The C₂H₂ + NO₂ reaction: Implications for high pressure oxidation of C₂H₂/NO_x mixtures

The reaction of C₂H₂ with NO₂ has been studied theoretically. It is a complex overall reaction with multiple wells and multiple product channels. The calculated rate constant for the preferred channel, formation of a CHOCHON adduct, is compatible with the only experimental determination. The CHOCHON adduct is assumed to dissociate rapidly to form the triplet carbene CHCHO and NO. An experimental and kinetic modeling study of the interaction between C₂H₂, O₂ and NO_x was performed under flow reactor conditions in the intermediate temperature range (600–900 K), high pressure (50–60 bar), and for stoichiometries ranging from reducing to strongly oxidizing. The results show that presence of NOx serves both to sensitize and inhibit oxidation of C₂H₂. Calculations with a detailed chemical kinetic model, partly established in the present work, confirm that C₂H₂ + NO₂ is the major initiation step, as well as the major sensitizing reaction. This reaction converts NO₂ to NO, which is then partly converted to HCN by reaction with C₂H₃ and CHCHOH. The latter reactions are both chain terminating and serve as the major inhibiting steps.