Effects of tertiary air injection on the reaction behavior and NOx emissions during pulverized coal air-staged MILD combustion

The combination of air-staged combustion with coal MILD combustion has the potential advantage to further reduce NO_x emissions, and the reduction effect is highly affected by the tertiary air injection parameters. In this study, the influence of tertiary air injection position (z_ter) and tertiary air proportion (P_ter) on the reaction behavior and NO_x formation during pulverized coal air-staged MILD combustion are studied. Results show that, the momentum introduced by tertiary air leads to the flue gas recirculation (FGR) broken into small eddies for P_ter ≥ 10 %. Compared with non-air-staged combustion, the average temperatures at different z_ter increase by 1–3 K when P_ter = 5 %, while they gradually decrease by 4–7 K when P_ter increases to 20 %. The MILD regime region distribution identified by internal recirculation and temperature decreases by 31.2–44.0 % with the increment in P_ter from 5 % to 20 %, indicating the inhabitation to FGR from strong tertiary air. While in terms of the maximum surface Damkӧhler number, the MILD combustion is enhanced for P_ter = 5–15 %, as the maximum surface Damkӧhler numbers in these cases are lower than that of the non-air-staged case. The char burnout region is expanded in air-staged combustion due to the entrainment of tertiary air to the fuel. In air-staged MILD combustion, the NO concentration in dry flue gas reaches minimum value of 168 ppm for z_ter = 1.0 m and P_ter = 10 %, which ascribed to the large oxy-lean region where NO is converted to N₂ through NH₃ reduction reaction of NH₃ + NO → N₂ + product 4.