Preparation Of Supported Fly Ash Based Hierarchical ZSM-5 Zeolite For Flue Gas Hg~0 Removal

Due to the volatility,toxicity,mobility and bioaccumulation,heavy metal mercury（Hg）may cause great harm to the environment and human body.Coal combustion is the main source of the global mercury pollution emission.At present,the removal of elemental mercury mainly relies on the adsorbent（such as activated carbon）injection technology from the flue gas.However,activated carbon is a typical carbon-based material.Its injection is high-cost.Also,it may affect the quality of fly ash which may restrict the further reuse of fly ash.With the characteristics of large specific surface area,abundant acidic sites,skeletal structure,and high hydrothermal stability,zeolites have the potential of being widely used in different fields.However,traditional zeolites only have single-pore channel structure.In order to improve the diffusion limitation in these micropores during the reaction,this paper studies the hierarchical ZSM-5 molecular sieve,which is prepared from two types of solid wastes,rice husk ash（RHA）and coal-fired fly ash（FA）.Utilizing these two raw materials could further reduce the synthesis cost of mercury removal adsorbents.Mercury removal adsorbents which has high-performance need to have abundant microporous structure,reasonable hierarchical pore distribution,sufficient surface functional groups and stable skeleton structure.Thus,four types of common adsorbent substrates with interrelated but different physicochemical properties were chosen,and a lateral comparative study was carried out to identify the optimal substrate material and modification method.Fe Cl₃ impregnated materials exhibited different pore structure and surface chemical characteristics.The mercury removal performance of the impregnated materials was furtherly compared and analyzed.In combination with BET,XRD,SEM and XPS,the interaction between Fe Cl₃ and adsorbent substrate can be detected.The adsorption active site for Hg⁰ over the surface of different materials can be further analyzed by Hg-TPD experiment.The results showed that the mercury removal ability of the pristine samples provided by physical adsorption is limited.The impregnation of Fe Cl₃ could significantly improve the mercury removal performance of different materials.The increase of reaction temperature is beneficial to the adsorption of Hg⁰ by the impregnated samples.The interaction between ZSM-5molecular sieve and Fe is relatively strong.In addition to the amorphous form of Fe species dispersed on the surface of other materials,part of Fe atoms are retained in the molecular sieve framework structure by ion replacement.Due to the abundant micropores and hierarchical pore structure of activated carbon（AC）,the loading and utilization of active Fe Cl₃ by activated carbon（AC）is the highest among the four types of substrates.The Hg⁰adsorption efficiency of AC is the highest,confirming that the hierarchical pore structure is beneficial to the reactants.Therefore,the ZSM-5 molecular sieve was subsequently selected as the base substrate to abstain the hierarchical pores thus utilizing the advantages of the silicon-alumina based material and the hierarchical pore structure.With FA as aluminum source and partial silicon source,the one-step hydrothermal synthesis process of ZSM-5 zeolite with hierarchical pores was studied in this work.Impacts of Si/Al,crystallization temperature,crystallization time,and external silicon source on the hierarchical ZSM-5 synthesis were investigated.The results showed that appropriate Si/Al could contribute to the construction of the framework during crystallization.Higher reaction temperature is helpful to provide the energy needed for crystallization reaction.48 h crystallization time is beneficial to the growth of crystal grain and the development of hierarchical pore structure.Selecting materials which could match well with silicon source and template agent is also beneficial to enhance the stability of crystal structure.In the one-step hydrothermal crystallization process,the silicon-aluminate material in the precursor solution is dissolved and dispersed under high temperature and pressure.Then the samples were assembled and arranged beside the micelle formed by the template agent,thus leading to the formation of crystal skeleton with a specific structure,the adsorption of mercury was significantly enhanced.Using RHA as the external silicon source,the suitable experimental method for the synthesis of molecular sieves were explored.The porosity structure,crystal structure,microscopic morphology and surface acidity of the hierarchical ZSM-5 synthesized by RHA and FA were compared and analyzed.The mercury removal efficiency of the pristine zeolite sample was poor.The pristine samples were further impregnated with Fe Cl₃ and then showed significantly enhanced mercury capture performance.The results showed that the concentration of the impregnating solution required for hierarchical ZSM-5 is much lower than that of the traditional ZSM-5.This is due to that the hierarchical pore structure could reduce the diffusion resistance of guest molecules（Fe Cl₃,Hg⁰）.And the utilization rate of the internal acidic sites and microporous structure becomes higher as well as the more adequate active sites combined with Hg⁰.In Hg-TPD experiment,compared with the traditional ZSM-5,the sample with hierarchical pore structure showed a new mesoporous Hg Cl₂ desorption peak,indicating that the existence of mesopores is conducive to the exposure of the active sites of the adsorbent.The diffusion path of the reaction can also be shortened.In this work,the synthesis of hierarchical zeolite by RHA and FA was explored.The resulting mercury removal ability and mechanism were also investigated.Based on the optimization of chemical structure and composition of adsorbents,highly efficient and low cost mercury removal adsorbents can be developed which could also provide an important reference for the high value-added utilization of solid waste.