To ensure the powder carrying capacity of primary air

when the boiler is running at a low load

the primary air volume cannot be reduced in proportion to the load

which leads to an increase in oxygen-to-coal ratio at initial stage of combustion and an increase in NO_x production. By using the method of primary air mixed with flue gas

the air volume and oxygen quantity are decoupled

the primary air volume and oxygen quantity are controlled individually

thus to reduce the oxygen concentration on the premise of ensuring the primary air volume

which can decrease the formation of NO_x from the source. According to the structural parameters and low-load operating parameters of a 330 MW unit boiler

numerical simulation was carried out to study the effects of flue gas mixing ratio on the average temperature in furnace section and the volume fraction of O_2 and CO as well as the mass concentration of NO_x produced. The results show that

blending flue gas in primary air at low load can effectively reduce the NO_x generation in the boiler. With the increase of the mixing ratio of flue gas

the average temperature in the furnace section and the CO volume fraction gradually decreases

the average temperature at the furnace outlet gradually increases

and the NO_x production reduces. Compared with the furnace with no flue gas mixing in primary air

the mass concentration of NO_x production in the furnace with flue gas's mixing ratio of 15% reduces by 79.92 mg/m~3.