Methanol oxy-combustion and supercritical water oxidation: A ReaxFF molecular dynamics study

Energy and environmental concerns are motivating the use of renewable fuels such as methanol. Furthermore, the implementation of the oxy-combustion and hydrothermal combustion technologies can help to improve the performance of power generation and reduce NOx emissions. These aspects can contribute to achieve the transition to cleaner sources of energy that is being sought worldwide, and thus we carried out the first molecular dynamics study of the oxidation of methanol at 2700 K and 3000 K in four supercritical environments with compositions CH3OH + O2, CH3OH + O2+CO2, CH3OH + O2+H2O, and CH3OH + O2+CO2+H2O. Reaction mechanisms were obtained and revealed that the initiation reaction is CH3OH unimolecular dissociation in all cases. The CH3OH oxidation chemistry changes when O2 is replaced by supercritical CO2 (sCO2) and/or H2O (sH2O), and a new route for the important oxidation sequence CH3OH→CH2OH→H2CO→CHO→CO→CO2 is reported. The rate constants for the CH3OH unimolecular dissociation were calculated, indicating a positive effect of sH2O. Furthermore, the collisions of CH3OH molecules with those of H2O and CO2 were analyzed with molecular dynamics simulations and quantum chemistry calculations, suggesting that collisions with H2O can activate more efficiently CH3OH for a prospective dissociation event. This study is aimed to help in the development of kinetic models for CH3OH oxidation/pyrolysis in sCO2 and sH2O, and thus in the implementation of the oxy-combustion and hydrothermal combustion techniques for this alternative fuel.