Study On The Mercury Removal Characteristics Of Flue Gas With Transition Metal Sulfide Modified Carbon Nanotubes

Coal combustion is the main source of atmospheric mercury pollution.Controlling mercury emissions from coal-fired flue gas is currently a hot research topic in atmospheric pollutant control research.Transitive metal sulfide is a highly efficient mercury adsorbent with great potential for application in the future,because it has more sulfur active sites and has a strong affinity with mercury.Moreover,microporous carbonaceous materials,such as multi-walled carbon nanotubes（MWCNTs）,have good pore structure and unique chemical properties,which can be used as a carrier for mercury removal adsorbents,playing a synergistic purification effect.Based on this background,the paper proposes to use the transitive metal sulfide and multi-wall carbon nanotubes to prepare modified composite adsorbents,and to conduct experimental research on its mercury removal performance under a simulated flue gas condition using a fixed bed adsorption performance test system.It will lay a foundation for further controlling the emissions of coal-fired mercury pollution.In this paper,FeS₂ and CuS were prepared by solvothermal synthesis,and FeS₂supported multi-walled carbon nanotubes（FeS₂/MWCNTs）and CuS supported multi-walled carbon nanotubes（CuS/MWCNTs）were prepared by ultrasonic impregnation.Scanning electron microscopy（SEM）,X-ray powder diffractometer（XRD）,specific surface area analyzer（BET）,and X-ray photoelectron spectroscopy（XPS）were used to perform microscopic characterization and physical and chemical analysis of the mercury removal adsorbents.The results showed that the prepared FeS₂ had good dispersibility,spherical crystals,pure cubic crystal pyrite,and the surface of the composite adsorbents were covered with MWCNTs,forming clusters.The presence of MWCNTs carrier provided rich specific surface area for FeS₂,improved its internal pore structure,which facilitated subsequent mercury removal.The Feelement in the adsorbent mainly existed in the form of Fe²⁺,while the S element existed in three forms of S^2-,S₂^2-,and SO₄^2-,which indicated that different S valence composition could be helpful for subsequent adsorption of Hg⁰.In addition,the characterization results of CuS and CuS/MWCNTs showed that the crystal phase of CuS was a hexagonal azurite structure,and MWCNTs were coated on the surface of CuS in clusters.The specific surface area of CuS was small,but the existence of MWCNTs made the specific surface area of the composite adsorbent greatly improved.The presence of Cu²⁺was detected in the adsorbent,no impurities such as Cu⁺or Cu⁰ were observed,and the S element was mainly attributed to S^2-and S₂^2-,contributing to the adsorption of Hg⁰.In a self-built fixed bed adsorption performance test system,the factors such as preparation parameters,fixed bed reaction temperature,initial mercury concentration,reaction space velocity,and flue gas component concentration on the mercury removal performance of the prepared composite adsorbents were studied.The results showed that the single MWCNTs had weak mercury removal performance,and the addition of FeS₂ greatly improved the removal efficiency.When the adsorption dose was 0.4g（10%loading ratio）,the reaction temperature was 70℃,and the initial mercury concentration was 25μg/m³,the adsorption effect of FeS₂/MWCNTs adsorbents was the best.The highest removal efficiency could be close to 100%,and the removal efficiency after adsorption for 60min was also 80%.The presence of O₂had a weak effect on mercury removal under complex flue gas conditions,and NO had a weak inhibitory effect on mercury removal.SO₂ and H₂O had a significant inhibitory effect on mercury removal efficiency.Temperature programmed desorption（TPD）and XPS analysis results further show that Hg⁰ in the flue gas was oxidized to Hg²⁺and attached to the surface of the adsorbent in the form of Hg S,further confirming that the adsorbent was mainly chemical adsorption.CuS exhibited extremely high sulfur coverage,which meaned that CuS/MWCNTs composite adsorbents also had strong mercury removal performance.In this paper,the factors such as preparation parameters,reaction temperature,system space velocity,and flue gas components on the mercury removal performance of the composite adsorbents were studied through a fixed-bed adsorption performance test system.The results showed that CuS had a stronger adsorption capacity relative to FeS₂.The maximum removal efficiency of CuS/MWCNTs adsorbents reached 93%,and the mercury adsorption capacity at 2h was 178μg/g in the case of 0.05g adsorption dose（10%loading ratio）,temperature of 70℃,and mercury concentration of 45μg/m³,which far exceeded that of FeS₂/MWCNTs adsorbents.For CuS/MWCNTs adsorbents,the effect of complex flue gas conditions on its mercury removal performance showed that O₂ had a weak effect on mercury removal,while NO and H₂O inhibited the Hg⁰ removal.Moreover,SO₂ had a weak inhibitory effect on mercury removal,which indicated that the adsorbent had strong sulfur resistance.After programmed temperature desorption and XPS characterization,the adsorbents,like the FeS₂/MWCNTs adsorbents,oxidized Hg⁰ to Hg²⁺by the active site of polysulfide on the surface of the active material CuS,with chemical adsorption playing a leading role.