Experimental Study On Evaporation,Drying Characteristics And Pollutant Transfer Of Desulfurization Wastewater

In this paper,the research on evaporation,drying characteristics and pollutant migration of desulfurization wastewater have important engineering application value for the development of zero-emission evaporation and flue drying technology of desulfurization wastewater.The evaporation laboratory study of desulfurization wastewater shows that the pH of desulfurization wastewater has limited effect on the evaporation rate,and the evaporation vacuum is the main factor affecting the evaporation rate;As the evaporation vacuum becomes larger,the evaporation rate increases rapidly,and the evaporation rate at a vacuum of-0.08MPa is 2.93 times that at atmospheric pressure.When the desulfurization wastewater is acidic,the evaporation rate at pH=5 is the smallest,and the minimum average evaporation rate is0.044 mL/?cm²·min?.The effects of desulfurization wastewater pH and evaporation vacuum on pollutant migration experiments show that the pH and evaporation pressure of desulfurization wastewater has small impact on the COD concentration,pH and ammonia nitrogen concentration of evaporation condensate;The vacuum degree has a great influence on the pH and ammonia nitrogen concentration of evaporation condensate which are negatively correlated with the evaporation vacuum.When the evaporation pressure is greater than-0.02Mpa,the condensate ammonia nitrogen concentration rises sharply;The pH of desulfurization wastewater and the evaporation pressure has a certain influence on Cl^-in the condensate which is small at pH=5 or 9.To ensure the evaporative condensate water quality superior to the"Urban Sewage Recycling Industrial Water Quality"?GB/T 19923-2005?standard,the optimal evaporation scheme is determined as pH=5 of desulfurization wastewater and-0.08MPa of evaporation vacuum.The convective drying characteristics of the thin layer of desulfurization wastewater concentrate were studied experimentally.The results show that the convective drying of the desulfurization wastewater concentrate has no obvious constant-speed drying stage;The desulfurization wastewater concentrate pH has small effect on the drying rate;The drying temperature is the main factor affecting the drying rate and the maximum drying rate is0.338g/?g·min?at a drying temperature of 200°C,1.72 times of the maximum drying rate at100°C.The hot air flow rate is another major factor affecting the drying rate,and the maximum drying rate of hot air wind speed of 6m/s is 0.340g/?g·min?1.27 times of hot air wind at speed of 2m/s.The thin layer convective drying model of the desulfurization wastewater concentrate is subjected to the Lewis model equation,and the relationship between the model equation coefficient k and the drying temperature,hot air wind speed and the concentrated liquid pH is obtained.The established general model prediction value agrees well with the experimental value,and the mean square error is 1.93135E-4.The experiment platform for drying pollutants of desulfurization wastewater concentrate was built.This research revealed the factors affecting the characteristics of dry condensate pollutants.The results show that the pH and drying temperature of the concentrate have small effect on the COD concentration in the dry condensate;the pH of the dry condensate is positively correlated with the concentrate pH.When the drying temperature is less than175°C,the condensate pH is almost alkaline and the Cl^-concentration in the condensate changes little with the concentrate pH.However,when the drying temperature is greater than175°C,the dry condensate pH drops sharply with the concentrate pH,and the concentration of Cl^-in the condensate is significantly negatively correlated with the concentrate pH.When the concentrate becomes acidic,the concentration of ammonia nitrogen in the dry condensate is almost independent of the drying temperature and the concentrate pH.While when the concentrate becomes alkaline,the drying temperature and the concentrate pH have a great influence on the ammonia nitrogen concentration of the dry condensate with exponential growth.The optimal solution for the desulfurization wastewater concentrate is as follows:the concentrate pH is 5 and the drying temperature is 200°C.The dry condensed water index can meet the standard of?Urban Water Recycling Industrial Water Quality??GB/T 19923-2005?.A pilot test bench for evaporation and inert particle drying fluidized bed was set up.At the evaporation temperature of 91.5°C and the drying temperature of 128°C,a pilot test was conducted on the desulfurization wastewater of a coal-fired power plant.The dry powder characteristics are as follows:The water content is 0.59%,the average particle size is 28.6?m,and the average particle size is 40.9?m.The dry powder mainly contains CaSO₄,CaSO₄·0.5H₂O,MgCl₂ and SiO₂.The mass fraction of chlorine in the dry powder is 7.53%,and chlorine elements exist in the form of MgCl₂ and NaCl,etc.95.8%of Cl^-and 25.3%of ammonia nitrogen in desulfurization wastewater are left in the dry powder,and 3.4%and23.1%are volatilized into the drying medium,and 0.8%and 51.6%are transferred to the evaporation condensate,respectively.The research results of this paper have certain reference value for the optimization design and operation of desulfurization wastewater evaporation and flue gas drying engineering.