Waste to Energy: Hydrothermal Carbonization of Agro-Waste for the Production of High Valuable Solid Biofuel

In the present study, hydrothermal carbonization (HTC) of banana peel waste, showing a moisture content of 85 wt%, was carried out in a 500 mL stirred stainless steel autoclave at a temperature ranging between 180 and 250 °C, for a residence time of 0.5 h. The reaction solid residue was recovered via vacuum filtration and washed with distilled water before drying it in a ventilated oven at 105 °C, until constant weight, and stored in 50 mL plastic vials, before analytical characterization. Banana hydrochars were characterized in terms of proximate, elemental analysis, FTIR and high calorific value (HHV). Hydrochar mass yield showed a maximum at 220 °C (32 wt%), while gas and liquid mass yields increased monotonically with reaction temperature, up to 70 and 2 wt%, respectively. HHV increased with reaction temperature up to 27.13 MJ/kg, corresponding to an energy densification ratio EDR of about 169%. Mass yield and energy properties of banana hydrochar prove that a HTC treatment carried out at 250 °C for 0.5 h is able to produce a valuable solid biofuel showing the characteristics of lignite.