Experiments with a small-scale furnace for a down-fired pulverized-coal 300-MWe utility boiler were carried out on a single-phase test facility to investigate the influence of different secondary air distributions on the aerodynamic field in the furnace. When the secondary air flux of tier E increased within a suitable range, it did not reverse the fuel-rich flow or shorten the residence time of coal particles in the furnace. Industrial experiments were also performed on a full-scale boiler. The gas temperature distribution along the primary air and coal mixture flow and in the furnace, and gas components such as O-2, CO, CO2, and NOx in the near-wall region, were measured with damper openings of the E-tier secondary air box at 0% and 30%. At 0%, ignition of the primary air and pulverized coal mixture was delayed and the gas temperature peak was above the burner arch, with high NOx emission. Increasing the damper opening to 30% provided the oxygen necessary for the initial combustion. This was advantageous for stable combustion and also lowered NOx emissions and carbon content in the fly ash.