Inhibition and sensitization of fuel oxidation by SO 2

Abstract

An experimental and theoretical study of the interaction of SO 2 with the radical pool under combustion conditions has been carried out. Experiments on moist CO oxidation were conducted in an isothermal quartz flow reactor at 1 atm; temperature ranged from 800 to 1,500 K and stoichiometries from fuel-rich to very lean. In addition, literature data on sulfur species concentration profiles and H atom decay in fuel-rich H 2/O 2 flames doped with SO 2 were analyzed. The results show that under flow-reactor conditions SO 2 may inhibit or promote oxidation of fuel, depending on conditions. In a narrow range of operating conditions close to stoichiometric SO 2 promotes oxidation through the sequence: SO 2 + H ⇌ SO + OH, SO + O 2 ⇌ SO 2 + O. Inhibition of oxidation by removal of radicals can be explained in terms of the SO 2+O+M reaction, even under fuel-rich conditions. From the shift in temperature for the onset of CO oxidation because of SO 2 addition under reducing conditions an upper limit of 3.0 × 10 14 cm 6 mol −2s −1 at 1,060 K can be estimated for the rate constant of H + SO 2 + N 2 ⇌ HOSO + N 2. This value is consistent with a significant barrier to reaction as proposed theoretically, but an order of magnitude lower than indicated by both ab initio calculations (Marshall and co-workers) and reaction rates derived from flames. However, we find that data on H atom decay in flames doped with SO 2 are not suitable for deriving rate constants because of uncertainty in important side reactions involving SO. Furthermore, we propose that the enhanced H atom decay observed in these flames may be attributed to recombination of H atoms with SO and S 2 species, rather than to a mechanism initiated by the H + SO 2 + M reaction.