Study On The Mechanism Of SO₂/H₂O Affecting The Denitrification Performance Of Rare Earth Tailings For NH₃-SCR

Selective catalytic reduction（SCR）is the most widely used technology for coal-fired flue gas denitrification.The core of NH₃-SCR technology lies in the selection and application of catalysts,and the research and development of new catalysts is one of the important initiatives in the development of nitrogen oxide（NO_X）emission reduction technology.Catalyst SO₂/H₂O tolerance is a key indicator for denitrification performance evaluation and subsequent application.Rare earth oxides and transition metal oxides are common raw materials for the preparation of new catalysts,and the preparation of rare earth tailings denitrification catalysts from Bayan Obo rare earth tailings contains a variety of rare earth elements and transition metal elements,which has important scientific significance and practical value for the research and development of new catalysts and the high-value utilization of rare earth tailings resources.In this thesis,the influence mechanism of SO₂/H₂O on the denitrification performance of rare earth tailings catalyst NH₃-SCR was investigated by using catalyst activity evaluation system,combined with XRF,BET,SEM&EDS,XRD,NH₃-TPD,H₂-TPR and in situ infrared experimental analysis.The rare earth tailings catalysts were prepared by ball milling and microwave roasting,and it was found that SO₂ and H₂O increased the denitrification efficiency of the catalysts by about 16%in the temperature range of 250°C to 450°C,and the denitrification efficiency was up to 66.7%at 350°C.The morphology,structure and chemical composition of the catalysts were more stable before and after denitrification.The presence of SO₂/H₂O increased the number and strength of weak and medium acidic sites on the catalyst surface,which improved the catalytic activity of rare earth tailings and enhanced the redox ability.The XRD results showed that the adsorption and conversion of SO₂/H₂O,NH₃ and O₂ on the catalyst surface resulted in the appearance of sulfamic acid（NH₃SO₃）species,and NH₃SO₃ has a catalytic effect on enhancing catalytic denitrification.Combining the catalyst denitrification performance analysis and in situ infrared experimental results,a reaction equation for the SO₂/H₂O tolerance mechanism of NH₃-SCR denitrification with rare earth tailings catalyst was established.In the experimental rare earth tailings catalyst,iron oxide minerals should be the main active component,and H₂O enhances the adsorption process of SO₂ on the catalyst surface and promotes the reaction of SO₂ to form SO₄^2-.Because of the complex succession of active minerals and numerous active sites,SO₂ adsorption and conversion under O₂ conditions enriched the catalyst surface with SO₃,and reacted with SO₄^2-group to form metabisulfate group(S₂O₇^2-).The S₂O₇^2-group promotes the production of amino（NH₂）and ammonium（NH₄⁺）groups,thus increasing the denitrification efficiency of the catalyst.The SO₂/H₂O tolerance mechanism of the rare earth tailing catalyst follows the L-H and E-R mechanisms together in the medium and high temperature section.In addition the conversion of S₂O₇^2-group to NH₃SO₃ and（NH₄）₂SO₄ in the presence of reducing gas NH₃ inhibits the sulfation of the active component.The SO₂/H₂O tolerance mechanism step of rare earth tailings catalyst was analyzed and NH₃SO₃ modified rare earth tailings was used to further improve the catalyst denitrification performance.The NH₃SO₃-modified rare-earth tailings catalysts were prepared by ball milling and microwave roasting.under the effect of NH₃SO₃,the catalyst mineral crystallization weakened,the specific surface area,pore volume and average pore size increased,the acidic sites increased,the acidic site intensity enhanced,and the catalysts modified with higher quality fractions（8%,10%,12%）of NH₃SO₃ showed stronger oxidation ability.10%sulfamic acid modified rare earth tailings catalyst showed the best denitrification performance,with denitrification efficiency of up to 97%at 300°C in the presence of SO₂ and H₂O.In this thesis,the influence process of SO₂/H₂O on the denitrification performance of rare earth tailings catalyst is explored,and the SO₂/H₂O tolerance mechanism of rare earth tailings NH₃-SCR denitrification is revealed,and the results can provide theoretical support for the application of rare earth tailings catalyst NH₃-SCR denitrification in Bayan Obo.