Inhibition and sensitization of fuel oxidation by SO2

An experimental and theoretical study of the interaction of SO2 with the radical pool under combustion conditions has been carried out. Experiments on moist CO oxidation were conducted in an isothermal quartz Row reactor at 1 arm; 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 SO2 were analyzed. The results show that under flow-reactor conditions SO2 may inhibit or promote oxidation of fuel, depending on conditions. In a narrow range of operating conditions close to stoichiometric SO2 promotes oxidation through the sequence: SO2 + H reversible arrow SO + OH, SO + O-2 reversible arrow SO2 + O. Inhibition of oxidation by removal of radicals can be explained in terms of the SO2 + O + M reaction, even under fuel-rich conditions. From the shift in temperature for the onset of CO oxidation because of SO2 addition under reducing conditions an upper limit of 3.0 x 10(14) cm(6) mol(-2) s(-1) at 1,060 K can be estimated for the rate constant of H + SO2 + N-2 reversible arrow 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 SO2 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. species, rather than to a mechanism initiated by the H + SO2 + M reaction. (C) 2001 by The Combustion Institute.