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This work addresses the management of Brewers' Spent Grain (BSG) using a state-of-the-art, microwave-assisted, hydrothermal carbonization (MA-HTC) process, for the production of hydrochar, i.e., a renewable solid biomaterial with many industrial applications. For the first time, a detailed relationship has been established between the processing conditions and the properties of the hydrochar via a thorough physicochemical characterization. The experimental results revealed that the temperature (180−250 °C) and reaction time (0−2 h) used in the MA-HTC process exerted a significant influence on the yield and properties of the hydrochar. An increase in these variables (process severity) diminished the hydrochar yield. However, such increments were beneficial to enhance the fuel properties of this product, as the proportions of O and C decreased and increased, respectively. As a result, this process was capable of transforming up to 47% of the original BSG into a high-energy hydrochar with a calorific value of 32 MJ/kg. The characterization of the hydrochar revelated that it was a mesoporous, hydrophilic, rough material, containing several cavities and oxygen functionalities on the surface. These features not only provide the hydrochar with a high aromatic character but also are fundamental for its potential applicability as a bioadsorbent material. An increase in the MA-HTC severity augmented the amounts of aliphatic and aromatic structures in the hydrochar as well as the roughness and the presence of cavities, thus highlighting the excellent flexibility of the process. Therefore, these promising results, together with the energy-efficient and bespoke nature of the MA-HTC process, which substantially reduces the reaction temperature and processing times in comparison to standard carbonization procedures reported to date, represent a step-change not only for the production of biofuels and biomaterials but also for the management of BSG.

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