Investigation On The Drying Characteristic And Combustion Treatment Of Sludge

With increasing urban sewage treatment year by year, the amount of sludge continues toincrease too. How to dispose sewage sludge safely has become an urgent problem inChina. Desiccation and combustion treatmemt of sludge is an effective way for achieving thetarget of reduction and stabilization and harmlessness of sludge treatment. Sludge dryingcharacteristics under conditions of constant temperature and poikilothermic temperature arediscussed in this paper.Sludge stickiness effects on its drying characteristics and thermaldynamics models about combustion and desiccation and combustion system design schemewill provide reference frames to practical application for sludge treatment.Constant temperature drying experiments about different thickness and kinds of sludgelayer were conducted on the temperature ranges of40～160℃,which were meanwhile fittedin terms of drying kinetics models of sludge. The results show that the higher dryingtemperature or the thinner thickness is, then the faster drying is. In low temperatures, thedrying rate under the ventilation state is faster obviously than no ventilation. After comparingfive different kinds of drying model, Modified Page Model is the most suitable for fittingsludge drying process. The relation between sludge constant drying rate and sludge thicknessconforms to Power function relationship. Activation energy of sewage sludge and plasticsludge are22.23kJ/mol、21.97kJ/mol respectively.The sludge stickiness experiments were conducted by NDJ Series Digital Viscometer.The apparent viscosity of different kinds of sludge and different moisture contents andtemperatures of sludge were studied. The effects of stickiness of sludge on sludge drying werestudied in different conditions too. The results show that sludge apparent viscosity increaseswith decrease of sludge moisture content and the relationship accords with Exponential Model.On the other hand, sludge apparent viscosity decreases with increase of sludge averagetemperature, and its relationship accords with Quadratic Model. The apparent viscosity ofsewge sludge is more than dyeing sludge’s probably because of the high content of organicmatter in sewge sludge, The sludge moisture diffusivity increases with moisture content ortemperature, which they accord with Exponential Model.Sludge characteristics are studied for inconstant temperature desiccation and the pyrolysis and combustion characteristics of sludge by thermogravimetric analysis indifferent heating rate and atmosphere conditions. The sludge apparent kinetic models andkinetic parameters in different stages were fitted and calculated. The results showed thatthermogravimetric process of sludge under the condition of constant heating rate can bedivided five stages. Sewage sludge turns into fast weight loss stage after temperature rising to100℃. Thermogravimetric process has a special stage including both evaporation of waterand effusion of volatile. When the process generates combustion reaction, the evaporation ofwater is in few state. The weight loss process of wet sludge can be divided into drying stage,flammable stage and difficult-flammable stage. Numerations of kinetic parameters show thatmechanism function of phase interfacial reaction suit for drying stage, and its function isg(a)=1-(1-a)n. Heat in drying process and activation energy and pre-exponential factor alldecrease versus heating rate increases, but reaction order increases. Chemical kineticsequation is suited to pyrolysis and combustion reaction of sludge, and the equation isf(a)=(1-a)n. The results show that activation energy of flammable stage is bigger thandifficult-flammable stage’s. The weight loss rate of flammable stage is mainly controlled bychemical reaction factor, and difficult-flammable stage is by diffusion reaction factor. Theweight loss rate of combustion stage is mainly influenced by heating rate and atmosphereconditions. Combustibility and combustion properties of sludge can be improved with suiteoxygen concentration. On the whole, the weight loss rate is gentler and smaller in purepyrolysis process than which in combustion process.The scheme that using flue gas waste heat of boiler to dry sludge in fluidized bed dryingtower and using paddle dryer-fluidized bed incinerator system were proposed and calculatedwith heat balance theory. The results of first scheme show that quantity of sludge treatmentcan be improved when increasing the inlet gas temperature or flow rate or outlet water contentof sludge. The inlet gas temperature has a higher influence on quantity of sludge treatmentunder conditions of higher outlet moisture content of sludge or inlet gas flow rate. The outletmoisture content of sludge or inlet gas flow rate has a higher influence on quantity of sludgetreatment under conditions of higher inlet gas temperature.Under the condition of the systemand material parameters of scheme2, the results show that the incinerator temperature cankeep in above850℃only when paddle dryer dry the sludge less than moisture content of35% and heat efficiency of fluidized bed incinerator is75%, or which need to add the assisted hotoil to ensure the provide of heat.