Cold-modeling flow characteristics for a 300-MWe down-fired furnace at different secondary-air distributions

Deflected flow fields and large combustion differences between zones near front and rear walls have been found in down-fired pulverized-coal boilers under symmetric air distribution modes. To eliminate or mitigate the flow-field deflection and achieve relatively symmetric combustion in these boilers, the secondary-air distribution ejected through the front and rear arches was adjusted to construct an asymmetric secondary-air distribution mode. Cold-modeling airflow experiments over a wide range of asymmetric secondary-air distributions (i.e., differences in the ratio of secondary-air mass flow rate between the front and rear arches (Rd) of −16%, –8%, 0%, 5%, 8%, 16%, and 32%) were conducted within a small-scale model of a down-fired pulverized-coal 300 MWe utility boiler. Results revealed that a steady and symmetric flow field could not be achieved simply by adjusting the secondary-air distribution between the front and rear arches. To establish a flow field along with an appropriate airflow reach for more economical operation, an optimal setting of Rd = 5% was found for the secondary-air distribution between the front and rear arches. Industrial-size measurements revealed that a secondary-air distribution setting of Rd = 6.3% (i.e., approaching to the cold-modeling optimized result of Rd = 5%) was applicable if applied in the real furnace to deal with asymmetric combustion, low burnout, and high NOx emissions.

► Asymmetric combustion and flow-field deflections are universal in MBEL down-fired boilers.
► With focus on this subject, an asymmetric secondary-air distribution method has been developed.
► Cold airflow experiments were conducted to establish an optimal secondary-air distribution model.
► An optimal setting for asymmetric secondary-air distributions was found at Rd = 5%.
► Real-furnace results confirmed that this method was applicable in combustion improvement.