MECHANISM AND MODELING OF HYDROGEN-CYANIDE OXIDATION IN A FLOW REACTOR

The oxidation of hydrogen cyanide under flow reactor conditions (atmospheric pressure, 900-1400 K) has been examined. The study is based mainly on experimental data from the literature on the effect of NO and CO on HCN oxidation, emphasizing N2O formation. However, additional experiments were conducted during the present work in order to investigate the importance of HNCO as an intermediate. The experimental data are compared with model predictions, using a revised reaction mechanism for HCN oxidation. Recent advances in our knowledge of thermodynamic properties for CN, NCO, and HNCO, as well as of the mechanism and rate constants for a number of key reactions have reduced the uncertainty in the model considerably: now model predictions are in good agreement with the experimental data. Compared with the previous HCN oxidation models, particularly the prediction of N2O is significantly improved, aided by better knowledge about the NCO + NO reaction. Under the conditions investigated, the main oxidation route for HCN proceeds through NCO, formed by the reaction the sequence HCN + OH --> CN + H2O, CN + O-2 --> NCO + O. The subsequent reactions of NCO determine the fate of the nitrogen atom. Depending on the gas composition and temperature, NCO is converted to HNCO (by reaction With H2O or HCN), N2O/N-2 (by reaction with NO) or NO (by reaction with O). Both HNCO and N2O are important intermediates in HCN oxidation under these conditions. The present results are significant for understanding the fate of reactive nitrogen in fluidized bed combustion and staged combustion, particularly the formation and destruction of N2O.