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

Abstract 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 CH 3 OH + HO 2 reaction dominates the uncertainty in the predicted ignition delay for stoichiometric CH 3 OH combustion at 1100 K and 20 bar. The rate coefficients for both product channels (CH 2 OH + H 2 O 2 and CH 3 O + H 2 O 2 ) 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 CH 3 OH + O 2 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 CH 2 OH … HO 2 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 CH 3 OH + HO 2 reaction and new predictions for the HO 2  + HO 2 reaction. The predictions for the CH 3 OH + HO 2  → CH 2 OH + H 2 O 2 , CH 3 OH + HO 2  → CH 3 O + H 2 O 2 , and CH 3 OH + O 2  → CH 2 OH + HO 2 rate constants are well represented over the 400–2500 K temperature range, by the expressions 3.78 × 10 −29 T 5.06 exp(−5140/ T ), 5.54 × 10 −26 T 4.12 exp(−8170/ T ) and 5.95 × 10 −19 T 2.27 exp(−21520/ T ) cm 3  molecule −1  s −1 , respectively, where T is in K.

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