Effect of CO2 and methyl groups reaction kinetics on the ignition and combustion of diesel surrogate fuel: Part Ⅰ. Reaction mechanisms

Abstract

The study aims to examine the impact of CO2 concentration gradient on the ignition and combustion of a diesel surrogate fuel composed of 70% C7H16 and 30% C7H8. The investigation focuses on the weak oxidation of CO2 through its reaction with methyl groups (CH, CH2, and CH3 radicals). Firstly, the molecular bond-breaking recombination process between CO2 and methyl groups is calculated using Born-Oppenheimer molecular dynamics with the UB3LYP/6–311++g (d, p) level of theory. Subsequently, the electronic distribution and competition of the molecule surface are analyzed through various wave function analyses to identify the reactive sites. Secondly, the precise potential energy surface and reaction rate are calculated based on the acquired reaction process information. This calculation employs the B2PLYP/def2-TZVP level of theory and transition state theory. Finally, extensive molecular dynamics simulations of CO2 and methyl groups are conducted using reaction force fields. These simulations provide insights into the temperature-energy variation within the system and the resulting chemical reaction network. The optimized combustion mechanism for the binary fuel is derived, focusing exclusively on the influences of CO2 and O2 while excluding those of N2 and other extraneous gases. The results demonstrate exothermic reactions of CO2 with CH and CH2 radicals at 850 K, yielding ΔG values of −238.91 kJ/mol and −209.96 kJ/mol, respectively. Conversely, CO2 reacts with CH3 radicals in heat-absorbing reactions at 850 K, resulting in a ΔG of 491.73 kJ/mol. Additionally, the chemical impact of CO2 exhibits both promotional and inhibitory effects on combustion. The CO2 molecule exhibits weak oxidation properties due to the presence of active sites on its surface characterized by minimal values of electrostatic potential (-50.17 kJ/mol) and average local ionization energy (61.92 kJ/mol) at the O-atomic end. The molecular dynamics process involving the methyl groups and CO2 system exhibits a heat absorption phenomenon. This behavior can be attributed to the participation of CO2 not only in the reactions with methyl groups but also with formaldehyde and ethylene, among others.