Pyrolysis of automotive plastic waste (APW) recovered from end-of-life vehicles (ELVs) can produce an oil with several properties comparable to commercial diesel, according to a study that characterized the feedstock before processing rather than relying on mixed automotive shredder residue. The work compared thermal and catalytic pyrolysis and found that an iron-modified clay catalyst sharply raised oil yield while leaving the hydrocarbon distribution largely unchanged.
Pyrolysis thermally decomposes plastics at elevated temperatures without oxygen, breaking polymer chains into liquid, gaseous and solid products. Earlier research has shown that common automotive plastics such as low-density polyethylene (LDPE), high-density polyethylene (HDPE) and polypropylene (PP) can be converted into hydrocarbons at relatively high yields, but much of that work focused on automotive shredder residue, a heterogeneous mixture of plastics, metals, textiles and other materials. Less is known about plastics recovered directly from components such as dashboards, bumpers and headlamps, which can contain mineral fillers, flame retardants, fiberglass and other additives that affect degradation behavior and product composition.
Feedstock and catalyst
In the study, researchers characterized plastics taken directly from ELVs. The feedstock contained acrylonitrile butadiene styrene (ABS), HDPE, LDPE, PP, polystyrene (PS) and fiberglass, and also had substantial volatile and ash fractions. Front and rear bumpers accounted for 86.6% of the sample by weight, and fiberglass, commonly used to reinforce automotive components, contributed to the relatively high ash content.
Instead of using bentonite clay in its original form, the team modified it through iron pillaring, introducing iron species into the clay structure and changing characteristics such as surface acidity and pore structure that can influence pyrolysis reactions. Iron-modified clays are attractive because they may improve oil production without relying on more expensive commercial catalysts; pillared clays modified with metals such as iron, titanium or zirconium have shown potential to increase liquid yields and influence the formation of fuel-range hydrocarbons.
Yield versus composition
The researchers tested thermal and catalytic pyrolysis at temperatures from roughly 350°C to 521°C and catalyst loadings of about 6% to 34% by weight, using a statistical experimental design to evaluate the interaction between temperature and catalyst concentration. The catalyst strongly affected liquid yield. Catalytic pyrolysis produced an average oil yield of approximately 68%, compared with 41% for thermal pyrolysis alone, and the highest oil yield, 78.6%, was achieved at about 416°C with a catalyst loading of roughly 24%.
However, the catalyst had much less influence on oil composition. Thermal and catalytic pyrolysis produced similar proportions of hydrocarbons in the C8–C24 range associated with gasoline- and diesel-range compounds. In other words, the catalyst helped convert more feedstock into liquid product but did not substantially shift that liquid toward a higher proportion of diesel-range molecules. Temperature had a greater effect on composition: lower pyrolysis temperatures favored diesel-range organics, while temperatures above 450°C produced a heavier, waxier oil.
Fuel properties and next steps
The resulting oil showed several properties comparable to commercial diesel, including a calorific value of 49.85 MJ/kg, a flash point of 113°C and total aromatics of 3.55% by area, values within or comparable to ranges specified for conventional fuels. Branched heptenes and heptanes dominated the oil, compounds also identified in previous studies of polypropylene and automotive-plastic pyrolysis.
Not all properties met diesel specifications. Both density and viscosity exceeded the limits typically specified for diesel fuel, which the researchers attributed partly to heavier, long-chain hydrocarbon fractions remaining in the oil. Additional processing could therefore be required before the oil can be used as a diesel-range fuel; the researchers suggested distillation and centrifugation, as well as optimizing reaction conditions through lower temperatures and longer residence times, to reduce heavier fractions and improve fuel properties.
Future work could examine different iron concentrations in the catalyst, improve reactor designs and heat transfer, and combine pyrolysis with downstream distillation to separate gasoline- and diesel-range fractions. Improved collection and sorting of automotive plastics will also be important: without systematic removal and classification of plastic components from ELVs, reliable data on the volume and composition of available APW remain limited. Better information on these waste streams would help researchers and recyclers assess feedstock availability and plan potential industrial-scale pyrolysis operations.