Shock tube studies and chemical kinetic modeling of oxygenated hydrocarbon ignition

As a contribution towards understanding, modeling and controlling the combustion of oxygenated hydrocarbons such as biofuels, the high-temperature ignition of a series of relevant molecules has been investigated behind reflected shock waves at pressures ranging from 1 atm to 13 atm. Short chain biodiesel surrogates, methyl and ethyl esters, have been investigated. Methyl esters of formic to butanoic acids have been investigated in order to uncover the trends in their ignition delay times. The trends have further been explored by means of computational quantum chemical calculations. While most of these surrogates portray similar ignition behavior, the influence of structure with respect to terminal methyl groups and the presence or absence of secondary C–H bonds have been observed as in the case of methyl acetate with longer ignition delay times. The role of the alkyl group on the ester reactivity has been investigated by comparing methyl and ethyl esters, with the result that ethyl esters are generally more reactive. Apart from these biodiesel surrogates, selected C3 oxygenates, relevant to combustion have been investigated. A chemical kinetic mechanism for the high-temperature oxidation of propanal is developed and tested. Propanal, like other aldehydes, belongs to the group of intermediate species which occur in the combustion of almost all hydrocarbons, but their accurate prediction in combustion modeling is challenging. Targeted studies of the submodels of these compounds are expected to contribute towards predictive chemical kinetic modeling. The developed mechanism also shows encouraging performance in the prediction of acetaldehyde (ethanal) ignition. Ethanol is another biofuel widely used in spark-ignition engines. There is also interest in using this fuel in diesel and Homogeneous Charge Compression Ignition (HCCI) engine concepts. This is in line with the need for tailor-made, flexible fuels for wide range applications in energy conversion. Ethanol ignition modification by isopropyl nitrate (IPN), isopropyl formate (IPF) and water has been investigated. It is found that whereas IPN improves the ignition performance of ethanol (shorter ignition delay times), IPF increases its ignition resistance (longer ignition delay times), so that it can be used as an anti-knock agent. It is further observed that at temperatures above 1400 K, IPN addition ceases to improve the ignition of ethanol. Wet ethanol ignition reveals that at the same post-shock temperature water has an ignition promoting effect. The feasibility of igniting wet ethanol raises the prospect of reducing ethanol production cost (distillation) by using ethanol with allowable water content, albeit with a lower specific energy content. The ignition behavior of the biodiesel surrogate, methyl butanoate, and the diesel surrogate, n-heptane, is compared. Similar behavior is observed under stoichiometric conditions, with slight differences under rich conditions. A skeletal mechanism is proposed for the combustion of blends of the two surrogates. The skeletal mechanism is derived from reduced skeletal mechanisms of literature mechanisms for n-heptane and methyl butanoate obtained on the basis of extensive ignition sensitivity analyses and chemical kinetic insight. These reduced skeletal models have been found to perform reasonably well when compared to predictions by their original detailed mechanisms with respect to ignition, flame propagation and the structure of an opposed flow flame in the mixture fraction space. A systematic approach has been taken in this work to compare the reactivity of fuels, which leads to insight on trends, similarities and differences in global ignition behavior. The combination of experiments, analyses, computations and modeling demonstrates the synergy required to address problems in modern combustion science and technology.En tant que contribution à la compréhension, la modélisation et le contrôle de la combustion des hydrocarbures oxygénés tels que les biocarburants, l'auto-allumage à haute température d'une série de molécules a été étudiée avec la méthode de tube a onde de choc pour les pressions entre 1 atm et 13 atm. Les molécules représentatives du biodiésel, c'est à dire des esters méthyliques et éthyliques, ont été étudiées. Les esters méthyliques d'acide formique jusqu'à butanoique ont été étudiés afin de découvrir l'influence de leurs structures sur l'auto-allumage. Cette relation a aussi été examinée avec les calculs de la chimique quantique. Alors que la pluparts de ces esters sont marqués par des délais d'auto-allumage similaires, les influences des groupes méthyliques terminales, et la présence ou absence des liaisons secondaires de C-H, ont été identifiées, comme dans le cas d'acétate de méthyle, caractérisé par les plus longs délais. Le rôle du groupe alkyle sur la réactivité d'ester a été étudié en comparant des esters méthyliques avec les esters éthyliques. Les esters éthyliques sont généralement plus réactifs que les esters méthyliques du même acide. De la même manière, sont investigués quelques hydrocarbures oxygénés, dont leur cinétique d'oxydation est impliquée dans la combustion des biocarburants et carburants pétrolifères. Un mécanisme de la cinétique chimique pour la combustion du propanal à haute température a été développé et validé. Le propanal, comme d'autres aldéhydes, appartient au groupe des espèces intermédiaires qui se forment pendant la combustion de presque tous les hydrocarbures, mais leur modélisation reste imprécise. Des études consacrées à la compréhension des sous-modèles de ces molécules devraient contribuer à la modélisation avancée de la cinétique chimique de la combustion. Le mécanisme proposé prédit aussi les délais d'auto-allumage d'acétaldéhyde, dont le sous-mécanisme est inclus. L'éthanol est un biocarburant largement utilisé dans les moteurs à allumage commandé. Il y a également intérêt à utiliser ce carburant dans les moteurs à allumage par compression. Ceci est en accord avec la nécessité de développer des carburants flexibles pour des moteurs divers. La modification de l'auto-allumage de l'éthanol par des additifs chimiques comme le nitrate d'isopropyle (IPN), le formiate d'isopropyle (IPF) et l'eau a été investiguée. Il se trouve que, alors que l'IPN améliore la tendance à l'auto-allumage de l'éthanol (délais plus courts), l'IPF augmente sa résistance à l'autoallumage, de sorte que ce dernier peut être utilisé comme additif pour supprimer l'autoallumage. Pour une même température, l'auto-allumage de l'éthanol contenant de l'eau se révèle accélérée. Un mécanisme pour la combustion des mélanges de diesel et du biodiesel est également proposé. Le mécanisme est dérivé de la réduction des mécanismes détaillés pour le n-heptane et le butanoate de méthyle obtenus sur la base de l'analyse de sensitivité de l'auto-allumage. Cette méthode comparative systématique et innovatrice cherche à caractériser les propriétés des carburants oxygénés en vue de révéler les similitudes et les différences. Les résultats servent à l'optimisation des modèles cinétiques chimiques ainsi qu'à la compréhension de la cinétique de combustion d'une série d'espèces oxygénées. Des corrélations de délais d'auto-allumage sont également proposées pour l'application pratique. Le mécanisme proposé pour les mélanges diesel et biodiesel se prête à l'étude de la combustion dans les écoulements turbulents

Recent Trends in the Production, Combustion and Modeling of Furan-Based Fuels

There is growing interest in the use of furans, a class of alternative fuels derived from biomass, as transportation fuels. This paper reviews recent progress in the characterization of its combustion properties. It reviews their production processes, theoretical kinetic explorations and fundamental combustion properties. The theoretical efforts are focused on the mechanistic pathways for furan decomposition and oxidation, as well as the development of detailed chemical kinetic models. The experiments reviewed are mostly concerned with the temporal evolutions of homogeneous reactors and the propagation of laminar flames. The main thrust in homogeneous reactors is to determine global chemical time scales such as ignition delay times. Some studies have adopted a comparative approach to bring out reactivity differences. Chemical kinetic models with varying degrees of predictive success have been established. Experiments have revealed the relative behavior of their combustion. The growing body of literature in this area of combustion chemistry of alternative fuels shows a great potential for these fuels in terms of sustainable production and engine performance. However, these studies raise further questions regarding the chemical interactions of furans with other hydrocarbons. There are also open questions about the toxicity of the byproducts of combustion

Reactivity Trends in Furan and Alkyl Furan Combustion

A systematic study of the ignition behavior of furan and the substituted furans 2-methyl furan (2-MF) and 2,5-dimethyl furan (DMF) is presented. Ignition delay times are measured over a temperature range from 977 to 1570 K and pressures up to 12 atm for lean, stoichiometric, and rich mixtures of fuel, oxygen, and argon. It is found that when the equivalence ratio ϕ, the argon-to-oxygen ratio <i>D</i>, and pressure <i>p</i> are kept constant over a range of temperatures <i>T</i>, DMF generally has the longest, while 2-MF has the shortest, ignition delay times, and furan shows intermediate reactivity. Ignition delay times decrease with increasing equivalence ratios, except for DMF, which does not show a conclusive trend over the temperature range investigated. The experimental data are also found to agree with published ignition data, showing differences in some cases partly related to disparities in endwall and sidewall ignition measurements. The ignition delay times of 2-MF and DMF are compared to predictions using furan chemical kinetic models by Sirjean et al. and Somers et al. The models show qualitatively that DMF has longer ignition delay times than 2-MF under similar conditions of ϕ, <i>D</i>, <i>p</i>, and <i>T</i>, as revealed by the experiments. Quantitatively, the model predictions agree with experimental data at conditions similar to those used in their development, and deviations from experiment at other conditions are mostly related to unmatched temperature sensitivities over a wider temperature range, revealed by varying pressure and reduced dilution. The reported experimental data set contributes toward further understanding and improved modeling of the combustion of furans, a promising class of alternative fuels

Skeletal Chemical Kinetic Mechanisms for Syngas, Methyl Butanoate, <i>n</i>‑Heptane, and <i>n</i>‑Decane

Skeletal chemical kinetic mechanisms are presented for combustion analysis of a series of fuels of interest in combustion systems. These models are obtained from their respective detailed chemical kinetic models using the global species sensitivity method in a formulation referred to here as alternate species elimination (ASE), reflecting the alternate elimination of chemical species from a mechanism in order to assess the resulting effect on the prediction ability of the model. Ignition delay times are used as the target global combustion property for the assessment of the chemical influence of a species. Three ignition conditions of lean, stoichiometric, and rich fuel/air mixtures at a temperature and pressure of 1050 K and 15 atm, respectively, are used to generate data for the model reduction process. The skeletal mechanisms obtained from this ignition-based reduction are tested for their ability to predict premixed flame propagation and diffusion flame structure. It is found that, by imposing an appropriate threshold on the ranked normalized changes in ignition delay times, these skeletal models capture a broad range of combustion processes beyond the homogeneous ignition process used to deduce them. The skeletal mechanisms presented in this work include syngas (31 species), methyl butanoate (MB) (88 species), <i>n</i>-heptane (122 species), and <i>n</i>-decane (89 species). These skeletal models reflect a reduction of at least 60% in the number of chemical species with respect to the detailed model. They are recommended for use in further computational combustion analysis since they result in a reduction in computational costs, and are provided as Supporting Information to this article

Comparative High Temperature Shock Tube Ignition of C1−C4 Primary Alcohols

The high temperature ignition of C1−C4 primary alcohols, methanol, ethanol, <i>n</i>-propanol, and <i>n</i>-butanol, is studied behind reflected shock waves. The experiments are carried out at pressures of 2, 10, and 12 atm with argon/oxygen ratios of 10, 15, and 20 under lean, ϕ = 0.5, stoichiometric, ϕ = 1, and rich, ϕ = 2, conditions between 1070 and 1760 K. It is observed that the ignition delay time data for ethanol, <i>n</i>-propanol, and <i>n</i>-butanol collapse under conditions of constant equivalence ratio, pressure, and dilution. The ignition delay times of methanol are comparable with the other alcohols but show a slightly lower activation energy than the other fuels. The observed collapse of the ignition delay times for the four alcohols under lean-to-stoichiometric conditions is comparable with recent observations by Veloo et al. (Veloo, P. S.; Wang, Y. L.; Egolfopoulos, F. N.; Westbrook, C. K. <i>Combust. Flame</i> <b>2010</b>, <i>157</i>, 1989−2004) that over a range of equivalence ratios less than one, methanol, ethanol, and <i>n</i>-butanol have similar laminar flame speeds. Measured ignition delay times for selected conditions are compared to simulated delay times using their corresponding chemical kinetic models developed in previous studies: methanol by Li et al. (Li, J.; Zhao, Z.; Kazakov, A.; Chaos, M.; Dryer, F.; Scire, J. <i>Int. J. Chem. Kinet.</i> <b>2007</b>, <i>39</i>, 109−136), ethanol by Marinov (Marinov, N. <i>Int. J. Chem. Kinet.</i> <b>1999</b>, <i>31</i>, 183−220), <i>n</i>-propanol by Johnson et al. (Johnson, M. V.; Goldsborough, S. S.; Serinyel, Z.; O’Toole, P.; Larkin, E.; Malley, G.; Curran, H. J. <i>Energy Fuels</i> <b>2009</b>, <i>23</i>, 5886−5898), and <i>n</i>-butanol by Sarathy et al. (Sarathy, S. M.; Thomson, M. J.; Togbé, C.; Dagaut, P.; Halter, F.; Mounaim-Rousselle, C. <i>Combust. Flame</i> <b>2009</b>, <i>156</i>, 852−864). The agreement of the various mechanisms with experiment is reasonable; however, the lower temperature ignition delay times for <i>n</i>-propanol and <i>n</i>-butanol tend to be longer than measured. The closest agreement between experiment and model predictions is observed for ethanol with the Marinov mechanism. Ignition delay time correlations for the alcohols are obtained by linear regression of the experimental data. This correlation method is also applied to the chemical kinetic mechanisms to obtain simplified expressions for their ignition delay times that allow a general assessment of their performance relative to experiment. Although sensitivity and reaction pathway analyses indicate similar modeling approaches for the four alcohols, the models do not capture the quantitative similarity observed in the experiment. These results will be useful in the process of developing a generalized chemical kinetic model for C1−C4 primary alcohol combustion as well as reduced models for combustion engineering