A new overfire air (OFA) technology was proposed by the authors for a Foster Wheeler down-fired boiler, and a small-scale cold experimental system for a 660-MWe unit was established. The velocity field distribution was measured in the furnace to study the characteristics of single-phase flow in the furnace at different OFA nozzle angles. The furnace temperature and distributions of the O-2 concentration and NO concentration at different OFA nozzle angles were simulated using Fluent 6.3.26, and the average temperature of the furnace outlet, the O-2 and NOx concentrations and carbon content in the fly ash were calculated. As the OFA nozzle angle increased, the vertical penetration depth of OFA increased and the horizontal penetration depth of OFA gradually decreased. The carbon content in the fly ash and NOx concentration initially rose and then fell at the furnace outlet, and they were lowest when the OFA angle was set at 20 degrees. Thus, according to the findings of this article, an optimized OFA angle of 20 degrees was chosen. (C) 2013 Elsevier Ltd. All rights reserved.