Effect of secondary air mass flow rate on the airflow and combustion characteristics and NOx 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 O-2 consumption rate and CO concentration increase, while the NOx concentration decreases. In the later stage, O-2 concentration remains almost constant below 2%, and the CO concentrations exceed 15 000 ppm, which restrain NOx formation notably. Near the water-cooled wall, flue gas temperature varies slightly, and O-2 concentration exceeds 3%. Along radial direction 1.5364 <= R-0/D-0 <= 1.8708, the O-2 concentrations are below 2%, and CO concentrations exceed 6000 ppm. A decreasing damper opening decreases furnace temperature in the primary combustion zone slightly. NOx emission decreases from 833.4 to 769.9 mg/m(3) (6% O-2), unburnt carbon increases from 5.18% to 6.84%, and boiler efficiency decreases from 91.53% to 90.99%. (C) 2015 Curtin University of Technology and John Wiley & Sons, Ltd.