Revealing roles of CO <subscript>2</subscript> and N <subscript>2</subscript> in pressurized hydrothermal carbonization process for enhancing energy recovery and carbon sequestration.

In this study, CO 2 - and N 2 -pressurized hydrothermal carbonization processes were investigated to understand the catalytic effects of CO 2 on hydrochar production and its quality (e.g., surface properties, energy recovery, and combustion behaviour). Both CO 2 - and N 2 -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 N 2 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 CO 2 , 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 CO 2 -rich flue gas in the HTC process to produce high-quality hydrochar for renewable energy and carbon recovery.

Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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