Hydrothermal carbonization of waste Rosa roxburghii Tratt pomace into solid fuel: mechanisms, combustion characteristics and thermodynamic studies

Rosa roxburghii Tratt (RRT) pomace, a major byproduct of juice processing, exhibits a high specific surface area and roughness, demonstrating significant potential for biomass energy production. Improper disposal may lead to environmental pollution and resource waste; thus, high-value utilization offers both economic and environmental benefits. This study employed hydrothermal carbonization (HTC) to convert RRT pomace into highly stable biomass fuel, aiming to elucidate the reaction mechanism and identify the optimal process conditions. Proximate and ultimate analyses revealed that, with increasing temperature and duration, the hydrochar yield and volatile matter content decreased, while the fixed carbon content and calorific value significantly increased, exhibiting coal-like characteristics. Structural evolution analysis revealed that deoxygenation and aromatization played dominant roles, with an increase in tetra-substituted aromatic rings and a decrease in oxygen-containing functional groups, confirming the transformation of the carbon skeleton into a highly stable bituminous coal-like structure. Under the optimal condition of 225 ℃ for 6 h, the resulting hydrochar exhibited an elevated ignition temperature of 323.14 ℃, significantly improved storage safety, a 25.24 % reduction in average activation energy, and a negative entropy change, indicating enhanced system ordering and reduced reaction energy barriers. Mechanistic studies indicated that the HTC process involved three stages: hydrolysis-dehydration, condensation-aromatization, and carbonization, with 225 ℃ identified as the threshold temperature that balances carbon enrichment and ash suppression. This study offers a theoretical foundation for high-value utilization of RRT pomace, thereby contributing to the resource recovery of agricultural waste and the development of clean energy.