A two-stage conversion process for the production of a valuable product from biomass waste, i.e., grape seeds activated carbon (GSAC) was investigated. Such process involved hydrothermal carbonization (HTC) of grape seeds, followed by chemical activation with potassium hydroxide (KOH). Different HTC temperatures (THTC
= 180–250 °C), as well as different KOH:hydrochar ratios (R = 0.25:1–1:1), were explored. The samples that were obtained from both stages of the biomass conversion process were analyzed in terms of textural characterization (apparent total and micro-pore surface areas, total and micro-pore volumes, pore size distribution), proximate and ultimate compositions, thermal stability, surface morphology (via SEM), and surface chemistry characterization (via FTIR). Overall yields of approximately 35% were achieved, which are comparable to those obtained with the state-of-art one-stage process. In a wide range of operating conditions, the higher THTC
and R, the higher was the surface area of the GSAC, which was maximal (above 1000 m2
/g) for THTC
= 250 °C and R = 0.5. At such optimal conditions, around 90% of the total porosity was due to micro-pores. Such a trend was not fulfilled at the most severe operating conditions (THTC
= 250 °C; R = 1), which resulted in larger pore size, causing surface area reduction. A proper selection of the process parameters of both the process stages gives great opportunities of tuning and optimizing the overall process. The produced GSACs showed a remarkable thermal stability, and their surface appeared rather free of functional groups.
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