Effect of separated over-fire air angle on combustion and NOx emissions in a down-fired utility boiler with a novel combustion system.

• Effects of SOFA angles on combustion, NO x and steam parameters are evaluated. • A SOFA angle of 30° is advisable for the novel combustion system. • Significant NO x reduction are achieved while limiting the carbon in the fly ash. 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 MW e Foster Wheeler down-fired boilers to reduce high NO x emissions while limiting the carbon content in the fly ash. This study comprehensively evaluates the effect of SOFA angles on flow, combustion characteristics and NO x 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 NO x 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° to 40°. NO x emissions increase slightly and the average temperature at the re-heater entrance changes slightly with the increase of SOFA angle from 0° to 30°. However, with further increasing SOFA angle from 30° to 40°, NO x 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, NO x and steam parameters into account, a SOFA angle of 30° is advisable. The actual industrial measurements also show that the steam can achieve the designed values when the SOFA angle is set at 30° instead of 40°. Significant NO x reduction (over 50 %) and good performance are attained after adopting the novel combustion technology of 30 °SOFA angle. [ABSTRACT FROM AUTHOR]