Research On Sludge Dewatering And Resource Utilization In Power Plant Desulfurization Wastewater Treatment System

The sludge produced by flue gas desulfurization（FGD）wastewater treatment system in coal-fired power plant is a typical chemical sludge.At present,little research has been done on the settleability and dewaterability of this sludge at home and abroad.In the sludge produced by FGD wastewater treatment system,there are a lot of inorganic substances,which have a great difference in compositions and properties in different treatment processes.The sludge with poor settleability and dewaterability has been transported to landfill after dewatering,which causes the large land occupancy.Therefore,the improvement of the sludge separation performance in FGD wastewater treatment system and development of sludge recovery route are one of the technical difficulties needed to be solved urgently.The operation conditions of FGD wastewater treatment process will affect the properties of sludge,thus affecting the treatment and disposal of chemical sludge.Therefore,the process operation conditions and sludge conditioning technology can both improve the settleability and dewaterability of chemical sludge.In this paper,standardization research was carried out on the determination method of chemical sludge dewatering performance index.It was found that the particle size distribution of chemical sludge after high speed stirring pretreatment was more uniform,and the specific resistance fitting degree was better.The correlation between capillary suction time,filtration time,specific resistance and particle size was higher,which could more accurately describe the dewatering performance of chemical sludge.In order to further reduce the water content of desulfurized gypsum,the effects of initial sludge concentration,polyacrylamide（PAM）type and PAM dosage on the sludge dewatering performance of gypsum cyclone underflow sludge were studied.The higher the initial concentration of the sludge,the lower the water content of the sludge cake after conditioning and dewatering,but too high sludge concentration will block the pipeline.In the actual production process,the sludge in the gypsum cyclone can be concentrated to400-500 g/L（40-50%）for conditioning and dewatering.The anionic PAM with higher molecular weight and degree of hydrolysis had better conditioning effect on cyclone underflow sludge,and when the dosage of anionic PAM was0.20 mg/g dry solid（DS）,the water content of desulfurization gypsum can be reduced to 25.63%.The pH value of the"triple tank"process will affect the composition and yield of the sludge,thus affecting the settling and dewatering performance of the sludge.The settling performance of sludge was improved with the increase of p H value.And the dewatering performance was the best when p H value was8.5.When the p H value increased to 10.0,the sludge dewatering performance was deteriorated due to the increase of Mg（OH）₂ production.With the further increase of p H value to 11.0,the sludge dewaterng performance was improved because of the proportion of Ca（OH）₂ in sludge increased.When adding anionic PAM at p H 8.5,the water content of sludge cake can be reduced to41.1%.Effects of temperature,reaction time,PAM type and dosage on settling and dewateing performance of two-stage desalination process sludge were investigated to explore improving strategies for separation by using actual flue gas desulfurization wastewater.The sludge of the Cl^-removal unit is mainly composed of Friedel’s salt（FS）.Raw sludge was difficult to separate by gravity settling and mechanical dewatering owing to their small particle size,light weight and high yield.Anionic PAM with higher hydrolysis degree and molecular weight can greatly enlarge particle size and improve settling performance of sludge.Increasing temperature to 40℃obtained the highest Cl^-removal,and improved settleability because layers spacing of FS decreased.The sludge was gradually broken down above 40℃,resulting in a decrease in the removal rate of Cl^-.Prolonging the reaction time can improve the removal rate of Cl^-by adsorption,and promote the crystal growth to improve the sludge settling performance.The 0.5 m³/h pilot-scale experiment confirmed that the FS precipitation process was operated stably with improved settling and dewateing performance of sludge by dosing PAM and elevating temperature.Techno-economic analysis showed that 37℃was optimal for the two-stage process.In order to realize the resource utilization of the sludge,the sludge of the Cl^-removal unit was used for the excess sludge conditioning,and it was used together with Fe Cl₃ to realize the deep dewatering of the excess sludge.When the dosage of Fe Cl₃ and Cl^-removal unit sludge was 60 mg/g DS and 0.15 g/g DS,respectively,the water content of sludge cake can be reduced to 58.63%.Among them,Fe Cl₃ can destroy the cellular structure of the excess sludge and release extracellular polymeric substance.The sludge of Cl^-removal unit can adsorb the extracellular polymeric substance and serve as the framework in the excess sludge to improve the dewatering performance of the excess sludge.