A numerical study on extinction and NOx formation in nonpremixed flames with syngas fuel

The flame structure, extinction, and NO_x emission characteristics of syngas/air nonpremixed flames, have been investigated numerically. The extinction stretch rate increased with the increase in the hydrogen proportion in the syngas and with lower fuel dilution and higher initial temperature. It also increased with pressure, except for the case of highly diluted fuel at high pressure. The maximum temperature and the emission index of nitric oxides (EINO_x) also increased in aforementioned conditions. The EINO_x decreased with stretch rate in general, while the decreasing rate was found to be somewhat different between the cases of N₂ and CO₂ dilutions. The reaction paths of NO_x formation were analyzed and represented as NO reaction path diagram. The increase in N radical resulted in larger NO_x production at high initial temperature and pressure. As the pressure increases, EINO_x increases slower due to the third-body recombination. The thermal NO mechanism is weakened for high dilution cases and non-thermal mechanisms prevail. The combustion conditions achieving higher extinction stretch rate can be lead to more NO_x emission, therefore that the selection of optimum operation range is needed in syngas combustion.