A new air-staged and low-NOx emission combustion technology has been applied to a 300 MW, anthracite- and down-fired boiler with swirl burners. To achieve the optimum air distribution in the primary combustion zone and further reduce NOx emissions, full-scale industrial experiments varying the ratios between burner secondary air ratio (R-bsa) and staged air ratio (R-sa) (i.e., R-bsa/R-sa ratios of 42.9/17.8, 47.1/13.5 and 51.5/9.3), on the premise that the overfire air (OFA) ratio was approximately 20%, were performed to evaluate the overall performance of retrofitted boiler. The improved ignition of coal/air flow was at a distance of 0.8-1.4 m to the burner outlet with increased R-bsa/R-sa ratio. Compared with the boiler before retrofit, under R-bsa/R-sa, ratios of 47.1/13.5 and 51.5/9.3, the flue gas temperature in the primary combustion zone of the retrofitted boiler increased slightly. Oxygen and carbon monoxide concentrations in the near-sidewall region revealed the furnace flame fullness varied under different R-bsa/R-sa, ratios. With increased the air staged level in the lower furnace, NOx, emissions decreased continually and the carbon content in fly ash did not necessarily increased. Under the optimum R-bsa/R-sa ratio of 47.1/13.5, the NOx, reduction efficiency was approximately 46% with increasing boiler thermal efficiency slightly. (C) 2018 Elsevier Ltd. All rights reserved.