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.