Application Research On Moisture Removal System Of Coal-fired Flue Gas By Composite Membranes

The flue gas discharged from coal-fired units using wet desulfurization process contains a large amount of water vapor.Direct emission not only cause waste of water resources,but also aggravate the pollution of local atmospheric environment.It is a feasible way for thermal power plants to realize deep water saving/energy saving and emission reduction by using porous composite ceramic membrane for flue gas moisture and waste heat recovery.In this paper,the transport membrane condenser constructed by micro-and nano-scale ceramic membranes is used to recover flue gas moisture and waste heat.The performance of composite ceramic membrane method for flue gas moisture and waste heat recovery was explored through numerical simulation analysis and calculation,laboratory test and pilot test of coal-fired units.An experimental system of flue gas moisture and waste heat recovery with nano-ceramic composite membrane was established.The performance of the system was studied by using simulated flue gas for ceramic membranes with different pore sizes(30nm,50nm and 200nm).The results show that the water recovery capacity of ceramic membranes with three pore sizes is similar.The water and waste heat recovery performance of ceramic membrane tube with pore diameter of 50nm is better than that of the other two pore sizes.The maximum recovered water flux and the maximum recovered heat flux are 4.82 kg/(m2·h)and 10.66 MJ/(m2·h),respectively.The recovered water flux,recovered heat flux and heat recovery rate of ceramic membrane with all pore sizes increase with the increase of flue gas temperature,flue gas flow rate and cooling water flow rate,and decrease with the increase of cooling water temperature.The ceramic membrane with smaller pore size can block more SO₂ permeation,while the ceramic membrane with larger pore size can remove SO₂ better.A calculation model for system performance analysis was established,and the heat and mass transport process of water vapor across the membrane was simulated and analyzed.Numerical calculations were carried out with the membrane module with a longitudinal spacing of 5cm as the prototype.The results show that with the increase of cooling water temperature from 25? to 36?,the recovered water flux gradually decreases from 29.45kg/h to 18.13kg/h,which is consistent with the experimental results.The difference between the calculated value and the experimental value at different cooling water temperatures is small,and the deviation is within 10%,which indicates that the established mathematical model can accurately describe the mass and heat transfer process of water vapor in ceramic membrane.Simulation results show that with the increase of inlet flue gas temperature,the recovered water flux increases linearly,and the temperature difference between inlet and outlet of flue gas temperature decreases.As the flue gas flow rate increases the recovered water flux increases,however,the growth rate gradually slows down.A pilot-scale test platform for 330MW coal-fired units was built,and the experimental research was carried out on ceramic membrane modules with 1?m pore size by using the flue gas after desulfurization tower.The experimental results show that by increase of cooling water temperature,the recovered water flux of membrane modules with longitudinal spacing of 8cm,5cm and 3cm first decreases slowly,when the cooling water temperature reaches 36?,the recovered water flux decreases sharply.Under relevant experimental conditions,the maximum recovered water flux is 22.23kg/(m2·h),16.49kg/(m2·h)and 10.95kg/(m2·h),respectively.The heat flux and total heat transfer coefficient are consistent with the change trend of water recovery.The maximum total heat transfer coefficients of the TMC with longitudinal tube spacing of 8cm,5cm and 3cm are 1068.2W/(m2·k),784.5W/(m2·k)and 504.1W/(m2·k),respectively.The flue gas moisture and waste heat recovery system of coal-fired units was designed and built by using the composite membrane method flue gas moisture and waste heat recovery technology.The number of membrane modules required was calculated according to the unit load and flue gas flow rate,and the arrangement of membrane modules was designed according to the shape and size of flue gas duct.According to the layout,the resistance calculation model and water recovery calculation model were constructed,and the water recovery was calculated.The results show that under the same operating parameters,with a growing unit load,the recovered water flux and outlet temperature of internal circulating cooling water show a linear growth trend.In addition,under the same load,by increase of the internal circulating cooling water temperature,the recovered water flux gradually decreases,and the temperature difference between outlet and inlet temperature of the internal circulating cooling water gradually decreases.