Investigation Of SO₃ Emission And Control Strategy In Coal-fired Power Plants

The present situation of air pollution in China is very severe.Energy saving and emission reduction is still the theme of energy industry for a long time.Under the extensive implementation of ultra-low emission（ULE）facilities in coal-fired power plants（CFPPs）of China,the emission of SO₃ has received increasing attention due to its impact on human health and operation safety of power plants.The further study of its control and emission characteristics is the basis for the next step of formulating relevant policies and implementing emission control.In this paper,563 field tests have been carried out in 215 CFPPs,focusing on the changes of SO₃ emission level before and after ULE retrofit,the SO₃ synergistic control performance and key influencing factors of environmental protection equipment,and the countermeasures and technical routes for the SO₃ emission control.Major findings are as follows.Firstly,the influence of ULE on the control and emission level of SO₃ was systematically studied.The results showed that the average emission concentration of SO₃ decreased by 51.8%before and after the implementation of ULE,and ULE effectively improved the emission control level of SO₃.The comprehensive removal efficiency of SO₃ varied greatly from 26.9%to 94.8%adopting different ULE technical routes.Secondly,the key influencing factors and characteristics of SO₂/SO₃ conversion in SCR equipment were analyzed.The results showed that the SO₂/SO₃ conversion rate increased with the increase of V₂O₅ content,WO₃ content and inlet flue gas temperature,but decreased with the increase of inlet SO₂ concentration,MoO₃ content and surface velocity.V₂O₅ content had the greatest influence weight,reaching 30.6-33.6%;SO₂/SO₃ conversion rate had the strongest sensitivity to flue gas temperature,reaching±42.4%.The SO₂/SO₃ conversion rate of in-service catalyst generally decreased with the decrease of catalyst activity.In the regeneration process,it is necessary to remove the deposits on catalyst surface that promote SO₂/SO₃ conversion.Thirdly,the key influencing factors and characteristics of SO₃ removal performance of wet limestone-gypsum flue gas desulfurization（WFGD）was analyzed.The results showed that the SO₃ removal efficiency is in the range of 31.0-80.9%,the average value was 53.1%,the SO₃removal efficiency after ULE was increased by about 5.89%,and the efficiency of double scrubbers was about 8.69%higher than that of single scrubber.In addition to the inlet flue gas temperature,other parameters were positively correlated with SO₃ removal efficiency,and the influence weight of flue gas residence time and liquid-gas ratio was relatively large,reaching27.4%and 23.1%respectively.The sensitivity of SO₃ removal efficiency to inlet flue gas temperature was the strongest,reaching±23.1%.Fourthly,SO₃ removal performance of the dedusting equipment and the influence of the factors were analyzed.The results showed that the SO₃ removal efficiency of the dry dedusting equipmentis about 20%,while that of the low-low-temperature electrostatic precipitator（ESP）was over 70%.The SO₃ removal efficiency of wet ESP（WESP）was between 50.9%and 91.8%,with an average of 76.9%.Among the influencing factors,the specific collection area had the largest influence weight,reaching 38.4-42.4%,and had a positive correlation with SO₃ removal performance.The sensitivity of SO₃ removal efficiency to the inlet flue gas temperature was the strongest,followed by the specific collection area,which were±18.9%and±7.1%respectively.Fifthly,a multiple linear regression model was established for SCR equipment,a single nonlinear regression model was established for dry dedusting equipment;an exponential regression model was established for WFGD equipment;a quadratic polynomial model was established for WESP equipment.Finally,the prediction model and prediction software of SO₃emission and control in the whole process of CFPPs were established.On this basis,the adaptability of different ULE technology routes to different SO₃ emission requirements and the emission capacity of 148 sample CFPPs were analyzed.The SO₃ control routes were proposed according to different furnace types,sulfur content of coal and emission limits.Finally,in view of the problem of SO₃ emission control of high-sulfur-coal CFPPs,an integrated collaborative SO₃ removal technology is proposed,which combined the flue injection of base absorbent with the technology of low-low-temperature ESP.The results of technical and economic demonstration showed that the technology has stronger emission control ability and better economy than the conventional technology,which provides a solution for the subsequent high-sulfur-coal CFPPs to achieve ULE and SO₃ collaborative control.