Experimental Study On High-temperature Pyrolysis Of Divalent Mercury In Hg-CEMS

China’s energy structure of“rich coal,lean oil and less gas”determines the current energy consumption of China is still dominated by coal.The mercury pollution caused by coal combustion,which is also known as the largest anthropogenic mercury emission sources,has attracted wide public attention.With the reduction policy of atmospheric mercury pollution becoming increasingly stricter in China,the demand on mercury speciation monitoring facilities is also daily growing for coal-fired boilers.Owing to the unique advantage of on-line measuring the concentration of mercury in flue gas,mercury continuous emission measurement system（Hg-CEMS）has been recognized as the dominant mercury detection facility in coal-fired power plants.Restricted by the fact that the current monitoring technology can only detect elemental mercury（Hg⁰）,Hg-CEMS converts Hg²⁺to Hg⁰ by the means of the key Hg²⁺conversion unit to measure the Hg²⁺indirectly.The method of Hg²⁺high temperature pyrolysis(Hg²⁺-HTP)is reliable and has high conversion efficiency.However,on the one hand,the reaction temperature scope of Hg²⁺-HTP has not been investigated systemataciallly.On the other hand,the problem of Hg⁰ reoxidation by acid gas in flue gas is seem as the difficulty of Hg²⁺-HTP.Therefore,this research studied the temperature range of Hg²⁺-HTP and then investigated the Hg²⁺reduction properties of Hg²⁺-HTP acid gas scavenger systemataciallly.Lastly,a new mercury speciation conversion facility for Hg-CEMS was proposed based on Hg²⁺-HTP.Firstly,the effects of temperature,filling material height and mesh number on Hg²⁺-HTP were investigated.Then the re-oxidation caused by acidic gas（O₂,HCl）was systematically explored.The results show that the pyrolysis efficiency of Hg²⁺is the result of the dual action of reaction temperature and contact site.With the increase of temperature,the reduction efficiency of Hg²⁺increases correspondingly.The Hg²⁺was completely cracked at 800℃by increasing the contact site and residence time resulted form filling quartz plate.It was found that the higher the bed height was filled,the higher the reduction efficiency was at the temperature of 400-600℃.The reduction efficiency of Hg²⁺at 3 cm and 6 cm bed was 84.78%and 91.29%at the temperature of 600℃,respectively.The temperature required for complete pyrolysis of Hg²⁺can be reduced by filling proper mesh number materials.When the reaction temperature was only 700℃,the quartz glass was filled with 10～16 mesh and the bed height was 3 cm,the Hg²⁺in gas stream was completely converted to Hg⁰.O₂ has a certain inhibitory effect on Hg²⁺-HTP,and the degree of inhibition is independent of O₂ concentration because the reaction is controlled by thermodynamics.The reduction efficiency of Hg²⁺decreased to about 92%after O₂ injection.The inhibitory effect of HCl on Hg²⁺-HTP increases with the growth of the concentration of HCl.When the concentration of HCl increases from 10 ppm to50 ppm,the reduction efficiency decreases from 53.1%to 21.24%.When HCl and O₂ exist at the same time,the Deacon reaction will occur to generate strong oxidizing Cl2,which leads to the complete oxidation of Hg⁰ after the conversion of Hg²⁺to Hg⁰,therefore,it should be focused on the re-oxidation.Secondly,in order to solve the problem of the re-oxidation caused by acid gas,the active component and carrier material was screened and a series of acid gas scavengers were prepared.Tthe results show that Ca O has better ability of Hg²⁺reduction because the specific surface area and the space structure of Ca O is obviously better than Mg O.Thus,Ca O was chosen as the active component of acid scavengers.As a result of the rich strong alkaline level ofγ-Al₂O₃surface and good gas structure and specific surface area,theγ-Al₂O₃ showed the highest Hg²⁺reduction efficiency,which was 68.36%.Therefore,theγ-Al₂O₃ was selected as the sintering resistance carrier materials.The acid scavengers prepared by different precursors show different specific surface area and void structure.The process of the calcium acetate decomposition releases a huge amount gas and the Si O₂ in lithium silicate has the effect of resisting sintering.The representation results indicate that the surface particle of（CH₃COO）₂Ca-Ca O is highly dispered and the gap between particles is bigger,which represents a good surface structure.Hence,（CH₃COO）₂Ca-Ca O has the highest Hg²⁺reduction efficiency（84.61%）and calcium acetate was chosed as Ca O precursor material.A series of load-carrying acid gas scavengers were prepared by chemical impregnation method.With the increase ofγ-Al₂O₃ loading,the reduction efficiency of Hg²⁺increased significantly.The addition ofγ-Al₂O₃ makes the specific surface area of the acid gas scavenger increases,andγ-Al₂O₃ increases the anti-sintering ability of active component.Therefore,the Hg²⁺reduction efficiency of Ca O/_50%γ-Al₂O₃ was the highest.With the addition of HCl,the Hg²⁺adsorped on the material surface released in the form of Hg⁰,leading to the Ca O/_50%γ-Al₂O₃ reduction efficiency is higher than the normal value.Through the cooperation of quartz filler material and acid gas scavenger,the phenomenon of unusually higher reduction efficiency caused by Hg²⁺adsorption is effectively avoided.The Ca O-KO surface modified by KOH doping has a certain void structure,and the structure reforming occurs during the reaction process,resulting in a new reaction surface,which greatly increases the surface void structure after the reaction,which is conducive to the lasting reaction.Finally,the influence of metal doping on the performance of deacidifier was explored,and several factors affecting the service life of deacidifier were identified,including simulated flue gas atmosphere and space velocity.The main conclusions are as follows:Ca O-KO has a certain void structure,which occurs in the reaction process of structural restructuring,resulting in the formation of a new reaction surface,the surface void structure after the reaction greatly increased,which is conducive to the lasting and efficient reaction.The space velocity of Ca O-KO should not be more than 23700 h^-1.The material after reaction still has a good specific surface area.The reaction of deacidifier and SO₂ to form Ca SO₄ in simulated flue gas is a major factor affecting the life of deacidifier.