Using a phase-Doppler anemometry (PDA) on a two-phase small-scale model for a down-fired 600-MW, supercritical utility boiler, experiments were conducted to compare the gas/particle flow characteristics between the prior and newly designed deep-air-staging combustion systems. The distributions of mean velocity, particle volume flux, and particle number concentration along several cross sections were compared between the two combustion systems, in addition to the decay and trajectory of the downward gas/particle flow. With the prior combustion system, asymmetric gas/particle flow characteristics appear in the furnace, with the gas/particle flows in the front-half of the furnace penetrating greatly further and occupying much more furnace volume than those in the rear-half of the furnace. The longitudinal gas/particle velocity components, particle volume flux, particle number concentration, decay curve, and trajectory of the downward gas/particle flow all display a severe asymmetric pattern along the furnace center. In applying the deep-air-staging combustion technology as a replacement for the prior art, the original asymmetric gas/particle flow characteristics that are seen in the furnace all develop a symmetric pattern. Industrial-sized measurements performed within the full-scale furnace uncovered that asymmetric combustion characterized by gas temperatures being much higher near the rear wall than near the front wall, developed in the boiler with the prior art. In comparison with the boiler with the prior art, the newly designed boiler applying the deep-air-staging combustion system achieved perfectly symmetric combustion, NOx emission reduction by around 40%, and large improvement in burnout.