Ozone effect on the flammability limit and near-limit combustion of syngas/air flames with N₂, CO₂, and H₂O dilutions

The effect of the presence of ozone on the flammability limit and near-limit combustion of syngas (H₂/CO)/O₂ flames with N₂, CO₂, and H₂O dilutions was numerically studied. This study involved systematic simulations of 1-D planer laminar premixed flames under near-limit conditions using the PREMIX code considering the detailed description of chemical kinetics and molecular transport, and the intrinsic radiation as the only heat loss mechanism. The flammability limits were determined by solving the singular turning points using the one-point continuation mathematical approach. The results showed that the presence of O₃ expanded the flammability limit of syngas as O₃ reduced the lower flammability limit and increased the upper flammability limit. With increasing O₃, the H₂/CO/O₂ flames could be sustained at higher N₂, CO₂ and H₂O dilution ratios. The laminar flame speed was also enhanced by the presence of O₃, and the enhancement effect of O₃ on the laminar flame speed was much more profound under near-limit conditions, being 4–11 times greater than that under near-stoichiometric conditions. Detailed near-limit flame structure and kinetic studies revealed that O₃ was consumed at relatively low temperatures (∼700 K), producing active radicals, such as H and OH. These radicals interfered with the H₂/CO/O₂ chemistry and thereby significantly accelerated the overall combustion process under near-limit conditions. For near-stoichiometric flames, the large disparity between the characteristic temperatures of O₃ related reactions (∼500 K) and those of H₂/CO/O₂ chemistry (>1250 K) depressed the interaction between O₃ related reactions and H₂/CO/O₂ chemistry and thus only limited O₃ enhancement effect was observed under near-stoichiometric conditions.