Study On Efficient Sludge Drving Method And Optimal Energy Efficiency Of Sludge Drying-Incineration Combined System

This thesis presents a study on efficient sludge drying and optimal energy efficient of sludge drying-incineration system. To achieve these purposes, a sludge shear test method and apparatus was built; a bench scale sludge drying test system with paddle dryer, disc dryer and diagonal paddle dryer was developed; sludge drying model and energy balance model of sludge drying-incineration system was established.The contributions of organic matters, inorganic minerals with fine particle size and crystalline metal salts to the sludge adhesion and cohesion were discussed, based on the detailed measurement of sludge contents and related physical properties. The adhesion and cohesion characteristics of the sludge were studied by using the shear test apparatus. This test method for sludge cohesion characteristic is reported for the time. Results show that the sludge sample exhibit significant adhesiveness and cohesiveness in the moisture range of45%-65%, and the moisture range varies with different sludge samples. The cohesion forces of sludge are greater than the adhesion force during the whole process of sludge drying. This indicating that when the mechanical force applied to the sludge is large enough, the sludge will firstly stripped away from the heating surface; a further increasing of the mechanical force is needed to break the sludge bulk into pieces. The temperature of heating surface and additives can affect the sludge adhesion and adhesion characteristics.The drying characteristics of sludge samples were studied by using bench scale paddle dryer. A significant fluctuation period between the first and second falling rate period was observed. It is found that the moisture range of fluctuation period is overlap with that when adhesion and cohesion force shows peak, after the comparison of sludge drying characteristics and adhesion/cohesion characteristics. The adhesiveness and cohesiveness of sludge is the basic reason for fluctuation period, and the stirring of paddles is the direct cause of fluctuation period. The fluctuation period is a direct reflection of sludge drying sticky phase, and the stirring of paddles helps to overcoming the negative impact of sticky phase. It is easier for the stirring paddles to strip the sludge away from the heating surface than to break the sludge bulk into pieces, because the cohesiveness of sludge is larger than its adhesiveness. Therefore, it is more important to break the sludge into pieces for the purpose of improving sludge drying rate. The impact of stirring intensity is significant in the second falling rate period. Rise the temperature of the heating source can not only improve the sludge drying rate by increase heat transfer, but also reduce the negative impact of the sludge sticky phase. Lime as additive can increase sludge drying rate in fluctuation period and second falling rate period. Tryptone as a kind of organics can largely improve the adhesiveness and cohesiveness of sludge. Silica as additive can reduce the adhesiveness and cohesiveness of sludge in higher moisture content, but increase the adhesiveness and cohesiveness at lower moisture content. Quartz sand as additive can effectively reduce the cohesiveness of sludge, make the broken of sludge more easily.In order to compare the differences between the disc dryer and paddle dryer, a bench scale disc dryer was established. The drying characteristics of sludge samples in bench scale disc dryer were studied and compared with the results of paddle dryer. In the case of using heating oil as the heat source, the sludge drying rate is higher in the disc dryer. Due to simple setup of oil pipes in disc dryer, the flow of heating oil is more efficient, and makes it maintaining a higher temperature, thus benefits the drying of sludge. When it comes to sticky phase, the sludge adheres on the disc surface and stick together to form a plastic sludge layer. The rotation of disc could barely extruding, shear and break the sludge layer, and play no role in maintaining the drying rate. In the real application of continuous feeding disc dryer, the sludge would probably sticky seriously, difficult to move towards the discharge, drying ability of sludge dryer may decline.On account of the positive effects of paddle sludge dryer on overcoming the negative impact of sticky phase, a modified bench scale diagonal paddle dryer was developed. The drying characteristics of sludge samples in this modified dryer were studied and compared with the results of traditional paddle dryer. The sludge drying rate and stirring effect of diagonal paddle dryer is effectively improved in the moisture range between72%and83%. When the sludge drying comes to sticky phase, the diagonal paddles could embed in the sludge bulk and form ravines, at a followed moisture point, diagonal paddles can largely improve the striping and breaking of sludge bulk and improve the drying rate significantly.A drying model for bench scale sludge dryer, which is more consistent with actual drying process, was established based on the fundamental of penetration model. A thermal resistance of stickiness was introduced to the model at corresponding region, based on the responsible moisture range of sticky region. The simulation results can better match the actual drying process. The drying characteristics of sludge in varieties of operation conditions of indirect dryer can be simulated by adjustment of operation parameters in the model.The operation mode of a real sludge drying and incineration project was analyzed, based on the established energy balance model for sludge drying and incineration system. The optimal moisture content of sludge sent to the furnace is determined. The impacts of the variation of sludge nature and operating parameters could be predictable. Through the energy loss analysis of the sludge drying and incineration project, eight kinds of greatest energy loss is determined. Targeted methods for energy recovery and saving can be further studied.Further works are summarized and presented at the last of this thesis.