Ionic liquid-catalyzed hydrothermal carbonization of sewage sludge: Effect of residence time and liquid phase circulation on hydrochar characteristic

The ionic liquid (IL, [DMAPA]HSO4) was used to catalyze hydrothermal carbonization (HTC) of sewage sludge (SSL), the study discussed how residence time and liquid-phase circulation influence the characteristics of the hydrochar. The decarboxylation process in IL-catalyzed HTC led to lower hydrochar yield but higher higher heating value (HHV). And the secondary hydrochar formation promoted by IL favored the system for high energy feedback (EF), but iron was involved in the secondary hydrochar formation. At the optimum hydrothermal time (90 min), the EF reached 124 % and the removal efficiencies of Fe, Mg, Mn, and Zn were 66 %, 73 %, 96 %, and 90 %, respectively. Functional groups on organic acids (COO–), amides (CO), and polysaccharides (C–O–C) in the SSL were sequentially removed during the IL-catalyzed HTC process, while cross-linking and graphitization occurred, resulting in higher ignition temperatures and lower metal contents. However, multiple cycles of the liquid phase resulted in a decrease in the catalytic properties of IL, resulting in low EF hydrochar with high metal content and functional group intensity. Catalytic graphitization is the key to obtaining hydrochar with high HHV and low metal content. Therefore, future development of catalysts is needed to facilitate the dehydration, decarboxylation and graphitization process of HTC. •IL-catalyzed HTC of SSL achieves high-energy feedback while removing metals.•The more funtional groups there are, the higher the metal content of the hydrochar.•IL-catalyzed deoxygenation and denitrification leads to graphitization process.•Graphitization increases ignition temperature and reduce metal content.•Liquid phase circulation verifies the role of hydrochar structures. [Display omitted]