Extreme Low-Temperature Combustion Chemistry: Ozone-Initiated Oxidation of Methyl Hexanoate

Abstract

The accelerating chemical effect of ozone addition on the oxidation chemistr y of methyl hexanoate [CH 3 (CH 2 ) 4 C(O)OCH 3 ] was investigated over a temperature range from 460 to 940 K. Using an externally heated jet-stirred reactor at p = 700 Torr (residence time τ = 1.3 s, stoichiometry φ = 0.5, 80% argon dilution), we explored the relevant chemical pathways by employing molecular-beam mass spectrometry with electron and single-photon ionization to trace the temperature dependencies of key intermediates, including many hydroperoxides. In the absence of ozone, reactivity is observed in the so-called low-temperature chemistry (LTC) regime between 550 and 700 K, which is governed by hydroperoxides formed from sequential O 2 addition and isomerization reactions. At temperatures above 700 K, we observed the negative temperature coefficient (NTC) regime, in which the reactivity decreases with increasing temperatures, until near 800 K, where the reactivity increases again. Upon addition of ozone (1000 ppm), the overall reactivity of the system is dramatically changed due to the time scale of ozone decomposition in comparison to fuel oxidation time scales of the mixtures at different temperatures. While the LTC regime seems to be only slightly affected by the addition of ozone with respect to the identity and quantity of the observed intermediates, we observed an increased reactivity in the intermediate NTC temperature range. Furthermore, we observed experimental evidence for an additional oxidation regime in the range near 500 K, herein referred to as the extreme low-temperature chemistry (ELTC) regime. Experimental evidence and theoretical rate constant calculations indicate that this ELTC regime is likely to be initiated by H abstraction from methyl hexanoate via O atoms, which originate from thermal O 3 decomposition. The theoretical calculations show that the rate constants for methyl ester initiation via abstraction by O atoms increase dramatically with the size of the methyl ester, suggesting that ELTC is likely not important for the smaller methyl esters. Experimental evidence is provided indicating that, similar to the LTC regime, the chemistry in the ELTC regime is dominated by hydroperoxide chemistry. However, mass spectra recorded at various reactor temperatures and at different photon energies provide experimental evidence of some differences in chemical species between the ELTC and the LTC temperature ranges.

Exploring the Effect of Different Reactivity Promoters on the Oxidation of Ammonia in a Jet-Stirred Reactor

“…Ozone is a strong oxidizer and can be produced at high pressures, which is suitable for most practical combustion facilities . The fundamental research for ozone enhancement on combustion has been conducted for various fuels, such as CH 4 , , C 2 H 4 , , dimethyl ether (DME), ethanol, n -heptane, methyl crotonate, and methyl hexanoate . The effect of ozone addition on ammonia combustion has only been studied regarding laminar burning velocity (LBV) by Chen et al The enhancement of the LBV of the NH 3 /O 2 /N 2 mixture by O 3 addition is more significant in fuel-lean conditions than in fuel-rich and stoichiometric conditions.…”

“…33 The fundamental research for ozone enhancement on combustion has been conducted for various fuels, such as CH 4 , 34,35 C 2 H 4 , 36,37 dimethyl ether (DME), 38 ethanol, 39 nheptane, 40 methyl crotonate, 41 and methyl hexanoate. 42 The effect of ozone addition on ammonia combustion has only been studied regarding laminar burning velocity (LBV) by Chen et al 43 The enhancement of the LBV of the NH 3 /O 2 /N 2 mixture by O 3 addition is more significant in fuel-lean conditions than in fuel-rich and stoichiometric conditions. They also observed the LBV enhancement by ozone addition in NH 3 + CH 4 /C 2 H 6 / C 3 H 8 flames and developed an O 3 -related submechanism to reproduce their measurements.…”

1-Hexene Ozonolysis across Atmospheric and Combustion Temperatures via Synchrotron-Based Photoelectron Spectroscopy and Chemical Ionization Mass Spectrometry

“…Above 650 K (in the NTC zone), the enhancement increases with temperature, up to 40% at 750 K, making a significant boost in reactivity compared to the non-O 3 case. This enhancement, due to thermal decomposition of O 3 into O atoms, is more marked than for other fuels (dimethyl ether, 24 methylhexanoate, 25 and propane 23 ), for which such an O 3 effect was reported.…”

Plasma-Assisted Chemical-Looping Combustion: Low-Temperature Methane and Ethylene Oxidation with Nickel Oxide

“…Here, the molecules are intersected with the energized electrons, and the resulting ions are mass-separated by a reflectron time-of-flight mass spectrometer with a mass resolution of about 3500, which is sufficient for separating species containing C/H/O by their precise mass. Further details of this MBMS can be found elsewhere. − …”