Preparation of gas-liquid-solid three-phase products from eucalyptus sawdust pretreated with the supercritical CO₂ atmosphere

The combustion of fossil fuels has triggered a series of environmental issues, and the resource utilization of biomass energy as a renewable energy source is attracting widespread attention. Converting waste biomass into activated biochar with a high specific surface area and a well-developed pore structure for pollutant adsorption has emerged as an approach that offers both environmental and economic benefits. This work aimed to elucidate the regulatory mechanisms by which a supercritical CO₂ (scCO₂) atmosphere regulates the thermochemical conversion of biomass and the structural evolution of activated biochar, thereby exploring pathways for producing high specific surface area biochar. The variations of gas-liquid-solid three-phase products at different temperatures and residence times were summarized. The specific surface area of activated biochar treated under the scCO₂ atmosphere reached 1831.55 m²/g at 500 °C for 40 min, approximately 11.77 % higher than that under the N₂ atmosphere. Activated biochar obtained from N₂ and scCO₂ pretreatments exhibited a rich microporous structure (<2 nm). Compared to an N₂ atmosphere, the liquid product composition was more diverse under the scCO₂ atmosphere. At a temperature of 700 °C for 20 min, CO₂ consumption reached 2.23 mol/kg, indicating its significant involvement in the gasification reaction. Overall, it provides novel technical approaches and scientific rationale for the high-value utilization of waste biomass and the recycling of carbon resources.