Optimisation of hydrothermal carbonisation of olive stones for enhanced CO₂ capture: Impact of zinc chloride activation
L. Gallego-Mena, Roberto Campana, A. Villardon, Fernando Dorado, L. Sánchez-Silva
Abstract
Hydrothermal carbonization (HTC) is a thermochemical conversion process that enables the production of renewable solid biofuels from various types of waste. In this study, olive stones (OS) were converted into hydrochars via HTC to develop high-performance adsorbents for CO₂ capture. Unlike previous studies, this research systematically optimizes HTC conditions-including residence time (2, 4, and 8 h), temperature (200, 220, and 240 ºC), and water-to-biomass ratio (10:1, 20:1)-to tailor the physicochemical properties of hydrochars for subsequent activation. The results indicate that extending the residence time to 8 h and increasing the temperature to 240 ºC promotes the decomposition of hemicellulose and cellulose, increases lignin concentration, enhances fixed carbon content (48.73 %), and improves the higher heating value (23.0 MJ/kg), while reducing mass yield (62.8 %). The optimized hydrochar (OS-8–240–10) was chemically activated with zinc chloride (ZnCl₂) at a 1:6 ratio and heated at 700 °C for 1 h under a nitrogen flow, leading to a remarkable increase in surface area (1281.38 m²/g) and total pore volume (0.67 cm³/g). CO₂ adsorption analysis confirmed that the activated hydrochar exhibited superior adsorption capacity due to the development of a highly porous structure with micropores (0.024 cm³/g) and mesopores (0.65 cm³/g). This study demonstrates, for the first time, that the strategic combination of optimized HTC and chemical activation can effectively convert agricultural residues into highly efficient carbon capture materials, providing a scalable and sustainable solution for climate change mitigation .