| Sludge is particularly burdensome for both the natural environment and human health. It is widely emitted from the treatment processes of municipal wastewater, industrial wastewater and urban waterbody sediment. Mechanically dewatered sludge usually contains about 80% of water,10% of organic matter and 10% of ash. Land-filling, agricultural utilization and incineration are the most widely used method for sludge disposal in the world. Drying and incineration has been recognized as the most effective method for sludge volume reduction and energy recovery. However, the drying mechanism of sludge is not well studied in the published documents. Based on above, in current dissertation the drying and pollutants emission mechanisms of sludge is studied:a theoretical system which can be used for the guidance of industrial application is built based on the theoretical analysis and experimental researchs.Chapter one is the background review, mainly including the definitions of sludge, the categories of sludge, the harm of sludge, the available sludge disposal methods, the concept of sludge drying, the types of sludge drying technologies, the research status of sludge drying mechanism, and the research status of pollutants emission during sludge drying and incineration processes, etc. In this chapter, the framework of the full text and the structure of this dissertation are also introduced.Chapter two is the researches on moisture distribution characteristics in sludges. Different methods for determination of moisture distribution in sludge are introduced. Thermal drying method, thermogravimetric-differential thermal analysis method (TG-DTA), thermogravimetric-differential scanning calorimetry method (TG-DSC), and water activity method have been paid a lot of attentions, focusing on their measuring principles and methods. The moisture distribution characteristics of different sludges are studied by these four methods respectively. The differences in moisture distributions of different sludges are compared, as well as the advantages and disadvantages of the four measuring methods.Chapter three is on the sludge drying characteristics in a nara-type paddle dryer. The drying mechanisms of sewage sludge and paper mill sludge are studied based on the laboratory experiments and penetration model. Physical parameters of sludge, including thermal diffusivity, thermal conductivity, specific heat capacity, and particle diameter, are experimentally determined for theoretical calculation. The experimental results are compared with those from theoretical calculation. The effects of drying temperature, stirrer speed and gas flowrate on drying characteristics of sludge are also studied. Furthermore, the effects of additives, including coal, waste oil, CaO, and dried sludge, on sludge drying are studied. The result indicates that the drying rate of sewage sludge increases significantly when CaO content in sludge is higher than 0.5%. The effects of heat-transfer media, including saturated water vapor, thermal oil and flue gas, on sludge drying are also studied. The heat transfer coefficient of flue gas in the hollow shaft of paddle dryer is measured. The effects of flue gas temperature and flowrate on the heat transfer coefficient of flue gas in are studied. The result indicates that when the flowrate increases from 3.5 to 13 m/s, the heat transfer coefficient will increase from 18.6 to 53.0 W/(m2 K). A comparison of overall heat transfer resistances from flue gas heated, thermal oil heated and saturated water vapor heated paddle dryers is presented.Chapter four is on drying characteristics of sludge under partial vacuum condition. The agitated drying mechanism of sewage sludge under partial vacuum condition based on penetration model is presented. The effects of system pressure, drying temperature, stirrer speed, and filling ratio on sludge drying characteristics are experimentally studied. The role of the scrappers in sludge dryer is also discussed. Furthermore, comparisons of drying characteristics and condensate properties between partial vacuum contact drying and atmospheric contact drying are presented.Chapter five is on emission characteristics of volatile compounds during sludge drying process. The drying experiments of sewage sludge and paper mill sludge are conducted on a tubular furnace, and the main components of drying gas are identified to be NH3,CH4,C7H16,CO2 by an infra-red spectrum analyzer. The formation mechanism of these compounds are also analysed. Furthermore, sludge drying experiments are carried out in a nara-type paddle dryer with 2m2 heating area. The drying gas and condensate from the dryer are analysed. The result indicates that condensate from sewage sludge drying is weakly alkaline, while the condensate from paper mill sludge drying is weakly acidic.Chapter six is on emission characteristics of pollutants during sludge combustion in a fluidized bed incinerator. In the first part, the emission characteristics of PAHs and dioxins, and the effects of coal and CaO on PAHs and dioxins emissions during sewage sludge combustion are studied, are also studied. In the second part, pollutants emission during incineration of Dianchi sludge in a 0.5 MW pilot-scale fluidized bed are studied. The result indicates that the heavy metal, dioxins and nitrogen oxide concentration in flue gas can meet the national standards.Chapter seven is on computer-aided sludge drying system and sludge drying incineration system design. The drying softwares available in the market are introduced. The method for sludge batch drying transferring to sludge continuous drying is presented. The process model of sludge drying and visualizing window are developed based on Excel spreadsheet software. Based on numerical simulation of spreadsheet software, it is found that energy utilization ratio of sludge drying system will increase from 69.8% to 83.3%, when the waste heat in drying gas is utilized. The process model of sludge drying and incineration system and visualizing are also developed based on Excel spreadsheet software. Optimizational calculation of the drying and incineration system is carried out based on the sewage sludge from Hangzhou city. The result indicates that, during incineration process, no auxiliary fuel is needed when the moisture content of sludge incinerated is not more than 60%; and that the sludge drying and incineration system consumes minimal auxiliary fuel when moisture content of sludge is 60%.Chapter eight is on a 60 t/d sludge drying and incineration project built in Wenzhou city. The project is composed of a sludge drying system and a sludge incineration system. Saturate water vapor produced from sludge incineration is supplied for sludge drying, and sludge is dried to a water content of 60% before being incinerated. The sludge drying and incineration system design is carried out based on Excel Spreadsheet software. The result indicates that the sludge drying and incineration system consumes minimal auxiliary fuel (2.32 t coal/d) when moisture content of sludge is 60%.Chapter nine mainly includes the conclusions of full text, the innovative point of this dissertation, and the prospect of future work.
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