The sustainable conversion of agricultural waste biomass, particularly crop residues such as corn stover, into high-value products is vital for reducing their open-field burning and mitigating environmental hazards. The hydrothermal carbonization (HTC) process integrated with natural deep eutectic solvents (NADES) presents an alternative approach for valorizing biomass into lignin-rich solid fuels and fermentable sugars for bioethanol production. In this study, corn stover was subjected to HTC using deionized (DI) water, a xylose-based NADES (ChCl:Xy:W), and an oxalic acid-based NADES (ChCl:OA:W) in a 150–300 °C temperature range to optimize both solid fuel and sugar stream yields. Characterization, including fiber analysis, SEM, FTIR, EDS, and bomb calorimetry, was conducted to evaluate structural, compositional, and energetic transformations. The results explored the HTC process, restructuring the biomass, promoting extensive hemicellulose solubilization and cellulose depolymerization, as well as substantially enriching lignin and polymerized compounds with increasing temperature. In addition, the DI water at 300 °C generated a lignin-rich residue, the Xy-based NADES effectively removed ash and extractives, and the OA-based NADES produced the most carbon-dense hydrochar with the highest calorific value. Collectively, these findings demonstrate that solvent-assisted HTC may be employed as a possible strategy for the valorization of agricultural residues into high-energy solid fuels.

At present, many secondary agricultural residues are landfilled or burned in the open field, leading to the loss of the valuable nutrients they contain. The valorization of agricultural biomass provides a promising route for resource recovery and circular bioeconomy development, allowing for symmetry in their reuse. Hydrothermal carbonization (HTC) offers a sustainable method for converting agricultural waste to both solid and liquid carbon-containing products. Since the liquid product is highly dilute, it is a challenge to apply it at large scales. In this work, a low-energy separation process, direct contact membrane distillation (DCMD), has been used to concentrate the liquid product from HTC of rice husks while recovering water from it. The effect of feed temperature on DCMD performance in terms of water recovery percentage and concentration factors for the fermentable sugars xylose and glucose was investigated at feed temperatures of 50 °C, 65 °C, and 80 °C. DCMD conducted at a feed temperature of 80 °C yielded the highest value for average water flux at 3.4 L/m²·h. DCMD at 80 °C also showed the highest water recovery (56.9%) compared to the lower temperatures, and concentrated xylose and glucose by 2.06 and 3.20 times, respectively³. Incremental flux measurements revealed evidence of limiting mechanisms, such as temperature and concentration polarization, with prolonged operation time. The results from this study thus establish an integrated HTC–DCMD system for efficient water recovery and the concentration of HTC liquor, thereby enhancing its potential for commercial use.