Study On Condensation Heat Transfer Mechanism Of Wet Flue Gas Containing Dust In The Transport Membrane Condenser

Recovering moisture and capturing fine particles from flue gas is an important way to alleviate the shortage of water resources in the thermal power industry and promote the clean development of coal-fired units.Based on this,this paper proposes that using the staggered arrangement of hydrophilic porous ceramic membrane tube bundles to manufacture a new type of transport membrane condenser.During the operation of transport membrane condenser,the hot and cold fluids separate on both sides of the ceramic membrane.On the hot fluid side,after the water vapor in the flue gas reaches the saturated state,it will condense on the surface or in the pore channels of the ceramic membrane to form the condensate.When the internal pressure of ceramic membrane is lower than the external pressure,under the action of pressure gradient,the formed condensate penetrates into the inside of ceramic membrane to achieve the recovery process of moisture.When the internal pressure of ceramic membrane is equal to or higher than the external pressure,the condensate cannot penetrate into the membrane,and it can only cover the surface of ceramic membrane.During the movement of flue gas,some fine particles can be captured by the condensate film,so as to achieve capture process of fine particles.In this paper,the experimental and theoretical methods are both used to study the condensation heat transfer mechanism of wet flue gas containing dust during the operation of transport membrane condenser.The specific research contents are as follows.This paper designs and build flue gas moisture capture experimental platforms for natural gas and coal-fired boilers,and analyses the water vapor capture mechanism based on ceramic membranes with different pore diameters.The results show that the condensation resistance of the macroporous transport membrane condenser is 2.55?22.51 times the permeation resistance.The condensation resistance is a key factor affecting the water vapor capture performance.Increasing the temperature difference between the hot and cold fluid or increasing the inlet flowrate of the hot and cold fluid can reduce the condensation resistance,thereby improving the water vapor capture performance.The water vapor capture mechanism of the microporous transport membrane condenser is film condensation and Knudsen diffusion.The water vapor capture flux under the action of the film condensation mechanism is 2?22kg/(m2·h),which is far better than the capturing flux of 0.8?0.9 kg/(m2·h)under the action of Knudsen diffusion mechanism,and the temperature gradient is the main driving force for the condensation of water vapor on the surface of the microporous ceramic membrane.This paper breaks through the traditional transport membrane condenser ceramic membrane pore size limit of 2?50nm,expands the upper limit of the pore size suitable for flue gas moisture capture scenarios to 1 ?m,improves the moisture capture performance and reduces the production cost of the ceramic membrane.The lower limit of the pore size is extended to 0.4 nm,which improves the quality of water obtained in the capturing process.Based on the micro-nano structure characteristics of porous ceramic membranes and the physical and chemical properties of hot and cold fluids,a mathematical model of heat and mass transfer of water vapor across the membrane under the action of capillary condensation mechanism and film condensation mechanism is established,and the performance calculation methods of transport membrane condensers with different structural characteristics under different operating conditions are obtained.The performance calculation method of the transport condenser under different operating conditions.Under the action of two different condensation mechanisms,the differences in the effects of temperature field,velocity field and their synergy on heat and mass transfer performance are comparatively analyzed.In addition,this paper reveals the key factors affecting heat and mass transfer fluxes of condensate transport process across membranes and action mechanisms,and optimizes the performance prediction method of transport membrane condensers in engineering applications.A technical solution for capturing fine particles from wet flue gas containing dust based on the transport membrane condenser is proposed.A transport membrane condenser made of porous ceramic membranes with a pore size of 0.4 nm is used to capture the fine particles in the exhaust emitted by a natural gas-fired boiler.The influence of different operating conditions on the capture efficiency of fine particles,the intensity of condensation heat transfer,and the quality of waste liquid formed during the collection process are studied experimentally.The maximum capture efficiency of fine particles in the experiments is 61.43%.The results show that there is a positive correlation between the intensity of condensation heat transfer and the fine particle capture efficiency.Reducing the thickness of condensed liquid film on the ceramic membrane surface,increasing the cooling water flowrate or reducing the wall thickness of ceramic membrane can all reduce the overall heat transfer resistances of transport membrane condenser,thereby increasing the intensity of condensation heat transfer.The quality of condensed water formed in the process of capturing fine particles is negatively related to the fine particle capture efficiency.Without disrupting the water balance of power plant,these condensed water can be used as make-up water for the desulfurization tower of coal-fired power plants.From the perspective of the second law of thermodynamics,a theoretical model of system irreversible losses based on the analysis of entransy dissipation and entropy increase is established.Based on this model,the influence of a single factor on the system irreversible losses under operating conditions is explored,and from the dimensions of operating modes and structure sizes,with the goal of reducing system irreversible losses and improving thermal economy,the performance of transport membrane condenser has been optimized.The results show that the irreversible losses are much more sensitive to the influence of the cooling water temperature than the cooling water flowrate.Therefore,cooling water flowrate can be increased appropriately in the project.For a specific project,the operating parameters of the flue gas side may have an inflection point with the least irreversible losses.Furthermore,increasing the length of ceramic membrane can reduce the system irreversible losses and the total area of ceramic membrane required for engineering.