A high-temperature study of 2-pentanone oxidation: experiment and kinetic modeling

doi:10.1016/j.proci.2018.05.039

Proceedings of the Combustion Institute

Volume 37, Issue 2, 2019, Pages 1683-1690

https://doi.org/10.1016/j.proci.2018.05.039 Get rights and content

Abstract

Small methyl ketones are known to have high octane numbers, impressive knock resistance, and show low emissions of soot, NO_x, and unburnt hydrocarbons. However, previous studies have focused on the analysis of smaller ketones and 3-pentanone, while the asymmetric 2-pentanone (methyl propyl ketone) has not gained much attention before. Considering ketones as possible fuels or additives, it is of particular importance to fully understand the combustion kinetics and the effect of the functional carbonyl group. Due to the higher energy density in a C₅-ketone compared to the potential biofuel 2-butanone, the flame structure and the mole fraction profiles of species formed in 2-pentanone combustion are of high interest, especially to evaluate harmful species formations. In this study, a laminar premixed low-pressure (p = 40 mbar) fuel-rich (ϕ = 1.6) flat flame of 2-pentanone has been analyzed by vacuum-ultraviolet photoionization molecular-beam mass-spectrometry (VUV-PI-MBMS) enabling isomer separation. Quantitative mole fraction profiles of 47 species were obtained and compared to a model consisting of an existing base mechanism and a newly developed high-temperature sub-mechanism for 2-pentanone. High-temperature reactions for 2-pentanone were adapted in analogy to 2-butanone and n-pentane, and the thermochemistry for 2-pentanone and the respective fuel radicals was derived by ab initio calculations. Good agreement was found between experiment and simulation for the first decomposition products, supporting the initial branching reactions of the 2-pentanone sub-mechanism. Also, species indicating low-temperature chemistry in the preheating zone of the flame have been observed. The present measurements of a 2-pentanone flame provide useful validation targets for further kinetic model development.

Introduction

With a rising energy demand and the objective to reduce global warming, a transition from the combustion of fossil fuels in the transport sector to second-generation biofuels is desired, because it may eventually contribute to a reduced net carbon emission. As representatives of this category of fuels, small methyl ketones like acetone (RON = 110-117 [1], [2]) and 2-butanone (RON = 117 [3]) show impressive knock resistance. 2-butanone, for example, was tested in a SI engine as neat fuel and showed low emissions of soot, NO_x, and unburnt hydrocarbons compared to a RON 95 fuel blend, ethanol, and 2-methylfuran [3]. Nonetheless, studies investigating the combustion behavior and properties of 2-pentanone (methyl propyl ketone, MPK) are scarce, while the symmetric isomer 3-pentanone (diethyl ketone, DEK) gained more attention [4], [5], [6]. Compared to 2-butanone and 3-pentanone with one or two ethyl side chains, respectively, 2-pentanone has a propyl side chain, which could lead to changes in the underlying kinetics and to a lower effect of the carbonyl group. With a higher energy density, C₅-ketones could be preferred in engine applications, with unknown formation of toxic and harmful species, however. While, to the best of our knowledge, (mass) production pathways from biomass for 2-pentanone remain to be developed, this study attempts to gain further insight into the combustion behavior of small methyl ketones.

In a premixed flame study [7], it was shown that 2-butanone exhibits very low emissions of oxygenated intermediates and soot precursors. With a linear alkyl chain of three carbon atoms in 2-pentanone, the formation of soot precursors like C₃H₃ could be increased. Minwegen et al. [8] measured the ignition delay times of a series of small linear ketones, including 2-pentanone, at 20 and 40 bar in a shock tube. High temperature measurements of reactions of small linear ketones with OH were performed at 1-2 atm by Lam et al. [9]. Furthermore, Badra et al. [10] experimentally investigated the H-abstraction by OH of a series of larger ketones. In theoretical work by Hudzik and Bozzelli [11], the thermochemistry and bond dissociation energies of ketones were calculated.

In this study, a laminar premixed low-pressure (40 mbar) fuel-rich (ϕ = 1.6) flat flame of 2-pentanone was quantitatively analyzed by vacuum-ultraviolet photoionization molecular-beam mass-spectrometry (VUV-PI-MBMS). To facilitate comparison with published data on 2-butanone [7], identical conditions (setup, stoichiometry, pressure, argon dilution) were chosen. For the first time for 2-pentanone, 47 species were measured, quantified and isomers were separated whenever possible.

Complementing the experimental data set, a kinetic model, representing the high-temperature chemistry, is presented here. For this kinetic model, the thermochemistry (heat of formation, entropy, and heat capacity) of the fuel and the corresponding fuel radicals was determined by ab initio quantum mechanical calculations.

Section snippets

Experiments

A laminar premixed fuel-rich flame of 2-pentanone/oxygen/argon (0.093/0.407/0.500) was investigated at 40 mbar, equivalence ratio of 1.6 and cold gas velocity of 73.85 cm/s (at inlet conditions of 333 K and 40 mbar, 2.574 cm/s at 298 K and 1 atm) by VUV-PI-MBMS [12], [13] at the Advanced Light Source (ALS) in Berkeley. Gas flow rates of oxygen and argon were metered by calibrated mass-flow controllers with a precision of 5%, while liquid 2-pentanone was injected into a heated vaporizer system

Kinetic model and flame simulations

The presented kinetic model is based on the latest AramcoMech 2.0 [19] updated with the high-temperature sub-mechanism of 2-butanone by Hemken et al. [20] and a small-aromatic sub-mechanism by Zhang et al. [21]. This model represents the high-temperature kinetics of 2-pentanone, consists of 520 species and 2960 reactions and is provided in Chemkin format in Supplemental Material 2. The high-temperature reactions include the unimolecular decomposition of the fuel, H-atom abstraction, and fuel

Results and discussion

The following section provides the experimental results and the comparison with the simulations using the present model. First, the main species occurring during the combustion of 2-pentanone will be discussed and then the primary intermediates will be presented along with the fuel decomposition scheme. Finally, some species indicating low-temperature chemistry will be discussed which normally are not observable in the hot flame. Because only selected mole fraction profiles are shown here,

Summary and conclusions

Mole fraction profiles for 47 species from a laminar premixed low-pressure flame fueled by 2-pentanone were obtained by VUV-PI-MBMS including isomer separation. A new high-temperature sub-mechanism has been developed for 2-pentanone and has been coupled to an existing base mechanism. High-temperature reactions for 2-pentanone were adapted in analogy to 2-butanone and n-pentane, and the thermochemistry for 2-pentanone and the fuel radicals was derived by ab initio calculations.

A flux analysis

Acknowledgments

The authors thank all participants of the ALS “Flame Team” for their contribution to the PI-MBMS measurements. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under contract DE-AC02-05CH11231. NH was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology

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A high-temperature study of 2-pentanone oxidation: experiment and kinetic modeling

Abstract

Introduction

Section snippets

Experiments

Kinetic model and flame simulations

Results and discussion

Summary and conclusions

Acknowledgments

Eng. Sci. Technol. Int. J.

Energy

Fuel

Combust. Flame

Proc. Combust. Inst

Proc. Combust. Inst

Prog. Energy Combust. Sci.

Combust. Flame

Combust. Flame

Proc. Combust. Inst.

Proc. Combust. Inst.

Proc. Combust. Inst.

Combust. Flame

Proc. Combust. Inst.

Proc. Combust. Inst.

Combust. Flame

Combust. Flame

Combust. Flame