HYDROTHERMAL CARBONIZATION OF CITRUS WASTES: HYDROCHARS PROPERTIES AND HYDROGEN PRODUCTION VIA PHOTOCATALYTIC REFORMING OF PROCESS WATER

This study investigates hydrothermal carbonization (HTC) of citrus wastes to obtain high energy dense hydrochars to be valorized as solid biofuels while producing hydrogen via photocatalytic reforming of HTC process water (PW). The study examines HTC of lemon (LP) and grapefruit (GP) peel waste at temperature between 180 and 260 °C, residence time of 0.5 h and solid/biomass ratio of 10%. LP and GP hydrochars are characterized in terms of mass yields, energy and chemical properties via proximate and elemental analysis, surface morphology via FTIR ATR, calorific values, combustion properties via thermogravimetry analysis. Results show that hydrochars high calorific values (HHVs) increase up to about 26.3 and 26.5 MJ/kg for LP and GP hydrochars at 260 °C, respectively. HTC process waters (PWs) are characterized in terms of pH, total organic carbon (TOC), chemical composition via HPLC. Photocatalytic reforming of LP and GP PWs, using Pt-TiO2 under UV light, shows the production of H2 with a concentration ranging between 0.13 and 0.30 mM depending on the starting material and HTC temperature. The present study demonstrates that HTC could lead to a complete valorization, in terms of renewable energy sources, of process residues ensuring economic circularity and sustainability of the citrus juice industry.