Effects of the inner-secondary-air damper opening on flow and combustion in a 600-MWe down-fired boiler incorporating multiple-injection and multiple-staging

Effects of adjusting the inner-secondary-air damper opening among 100%, 70%, and 50% on the flow, combustion, and NOx emissions of a 600-MWe down-fired boiler incorporating multiple-injection and multiple-staging technology were investigated. Both 1 : 20-scale aerodynamic tests and industrial experiments were conducted. The small-scale tests showed that symmetric flow patterns formed in all cases, and the velocities of boundary and outer secondary air decayed more quickly than the velocity of inner secondary air. As the inner-secondary-air damper opening decreased from 100% to 50%, the average dimensionless depth of downward airflow reduced from 0.830 to 0.788 while the horizontal velocity in the recirculation zone increased. Industrial experiments demonstrated that, as the inner-secondary-air damper opening decreased from 100% to 50%, the heating rate of fuel-rich coal/air flow increased from 572 to 687 degrees C/m, the corresponding ignition distance shortened from 1.36 to 1.11 m, and the hopper near-wall temperature decreased by approximately 50 degrees C. Carbon in fly ash decreased from 7.31% to 6.93%, and NOx emissions rose from 343 to 371 ppm at 6% O-2. The boiler efficiency improved from 89.78% to 89.92%. An inner-secondary-air damper opening of 70% is more reasonable than 50% and 100% considering the ignition distance, carbon in fly ash, and NOx emissions. Copyright (C) 2017 Curtin University of Technology and John Wiley & Sons, Ltd.