'Minor' Molecular Thermochemistry Effects Significantly Alter Predictions of Global Combustion Phenomena

Abstract

CH3, the simplest alkyl radical, is an extremely stable intermediate with a C-H bond energy of D0(H-CH2) = 109.28 ± 0.03 kcal/mol, according to Active Thermochemical Tables (ATcT). Consequently, it persists in large concentrations even in high temperature chemically reacting systems such as flames. The dominant process for CH3 removal in combustion and flame processes is through radical-radical recombination, either with H-atoms (R1: CH3 + H –> CH4 ; ∆H0K = -103.34 ± 0.02 kcal/mol, ATcT) or with itself (R2: CH3 + CH3 –> C2H6; ∆H0K = -87.92 ± 0.05 kcal/mol, ATcT). For reaction (R1), direct measurements for k1 are largely limited to low temperatures, while direct shock tube measurements for k-1 span a temperature range from 1700 to 4500 K. Reconciling the apparently disparate experimental databases on k1 and k-1 requires accurate equilibrium constants for reaction (R1) spanning an extended range of temperatures (300-4500K). While several thermochemical databases have accounted for anharmonic effects in calculations of CH4 thermochemistry, analogous corrections have not been included in CH3 radical thermochemistry. As a consequence, existing literature assessments of the equilibrium and rate constants for recombination/dissociation reaction (R1) have notable inaccuracies.

In the present work, we include all relevant anharmonic corrections to calculate an accurate ATcT partition function for CH3. The resulting nonrigid-rotator-anharmonic-oscillator partition function is used to determine thermochemical parameters over a wide temperature range (200-6000 K) of relevance to atmospheric and combustion chemistry. The new ATcT thermochemistry is also used to prescribe Keq values for reaction R1. Furthermore, 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 k1,-1(T,P) for subsequent use in combustion modeling. Theory is also used to predict the effects of bath-gas colliders on k1,-1(T,P). Lastly, we assess the impact of incorporating anharmonic thermochemistry for CH3 and the updated fits for k1,-1 in current widely used literature models for simulations of CH4-air laminar flame speeds.

NICOLE J. LABBE

University of Colorado Boulder