Experimental And Theoretical Study On The Low-temperature Sulfur-containing Flue Gas Denitration Based On Fast SCR Reaction

Energy is the foundation for the development of modern economy and society.In current stage,the burning of fossil fuels is still the most significant way to obtain energy in China.Nitrogen oxides(NO_x)are one of the main air pollutants produced during the combustion of fossil fuels,and their emission can cause a lot of environmental pollution problems,such as acid rain,photochemical smog,and haze.The Chinese government pays great attention to the control of NO_x emissions from industrial sources,and the annual NOx emission amount has decreased significantly since 2011.Selective catalytic reduction of NO_x with NH₃(NH₃-SCR)is the most mature and efficient technology to reduce the emission of NO_x from coal-fired power plants.In recent years,the focus of the industrial NO_x emission control in our country has gradually shifted from power plants to non-power industries.However,the medium and high temperature SCR de NO_xsystems currently used in power plants cannot operate stably and efficiently on the boilers in non-power industries,because the temperature of the flue gas in non-power industries is lower than power plants.A major obstacle to the industrial application of low-temperature SCR system is the deactivation of the SCR catalysts during the SCR operation process under water and sulfur-containing flue gas in low temperatures.This Ph.D.program mainly aimed to solve the industrial application problems of the low-temperature SCR system.The studies in this paper start from solving the deposition problem of ammonium bisulfate(NH₄HSO₄)on the surface of the SCR catalyst.“Fast SCR”reaction is used to improve the low-temperature activity of the catalyst and simultaneously efficiently decompose the deposited NH₄HSO₄ on the catalyst surface.An efficient and stable de NO_x method which based on the“fast SCR”reaction for low-temperature sulfur-containing industrial flue gas is proposed.Moreover,this thesis also focuses on the reaction mechanism of the“fast SCR”reaction,the formation of N₂O,and the effect of H₂O that are generally concerned during the SCR process.The main contents of this thesis are as follows:1.The formation,deposition,and reaction characteristics and mechanism of NH₄HSO₄ on the surface of V-based catalysts were studied.It was found that the NH₄HSO₄ can be easily formed in the gas phase through the nucleation of NH₃,H₂O,and SO₃ in NH₃-SCR processes and the deposition of the formed NH₄HSO₄ on the VTi catalyst is extremely favorable with a tight Ti-O-S bond.The above conclusions indicate that it is difficult to solve the problem of the poisoning and deactivation effect of NH₄HSO₄ on the catalyst surface by only optimizing the formulation of the catalyst,accelerating the decomposition of the NH₄HSO₄ on the catalyst surface is significant to solve the NH₄HSO₄ deposition problem.The NH₄⁺of NH₄HSO₄ can be consumed by NO,while the sulfate ion of NH₄HSO₄ remains on the catalyst surface in the form of a metal sulfate.The reaction of NH₄HSO₄ with NO_x was found as an effective way to decompose NH₄HSO₄ on the surface of the SCR catalyst.However,this reaction between NH₄HSO₄ with NO can only rapidly reacts at temperatures above 300?,and the reaction at low temperatures was limited by the catalyst reoxidation step.Through the study on the reaction mechanism of the NH₄HSO₄ with NO_x,we found that the presence of NO₂ in the flue gas significantly promotes the reaction of NH₄HSO₄ with NO_x on the surface at low temperatures.The role of the NO₂ in the NH₄HSO₄ fast decomposition reactions is accelerating the reoxidation of the V active sites in the V-based catalyst.2.The potential of the NH₄HSO₄ rapid decomposition method for industrial applications was investigated.It was found that the“fast SCR atmosphere”which containing NO₂ can effectively regenerate the low NH₄HSO₄-loading V-based catalyst at a low-temperature of 250?.When the loading amount of NH₄HSO₄ on the catalyst surface is too much,it is difficult to regenerate the activity of the poisoned catalyst through chemical reactions because the small pores in the catalyst were blocked by the NH₄HSO₄.Based on the above conclusion,we conclude that avoiding the excessive deposition of NH₄HSO₄ on the catalyst surface is vital for the stable operation of the low-temperature SCR system.Therefore,the sulfur poisoning tests on the commercial V₂O₅-WO₃/TiO₂ catalyst were conducted to verify the promotion effect of NH₄HSO₄rapid decomposition on the sulfur resistance of the catalyst.The experiments showed that the V₂O₅-WO₃/TiO₂ catalyst could show excellent sulfur resistance in the reaction atmospheres contained a certain proportion of NO₂ at 250?.The presence of NO₂ can accelerate the decomposition of NH₄HSO₄ on the catalyst surface to avoid the continuous deposition of the NH₄HSO₄,thereby avoiding the catalyst from being poisoned by NH₄HSO₄ deposition.In addition,the addition of NO₂ could also promote the activity of the SCR catalyst through the“fast SCR”reaction.Finally,we proposed an option method for NO_x removal in H₂O/SO₂-contained flue gas at low temperatures which could simultaneous fast decomposition of NH₄HSO₄ and efficient NO_x removal by NO₂ addition.The required NO₂ gas which used in our new method can be obtained by the oxidation of the NO in the flue gas.3.The roles of NO₂ in the“fast SCR”reaction was studied by using experimental and theoretical methods.The study starts from the investigating of the chemical adsorption characteristics of NO₂ on the surface of the V/Ti catalyst.The theoretical results show that NO₂ tends to be adsorbed on the surface of TiO₂ support rather than on the V active sites of fresh V/Ti catalyst,a corresponding NO₂-TPD experiment was conducted to prove the calculation results.For the reduced V/Ti catalyst,NO₂ will strongly interact with the reduced V⁴⁺O_x species on the catalyst surface.On the basis of the NO₂ adsorption behaviors,we concluded two different reaction mechanisms with the participation of NO₂ in the“fast SCR”reaction.One is that NO₂ significantly accelerates the reoxidation process of the supported vanadia sites,which is directly proved by the designed experiments and the XPS characterizations in this study.The other is that the NO₂ is first adsorbed on TiO₂ and then reacts with NH₃ and NO in the path of the“nitrate route”proposed by previous studies.4.The formation characteristics and mechanism of the N₂O in standard SCR and fast SCR reactions on V-based catalysts were studied.We have compared four differently probable N₂O formation routes in different reaction atmospheres.For the fast SCR reaction,N₂O is produced by the thermal decomposition of solid NH₄NO₃ which formed by the reaction between NH₃ and NO₂ at low temperature.For the standard SCR reaction,N₂O is mainly generated at high temperatures.Using a combination of kinetic experiments and DFT calculations,the NH is found to be formed by NH₃ oxidative dehydrogenation over Lewis acid sites of the vanadium oxide and subsequently reacts with NO to produce N₂O.In this process,the deep oxidation of the adsorbed NH₃ is the rate-determined step,which can be aggravated by the high temperature and high V content.5.The influence of water vapor in the reaction atmosphere on the activity of both standard SCR and fast SCR reactions over V-based catalysts was investigated.It was found that the fast SCR reaction showed stronger water resistance than the standard SCR reaction at low space velocity.Through a series of fundamental kinetic(turnover frequency,apparent activation energy,NH₃ reaction order,transient response method)tests combined with theoretical calculations,we concluded that the competitive adsorption between H₂O and NH₃ on the catalyst surface is one of the important factors that H₂O inhibits the SCR reaction activity of the catalyst.For the fast SCR reaction,the rapid oxidation effect of NO₂ can keep the fast SCR reaction at a high reaction rate level,thus enabling fast SCR reaction to exhibit good resistance to H₂O.