Arch- and wall-air distribution optimization for a down-fired 350 MW _e utility boiler: A cold-modeling experimental study accompanied by real-furnace measurements

To determine the effect of the arch- and wall-air distribution on flow characteristics, experiments were conducted within a 1:15-scaled model for a down-fired 350 MW_e furnace at various settings, i.e. ratios (denoted by R_d) of the mass flow rate of staged air to that of total secondary air of 0%, 7%, 16%, 23%, and 29%. Meanwhile, industrial scale measurements were performed at full load with staged-air damper openings of 15% and 35% (equaling to R_d at about 10% and 20%), respectively. At settings of R _d = 0% and 7%, an essentially symmetric flow field appeared. At the left three higher settings (i.e., R_d = 16%, 23%, and 29%), a deflected flow field developed, with the airflow near the front wall penetrating much further than that near the rear wall. At this time, increasing R _d deteriorates the flow-field deflection. By means of cold-modeling experiments to evaluate the flow-field symmetric extent and downward airflow penetration depths in the lower furnace, the appropriate R_d was found to be in the range of 7-16%. Real-furnace measurements revealed that although the 15% opening was inapplicable in boiler operations for a long time, relatively symmetric combustion could developed at the this opening setting. At the 35% opening setting, an asymmetric combustion pattern developed in the furnace, with temperatures near the front wall being clearly higher that those near the rear wall. However, particularly high NO_x emissions and good burnout developed at both two openings. In considering that numerical simulations and industrial scale measurements in published work have confirmed the validity of a previously-proposed deep-air-staging combustion technology in achieving excellent furnace performance within down-fired furnaces, retrofitting the present furnace with the technology is thus recommended if symmetric combustion, good burnout, and low NO_x emissions are to be achieved.