Kinetic investigations of combustion of small oxygenated aliphatic hydrocarbons – modeling and experiments

The depletion of oil reserves and the increasingly stringent European Union regulation of air pollution have forced researchers and manufacturers to search for cleaner and sustainable substitutes to petroleum‐based transportation fuels. Biofuels like small aliphatic alcohols are of increasing interest as alternatives to fossil fuels, as they offer long‐term fuel‐source regenerability. As major advantages, bio-alcohols promise to reduce environmental impact and to be ready-to-use, as their employment requires minor adjustments in internal combustion engines. However, only methanol and ethanol have established themselves on the fuel market, while the use of higher homologous is still a research matter. To this end, to make a proper selection of alternative fuels, the main aim of the thesis was to increase the understanding of chemical reaction networks of propyl alcohols combustion. The thesis focused on building a detailed kinetic reaction model capable to predict the decomposition and oxidation processes, as well as the formation of undesired and harmful pollutant, such as NO. Kinetic investigation also included the study of propanal, which is a critical stable intermediate derived from the oxidation of 1-propanol. Moreover, my research was also comprehensive of the experimental investigation of NO formation in methanol flames. The combustion kinetic models developed during my PhD studies were assessed against both new and available burning velocities, as well as against other combustion properties performed with different devices and methods from literature.New laminar burning measurements were performed at atmospheric pressure and different temperature using the heat flux method.NO predictions from the kinetic model were assessed against new quantitative NO mole fraction measurements in the post-flame region. Experiments were performed using saturated laser-induced fluorescence and flames were stabilized using the heat flux burner.The presented combustion models were also compared with the most reliable models from literature and the strengths and weaknesses in the combustion chemistry predictions of such mechanisms were evaluated and discussed

Combustion of propanol isomers : Experimental and kinetic modeling study

In this work an experimental and kinetic modeling study on n-propanol and i-propanol combustion has been performed. New burning velocity measurements were carried out using the heat flux method at 1 atm over the temperature range of 323–393 K. Analysis of the temperature dependence was conducted with to verify the data consistency of the new and available data from the literature. Important inconsistencies were identified with the literature experiments performed using the spherical flame method and the nature of such inconsistencies was discussed. Moreover, a new kinetic mechanism, based on the most recent Konnov model and extended to include C3 alcohol isomers chemistry subset, was validated against new and all available literature data obtained at different combustion regimes. Rate constant parameters were carefully selected by evaluating all experimental and theoretical sources. Moreover, Sarathy et al. (2014) detailed kinetic mechanism was also tested. Overall, both kinetic models reproduce experimental data with good fidelity, but the presented model was found superior in representing ignition delay times data performed at high-pressure conditions

An experimental and kinetic study of propanal oxidation

Propanal is a critical stable intermediate derived from the oxidation of 1-propanol, a promising alcohol fuel additive. To deepen the knowledge and accurately describe propanal combustion characteristics, new burning velocity measurements at different temperatures were carried out and a new detailed kinetic mechanism for propanal was proposed. Experiments were performed using the heat flux method and compared with literature data. Important discrepancies were noted between the new and available data, and possible reasons were suggested. Flow rate sensitivity analysis highlighted that, as expected, the important reactions influencing the propanal oxidation in flames are pertinent to H2 and CO sub-mechanism. Current mechanism is based on the most recent Konnov model, extended to include propanal chemistry subset. Rate constant parameters were selected based on careful evaluation of experimental and theoretical data available in literature. Model validation included assessment against a large set of combustion experiments obtained at different regimes, i.e. flames, shock tubes, and well stirred reactor, as well as comparison with the semi-detailed (lumped) kinetic mechanism for hydrocarbon and oxygenated fuels from Politecnico di Milano, detailed kinetic model from Veloo et al. and low temperature oxidation of aldehydes kinetic model of Pelucchi et al. The proposed model reproduced experimental burning velocities, ignition delay times, flame structure and JSR data with an overall good fidelity, while it reproduces only qualitatively the species distribution of propanal pyrolysis

Experimental and modeling study of nitric oxide formation in premixed methanol + air flames

Validation and further development of models for alcohol combustion requires accurate experimental data obtained under well-controlled conditions. To this end, measurements of nitric oxide, NO, mole fractions in premixed laminar methanol + air flames have been performed using saturated laser-induced fluorescence, LIF. The methanol flames have been stabilized at atmospheric pressure and initial gas temperature of 318 K at equivalence ratios ɸ = 0.7–1.5 using the heat flux method that allows for simultaneous determination of their laminar burning velocity. The LIF signal is converted into NO mole fraction via calibration measurements, which have been performed in flames of methane, methanol and syngas seeded with known amounts of NO. The experimental approach is verified by the measurements of NO mole fractions in the post flame zone of methane flames, investigated in previous studies at similar conditions. Data on the NO formation together with burning velocities for methanol and methane obtained under adiabatic flame conditions provide highly valuable input for model validation. They have been compared with predictions of six different chemical kinetic mechanisms. Summarizing the behavior of all models tested with respect to burning velocities and NO formation in flames of methane and methanol, the mechanism of Glarborg et al. (2018) and the San Diego mechanism (2019) demonstrate uniformly satisfactory performance

Measurements of the laminar burning velocities and NO concentrations in neat and blended ethanol and n-heptane flames

Adiabatic laminar burning velocities and post-flame NO mole fractions for neat and blended ethanol and n-heptane premixed flames were experimentally determined using a heat flux burner and laser-induced fluorescence. The flames were stabilized at atmospheric pressure and at an initial temperature of 338 K, over equivalence ratios ranging from 0.6 to 1.5. These experiments are essential for the development, validation and optimization of chemical kinetic models, e.g. for the combustion of gasoline-ethanol fuel mixtures. It was observed that the addition of ethanol to n-heptane leads to an increase in laminar burning velocity that is not proportional to the ethanol content and to a decrease of NO formation. Such a NO reduction is due to the slightly lower flame temperatures of ethanol, which decrease the production of thermal-NO at 0.6 1.3, the lower NO formation through the prompt mechanism in the ethanol flames is partially offset by a lower rate of NO consumption through the reburning mechanism. New experimental results were compared with predictions of the POLIMI detailed chemical kinetic mechanism. An excellent agreement between measurements and simulated results was observed for the laminar burning velocities over the equivalence ratio range investigated; however, discrepancies were found for the NO mole fractions, especially under rich conditions. Further numerical analyses were performed to identify the main causes of the observed differences. Differences found at close-to stoichiometric conditions were attributed to an uncertainty in the thermal-NO mechanism. In addition, disagreement under rich conditions could be explained by the relative importance of reactions in hydrogen cyanide consumption pathways

Experimental and kinetic modeling study of para-xylene chemistry in laminar premixed flames

The chemistry of para-xylene oxidation in laminar premixed flames has been analyzed using new experimental data on flame propagation at atmospheric pressure and flame structure of low-pressure stoichiometric flame. Atmospheric pressure laminar burning velocities of para-xylene + air flames were determined using the heat flux method at initial temperatures of 328 and 353 K over the equivalence ratio range of ϕ = 0.7–1.4 and of ϕ = 0.7–1.3, respectively. Temperature and mole fraction profiles of reactants, final products, and reactive and stable intermediate species have been measured in laminar premixed CH4/O2/N2 and CH4/1.5%C8H10/O2/N2 flames at low pressure (40 Torr) using thermocouple, molecular beam/mass spectrometry, and gas chromatography/mass spectrometry techniques. These new experimental results have been modeled with our previous model including sub-mechanisms for aromatics (benzene up to p-xylene) and aliphatic (C1 up to C7) oxidation. Good agreement has been observed for the profiles of the main species analyzed. Moreover, chemical pathways for common species in methane flame with and without 1.5% of benzene or 1.5% toluene investigated earlier under similar conditions were analysed and compared to the present flame doped with para-xylene. Key reactions of aromatics degradation in CH4/O2/N2 flames were identified and discussed. Burning velocities of para-xylene + air flames were also reproduced by the kinetic model

The combustion kinetics of the lignocellulosic biofuel, ethyl levulinate

Ethyl levulinate (Ethyl 4-oxopentanoate) is a liquid molecule at ambient temperature, comprising of ketone and ethyl ester functionalities and is one of the prominent liquid fuel candidates that may be easily obtained from lignocellulosic biomass. The combustion kinetics of ethyl levulinate have been investigated. Shock tube and rapid compression machine apparatuses are utilised to acquire gas phase ignition delay measurements of 0.5% ethyl levulinate/O-2 mixtures at phi = 1.0 and phi = 0.5 at similar to 10 atm over the temperature range 1000-1400K. Ethyl levulinate is observed not to ignite at temperatures lower than similar to 1040 K in the rapid compression machine. The shock tube and rapid compression machine data are closely consistent and show ethyl levulinate ignition delay to exhibit an Arrhenius dependence to temperature. These measurements are explained by the construction and analysis of a detailed chemical kinetic model. The kinetic model is completed by establishing thermochemical-kinetic analogies to 2-butanone, for the ethyl levulinate ketone functionality, and to ethyl propanoate for the ethyl ester functionality. The so constructed model is observed to describe the shock tube data very accurately, but computes the rapid compression machine data set to a lesser but still applicable fidelity. Analysis of the model suggests the autooxidation mechanism of ethyl levulinate to be entirely dominated by the propensity for the ethyl ester functionality to unimolecularly decompose to form levulinic acid and ethylene. The subsequent reaction kinetics of these species is shown to dictate the overall rate of the global combustion reaction. This model is then use to estimate the Research and Motored Octane Numbers of ethyl levulinate to be >= 97.7 and >= 93, respectively. With this analysis ethyl levulinate would be best suited as a gasoline fuel component, rather than as a diesel fuel as suggested in the literature. Indeed it may be considered to be useful as an octane booster. The ethyl levulinate kinetic model is constructed within a state-of-the-art gasoline surrogate combustion kinetic model and is thus available as a tool with which to investigate the use of ethyl levulinate as a gasoline additive. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.Research (University of Limerick, Trinity College Dublin & KAUST) reported in this publication was carried out under the Future Fuels project supported by the Competitive Center Funding (CCF) program at King Abdullah University of Science and Technology (KAUST). Research conducted at National University of Ireland, Galway and Trinity College Dublin was supported by Science Foundation Ireland. Computational resources were provided by the Irish Centre for High-End Computing, ICHEC.2020-04-0

The combustion kinetics of the lignocellulosic biofuel, ethyl levulinate

Ethyl levulinate (Ethyl 4-oxopentanoate) is a liquid molecule at ambient temperature, comprising of ketone and ethyl ester functionalities and is one of the prominent liquid fuel candidates that may be easily obtained from lignocellulosic biomass. The combustion kinetics of ethyl levulinate have been investigated. Shock tube and rapid compression machine apparatuses are utilised to acquire gas phase ignition delay measurements of 0.5% ethyl levulinate/O-2 mixtures at phi = 1.0 and phi = 0.5 at similar to 10 atm over the temperature range 1000-1400K. Ethyl levulinate is observed not to ignite at temperatures lower than similar to 1040 K in the rapid compression machine. The shock tube and rapid compression machine data are closely consistent and show ethyl levulinate ignition delay to exhibit an Arrhenius dependence to temperature. These measurements are explained by the construction and analysis of a detailed chemical kinetic model. The kinetic model is completed by establishing thermochemical-kinetic analogies to 2-butanone, for the ethyl levulinate ketone functionality, and to ethyl propanoate for the ethyl ester functionality. The so constructed model is observed to describe the shock tube data very accurately, but computes the rapid compression machine data set to a lesser but still applicable fidelity. Analysis of the model suggests the autooxidation mechanism of ethyl levulinate to be entirely dominated by the propensity for the ethyl ester functionality to unimolecularly decompose to form levulinic acid and ethylene. The subsequent reaction kinetics of these species is shown to dictate the overall rate of the global combustion reaction. This model is then use to estimate the Research and Motored Octane Numbers of ethyl levulinate to be >= 97.7 and >= 93, respectively. With this analysis ethyl levulinate would be best suited as a gasoline fuel component, rather than as a diesel fuel as suggested in the literature. Indeed it may be considered to be useful as an octane booster. The ethyl levulinate kinetic model is constructed within a state-of-the-art gasoline surrogate combustion kinetic model and is thus available as a tool with which to investigate the use of ethyl levulinate as a gasoline additive. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.Research (University of Limerick, Trinity College Dublin & KAUST) reported in this publication was carried out under the Future Fuels project supported by the Competitive Center Funding (CCF) program at King Abdullah University of Science and Technology (KAUST). Research conducted at National University of Ireland, Galway and Trinity College Dublin was supported by Science Foundation Ireland. Computational resources were provided by the Irish Centre for High-End Computing, ICHEC.peer-reviewed2020-04-0

Experimental and Kinetic Modeling Study of Laminar Burning Velocities of Cyclopentanone and Its Binary Mixtures with Ethanol and nPropanol

Cyclopentanone is a promising biofuel that can enable more efficient engine operation and increase the fuel economy of the light duty fleet over current and planned technology developments. While the ignition of cyclopentanone has been investigated in detail, more studies on the laminar burning velocities of cyclopentanone are called for. In this work, the laminar burning velocities of cyclopentanone (C5H8O) have been measured using the heat flux and spherical flame methods at 1 atm, equivalence ratios from 0.7 to 1.6, and initial temperatures of 328, 353, and 428 K. To further investigate the relationship between the molecular structure and laminar burning velocity, identical experiments were also performed for binary mixtures of cyclopentanone with ethanol and n-propanol at 1:1 (mol). The consistency between the experimental data sets obtained in this work and literature data sets has been evaluated. A recently published mechanism of cyclopentanone was used for simulation after adopting the submechanism of n-propanol. Good agreement has been seen between experimental and simulated results for all flames. To qualitatively explain the characteristics of the laminar burning velocity of cyclopentanone and the differences with those of ethanol and n-propanol, sensitivity analysis and reaction pathway analysis have been performed to compare the chemistry of the fuels under flame conditions, which revealed how the molecular structure of cyclopentanone could affect its laminar burning velocity. Compared to ethanol and n-propanol, cyclopentanone does not have primary carbon atoms in its molecule, leading to lower production of methyl radicals. Meanwhile, the carbonyl group in the cyclopentanone molecule is mostly released as CO in the decomposition of multiple intermediates accompanied by the production of unsaturated C2 and C4 species, especially C2H4 and C2H3. Both features contribute to the high laminar burning velocity of cyclopentanone