Sustainable solid fuel from coffee agro-industry residues: role of mild hydrothermal duration in enhancing the fuel properties

The increasing volume of agricultural waste, such as coffee pulp and coffee parchment, has driven the development of biomass conversion technology into a renewable solid fuel. One promising approach was the Mild Hydrothermal Carbonization (MHTC) process, which was applied to improve the fuel characteristics of biomass through thermochemical modification. The main objective of this study was to produce and characterize bio-pellet products from coffee pulp and coffee parchment residues, as well as to evaluate the influence of residence time variation (30, 60, and 90 min) during mild hydrothermal processing on the final quality of the bio-pellets. The mild hydrothermal process was carried out at 130 °C using a 1.8 M acetic acid solution with a biomass-to-solution ratio of 1:3 (b/v). The investigated parameters included mass balance, particle size distribution, proximate analysis, apparent density, calorific value, burning rate, as well as heating rate and mass change during heating. The results showed that increasing residence time was followed by a decrease in solid mass balance due to the dissolution of the hemicellulose fraction and organic matter. The particle size was made more homogeneous, while moisture and ash contents were reduced. In contrast, fixed carbon and calorific value were increased. The burning rate of the bio-pellets ranged between 0.48 and 1.13 g/min. The best bio-pellet product was obtained from coffee pulp hydrothermally treated for 90 min, where the calorific value was measured at 5130 cal/g, fixed carbon at 21.79%, and burning rate at 0.48 g/min. It was concluded that residence time up to 90 min significantly improved the bio-pellet quality, while proving that this fuel was clean, efficient, and environmentally friendly.