Upgrading empty fruit bunches into carbon-rich hydrochar via CO₂-assisted hydrothermal carbonization

This study evaluates the effectiveness of CO₂-assisted hydrothermal carbonization (HTC) in enhancing the carbon densification and carbon sequestration potential of empty fruit bunches (EFB) within a biomass-based carbon capture and storage (Bio-CCS) framework. HTC experiments were conducted at 180-300 °C for 1-24 h under CO₂ and N₂ atmospheres. The results demonstrated that CO₂-assisted HTC significantly improved carbon enrichment, increasing the carbon content from 46.6 to 68.9 wt% while reducing the oxygen content. The energy densification ratio increased with temperature, whereas the energy yield remained relatively stable (83-94%). Van Krevelen analysis showed a progressive shift from biomass toward sub-bituminous coal-like characteristics, which was further supported by fourier transform infrared (FTIR) spectroscopy results indicating enhanced aromatic structures and more stable carbon bonds. Notably, the intermediate temperature range of 200-240 °C exhibited optimal performance, achieving a balance between carbon densification and energy retention, with approximately 4 h identified as the optimal reaction time. Furthermore, the carbon capture efficiency exceeded 75% under CO₂ conditions. These findings indicate that CO₂-assisted HTC not only upgrades biomass into a high-quality solid fuel but also promotes the formation of structurally stable carbon, highlighting its strong potential as a Bio-CCS technology for sustainable carbon management and energy recovery.