Mechanism Analysis And Experimental Research On Recovering Water And Latent Heat From Flue Gas By Applying Ceramic Membrane

Recovering water vapor from flue gas exhausted from thermal power plant is beneficial for saving water and eliminating“colorful smoke".In addition,the recovered water vapor contains substantial sensible heat and latent heat.If the substantial heat and latent heat can be recovered efficiently and applied for thermal recycle in power plants,coal consumption and economical efficiency can be improved apparently.In thermal power plants,recovering water and latent heat by installing condensation-heat-transfer equipment in tail flue is a popular method.In contrast with traditional uniform condensation-heat-transfer material,ceramic membrane called as porous condensation-heat-transfer material holds a outstanding heat and mass-transfer property.Therefore,condensation heat and mass-transfer process of flue gas within ceramic membrane was researched in the paper.Main research of this paper includes:Based on the Lagrange multiplier method,a numerical model of condensation heat and mass-transfer process within ceramic membrane was built in the study.In the numerical model,the entransy dissipation and momentum consumption were set as optimization objects,the Navier-Stokes equation was set as constraint condition.In this section,influences of heated-fluid properties on condensation heat and mass-transfer performance of porous ceramic membrane was the research object.During the experiment,temperature variation of flue gas and cooling water was tested,impacts of flue gas velocity on water-recovery process of ceramic membrane was also investigated.During the numerical simulation,velocity and temperature distribution in ceramic membrane were studied,the variation of velocity boundary layer and temperature boundary layer were also analyzed.The results showed that heat-transfer process of gas mixture within ceramic membrane is a gradual changing process.The forced convection heat transfer would transferred to the condensation heat transfer when velocity of gas mixture increased to a certain value.Nevertheless,a relatively higher velocity of gas mixture would result in a relatively larger momentum consumption.Formation and attaching type of fouling cake layer on the ceramic membranes with various using time(0,400,800 hours)was investigated in this section.And the corresponding numerical model was built based on the material characterization results.The numerical model was used to predict influences of fouling cake layer on water and heat-recovery performance of ceramic membrane.In order to investigate influence degree of operational conditions on water and heat-recovery capability of ceramic membrane with various using time(0,400,800 hours),the surface corresponding method was used to analyze numerical and experimental data.The experimental results showed that relationship between fouling cake and inner wall surface of ceramic membrane is a physical contact process,instead of a chemical reaction.The fouling cake adhering on the inner wall will restrain condensation heat and mass transfer in ceramic membrane.Moreover,temperature of fluids flowing along with inner wall and outer wall have a minor impact on water and heat-recovery properties of ceramic membrane.However,ordinary water content of gas mixture has a prominent impact on water and heat-recovery performance of ceramic membrane,especially for the ceramic membrane with a relatively thicker fouling cake layer.A contrast experiment by using a ceramic membrane and a stainless steel tube was performed in this section.In order to make the experiment is trustworthy,structure parameters of the experimental ceramic membrane and the stainless steel tube are just the same.In the experiment,inclination angle was the prominent operational condition.The variation of cooling water local temperature,bulk temperature on ceramic membrane and stainless steel tube were investigated respectively when values of inclination angle were changed.In addition,the force characteristic of fluids in ceramic membrane and stainless steel tube were discussed by pressure balance analysis,and effects of pressure difference between the condensation tubes(ceramic membrane and stainless steel tube)on condensation heat and mass-transfer properties were also studied.The experimental results showed that the traditional continuous condensation-heat-transfer tube can only exchange heat with heated gas mixture by heat conduction process and heat convection process.Nevertheless,the new-type porous condensation-heat-transfer tube can achieve heat not only by heat conduction process and heat convection process,but also by significant amount of latent heat from recovered condensate.As a result,porous ceramic membrane owns a outstanding thermal property compared with stainless steel tube.Moreover,the radical flow caused by porosity and velocity gradient in the near-wall region is obvious,which leads to a relatively intenser condensation heat-transfer process occurring within ceramic membrane.Typically,influence of the gravity force on the trans-membrane pressure difference is relatively apparent when ceramic membrane was installed horizontally.Therefore,condensation-heat-transfer efficiency is relatively higher when ceramic membrane was installed horizontallyThe application of water and heat-recovery module on gas-fired boiler was introduced in this section.The simultaneous drop-wise/film-wise condensation heat transfer occurring on membrane wall was proposed,and corresponding thermal resistance was also analyzed.Above all,water and heat-recovery capacity of ceramic membrane bundle in the real flue gas was examined in this section.The experimental results showed that,average temperature of cooling water decreased from 20.573 to 19.196? when inlet volume flow rate of cooling water increased from 0.3 to 0.7m3/h,meanwhile average temperature difference of cooling water decreased from 2.073 to 0.696?.In other words,changing the operational conditions of cooling water(inlet temperature and inlet flow rate)has a minor impact on water and heat-recovery module.Additionally,disposable amount of flue gas is limited by membrane area in the study.The experimental results showed that variation tendencies of water-recovery amount and efficiency are similar when inlet flow rate of cooling water is relatively lower,while the variation tendencies change oppositely when inlet flow rate of cooling water is larger than 0.5m3/h.