| With the implementation of ultra-low emission retrofits in the coal-fired power plants,in the end of 2019,the installation rate of SCR denitrification devices was close to 100%.The large-scale installation of SCR denitrification devices has resulted in a significant increase in the concentration of SO3 in coal-fired flue gas.It adversely affects the operation of the equipment,such as corrosion of the equipment and blockage of the air preheater.In addition,it causes adverse environmental problems,such as colored plumes.Therefore,the control of SO3 in flue gas is very important.The accurate detection of SO3 is the premise of SO3 control.However,SO3 has a lively nature and about two orders of magnitude lower than the SO2 concentration,making accurate measurement difficult.Therefore,it is necessary to explore the influencing factors of SO3 detection and find ways to reduce test errors.At present,the generation mechanism of SO3 is not clear,and in-depth exploration is needed to provide a theoretical basis for reducing SO3 at the source.In addition,due to the low utilization rate of alkaline absorbents in the SO3 removal process,it is urgent to explore the influencing factors of the alkaline absorbents' absorption of SO3 in order to guide the process optimization.The main research contents and conclusions of the paper are as follows.(1)The influencing factors of the online monitoring of SO3 by the isopropanol absorption method were investigated.The measurement error of SO3 mainly comes from two aspects.On the one hand,several measurement errors of SO3 come from the instrument,including the deformation of the peristaltic pump tube,bubbles and leakage of BaC6O4Cl2,which can be eliminated by adjusting and optimizing the instrument.On the other hand,several measurement errors of SO3 come from the nature of the components,including sampling gas flow rate error,IPA solution volatility loss,SO3 side reaction and SO2 oxidation reaction.Correction coefficients were proposed for these four sources of error,and a correction formula was established to correct the SO3 concentration.Test instrument optimization combined with correction formula significantly reduces SO3 measurement error.The measurement error is reduced from 25 to 54%to less than 25%within the range of 10 to 40 ppm,and the error is reduced to less than 5%when it exceeds 20 ppm.(2)The factors influencing the generation of SO3 on the surface of V2O5/TiO2 catalyst were investigated,and the kinetic parameters of SO3 generation were obtained.The apparent reaction orders of O2,SO2 and SO3 are 0,0.77 and-0.19,respectively,and the apparent activation energy is 74.3 kJ/mol.By testing the amount of SO2 conversion,the amount of SO3 generation and the amount of sulfate depisition at different temperatures,the existential forms of SO2 oxidation products at different temperatures were clarified.Above 270?,the main product is gaseous SO3;below 270?,the main product is solid NH4HSO4.The total reaction rate of gaseous SO3 and NH4HSO4 first decreases and then increases as the temperature rises,,and the lowest point occurs near 250?270 ?.(3)The SO2 adsorption,oxidation and desorption processes on the surface of V2O5/TiO2 catalyst were studied.The formation mechanism of SO3 was revealed.Firstly,gaseous SO2 is chemically adsorbed on the surface of the carrier through the hydroxyl groups,and H2O is released at the same time.Secondly,the chemically adsorbed SO2 is oxidized by V5+ to tridentate sulfate,while V5+is reduced to V3+.Finally,the tridentate sulfate decomposes to form gaseous SO3,and the V3+ is converted into V5+under the action of O2.Tridentate sulfate is a key intermediate product,which begins to form above 150? and begins to decompose and desorb to form gaseous SO3 above 270?.NOx obviously promotes the generation of SO3,which originates from the oxidation of V3+to V5+by NO2.H2O slightly inhibits the generation of SO3,and NH3 strongly inhibits the generation of SO3.The inhibitory effect comes from their combination with tridentate sulfate to form more stable bidentate sulfate and ammonium sulfate.It is proved that there are two paths for the formation of ammonium bisulfate at low temperature.Oxidation followed by ammonification and oxidation followed by ammonification.(4)Finally,the factors affecting the absorption of SO3 by three alkaline absorbents Ca(OH)2,MgO and NaHCO3 were investigated.The temperature windows for different absorbents is proposed.Ca(OH)2 should be used in the high temperature above 500?.MgO should be used in the low temperature below 320?.And avoid using NaHCO3 in the temperature range of 180?260 ?.The utilization rate of NaHCO3 to absorb SO3 is close to 100%,and the reaction process conforms to the shrinking core model.However,the utilization rate of Ca(OH)2 or MgO to absorb SO3 is less than 50%,and the reaction process conforms to the grain model.The reason for the low utilization rate of the latter two is that the dense product layer hinders the diffusion of SO3 to the unreacted core. |
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