Investigation On The Formation Characteristics Of Ammonium Sulfate In Flue Gas Phase During SCR Process

At present,the Selective Catalytic Reduction（SCR）DeNOx technology is large-scale used in coal-fired power plants.However,SCR reactors also produce particles of ammonium sulfate（（NH₄）₂SO₄）and ammonium hydrogen sulfate（NH₄HSO₄）besides removing NOx.Previous researches mainly focused on the formation mechanism of ammonium sulfates in the presence of catalyst for the purpose of catalyst clogging while the formation characteristics of ammonium sulfates in gas phase during SCR DeNOx was ignored.In this paper,the object of study is the formation characteristic of ammonium sulfates in flue gas phase during SCR process.Firstly,the generation temperature of（NH₄）₂SO₄ and NH₄HSO₄ in SCR gas phase has been calculated through HSC Chemistry software.Then,the simulative gas-phase reaction system of SCR DeNOx was utilized to study the emission characteristics of fine particles of（NH₄）₂SO₄ and NH₄HSO₄.Lastly,it was researched of the mechanism of ammonium sulfates deposited on the catalyst surface decomposing into the gas phase.The formation characteristic of ammonium sulfates in flue gas phase during SCR process was obtained through thermodynamic calculation combined with experiment.The main results were summarized as follows:In normal condition,it has been calculated from 300 to 400 ℃ by HSC Chemistry that（NH₄）₂SO₄ could generate in SCR gas phase through reaction between NH₃,SO₃ and H₂O while NH₄HSO₄ only forming under decreased load（about 284 ℃,depending on the specific conditions）.But NH₄HSO₄ would exist through decomposition of（NH₄）₂SO₄ due to heat instability above 300℃.Therefore,（NH₄）₂SO₄ and NH₄HSO₄ were both present in SCR gas phase.In addition,the formation temperature of ammonium sulfates increased along with the increased concentration of NH₃ and SO₃.A portion of NH₃ in SCR gas phase was derived from the free NH₃ which did not adsorb on while some of it originated in NH₃ desorbed from the catalyst surface due to temperature rise.SO₃ was from combustion oxidation and catalytic oxidation of SO₂ in flue gas.It has been studied by in situ Fourier Transform infrared spectroscopy（FTIR）that SO₂ in flue gas firstly reacted with V⁵⁺-OH on the catalyst surface,generating intermediate product of metal sulfate（VOSO₄）,and then decomposed into SO₃.It was found that fine particles formed by reaction between NH₃,SO₃ and H₂O in SCR gas phase.The consist of fine particles was mainly（NH₄）₂SO₄ and a bit of NH₄HSO₄ which mostly were rectangular particles with size under 1 μm.Temperature was the decisive factor in the formation of ammonium sulfates.The particulate matter did almost not generate at 400℃ when the number concentration of them was 106/cm3 and the mass concentration of them was 102mg/m3 at 300 ℃.With decrease of temperature and increase in a certain range of residence time,SO₃ concentration,NH₃/SO₃ and water vapor content,the concentration of fine particles of ammonium sulfates increased gradually.Among these factors,the concentration of ammonium sulfates was linearly related to SO₃ concentration.The catalyst made change of the decomposition of（NH₄）₂SO₄ and NH₄HSO₄ and broadened the decomposition temperature window of them.When it was heated,NH₄HSO₄ deposited on the catalyst surface firstly volatilized at about 150℃ and then decomposed through two steps.（NH₄）₂SO₄ deposited on the catalyst surface did not volatilized and decomposed directly through four steps.The first step of deamination was carried out and overlapped with NH₄HSO₄.The release of SO₂ was delayed by the catalyst.The TiO₂ in the catalyst postponed the release of SO₂ and increased the complete decomposition temperature.