Quantum Chemical Study of Supercritical Carbon Dioxide Effects on Combustion Kinetics

In oxy-fuel combustion, the pure oxygen (O₂), diluted with CO₂ is used as oxidant instead air. Hence, the combustion products (CO₂ and H₂O) are free from pollution by nitrogen oxides. Moreover, high pressures result in the near-liquid density of CO₂ at supercritical state (sCO₂). Unfortunately, the effects of sCO₂ on the combustion kinetics are far from being understood. To assist in this understanding, in this work we are using quantum chemistry methods. Here we investigate potential energy surfaces of important combustion reactions in the presence of the carbon dioxide molecule. All transition states and reactant and product complexes are reported for three reactions: H₂CO + HO₂ → HCO + H₂O₂ (R1), 2HO₂ → H₂O₂ + O₂ (R2), and CO + OH → CO₂ + H (R3). In reaction R3, covalent binding of CO₂ to the OH radical and then the CO molecule opens a new pathway, including hydrogen transfer from oxygen to carbon atoms followed by CH bond dissociation. Compared to the bimolecular OH + CO mechanism, this pathway reduces the activation barrier by 5 kcal/mol and is expected to accelerate the reaction. In the case of hydroperoxyl self-reaction 2HO₂ → H₂O₂ + O₂ the intermediates, containing covalent bonds to CO₂ are found not to be competitive. However, the spectator CO₂ molecule can stabilize the cyclic transition state and lower the barrier by 3 kcal/mol. Formation of covalent intermediates is also discovered in the H₂CO + HO₂ → HCO + H₂O₂ reaction, but these species lead to substantially higher activation barriers, which makes them unlikely to play a role in hydrogen transfer kinetics. The van der Waals complexation with carbon dioxide also stabilizes the transition state and reduces the reaction barrier. These results indicate that the CO₂ environment is likely to have a catalytic effect on combustion reactions, which needs to be included in kinetic combustion mechanisms in supercritical CO₂.