Study On Migration And Transformation Of SO₃ In FCC Flue Gas And The Control Technology Of Blue Plume

In recent years,the blue plume emissions caused by the wet desulfurization of FCC flue gas in refineries are becoming one of the new concerns of the public.The present pollutant treatment facilities in FCC plants have low removal rate of SO₃which causes blue plumes,it is difficult to meet future emission requirements of environmental protection standard.Firstly,the different gas sampling methods,i.e.isopropyl alcohol absorption method and controlled condensation method were analysed for the measuring accuracy of SO₃concentration in FCC flue gas.The results demonstrated that the capture effiency of SO₃in flue gas by the isopropanol absorption method was less than 73.1%.In the case where the acidic mist of SO₃had not condensed on the surface of pipeline,the lower reheat temperature of flue gas was benefical to the SO₃captured by the isopropyl alcohol absorption method.The controlled condensation method could capture 74.9%of SO₃in flue gas.The sampling time and the flow rate of simulated flue gas had a significant impact on the SO₃capture efficiency,while the longer sampling time and the larger flow rate of flue gas was conducive to the controlled condensation method for SO₃capture in flue gas.Based on the summary for the advantages and disadvantages of the two above methods and their maily influencing factors,the improved controlled-condensation+isopropanol absorption method could achieve a high efficiency of 97.3%for the capture of SO₃in flue gas.Subsequently,the flue gases from different locations of 4 sets of FCC devices in two refineries were sampled and analyzed,and the generation,transformation and migration of SO₃in flue gas were studied.For the two sets of FCC device in refinery I,the ozone method was applied to denitration for their flue gas.From the outlet of the third cyclone separator,through the waste heat boiler,to the outlet of wet desulfurization tower,the SO₃concentration in the flue gas reduced gradually.In the waste heat boiler,the SO₃concentrations were reduced by 4.5%and 6.3%,respectively.By the wet desulfurization process of flue gas,the SO₃concentrations were reduced by63.8%and 54.9%,respectively.For the two sets of FCC device in Refinery II,the SCR denitration process was used to their flue gas.From the outlet of the third cyclone separator,through the waste heat boiler and SCR denitration process,to the outlet of wet desulfurization tower,except for the concentration of SO₃in the flue gas at the SCR outlet increased by 8.1～18.1%,the SO₃concentration in flue gas had a generally decreasing trend by the wet desulfurization of flue gas,the SO₃concentration in flue gas reduced about 79.7～91.8%and 57.1～82.8%,respectively.Among the four FCC devices investigated,influenced by the factors such as raw material sulfur content and excess oxygen content,0.26～2.22wt.%sulfur in feed is converted into SO₃in the flue gas,and the amout of SO₃was accounted for 12.2～19.1vol.%of total SO_xin flue gas,which was much higher than the ratio of SO₃to total SO_xin flue gas of coal-fired power plants.Finally,the removal performance of SO₃ from flue gas by MgO was investigated by using the fixed bed reaction device in the laboratory.The results demonstrated that the increasing of reaction temperature,MgO loading amount and steam content in the flue gas were beneficial to the removal of SO₃in the flue gas by MgO,and the increasing of SO₃concentration in the flue gas would promote the reaction between SO₃and MgO,but decreased the removal rate of SO₃in the flue gas.The SO₂and SO₃in the flue gas would compete with the reactions of MgO.Increasing the reaction temperature and concentration of SO₂in flue gas would reduce the SO₃selectivity of the reaction with MgO,that is,the SO₂in flue gas would inhibit the reaction between SO₃and MgO.The surface of MgO changed significantly before and after the reaction.The reactive site deactivation model could be used to describe the reaction process between MgO and SO₃.All the correlation coefficients fitted to the experimental results were greater than 0.9952.