A critical review on preparation, property prediction and application of sludge co-hydrothermal carbonization hydrochar as solid fuel

Abstract

This paper reviews the preparation of sludge-based hydrochar through co-hydrothermal carbonization (co-HTC) and its application as a solid fuel. Sewage sludge (SS), a byproduct of wastewater biological treatment, possesses potential for energy utilization; however, hydrochar derived directly from hydrothermal carbonization (HTC) exhibits suboptimal fuel properties. Co-HTC, by processing SS with biomass rich in organic matter, significantly enhances the combustion performance of hydrochar. The study provides a detailed overview of the carbonization mechanisms and the interactions among components in the co-HTC. It evaluates the performance of co-hydrochar as a solid fuel from two perspectives: physicochemical properties and combustion characteristics. Special attention is given to the effects of biomass blending materials, mixing ratios, and hydrothermal conditions on the properties of hydrochar. The findings indicate that the elemental and organic composition of blending materials directly influence hydrochar quality, and optimizing material ratios and hydrothermal conditions can improve hydrochar properties to meet solid fuel requirements. Furthermore, the integration of machine learning is proposed to predict hydrochar properties based on feedstock composition and hydrothermal conditions. The review discusses future directions for SS co-HTC technology, aiming to provide theoretical foundations and technical support for SS resource utilization and hydrochar property prediction.