Revisiting diacetyl and acetic acid flames : The role of the ketene + OH reaction

The mechanism of the reaction of ketene with hydroxyl radical has been studied by ab initio CCSD(T)-F12/cc-pVQZ-F12//B3LYP/6-311G(d,p) calculations of the potential energy surface. Temperature- and pressure-dependent reaction rate constants have been computed using the RRKM-Master Equation and transition state theory methods in the temperature range of 300–3000 K and in the pressure range of 0.01–100 atm. Three main channels have been analyzed: through direct abstraction of H atoms or starting with OH addition to the terminal carbon and to the central carbon atoms. Major products identified agree with the recent theoretical studies, however, significant difference was found with the rate constants derived by Xu et al. [13] and Cavallotti et al. [11]. To investigate the impact of the choice of reactions between CH2CO and OH radicals on the predicted burning velocities of the flames sensitive to ketene chemistry, namely diacetyl and acetic acid flames, a detailed kinetic mechanism was updated with pertinent reactions suggested in the literature. Then the rate constants of four most important product channels of reaction CH2CO + OH forming HCCO + H2O, CH2OH + CO, CH3 + CO2 and CH2COOH from the present and from the recent theoretical studies were tested. Good agreement with the burning velocities of diacetyl + air flames was found for the present model, while the expressions from the literature underestimate them. On the contrary, any combination of the rate constants of reactions between ketene and hydroxyl radical overpredicts burning velocities of acetic acid + air flames, which strongly indicates that the kinetic model of acetic acid is most probably incomplete and requires consideration of additional reactions