Numerical Study of NOx Formation Characteristics on the 35 MW Staged Oxy-Coal Combustion

In this study, the NOx formation has been numerically investigated on a 35 MW large-pilot oxy-fuel combustion boiler by employing an improved gas phase reaction mechanism and radiation property models. The fuel is sub-bituminous coal. After verifying the prediction with experimental data, the in-furnace flow patterns, temperature distribution, and NOx formation characteristics are analyzed in detail in the conditions of air-fuel combustion, oxy-fuel and staged oxy-fuel combustion. The results show that aerodynamic and temperature field in staged oxy-fuel combustion are similar to those in air-fuel combustion, which ensures the smoothly transition and similar heat transform characteristics between oxy-fuel combustion and air-fuel combustion. Staged oxy-fuel combustion can effectively reduce the NOx formation. More precisely, fuel staged combustion and fuel-oxygen two-way staged combustion can reduce 16% and 22-30% NOx emission, respectively. With the increase of the equivalent ratio in the burner zone, the reduction of NO becomes more significant. The fuel-oxygen two-way staged combustion can effectively reduce NOx, and the maximum NOx reduction ratio can reach 30%, then, the NOx emission in this condition is 69% lower than that in air-fuel combustion. The results in this study could provide a reference for low NOx operation of oxy-fuel combustion.