In oxy-fuel combustion, the pure oxygen (O), diluted with CO is used as oxidant instead air. Hence, the combustion products (CO and HO) are free from pollution by nitrogen oxides. Moreover, high pressures result in the near-liquid density of CO at supercritical state (sCO). Unfortunately, the effects of sCO 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: HCO + HO → HCO + HO (R1), 2HO → HO + O (R2), and CO + OH → CO + H (R3). In reaction R3, covalent binding of CO 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 2HO → HO + O the intermediates, containing covalent bonds to CO are found not to be competitive. However, the spectator CO molecule can stabilize the cyclic transition state and lower the barrier by 3 kcal/mol. Formation of covalent intermediates is also discovered in the HCO + HO → HCO + HO 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 CO environment is likely to have a catalytic effect on combustion reactions, which needs to be included in kinetic combustion mechanisms in supercritical CO.
We report on potential energies for the transition state, reactant, and product complexes along the reaction pathways for hydrogen transfer reactions to hydroperoxyl radical from formaldehyde HCO + HO → HCO + HO and another hydroperoxyl radical 2HO → HO + O in the presence of one carbon dioxide molecule. Both covalently bonded intermediates and weak intermolecular complexes are identified and characterized. We found that reactions that involve covalent intermediates have substantially higher activation barriers and are not likely to play a role in hydrogen transfer kinetics. The van der Waals complexation with carbon dioxide does not affect hydrogen transfer from formaldehyde, but it lowers the barrier for hydroperoxyl self-reaction by nearly 3 kcal/mol. This indicates that CO environment is likely to have catalytic effect on HO self-reaction, which needs to be included in kinetic combustion mechanisms in supercritical CO.
The oxy-fuel–carbon dioxide combustion process is expected to drastically increase the energy efficiency and enable easy carbon sequestration. In this technology, the combustion products (carbon dioxide and water) are used to control the temperature and nitrogen is excluded from the combustion chamber, so that nitrogen oxide pollutants do not form. Therefore, in oxy-combustion, carbon dioxide and water are present in large concentrations in their transcritical state and may play an important role in kinetics. The computational chemistry methods may assist in understanding these effects, and molecular dynamics with a reactive force field (ReaxFF) seems to be a suitable tool for such a study. Here, we investigate applicability of the ReaxFF to describe the critical phenomena in carbon dioxide and water and find that several non-bonding parameters need adjustment. We report the new parameter set, capable of reproducing the critical temperatures and pressures. The critical isotherms of CO2/H2O binary mixtures are computationally studied here for the first time, and their critical parameters are reported.
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