Research On The Performance And Mechanism Of Mercury Removal By Magnetic Sorbent From Coal-fired Flue Gas

Mercury emission from coal-fired flue gas is one of the largest man-made mercury emission sources in our country.The limitation of its mercury emission is an urgent problem that needs to be solved at present.Currently,activated carbon injection(ACI)for mercury removal is considered to be the most effective technology,but this technology faces problems such as a high cost of operation,difficulty in adsorbent recovery and regeneration,pollution of fly ash during the application process.Using magnetic adsorbent can magnetically separate the magnetic adsorbent from fly ash under an external magnetic field,and the recovered sorbent can be reused,which can effectively solve the drawbacks of ACI technology.In this work,Fe₃O₄ and?-Fe₂O₃ were used as magnetic media to prepare a variety of magnetic sorbents.Through the combination of experimental study,characterization analysis and quantum chemical simulation,the mercury removal mechanism of the prepared sorbents and the effect of multiple flue gas components on mercury adsorption were explored.The study in this work has important scientific significance and practical value for the development of efficient and economical mercury removal sorbents preparation and the development of sorbent mercury removal mechanism theory.This work explored the preparation of Ag based sorbent supported by magnetic media and its mercury removal characteristics.Nano Ag particles were loaded on the surface of Fe₃O₄,then the Ag/Si O₂-Fe₃O₄ sorbent was prepared.Ag had a strong binding ability to mercury and acted as the adsorption active site.The amalgam between Ag and Hg was prone to decompose at higher temperatures and the mercury removal performance decreased.The theoretical calculation showed that the adsorption energy of Hg on the Ag/Fe₃O₄(111)surface is-86.82 k J/mol,which was much higher than that on the surface of Fe₃O₄(111).In view of the deterioration of the mercury removal performance of Ag/Si O₂-Fe₃O₄ at higher temperatures,a magnetic 0.5Fe1ATT sorbent with?-Fe₂O₃ incorporated with palygorskite was prepared,which had a large specific surface area.The 0.5Fe1ATT showed good mercury removal performance in the temperature range of 150-250 ^oC.The Fe³⁺and O*species on the surface of the sorbent were the keys to Hg⁰ oxidation.The surface O*was consumed and Fe³⁺was reduced after Hg⁰ oxidation.O₂ in the flue gas could replenish O*,and re-oxidized Fe²⁺to Fe³⁺.NO could promote the oxidation of Hg⁰,while SO₂ and H₂O inhibited the removal of Hg⁰,however,NO could weaken the inhibitory effect of SO₂.In order to improve the Hg⁰ oxidation removal performance and the flue gas composition applicability of the aforementioned sorbent.The as-synthesized magnetic palygorskite was further modified with Mn O₂ and Cu Cl₂,respectively.Mn⁴⁺and O*species were introduced on the surface of the synthesized 0.1Mn FA,Cl*species were introduced on the surface of 0.05Cu FA.The Hg⁰ oxidation removal performance and flue gas composition applicability of the modified sorbents was then remarkablely improved in comparison to0.5Fe1FA sorbent.Through the fixed bed mercury removal experiment combined with XPS and in situ DRIFTs characterization analysis,the mechanism of the effect of the flue gas components on the adsorption of Hg⁰ was obtained:It is found that in the NO atmosphere,NO₂ and nitrate were generated on the surface of 0.1Mn FA,which promoted the oxidative removal of Hg⁰,while no nitrate was formed on the surface of 0.05Cu FA in the presence of NO.Besides,NO could competitively adsorb with Hg⁰for the active sites on the 0.05Cu FA sorbent,hence,NO had no obviously effect on the removal of Hg⁰.SO₂ could cause sulfation and O*loss on the surface of the two adsorbents,but SO₂ and Hg⁰ were competitively adsorbed on the surface of 0.1Mn FA,but not on the surface of 0.05Cu FA.In the SO₂ atmosphere,the surface of 0.1Mn FA was sulfated,while no Cu sulfate and bulk sulfate was generated the surface of 0.05Cu FA,so the latter one has stronger sulfur resistance.In addition,the HCl and O₂ in the flue gas could help the surface of the reacted0.05Cu FA to return to the initial state,which makes it can be effectively regenerated.Finally,quantum chemistry was used to explore the Hg⁰ adsorption mechanism on three typical metal compounds,namely?-Fe₂O₃,Mn O₂ and Cu Cl₂.Hg on the surface of?-Fe₂O₃(001),?-Mn O₂(100)and Cu Cl₂(001)belonged to physical adsorption,weak chemical adsorption and strong chemical adsorption,respectively.Hg tended to interact with Fe sites on the surface of?-Fe₂O₃(001).The presence of surface dissociated O in?-Fe₂O₃(001)and?-Mn O₂(100)significantly enhanced the adsorption of Hg.As to the adsorption of Hg on Cu Cl₂(001)surface,orbital hybridization between Hg and the adjacent Cu and Cl atoms on the surface of Cu Cl₂(001)was found,and a stable chemical bond was formed between them.