Down-fired boilers, designed especially for industry-firing low-volatile coals such as anthracite and lean coal, suffer similarly from various problems, such as poor burnout, high NOx emissions, and asymmetric combustion. A new down-fired combustion technology was developed specially for these, problems and finally trialed in a 600-MWe supercritical boiler. To evaluate the flexibility of this technology application with respect to boiler load and air-staging conditions at partial loads, industrial-size measurements (taken of gas temperatures and species concentrations in the furnace, CO and NOx emissions in the flue gas, and carbon in fly ash) were performed within the furnace at different boiler loads (i.e., 370, 450, and 600 MWe) and staged-air damper openings (i.e., 10, 30, and 45% at 450 MWe), respectively. It was found that a relatively symmetric gas temperature distribution pattern developed for all settings, despite asymmetric burner operation models at 450 and 370 MWe. Decreasing boiler load reduced gas temperature levels in the furnace to weaken coal combustion, thereby increasing combustible loss and reducing both levels of NOx emissions and exhaust gas temperature. On increasing the staged-air damper opening at 450 MWe, both gas temperatures in the furnace and NOx emissions increased initially but then decreased, while carbon in fly ash and exhaust gas temperature respectively decreased and increased. A comprehensive evaluation of results in this work suggested that (i) the boiler flexibility was poorer at 370 MWe than at 450 and 600 MWe, (ii) relatively high-O-2 operation conditions needed to be set for improving the poor burnout at the low 370-MWe load, and (iii) increasing the staged-air damper opening was favorable for the furnace performance at the moderate 450-MWe load and the optimal damper opening at the load was 45%. Again, countermeasures were recommend respectively for improving the poor furnace performance at the low 370-MWe load and for reducing the still high NOx emissions at moderate and full loads.