Enhancing cool-flame oxidation of cyclic ethers via linear ether addition: a case study on tetrahydrofuran and diethyl ether in a jet-stirred reactor

Diethyl ether (DEE) and tetrahydrofuran (THF) are two promising candidates for second-generation biofuels. Although both are ethers and have the same number of carbon atoms, their molecular structures lead to markedly different reactivity. While DEE is highly reactive at very low temperatures, THF exhibits weak and delayed low-temperature reactivity under identical conditions. The present work aims to enhance THF cool-flame oxidation and to trigger it at lower temperatures by adding DEE. This novel study allows us to explore, for the first time, the interactions between a linear ether (DEE) and a cyclic ether (THF), and to investigate more deeply the low-temperature reaction pathways of the latter, rarely studied in detail in the literature. Three DEE/THF blends – differing only in DEE content – were considered, and a neat THF case served as a reference. All experiments were conducted in a jet-stirred reactor for stoichiometric mixtures between 450 and 1260 K, at a pressure of 10 bar, and a residence time of 0.7 s. Gas chromatography and Fourier transform infrared spectroscopy were used to determine the mole fraction profiles of the reactants, products, and intermediates observed during oxidation. As expected, increasing DEE proportion in the blend enhanced THF reactivity and shifted its onset to lower temperatures. A detailed kinetic mechanism was developed to reproduce the experimental results and to elucidate the underlying reaction pathways. Overall, the present mechanism predicts well the general trends of the detected species. This mechanism also reveals the significance of a quasi-latent low-temperature pathway of THF promoted by the earlier reactivity initiation induced by DEE addition: the disproportionation of its peroxy radicals adjacent to the ether group. Finally, the mechanism is further tested against previously reported data on the oxidation and ignition of DEE and THF to assess its ability to accurately represent the kinetic behavior of both ethers. Novelty and significance statement Based on current knowledge, the ether blends investigated in this work show significant potential as second-generation biofuels for compression-ignition engines. Such fuels present promising features: renewability, lower emissions, production from non-edible feedstocks, and compatibility with existing combustion systems. While the oxidation of DEE and THF has been widely studied separately, the present study presents the first characterization of the oxidation behavior of their blends, combining detailed speciation and kinetic modeling. This work also explores the interactions between linear and cyclic ethers and helps elucidate the cool-flame chemistry of cyclic ethers, and consequently that of hydrocarbons, where cyclic ethers act as key oxidation intermediates.