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Abstract
In an ongoing effort to mitigate climate change concerns from the transportation sector, development of next-generation biofuels concurrently with cleaner-burning, higher-efficiency engines that operate at low temperature (< 1200 K) remains a top priority of the U.S. and international entities. Balancing climate concerns with rising transportation energy demands has created a need for diversifying biofuel beyond ethanol and biodiesel. However, the impact on ignition from using new biofuels is an ongoing area of research. Such insight is critical for developing predictive modeling tools and is supported by isomer-resolved speciation measurements. The experiments of the work herein use a jet-stirred reactor (JSR) paired with vacuum-ultraviolet absorption spectroscopy and electron-impact mass spectrometry to provide isomer-resolved speciation measurements, a process that was developed as described in this work. Low-temperature combustion of tetrahydrofuran, a next-generation biofuel, involves competing reactions that depend on temperature, pressure, and oxygen concentration, including ring-opening and subsequent oxidation of initial radicals (Ṙ), HOȮ-elimination yielding dihydrofuran isomers, and the formation of peroxy radicals (ROȮ). The latter species can isomerize to hydroperoxy-substituted radicals (Q̇OOH) that undergo either unimolecular decomposition or second-O2-addition. To examine the influence of temperature and oxygen concentration on intermediates from tetrahydrofuran, isomer-resolved speciation measurements were conducted at 810 Torr in a JSR from 500 – 1000 K. Resulting from negative-temperature coefficient behavior, species concentrations peaked at two temperatures, 600 K and 800 K, which were then selected for separate experiments to quantify O2-dependence using O2 concentrations of 0.37 – 7.40 · 1018 molecules cm–3.
Several species were detected for the first time, including constitutional isomers tetrahydrofuran-3-one, butanedial, and allyl formate; the latter two resulting from ring-opening reactions of Q̇OOH radicals. For the majority of species, clear dependence on O2 exists that is not captured quantitatively by chemical kinetics mechanisms. The experiments herein provide new targets for refinement of chemical kinetics mechanisms of tetrahydrofuran. The discrepancies existing between the measured and model-predicted species profiles indicate that sub-mechanisms for important intermediates may require additional elementary reactions, such as for dihydrofuran isomers. In addition, rates for O2-addition to tetrahydrofuranyl radicals in chemical kinetics mechanisms, which employ rate rules from alkyl radicals, require scrutiny.