Thermal treatment on sewage sludge by electromagnetic induction heating: Methodology and drying characterization

Abstract

Thermal drying of sewage sludge is not only an effective way to treat the waste, but also an essential step for further energy utilization. This work focused on drying property of sewage sludge by using electromagnetic induction heating. A novel drying method for sewage sludge was designed. The effects of different electromagnetic-induction media materials, working parameters and conditioning reagents on the efficiency of drying were investigated. Then, the kinetics was analyzed. The change of temperature and heat-transfer was analyzed during the drying process. Experimental results showed that sewage sludge combined with three kinds of induction medias can be efficiently dried by applying electromagnetic induction heating. Fast formation and development of cracks indicated that an increase of drying rates of sludge can be obtained. Considering the release of volatile organic compounds from sludge during drying process, estimated moisture content was used to evaluate the drying effect. A Higher working voltage leaded to a more weight reduction of sludge during a shorter drying time, but a lower voltage prolonged the drying time. It was noted that the estimated moisture rate was very close to the experimental moisture content. Sludge content, forming and induction media significantly affected the drying process. Plate and net media were fitted for thin layer and piled sludge, respectively. However, fiber media seemed to show lower drying rate due to no circuit for induction current. An addition of CaO and sawdust improved the drying process. As a result, few volatile organic compounds released from sludge. For kinetics, three periods (warm-up, constant rate and falling rate period) can be observed and the data fitted linear regression of Lewis drying model very well. The effective moisture diffusivity was influenced by the different induction media and the thickness of sludge. Infrared images showed that outside temperature was higher than central part temperature for all sludge samples. A higher evaporation rate and diffusion of moisture can be obtained from outside part of sludge due to the easy collapse of porous structure.