Effects of the gas/particle flow and combustion characteristics on water-wall temperature and energy conversion in a supercritical down-fired boiler at different secondary-air distributions

Abstract To alleviate the over-temperature of water-wall and increase the energy conversion efficiency of supercritical down-fired boilers, cold-model air/particle flow experiments and in-situ trials were performed to investigate the effects of the secondary-air distribution wall-side deflection coefficient (ws) on air/particle flow and combustion characteristics. As ws increased from 0 to 0.33, the amount of air/particle flow diffused toward water-wall and the corresponding particle concentration both decreased, while the velocity decay was accelerated. At a ws of 0.16, due to prior ignition of fuel-rich flow and a longer flame stroke in lower furnace, the carbon in fly ash and slag were lowest. As ws increased, the thermal load was gradually focused at the furnace center, and the maximum value and deviation of water-wall temperature in lower furnace decreased. Consequently, the temperatures of superheated and reheated steam respectively increased from 542.7 °C to 543.9 °C–560.2 °C and 560.1 °C, respectively. Furthermore, increasing ws from 0 to 0.28 decreased the specific coal consumption from 346.29 to 341.09 g (kW h)−1, indicating the energy conversion efficiency was improved. However, adjusting ws had only a minimal effect on NOx emissions. A ws of 0.28 is recommended to optimize the water-wall temperature, pulverized-coal burnout, and economic performance.