A FLOW REACTOR STUDY OF HNCO OXIDATION CHEMISTRY

An experimental and theoretical study of HNCO oxidation has been carri ed out. The experiments were performed in an isothermal quartz flow re actor, and the effects of temperature, CO concentration, and NO concen tration were investigated at atmospheric pressure in the temperature r ange 1025-1425 K. The reaction mechanism for RAPRENO(x) proposed by Mi ller and Bowman (1991) has been updated based on the present results a s well as recent advances in the understanding of important elementary steps. Model predictions with the revised mechanism are in good agree ment with our experimental data as well as data from the literature. O xidation of HNCO proceeds mainly through NCO, which subsequently is ox idized to NO or reacts with NO to form N2 and N2O. This sequence of re actions is chain terminating, and for reaction to occur, radicals must be generated either by alternative oxidation pathways or by the prese nce of other combustibles. A chain-branching oxidation route initiated by reaction of HNCO with O2 is proposed in order to explain the obser ved HNCO decay in the absence of inlet CO. Addition of CO enhances HNC O oxidation and the RAPRENO(x) chemistry, since CO oxidation acts to r eplenish the radical pool. The experimental results show that the mutu al presence of HNCO and NO strongly inhibits CO oxidation at lower tem peratures. In addition to the chain terminating HNCO/NCO reactions, a second inhibition mechanism involving NO is necessary to explain this behavior. This mechanism is presently believed to be NO/NO2 interconve rsion, but additional work is needed to confirm this. Further progress in the understanding of the HNCO chemistry is dependent on an accurat e determination of the rate and/or mechanism of a number of key reacti ons, including HNCO + OH, HNCO + O2, NCO + NO and NO + O + M.