significantly revise the mechanism in order to provide a more robust version, able to reproduce the wide range of experimental data. Particularly, in order to adequately predict the laminar burning velocities, the base hydrocarbon chemistry in the original mechanism is replaced with the corresponding set of reactions adopted from the AramcoMech 3.0 [23]. The revision of thermochemistry data for the fuel molecule and fuel radicals allowed significant improvement of predictions of chemical speciation in the JSR at 1 atm and ignition delays measured at 20 and 40 bar. Further details are given in the sections below.
Section snippets
Speciation measurements in jet-stirred reactor at 1 atm
Mhx3d oxidation under atmospheric pressure has been conducted in the experimental facility described in detail in previous publications [21,24,25]. The JSR, 4 cm in diameter, was made of fused silica and shown schematically in the Supplementary Material 2 in Fig. 2.1S. This reactor was used earlier for studies of mhx3d oxidation at 10 atm [21]. In the current work, similarly to the previous study [21], three mixtures of different equivalence ratios (φ = 0.6, 1.0 and 2.0) were studied with a
Kinetic mechanism development
Substantial efforts have been undertaken in this work to update the detailed chemical kinetic mechanism for mhx3d oxidation developed earlier by Zhang et al. [21]. The base hydrocarbon chemistry in the updated version is replaced by that of the recent version of Aramco mechanism 3.0 [23]. The sub-mechanism for mhx3d and smaller methyl esters (from methyl acetate up to methyl pentenoate) was also considerably revised to achieve an adequate reproduction of all available experimental data related
Results and discussion
In this section, we provide a comparison between predictions with the updated mechanism and the experimental data, which are relevant to oxidation and combustion of mhx3d, available in the literature and the novel data reported in this study. All new experimental data presented in this work can be found in Supplementary Material 4 in digital form.
Figure 3 demonstrates a comparison between the measurements and numerical simulation of laminar burning velocity of mhx3d/air mixtures in a range of
Conclusion
An updated detailed chemical kinetic mechanism for oxidation and combustion of methyl-3-hexenoate was proposed in this work. The AramcoMech 3.0 detailed chemical kinetic model for oxidation of small hydrocarbon and oxygenated intermediates was used as a base submechanism. The reactions for mhx3d and smaller methyl esters (from methyl acetate up to methyl pentenoate) were adopted from the mechanism proposed earlier [21] for mhx3d oxidation. The molecule geometries of the fuel and the fuel
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Support from the CAPRYSSES project (ANR- 11-LABX-006–01) funded by ANR through the PIA (Programme d'Investissement d'Avenir) and Région Bourgogne-Franche Comté are gratefully acknowledged. D.A.K. is grateful to the Ministry of Science and Higher Education of the Russian Federation for the financial support (project №075-15-2019-1878).
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