Effects of the air-staging degree on performances of a supercritical down-fired boiler at low loads: Air/particle flow, combustion, water wall temperature, energy conversion and NO_x emissions

Low load operation of down-fired boilers has become the norm such that the comprehensive performances of such boilers at low loads need to be investigated to increase the energy conversion efficiency and reduce carbon and NO_x emissions. In this work, cold-model air/particle flow experiments and industrial-scale trials were performed to study the effects of the air-staging degree (ASD) on the air/particle flow, combustion, water wall temperature, energy conversion and NO_x emission characteristics of a 350 MW supercritical down-fired boiler at low loads. The experimental conditions are four different combinations of damper openings associated with the secondary-air, tertiary-air and OFA (i.e. 60%/40%/15%, 50%/50%/15%, 35%/70%/15% and 30%/55%/55%), and are referred to as ASD = I, II, III and IV, respectively. The results showed that, as the ASD was increased from I to IV, the injection effect of the secondary-air on the fuel-rich flow was weakened. In addition, the downward diffusion of the air/particle flow toward the furnace center increased, while the particle concentration near water wall decreased and the velocity decay was accelerated. Decreasing the ASD shortened the ignition distance of the fuel-rich flow, reduced the carbon in fly ash and slag and improved the boiler efficiency. At an ASD setting of I, a higher thermal load and increased temperature near the water wall were obtained in the lower furnace. These conditions gave the highest superheated and reheated steam temperatures of 568.1 and 570.7 °C, respectively. As the ASD was increased from II to III, the cooling effect of the tertiary-air reduced the maximum water wall temperature. The reduction, in turn, increased the regulating margin of superheating degree, which raised the steam temperature. Increasing the ASD from I to IV increased the specific coal consumption from 337.36 to 348.12 g·(kW·h)⁻¹, indicating that the energy conversion efficiency of the unit was decreased. Additionally, the NO_x emissions at the furnace exit were reduced from 770 to 528 mg/m³ at 6% O₂. On the basis of the present results, an ASD setting of II is recommended to ensure the safe and economical operation of such units while minimizing environmental impact.