Effect of secondary air mass flow rate on the airflow and combustion characteristics and NO_x formation of the low-volatile coal-fired swirl burner

Laboratory experiments and industrial-scale experiments on 300-MW low-volatile coal-fired boiler were performed under deep air staging. Aerodynamic characteristics, gas temperature and concentrations, furnace temperature, and boiler efficiency were measured for various secondary air mass flow rates. Under deep air staging, a steady central recirculation zone forms near the burner nozzle. With a decreasing flow rate, the swirl intensity and maximum axial, radial, and tangential velocities decrease; between X/D=0-0.8, the velocity decay rate decreases, and the relative reverse flow rate decreases. In the early stage, the maximum axial mixing rate decreases; the primary air concentration increases slightly. A decrease in secondary air-box damper opening decreases the gas temperature and its increasing rate, generating a farther ignition position. In the initial stage, the O₂ consumption rate and CO concentration increase, while the NO_x concentration decreases. In the later stage, O₂ concentration remains almost constant below 2%, and the CO concentrations exceed 15000ppm, which restrain NO_x formation notably. Near the water-cooled wall, flue gas temperature varies slightly, and O₂ concentration exceeds 3%. Along radial direction 1.5364≤R₀/D₀≤1.8708, the O₂ concentrations are below 2%, and CO concentrations exceed 6000ppm. A decreasing damper opening decreases furnace temperature in the primary combustion zone slightly. NO_x emission decreases from 833.4 to 769.9mg/m³ (6% O₂), unburnt carbon increases from 5.18% to 6.84%, and boiler efficiency decreases from 91.53% to 90.99%.