Kinetic roles of excited state species in the oxidation of n-pentane/air assisted by nanosecond-pulsed discharge

Despite the significant advancement in plasma assisted combustion recently, many of the underlying plasma/ combustion interaction mechanisms remain unknown. Critical data such as cross sections for large hydrocarbons and reaction rate constants involving excited species are still limited. To address these issues, a detailed plasma chemistry mechanism governing the oxidation processes in a n-pentane/air combustion mixture was proposed and studied by including a complete set of e -n-pentane cross section and excited state chemistry of n-pentane. The concentrations of alkane (CH4, C2H6, C3H8), alkene (C2H4, C3H6), methanol, NO2 and NH3 were measured by GC and a small amount of liquid products are qualitatively characterized by GC-MS. The electron-scattering cross sections of n-pentane were obtained by the R-matrix method. The rate constants of excited species of n-pentane which involved the chemical reactions were calculated via Fridman alpha-model and MMVT method. A linear in-crease in reactants consumption with increased voltages was observed experimentally, suggesting that n-pentane excitation by plasma was decoupled from the low-temperature chemistry. The kinetic mechanism accurately predicted the generation of alkane and hydrogen and captured the overall trend. However, it underestimated the generation of NH3 and NO2, indicating certain reaction pathways in NH3 and NO2 generation were missed. The results showed that because of the introduction nitrogen in the system, the excited states of the oxidant molecules O(1D), O(1S), N2(B) and N2(C) reacted with n-pentane to form fuel radicals via H-atom abstraction reactions RH + X* -> R + XH. The excited species of n-pentane reacted with O, H, OH to form n-pentane radicals via RH* + X -> R + XH, accordingly enhancing the chain initial reactions. Importantly, ammonia was produced in a n- pentane/air mixture with plasma assistance through the pathway of N2 -> N(2D) -> NH -> NH2 -> NH3. These results highlight strong roles of excited species in a complex n-pentane/air plasma chemistry system.