Technological Study Of NO Removal From Simulated Flue Gas By Iron-activated Persulfate Solution Under High Gravity Environment

NOx generated by coal combustion is one of the main pollutants in the atmosphere,causing serious harm to human health and the ecological environment.Due to the relatively large land area and complex equipment,dry denitration technology leads to higher investment and operation costs.Wet oxidation denitrification technology is simple in operation and easy to achieve collaborative removal of NO,SO2 and Hg in flue gas,which is an important direction of denitrification technology development.However,there are still problems such as poor treatment capacity and secondary pollution.The advanced oxidation technology of persulfate is the activation of persulfate to produce sulfate radicals(·SO4-)and hydroxyl radicals(·OH)with stronger oxidizing ability,which accelerates the liquid phase dissolution and absorption of NO.Compared with other oxidation methods,it has the advantages of strong oxidizing capacity,good stability and no secondary pollution.In the process of NO removal by persulfate(S2O82-)oxidation,it is crucial to increase the radical yield and utilization rate as well as to enhance the mass transfer of NO to improve the NO removal efficiency.Ferric salt has become the most widely used means of activating persulfates due to their low price,easy availability and high catalytic efficiency.Meanwhile,the introduction of rotating packed bed can also strengthen the gas-liquid and gas-liquid-solid mass transfer processes.Therefore,the removal of NO from simulated flue gas by Fe2+/S2O82-,(Fe2+-EDTA)/S2O82-and(Fe/γ-Al2O3)/S2O82-systems under high gravity environment was proposed.Then,an efficient and green wet denitrification technology could be explored.The main works of this paper are as follows:(1)Fe2+/S2O82-system removed NO from simulated flue gas in bubble reactor.The chemical reaction enthalpy change Δr/HmΘ,entropy change ΔrSmΘ,Gibbs free energy changeΔrGmΘ,NO gas phase equilibrium partial pressure pNO/pΘ and chemical balance constantin KΘthe process of removing NO from simulated flue gas by activating S2O82-with Fe2+were calculated and analyzed.Combined with the experimental results of the variation of NO removal efficiency with reaction temperature,initial pH,Na2S2O8 concentration and Fe2+concentration in the bubbling reactor,it was shown that the Fe2+activation of S2O82-to remove NO from the simulated flue gas reaction process was feasible,which could be carried out spontaneously and the reaction was carried out relatively completely.The highest NO removal efficiency was 68%within the experimental range.(2)Fe2+/S2O82-system removed NO from simulated flue gas in the rotating packed bed.The effects of packing type,flue gas flow,high gravity factor,S2O82-concentration,Fe2+concentration,initial pH value and reaction temperature on NO removal efficiency were studied with the rotating packed bed as the absorption device,and a systematic analysis of NO mass transfer-reaction kinetics was carried out.The results showed that the rotating packed bed had an obvious strengthening effect on the NO mass transfer-reaction process,obtaining NO removal efficiency of about 70%.The liquid-phase mass transfer coefficient kNO,L in the rotating packed bed(2.52～2.96×10-3 m·s-1)was significantly improved by about 1～2 orders of magnitude compared with the other reaction devices such as bubbing reactor,and the the phase interfacial area aNO was also much larger than that of the impinging flow and bubbling reactors.The NO removal efficiency under the screen packing was about 5%larger than that of the Ball ring packing.The increase of gas flow was not favorable to NO removal,and the increase of liquid volume could improve the NO absorption rate NNO,L and the reaction rate constant kov1.When the high gravity factor and the concentration of S2O82-increased,the NO removal efficiency increased.Appropriately increasing the Fe2+concentration and elevating the temperature could enhance the oxidizing ability of S2O82-to remove NO.The optimum pH for the Fe2+/S2O82-system to remove NO from the simulated flue gas was 3.The kinetic equation of NO removal reaction was established with the help of steady state approximation,and it was obtained that the reaction of NO removal by Fe2+/Na2S2O8 system was proposed to be a first-order reaction,while activation energy Ea>20 kJ·mol-1,which indicated that the rate of the reaction between S2O82-and NO was controlled by the chemical reaction.(3)(Fe2+-EDTA)/S2O82-system removed NO from simulated flue gas in the rotating packed bed.In order to improve the utilization rate of Fe2+and the NO removal,the NO removal efficiency by Fe2+-EDTA activated S2O82-in the rotating packed bed was investigated.The results showed that the addition of EDTA increased the Fe2+content in the liquid phase and improved the NO removal efficiency.The simultaneous presence of ·SO4-,·OH and ·O2reactive radicals in the system was obtained by free radical detection,and Fe2+-EDTA could maintain the free radical level in the liquid phase for a longer period of time,and the peak intensity of the ·SO4-and ·OH radical was also enhanced by increasing the temperature.The presence of Fe2+-EDTA and the increase in reaction temperature favored the NO removal process.The ·SO4-radical played a dominant role in the NO removal process with a contribution of 45.6%.The negative effect of gas-liquid contact time reduction and reaction insufficiency due to increased gas flow rate was much larger than the positive effect of increasing the volumetric mass transfer coefficient KGa and decreasing the mass transfer resistance,which decreased the NO removal efficiency and increased the height of the mass transfer unit HTU.The increase of liquid-gas ratio and Fe2+-EDTA concentration favored the NO mass transfer-reaction process.The optimum NO removal was achieved when the ratio of Fe2+and EDTA was 1:1,and the optimum high gravity factor for the experiments in this chapter was determined to be 77.61.Within the experimental range,the 0.1 mol·L-1(NH4)2S2O8 solution had a good absorptive capacity for the variation of NO concentration.The presence of O2 in the system caused the depletion of Fe2+,and a high level of high gravity factor could not only enhance the mass transfer process of NO,but also effectively inhibit the negative effect of O2 on the removal of NO from this system.The results of ion chromatographic analysis and elemental N material balance calculation of the liquid phase products showed that NO in the simulated flue gas was finally oxidized to NO3-.Under suitable operating conditions,the maximum removal efficiency of NO was more than 84%.(4)(Fe/γ-Al2O3)/S2O82-system removed NO from simulated flue gas in the rotating packed bed.Fe/γ-Al2O3 catalyst was successfully prepared by impregnation-sodium borohydride reduction method,and it was placed in a rotating packed bed for NO removal experiments with Fe/γ-Al2O3 catalyzed activation of S2O82-.The study showed that under the conditions of gas flow of 100 L·h-1,liquid-gas ratio of 0.06,high gravity factor of 9.27,NO concentration of 500 ppm,Fe/γ-Al2O3 catalyst of 10 g,S2O82-concentration of 0.1 mol·L-1,reaction temperature and initial pH value of 50℃ and 3,respectively,the NO removal efficiency could reach 68.2%in the optimum working condition.The experiments of Fe/γ-Al2O3 catalyst recycling and the determination of Fe dissolution in the liquid phase showed that Fe/γ-Al2O3 had good catalytic activity,reusability and stability.Combined with the results of catalyst characterization,free radical test and product analysis,it was shown that the active components of Fe/γ-Al2O3,namely Fe0 and Fe3O4,could realize the catalytic cycling of Fe2+and effectively catalyze the decomposition of(NH4)2S2O8 to produce·SO4and ·OH free radicals to promote the process of NO removal,which meaned that NO removal followed a free radical mechanism.The final oxidation product of NO was NO3-.(5)Effect of different activation systems on the NO removal in the rotating packed bed.The ability of Fe2+/S2O82-,(Fe2+-EDTA)/S2O82-and(Fe/γ-Al2O3)/S2O82-to produce·SO4-and·OH free radicals was semi-quantitatively analyzed by electron spin resonance spectrometer,and the activation of S2O82-was obtained in the order of strongest to weakest for Fe/γ-Al2O3,Fe2+-EDTA and Fe2+,respectively.At the same time,the parameters involved in the removal process of NO in different activation systems were compared and analyzed.The results showed that the rotating packed bed significantly enhanced the process of NO mass transfer and reaction,and the NO removal efficiency of(Fe2+-EDTA)/S2O82-system was the highest.The difference in NO removal efficiency between Fe2+/S2O82-and(Fe/γ-Al2O3)/S2O82systems was not significant,but the Fe dissolution and liquid-gas of(Fe/γ-Al2O3)/S2O82-was relatively small,which avoided the generation of iron-containing wastewater to a great extent,and also reduced energy consumption and treatment costs.Fe/γ-Al2O3 catalytic activation of S2O82-for NO removal in the rotating packed bed has more application potential.