Experimental Study On The Drying Characteristic Of Sewage Sludge And Papermaking Sludge

The yield of sludge soars has been increased in recent years with the advancement of urbanization and development of economy in China.. Desiccation and burning of sludge, which are the efficient solutions to reducing quantity, hazard-free treatment and reclamation for sludge have been paid much more attention. The researches on the law of sludge evaporation and the techniques of sludge evaporation have always been the research emphasis. The objective of this paper is to identify the law of desiccation for sludge taking City sludge and papermaking as the research objection and than take a useful exploration on the law of sludge evaporation in fluidized bed system based on experimental researches on fluidized bed spray drying tower .The research results obtained by this paper propose a theoretical reference for the industrial application of sludge treatment .The efficiency of sludge evaporation is greatly influenced by the temperature, specific surface area as well as the sludge samples. The law of water content changing with time is researched on the condition that temperature, specific surface area as well as the sludge samples is altered in the loft drier. It concludes that evaporation speed of city sludge is less 0.01%/s than papermaking one in express evaporation section, fixed-speed evaporation section and down-speed evaporation section. And the water loss velocity constant (k) of city sludge is far less than papermaking one through drying thermodynamics analysis. In the fixed-temperature evaporation, as the temperature increase, the velocity of heat transfer rate and the water volatilization is accelerated. As the water loss velocity of sludge increase, the water loss velocity constant(k) of sludge is accelerated. As the specific surface area exaggeration, the water loss velocity is also accelerated. The experiment suggests that when the temperature is 30℃and specific surface area is 3.24 cm2/g, it requires approximately 13h that papermaking sludge is evaporated to only content 1% water whereas city sludge requires 15h. When the temperature is 130℃and specific surface area is 3.24 cm2/g, it requires approximately 33minute that papermaking sludge is evaporated to only content 1% water whereas city sludge requires 83 minute. Because 130℃is close to smoke evacuation temperature of boiler, it can increase the rate of energy utilization when the tail flue gas of large boiler is used to evaporate sludge.Before incineration in boiler, wet sludge should be evaporated. The research suggests that when water content of wet sludge declines to about 40%, the net calorific power can reach 3000～5000kJ/kg which can meet the burn request. The maximum rate of blending between coal and half evaporated sludge is affected by evaporation degree. It calculates that the tail flue gas heat of large boiler can meet the sludge evaporation request. The maximum rate of blending on different evaporation degree can calculate by thermal force calculation. As a result, it can calculate the maximum sludge production on the premise of boiler burn stability. The maximum rate of blending is 61.5% coal and 38.5% half evaporation sludge when the water content of water sludge is between 71.46% and 60% as well as the temperature of tail flue gas is 180℃and declines to 130℃in drying tower. Under the design conditions, the largest blending quality ratio is 38.5(evaporated sludge): 61.5(coal).Under the design conditions of inlet gas temperature/sludge water content/aerosol particle diameter of sludge, the residence time of sludge particles in the furnace and drying tower geometry could be obtained based on thermal calculation. By using the circulating fluidized bed drying tower and layout of bed material, the furnace heat transfer can be greatly enhanced which can improve the drying efficiency and reduce product moisture content of sludge.Experiments have been taken in the self-designed Fluidized bed spray drying tower. The experiment results suggest that it can acquire ideal atomization result when the water content of sludge is just over 78%. Quartz sand is used in drying tower to strengthen the transfer heat. The water content of sludge is 80% can be evaporated to 17.43% in fluidized bed spray drying tower with the height of 4m and diameter of 0.3. So a conclusion can be made that Heat transfer is greatly enhanced in furnace and evaporated sludge can meet the requirements of higher blending ratio of sludge and coal by using the circulating fluidized bed drying tower and layout of bed material.Based on all researches in this paper it present a scientific data for sludge evaporation and provides a useful reference for optimizing the technology of sludge evaporation .