Quantum Chemical Study of Supercritical Carbon Dioxide Effects on Combustion Kinetics

Abstract

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.

Quantum chemical and master equation study of OH + CH2O → H2O + CHO reaction rates in supercritical CO2 environment

“…They calculated activation barrier to be 4.3 kcal/mol. In this work, we will refine that result using higher theory level CBS‐QM11, introduced earlier . Finally, Xu et al .…”

“…In the following papers, we reported lowering of the activation barrier of seven other combustion reactions via van der Waals (vdW) interactions with CO 2 . To obtain a better description of these vdW interactions, we used a higher theory level (CBS‐QM11) and refined the reaction pathways . Next, we applied the master equation solver to obtain rate constants for the OH + CO → H + CO 2 reaction with and without an additional CO 2 molecule …”

“…10,11 To obtain a better description of these vdW interactions, we used a higher theory level (CBS-QM11) and refined the reaction pathways. 9,12 Next, we applied the master equation solver to obtain rate constants for the OH + CO → H + CO 2 reaction with and without an additional CO 2 molecule. 13 In this contribution, we investigate the rate of another reaction R1:…”

“…In this work, we will refine that result using higher theory level CBS-QM11, introduced earlier. 9,12 Finally, Xu et al 20 predicted R1 reaction rates by fitting the minimum energy pathway (MEP) to a Morse potential using three different methods. Their Method 1 was CCSD(T)/6-311+G(3df,2p)//CCSD/6-311++G(d,p) theory level and used multiple reflections correction, Method 2 was CCSD(T)/6-311+G(3df,2p)//B3LYP/6-311+G(3df,2p) theory level with multiple reflections, and Method 3 was CCSD(T)/6-311+G(3df,2p)//CCSD/6-311++G(d,p) theory level without multiple reflections.…”

Molecular Dynamics of Combustion Reactions in Supercritical Carbon Dioxide. 6. Computational Kinetics of Reactions between Hydrogen Atom and Oxygen Molecule H + O2 ⇌ HO + O and H + O2 ⇌ HO2

“…As a part of our efforts to further develop this technology, we have modeled equation-of-state parameters, developed reduced combustion mechanism, and performed counterflow diffusion flame analyses in sCO 2 environment. We also developed the force field to describe water and carbon dioxide in a mixture supercritical state, investigated potential energy surfaces (PESs) of reactions with the existence of CO 2 by density-functional theory (DFT) calculation, − and predicted the rate constant k of several important combustion reactions in high pressure of CO 2 by molecular dynamics (MD) − and DFT simulations. , In this contribution, we report advances in computational methods of reaction rate prediction in supercritical CO 2 and apply these methods to study R1 and R2.…”