Hydrothermal carbonization of hemp digestate: influence of operating parameters

In the last decade, great attention has been given to hydrothermal carbonization (HTC) as a suitable process for residual biomass valorization, able to convert organic waste into useful materials or energy carriers. However, the involved conversion reactions that biomass components undergo are influenced by the characteristics of the treated residue along with the HTC process conditions, particularly in terms of temperature and holding time. In this paper, the potential valorization of hemp digestate via HTC was investigated. The study was aimed at evaluating the effect of reaction temperature and holding time on the yield and composition of produced hydrochar and on the process water characteristics. Three temperatures (180, 200, and 220 °C) were investigated along with three holding times (1, 3, and 6 h) and the obtained solid and liquid phases were characterized. Results show that the investigated operating parameters affect both the solid yield and the hydrochar and process water composition. By increasing process severity conditions through an increase in temperature and/or holding time, a drop in solid yield (89–60 wt%) and an increase in carbon content (+ 15– + 30%) and energy content (up to + 22%) were achieved for the produced hydrochar compared to the feedstock, which showed suitable properties in view of its use as a fuel. Process water characteristics suggest a potential valorization in terms of nutrient recovery or biogas production by anaerobic digestion, while the feasibility of an aerobic treatment should be carefully evaluated. Graphical abstract: (Figure presented.)