Sustainable Valorization of Himalayan Horse Chestnuts via Microwave-Assisted Hydrothermal Carbonization: Insights into Structural Evolution and Pyrolysis behavior

Microwave-assisted hydrothermal carbonization (MHTC) represents an advanced hydrocarbon-based approach for rapidly upgrading various types of biomasses into high-quality carbon-rich solid fuels. Herein, we utilized Himalayan horse chestnut to examine the effects of hydrothermal conditions on hydrochar characteristics and structure, including thermal stability. Hydrochar were produced via MHTC using different hydrothermal temperatures (120-200 °C). As hydrothermal temperature increased from 120 to 200 °C, the yield of hydrochar decreased from 62.74 to 46.68 %. However, with increasing temperature, resulting hydrochar contained more carbon, had greater fixed carbon content, better Fuel Ratio, and higher Heating Values with HV’s for HC200 reaching up to 27.63 MJ/kg. Spectroscopic and structural analysis demonstrated dehydration, de-oxygenation reactions and reorganization of the carbon matrix. An increase in surface area (120 °C to 180 °C) of the intermediate sample demonstrated largest degree of textural development. Thermal decomposition and iso-conversional kinetic analyses confirmed that samples treated at highest hydrothermal temperatures (HC200) exhibit temperature dependent activation energy values, indicating a multi-step pyrolysis rather than a single-step reaction pathway. The larger activation energy values also demonstrated that MHTC generates highly carbonized and thermally stable hydrochar products. The results demonstrate that low-temperature MHTC can be used as a process intensification strategy to generate value-added hydrochar products from HHCN feedstocks. Additionally, the results provided herein demonstrate that the temperature conditions used during hydrothermal treatment are key factors in determining both the characteristics of hydrochar produced via MHTC and their subsequent thermal stability.