Effects of Liquid Fraction Recirculation during Microwave-Assisted Hydrothermal Carbonization on Properties of Coffee Parchment

The depletion of fossil fuels and the environmental burden of biomass residues have driven interest in hydrothermal carbonization as a sustainable pathway for energy and material production. This study investigates the valorization of coffee parchment, an underutilized by-product of coffee processing, through microwave-assisted hydrothermal carbonization with liquid fraction recirculation. The primary aim was to evaluate how operating conditions and liquid reuse affect hydrochar properties and process efficiency. Experiments were conducted at temperatures between 70 and 110 °C and residence times from 60 to 180 minutes, using a modified microwave reactor. Hydrochar samples were characterized for surface acidity, functional groups, iodine number, and morphology, while the recirculated liquid fraction was monitored for its influence on subsequent reactions. Results show that total acid density increased with temperature and time, peaking at 3.39 mmol g⁻¹ at 110 °C and 180 minutes, while sulfonic and weak acid densities remained unchanged. Liquid fraction recirculation enhanced surface acidity but reduced iodine number and porosity due to pore blockage from soluble organics. FTIR analysis confirmed enrichment of oxygenated groups, and SEM images revealed cavity formation and pore collapse across recirculation cycles. These findings highlight the potential of liquid fraction recirculation to tailor hydrochar properties for catalytic rather than adsorptive applications, advancing sustainable strategies for coffee waste utilization.