Inner and outer secondary-air distance-effect study within a cold small-scale model of a new down-fired 600 MW _e supercritical utility boiler

To achieve significant reductions in particularly high NO _x emissions and to eliminate severe asymmetric combustion in down-fired boilers, a multiple-injection and multiple-staging combustion technology was developed in our previous study. That technology was trialed in a newly designed down-fired 600 MW _e supercritical utility boiler, without applying overfire air. In consequence, a reconfiguring of the secondary air for the redesigned burners on the arches, composed of direct-flow inner and outer secondary air, had to be performed for the newly reconfigured furnace. The present work reports our experimental investigation on the impact of the distance between inner and outer secondary air on aerodynamic characteristics within the furnace, determined by cold airflow experiments within a cold 1:20-scaled model of the furnace. Aerodynamic field measurements were conducted at four different settings, each distinguished by the ratio (denoted by C _s) of the distance between the inner and outer secondary air to the arch depth; values for C _s were fixed at 7, 9, 12, and 15%. At the two lower settings, the aerodynamic field and velocity distributions at certain cross-sections as well as airflow penetration depths were symmetric along the furnace center. At the two higher settings, a deflected flow field appeared in the lower furnace. In considering a symmetric flow field along with an appropriate airflow penetration depth, an optimal distance between the inner and outer secondary-air ports on the arch should be at C _s = 7-9% for the reconfigured furnace.