Study On Preparation And Property Of The Modified Semi-coke Catalyst For NH₃-SCR At Low Temperatures

With the continuous improvement of China's industrialization,the nitrogen oxides(NO_x)in the industrial flue gas have a serious impact on the air quality.The control technology of NO_x in the high temperature flue gas from thermal power plant has developed significantly because the NO_x emission standard become strict around the world.But there are many technical problems for the removal of NO_x in the low temperature flue gas from the steel,cement and other industrial furnaces.It has restricted the development of China's industrial flue gas de-NO_x.In view of the abundant pores,large specific surface area and excellent low-temperature adsorption property of carbon materials,metallurgical coke,semi-coke and biomass coke are selected as new-type low-temperature SCR catalyst support in this paper.The main operating factors on the support activation process are studied.Also,the effects of different operating factors on the nature of catalyst support are analyzed.The new support and the optimum activation process are determined.In this study,the semi-coke support with a particle size of 20-40 mesh is activated by 10 mol/L nitric acid solution at 80?for 3 h.Then the obtained activated semi-coke support is most suitable for the performance requirements of new-type semi-coke catalyst.A new-type modified semi-coke catalyst is developed by loading active component on the active semi-coke with nitrogen doping.In this paper,the influence of modification treatment conditions and recipe ingredients,such as modification method,modification sequence,impregnation time,heat treatment temperature and time,different Mn O_x content,active metal combination,nitrogen source and N-doping amount on the de-NO_x activity and physicochemical properties of the catalyst at low temperature was systematically studied.The optimal modification treatment and formula of the catalyst are confirmed as follows:the activated semi-coke were impregnated for 3 h in the mixed precursor solution containing 10%urea and 10%Mn(CH₃COO)₂·4H₂O,and then roasted in N₂ atmosphere at 400?for 6 h after fully drying.The obtained ASC-10U10Mn has stably high activity with 16500 h^-1,the NO conversion rate is 82.5%at 200?and 94.5%at 275?.Meanwhile,the active temperature range is expanded to 75-300?.SEM,XRD,BET,NH₃-TPD,H₂-TPR,XPS,in situ DRIFTS and other characterization techniques are employed.The catalyst ASC-10U10Mn has a clear and dense layered structure and micropores,mesopores and narrow crack holes,with a specific surface area of 38.0 m²/g.A large number of lattice oxygen,Mn⁴⁺and nitrogen functional groups like pyridine-N,pyrrole-N and quaternary-N are formed on the surface.Besides,abundant strong acid sites and lower reduction temperature also explain its excellent low-temperature de-NO_x performance.The enhancement mechanism of the low-temperature de-NO_x activity of the catalyst by modification is showed that abundant Lewis acid sites are formed on the catalyst,which improves to adsorb NH₃.Mn-O-Mn structure promotes the generation of oxygen vacancy and the oxidation of coordinated NH₃ to the intermediate NH₂ to participate SCR reaction instead of NH₃ by promoting the electron mobility between different valence states of Mn.In addition,the non-paired electrons in pyridine-N,pyrrole-N and quaternary-N interact with the?antibonding orbital of NO molecule,effectively promoting the adsorption of NO.The increase of Mn⁴⁺content and Mn⁴⁺/Mn³⁺ratio contribute to the further oxidation of NO into NO₂ and nitro.As an important active intermediate,bridging nitrate also promotes the development of low-temperature SCR reaction.Based on the excellent properties of ASC-10U10Mn,the effects of different alkali metals(KCl,Na Cl,K₂SO₄,Na₂SO₄)in industrial flue gas on its properties were deeply discussed and the alkali metals poisoning mechanism was revealed.K and Na significantly decrease the activity of the ASC-10U10Mn,which is 52%lower than that of the fresh catalyst at 150?.The NO conversion of the catalyst decreases obviously with the increase of alkali metal.The K makes de-NO_x activity decrease greater than Na,but the co-existing K and Na could conjointly influence on NO conversion.Besides,the influence of alkali sulfate on the pore structure of catalyst is greater than that of alkali chloride,among which Na₂SO₄ has the greatest influence.The poisoning and deactivation of the catalyst is mainly due to the presence and adsorption of alkali metals,which leads to the decrease of specific surface area and pore volume and the increase of average pore diameter.The reduction of the O_?content hinders the electron mobility and thus weakens the oxidation of NO to NO₂,which ultimately resulted in reduced de-NO_xactivity of the modified semi-coke catalyst.