H-Atom-Forming Reaction Pathways in the Pyrolysis of Furan, 2-Methylfuran, and 2,5-Dimethylfuran: A Shock-Tube and Modeling Study

The methyl-substituted furan derivatives 2-methylfuran (2-MF) and 2,5-dimethylfuran (2,5-DMF) are often discussed as alternative fuels. Despite the large number of mechanistic studies on the pyrolysis and oxidation of 2-MF, 2,5-DMF, and unsubstituted furan (F), detailed kinetic investigations of the initial reaction steps are scarce. In this work, we report on shock-tube studies with detection of hydrogen atoms by atom resonance absorption spectroscopy to investigate the thermal decomposition of F, 2-MF, and 2,5-DMF. Hydrogen atom concentration-time profiles were recorded behind reflected shock waves at temperatures between 1200 and 1900 K and pressures between 0.7 and 1.6 bar with Ar as the bath gas. The recorded profiles were compared with results from kinetic simulations performed on the basis of a joint F/2-MF/2,5-DMF oxidation mechanism recently published. Kinetic parameters for a small number of reactions with high sensitivities for the formation and consumption of H atoms were adapted by taking values from other references to improve the agreement of the experimentally determined and simulated concentration-time profiles. In this way, an adequate description of the H atom concentration-time profiles for all three furan derivatives with the joint mechanism could be achieved. On the basis of this adapted mechanism, the formation pathways of H atoms in the pyrolysis of all three furan derivatives were identified and analyzed. It turned out that the formation of H atoms in the case of 2-MF and 2,5-DMF is governed by a competition between H split-off from the methyl group(s) of the reactant molecule as well as from the primary ring-opening product. In the case of F, only decomposition steps of the ring-opening product are relevant. The adapted mechanism is given in machine-readable form for modeling purposes, and the alterations made are discussed.