Internal association between combustion behavior and NOx emissions of pulverized coal MILD-oxy combustion affected by adding H2O

In practical pulverized coal Moderate or Intense Low-oxygen Dilution oxy-fuel combustion (MILD-oxy) process, the accumulation of steam results in the recycled flue gas consisting of highly concentrated H2O, particularly under wet flue gas recirculation. This work numerically studies the individual effect of H2O on the turbulence-chemistry interaction, aiming at revealing the internal association between combustion behavior and NOx emissions. Results show that H2O addition reduces the flame temperature by enhancing the radiative heat transfer. The extremum values of turbulent Damkӧhler number (Dat,max) and Karlovitz number (Kamin) are reduced and increased, respectively, denoting the slow-chemistry feature (Dat < 10 and Ka >> 1) in the reaction zone. With the promoted H2O level, the heterogeneous reactions on the char surface tend to be governed by kinetics due to the reduced reaction rate under low flame temperature. The gasification reaction is enhanced to produce more synthesis gas, and the burnout of char particle is prolonged. The in-furnace NO formation is inhibited under low Dat,max, while the NOx emissions are reduced directly due to the enhanced gasification reaction under low maximum surface Damkӧhler number (Das,max). The addition of H2O essentially establishes a moderate diffusion and reaction process under the kinetics-controlled regime to obtain the uniform thermal field and low-NOx emissions.