A novel combustion system, consisting of moving fuel-lean nozzles from the arches to the front/rear walls, rearranging staged air, and introducing separated over-fire air (SOFA), has been proposed and successfully applied in a 600 MWe Foster Wheeler down-fired boilers to reduce high NOx emissions while limiting the carbon content in the fly ash. This study comprehensively evaluates the effect of SOFA angles on flow, combustion characteristics and NOx emissions for the novel combustion system by numerical simulation and in-situ experiment. The numerical simulation shows in acceptable consistence with the in-situ experiment. The simulated results show that significant NOx reduction is obtained for all five SOFA angles compared to the original combustion system. O-2 and CO concentrations at the furnace outlet, as well as the carbon content in the fly ash decrease significantly with increasing SOFA angle from 0 degrees to 40 degrees. NOx emissions increase slightly and the average temperature at the re-heater entrance changes slightly with the increase of SOFA angle from 0 degrees to 30 degrees. However, with further increasing SOFA angle from 30 degrees to 40 degrees, NOx emissions increase substantially due to the smaller reducing region below the SOFA and the average temperature at the re-heater entrance falls significantly. Taking combustion, NOx and steam parameters into account, a SOFA angle of 30 degrees is advisable. The actual industrial measurements also show that the steam can achieve the designed values when the SOFA angle is set at 30 degrees instead of 40 degrees. Significant NOx reduction (over 50 %) and good performance are attained after adopting the novel combustion technology of 30 degrees SOFA angle. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.