Influence of staged-air on airflow, combustion characteristics and NO _x emissions of a down-fired pulverized-coal 300 MW_e utility boiler with direct flow split burners

Cold airflow experiments were conducted to investigate the aerodynamic field in a small-scale furnace of a down-fired pulverized-coal 300 MW _e utility boiler arranged with direct flow split burners enriched by cyclones. By increasing the staged-air ratio, a deflected flow field appeared in the lower furnace; larger staged-air ratios produced larger deflections. Industrial-sized experiments on a full-scale boiler were also performed at different staged-air damper openings with measurements taken of gas temperatures in the burner region and near the right-side wall, wall heat fluxes, and gas components (O₂, CO, and NO_x) in the near-wall region. Combustion was unstable at staged-air damper openings below 30%. For openings of 30% and 40%, late ignition of the pulverized coal developed and large differences arose in gas temperatures and heat fluxes between the regions near the front and rear walls. In conjunction, carbon content in the fly ash was high and boiler efficiency was low with high NO_x emission above 1200 mg/m³ (at 6% O₂ dry). For fully open dampers, differences in gas temperatures and heat fluxes, carbon in fly ash and NO_x emission decreased yielding an increase in boiler efficiency. The optimal setting is fully open staged-air dampers.