Integrated valorization of oil palm waste via CO2-Assisted slow Pyrolysis: Enhanced biochar, tailored bio-oil, and economic viability

Abstract

This study systematically investigates the CO<inf>2</inf>-assisted slow pyrolysis of oil palm leaf biomass, focusing on the influence of operating parameters—including carrier gas flow rate, reaction time, temperature (400–800 °C), CO<inf>2</inf>/N<inf>2</inf> gas composition, and limestone catalyst loading—on the yield and properties of biochar, heavier bio-oil (HBO), lighter bio-oil (LBO), and syngas. Pyrolysis temperature was identified as the dominant factor controlling product distribution, while the introduction of CO<inf>2</inf> significantly increased LBO and biochar yields and altered the physicochemical pathways of decomposition. The BET surface area of the biochar was enhanced from 4.78 to 333.35 m²/g with the combined effect of high temperature, CO<inf>2</inf>-rich atmosphere, and catalyst addition, which resulted in the highest heating values of HBO (25.80 MJ/kg) and syngas (4.27 MJ/Nm³). The maximum yields of HBO (18.65 wt%) and LBO (13.53 wt%) occurred at 700 °C under a CO<inf>2</inf>-rich atmosphere. CO<inf>2</inf> atmosphere also promoted the formation of acetic acid in bio-oils, while increasing CO content in the syngas fraction. Catalyst addition (CaCO<inf>3</inf>) induced in situ neutralization of carboxylic groups in the bio-oil, reducing acid content and enriching ketonic and phenolic species. GC-MS analysis revealed marked shifts in oxygenated and N-heterocyclic compound profiles in bio-oil products across pyrolysis conditions and aging periods. Techno-economic analysis using Aspen Plus shows that the integrated CO<inf>2</inf>/catalyst system achieves the lowest total investment cost over a 10-year operation period with payback period in 8.3 years. These results provide key insights into the design of CO<inf>2</inf>-mediated pyrolysis systems for integrated biomass valorization and negative-emissions carbon materials.