Synthesis Of X-Zeolite From Coal Fly Ash And Its Application On NH₃-SCR Reaction

As the rapid developing of social economy,energy consumption is also growing rapidly.In China,about 70 percent of the energy currently comes from burning coal.In the process of coal utilization,a large amount of coal fly ash and pollution gas will be produced.In recent years,the total amount of fly ash emissions has continued growing,at the same time,some fly ash is still not properly disposed every year,causing harm to land,air,water resources and human health.On the other hand,the coal fly ash is also a kind of enormous resources.The comprehensive utilization of fly ash is more and more attention at present.In China,the utilization of coal fly ash is focused on the low added value like the construction industry and agriculture,while the application of high value-added utilization of fly ash is small,comparing the developed countries.Therefore,exploring ways to use fly ash with high added value,regardless of starting from environmental protection and social and economic development,is very promising.Zeolite is widely used in SCR reaction as the catalyst carrier for selective catalytic reduction of nitrogen oxides?SCR?.However,due to its complex synthesis process and high cost,its large-scale application is limited.Therefore,fly ash zeolite based catalysts have great application potential in NH3-SCR reaction.It is of great value and significance to study the method of removing NOx by using the synthetic zeolite prepared from fly ash.However,there is still no unified theory on the method and conditions of fly ash synthesis,and the application of zeolite based catalyst for fly ash in denitrification is rarely reported.Since the main components of fly ash are silicon and aluminum,and they are also the main components of zeolite,it is theoretically feasible to use fly ash to prepare zeolite.Zeolite is widely used in SCR reaction as the catalyst carrier for selective catalytic reduction of nitrogen oxides?SCR?.However,due to its complex synthesis process and high cost,its large-scale application is limited.Therefore,fly ash zeolite based catalysts have great application potential in NH3-SCR reaction.It is of great value and significance to study the method of removing NOx by using the synthetic zeolite prepared from fly ash.However,there is still no unified theory on the method and conditions of fly ash synthesis,and the application of zeolite based catalyst for fly ash in denitrification is rarely reported.In this paper,X zeolite from fly ash was synthesized by hydrothermal method,and the pure phase X zeolite was further synthesized by optimizing the synthesis conditions.On this basis,Mn and Ce were loaded to prepare denitrification catalyst.By testing its catalytic activity and catalytic characterization experiments,the relationship between the physical structure,physicochemical characteristics and catalytic activity of the catalyst was studied,and the key factors affecting the activity of the catalyst were revealed.Finally,in combination with in-situ infrared spectrum technology,the reaction path of NH₃-SCR reaction over zeolite-based catalyst loaded with Mn and Ce was studied.With the conditions of alkali fly ash ratio of 1.2,liquid-solid ratio of 9,silicon aluminum mole ratio of 2.4,the crystallization temperature is 90?,crystallization time is 18h.the pure phase fly ash X zeolite was successfully synthesized.The physical and chemical properties as well as the microstructure and morphology of the crystal were studied.The crystal phase of the product was single phase,and the position and intensity of the main diffraction peaks of XRD were basically consistent with the standard sample of commercial zeolite X.The particles of the synthesized product presented a relatively complete octahedral structure with complete crystallization and uniform particle distribution.The crystal morphology was basically the same as that of commercial zeolite X.At the same time,the yield is high and has good economy.On the basis of the synthesized fly ash zeolite X and the commercial zeolite X,two kinds of active component Mn and Ce have been load over the fly ash X zeolite and commercial X zeolite and according to the study of its catalytic activity in NH₃-SCR reaction,It was found that cerium in composite molecular sieve catalyst,manganese and cerium has obvious synergistic effect,showing a higher SCR catalytic activity than single one,weighing against commercial X zeolite denitration catalyst performance.Based on the characterization results of these catalysts,the relationship between the physical structure,physicochemical properties and catalytic activity of the catalysts was studied.We found that the composite molecular sieve catalyst has larger specific surface area,cerium and manganese in the catalyst surface can form an amorphous structure,and have good dispersion,at the same time,the catalyst with lower reduction temperature,therefore because of the advantage of the catalyst on the physical structure and chemical properties,make its have superior denitration catalytic activity.At the same time,the content of the species valence state of the surface active catalyst plays an important role in the low temperature activity of the catalyst.There is a strong interaction between MnOx species and CeO₂,which will change the electronic properties of the surface active components and improve the reduction capacity of the catalyst,so as to promote the oxidation of NO to NO₂ at low temperature and improve the low temperature activity of the catalyst.Finally,we studied the reaction mechanism of NH₃-SCR reaction on Mn-Ce-HX catalyst,identified the key species and intermediate species involved in the reaction,and explored the reaction path of this reaction on denitrification catalyst.We found that on the surface of Mn-Ce-HX catalyst,the mechanism of E-R and the mechanism of L-H were carried out simultaneously in the SCR reaction.Among them,Br?nsted acid sites play a major role in the E-R mechanism.In the L-H mechanism,monodentate nitrate species and NO₂ have catalytic activity,while bidentate nitrate and bridged nitrate will hinder the catalytic reaction.In the low temperature section,the E-R mechanism has a great influence on the reaction.At low temperatures?under 250??,the NH₃adsorption quantity is big.As the temperature increases,NH₃ adsorption decreased slowly,at the same time,surface oxygen species such as nitrate activity enhancement,in the reaction proportion of mechanism of E-R slowly decrease,proportion of L-H increased,therefore,the catalyst activity is high at low temperature.In the high temperature section?above 250??,Br?nsted acid have a poor thermal stability.As the temperature increases,NH₃ adsorption decreased rapidly,at the same time,surface oxygen species such as catalyst of nitrate activity is reduced,thus makes the high temperature of catalyst activity is abate,which is accorded with the results of activity..