An orthogonal experimental study on solid fuel production from sewage sludge by employing steam explosion

The treatment and disposal of sewage sludge have been one of the most significant environmental problems in China. Production of solid fuel from sewage sludge could be an effective way to work out this problem as it can realize the harmlessness treatment and recycling of these wastes simultaneously. This study proposed a novelty process, including the steam explosion (SE) pretreatment, mechanical dewatering, natural drying and pelleting, to produce solid fuel from sewage sludge. Dewatered sewage sludge with moisture content about 80% and the leaves of Chinese parasol were used as raw materials. A L16(45) orthogonal table was employed to arrange the experiments to investigate the feasibility of the proposed process and optimize the operating parameters such as the ratio of sludge to leaves α and size of leaves S, pressure P and holding time H of the SE process. The range analysis and Analysis of Variance (ANOVA) were carried out to detect the optimal level within the observed range and the predominance of each factor. The thermogravimetric (TG) method was also adopted to characterize the combustion behaviors of these sludge derived fuel (SDF). The results show that no important factor has been omitted from the orthogonal design and the experimental results are consequently reliable. Considering only the SE operating parameters, the holding time H with a percentage contribution φH of 31.75% is the main influencing factor on the dewaterability of the SE treated products. With a holding time of 55s, the moisture of all the solid product is lower than 46.5% after mechanical dewatering with a press pressure of 1.0MPa for 1.5h. Moreover, the dewaterability of the SE products becomes better with the increase of the SE pressure. The optimal parameters to produce solid fuel from sewage sludge and leaves were α=2:1, H=70s, P=2.4MPa, and S=40mm within the range observed in this study. The wet base heating value of these fuels is more than 4000kJ·kg−1, which are capable of self-sustaining combustion. The TG analysis results show that these fuels have a good combustion performance with a low ignition point, narrow range of combustion temperature. The combustion process of these fuels includes three stages, which were evaporation of water, combustion of the volatile matters and fixed carbon respectively. Finally, an energy consumption model considering the main advantage was employed to investigate the economy of the process proposed in this study. All the data demonstrate the proposed process is powerful to control the water content and also an energy-savings method to produce solid fuel from bio-wastes.