Hydrochar Derived from Zinc Chloride Aided Hydrothermal Co-carbonization of Sewage Sludge and Coconut Shells (Cocos nucifera)

Abstract

The hydrothermal co-carbonization (co-HTC) process is a promising method for improving the fuel properties of hydrochar. This process leverages synergistic interactions, facilitated by the Maillard reaction, which is believed to be catalyzed under acidic conditions. In this study, the co-HTC of sewage sludge (SS) and coconut shells, with ZnCl₂ as a catalyst, was investigated to evaluate its effectiveness in enhancing the co-HTC process. Response surface methodology was employed to optimize key parameters, including the raw material ratio, temperature and reaction time. The optimized parameters were subsequently applied to the co-HTC process. The raw materials and resulting hydrochars were characterized using proximate and ultimate analyses, higher heating value (HHV) determination, Fourier transform infrared spectroscopy and thermogravimetric analysis. The results revealed that the fuel ratio of raw SS increased significantly, from 0.09 in untreated SS to 0.26 in co-HTC, and further to 0.41 in ZnCl₂-aided co-HTC. The carbon content of the hydrochar increased, accompanied by a notable reduction in the H/C and O/C atomic ratios. The HHV improved substantially from 5.8 MJ/kg for raw SS to 11.1 MJ/kg in co-HTC and 14.0 MJ/kg in ZnCl₂-aided co-HTC. The combustion characteristic index demonstrated superior combustion performance for the ZnCl₂-aided co-HTC process, achieving a value of 33 × 10⁻⁷ min⁻² °C⁻³. Additionally, the synergistic effects on HHV were significantly enhanced, with the synergistic coefficient increasing from 0.79% in co-HTC to 27.00% in ZnCl₂-aided co-HTC. Overall, ZnCl₂ effectively catalyzes the co-HTC process, enabling the production of higher-quality solid fuels.