Chemical Kinetics of H-Atom Abstraction from Ethanol by HȮ2: Implication for Combustion Modeling.

ABSTRACT

As a renewable source of energy, ethanol has been widely used in internal combustion engines as either a gasoline alternative fuel or a fuel additive. However, as the chemical source term of the computational fluid dynamics simulation of combustors, there remains a disagreement in understanding the chemical kinetic mechanism of ethanol. The reaction mechanism of ethanol + HÈ®2 is a well-known crucial reaction class in terms of predicting the reactivity of ethanol as well as ethylene formation under engine-relevant conditions. However, the kinetic parameters of the reactions are basically extrapolated by analogy to the n-butanol + HÈ®2 system calculated by Zhou et al. (Zhou et al. Int. J. Chem. Kinet. 2012, 44 (3), 155-164). The reliability of such an analogy remains to be seen because no direct theoretical or experimental evidence is available in the literature to date. In this study, thermal rate coefficients of H-atom abstraction reactions for the ethanol + HÈ®2 system were determined by using both conventional transition-state theory and canonical variational transition-state theory, with the potential energy surface evaluated at the CCSD(T)/cc-pVTZ//M06-2x/def-TZVP level. The quantum-mechanical effects were corrected by the zero-curvature tunneling method at low temperatures (<750 K), and difference schemes of two Eckart functions were fitted to optimize the minimum energy path curves. Torsional modes of the -CH3 and -OH groups were treated by using the hindered-internal-rotation approximation. Furthermore, the rate coefficients of the title reaction were calculated at both the CCSD(T)/cc-pVTZ//M06-2x/def-TZVP and CCSD(T)/CBS//M06-2x/def-TZVP levels of theory with uncertainty of a factor of 3. Similar to the n-butanol + HÈ®2 system, the title system is dominated by α-site H-atom abstraction, but the rate coefficients of the three channels are slightly slower than that of the n-butanol + HÈ®2 system. In general, the new calculations show only a limited effect on the ethanol reactivity at low pressures and high temperatures (>1300 K), but they prevent the kinetic mechanisms from overpredicting ignition delay times under engine-relevant conditions.