Study On The Performance Of Fly Ash-based Zeolite Catalysts For Simultaneous Desulphurization And Denitrification

Carbon peaking and carbon neutrality goals requires flexible operation and deep peaking of coal-fired units,which places higher demands on coal-fired gas SO₂ and NO_X emission control.Conventional SO₂ and NO_X emission control is usually achieved using calcium-based wet flue gas desulfurization（FGD）technology and selective catalytic reduction（SCR）denitrification technology.This one-to-one treatment has a range of disadvantages such as complex equipment,large footprint,high investment and operating costs,and high pollution of downstream equipment from upstream treatment.In addition,fly ash,as a solid waste rich in Si and Al,should be fully utilized,and high value synthesis of various catalysts is one of the ways to utilize it.Therefore,taking into account the advantages of H₂O₂ oxidation for simultaneous desulfurization and denitrification and the high value-added application of fly ash,fly ash-based zeolite catalysts were prepared to catalyze the production of active oxides from H₂O₂,coupled with alkali solution absorption to achieve simultaneous desulfurization and denitrification,and to achiece the purpose of"waste-based pollution control".In this paper,three fly ash-based zeolite catalysts,FA-SSZ-13 zeolite,FA-ZSM-5 zeolite and FA-βzeolite,were firstly synthesized successfully by acid leaching and alkali melting of fly ash as raw materials,combined with hydrothermal synthesis methods.In the experiments,the synthesis conditions（Na OH/FA,Si O₂/Al₂O₃,templating agent/Al₂O₃,H₂O/Al₂O₃,hydrothermal temperature and time,and seed addition amount）were investigated,and the optimum synthesis parameters for the various zeolites were obtained by combining SEM,XRD,and FTIR characterization means.The optimum synthesis parameters for FA-SSZ-13 zeolite are:Na OH/FA mass ratio is1.4,Si O₂/Al₂O₃=50,H₂O/Al₂O₃=1800,TMAda-OH/Al₂O₃=7,SSZ-13-seed addition of 7.5%,and hydrothermal temperature and time are 160℃and 48 h;the optimum synthesis parameters for FA-ZSM-5 zeolite are:Na OH/FA mass ratio is 1.2,Si O₂/Al₂O₃=40,H₂O/Al₂O₃=1325,TPAOH/Al₂O₃=10,the addition amount of ZSM-5-seed is5%,and hydrothermal temperature and time are 140℃and 24 h;the optimum synthesis parameters for FA-βzeolite are:Na OH/FA mass ratio is 1.2,Si O₂/Al₂O₃=30,H₂O/Al₂O₃=1200,TEAOH/Al₂O₃=14,β-seed addition amount is 7.5%,and hydrothermal temperature and time are 140℃and 72 h.Using fly ash-based zeolites as catalysts,the catalytic decomposition of H₂O₂ to produce strong oxidizing substances to oxidize NO and SO₂,and then synergistically removed of sulfur and nitrogen oxides by alkali absorption.Simultaneous oxidation experiments and synergistic removal experiments were carried out to investigate the effects of reaction temperature,H₂O₂concentration,catalyst dose,and each each gas concentration in flue gas on the simultaneous oxidation efficiency and desulfurization and denitrification efficiency,to analyze the effect of the presence of SO₂ on NO removal,and to investigate the synergistic removal mechanism.The results of the oxidation experiments showed that NO was oxidized in preference to SO₂,while the simultaneous removal experiments showed that the removal of nitrogen oxides depended on full oxidation of NO,while the sulfur oxides were almost completely removed due to their high solubility.The deactivation and stability tests were carried out on various fly ash-based zeolites to determine the deactivation mechanism and stability performance.The catalytic oxidation mechanism is as follows:the FA-zeolite can catalyze the decomposition of H₂O₂ to produce hydroxyl radicals（·OH）,·OH oxidizes NO to generate NO₂,NO₂ will react reversibly with N₂O₄,and N₂O₄ dissociates non-homogeneously at the Br?nsted acid site on the zeolite to produce NO⁺,which is produced as an important reaction intermediate for NO oxidation,and it is accompanied by the Br?nsted acid is consumed,leaving the Br?nsted acid with a single electron orbital（Si-O·-Al）as the catalytically active center.In the simultaneous desulfurization and denitrification experiments,100%removal of SO₂ can be obtained and NO removal efficiency increased due to the addition of SO₂.This is because the presence of moderate amount of SO₂ favours the absorption of NO₂ by alkali solution and the zeolite has a large specific surface area,which helps to reduce the competitive adsorption of NO and SO₂ on the active sites.Fe/FA-zeolite catalysts were prepared by equal volume impregnation,liquid ion exchange,and solid-state ion exchange methods using fly ash-based zeolites loaded with Fe as a carrier.The simultaneous desulfurization and denitrification performance of Fe/FA-β,Fe/FA-SSZ-13,and Fe/FA-ZSM-5 catalysts were investigated experimentally based on a fixed bed experimental setup,and the results showed that the introduction of Fe significantly improved the catalytic performance of the fly ash-based zeolites.The removal experiments investigated the effects of different Fe/FA-zeolite catalysts on the removal efficiency in terms of H₂O₂concentration,catalyst dosage,and flue gas composition,to determine the optimal reaction conditions and to elucidate the mechanism of simultaneous desulfurization and denitrification by means of characterization methods.Firstly,the redox cycle of Fe²⁺/Fe³⁺occurs simultaneously on the catalyst surface,reacting with H₂O₂ and releasing·OH.The highly dispersed Fe species on the zeolite surface provides sufficient active sites and the zeolite as a carrier provides sufficient contact area for the H₂O₂,stabilizing the Fe species in the catalyst,while having a high loading capacity.Thus,there is a strong interaction between Fe species and the zeolite carrier,which shows a positive effect on the catalytic performance.Secondly,Fe species in the zeolite framework enhances the concentration of Br?nsted acid sites,and Br?nsted acid can act as a promoter and facilitates the catalytic oxidation reaction.Moreover,Br?nsted acid creates an acidic environment around the catalyst surface,which can greatly facilitate the rate of·OH production due to the presence of protons that balance the electronegativity of the framework.Thus,the combination of having more Fe³⁺species and Br?nsted acid sites gives the Fe/FA-zeolite catalyst better catalytic performance.In the simultaneous desulfurization and denitrification experiments,the desulfurization efficiency remained close to 100%,while the presence of moderate amounts of SO₂ facilitates the absorption of NO₂ by alkaline solution and accelerates the Fe³⁺to Fe²⁺transformation and provide an acidic atmosphere,which facilitates the decomposition of H₂O₂ to generate·OH;in addition,the addition of SO₂ generates H₂SO₄,which in turn leads to the formation of·OH and SO₄^-·generation,both of which can realized to promote the oxidation of NO.Excess SO₂ will,of course,compete with NO for the active site,leading to a reduction in NO removal efficiency.Fe/FA-βcatalysts with good catalytic performance and stability were selected for the experimental study of NO and H₂O₂ reaction levels,chemical reaction rate constants,and apparent activation energy,and macroscopic kinetic equations were obtained.The effects of experimental parameters such as SO₂ concentration,NO concentration,H₂O₂ concentration,and catalyst dosage on the chemical reaction rate constants were analyzed in detail.With the elimination of the effects of internal and external diffusion,the reaction level of NO and H₂O₂are 1.08 and 0.27,respectively,and the apparent activation energy is 10.27 k J/mol.The performance of fly ash-based catalysts for simultaneous desulfurization and denitrification was investigated experimentally,relying on a flue gas circulating fluidized bed experimental rig with a pre-oxidation unit.The optimal removal conditions were determined and the possible simultaneous reaction paths were speculated.The NO removal efficiency and SO₂ removal efficiency could reach 77%and 100%when the reaction temperature was 130℃,the humidity was 6%,initial concentrations of NO and SO₂ were 600 mg/m³ and 600 mg/m³,H₂O₂ concentration was 25%and Ca/（S+N）was 1.4,where the SO₂ removal efficiency increases and then decreases with increasing inlet flue gas temperature,flue gas humidity and Ca/（S+N）,and decreases slightly with increasing initial SO₂ concentration,but is not significantly affected by residence time,NO concentration and H₂O₂ concentration.In addition,since both SO₂ and its oxidation product SO₃ have higher solubility,higher removal efficiency than NO can be obtained.The removal efficiency of NO increases with increasing H₂O₂concentration and residence time,and tends to increase and then decrease with increasing NO concentration,Ca/（S+N）,SO₂ concentration,and flue gas humidity.The pilot test showed that the technical route of fly ash-based catalyst for simultaneous desulfurization and denitrification is feasible,and this study provides important technical data for industrial scale-up.