Technical retrofit was conducted on a 300 MW Foster Wheeler down-fired boiler for burning bituminous coal. Firstly

the area of refractory belt was reduced from 550 m~2 to 450

350 and 300 m~2. Then

numerical simulation was carried out and the result shows that the reduction of the area of the refractory zone helps to reduce the carbon content in fly ash

while it affects little on NOx emission mass concentration(the NOx mass concentration is 571

567

563 and 583 mg/m~3

respectively). Via introducing the over fire air(OFA) containing 20% total volume of secondary air at 3 m high from the furnace top

the NOx mass concentration decreased from 581 mg/m~3 to 423 mg/m~3. When the downdip angle of the OFA changed from 0° to 25°

the carbon content in fly ash decreased from 2.71% to 2.02% at first

and then increased to 2.17%. The NOx emission mass concentration at three downdip angles was 423

435 and 424 mg/m~3

respectively

indicating the best OFA downdip angle was 25°.