(273a) Implications of High Accuracy Thermochemical Kinetics for H + CH3 (+M) ? CH4 (+M) on Combustion Models

CH₃, the simplest alkyl radical, is an extremely stable intermediate that persists in large concentrations even in high temperature chemically reacting systems such as flames. Radical-radical recombination with H-atoms (R1: H + CH₃⇄ CH₄) is the dominant process for CH₃ removal in high temperature combustion and flame processes. As such, the net result of this reaction is to terminate reactive H-atoms and CH₃ radicals in flames. Numerous studeies have been conducted to directly measure k₁, however, k₁ has only been measured at low temperatures (< 700 K) while all data at high temperatures (> 1300 K) relevant to the pre-flame regime have been measured in the reverse direction. Hence, reconciling k₁ and k_-1 requires accurate equilibrium constants spanning an extended range of temperatures. In the present work, we include all relevant anharmonic corrections to calculate an accurate Active Thermochemical Tables (ATcT) partition function for CH₃. The resulting nonrigid-rotator-anharmonic-oscillator partition function is used to determine thermochemical parameters (heat capacity, entropy, and enthalpy increment values) for CH₃ and K_eq values for reaction R1 over a wide temperature range (200-6000 K). With the newly evaluated equilibrium constant, literature experiments and theory (based on two-dimensional master equation calculations with a first principles energy and angular momentum transfer kernel) are used to obtain an accurate representation of the kinetics for the title reaction. In particular, the present work provides fits to k₁(T,P) for N₂, Ar, and He as colliders for direct use in combustion modeling. The results of the new rate constant evaluation for R1 has a dramatic impact on combustion modeling. In particular, we assess the impact of incorporating anharmonic thermochemistry for CH₃ and the updated fits for k₁ (and k_-1) in current widely used literature models for simulations of CH₄/air and n-heptane/air laminar flame speeds and find that flame speed predictions can be affected by as much as 50%.

This work was supported by the Office of Basic Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, U. S. Department of Energy, under Contract No. DE-AC02-06CH11357.