Abstract: In order to meet the strict emission standards,most of the power station boilers have implemented NOxemission control technology. A 420 t/h tangential coal fired boiler of utilizing bituminous coal was retrofitted by replacing original burners with horizontal bias combustion( HBC) ones as well as introducing separated over-fire air( SOFA) to achieve the effect of low nitrogen combustion. The numerical simulation method was used to demonstrate the scheme. The effects of the primary air concentration ratio,SOFA air rate and SOFA air jet angle on the combustion status and NOxemission of the boiler were studied,and the optimum modification scheme was put forward. As the rich-lean ratios increasing,the furnace outlet temperature gradually increases,while the NOxcontent gradually decreases. When the richlean ratios is 4 ∶ 1,the carbon content in fly ash is the lowest. As rich-lean ratios increasing,the concentration of CO increases,which enhances the reduction of the main combustion zone and inhibits the oxidation of volatile nitrogen-containing intermediates into NO. Moreover,the increase of rich-lean ratios makes the rate of volatilization of concentrated coal powder increase. The volatile nitrogen-containing intermediates HCN and NH3 are used to reduce the produced NO to N2. At the same time,the lean side gas pulverized coal concentration is low,the amount of the nitrogen-containing group precipitation is small,and the amount of NO formed by reaction with oxygen is reduced.With the increase of SOFA ratios,the flue gas temperature of the furnace exit and the carbon content of the fly ash increase,and the NOxconcentration is higher at the 20% SOFA ratios,and the NOxconcentration remains basically unchanged when the SOFA wind rate increases from 30% to 40%. With the increase of SOFA ratios,the strong reducing atmosphere with low O2 and high CO concentration formed in the main combustion zone inhibits the oxidation of HCN and NH3 to NO,which in turn promotes the reaction with the generated NO to form N2. In addition,under the high SOFA ratios,the high temperature zone of the main combustion zone is reduced,and the thermal NOx is also reduced accordingly. As the SOFA jet angle increases,the reduction zone lengthens,which is conducive to reducing the NOxconcentration,but the flame center in the burnout zone rises,and the pulverized coal burnout time become shorter,and the furnace exit temperature and fly ash carbon content increase. As the jet angle increases,the O2 concentration decreases and the CO concentration increases. Because the increase of jet angle delays the pulverized coal burnout process and increases the chemical incomplete combustion loss. The strong reducing atmosphere with low O2 and high CO concentration greatly suppresses NOxgeneration. According to the numerical simulation results,the low-nitrogen combustion reforming scheme of the boiler is determined as: a horizontal rich-lean burner with a ratio of 4 ∶ 1 is selected as the transformation burner,the SOFA ratios is set at 30%,and the SOFA jet angle is raised by 15°. The on-site measurement after retrofitting showed that combustion characteristic of the furnace was stable. The NOxemissions decreased significantly to 220 mg/Nm3( The decline is 65%-70%). The unburned carbon content in fly ash was as low as 3%-4% at the same time. It shows that the retrofitting scheme can achieve good low nitrogen combustion effect.