Hydrothermal carbonization for extracting lignin from agricultural biomass rice husks and cotton gin trash

Rice husks (RH) and cotton gin trash (CGT), two abundant agricultural residues, were subjected to hydrothermal carbonization (HTC) to enhance lignin recovery. HTC was conducted at 220 °C, 250 °C, and 280 °C for reaction times ranging from 10 min to 6 h. The resulting hydrochars were characterized using higher heating value (HHV) measurements, Fourier-transform infrared (FTIR) spectroscopy, fiber analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and energy dispersive spectroscopy (EDS). The results showed that increasing HTC temperature reduced lignin yield while improving its properties. The HHV of untreated RH and CGT were 14.13 MJ/kg and 14.59 MJ/kg, respectively, increasing by 43 % (RH at 280 °C) and 28 % (CGT at 250 °C) after HTC. Fiber analysis showed increasing acid insoluble lignin (AIL) concentration with temperature, whereas acid soluble lignin (ASL) decreased. Sugar content declined significantly (RH: 2948 ppm to 140 ppm; CGT: 930 ppm to 244 ppm), while the lignin-to-cellulose ratio increased (RH: 0.48 to 19.00; CGT: 0.39 to 0.75). Additionally, HTC led to a reduction in RH crystallinity index (0.460 to 0.445), increased thermal decomposition temperatures (RH: 364.63 °C to 437.14 °C; CGT: 340.70 °C to 451.94 °C), and elevated carbon content (RH: 25.85 % to 48.14 %; CGT: 50.98 % to 67.57 %). These results demonstrate that hydrochar produced at elevated HTC temperatures exhibits enhanced thermal stability, carbon enrichment, and improved lignin recovery, making it a promising material for energy and bioproduct applications.