Numerical simulation of NO_x production under deep air staged combustion in furnace

With the CFD(computational fluid dynamics) software FLUENT, numerical simulation was made in a 600 MW ultra supercritical wall-tangentially fired boiler. The distribution characteristics of fuel NO, thermal NO and total NO in the furnace were analyzed in detail under the condition of deep air staged combustion, and the production characteristics of NO and HCN at the beginning of combustion were also studied. The result shows that a large amount of fuel NO is formed at the stage of evaporation. Homogenous subtractive reaction will occur between HCN which has been produced by char combustion and NO which has been produced previously to form N₂ under lean oxygen. In both fierce combustion zone of char where temperature is high and SOFA zone where temperature and oxygen concentration are both high, thermal NO is high. Total NO and fuel NO production rate distributions are generally similar in the main combustion zone. Total NO production rate in SOFA zone is generally the superposition of both fuel NO and thermal NO. The concentrated secondary air in the main combustion zone is unfavorable for NO_x reduction. Within the distance of 3 m, NO production rate changes fast. The changing trend of the production rate of HCN and NO is the same, which has a close relationship with evaporation and char combustion. From the results of air staged optimization, the concentration of NO_x is lowest at the SOFA ratio of 25%. The key for organizing air staged low NO_x combustion is that char combusts under the condition of lean oxygen in the main combustion zone. The oxygen concentration in SOFA zone shouldn't be too high, so the key for low NO_x technique is the optimization of oxygen ratios between main combustion zone and SOFA zone.