Simultaneous Removal NO_x And SO₂ With H₂O₂ Catalyzed By Alkali-Magnetic Modified Fly Ash

At present,wet flue-gas desulfurization（WFGD）and selective catalytic reduction（SCR）are the main methods to control flue gas pollutants in power plants,and ESP equipments are used to capture fly ash.These techniques have been industrially applied on a large scale for flue-gas purification in coal-fired power plants and are considered the most effective processes for addressing contaminants.They require large and complex systems as well as high investment and operating costs,thereby limiting their wide utilization in developing countries.In addition,the treatment and comprehensive utilization of fly ash is also a major problem puzzling power plant.Therefore,low cost and high efficiency simultaneous removal of flue gas pollutants has become a hot research topic in the control of flue gas pollutants in coal-fired power plants.There are many problems in the study of simultaneous removal of flue gas pollutants:low efficiency of combined removal（especially denitrification）,high price of catalyst and easy deactivation,huge consumption of oxidants,high operating costs,etc.Most of the research is still in the laboratory stage,and can not be developed into practical technology,so it is difficult to industrialize application.Based on this background,a set of efficient and low-cost flue gas simultaneous removal method,namely alkali-magnetic modified fly ash catalytic hydrogen peroxide simultaneous desulfurization and denitrification method,is proposed in this paper.In this method,the silica-aluminium shell of fly ash was destroyed by alkali solution,and the activity of fly ash was stimulated.The magnetic field was used to enrich the iron-containing components in fly ash,making it a highly efficient and low-cost heterogeneous Fenton catalyst.The simultaneous desulfurization and denitrification of hydrogen peroxide catalyzed by alkali-magnetic modified fly ash was studied with low concentration of hydrogen peroxide as oxidant.In addition,this paper innovatively used ultrasonic atomizer to atomize hydrogen peroxide.The fly ash and its catalysts were characterized by XRF,XRD,FTIR and SEM.Combining with the simultaneous removal performance of pollutants,the optimum modification process conditions and reaction conditions were obtained,and the simultaneous desulfurization and denitrification mechanism of hydrogen peroxide catalyzed by alkali-magnetic modified fly ash was analyzed.The main conclusions of this paper are as follows:（1）The modification process consisted of ball milling,alkali modification,magnetic separation,pH adjustment and drying.Ball milling destroyed the hard Si-Al shell of fly ash,increased the roughness and porosity of fly ash surface,and greatly shortened the time required for alkali modification.Fly ash was mainly composed of mullite,quartz shell,iron oxide（FeOx）and inactive SiO₂ and Al₂O₃.Alkali modification process seriously damaged the structure of fly ash and exposed the internal FeOx.Besides,Alkali solution reacted with SiO₂ and Al₂O₃in the fly ash to increase its activity.From the analysis of SEM and BET,it could be seen that they provided a large number of reaction sites for catalytic reaction and greatly increased the specific surface area.FeOx had strong alkali stability and could be enriched by wet magnetic separation process.（2）According to ESR analysis,increasing the content of FeOx could enhance the oxidation ability of H₂O₂ solution,thus enhancing the removal ability of NO_x.FeOx could catalyze hydrogen peroxide to produce hydroxyl radicals（·OH）with strong oxidation ability.The order of concentration of hydroxyl radicals was AMA>DMA>RA>H₂O₂ alone.The key of simultaneous desulfurization and denitrification was to increase the content of FeOx by magnetic separation so as to catalyze more·OH from H₂O₂.（3）The wet grinding process of alkali liquor combined the two processes of ball milling and alkali modification,so that the process of ball milling and alkali modification could promote each other,effectively improved the modification effect and greatly reduced the modification time.The optimum economic modification time was 4-6 hours;the increase of alkali content could improve the catalytic efficiency,but excessive alkali content could block the fly ash channel,increase the difficulty of pH regulation and increase the cost of modification;the enhancement of magnetic field intensity could not only select Fe₃O₄ with strong magnetic ability,but also selectγ-Fe₂O₃ with weak magnetic ability,and increased the content of Fe（III）and FeOH,which could improve the modification.The catalytic capacity of fly ash and the roasting process destroyed the active sites and functional groups on the fly ash surface,so this process does not need roasting.（4）The molar ratio of H₂O₂ to NO was determined by the concentration of H₂O₂.Excessive concentration of H₂O₂ will reduce the effective utilization rate of oxidant.The removal efficiency increased first and then decreased with the increase of reaction temperature,and the optimal reaction temperature was between 100°C–150°C.The addition of oxygen has promoted NO removal,but when·OH existed in the system,the addition of oxygen was almost negligible.SO₂ gas could increase the acidity of the reaction system,help to enhance the oxidation ability of H₂O₂/·OH,and thus help to improve the denitrification efficiency.（5）By comparing the simultaneous removal efficiency of hydrogen peroxide catalyzed by AMA and purchased Fe₃O₄ in different temperature ranges,and combining with ESR,inhibitor addition experiment,XPS,TG-DSC,calcination experiment and PL characterization,the catalytic mechanism of modified fly ash was obtained.In low temperature region,iron ions dissolved on catalyst surface played a decisive role in the production of·OH;In high temperature region,FeOH on the catalyst surface played a decisive role in catalytic decomposition of H₂O₂,and the alkali modification process contributed to the formation of FeOH and oxygen vacancies.（6）The difference of crystal phase composition,particle size and micro-morphology of fly ash from different pulverized coal fired boilers was small,while the difference of element composition,crystal phase composition,iron phase composition,particle size and micro-morphology of fly ash from different fluidized beds was large.The iron oxide of fly ash from coal fired boilers was mainly magnetic Fe₃O₄,while the iron oxide of fly ash from circulating fluidized beds was mainly non-magnetic Fe₂O₃.Therefore,the yield of catalyst prepared from fly ash of pulverized coal fired boiler was higher than that of fly ash of circulating fluidized bed boiler;the denitrification efficiency of catalyst prepared from fly ash of pulverized coal fired furnace was very close,while the removal efficiency of catalyst prepared from fly ash of fluidized bed was different,the main influencing factors were particle size and specific surface area of catalyst.（7）The stability test of the catalyst showed that no new phase was formed and the iron content of the catalyst did not change significantly before and after the reaction,which indicated that the catalyst had excellent stability.Taking SCR denitrification system of 600 MW unit in Datong Power Plant as a comparison object,the technical economy of the process was evaluated.Through comparison,it is found that the cost of catalyst in this study is much lower than that of SCR catalyst by 5000 yuan/ton,and the consumption of hydrogen peroxide is also lower than that of ammonia.Therefore,compared with SCR system,the cost of denitrification is very low,and this process has a good prospect of industrialization.The results show that the alkali-magnetic modified fly ash catalytic desulfurization and denitrification system not only has high efficiency of simultanous removal,but also has the advantages of good adaptability of ash species,strong stability of catalyst and low cost of catalyst and oxidant.This process not only achieves the goal of low cost and ultra-low emission of boiler flue gas in coal-fired power plants,but also realizes the goal of"solid wastes for productive salvaging and recycling wastes for pollution recovery".