Research On Recovery And Utilization Of Water From Coal-fired Boiler Flue Gas

China is a country which is short of water resource.With the growth of population and development of economic,also the problem of water pollution been not effectively solved,water shortage is bound to be the hot issues that need to be focused in the future.In such a situation,water saving and emission reduction is the fundamental way to solve the problem.Therefore the state has issued srict water policy,and the coal-fired power plant is facing considerable pressure of water saving.The flue gas at outlet of the wet desulphurization system has plenty of water vapor,if the moisture can be effectively utilized,it will greatly relieve the water saving pressure of the power plant.So the utilization of the moisture in the flue gas is the key way to save water in coal-fired power plants.This paper took water recovery from the flue gas as the goal,and based on the cooling-condensation heater technology,using the gas cooler to recover the moisture in the flue gas.By the ways of comprehensive theoretical study,technology development and pilot plant tests,the key data of the related technology were obtained,which laid the foundation for the application of the engineering.First,theoretical calculation was carried out to analyze the moisture content of flue gas in different coal types,study the state of the flue gas at the outlet of WFGD and evaluate the potential of the moisture recovery from the wet flue gas.It was calculated that the flue gas at the outlet of WFGD is saturated or supersaturated,which has nothing to do with the coal type.So it is a great potential to recover water from the flue gas.For a 600MW unit,whose flue gas flow rate is 1917886 cubic metres per hour,when the flue gas has a temperature drop of 8-12 degrees,the condensation flow rate would be 62-85 tons per hour.On the basis of theoretical analysis,a pilot system of flue gas water recovery was designed and built in Yueqing power plant to study the influence of the ratio of cooling water flow rate and flue gas flow rate on convective condensation heat transfer process.The best operation condition of flue gas cooler was obtained,that is,there is a value(X),when the ratio of cooling water flow rate to gas flow rate is greater than or equal to X,the larger condensation rate can be obtained.For this pilot system,X equals two.Under this operating condition,the condensation rate is 38 to 43 percent,and the condensation flow rate is 60 to 84 tons per hour(which is converted to a 600MW unit).Through the pilot tests,the empirical formula of condensation heat transfer coefficient under the experimental conditions was also obtained.The error between the calculated value of the empirical formula and the experimental value is less than 10 percent,which means the empirical formula can be used for the design and calculation of the flue gas cooler of the flue gas water recovery system.The quality of the recovered condensation is good which ranks in the III class water,and whose PH is weakly acidic.By simply adjusting the condensation's PH,it can be used in most industrial water consumption occasions,such as cooling tower water supply,circulating cooling water,desulfurization tower replenishment,etc.Finally,this paper took a 600MW coal-fired unit as an example,analyzed the benfits of water recovery from the flue gas.The results show that the unit can save 284 thousand tons water and 1305 tons coal per year,whose economic benefits were 1193 thousand and 750 thousand yuan per year respectively,at the same time,the system can reduce the emission of fine particulate matter.Water recovery from the flue gas of the coal-fired plant has the great significance of water saving,energy saving and environmental protection.