Evolution characteristics and mechanism of carbon structure during co-hydrothermal carbonization of sewage sludge and sub-bituminous coal

Abstract

Co-hydrothermal carbonization (Co-HTC) is a promising method for the conversion of organic solid waste with high moisture content into high-value-added products (hydrochar). Herein, the Co-HTC of sewage sludge (SS) and sub-bituminous coal (SC) was studied, and the structural evolution at different blending ratios and hydrothermal temperatures during Co-HTC was further investigated. The physicochemical structure of the hydrochar was characterized using SEM-EDS, Raman spectroscopy, XPS, FTIR, and 13C NMR. The results indicated that with the blending ratio of SC increasing, the oxygen-containing functional groups and aliphatic content of co-hydrochar decreased, whereas the aromatic structure content and apparent aromaticity increased. Additionally, the numbers of aliphatic branches and aromatic substituents decreased during Co-HTC. Furthermore, the interaction between the SS and SC promoted the expansion of aromatics and removed the oxygen-containing functional groups from the co-hydrochars during Co-HTC. In addition, a potential mechanism for the interaction of SS and SC on the hydrochar carbon structure during Co-HTC was proposed. The small aromatic rings could be linked to other rings by bridges of short-chain aliphatics in SS, which may be the key step in the interaction. The results may contribute to the better understanding of the structural characteristics of hydrochar from SS and SC, and provide a theoretical basis for the resource utilization of solid waste.