Study On The Formation Mechanism Of Ammonium Sulfate Salt In Flue Gas Denitrification Process Of Coal-fired Units

Nitrogen oxides is one of the major atomospherical pollutants.The national pollution control requirements has been improved progrssively.The Ministry of Environmental Protection has tightened controls on emissions of nitrogen oxides?NOx?from coal-fired power station boilers.Selective catalytic reduction?SCR?is the most widely applied flue gas denitrification technology around the world.In recent years,in order to adapt to the change of power grid structure,the development of new energy,coal-fired units is more responsible for the depth peak load cycling,and at the same time in order to meet the increasingly stringent environmental requirements for ultra-low emissions,it has become an inevitable trend to put the denitrification system into operation under lower load.However,if the load is too low?flue gas temperature is too low?in the SCR denitrification process,NH₃ in the flue gas will react with H₂O and SO₃ to produce ammonium sulfate and ammonium bisulfate,which will affect the service life of the catalyst and cause cause the blockage and corrosion of the air preheater.To learn about those peoblems,this paper researches the formation mechanism and formation ratio quantification model of ammonium sulfate in the denitrification gas at low flue gas temperature,as well as the law of ammonium salt deposition and decomposition on the catalyst surface in the actual operation,which will provide theoretical basis for denitrification under full load.In this paper,a laboratorial flue gas simulation system was established to obtain a series experimental data of ammonium bisulfate and ammonium sulfate formation ratio by adjusting flue gas temperature and concentration of NH₃ and SO₃.On the basis of the experimental data,the Gompertz model was found to best descrtibe the formation mechanism of ammonium bisulfate and ammonium sulfate,and the quantification model of formation ratio of ammonium bisulfate and ammonium sulfate is further obtained.This model can predict the deposition position and formation trend of ammonium sulfate in actual production.Compared with the previous qualitative analysis method for ammonium sulfate,it can provide a more general and quantifiable technical analysis method for preventing the blockage of air preheater.After that,the field test was conducted in Lanxi power plant.The change of SCR flue gas temperature,ammonia escape ratio and SO₃ concentration with load was studied through ammonia and SO₃ collection device.The ignition and steady-stage operation test apparatuses were used to respectively research the the deposition law of ammonium sulfate on the catalyst surface under ignition and steady-state operation conditions.It was found that the formation of ammonium sulfate on the catalyst surface stabilized first and then decreased with the load decreasing,and the peak value appeared at 310?,that is,the catalyst is under very dangerous condition when the flue gas temperature was 310?.As the normal operating temperature range?300?⁴00??of the SCR denitrification unit generally includes 310?,an appropriate reduction of the lower limit of the SCR temperature window will not seriously affect the performance of the catalyst.The catalyst from the low temperature test was placed in laboratory for pyrolytic decomposition.The results showed that the lower the reaction temperature of the catalyst,the longer the reaction time,the worse conducive to pyrolytic decompositions.According to the research results,the lower limit of the SCR operating temperature of the 660MW supercritical unit was adjusted from 300?to 280?,and the unit still performs normally,without abnormal increase of resistance in air preheater and abnormal failure tendency of the catalyst.