Sustainable synthesis of energy-dense hydrochar from food waste blends via hydrothermal carbonisation : process optimisation and characterisation using response surface methodology

Abstract

Hydrothermal carbonisation (HTC) was used to convert food waste (FW), comprising butternut waste (BW) and potato peels (PP), into hydrochar (HC). A central composite design (CCD) with response surface methodology (RSM) was used to investigate the effects of HTC temperature (140–300 °C), residence time (22–248 min), solid-to-liquid (S/L) ratio (1:11–1:15), and BW/PP ratio on HC yield, surface area (SA), and higher heating value (HHV). The quadratic model well-described the yield, SA, and HHV responses. The RSM-CCD maximum yield (40.22 %), SA (7.86 m2/g), and HHV (29.351 MJ/kg) was achieved at temperature of 204 °C, residence time of 131.39 min, S/L ratio of 1:12.89, and BW/PP ratio (0.5:0.5). The predicted responses were closer to experimental runs, proving good model reproducibility. Notably, temperature had the most significant influence on SA and HHV, except for yield, which depends greatly on the FW ratio. This is because at higher temperatures, volatiles are more effectively driven off, concentrating the carbon-rich components that enhance both SA and HHV. At the same time, the yield is more dependent upon the FW ratio due to variations in feedstock composition. Elemental analysis showed lower H/C and O/C ratios at elevated temperatures, indicating improved carbonisation typically leads to a higher HHV and improved fuel quality. SEM and FTIR confirmed the flask-like structures and functional groups, which can play a significant role in the adsorption capabilities of HC for pollutant removal. The valorisation of FW into adsorbents and biofuels represents a dual breakthrough, transforming waste management practices while simultaneously reducing greenhouse gas emissions.

HIGHLIGHTS • Hydrothermal carbonisation of butternut and potato peel waste produces energy-dense hydrochar. • Response surface methodology optimised temperature, time, S/L ratio, and feedstock blend for yield, HHV, and surface area. • Optimised hydrochar at 220 °C, 113 min, 1:13 S/L ratio, and equal butternut and potato peel blend shows high HHV, surface area, and thermal stability.