Probing the low-temperature chemistry of di-n-butyl ether: Detection of previously unobserved intermediates

Abstract

Din -butyl ether (DBE, C8H18O) has been proposed as a promising biofuel for diesel engines, but details of its low-temperature (LT) oxidation chemistry are not well understood. This paper reports new speciation data obtained in the temperature range of 400-1100 K at ϕ=1 and nearly-atmospheric pressure, using a plug flow reactor (PFR) combined with electron ionization (EI) molecular-beam mass spectrometry (MBMS) and two different jet-stirred reactors (JSRs) coupled with either online gas chromatography (GC) or tunable synchrotron vacuum ultraviolet (SVUV) photoionization (PI)-MBMS. The experimental results confirm that DBE is very reactive and exhibits two negativetemperature coefficient (NTC) zones around 500-550 K and 650-750 K. Speciation data with about 40 C0-C8 species are presented, including about 20 LT species not reported previously. Among those, fuel-specific C8H16O2 cyclic ethers were quantified. Also, butanoic acid, which is present in highest amounts among the detected LT intermediates, and C8H14O3 diones, were found to peak already near 500 K, suggesting their importance in the LT chemistry of DBE. Signals of several highly oxygenated peroxides (e.g., C8H14O5 and C8H16O6) were detected, indicating third O2 addition steps. Respective reaction pathways are suggested and discussed based on these experimental results. To better understand the LT chemistry of DBE, the present data were compared to two recent DBE models [L. Cai et al. Combust. Flame. 161 (2014) 798-809 and S. Thion et al. Combust. Flame 185 (2017) 4-15]. Significant discrepancies between the experimental data and both models were found for important LT intermediates, of which many were not included in the respective mechanisms. The results reported in the present study thus provide new opportunities for refining DBE kinetic models.

Combustion in the future: The importance of chemistry

“…For many fuels, the LT oxidation features an NTC region that reflects the changing reactivity with temperature as a consequence of fuel-specific pathways, including oxygen addition and chain-branching reactions. Low-temperature chemistry can be even more complex with two NTC zones as recently observed for DEE and DBE [169] , [170] , [171] . The temperature-dependent fuel and oxygen consumption for DBE measured in two JSRs and a plug-flow reactor (PFR) is shown in Fig.…”

“…Low-temperature oxidation has also been investigated for various oxygenated fuels [92 , 163 , [165] , [166] , [167] , [168] , [169] , [170] , [171] , [172] , [173] , [174] . Regarding the discussion on OMEs and their potential as clean alternative fuels, the LTC of ethers has received particular attention, including DME (as OME 0 ) [92 , 166 , 167] , diethyl ether (DEE) [168 , 169] , and di- n -butyl ether (DBE) [170 , 171] ; further studies have been devoted to e.g., dimethoxyethane [172] and aldehydes [173 , 174] . Recent work includes high-pressure investigations [175] , ab initio kinetics studies [176] , as well as quantitative measurements of key intermediates, in part with optical methods [160 , 177] .…”

“…The temperature-dependent fuel and oxygen consumption for DBE measured in two JSRs and a plug-flow reactor (PFR) is shown in Fig. 14 [171] . Good general agreement is seen between PI-MBMS-JSR, GC-JSR, and EI-MBMS-PFR results.…”

“…( a ) JSR ( , 1% DBE), ( b ) PFR ( , 0.5% DBE), ( c ) DBE profiles in JSR-GC and PFR-EI-MBMS experiments, ( d ) DBE profiles for different equivalence ratios Φ and PFR lengths L. Profiles in ( c ) and ( d ) are normalized by the respective inlet DBE mole fractions. Reprinted from [171] with permission from Elsevier/The Combustion Institute. …”

“…The double-NTC behavior was analyzed following the main reaction path starting from the C 4 H 9 OC 4 H 8 -a radical, formed by H-abstraction from the fuel molecule at the C α -position next to the ether function. Reactions start well below 500 K; here, C 4 H 9 OC 4 H 8 -a can react by two sequential O 2 additions that result through several steps in the production of n -C 3 H 7 and two OH radicals, thus enhancing the reactivity [171] . Both NTC zones are characterized by decreasing reactivity with temperature, and the overall behavior reflects delicate competitions between decomposition reactions of the formed intermediates to produce mainly stable products (as e.g., cyclic ethers) versus those reactions that can form chain carriers (as e.g., OH radicals) [171] .…”