Abstract: A novel combustion system has been applied to a 600 MW Babcock & Wilcox (B & W) down-fired boiler to reduce NOx emissions without obvious increasing of the carbon content in fly ash. The modification mainly includes rearranging secondary air,and introducing separated over-fire air ( SOFA). The aim of this work is to evaluate the overall performance of the novel combustion system with different SOFA ratios (e. g. ,15% ,20% ,and 25% ) and dif- ferent SOFA locations in the upper furnace (e. g. ,3. 0,4. 0,5. 0,and 6. 0 m above the arches) using numerical simu- lations. In order to calculate the carbon content of fly ash more accurately, this paper used CBK coke combustion mode. Results showed that not only the NOx emissions were greatly reduced from 823 mg / m3 to 738 mg / m3 ,but also the carbon content in the fly ash decreased slightly in 0. 264% with increasing the SOFA ratio from 15% to 20% . When the SOFA ratio increased from 20% to 25% ,NOx emissions only decreased 32 mg / m3 ,but the combustible con- tent in the fly ash increased obviously in 0. 41% . Considering the NOx emissions and combustible content in the fly ash,an SOFA ratio of 20% was advisable. With increasing the height of SOFA from 3. 0 to 6. 0 m above the arches,the carbon content in the fly ash somewhat increased while NOx emissions decreased. Taking economic efficiency and NOx emissions into account,the location (5. 0 m above the arches) was the optimal SOFA location.