Uncertainty driven theoretical kinetics studies for CH3OH ignition: HO2 + CH3OH and O2 + CH3OH

A combination of global uncertainty screening and ab initio theoretical chemical kinetics is used to iteratively improve the mechanism of Li et al. [3] for the ignition of methanol at high pressure. The initial application of the screening analysis indicates that the CH3OH + HO2 reaction dominates the uncertainty in the predicted ignition delay for stoichiometric CH3OH combustion at 1100 K and 20 bar. The rate coefficients for both product channels (CH2OH + H2O2 and CH3O + H2O2) in this reaction are predicted with ab initio transition state theory employing barriers and rovibrational properties obtained at the CCSD(T)/CBS//CASPT2/cc-pvtz level. The estimated uncertainty in these predictions is a factor of 2. The second iteration of the screening analysis indicates that the CH3OH + O2 reaction next dominates the uncertainty in the ignition delay at high pressure. The associated rate coefficient is updated using a two transition state model that employs CCSD(T)/CBS//CASPT2/cc-pvtz properties for the tight transition state and direct CASPT2/aug-cc-pvdz based variable reaction coordinate transition state theory for the barrierless formation of the long-range CH2OH … HO2 complex. The final predictions for the ignition delay are a factor of 4 greater than those with the original model and the width of the distributions of ignition delay relative to its peak value decreases by a factor of 3. Further reduction in the uncertainty will require more accurate predictions for the CH3OH + HO2 reaction and new predictions for the HO2 + HO2 reaction. The predictions for the CH3OH + HO2 → CH2OH + H2O2, CH3OH + HO2 → CH3O + H2O2, and CH3OH + O2 → CH2OH + HO2 rate constants are well represented over the 400–2500 K temperature range, by the expressions 3.78 × 10−29T5.06exp(−5140/T), 5.54 × 10−26T4.12exp(−8170/T) and 5.95 × 10−19T2.27exp(−21520/T) cm3 molecule−1 s−1, respectively, where T is in K.