Effect of fuel injection velocity on MILD combustion of syngas in axially-staged combustor

The role of fuel injection velocity on MILD (Moderate or Intense Low-oxygen Dilution) combustion of coal-derived syngas was examined in an axially staged combustor, where the secondary air was mixed with the flue gases from the gas generation zone to produce hot and diluted oxidant prior to its mixing with the secondary fuel. The global flame signatures, OH∗ radicals distribution, and exhaust emissions were obtained through experimental measurements, while the mixing behavior between the secondary fuel and oxidant was numerically studied. Higher secondary fuel injection velocity within 199–299 m/s facilitated the earlier entrainment of oxidizer into the secondary fuel and increased the flame lift-off height, resulting in a lower flame temperature, a more distributed reaction zone and reduced NOx emissions, but higher pressure loss and CO formation. The MILD regime yields lower NOx emissions compared to the traditional diffusion combustion mode, and the N2O-intermediate mechanism dominates the NO production in the syngas MILD flame with adiabatic flame temperature lower than 1565 K according to the prediction of the chemical reactor network model. •Higher secondary fuel injection velocity is beneficial for the establishment of MILD scheme.•NOx emissions of MILD combustion are lower than those of conventional diffusion flame.•N2O-intermediate mechanism dominates the NO production in syngas flame with adiabatic flame temperature lower than 1565 K.