Contributions to improving small ester combustion chemistry: Theory, model and experiments

Abstract

Biodiesel combustion models demand detailed understanding of the reactions undertaken by the ester functional group in the molecule. Investigations of the chemistry of small methyl esters can contribute to this goal. We have thus chosen methyl propanonate (MP) as a representative ester molecule in a study combining theory, model, and experiments. As an advantage, its reactions are also amenable to high-level theoretical calculations. Based on recent theoretical calculations , a new kinetics model for small ester combustion was developed and validated. New experimental results were obtained here in an extensive range of conditions, including full speciation in laminar low-pressure flames at two different stoichiometries ( φ = 0.8 and 1.5) using electron ionization (EI) molecular-beam mass spectrometry (MBMS) and flame speed measurements in a spherical confined chamber (1-6 atm). Comparison of the experimental data to the present model shows overall improved performance. Some specific new reaction pathways to form methanol, methylketene, methyl acetate, and acetic acid from the fuel radicals were identified and will permit more detailed insights into the combustion properties of methyl propanoate.

Relative Reactivity of Oxygenated Fuels: Alcohols, Aldehydes, Ketones, and Methyl Esters

“…In fact, according to the rate constants calculated by Tan et al [50], the relative importance of the H-abstraction channels in MP at 1000 K, does not reflect the hierarchy in BDEs. Despite the surprising trend, it has to be mentioned that the same rate constants were recently found to have a positive effect on MP laminar flames prediction [52].…”

Shock tube ignition delay time measurements for methyl propanoate and methyl acrylate: Influence of saturation on small methyl ester high‐temperature reactivity

“…The ignition delay times for MP and MA are compared with predictions of five kinetic models in Figures 5 and 6. The models considered include MP models from Zhao et al 37 (98 species and 479 reactions), Zhang et al 32 (318 species and 1 668 reactions), and Felsmann et al 36 (128 species and 934 reactions) and MA models from Bennadji et al 52 (103 species and 826 reactions) and Grana et al 53 (402 species and 16 118 reactions). Ignition delay time calculations were carried out in Ansys Chemkin Pro 58 under an adiabatic constant volume constraint, a suitable boundary condition for ignition delay times of 1‐2 ms or shorter 59 …”

“…Kinetic studies, both experimental and modeling, have been reported on several model small esters, including: methyl formate, 25‐28 methyl acetate, 27‐30 methyl propanoate (MP), 31‐41 and methyl butanoate 28,42‐51 . There have been relatively fewer studies for small unsaturated esters, including only two studies, 52,53 to our knowledge, for the unsaturated methyl acrylate (MA), which has similar structure to MP but with a double bond in the carbon chain.…”

“…In addition to shock tube and RCM experiments, kinetic studies for MP have been carried out in flames and flow reactors. Laminar flame speed measurements have been made for MP and other small ester species by Wang et al, 33 Yang et al, 34 and Felsmann et al 36 Detailed speciation measurements have been reported for low‐pressure MP flames by Felsmann et al 36 and for low‐pressure MP pyrolysis in a flow reactor by Zhao et al 37 . Diévart et al 25 characterized diffusive extinction limits for MP and other methyl ester flames in the counterflow flame configuration.…”

“…There are several kinetic models available in the literature for MP including comprehensive models from Zhao et al, 37 Zhang et al, 32 and Felsmann et al, 36 which are examined here. These models are all built on chemistries previously developed in the literature for small hydrocarbons and alkyl esters and implement improvements for MP chemistry based on quantum chemistry predictions for rate coefficients.…”