Hydrothermal carbonization of kitchen wastes: Physicochemical properties and combustion performance of hydrochars

Hydrothermal carbonization (HTC) is considered a promising method for treating kitchen wastes (KWs), and hydrochars are a relatively good energy fuel. Therefore, this work investigated the physicochemical properties and combustion performance of hydrochars obtained from HTC of KW under different conditions. The results showed that dehydration and decarboxylation are the key steps during the HTC process, while demethylation had little effect. Fourier transform-infrared spectroscopy and x-ray photoelectron spectroscopy results showed that increasing temperature and prolonged residence time could promote dehydration, decarboxylation, demethylation, and deamination of KW, and the liquid–solid ratio had little effect on surface functional groups. Higher HTC temperature promoted the movement of thermogravimetric and differential thermogravimetric curves to the right, and reduced the composite combustion index of hydrochars. Kinetic analysis showed that with increase in temperature, the activation energy of hydrochars in the three stages gradually decreased from 20.95, −0.17, and 30.35 to 16.38, −3.99, and 27.19 kJ/mol, respectively, and there may be exothermic reactions in stage II.