Preparation And Adsorption Properties Of Modified Chitosan Adsorbents To Simultaneous Removal Of CO₂ And Mercury In Flue Gas

The emission of heavy metal mercury is very harmful to the environment and people's health,and the greenhouse gas CO₂ emission causes global climate variability.It is very important to control the emission of mercury and capture and storage of CO₂.As one of the largest sources of CO₂ emission and mercury pollution,coal-fired power plants need to find an efficient,safe and economical way for CO₂ and mercury removal.That will solve the problem of high energy consumption and large investment caused by mercury emission and carbon capture and storage.At present,there is no effective technology to remove carbon and mercury together.The adsorption method has the similar characteristics of mercury removal and decarbonization technology.However,the application of the same adsorbent for the removal of carbon and mercury has not been studied in depth.In this thesis,according to the characteristics of CO₂ and heavy metal mercury emission in the flue gas of coal-fired power plant,the natural polymer chitosan,which is rich in amino and hydroxy functional groups,was modified as a new type of adsorbent with simultaneous decarburization and mercury removal.In order to find an efficient,safe and economical method for removing mercury and carbon dioxide,a study was made on the performance and mechanism of combined removal of carbon and mercury.In this thesis,the sol-Gel method was used to prepare a series of chitosan adsorbents which own photocatalytic properties to mix with KI and TiO₂ by different acid.Specific surface area?BET?analysis,X-ray diffraction?XRD?analysis,scanning electron microscopy?SEM?and Fourier transform infrared spectroscopy?FTIR?were used to characterize the samples before and after adsorption of Hg²⁺.Then adsorbent suitable for mercury removal from coal combustion flue gas was optimized.Using the molecular imprinting process,a one-step high-temperature activation modification method was developed,the relative high specific surface area of chitosan modified adsorbent was obtained,and the removal performance of CO₂ was studied systematically.The parameters of high temperature modification of adsorbent were obtained.The regeneration performance was studied under the pure carbon gas flow,and the regeneration performance is good,and the energy consumption is low.The results show that chitosan as the carrier of photocatalyst TiO₂ is very suitable,which can strengthen the anatase phase and enhance the photocatalytic properties of TiO₂.The Br-can prevent the growth of TiO₂ crystal grains to gain high Specific surface area and strengthen the ability of I-to remove mercury.The addition of TiO₂ greatly enhanced the adsorption capacity of mercury.When PH is 5-7,it is beneficial to the adsorption of mercury,and the mercury adsorption capacity is larger when the temperature is about 55?.The adsorption isotherm and kinetic results indicated that the adsorption behaviour of CS-KI/TiO₂-HBr as it removes Hg²⁺ is an inhomogeneous multilayer adsorption process.The surface adsorption and intraparticle diffusion effects are both important in the Hg²⁺ adsorption process.Dilute sulfuric acid can be eluted Hg²⁺ to make the adsorbent regeneration,the regeneration process is the molecular imprinting process,is conducive to the selective absorption of mercury,increase its adsorption capacity.The chitosan based carbon material with large surface area and micropore can be obtained by the method of alkali activated high temperature modification.If the pyrolysis temperature is below 500?,the amino functional groups can be effectively retained.The preparation conditions of the best adsorbent which is favorable for the decarbonization reaction are KOH as the activating agent,and the temperature rising process in the N2 atmosphere from 25? raised to 500? for 20 min,TiO₂ content of about 10%,TiO₂ can accelerate the pyrolysis process The adsorption of CO₂ by adsorbent is a combination of physical adsorption and chemical adsorption,and the optimum reaction temperature is 50?.The maximum adsorption capacity of 20%CO₂ at atmospheric pressure was 2.86mmol/g,and the adsorption capacity decreased sharply at 110?.The adsorption capacity of adsorption regeneration was almost unchanged under pure carbon flow.The IR spectra showed that the adsorption mechanism of CO₂ was the reaction of CO₂ with basic amino group-NH,-NH₂ and the formation of carbamate or amino acid ammonium salt.The residual activated agent promotes the formation of carbonate or bicarbonate.In this thesis,four kinds of activated adsorbents CS-Ti-Na-N2,CS-Ti-Na-CO₂,CS-Ti-K-N2,and CS-Ti-K-CO₂ were used to carry out the synergistic removal of carbon and mercury in the photocatalytic reactor by self-designed.Combined with XRD,SEM,BET,FT-IR,the mechanism of carbon and mercury removal was obtained,and the reactive sites were found.Infrared spectrum analysis showed that the adsorption of mercury and CO₂ on the modified chitosan adsorbent was different from that of the O-H stretching vibration in the vicinity of 3500cm-1.The results showed that UV irradiation could promote the synergistic removal of mercury and CO₂.When CO₂ and elemental mercury coexist,it hinders the recombination of photogenerated electron hole pairs,and consumes electrons and holes,which promotes the extension of photocatalytic oxidation and reduction,and increases the removal efficiency of elemental mercury and CO₂.However,when the elemental mercury captures the photogenerated electrons and the oxygen molecules adsorbed on the surface of the adsorbent,the mercury removal reaction is promoted,and the decarburization reaction is inhibited.The presence of H₂ O can promote the synergistic removal of carbon and mercury.KOH as an activator is more conducive to the removal of CO₂ than NaOH.When the carbon and mercury coexist,the maximum decarburization capacity of 20 min is 29.35mmol/g,which is much larger than that of thermogravimetric?TG?experiment under the pure carbon flow in the 1.02mmol/g.The effect of CS-Ti-K-CO₂ on mercury removal was better than that of CS-Ti-K-N2,and the adsorption removal of CO₂ was slightly decreased due to the presence of K2CO₃.Modification of the adsorbent by high temperature promotes the formation of titanium dioxide nano-crystal.That improves the visible light catalytic performance.Under sunlight,when CO₂ and elemental mercury coexist,the adsorbent owns a higher carbon dioxide and mercury removal capacity.The mercury removal efficiency of CS-Ti-Na-CO₂ in the presence of CO₂ is as high as 100%,which is more conducive to the removal of mercury in pure carbon flow,and then the purity of CO₂ is obtained.This will be beneficial to the purification and utilization of CO₂ in oxygen enriched combustion and chemical looping combustion technology.In a word,under the sunlight or ultraviolet light,CO₂ and mercury can promote the occurrence of redox reaction in the presence of co-existing,which is beneficial to the removal of carbon dioxie and mercury by modified chitosan adsorbent.