Impact of Cooling Phases on Hydrothermal Carbonization of Spent Coffee Grounds: A Study on Hydrochar and Process Water Properties

This work contributes to the expanding body of research aimed at refining HTC processes for the development of eco-friendly materials from organic residues. Specifically, the influence of cooling duration on the HTC of spent coffee grounds is examined, in terms of process yield and quality of the resulting products. HTC experiments were conducted at temperatures ranging from 180°C to 220°C, with a one-hour reaction time. Cooling was performed either naturally, until room temperature was reached, or using a forced method via a chiller. The produced hydrochar was analyzed using elemental and thermogravimetric analysis, surface area measurements, and calorific value determination. Differences in cooling times were examined to determine their effect on material quality. Additionally, the process water was analyzed for environmental impact using bioindicator species, such as Cress seeds, Daphnia magna and nitrifying bacteria. The results demonstrate that cooling duration significantly influences both the physical and chemical properties of hydrochar. Shorter cooling times yielded denser hydrochar with higher energy content, while extended cooling enhanced solid yield and surface functionality. Furthermore, variations in cooling time affected the composition of process water, highlighting potential environmental concerns and opportunities for its reuse or treatment. These findings suggest that optimizing the cooling phase in HTC can improve both product quality and process efficiency, offering valuable insights for the sustainable valorization of spent coffee grounds in the context of circular economy practices..