The primary aim of this study was to investigate the hydrothermal carbonization (HTC) of lignin-rich residue (LRR) obtained after enzymatic hydrolysis of forest residue biomass (FRB). Biomass was pretreated using a two-step hydrothermal pretreatment process under process conditions optimized in our previous study, followed by hydrolysis using commercial cellulase enzymes. LRR was obtained by filtering the slurry at the end of hydrolysis and used for the HTC experiments. The HTC was conducted at temperatures ranging from 200°C to 280°C, with a fixed reaction time of one hour. Comprehensive characterization techniques, including proximate and ultimate analysis, were utilized to evaluate the chemical composition of the samples. Additionally, structural, chemical, and thermal properties of the LRR-derived hydrochar were examined using scanning electron microscopy (SEM), nitrogen adsorption for porosity assessment, and Fourier transform infrared spectroscopy (FTIR). Non-isothermal thermogravimetric analysis (TGA) was employed to investigate the pyrolysis properties of the HTC products. The findings revealed a decrease in hydrochar mass yield with increasing HTC temperature. Notably, the high heating value (HHV) of the hydrochar produced at 280°C was 1.75 times higher than that of the raw LRR. Van Krevelen plot analysis highlighted the efficacy of HTC in accelerating the coalification process of LRR, particularly at elevated temperatures, resulting in a product comparable to more mature coal types. The progressive increase in carbon content, coupled with reductions in oxygen and hydrogen ratios, demonstrated the successful transformation of LRR into a higher-energy material with enhanced fuel potential. The study also revealed that the porosity and specific surface area of hydrochar initially increased with temperature, peaking at 200°C, before declining at higher temperatures. The findings provide valuable insights into the potential of converting lignin-rich residues into high-energy materials, paving the way for broader applications of these renewable resources in energy and material science.
