The main component of NOx in flue gas is NO (with a volume concentration of 95%), which is difficult to dissolve in water and has poor reaction activity. As a strong oxidant, ozone can easily oxidize NO into high valence NO2, N2O3, N2O5, etc., and dissolve in water to generate HNO2 and HNO3. As a clean strong oxidant, ozone can quickly and effectively oxidize NO to high valence nitrogen oxides.