Kinetic modeling of the CO/H2O/O2/NO/SO2 system: Implications for high-pressure fall-off in the SO2 + O(+M) = SO3(+M) reaction

Abstract

Flow reactor experiments were performed to study moist CO oxidation in the presence of trace quantities of NO (0-400 ppm) and SO 2 (0-1300 ppm) at pressures and temperatures ranging from 0.5-10.0 atm and 950-1040 K, respectively. Reaction profile measurements of CO, CO 2 , O 2 , NO, NO 2 , SO 2 , and temperature were used to further develop and validate a detailed chemical kinetic reaction mechanism in a manner consistent with previous studies of the CO/H 2 /O 2 /NO X and CO/H 2 O/N 2 O systems. In particular, the experimental data indicate that the spin-forbidden dissociation-recombination reaction between SO 2 and Oatoms is in the fall-off regime at pressures above 1 atm. The inclusion of a pressure-dependent rate constant for this reaction, using a high-pressure limit determined from modeling the consumption of SO 2 in a N 2 O/SO 2 /N 2 mixture at 10.0 atm and 1000 K, brings model predictions into much better agreement with experimentally measured CO profiles over the entire pressure range. Kinetic coupling of NO X and SO X chemistry via the radical pool significantly reduces the ability of SO 2 to inhibit oxidative processes. Measurements of SO 2 indicate fractional conversions of SO 2 to SO 3 on the order of a few percent, in good agreement with previous measurements at atmospheric pressure. Modeling results suggest that, at low pressures, SO 3 formation occurs primarily through SO 2 ϩ O(ϩM) ϭ SO 3 (ϩM), but at higher pressures where the fractional conversion of NO to NO 2 increases, SO 3 formation via SO 2 ϩ NO 2 ϭ SO 3 ϩ NO becomes important. For the conditions explored in this study, the primary consumption pathways for SO 3 appear to be SO 3 ϩ HO 2 ϭ HOSO 2 ϩ O 2 and SO 3 ϩ H ϭ SO 2 ϩ OH. Further study of these reactions would increase the confidence with which model predictions of SO 3 can be viewed.

Thermal Dissociation of SO3 at 1000−1400 K

“…The product gas from the homogeneous reactor is analyzed. (4)(5)(6)(7)(8)(9)(10) Step 3 is repeated for temperatures of 948, 973, 998, 1023, 1048, 1073, and 1176 K. , from which k 1b can easily be obtained. Since R2 is comparatively fast according to the flame measurements, the inequality is almost fulfilled and the error in the values in k 1b from the simple analysis, compared to the detailed analysis, is only of the order of 30%.…”

“…The latter include the room-temperature result of Atkinson and Pitts, Jr. 8 and indirect measurements by Merryman and Levy from flames. 14 The results from the flow reactor study of Mueller et al 7 and the shock tube study of Hwang et al 15 on the reaction are not included, since they were obtained at higher pressures. The hightemperature data of Astholz et al 10 and Smith et al 12 are shown, even though they relate to argon as a bath gas.…”

“…According to the present study, the rate constant using N 2 as collision partner does not deviate much from that using Ar. It is worthwhile to compare the present results with those of Mueller et al, 7 who studied the high-pressure falloff behavior of the reaction. Under the conditions of the present work (1000-1400 K, atmospheric pressure), their proposed rate constant for SO 2 + O (+N 2 ) is in very good agreement with the present work, being within 25% of our theoretical value.…”

“…[5][6][7] In the presence of nitrogen oxides, there is a kinetic coupling of NO x and SO x chemistry via the radical pool. 5,7 While the overall mechanisms for SO 3 formation are fairly well-known, kinetic modeling of the process still suffers from a lack of accurate kinetic data and refinements are required in order to establish a reliable mechanism. 4,5 Flow reactor studies of SO 2 oxidation under post-flame conditions 5,7 indicate fractional conversions of SO 2 to SO 3 of the order of a few percent.…”

“…5,7 While the overall mechanisms for SO 3 formation are fairly well-known, kinetic modeling of the process still suffers from a lack of accurate kinetic data and refinements are required in order to establish a reliable mechanism. 4,5 Flow reactor studies of SO 2 oxidation under post-flame conditions 5,7 indicate fractional conversions of SO 2 to SO 3 of the order of a few percent. According to these studies, SO 3 formation occurs primarily through the reaction Direct measurements are available for this reaction in the 300-800 K range 8,9 and for the reverse reaction, thermal dissociation of SO 3 , in the temperature range of 1700-2500 K. 10 Indirect determinations are available from flames [12][13][14] and shock tube experiments.…”