| The wet desulphurization technology is promoted and applied rapidly in coal-fired power plants to meet the strict ultra-low emission requirements.However,the flue gas reaches wet saturation after scrubbed by the wet desulphurization tower and direct discharges can result in significant water loss and low temperature waste heat losses.The membrane separation method combines flue gas dehydration and waste heat recovery,and it has stronger heat transfer capacity and higher condensation water quality compared with low temperature condensation method and solution absorption method.In this paper,the porous ceramic membrane flue gas water and waste heat recovery system is used to study the heat/mass transfer characteristics with different cooling medium,and the theoretical analysis experimental investigation,statistical analysis,numerical simulation and application test are used to reveal the variation rules of flue gas water and waste heat recovery with porous ceramic membranes.The specific research contents are as follows:The experimental platform for nanometer ceramic membrane flue gas dehydration and waste heat recovery was built.With circulating water as the membrane module cooling medium,the influence of flue gas temperature and flow,circulating water temperature and flow on the flue gas water and waste heat recovery performance was analyzed.The evaluation indicators for flue gas water and waste heat recovery performance are water recovery rate and efficiency and heat recovery power and efficiency.Combining the ceramic membrane permeation characteristics for condensation water,the actual water recovery efficiency and the theoretical water recovery efficiency are also defined.Experimental results show that increasing the flue gas temperature or reducing the circulating water temperature can help to improve the flue gas water and heat recovery indicator,while increasing the flue gas flow does not consistently improve the flue gas water and heat recovery efficiency.The ceramic membrane permeation efficiency is greatly affected by the flue gas temperature,and the difference between the actual water recovery efficiency and the theoretical water recovery efficiency ranges from 5.93%to 24.98%.At the same time,it has been found that excessive circulating water flow can cause non-condensable gases to seep into the ceramic membrane.The 10 nm ceramic membrane has higher water recovery rate and efficiency than the 0.4 nm ceramic membrane,with water recovery rate up to 6.45 L/(h·m2)and heat recovery power up to 129.45 W.Considering the wet saturated flue gas temperature is low,it is proposed to use the purge gas as the cooling medium of the membrane module.It is also used as boiler secondary air to achieve low temperature flue gas waste heat reuse.With positive and negative pressure purge gas as the cooling medium,the experimental platform for the ceramic membrane flue gas water and waste heat recovery has been set up.Experimental studies were carried out with positive or negative pressure purge gas respectively.The feasibility of positive or negative pressure purge gas as the cooling medium of membrane modules was verified,and the dominant factors affecting flue gas water and waste heat recovery performance were investigated,and the positive and negative pressure purge gas outlet parameters in membrane modules and the variation patterns were analyzed.Experimental results show that positive or negative pressure purge gas as the cooling medium of membrane modules can also achieve flue gas water and waste heat recovery.Increasing the flue gas temperature can help to increases both the water recovery rate and the heat recovery power.Increasing the purge gas flow,the water and heat recovery performance indicators all decrease and then increase with negative pressure purge gas,the water and heat recovery performance indicators all increase consistently with positive pressure purge gas.The positive and negative pressure purge gas outlet temperature and moisture content are significantly increased with significant heat and mass transfer processes.At the membrane module outlet position,the negative pressure purge gas is supersaturated,while the relative humidity of positive pressure purge gas is slightly below 100%.Based on flue gas water and waste heat recovery experiment platform,the variation patterns of water vapor pressure difference,transmembrane pressure difference and Reynolds number and their linear correlation coefficients with water recovery rate were investigated with statistical linear correlation methods.At the same time,the heat transfer comparative experiments between porous ceramic membranes and 316L stainless steel were carried out.The aim is to analyse the enhanced effect of porous ceramic membranes for flue gas waste heat recovery.Statistical analysis shows that the water vapor pressure difference is strongly influenced with flue gas temperature and has a significant linear correlation with the water recovery rate.The Reynolds number is mainly influenced with fluid flow and has a linear correlation with the water recovery rate.The transmembrane pressure difference is related with the cooling medium type and flow,but it does not have a linear correlation with the water recovery rate.Comparative experiments show that the composite heat transfer coefficient of porous ceramic membrane is higher than 316L stainless steel tube.In particular,the composite heat transfer coefficient of ceramic membrane is up to 8 times when purge gas is used as the cooling medium.Flue gas waste heat recovery is dominated by latent heat recovery,which accounts for over 90%.At the same time,porous ceramic membrane can significantly improve the condensation water quality.Based on the strongly hygroscopic characteristics of the dehumidifying solution,the semi-open absorbent heat pump flue gas water and waste heat recovery system is proposed with multi-channel ceramic membranes as the absorber.The regeneration of lithium bromide solution is realized with a high temperature generator,a reheat generator and a gas-liquid separator.Based on a 330 MW coal fired power plant,the effects of circulating medium parameters,return heat steam flow and heat exchange in the absorber on the operational performance of the semi-open absorption heat pump flue gas water and waste heat recovery system are simulated and analyzed.Simulation results show that the lithium bromide solution flow varies positively with the solution outlet temperature and the amount of water removed from the flue gas.Increasing the amount of heat absorbed by the condensation water in the absorber will increase the amount of water removed from the flue gas,but will reduce the lithium bromide solution outlet temperature.The return heat steam flow is controlled by the high temperature generator and diverter for regenerating the lithium solution in the return heat generator.Increasing the return steam flow can reduce the required drive heat source and improve the waste heat recovery from flue gas and the heat pump COP,but there are limits.Based on a 330 MW coal fired power plant,the flue gas water recovery system application test was carried out.The ceramic membrane modules with total membrane area of 1867.2 m2 were installed in the horizontal flue channel between the wet scrubber and the chimney.The application test showed that.the ceramic membrane modules had an overall permeation efficiency between 84.8%and 86.7%and part of the condensation water could not be absorbed by the ceramic membrane modules in time.The flue gas water recovery system operated continuously for 168 h,and effectively recovered flue gas water for 35.68～38.97 t/h.The application test has improved the technical guidance for the application of the flue gas water and waste heat recovery system by ceramic membrane separation. |
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