In situ measurements on a down-fired pulverized-coal 350 MW e utility boiler revealed that, aside from particularly high NO x emissions, an asymmetric combustion phenomenon characterized by gas temperatures near the front wall being clearly higher than those near the rear wall, had developed in the furnace. A deflected flow field, obtained by cold airflow experiments within a small-scale model of the furnace, accounts for this asymmetric combustion. With focus on these problems, a new combustion technology based on a concept of multiple-injection and multiple-staging has been developed. To establish optimal overfire air (OFA) ratio settings with this technology, cold airflow experiments were conducted recording aerodynamic field measurements within a small-scale model of the boiler at five different OFA ratio settings, i.e., 0%, 10%, 15%, 20%, and 25%. For all except the highest setting, well-formed symmetric flow fields appeared in the lower furnace and the furnace throat. Increasing the OFA ratio in 0–20% raised the reach of the OFA flow in the furnace throat without incurring adverse effects in the overall flow field. At the highest setting, a deflected flow field developed not only in the lower furnace but also in the furnace throat. To establish a symmetric flow field, along with an appropriate penetration depth of the OFA flow, an optimal setting of 20% was found for the OFA ratio.