Experimental Study On The Catalytic Oxidation For Removing Sulfur Dioxide,Nitrogen Oxides And Mercury Over Iron-based Catalysts

For controlling the emission of sulfur dioxide,nitrogen oxides and mercury in coal-fired flue gas,most coal-fired power stations generally adopted the wet desulfurization,selective catalytic reduction denitrification and activated carbon injection adsorption methods.This hierarchical tandem approach obviously has the defects of larger occupying area,ammonia escape,high investment and operating costs,and poor system stability.Therefore,the development of high-efficiency,economical and low-level pollution removal technology for the desulfurization,denitrification and mercury removal has become a research hotspot in the field of air pollution control,with very important theoretical significance and wide application value.The main work of this paper is as follows:The preparation of various catalysts were studied experimentally,and four kinds of iron-based catalysts were prepared.The optimal composite iron-type catalyst was screened out by comparing the removal efficiency and catalyst stability in a series of removal experiments.And the effects of different reaction conditions on the desulfurization,denitrification and mercury removal efficiency were investigated on the self-designed gas-like phase catalytic oxidation and liquid-absorption experimental platforms.Three reaction systems for simultaneous desulfurization,denitrification and mercury removal using different catalysts and radicals were formed:(1)a composite iron-type catalyst composed of Feoand Fe3O4 was used to activate the vaporized H2O2 containing a small amount of hydrochloric acid to produce oxidizing species including hydroxyl radical(’OH) and chlorine-containing radicals for the catalytic oxidation of poorly soluble gaseous pollutants NO and Hg0,meanwhile,the oxidation products and SO2 were absorbed in the solution and the average desulfurization,denitrification and mercury removal efficiencies were 99.8%,82.7% and 90.1%,respectively;(2)the composite iron-type catalyst composed of Fe0 and Fe3O4 was supported on ZSM-5 molecular sieve to form a supported iron-based catalyst(Fe/ZSM-5),which was used to catalyze the persulfate solution to produce·OH and SO4·- radicals that could oxidize the poorly soluble gaseous pollutants NO and Hg0,and the oxidation products and residual gaseous pollutants such as SO2 were absorbed by the solution.The average desulfurization,denitrification and mercury removal efficiencies were 100%,72.6% and 93.4%,respectively;(3)Fe/ZSM-5 was empolyed to catalyze the vaporized composite oxidant consisted of H2O2 and persulfate to generate the oxidizing species represented by ·OH and SO4·-for the high efficiency oxidation of NO and Hg0,their oxidation products and residual gaseous pollutants were absorbed by Ca(OH)2 solution.The average desulfurization,denitrification and mercury removal efficiencies were 99.8%,95.5% and 90.8%,respectively.Through the systematic experimental studies,three new methods for simultaneous removal of SO2,NO and Hg0 from the flue gas based on the catalytic oxidation and liquid phase absorption were proposed.According to the catalyst characterization methods including X-ray diffraction(XRD),scanning electron microscopy(SEM),X-ray photoelectron spectroscopy(XPS),Fourier transform infrared spectroscopy(FT-IR) and inductively coupled plasma mass spectrometry(ICP-MS),the changes in the iron-based catalyst during the catalytic oxidation process were analyzed,the simultaneous presence of Feoand Fe3O4 in catalyst D was confirmed,which could promote the electronic transfer between Fe0,Fe2+ and Fe3+ effectively,making more Fe2+ stable in the catalyst and promoting the catalytic activation of H2O2 and persulfate,being conducive to the formation of ·OH and SO4·- and increasing the removal efficiency.The characterization and analysis of Fe/ZSM-5 by transmission electron microscopy(TEM),XRD and FT-IR methods indicated that the spherical particles in the catalyst have the diameter of 40-90 nm and were uniformly dispersed on the surface of ZSM-5,effectively avoiding the agglomeration of active iron represented by Fe0 and Fe3O4,and part of the active iron entered the framework of the molecular sieve to enhance the stability of catalyst.The saturation saturation magnetometer(VSM)measured the saturation magnetization of Fe/ZSM-5 to be 55.19 emu/g,indicating that the prepared catalyst could be recycled under the applied magnetic field.The presence of ·OH and SO4·- in the catalytic process and the radical yields under different conditions were determined by means of UV-visible spectroscopy(Uv-Vis)，radical quenching experiments and electron paramagnetic resonance(EPR).The main removal products of catalytic oxidation for desulfurization,denitrification and mercury removal were determined as CaSO4,HgO,HgCl2,Ca(NO3)2 and HgSO4 by using fluorescence spectrometer(AFS),mercury temperature programmed desorption(Hg-TPD) and ion chromatography(IC).On this basis,with reference to a large number of related literatures,the catalytic oxidation mechanism for removing SO2,NOx and Hg0 was speculated.