Design And Optimization Of Supersaturated Environment Of Flue Gas Recirculation Coupled With Vapor Condensation Technology To Remove Fine Particles

Coal still accounts for the largest proportion of energy consumption in China.The extensive emission of fine particles into atmosphere from coal-fired power plants will cause environmental pollution and health hazards to the human body.Therefore,how to efficiently remove fine particles from the flue gas is a very important project.The particle pretreatment technology that uses physical or chemical methods to promote the growth of fine particles can improve the removal efficiency of fine particles by conventional dust collectors,and is currently one of the main technical routes for fine particle removal.Heterogeneous condensation process of water vapor on particles is simple and can be combined with wet flue gas desulfurization system to create a supersaturated atmosphere with low energy consumption,and is a promising pretreatment method.One of the key factors affecting the growth of fine particulate matter is the degree of supersaturation.This paper aims at the flue gas recirculation coupled with water vapor phase change fine particle growth technology proposed by our research group,and the characteristics of the saturation field constructed under the flue gas cooling method research,which has important research significance and practical value.In this work,a numerical analysis platform is established by FLUENT software to calculate the condensational characteristics in the flue gas cooling process.Based on this numerical model,taken the flue gas at the outlet of the desulfurization tower of a 660 MW power plant as the research object,the flow field in the heat exchanger is numerically analyzed.The distributions of temperature,velocity,water vapor mass fraction and supersaturation on the flue gas side are investigated respectively.The influence characteristics of operating parameters on the flow field are also explored.The dependence of reflux flow rate,reflux pipe arrangement and reflux extraction point on the supersaturation distribution is investigated.The results are as follows:(1)A numerical model of water vapor condensation in the flue gas cooling process is established by using FLUENT software,and the supersaturation in the flow field when condensation exists can be calculated based on this numerical model.Finally simulation results can be verified by compared with the experimental results from literature.(2)Taken the outlet flue gas of the desulfurization tower of a 660 MW power plant as the inlet flue gas of the heat exchanger,the temperature,velocity,water vapor mass fraction and supersaturation distribution can be obtained.The results show that the temperature gradually decreases along the flow direction.The temperature near the wall of the heat exchange tube is the lowest,and the temperature on the leeward side of the heat exchange tube is lower than that on the windward side;The supersaturation increases along the gas flow direction.Also the supersaturation on the leeward side of the heat exchange tube is higher than that on the windward side.The maximum supersaturation in the flow field is 1.43 while the average value at the heat exchanger outlet is 1.23.The more heat exchange tubes,which means the greater reduction of the temperature,so the greater supersaturation can be obtained;For example,when heat exchange tubes increase from 25 to 45,the maximum supersaturation increases from 1.31 to 1.5,the average outlet supersaturation increases from 1.17 to 1.31 accordingly by an increase of 11.9%.Meanwhile the flow resistance is also significantly increased.The higher velocity of the inlet flue gas,the shorter residence time of the flue gas in the flow field,and the smaller supersaturation in the heat exchanger can be obtained.When the velocity is 3.2 m/s,the average supersaturation of the outlet flue gas is 1.36.While when the velocity increases to 7.2 m/s,the average supersaturation decreases to 1.23.(3)A small amount of flue gas recirculation can significantly increase the supersaturation in the flow field.In this model,when the recirculation flow accounts for 2.2% of the inlet flow,the outlet supersaturation increases by 18% compared with the case of no recirculation flow.Under the same reflux,a multi-pipe arrangement is recommended to adopt,which is beneficial to a uniform supersaturation distribution in the flow field.The average outlet supersaturation has basically no relevance to the reflux extraction point.But when the extraction point is closer to the last row of heat exchange tubes,the minimum supersaturation in the flow field will increase and the maximum value will remain unchanged,so the average supersaturation will increase.Therefore,when arranging the reflux extraction point,it should be as close as possible to the last row of heat exchange tubes.