International audienceTetrahydrofuran (THF) is a well suited starting point fuel to study the combustion chemistry of saturated cyclic esters that are being considered as promising bio-fuels. To better understand the combustion chemistry of THF, laminar low-pressure premixed flame structure, atmospheric adiabatic laminar burning velocities, and high-pressure ignition delay times were investigated. The structure of laminar premixed low-pressure (6.7 kPa) argon-diluted (78%) flames of THF were studied at three equivalence ratios (0.7, 1.0 and 1.3) using on-line gas chromatography analyses. The results consist of temperature and mole fraction profiles (about 40 species) measured as a function of the height above the burner. Ethylene, propene, formaldehyde, acetaldehyde, and dihydrofurans were observed as important intermediates. Aromatic species were detected in very low amounts. The adiabatic laminar burning velocities of THF–air mixtures were measured using the heat flux burner method at atmospheric pressure (initial temperatures from 298 to 398 K, at equivalence ratios from 0.55 to 1.60). The maximum burning velocity of THF was comparable to that of ethanol and diethyl ether. The ignition delay times of THF–oxygen–argon mixtures were measured behind reflected shock waves (temperatures from 1300 to 1700 K, pressures around 8.5 atm, mixtures containing 0.25–1% of fuel for equivalence ratios of 0.5–2.0). A new detailed kinetic model for THF combustion was developed using a combination of automatic generation (EXGAS), Evans–Polanyi correlations (for H-abstraction kinetic data), and CBS-QB3 theoretical calculations (for unimolecular initiation, H-abstraction and β-scission kinetic data). An overall good agreement between simulations and the present experimental results has been found. The main THF reaction pathways under flame conditions have been identified from flow rate analyses

The hydrolysis of ketene (H2C═C═O) to form acetic acid involving two water molecules and also separately in the presence of one to two water molecules and formic acid (FA) was investigated. Our results show that, while the currently accepted indirect mechanism, involving addition of water across the carbonyl C═O bond of ketene to form an ene-diol followed by tautomerization of the ene-diol to form acetic acid, is the preferred pathway when water alone is present, with formic acid as catalyst, addition of water across the ketene C═C double bond to directly produce acetic acid becomes the kinetically favored pathway for temperatures below 400 K. We find not only that the overall barrier for ketene hydrolysis involving one water molecule and formic acid (H2C2O + H2O + FA) is significantly lower than that involving two water molecules (H2C2O + 2H2O) but also that FA is able to reduce the barrier height for the direct path, involving addition of water across the C═C double bond, so that it is essentially identical with (6.4 kcal/mol) that for the indirect ene-diol formation path involving addition of water across the C═O bond. For the case of ketene hydrolysis involving two water molecules and formic acid (H2C2O + 2H2O + FA), the barrier for the direct addition of water across the C═C double bond is reduced even further and is 2.5 kcal/mol lower relative to the ene-diol path involving addition of water across the C═O bond. In fact, the hydrolysis barrier for the H2C2O + 2H2O + FA reaction through the direct path is sufficiently low (2.5 kcal/mol) for it to be an energetically accessible pathway for acetic acid formation under atmospheric conditions. Given the structural similarity between acetic and formic acid, our results also have potential implications for aqueous-phase chemistry. Thus, in an aqueous environment, even in the absence of formic acid, though the initial mechanism for ketene hydrolysis is expected to involve addition of water across the carbonyl bond as is currently accepted, the production and accumulation of acetic acid will likely alter the preferred pathway to one involving addition of water across the ketene C═C double bond as the reaction proceeds.

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