Christyfani Sindhuwati, Mohamad A. Nahil, Paul T. Williams
The process of hydrothermal liquefaction of waste biomass was executed through a subcritical water reaction, subsequently followed by a supercritical water reaction, with the objective of improving both the yield of bio-oil and the quality of its composition. The bio-oil, aqueous phase, and gas compositions were meticulously analysed to provide an in-depth understanding of how the separation of subcritical and supercritical process conditions affects the yield and composition of the products. The findings indicated that the two-step process yielded a greater amount of bio-oil in comparison to the direct supercritical water processing of the biomass. The highest yield of bio-oil achieved through the two-step process was 25.44 wt%, realised with a biomass to water ratio of 1:10. This involved an initial subcritical water reaction at a temperature of 250 °C for 30 min, succeeded by a second stage at a supercritical water temperature of 400 °C for a duration of 60 min. The bio-oil generated through the two-step process exhibited a greater proportion of longer-chain hydrocarbons, alongside elevated concentrations of phenolic compounds, oxygenated substances, terpenoids, and furans. Furthermore, there was a notable decrease in both the concentration and complexity of nitrogen-containing compounds. The gas composition generated in the two-step process primarily consisted of carbon dioxide, methane, carbon monoxide, and hydrogen, with an enhanced yield observed in this method when compared to direct supercritical water liquefaction. The examination of the residual aqueous phase revealed that acetic acid was the predominant compound, succeeded by formic acid, furfural, guaiacol, furan, phenol, and cresols, with these substances being generated in greater concentrations through the two-step process. • Two-step subcritical-supercritical water liquefaction used to produce bio-oil. • Detailed analysis of the product bio-oil, gases and aqueous phase presented. • Bio-oil yield increased by ∼12 % compared to the one-step hydrothermal process. • Bio-oil nitrogen reduced, increased hydrocarbons, phenolics, terpenoids and furans. • Reaction mechanism proposed for the two-step hydrothermal process.