Mechanistic Study On The Sulfur Resistance Of SCR Catalyst With Wide Operation Temperature Window

In recent years,with the rapid development of economy,the demand for energy in China shows a tendency of increase.The burning of fossil fuels has brought serious environmental problems.As one of the main atmospheric pollutants,the emission of nitrigon oxides(NO_x)exerts a negative effect on human health and ecological environment.The selective catalytic reduction of NO with NH₃(NH₃-SCR)now is the most effective and widely used de NO_x technology.However,the adaption of operation load fluctuation constitutes the main barrier to the industrialization of this technology.In the present,the operation load of power plants in China continues to change.When operating in the low load mode,the temperature of the flue gases decreases in some extent.In this case,SO_x in the flue gases would inevitably react with NH₃ and H₂O to produce NH₄HSO₄.Formation of this product would cover the catalyst-active sites and interfere with the SCR reaction,hencing leading to the excess NO_x emmision and NH₃ slip.In this thesis,extensive research would be conducted to deal with these previously mentioned challenges.We extensively explored the detailed mechanism and the key factor in the decomposition of NH₄HSO₄ on the catalyst surfaces.By optimizing the formula and the mirco-structures of the catalysts,the decomposition behavior of NH₄HSO₄ would be promoted at lower temperatures,hence enhancing the sulfur-resistant ability of the catalysts in some extent.These results would lay a solid foundation for the commercialization of low-temperature SCR technology.Finally,certain kinds of catalysts which can operate under practical conditions in the power plant in the long run could be octained.Some important contents are listed as follows:1.It is known that the deposition of NH₄HSO₄ is the main reason for the serious deactivation of the catalysts in H₂O-and SO_x-containing flue gas at lower temperatures.In this part,commercial V₂O₅/TiO₂ was used as the model catalyst and wet impregnation method was adopted to simulate the deposition of NH₄HSO₄ on the catalysts.Under low NH₄HSO₄ deposition amounts,ammonium sulfate salts existed in the amorphous state.(NH₄)₂TiO(SO₄)₂ and NH₄HSO₄ phases began to come out with increasing NH₄HSO₄ deposition amount.Morever,it is found that NH₄HSO₄sometimes interacted with the V₂O₅/TiO₂ catalyst.On the catalyst surface,S-containing species were transformed from HSO₄^-to bidentate sulfate anions,which were subsequently bonded to TiO₂.Electrons around the catalyst surface metal atoms deviated to SO₄^2-because of this species'stronger electronegativity.Thanks to the exsistence of this interaction,the decomposition behavior of NH₄HSO₄ was varied on the catalyst surfaces.Besides,unlike pure NH₄HSO₄,NO_x could react with NH₄⁺belonging to the NH₄HSO₄ deposited on the catalyst surface,accelerating the consumption of NH₄⁺,thereby further improving the decomposition behavior of NH₄HSO₄.Compared with crystallite salts,amorphous ammonium sulfate salts decomposed more easily.2.In the second part,kinetic study of the decomposition reaction of pure NH₄HSO₄ and NH₄HSO₄ supported on the V₂O₅/TiO₂ catalyst was conducted.In the case of pure NH₄HSO₄,only one weight loss peak came out during the heat process.As the heating rate increased,temperature assigned to the weight loss peak continued to increase.According to Arrhenius equation,the corresponding activation energy and reaction order were calculated to be 112 k J/mol and 0.37,respectively.While for NH₄HSO₄ supported on the V₂O₅/TiO₂ catalyst,two weight loss peaks could be observed during heating.Similar with pure NH₄HSO₄,given that the heating rate increased,temperatures assigned to these two weight loss peaks continuously increased.The first weight loss peak of NH₄HSO₄ supported on the V₂O₅/TiO₂catalyst could be assigned to the decomposition of ammonium ions;the corresponding activation energy and reaction order were calculated to be 98 k J/mol and 1.25,respectively.The second weight loss peak could be attributed to the decomposition of sulfate anions;the corresponding activation energy and reaction order were calculated to be 122 k J/mol and 0.95,respectively.3.In the third part,commercial vanadia-titania-based catalysts were used as the model to study the main factor in determining the decomposition of NH₄HSO₄.It is found that electron deviation from the catalyst surface metal atoms to sulfate anions,together with dehydrogenation of NH₄⁺by the redox sites,played an important role in the decomposition of NH₄HSO₄ on the catalyst surface.Based on these results,wet impregnation method was used to synthesize Nb₂O₅-and Sb₂O₅-modified V₂O₅-WO₃/TiO₂ catalysts.It is found that adding Nb₂O₅ and Sb₂O₅ led to an obvious electron cloud deviation between catalyst surface metal atoms and sulfate anions,which promoted the transformation of sulfate anions to SO₂ at lower temperatures,thereby improving the decomposition of NH₄HSO₄.Compared with V₂O₅-WO₃/TiO₂sample,the decomposition temperature of NH₄HSO₄ on the Nb₂O₅-and Sb₂O₅-doped catalysts was decreased by 30 ^oC.Besides,the introduction of Nb₂O₅ and Sb₂O₅ also increased the relative ratio of V⁵⁺,which was beneficial for the activation of NH₄⁺and accelerated the reaction between NH₄⁺and NO,hence further promoting the decomposition behavior of NH₄HSO₄.This factor constituted the main reason for the inhibition of the excessive accumulation of NH₄HSO₄ on the promoters-doped catalysts,which enabled promoters-modified samples with an enhanced sulfur-tolerant ability.As a result,when 1000 ppm SO₂,10%H₂O were purged into the reaction system,the de NO_x activity belonging to Sb₂O₅-and Nb₂O₅-decorated catalysts at 250 ^oC remained to be 0.91 and 0.86 of the initial activity after 8 h,respectively,which were higher than that of VW/Ti sample(0.81).4.In the forth part,TiO₂ hollow spheres and solid spheres were synthesized using the sol-gel method and wet impregnation method was adopted to prapred the corresponding catalysts.Thanks to the high specific surface area of hollow TiO₂spheres,catalyst catalytic activity and sulfur-tolerant ability could be simultaneously improved.Compared with the solid-structured sample,de NO_x efficiency of the hollow-structured sample at 250 ^oC was increased from 64%to 83%and over 80%de NO_x efficiency could be obtained between 250 and 400 ^oC.When 1000 ppm SO₂,10%H₂O were purged into the reaction system,the de NO_x activity belonging to the hollow-structured sample at 250 ^oC still remained to be 0.93 of the initial activity after10 h,which was some higher than that of the solid-structured sample(0.80).Finally,we tested the stibility of the hollow-structured sample and commercial V₂O₅-WO₃/TiO₂ catalyst under simulated actual flue gas conditions.It was found that when 800 ppm SO₂,10%H₂O,15 ppm SO₃ were purged into the reaction system,the de NO_x activity belonging to the hollow-structured sample at 275 ^oC always remained above 0.96 of the initial activity in the whole sulfur deactivation cycle,much higher than that of commercial catalyst(0.79).This result laid a solid foundation for the industrialization of low-temperature SCR technology.