| Mercury is a global threat to human health and the enviroment due to its toxicity.In recent years,atmospheric mercury pollution problem is becoming more and more attention all over the world.Coal-fired utility boilers consume large quantities of coal and are the largest anthropogenic source of atmospheric mercury,therefore it is very important to control the mercury emision from the power plant.At present,the the sorbentinjection technology is considered to be the most effective technology to specifically to mercury removal.Activated carbon sorbent has become a hotspot in research related to mercury removal area all over the worlddue to its superior mercury adsorption performance.However,its high production cost and negative effect on fly ash reuse has become an obstacle to its widely use and application.Therefore,this thesis aims to develop a highly efficient,low-cost mercury activated carbon sorbent and explore its mechanism on mercury removal.Firstly,A high carbon content and cheap natural asphaltwas chosen as the raw materials,then original activated carbon samples were prepared under different activation methodsaccording to the principle of orthogonal experiment.Study the effect and mechanism of different preparation conditions on the mercury removal performance of obtained activated carbon samples.and obtained a best cost performance of the original activated carbon preparation plan bythe production cost economic analysis.Secondly,different chemical were used to modified the original activated carbon prepared in the optimal condtion to improve the mercury removal performance and evaluate the mercury adsorption performance under different simulated flue gas atmosphere.Thirdly,study the kinetic behavior of modified activated carbon.Finaly,In order to eliminate the negative effects on fly ash reuse and reduce the cost of activated carbon injection,a modification method of recycled magnetic modified activated carbon was put forward,and optimize its adaptive characteristics on real flue gas condition.In theprocess of original activated carbonpreparation,different experimental variables has a influence in activated carbon's specific surface area,yield and mercury adsorption performance.So this thesis mainly studies the mechanism of experimental variables impact on the properties of activated carbon and obtain an economic and efficient activated carbon sorbent preparation methods.The research content including:(1)According to the principle of single variable method,respectively,study the effect of different experimental conditions on the performance of activated carbon and concluded that the mixing proportion of KOH and carbonization temperature exist close correlation with the formation of surface functional groups,and activation temperature has a remarkable effect on the pore structure and yield.Improving the activation temperaturecan increase the activation reaction rate,which is conductive to the formation of inner pore structure of activated carbon.Howeverr,when the temperature is too high,a severe reaction rate will lead to the "collapse" of pore structure.(2)Three activation methods were used to prepare the activated carbon.Through evaluatethe pore structure,surface functional groups and mercury adsorption performance,studying the impact significance of different experimental variables(carbonization temperature,carbonization time,activation dosage,activation temperature,activation time,etc.)on the characteristicof obtained activated carbon.The result indicated that the single physical activation method will lead to most of the carbon loss in the form of volatile and extremely low yield of the activated carbon inthe pyrolysis process,therefore,this method does not apply to preparaed activated carbonfrom asphalt.The potassium hydroxide added in the chemical activation method reduced the formation of volatiles during the carbonization process.The KOH/asphaltene mix rate plays a decisive role on the specific surface area and Hg adsorption capacity of activated carbon.(3)According to the result of polar difference analysis and factor analysis,the most optimal experimental conditionsin different activation method were obtained and verified by the real experiment.(4)By establishing activated carbon preparation system model,and combining the energy consumption,yield and system cost,obtained a best cost performance of the original activated carbon preparation plan bythe production cost economic analysis.Because mercury adsorption capacity of the original activated carbon a is very limited,in order to improve its mercury adsorption performance,different methods were adopt to modifythe activated carbon.The research content including:(1)Based on a fixed bed experimental platform,combining with different characterization methods,mercury adsorption mechanism of the halogen modified activated carbon was studied and concluded that activated carbon modified by elemental iodine and HBrbehaves best mercury adsorption performance,Homogeneous reaction and Heterogeneous reaction were taken place at the same time in the process of adsorption.Under certain temperature,a part of halogen will release to the flue gas and react with the elemental mercury and the rest of halogen will retain in the activated carbon through chemical bonds.(2)The study on dynamics of elemental iodine modified activated carbon adsorption behavior,concluded that the real adsorption behavior is close to the accurate secondary adsorption model and the main adsorption control steps is the chemical active-site point on the surface of activated carbon.(3)In order to eliminate the negative effects on fly ash reuse and reduce the cost of activated carbon injection,a modification method of recycled magnetic modified activated carbon was put forward.the result have shown that MnFe2O4-AC has good magnetic property and enhenced elemental mercury oxidation ability.(4)In order to increase the adaption property of activated carbon in the actual flue gas,the thesis studied the magnetic activated carbon's mercury adsorption performance in different flue gas composition.It was concluded that O2 and HCl were conducive to mercury adsorption performance improvement of magnetic activated carbons,while SO2,SO3 and H2O in gas phase greatly reduces the mercury adsorption properties.In order to improving the magnetic activated carbon's adaption ability to the real flue gas,reducing SOx negative effect on its mercury adsorption performance,A method of addingalkali metal oxide to the magnetic activated carbon was put forward in this thesis.The results showed that the addition of alkali metal oxide can significantly lower SO3 concentration and help to reducign the nagative effect on the mercury adsorption performance of magnetic activated carbon.(5)in view of the sorbent applied in power plant only hasvery limited residence time in the flue gas,activated carbon's mercury adsorption capacity cannot be make full use of,repeatedly use the magnetic activated carbon was studied,the results showed that after mercury adsorption experiment take place many times,magnetic activated carbon can still maintain high levels of mercury adsorption efficiency,which will largely reduce the cost of activated carbon injection. |
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