Analysis on Degradation Mechanism and Product Recycle of Ex-service Wind Turbine Blades by Hydrothermal Liquefaction

The recycling of ex-service wind turbine blades (EWTBs) presents a significant challenge for the future. Hydrothermal liquefaction (HTL) has emerged as a promising approach for the recovery of resins and glass fibers (GFs) from EWTBs. This study offers a comprehensive analysis of the separation mechanisms and product characteristics under the catalytic effect of an acidic medium during the HTL tests. The primary factors were identified as hydrothermal temperature and H₂SO₄ concentration. The acidic medium facilitated disruption of the resin's structure by supplying hydronium ions, while temperature played a crucial role in determining the yield of oil-phase products. Operating conditions of 280 °C and 0.6 mol/L H₂SO₄ resulted in an oil-phase yield of 98.2%. The cleavage of cross-linked C-N and C-O-C bonds reacted with hydrogen ions to produce stable compounds, primarily phenol and <i>p</i>-cumyl alcohol. Based on these findings, two distinct mechanisms of resin depolymerization were proposed, depending on the sequential cleavage of C-O-C and C(benzene ring)-C(isopropyl) bonds. Intermediates, including bisphenol A and 2-propanol,1,3-diphenoxy-, were generated. They further decomposed into smaller units, eventually forming phenol. Moreover, temperature was found to be a critical factor in determining the strength of recycled glass fibers (RGFs). The optimal conditions of 260 °C and 0.6 mol/L H₂SO₄ concentration were identified as being capable of achieving complete resin depolymerization while preserving high-strength RGFs. These innovative findings provide valuable insights into the development of low-temperature, high-efficiency methods for the full-component recovery of EWTBs, offering significant guidance for future recycling efforts.