A deep-air-staging combustion technology was previously developed to achieve reduction in NOx emissions and to eliminate strongly asymmetric combustion found in down-fired boilers. Recently, one of two down-fired 600 MWe supercritical utility boilers using this technology (without applying overfire air) began commercial operations. To understand coal combustion and NOx emissions characteristics within the furnace, full-load industrial-size experiments were performed at different air-staging conditions with measurements taken of gas temperatures in the burner region, gas temperatures and species concentrations in the near wing-wall region, carbon content in fly ash, and NOx emissions. As expected, the furnace performance characterized by relatively timely coal ignition, symmetric combustion, and low levels of carbon in fly ash, developed in the furnace at all three settings. Deepening the air-staging conditions could reduce NOx emissions by one-fifth, but varied slightly carbon in fly ash. In view of the still high NOx production (i.e., 1036 mg/m(3) at 6% O-2), adding an overfire air system which was essentially a part of the technology, was recommended for the boiler to significantly reduce the present NOx emissions. (C) 2013 Elsevier Ltd. All rights reserved.