Research On Preparation And Application Of Ceramic Composite Membrane

Under the background of carbon neutrality and the current situation of coalbased energy consumption,clean and efficient utilization of coal is an important direction of energy development.The transport membrane condenser technology can efficiently recover water vapor from flue gas of power plant,which is of great significance for promoting water saving in power plant and the clean and efficient utilization of coal.Taking the transport membrane condenser as the research object,aiming at two key problems,the high cost of the ceramic membrane and the urgency to improve the water recovery performance,this dissertation focuses on the development of low-cost ceramic membrane with good performance and the configuration optimization of the ceramic membrane for the transport membrane condenser.This dissertation is committed to ascertain the performance control mechanism of ceramic membrane,reveal the influence law of ceramic membrane parameters on the performance of transport membrane condenser,and lay a scientific foundation for the optimization design of ceramic membrane for transport membrane condenser.Coal fly ash was used as the main raw material for developing low-cost ceramic membranes.Based on various characterization methods,the relationship between process parameters and ceramic membrane properties was explored from two aspects of ceramic paste water content and sintering holding time.Shrinkage,porosity and pore size increase and bending strength decreases with the ceramic paste water content increasing.The sintering holding time shows little effects of on membrane properties,the densification of ceramic membrane slightly increases with sintering holding time increasing.Coal fly ash with different particle sizes were adopted to reveal the working mechanism of particle size in the performance control of ceramic membrane.The wide particle size distribution of raw material reduces the membrane porosity.There is a trade-off between the permeability and bending strength of ceramic membrane.The permeability increases while the bending strength decreases with particle size increasing.The bending strength is mainly determined by the large particles in raw material.In order to overcome the trade-off between permeability and mechanical strength,means to control the performance of ceramic membrane were studied from the view of pore-forming agent.Dolomite has the best poreforming effects at high sintering temperature,which can significantly reduce the shrinkage and improve the porosity.The binder properties of the dextrin improve the bending strength of ceramic membrane and reduce the pore tortuosity,which can effectively overcome the trade-off between permeability and mechanical strength.The ceramic membrane sintered at 1200℃ with dextrin as pore-forming agent showed the best comprehensive performance,with porosity of 39.46%,bending strength of 35.56 MPa,and permeability of 3.84×10^-14 m²。Focusing on the problems that the wide pore size distribution of support makes the separation layer prone to defects when preparing ceramic composite membrane using coal fly ash-based membrane as the support,and the trade-off between the selectivity and permeability of the ceramic composite membrane is difficult to overcome,the characteristics of the wide pore size distribution of the coal fly ashbased support were analyzed,and the working mechanism of the coating particle size in preparing the separation layer is studied.Too small coating particles penetrate into the large pores of the support,resulting in defects and wide pore size distribution of the separation layer;while too large coating particles lead to little difference between the structures of the separation layer and the coal fly ash-based support,resulting in wide pore size distribution of the separation layer.Optimizing the coating particle size of the separation layer is a simple and effective way to overcome the trade-off between selectivity and permeability.A coal fly ash-based ceramic composite membrane with uniform pore size has been developed.With the coal fly ash-based ceramic membrane as the support,which shows a wide pore size distribution of 388 nm in average and 1398 nm in maximum,and the alumina powder with average particle size of 1.11 pm as the coating particles,the ceramic composite membrane with a narrow pore size distribution was prepared.The average pore size is 161 nm the water permeate coefficient is 879.71 L/（m²·h·bar）.The developed coal fly ash-based ceramic membrane was applied in the transport membrane condenser to recover flue gas moisture.The influence of operating parameters on water and heat recovery performance was experimentally studied to analyze the characteristics in variable conditions.Increasing the inlet cooling water flowrate,decreasing the inlet cooling water temperature and increasing the inlet flue gas temperature can simultaneously improve the water and heat recovery flux and efficiency.The inlet flue gas temperature shows the most significant effects on the water and heat recovery because the water content of saturated wet flue gas increases with the temperature rising.Increasing the inlet flue gas flowrate improves the convective heat transfer coefficient between flue gas and ceramic membrane surface,thus raising the recovered water flux and recovered heat flux,but simultaneously leads to insufficient heat exchange of a large proportion of flue gas.thus reducing the water recovery efficiency and heat recovery efficiency.Results show that the characteristics under variable conditions of the coal fly ashbased ceramic membrane in the transport membrane condenser are similar to that of the alumina ceramic membrane,which verifies the performances of using low-cost coal fly ash-based ceramic membranes in the transport membrane condenser to recover flue gas moisture.The low-cost advantage of coal fly ash-based ceramic membrane is promising to promote the large-scale application of transport membrane condenser.With the experimental route of "Preparation-Characterization-Application",ceramic membranes with different permeability coefficients,ceramic membrane raw materials and particle sizes were prepared to study the effects of the three parameters on the performance of transport membrane condenser.Besides,feasible suggestions are provided for the optimization design of ceramic membrane for transport membrane condenser considering practical applications.Increasing the permeate coefficient of ceramic membrane can reduce the heat and mass transfer resistance of the transport membrane condenser,thus improving the water and heat recovery performance.The improvement is more obvious under high recovered water flux conditions.It is necessary to evaluate whether the permeability coefficient of ceramic membrane meets the requirements of condensate permeation before engineering application.The thermal conductivity of the alumina ceramic membrane is 11.5 times that of the coal fly ash-based ceramic membrane while the alumina ceramic membrane improves the water recovery efficiency of transport membrane condenser by about 3～10%compared with coal fly ash-based ceramic membrane.Taking a 330MW coal-fired unit as an example,the technical and economic analysis of the transport membrane condenser shows that the payback periods of using coal fly ash,alumina and silicon carbide membranes are 3.0 years,4.7 years and 9.3 years,respectively.The coal fly ash based ceramic membranes shows better economic prospect.Increasing ceramic particle size shows two opposite effects on the performance of transport membrane condenser,promotion because of the permeability rising,and undermine because of the thermal conductivity reduction.When designing the ceramic membrane for the transport membrane condenser,priority should be given to the use of smaller particles for membrane preparation if the membrane permeability meets the application requirements.