During ultra low emission operation of the selective non-catalytic reduction-selective catalytic reduction(SNCR-SCR) combined denitrification system of a 600 MW thermal power unit

problems such as strong ammoniacal odor

heavy ammonia slip and large consumption of reducing agent(urea) occur. Thus

the reasons were investigated

and operation of the SNCR-SCR combined denitration system was optimized. The consumptions of urea and risk of ammonium bisulfate condensation were reduced. The operation diagnosis results show that

the current used catalysts could meet the design performance requirement

with no abnormal deactivation occurred. The irrational layout of spray guns led to excess ammonia injection

which was the main reason for heavy ammonia slip. By optimizing the configurations of layout of the spray guns and regulating the urea flow in four sections of the SNCR denitration system

the reduction of urea consumption exceeded 600 L/h at590 MW load. Through optimal combustion adjustments

especially reducing the oxygen content

the reduction of urea consumption exceeded 440 L/h at 550 MW load. The energy saving and emission reduction effect was obvious. This operation diagnosis and optimization measure could provide references for ultra low emission operation of the same type units.