Bio-coal Synthesis via Hydrothermal Carbonization of Giant Salvinia for a High-Quality Solid Biofuel

The rapid proliferation of giant Salvinia (GS; Salvinia molesta) in various hydrostatic environments, such as ponds and paddy fields, poses a threat to water quality due to light obstruction. Thus, this study aimed to transform GS biomass into hydrochar or solid biofuel via hydrothermal carbonization (HTC). Several parameters were examined, including residence time, reaction temperature, and liquid-to-solid mass ratio (L/S). The Box-Behnken Design (BBD) was also employed to set the experimental conditions at three levels and factors. The examinations of reaction temperature (200–220 °C), residence time (2–6 h), and L/S ratio (12–20) were conducted. The physical and chemical characteristics of hydrochar were further analyzed to encompass higher heating value (HHV), proximate analysis, ultimate analysis, functional group, and morphology. The percent energy recovery (ER, %) was remarked for the experimental design response. The kinetic analysis and a comprehensive combustibility index, calculated from TGA/DTG curves, were employed to elucidate the combustion behavior of hydrochar. The optimal condition for hydrochar production, resulting in maximal ER, was identified at 220 °C, 6 h, with an L/S ratio of 16. The corresponding fixed carbon (FC), HHV, and mass yield were approximately 17.2%, 23.5 MJ/kg, and 51.4%, respectively. The H/C and O/C mole ratios in the sub-bituminous coal region. This study affirms the feasibility of converting GS biomass into a renewable fuel resembling low-rank coal.