Study On The Performance And Mechanism Of Fly Ash-based Catalysts For Simultaneous Removal Of SO₂ And NO

Coal-fired power generation not only releases atmospheric pollutants such as NO_x and SO₂,but also emits a large amount of solid wastes such as fly ash.The removal of NO_x and SO₂ in coal-fired flue gas and the rationalization and resource utilization of fly ash are always related to the health of human life and the safety of the ecological environment.At present,coal-fired power plants mostly use one-to-one desulfurization and denitrification technologies,which have large footprints,high investment and operating costs,and difficult to solve defects such as ammonia escape,air preheater blockage,blue plume,and catalyst deactivation.At the same time,the utilization of fly ash in China has problems such as comprehensive utilization rate and low utilization rate of high added value.Based on the above research background,this paper proposes a new process for simultaneous removal of NO_x and SO₂ with fly ash-based catalysts,that is,the fly ash-based catalysts catalyzes the rapid oxidation of NO by H₂O₂,combining with the alkali absorption device to realize the flue gas synergistic desulfurization and denitration.This process can not only realize the high value-added utilization of fly ash,but also conform to the new development concept of“using waste to control pollution”and circular economy.In this paper,a variety of catalysts were prepared with fly ash as raw materials,tested their catalytic performance of H₂O₂ oxidation of NO and the removal efficiencies of NO and SO₂coupling alkali absorption,and combined various characterization methods to speculate the reaction mechanism.Finally,a pilot experiment of collaborative desulfurization and denitrification was carried out on a flue gas circulating fluidized bed experimental platform equipped with a flue gas pre-oxidation device to initially explore the feasibility of its industrial application.Improvement of fly ash microstructure through ball milling and alkali modification.The crushing effect of wet ball milling on fly ash is better than that of dry ball milling.The particle size of fly ash decreases continuously with the increase of milling time,fly ash solid content and the weight ratio of ball mill beads to fly ash.Alkali modification can continue to destroy the silica-alumina network structure in the fly ash glass body,release active silica and alumina,and at the same time make the fly ash become loose and porous,and increase the specific surface area and pore volume.Preparation of fly ash-based iron-based catalyst through magnetic separation and alkali-acid composite treatment.The iron removal effect of wet magnetic separation on pulverized coal boiler fly ash is better than that of fluidized bed boiler fly ash,and the total iron content of the magnetic beads obtained by magnetic separation increases with the increase of the number of magnetic separations,the intensity of the magnetic field,and the weight ratio of water to ash.When the particle size of fly ash decreases,it increases first and then decreases.The alkali-acid composite treatment causes that the aluminum and iron components are respectively enriched on the surface of the magnetic beads in the form of alumina and hematite,respectively.The increase in the amount of surface alumina not only improves the dispersion of hematite on the surface,but also increases the amount of Lewis acid on the surface which promoting the adsorption of H₂O₂ on the surface.The formation of Fe OAl results in a large number of oxygen vacancies in the catalyst,which is beneficial to the conversion of Fe³⁺to Fe²⁺during the catalytic reaction and promotes the generation rate of hydroxyl radicals,having a high catalytic performance of H₂O₂ oxidation of NO.Combined with lye absorption,it can achieve about 90%NO removal efficiency and 100%SO₂ removal efficiency.In the iron-based catalyst prepared with alkali-modified fly ash as the carrier,the dispersibility of iron oxide on the surface of the carrier is good,and there is an interaction force between the Si O₂ in the support and the iron oxide,which facilitates the transfer of electrons from H₂O₂ to Fe³⁺and the generation of Fe²⁺,which is beneficial to the generation of hydroxyl radicals.The NO removal efficiency is about60%.Synthesis of fly ash-based HY molecular sieve by alkali fusion-hydrothermal crystallization method.Fly ash-based HY molecular sieve can achieve 76%NO removal efficiency and 100%SO₂ removal efficiency.The NO removal efficiency is positively correlated with the Br(?)nsted acid content of the molecular sieve.Combined with the results of in-situ infrared testing and electron paramagnetic resonance characterization,it is speculated that NO⁺produced by oxidation and heterogeneous cleavage of NO first replaces the protons of Br(?)nsted acid in the HY molecular sieve,and then is oxidized by H₂O₂ to nitric acid and leaves the surface of the molecular sieve,leaving the Br(?)nsted acid with one-electron orbit,serves as the catalytic active center to catalyze the oxidation of NO to nitric acid by H₂O₂.However,nitric acid will remove the framework aluminum of the molecular sieve,which in turn leads to a decrease in crystallinity,a deterioration of the surface structure,and a decrease in the content of Br(?)nsted acid,so the catalytic stability is deteriorated.Preparation of solid acid catalyst from fly ash.High-temperature acid leaching with concentrated sulfuric acid not only dissolves a large amount of aluminum components in the fly ash of the fluidized bed boiler,but also reacts with the silanols of the dealumination residue to form a solid acid catalyst(Si O₂-O-SO₃H).Compared with raw ash,the prepared solid acid catalyst has a significantly larger surface area,enhanced water absorption,and increased surface Lewis acid content.The solid acid catalyst can catalyze the oxidation of NO by H₂O₂.The catalytic oxidation reaction is a zero-order reaction to the concentration of H₂O₂ and a first-order reaction to the concentration of NO.The index factor and activation energy are 1.1×10⁴s^-1 and 19,770 J/mol.It is speculated that the oxidation mechanism of NO is that the solid acid catalyst further enhances the electronegativity of oxygen atoms through the hydrogen bond formed with H₂O₂ adsorbed on its surface,and then the electronegative oxygen atoms can quickly oxidize NO to NO₂.In the case of combined alkali absorption,92%NO removal efficiency and 100%SO₂ removal efficiency can be achieved.The product of the NO and SO₂removal products are nitrate,nitrite and sulfate.The NO removal efficiency of fly ash-based HY molecular sieve,iron-based catalyst and solid acid catalyst decreases with the increase of the concentration of NO and SO₂ in the flue gas.When the oxygen content is relatively high,increasing the oxygen content is beneficial to the improvement of the NO removal efficiency of fly ash-based HY molecular sieve and iron-based catalyst,while the NO removal efficiency of fly ash-based solid acid catalyst is not affected by the change of oxygen content.The NO removal efficiency of fly ash-based solid acid and HY molecular sieve gradually increased with the increase of H₂O₂ concentration,while the NO removal efficiency of fly ash-based iron-based catalyst increased first and then remained unchanged with the increase of H₂O₂ concentration.The NO removal efficiency of the three catalysts first increased and then decreased with the increase of H₂O₂ flow rate and reaction temperature.The NO removal efficiency of fly ash-based solid acid and HY molecular sieve first increases and then remains unchanged with the increase of the amount of catalyst,while the NO removal efficiency of fly ash-based iron-based catalyst first increases and then decreases with the increase of the amount of catalyst.The fly ash-based solid acid catalyst has the most stable performance in long-term desulfurization and denitrification.Its process of catalyzing H₂O₂ to oxidize NO has economic advantages in terms of investment and operating cost.Based on the fly ash-based solid acid catalyst,a pilot experiment of simultaneous removal of NO and SO₂ in flue gas circulating fluidized bed with NO pre-oxidation was carried out.When the H₂O₂ concentration is 15%,the flue gas humidity is 6%,the SO₂ concentration is1000 mg/m³,the NO concentration is 500 mg/m³,Ca/(S+N)is 1.6,and the catalytic oxidation space velocity is 81400 h^-1,100%SO₂ removal efficiency and 81.6%NO removal efficiency can be achieved.In the simultaneous desulfurization and denitrification process,the concentration of NO and SO₂ in the flue gas,the concentration of hydrogen peroxide,the temperature of the inlet flue gas and the residence time affect the two processes of flue gas pre-oxidation and alkali absorption in the bed,thereby affecting the overall removal of SO₂ and NO_x.The NO_x removal efficiency increases with the increase of residence time and H₂O₂concentration,and decreases with the increase of flue gas NO and SO₂ concentration.As the flue gas temperature increases,it first increases and then decreases;while the SO₂ removal efficiency is mainly affected by the inlet temperature of the flue gas,with the increase of flue gas temperature it first increases and then decreases.Ca/(S+N)and flue gas humidity mainly affect the alkali absorption process in the bed.The desulfurization and denitrification efficiency first increases with the increase of Ca/(S+N)and then remains unchanged.As the flue gas humidity increases,increase first and then decrease.