Valorization of winery waste through hydrothermal carbonization: A sustainable route to solid biofuel production

The winemaking industry generates large quantities of seasonal waste creating significant environmental and management troubles. Hydrothermal carbonization (HTC) is one of the most promising thermochemical processes for converting wet biomass into added-value products for different applications. In this work, grape stalks, a considerable portion of winery waste, were used as starting feedstock for the HTC process to produce a solid biofuel. A 3-liter HTC reactor was used to investigate the effects of reaction temperature (180-250 degrees C) and residence time (1-8 h) on the product characteristics generated during the HTC process. The resulting solid product, hydrochar, was characterized for its proximate and elemental composition, heating value, and combustion behavior. In addition, the process water was evaluated by determining its total organic carbon and total nitrogen content, while the permanent gas was characterized by measuring the fractions of CO2, CO, H2, CH4, and C2-C4 hydrocarbons. The hydrochar obtained from the HTC tests exhibits enhanced physicochemical properties compared to the original feedstock. Under higher severity conditions, the carbon content of hydrochar increased to 64 wt%, representing a 39 % increase over the original raw biomass. This enhancement led to a notable increase in calorific value, rising to 26 MJ/kg (+23 %). However, the yield of hydrochar decreased by 22 wt%. The yield and calorific value influenced the energy yield of hydrochar, which resulted in higher values at lower severity conditions. Therefore, hydrochar produced under these less severe conditions is more beneficial for energy recovery and the economic sustainability of the HTC process.