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

- O3 substantially expands the flammability limits of syngas.
- O3 sustains the syngas flame at higher N2, CO2 and H2O dilution ratios.
- The O3 effect on increasing burning rate is stronger under near limit conditions.
- O3 is consumed at low temperatures (∼700 K), releasing active radicals.
- Active radicals released by O3 chemistry interfere with H2/CO/O2 chemistry.

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.