Study On Integrated Removal Of SO₂ And NO Using Free Radicals From VUV Photolysis Of Peroxide In Spraying Tower

At present,a large number of small medium-sized industrial coal-fired boilers are widely used in China's industrial and civilian sectors.The combustion consumption of coal from these small medium-sized coal-fired boilers accounts for about a third of total coal combustion consumption,which results in a huge air polluation.Due to technical and economic issues,the kind of hierarchical processing strategy(combination of Ca-WFGD and NH₃-SCR)for SO₂ and NO ramoval is difficult to be widely used for such boilers.Therefore,the development of an economical,efficient and simple technology for simultaneous removal of SO₂ and NO has the important theoretical significance and engineering value.Based on the previous researches on simultaneous removal of SO₂ and NO using UV/H₂O₂ AOP,a pilot-scale removal system of VUV-photochemical spraying tower were designed and built.The performance and mechanism of desulfurization and denitrification using free radicals from VUV photolysis peroxides were studied.To understand the removal performance of desulphurization and denitrification under the actual coal-fired operational conditions by VUV photolysis H₂O₂ oxidation process,the main influencing factors,removal products and economy of simultaneous desulfurization and denitrification were studied in a pilot-scale photochemical spraying tower.The results show that this process can achieve high removal of SO₂,but the different operational conditions have a significant effect on NO removal efficiencies.The increase of VUV irradiation intensity,H₂O₂concentration and solution p H will promote NO removal.Increasing liquid-gas ratio and solution temperature,NO removal efficiencies increase firstly,and then decrease.An increase of flue gas flow,NO concentration and SO₂concentration has a negative effect on NO removal.Under the optimal operational conditions with SO₂ concentration of 3000 mg/m³ and NO concentration of 500 mg/m³,the desulfurization and denitrification efficiencies are 100%and59.8%,respectively.The liquid phase ion products are mainly SO₄^2–and NO₃^–,and the removal processes have good adaptability to product accumulation.The cost of investment and operation of this technology is lower than that of combination process of NH₃-SCR and Ca-WFGD.To increase the denitrification performance and concentration of removal product of VUV photolysis H₂O₂ oxidation process,solid oxidant(NH₄)₂S₂O₈ was selected as additive to prepare dual oxidants(H₂O₂/S₂O₈^2–).The main influencing factors and removal products of simultaneous desulfurization and denitrification by VUV photolysis dual oxidants(H₂O₂/S₂O₈^2–)oxidation process were studied in a pilot-scale photochemical spraying tower.The results show that this process exhibits an excellent performance for NO,and achieves the complete removal of SO₂under different operating conditions.NO removal efficiencies increase with the increase of VUV irradiation intensity,H₂O₂ concentration and solution temperature.An increase of solution p H,S₂O₈^2–concentration and liquid-gas ratio L/G has dual effect on NO removal.NO removal efficiencies greatly decline with increasing flue gas flow and NO concentration.The competitive oxidation between high SO₂ concentration and NO is obvious,resulting in a decrease of NO removal efficiencies.Under the optimal operational conditions with SO₂concentration of 3000 mg/m³ and NO concentration of 500 mg/m³,the desulfurization and denitrification efficiencies are 100%and 71.2%,respectively.The main ions product in solution are NO₃^–and a large number of SO₄^2–.VUV photolysis of dual oxidants(H₂O₂/S₂O₈^2–)oxidation process displays excellent performance on simultaneous removal of SO₂ and NO.In order to understand the mutual relationship between chemical reaction and mass transfer process in the process of desulfurization and denitrification,and deeply understand the gas-liquid reaction mechanism of desulfurization and denitrification,the mass-transfer reaction kinetics of NO removal and simultaneous removal mechanism of SO₂ and NO were researched in a small-scale photochemical spraying tower.Based on the the reaction kinetics theory and two-film theory,the intrinsic kinetic equation of NO removal was deduced and the rate equation of NO absorption was established.The effect of operational conditions on NO absorption rate was discussed.The reaction rate constant and Hatta numbe of NO absorpation were calculated and measured.The results show that NO absorption rate increases with increasing H₂O₂concentration,S₂O₈^2–concentration and NO concentration.The increasing tendency of NO absorption rate following the increase of NO concentration is linear.Increasing the solution spray flux,NO absorption rate decreases significantly.A trend of NO absorption rate first increasing and then decreasing is showed with increasing solution p H.NO removal processes by VUV photolysis dual oxidants(H₂O₂/S₂O₈^2–)is a fast pseudo first order reaction.At the end,the empirical equations for the pseudo first order rate constant are obtained by fitting experimental data.The effect of different reaction systems on NO removal performance shows that the oxidation of the active groups is the main reaction routes for NO.The oxidation of H₂O₂ and S2O₈^2–is the secondary reaction routes for NO.There is the obvious synergy effect among VUV,H2O2 and S₂O₈^2–.The measurement results of free radicals show that the removal process of SO₂and NO by VUV photolysis of dual oxidants(H₂O₂/S₂O₈^2–)oxidation process is a typical free radical chain reaction because of the generation of·OH and SO₄^–·.The measurement results of liquid product and calculation results of the molar mass conservation of N element in NO further show that the oxidation raction is the main mechanism in SO₂ and NO removal.The simplified intrinsic dynamics model of NO removal was derived and verfied using the radical steady-state approximation theory.The calculated results of the mathematical model of the pseudo first order rate constant are in good agreement with the experimental results,suggesting that the mechanism model deduced is reliable.At the end,the reaction mechanism was proposed for simultaneous removal of SO₂ and NO using VUV photolysis dual oxidants(H₂O₂/S₂O₈^2–)oxidation process.For the effective removal of NO in urea-WFGD system,the VUV photolysis of H₂O₂technology was applied into urea-WFGD system.The denitration process of VUV photolysis of H₂O₂ technology in urea-WFGD system was investigated.The results show that SO₂ removal efficiencies always maintain 100%under all operational conditions.Increasing VUV irradiation intensity and H₂O₂ concentration can promote NO removal,but increasing flue gas flow and NO concentration has negative on NO removal.NO removal efficiencies increase first and then decrease with increasing solution temperature,liquid-gas ratio and urea concentration,while solution temperature functions less.The effect of SO₂ concentration on NO removal is very small.Under the optimal operational conditions with SO₂ concentration of 3000 mg/m³ and NO concentration of 500 mg/m³,the desulfurization and denitrification efficiencies are 100%and65.87%,respectively.The final removal products of SO₂ and NO are SO₄^2–and NO₃^–,respectively.NO is mainly removed by·OH.The oxidation of H₂O₂ and other active groups plays a minor role in NO removal.The VUV photolysis of H₂O₂/urea method shows a favorable development foreground in simultaneous removal of multiple pollutants from medium and small-sized coal-fired boilers.