Comprehensive insights into synergistic effects of cotton stalk and polyethylene in hydrothermal liquefaction process

This study focuses on the synergistic effects of cotton stalks (CS) and polyethylene (PE) during the hydrothermal co-liquefaction process. Bio-oil yield from co-liquefaction increased by 22.8% to 45.1 wt%, with solid residue decreased by 6.1 % and {gas + aqueous} products declined by 19.2 % under the optimal conditions (320 °C; CS:PE = 4:1; 40 % ethanol). Gas chromatography-mass spectrometry (GC–MS) presented oxygen-containing compounds in oil decreased but hydrocarbons (HCs from 6.2 % to 66.8 %) increased. Thermogravimetric (TG) analysis and Fourier transform-infrared spectroscopy (FT-IR) of solid residues revealed significant mutual promotion of the decomposition between CS and PE. The decomposition kinetics calculation demonstrated that co-treatment reduced the decomposition temperature by 33 °C, increased the mass loss by 7.8 wt%, and lowered the decomposition activation energy by 6.8 % (from 240.0 to 225.7 KJ/mol). Moreover, co-liquefaction of PE with biomass components further revealed that cellulose in CS predominantly facilitated the depolymerization of PE, followed by hemicellulose. This effect could be attributed to the acid from hydrolysis of cellulose and hemicellulose as well as high [H]⁺/[OH]^- concentration in from hydrothermal water, which promoted the β-scission of PE. The synergistic reaction pathways were proposed: CS and PE mutually promoting decomposition, with PE-derived olefins and hydrogen undergoing Diels-Alder, alkylation, and hydrodeoxygenation (HDO) reactions with CS intermediates, thereby enhancing HCs yield and inhibiting carbonization rearrangement of intermediates. This work reveals the causes and reaction pathways underlying synergistic effects, offering comprehensive guidance on producing crude bio-oil.