Valorization of LDPE–PET Plastic Waste through Co-Pyrolysis: Synergistic Effects on Liquid Product Quality

Abstract

This research evaluates the valorization of low-density polyethylene (LDPE) and polyethylene terephthalate (PET) waste through co-pyrolysis to obtain liquid fuel. The study analyzed the influence of temperature (500–600 °C), particle size (5–20 mm), and operation time (2–3 h) in a fixed-bed reactor under an inert atmosphere. Results from individual pyrolysis indicated that LDPE achieved a maximum liquid fuel yield of 77.45% w/w at 600 °C, with 5 mm particles and 3 hours of operation. In contrast, PET pyrolysis did not generate significant liquid fuel, producing mostly carbonaceous solids (up to 68.25%) and oxygenated waxes due to its complex aromatic structure and high oxygen content. In co-pyrolysis (1:1 mixture), no liquid fuel was obtained, though waxes (2.38% w/w), gases (50.50% w/w), and solids (47.12% w/w) were produced. It is confirmed that the presence of PET inhibits liquid formation due to its thermal stability and tendency to fragment into gaseous compounds and heavy carboxylic acid. Nevertheless, a synergistic effect was identified where LDPE acted as a hydrogen donor, moderating PET acidity and facilitating the formation of a hybrid wax with aliphatic chains and oxygenated group. The study concludes that while the linear structure of LDPE favors its conversion, PET requires additional strategies, such as the use of catalysts, to improve liquid yield.