Arch- and wall-air distribution optimization for a down-fired 350 MWe 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 MWe furnace at various settings, i.e. ratios (denoted by Rd) 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 Rd at about 10% and 20%), respectively. At settings of Rd = 0% and 7%, an essentially symmetric flow field appeared. At the left three higher settings (i.e., Rd = 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 Rd 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 Rd 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 NOx 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 NOx emissions are to be achieved.

► Uncovering flow-field deflection and asymmetric combustion at high ratios of wall-air to arch-air.
► Determining an optimal arch- and wall-air distribution of 7–16% by cold experiments.
► Evaluating the cold-modeling experiments by real-furnace measurements.