Theoretical and kinetic modeling study of H-abstraction reactions in NH3/CH4 and NH3/C2H4 combustion

Taking methane and ethylene as representative species of alkanes and alkenes, respectively, sixteen H-abstraction channels involving eight radicals were systematically investigated in NH3/CH4 and NH3/C2H4 combustion. The potential energy surfaces were determined using two complete basis set (CBS) extrapolation schemes based on CCSD(T)/cc-pVXZ (X = D, T, and Q) and MP2/cc-pVXZ (X = D, T, and Q) calculations. The average unsigned deviations of the forward barrier heights were computed using twelve methodological combinations comprising four functionals (M06-2X, M05-2X, M08-HX, and MN12SX) and three basis sets, namely 6–311++G (d, p), def2-TZVP, and aug-cc-pVTZ. Rate constants were computed using RRKM and TST theories. The results indicate that both computational approaches yield nearly identical energy barriers and rate constants. Compared with C2H4, H-abstraction pathways from CH4 are favorable owing to lower energy barriers. The OH-initiated abstraction exhibits the lowest energy barriers and highest rates for both fuels, whereas NO2-initiated pathways are kinetically hindered. The kinetic parameters derived from the modified Arrhenius expression were incorporated into the different combustion model, enhancing the prediction accuracy of ignition delay times (IDTs). This work furnishes reliable kinetic parameters for the further development of CH4 and C2H4 oxidation models.