Effect of Acrylic Acid Concentration on the Hydrothermal Carbonization of Stevia rebaudiana Biomass and Resulting Hydrochar Properties

In this study, Stevia rebaudiana biomass was hydrothermally carbonized (HTC) at 215 °C for 60 min with acrylic acid (AA) as a catalyst at concentrations of 0.25, 0.50, and 1.00 mol L−1. The maximum hydrochar yield (48.5%) was obtained at 0.25 mol L−1 AA, while fixed carbon contents ranged from 20.79% to 34.27%. Higher heating values (HHV) varied between 26.95 and 36.61 MJ kg−1, with the highest catalytic HHV (32.20 MJ kg−1) achieved at 1.00 mol L−1 AA (HC15). Acrylic acid addition significantly promoted deoxygenation, reducing the O/C ratio from 0.67 in raw biomass to 0.21, thereby improving fuel quality. FT-IR and XRD analyses indicated enhanced aromatization and partial graphitization with increasing acid concentration, while SEM images revealed carbon microspheres and porous morphologies. Thermogravimetric analysis showed that HC15 exhibited the lowest mass loss and highest residual carbon, indicating superior thermal stability. GC-MS analysis demonstrated that acrylic acid markedly increased phenolic derivatives, with phenol content rising from 19.47% (without catalyst) to 40.92% (1.00 mol L−1 AA). The aqueous phase contained TOC values of 14,280–28,728 mg/L and COD values of 43,227–113,920 mg/L. Overall, acrylic acid-assisted HTC enhances both the energy-related properties of hydrochars and the chemical diversity of liquid products, providing a sustainable route for valorizing Stevia rebaudiana waste into value-added fuels and chemicals.