Study On Generation Mechanism Of SO₃ And Ammounium Hydrogen Sulfate In Coal-fired Flue Gas

SO₃ brings serious problems to the atmospheric environment and power plant operation.The selective catalytic reduction（SCR）of NO with NH₃ technology has been widely used in China.V₂O₅-WO₃/TiO₂ catalyst is the mainstream catalyst for coal-fired power plants,which will lead to the catalytic oxidation of SO₂ to SO₃.The generation mechanism and control method of SO₃ on SCR catalyst are studied.The subject aims to study the SO₃ generation characteristics under various conditions,reveal the chemical reaction pathway of SO₂ catalytic oxidation to SO₃,study the mechanism of SO₃ reacting with NH₃ to form ABS,and clarify the ABS evolutionary path,study of SO₃ removal method based on SO₃ adsorption by alkaline-based absorbents to provide a theoretical basis for SO₃ control methods in coal-fired power plants.The main research contents of the subject are as follows:The fixed bed reaction system and in-situ Drifts test system were built,the effects of H₂O/CO₂/NO/NH₃ on the generation of SO₃ were investigated by temperature-programmed sesprption,X-ray diffraction,X-ray photoelectron spectroscopy and other test method and the conversion pathway of SO₃ on the catalyst was revealed.The catalyst component V promotes the formation of SO₃.The reaction order of SO₂ is 0.52,when the O₂concentration was more than 1.5%,the reaction order of O₂ is 0,and the activation energy of reaction is 118 KJ/mol.Low concentration of H₂O and CO₂ promotes the adsorption of SO₂ and increases the ratio of adsorbed oxygen on the catalyst surface,which is favorable for SO₃ generation.However,high concentration of H₂O will inhibit SO₂ adsorption.High concentration of CO₂ will lead to a decrease in the proportion of adsorbed oxygen on the catalyst surface,resulting in a decrease in SO₃ generation.NO and NH₃ will increase the ratio of adsorbed oxygen on the surface of the catalyst,promote the adsorption of SO₂,and promote the generation of SO₃.SO₃ generation path I:Most of the SO₂ will adsorb on the surface of the catalyst and react with V⁵⁺-OH to form the intermediate product VOSO₄.Path II:A part of the SO₂ adsorbs on the surface of the catalyst and reacts with V⁵⁺-OH to form HSO₄^-.The intermediate products will further react with the catalyst to form SO₃.The presence of O₂ promotes the reaction but does not change the reaction pathway.ABS/AS was loaded on the surface of the catalyst by equal volume impregnation method and in-situ reaction formation method.The deposition and decomposition characteristics of ABS on the catalyst surface were studied by Fourier transform infrared spectroscopy（FT-IR）and thermogravimetry-mass spectrometry（TG-MS）.ABS would be deposited in the microporous structure of the catalyst firstly and subsequently deposited in larger pores,resulting in a decrease in specific surface area,pore volume,and a significant increase in average pore size.After ABS formed on the surface of the catalyst,NH₄⁺mainly present in the surface of the catalyst in the forms of NH₂,NH₃ coordinated to the Lewis acid site,and NH₄⁺adsorbed on the Bronsted acid site.HSO₄^-would be partially converted to sulfate on the surface of the catalyst.The adsorpted NH₄⁺on the surface will react with NO,which is beneficial to NO removal.In addition,the sulfate formed by ABS conversion may form an acidic site on the surface of the catalyst,which is beneficial to the adsorption of NH₃ on the catalyst surface and promote NO removal.Because of its derivatization into a variety of substances on the surface of the catalyst,ABS has multi-stage decomposition characteristics,the decomposition temperature is higher than the pure ABS decomposition temperature,and the decomposition temperature moves to a higher direction as the ABS deposition increases.A special multi-stage temperature control reaction system for simulated air preheater was built,the ABS generation and deposition experiments were carried out under different SO₃ and NH₃ concentrations and different SO₃:NH₃ conditions,the X-ray diffraction,Fourier transform infrared spectroscopy and thermogravimetry were used to study the deposition and decomposition characteristics of ammonium bisulfate（ABS/AS）in the variable temperature space.The increase of SO₃ and NH₃ concentration in flue gas will promote the formation of ABS in the air preheater under low temperature conditions,and will increase the initial formation temperature of ABS.Under the conditions of different concentrations of SO₃ and NH₃,the products in the high temperature range of 390-300°C are ammonia hydrogen sulfate,which are deposited in liquid form.The products in the high temperature range of 300-296°C and 296-223°C are deposited in a white mist form.The low temperature product is related to the ratio of SO₃:NH₃,SO₃:NH₃ is 2:1,the low temperature section is 223-185°C and 185-131°C,the product is H₂O,H₂SO₄ and a small amount of NH₄HSO₄ mixture,which was deposited on the surface of reactor in the form of dense droplets;SO₃:NH₃ is 1:1 and 1:2,the main product in the low temperature section is（NH₄）₂SO₄,and contains a small amount of（NH₄）₃H（SO₄）₂,which was deposited in a white mist form.The deconposition characteristic curve of the high temperature section is basically consistent with the pure hydrogen sulfate ammonia decomposition characteristic curve.When SO₃:NH₃ is 2:1,the product in the low temperature section 4-5 is basically decomposed before 300°C.The lower the SO₃ concentration,the lower the temperature,the higher the H₂O content of the product,and the easier it is to decompose.When SO₃:NH₃ is 1:1 and 1:2,the decomposition characteristic curve of the product in the low temperature section is basically consistent with the decomposition curve of pure ammonia sulfate.In the fixed bed reaction system,different alkaline-based absorbents such as potassium,sodium and calcium were selected for SO₃ removal,the SO₃ removal experiments are carried out under different absorbents and SO₃ stoichiometric ratio and reaction temperature.The removal characteristics of SO₃ by absorbent under different reaction conditions were revealed,and the absorption kinetics model of SO₃ removal was established.The removal efficiency of SO3 from different absorbents from high to low is potassium-based absorbent>sodium-based absorbent>calcium-based absorbent,where K₂CO₃>KCl>Sodium sesquicarbonate>Na₂CO₃>NaCl>CaCO₃>Ca（OH）₂>CaCl₂>CaO.For the selected 9 kinds of absorbents,the removal efficiency of SO₃ can reach more than 95%when the chemical equivalent ratio of 5:1 is selected.The presence of SO₂ can significantly reduce the removal efficiency of SO₃ by the absorbent,and the increase of reaction temperature and stoichiometric ratio is beneficial to the removal of SO₃.The physical structure of the absorbent can significantly affect the removal efficiency of SO₃.The SO₃ absorption reaction kinetic model is the Bangham channel diffusion model.