Preparation Of Solid Waste-Based Functional Materials And Mechanism Study On Mercury Removal

As a kind of heavy metal with high toxicity,mercury can accumulate in the organism and transport long distances in the ecosystem.Coal-fired boiler is the largest anthropogenic mercury emission source.Therefore,the mercury control in power plant has become important research topic so far.Mercury adsorption and catalysis have high potential for promoting mercury removal efficiency due to the flexibility of choosing specific materials.Therefore,the development of cost-effective and highly-efficient adsorbents and catalysts for mercury removal is the research direction in the field of flue gas mercury pollution control.With the rapid development of social economy and the acceleration of industrialization,the production of solid waste is also continuously increasing,which has adverse effect on the water quality,soil environment and atmospheric environment as well as harm human health.In the“14th Five-year Plan”,Chinese government further proposed the development of solid waste recycling in order to reduce energy consumption and carbon emissions during process of mineral mining,transportation and transformation of raw materials.In this research,taking the reutilization of solid wastes into mercury control in coal-fired flue gas as the purpose,in combination with the technological concept of"detoxification by waste",focused on the performance factors such as pore structure and active sites on mercury removal technologies（adsorption method and catalytic oxidation method）,the cost-effective and highly-efficient flue gas mercury removal materials were developed with typical industrial and domestic solid wastes as raw materials.Firstly,low-cost waste polystyrene was selected as raw material to explore the synthesis of hyper-cross-linked microporous polymer（HCPs）material under mild conditions.The effects of reaction temperature,reaction time and crosslinker on pore structure and micromorphology of HCPs were studied by single factor analysis.The results showed that the formation of HCPs skeleton requires sufficient reaction time and temperature.The reaction under 80℃ for 24 h can fully build the cross-linking of the internal aromatic ring structure,thus creating a hierarchical system with interconnected micropores and mesopores.Therefore,an organic porous material HCPs with waste polystyrene as raw material was successfully developed by focusing on the performance factor（pore structure）in mercury adsorption materials.Subsequently,a novel surface chemical modification method of Lewis acid in situ construction was proposed to prepare HCPs-based mercury removal adsorbent.Different Lewis acid catalysts were utilized to catalyze the formation of HCPs skeleton structure.Simultaneously,Lewis acids were in-situ constructed in HCPs material as active site.The obtained HCP-Metal materials were characterized from the aspects of pore structure,organic structure,microscopic morphology,and surface chemical environment.Then,the differences of physical and chemical properties and mercury removal ability of different HCP-Metal materials were analyzed through fixed-bed mercury removal performance reactor.As a result,the superior in-situ construction catalyst FeCl₃ and corresponding in-situ construction adsorbent HCP-Fe were selected for Hg⁰ removal.HCP-Fe showed nearly 100%mercury removal efficiency under complicated flue gas conditions（N₂+6%O₂+1200 ppm SO₂+10%H₂O）.High mercury removal efficiency was maintained for 30 h during a long mercury removal stability test.Therefore,based on the synthesized HCPs with well-developed pore structure and focusing on the performance factor（active sites）in the adsorbent,the in-situ construction HCPs adsorbent for mercury removal was successfully created.Thirdly,based on the synthesized HCP-Fe material,the mechanism of mercury removal with the industrial application prospect of this material was further explored by changing the dosage of FeCl₃ in the in-situ construction process.Characterizations and fixed-bed Hg⁰ removal performance were further tested.The mechanism of Hg⁰ removal of HCP-Fe in various flue gas components was investigated by integrating DFT theoretical calculation,molecular simulation and Hg temperature desorption.HCP-2-Fe can embed a large number of active Fe sites and retain the original developed pore structure（specific surface area is about 1158 m²/g）at the same time.The Fe sites embedded in the skeleton can be remained in the HCPs throughπ-complexation.The micro-pores in the HCPs skeleton can cause the nano-confinement effect,thus the Fe sites can homogeneously disperse in the HCPs skeleton,which can prevent the high hydroscopicity of FeCl₃ impacting the hydrophobicity of HCP-Fe material.It can be concluded that the high mercury removal ability of the in-situ constructed HCP-Fe materials relies on the synergistic effect of the two aspects.On the one hand,the embedded Fe site has high activity,which can ensure the effective capture of Hg⁰ in different flue gas atmosphere.On the other hand,the well-developed and interconnected pore structure of HCPs can guarantee the mass transfer of Hg⁰ in HCP-Fe and promote the oxidation of Hg⁰.Fourthly,after discussing the synthesis of organic material（HCPs）and the mechanism of mercury,the application and mechanism of traditional inorganic porous molecular sieve in mercury removal were studied by means of material characterization and theoretical calculation.Hierarchical pores were introduced into traditional molecular sieve materials by acid/alkaline etching method in order to obtain hierarchical molecular sieve materials with optimized pore structure.Active sites were obtained via FeCl₃ impregnation.After chemical etching,the internal pore structure of ZSM-5 was changed.The desilication and dealumination caused by chemical etching can promote the formation of new micropore and mesoporous structure in the ZSM-5framework,improving the mass transfer rate of guest molecules in the h ZSM.Chemical etching can produce electron-rich unsaturated oxygen in the skeleton,thus affecting the chemical environment of adjacent Fe ³⁺thus enhancing the reactivity of corresponding Fe sites.Fe /h ZSM-a exhibits excellent mercury removal performance and high SO₂-resistance due to the introduction of additional Bronsted acid sites in the skeleton by HNO₃ treatment,which effectively protects Fe sites from SO₂ attacking.Then hierarchical ZSM-5 was prepared from rice husk ash and fly ash via one-step synthesis,which also showed excellent mercury removal performance compared with chemically etched ZSM-5.Similar to the study of developing HCPs adsorbents focused on pore structure and active site,the biomass ash-based hierarchical zeolites were successfully compounded from the aspect of inorganic materials.Finally,after exploring the mechanism of mercury adsorption on different porous materials,the Hg⁰ catalytic oxidation materials was further studied.Focusing on the performance factor（active sites）of catalytic mercury removal materials,inorganic crystal material Ti-bearing blast furnace slag was selected as a case study.Mn Ce/TBFS catalyst was prepared via A/B site chemical modification over internal CaTiO₃ for catalytic mercury removal from coal-fired flue gas.Combined with experimental research and theoretical calculation,the influence of A/B site metal doping on the crystal structure,electronic structure and redox performance of the catalyst were investigated.The doping of Mn and Ce into CaTiO₃ structure is based on the substitution over B site and A site,respectively.After doping,the crystallinity and grain size of CaTiO₃ are significantly reduced.Meanwhile,the phenomenon of lattice distortion and surface defects can be observed.During the process of Mn/Ce co-doping,the electrons in Mn 3d orbit will transfer to Ce 4f orbit,resulting in obvious interaction,which will optimize the electronic structure of the catalyst surface as well as optimize the surface acidity and oxidation reducibility,which is of great significance to improve the oxidation capacity of catalyst Hg⁰.Taking flue gas mercury removal and solid waste reutilization as the research purpose,three types of solid waste-based mercury removal materials were developed in this paper,including hyper-cross-linked polymer,hierarchical ZSM-5 and perovskite.Based on the surface chemical reactions over the three materials,the Hg⁰ removal mechanism was revealed from the perspective of adsorption and catalysis of Hg⁰,providing theoretical and experimental guidance for the design and application of novel solid waste-based functional materials.