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 p-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.