Hydrothermal Liquefaction of Coffee Silverskin and Spent Coffee Grounds: Bioenergy and Biochemical Potential

Coffee roasting by-products represent a significant, underutilized side-stream globally. This study investigates hydrothermal liquefaction (HTL) as a method to convert these materials into hydrochar, water-rich light oil, and heavy oil. Using HTL at 300 °C for 60 min, we evaluated the energy content and properties of the resulting hydrochars, finding energy values exceeding 33 MJ/kg—significantly higher than the 19–21 MJ/kg of the raw materials. Hydrothermal liquefaction of spent coffee grounds produced more hydrochar (18 g) and heavy oil (1.2 g) than silverskin (12–14 g hydrochar and 0.1–0.5 g heavy oil). In contrast, silverskin generated twice as much light oil (9.7 g) as spent coffee grounds (4.6 g). Silverskin hydrochars exhibited higher gross calorific value (Baqué 33.95 ± 0.06 MJ/kg, Mariposa 33.86 ± 0.07 MJ/kg, Meira 33.22 ± 0.00 MJ/kg), lower ash content (3–5%), and reduced volatile matter (57–61%) than their raw form. Spent coffee grounds produced hydrochar with the highest gross calorific value (34.27 ± 0.01 MJ/kg), lowest ash content (0.8%) and the most significant reduction in volatile matter. Light and heavy oils produced were rich in alkaloids, fatty acids, and phenolic compounds, with potential applications in cosmetics and pharmaceuticals. This work contributes to both bioenergy production and circular economy strategies, valorising the two main side-streams of the coffee industry. With broad implications for sustainable waste management, this study highlights the potential of HTL to advance global bioenergy goals.