Hydrothermal carbonization and photocatalytic reforming as a two-step combined strategy for the complete valorization of citrus waste

In the present study, the combined use of hydrothermal carbonization (HTC) technology and photocatalytic reforming is proposed as a promising strategy for the generation of energy-dense solid biofuel and green hydrogen (H2), starting from lemon (LP) and grapefruit (GP) peel waste. For the aim, HTC of LP and GP was carried out at temperatures between 180 and 260 ◦C, fixed reaction time of 0.5 h, and solid to liquid mass ratio of 10%. LP and GP hydrochars, recovered after HTC, were characterized in terms of mass yield, proximate and elemental composition, surface chemistry via FTIR-ATR, calorific values, and combustion properties via thermogravimetric analysis. HTC aqueous liquid residues (PWs) were characterized in terms of pH, total organic carbon (TOC) content, and chemical composition via HPLC. Hydrochars showed an increasing coalification character with process temperature (decreasing H/C and O/C atomic ratios) and a higher heating value (HHV) increase up to 26.3 and 26.6 MJ/kg, for LP and GP samples produced at 260 ◦C. Photocatalytic reforming of LP and GP PWs, using Pt-TiO2 under UV light, enabled the production of H2 with a concentration ranging between 0.13 and 0.30 mM, depending on the starting material and HTC process temperature. This work demonstrates that HTC of citrus residues and photocatalytic reforming of process waters by-products could ensure the complete valorization of typical lignocellulosic waste biomass, serving as a proof of concept for valuable biofuels generation and agro-industrial sustainability.