Study On The Migration And Transformation Characteristics Of Ammonium Sulfates And Ammonia Slip Formed In SCR In The Subsequent Flue Gas System

In coal-fired power plants(CFPPs),to reduce the emission of particle matters(PM),SO₂and NO_x,the flue gas is subjected to denitrification,dedusting,desulfurization and other air pollution control devices before being finally discharged into the atmosphere.Among them,ammonium sulfates and NH₃ slip formed in selective catalytic reduction(SCR)process will have an impact on the physical propterties and removal mechanism of fine particles(PM_2.5),as well as the emission characteristics of pollutants from coal-fired power plants(CFPPs),and will also cause low temperature deactivation of denitration catalyst.Due to the extensive installation of SCR system in CFPPs in China,it is of great significance to study the formation mechanism of ammonium sulfates in SCR process and its impact on the emission characteristics of CFPPs.Based on the experiments and density functional theory(DFT)calculation,the deposition configuration and decomposition mechanism of ammonium bisulfate on the catalyst were studied.The results showed that NH₄HSO₄ deposited on the surface of TiO₂(001)was broken into NH₄⁺group adsorbed on O atom and bidentate sulfate combined with Ti atoms by surface interaction.In the vicinity of V₂O₅ active groups,NH₄HSO₄ might bind to the carrier in a single bond,and NH₄⁺was adsorbed and activated by the adjacent V=O site.NO was reduced by NH₄⁺group forming N₂ and H₂O by transferring protons to the adjacent V=O and S=O sites.The bridging O atoms on the surface of Ti-Si support carried more negative charge,which is beneficial to attract H⁺from adsorbed NH₄⁺and promote the decomposition of NH₄⁺group at low temperature.In addition,the strong interaction between HSO₄^-group and Ti-Si support can weaken the adsorption of NH₄⁺by HSO₄^-group and enhance the activation of NH₄⁺at V=O site,thus promoting the reaction between NH₄HSO₄and NO.The formation,condensation and emission characteristcs of ammonium sulfate and ammonium bisulfate aerosols in air preheater were investigated in a simulated air preheater experimental system.The results showed that the formation of ammonium sulfate and ammonium bisulfate depends on the initial NH₃/SO₃ molar ratio,and the aerosol size was mainly in the submicron range.Ammonium bisulfate is the principal product formed in typical air preheater flue gas environment with NH₃ concentration lower than SO₃,and its initial formation temperature ranges from 231–260°C,calculated as follows:₃₂₄=7.9×10¹³exp?(-234968/)With the increase of NH₃ concentration,more ammonium sulfates were formed,which increased the probability of collision and agglomeration between fine particles,leading to the increase of particle size,the decrease of number concentration and the increase of mass concentration of emitted fine particles.The migration and transformation characteristics of the ammonium sulfates aerosols and NH₃ slip from SCR in the downstream electrostatic precipitator(ESP)and wet flue gas desulfurization(WFGD)unit were conducted both over simulated limestone-gypsum WFGD experimental system and actual coal-fired air pollution control experimental system.In the ESP system,the generated ammonium sulfate particles can be effectively captured by the electric field force,which can also improve the capture efficiency of ESP on fine particles.In the WFGD system,ammonium sulfates and ammonia can be partly removed by desulfurization through slurry scrubbing.However,the accumulated NH₄⁺in slurry will generate new ammonium species and ammonia through the entrainment and evaporation of the slurry.The ammonium-containing particles formed by desulfurization are not only derived from the entrainment of slurry droplets,but also from the re-adsorbtion of gaseous ammonia generated by slurry evaporation Therefore,even if the concentration of NH₄⁺in the desulfurization slurry is quite low,a high level of NH₄⁺was still contained in the fine particles at the outlet of the scrubber.When the accumulated NH₄⁺in the desulfurization slurry was high enough,the WFGD system promoted the conversion of NH₃ to NH₄⁺and increased the additional emission of primary NH₄⁺aerosols.SCR operation with the NH₃ slip will increase the emission of submicron particles,NH₄⁺and SO₄^2-at the outlet of WFGD system.While ensuring that the desulfurization efficiency meets the emission requirements,properly reducing the liquid/gas ratio,slurry concentration and inlet flue gas temperature,as well as adding tray in the desulfurization tower can help reduce the NH₃/NH₄⁺emission from the WFGD system.Finally,the effect of SCR on emission characteristics of CFPPs was investigated by field test.The results indicated that the particle-bound NH₄⁺accounted for more than 90%of the total ammonia emitted from SCR system.The downstream ESP system,which can reduce NH₄⁺by about 95%,is the most effective removal unit.WFGD and wet electrostatic precipitator(WESP)have a similar contribution to reducing NH₄⁺emission.About 2.41%of NH₄⁺eventually released into the atmosphere with flue gas.Reducing the concentration of NH₃ slip,controlling the operating parameters of the desulfurization system,and adding tray in the desulfurization tower are all effective methods to reduce the emission of ammonium-containing particles and gaseous NH₃ in the CFPPs.