Study On Flue Gas Hg²⁺ Selective Adsorbent In Hg-CEMS

In order to effectively control mercury pollution,a series of strict mercury emission standards and regulatory measures have been issued by countries in recent years.Accurate mercury speciation measurement in coal-fired flue gas is an important prerequisite for effective mercury control.Owing to the advantages of easy operation,high precision and real-time online monitoring,mercury continuous emissions monitoring system?Hg-CEMS?is widely applied in coal-fired power plants.However,due to the limitation of existing Hg detection techniques in Hg²⁺recognition,a pretreatment/conversion unit designed for achieving a complete separation and conversion of Hg²⁺to Hg⁰ should be used in Hg-CEMS.This paper proposed a new separation method for Hg²⁺and Hg⁰ in Hg-CEMS based on selective solid sorbents,the preparation and performance of solid sorbents for Hg²⁺selective adsorption are mainly focus in this paper.Firstly,Hg²⁺selective adsorption performances of three kinds of natural mineral materials?attapulgite,bentonite,zeolite?and three kinds of porous materials(SiO₂,TiO_2,?-Al₂O₃)were studied,so that the best Hg²⁺selective adsorbent carrier material can be screened out.The results show that the selected three natural mineral materials all had obvious Hg⁰ removal ability.The removal of Hg⁰ by zeolite is due to its catalytic oxidation ability while chemisorption is the cause of attapulgite and bentonite capturing Hg⁰.Different from natural mineral,Hg⁰ was scarcely adsorbed by three porous materials when Hg²⁺exhibited an apparent adsorption behavior.Moreover,?-Al₂O₃ was verified to be the best carrier material for selective adsorbents with a better Hg²⁺selective adsorption performance.The intrinsic mechanisms of mercury species adsorption and SO₂ competition over?-Al₂O₃ surface sites were further investigated.Hydroxyl?Al-OH?,Lewis Alⁿ⁺-Bronsted O^n-pairs and the coordinatively unsaturated oxygen(O^n-ions)were deduced to be effective basic sites for the chemisorption of HgCl₂ on?-Al₂O₃.Simultaneously,the competitive adsorption between SO₂ and HgCl₂in O^n-ions was confirmed to be responsible for the increased breakthrough of HgCl₂over?-Al₂O₃ layer with the existence of SO₂.Secondly,the active component CaO was loaded on the surface of carrier?-Al₂O₃to prepare the supported calcium-based adsorbent CaO/?-Al₂O₃.Effects of loading methods,calcination process and ratio of active component to carrier on the performance of adsorbent were studied.The results find that CaO/?-Al₂O₃ prepared by the sol-gel method exhibited a better Hg²⁺selective adsorption performance than the equal volume impregnation or excess impregnation methods.A three-step calcination method applied in this paper greatly improve the performance of the adsorbent as well,because the sintering of CaO particles during three-step calcination process can be effectively prevent,and the decomposition of the precursor and the escape of the decomposition products are effectively promoted.The ratio of active component to carrier has an important effect on the number of basic sites of the adsorbent CaO/?-Al₂O₃ and its surface morphology.The optimum mole ratio of CaO and?-Al₂O₃ was found to be 1.5.CaO/?-Al₂O₃ prepared by the optimal preparation method showed a good performance of Hg²⁺selective adsorption,the penetration rate to Hg²⁺in N₂atmosphere was only 8.53%.Which is beneficial from the active component CaO providing abundant basic sites,besides,calcium-aluminum compound Ca₁₂Al₁₄O₃₃formed during the preparation of the adsorbent can effectively support the pores of the adsorbent.Finally,based on mercury desorption experiments and characterization results,the Hg²⁺selective adsorption mechanism of calcium-based adsorbent CaO/?-Al₂O₃ was inferred.The optimal operating parameters of the adsorbent were determined considering the actual industrial application requirements.The results reveal that CaO/?-Al₂O₃ has only weak physical adsorption to Hg⁰,but can effectively capture Hg²⁺through chemical adsorption.After the adsorption,HgCl₂ form the monodentate,bidentate and tridentate adsorption configuration on the surfaces of the adsorbent.In practical industrial applications,the recommended optimum adsorption temperature of CaO/?-Al₂O₃ is 160°C.The optimal space velocity of CaO/?-Al₂O₃ for selective adsorption of Hg²⁺is 40000 h^-1,and the optimal replacement period is 60 min.