Revealing roles of CO₂ and N₂ in pressurized hydrothermal carbonization process for enhancing energy recovery and carbon sequestration

In this study, CO₂– and N₂-pressurized hydrothermal carbonization processes were investigated to understand the catalytic effects of CO₂ on hydrochar production and its quality (e.g., surface properties, energy recovery, and combustion behaviour). Both CO₂– and N₂-pressurized HTC processes could enhance the energy recovery (from 61.5% to 63.0–67.8%) in hydrochar by enhancing the dehydration reactions. Nonetheless, the two systems exhibited contrasting trends in volatile release, oxygen removal, and combustion performance as a function of increasing pressure. High N₂ pressure enhanced deoxygenation reaction, facilitating the release of volatiles and increasing the hydrochar aromaticity and combustion activation energy (172.7 kJ/mol for HC/5N). Without the contribution of CO₂, excessively high pressure may cause an adverse impact on the fuel performance owing to higher oxidation resistance. This study presents an important and feasible strategy to utilise CO₂-rich flue gas in the HTC process to produce high-quality hydrochar for renewable energy and carbon recovery.