Research On Mechanisms Of NH₃-SCR Reaction And SO₂&H₂O Resistance In Low-Temperature Over Fly Ash Supported Mn-Based Catalysts

More coal as fuel is being used to meet the energy demand.The process of coal combustion is accompanied by the release of NO_x and other environmentally toxic gases,as well as large quantities of fly ash.NO_x and fly ash pose a serious threat to human life and ecological environment.V₂O₅-WO₃(Mo O₃)/Ti O₂ is the most widely used commercial catalyst at present.However,this catalyst suffers from poor low-temperature activity,and it contains poisonous vanadium substances.As a result,China has introduced strict policies to regulate the existing denitrification catalysts and technology,as a result,prohibiting the production of vanadium/titanium-based catalysts and all the relevant industries were required to stop using these catalysts.The economic and environmental protection aspect of low-temperature denitrification technology has attracted the attention of scientific community.Herein,coal-based fly ash(FA)is used as a catalyst carrier of the active component.Mn,Fe,and Nd are used as the active species to synthesize Nd_0.05MnFe/FA catalyst series for low-temperature denitrification experiments.A fixed-bed flue gas denitrification experimental setup was used to investigate the performance of the catalyst and the influence of different factors,such as catalyst preparation condition,flue gas temperature,and flue gas composition on the denitrification performance of the catalyst.The physical and chemical structure of the synthesized material was studied using N₂ physisorption analysis,X-ray diffraction analysis(XRD),Temperature Programmed Desorption(TPD),Temperature Programmed Reduction(TPR),X-ray photoelectron spectroscopy(XPS),and In situ furious transmission infrared(in situ DRIFTs).Based on the obtained results,the mechanism of the catalytic reaction,kinetics,and the mechanism of water and sulfur poising were established.The main content and conclusions of this study are as follow:(1)The Mn_0.15Fe_0.05/FA(N)catalyst is prepared using fly ash as the carrier using traditional impregnation method.The NO_x conversion in the temperature range of 130-300? is maintained at more than 90%with a relatively good stability.With the results of XPS,NH₃/NO-TPD,H₂-TPR,and in situ DRIFTs,it was found that Langmuir-Hinshelwood mechanism was the reaction mechanism under which the denitrification reactions took place.(2)The oxidation-reduction characteristics of the catalyst surface are the main factors that affect the denitrification activity.In this study,KMn O₄ is used as the oxidant and Mn source to prepare the MnFe/FA catalysts.NO/NH₃-TPD,H₂-TPR,and other analysis showed that the addition of KMn O₄ increased the oxidation-reduction capacity of the catalyst by 7times,increased the acidity by 1.74 times,and improved the interaction between the active metal and the carrier.The addition of KMn O₄,therefore,significantly improved the low-temperature activity of the catalyst and improved H₂O resistance of the catalyst to a certain degree.The NO_xconversion reached 66%at 60? and remains above 90%in the temperature range of 80-230?.The catalyst performed well in a flue gas containing 10 vol.%H₂O at a temperature of 200?.The in situ DRIFTs analysis found that the catalytic reaction followed Langmuir-Hinshelwood and Eley-Rideal reaction mechanism during NH₃-SCR reaction.Kinetic calculations and in situ DRIFTs results were used to verify the reaction mechanisms.It was also found that the apparent activation energy of MnFe/FA catalyst to complete the NH₃-SCR denitrification reaction is 28.55 KJ·mol^-1.(3)The addition of neodymium to the catalyst can enhance the NO_x conversion of Nd_0.05MnFe/FA catalyst and exhibit 95%NO_x conversion at 80? and maintain 90%conversion in the temperature range of 75-250?.The presence of 5 vol.%or even 10 vol.%H₂O and 20 ppm SO₂ at 105,150,and 200? did not influence the NO_x conversion capability of the catalyst.When 5 vol.%H₂O and 20 ppm SO₂ were simultaneously used,the conversion rate reduced from ca.100%to ca.95,ca.93,and ca.90%after 4 hours at 200,150,and 105?,respectively.After the removal of H₂O and SO₂,the NO_x conversion remained unchanged.Based on the XPS,XRD,and NO/NH₃-TPD analysis,the addition of Nd improved the interaction between the active metal ions,and thereby improving the oxidation-reduction characteristics of the catalyst surface,weak acidity,and the ability to adsorb NO.In situ DRIFTs and kinetic calculations showed that E-R reaction pathway was the primary reaction pathway to reduce the SO₂ on the catalyst.It can be concluded that Nd_0.05MnFe/FA catalyst completes the NH₃-SCR reaction primarily following the E-R reaction pathway,which can reduce the poisoning effect of SO₂ on the catalyst.XPS and TG analysis were performed on a sample that was used during the SO₂/H₂O resistivity experiments.The results showed that the presence of Nd in the synthesized catalyst decreases the decomposition temperature of bisulfates that were formed during the presence of SO₂ and H₂O and therefore protecting the active material.Also,the kinetic investigation revealed that the apparent activation energy of the denitrification reaction of this catalyst is17.25 KJ·mol^-1,promoting the low-temperature reaction of the catalyst.