Kinetic modeling of the CO/H2O/O-2/NO/SO2 system: Implications for high pressure fall-off in the SO2+O(+M)=SO3(+M) reaction

Flow reactor experiments were performed to study moist CO oxidation in the presence of trace quantities of NO (0-400 ppm) and SO2 (0-1300 ppm) at pres sures and temperatures ranging from 0.5-10.0 atm and 950- 1040 K, respectiv ely. Reaction profile measurements of CO. CO2. O-2, NO, NO2, SO2, and tempe rature were used to further develop and validate a detailed chemical kineti c reaction mechanism in a manner consistent with previous studies of the CO /H-2/O-2/NOx and CO/H2O/N2O systems. in particular, the experimental data i ndicate that the spin-forbidden dissociation-recombination reaction between SO, and O-atoms is in the fall-off regime at pressures above 1 atm. The in clusion of a pressure-dependent rate constant for this reaction, using a hi gh-pressure limit determined from modeling the consumption of SO2 in a N2O/ SO2/N-2 mixture at 10.0 arm and 1000 K, brings model predictions into much better agreement with experimentally measured CO profiles over the entire p ressure range. Kinetic coupling of NOx and SOx chemistry via the radical po ol significantly reduces the ability of SO2 to inhibit oxidative processes. Measurements of SO2 indicate fractional conversions of SO2 to SO3 on the o rder of a few percent, in good agreement with previous measurements at atmo spheric pressure. Modeling results suggest that, at low pressures. SO3 form ation occurs primarily through SO2 + O(+M) = SO3(+M), but at higher pressur es where the fractional conversion of NO to NO2 increases. SO3 formation vi a SO2 + NO2 = SO3 + NO becomes important. For the conditions explored in th is study, the primary consumption pathways for SO3 appear to be SO3 + HO2 = HOSO2 + O-2 and SO3 + H = SO2 + OH. Further study of these reactions would increase the confidence with which model predictions of SO3 can be viewed. (C) 2000 John Wiley & Sons. Inc.