Pyrolysis can turn plastics recovered from end-of-life vehicles (ELVs) into diesel-range oil, and an iron-modified clay catalyst substantially raises the amount of liquid product, a study on thermal and catalytic pyrolysis of automotive plastic waste indicates.
Recovering plastics from ELVs is an important part of improving resource recovery and supporting more circular use of plastics. Pyrolysis has emerged as a promising route for recovering fuels and chemicals from waste polymers: it thermally decomposes plastics at elevated temperatures without oxygen, breaking polymer chains into liquid, gaseous and solid products.
Earlier work has shown that pyrolysis can convert common automotive plastics such as low-density polyethylene (LDPE), high-density polyethylene (HDPE) and polypropylene (PP) into valuable hydrocarbons at relatively high yields. Much of that research, however, has focused on automotive shredder residue, a heterogeneous mixture of plastics, metals, textiles and other materials, leaving less information on how plastics recovered directly from components such as dashboards, bumpers and headlamps behave.
The study characterised plastics taken directly from ELVs rather than mixed shredder residue. The feedstock contained acrylonitrile butadiene styrene (ABS), HDPE, LDPE, PP, polystyrene (PS) and fibreglass, along with substantial volatile and ash fractions. Front and rear bumpers accounted for 86.6% of the sample by weight, and fibreglass, commonly used to reinforce automotive components, contributed to the relatively high ash content.
Catalyst and experiments
Catalysts can improve pyrolysis by accelerating reactions, altering product distributions and potentially reducing the temperature or residence time required. Commercial zeolites have been widely studied, but lower-cost clay-based alternatives are drawing interest. Pillared clays modified with metals such as iron, titanium or zirconium have shown potential to raise liquid yields and influence the formation of fuel-range hydrocarbons; iron-modified clays are particularly attractive because they may improve oil production without relying on more expensive commercial catalysts.
Rather than using bentonite clay in its original form, the researchers 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. They compared 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 design to evaluate the interaction between temperature and catalyst concentration.
The catalyst strongly affected liquid yield. Catalytic pyrolysis produced an average oil yield of about 68%, compared with 41% for thermal pyrolysis alone. The highest oil yield, 78.6%, was reached at approximately 416°C with a catalyst loading of about 24%.
Limited effect on oil composition
The catalyst had much less influence on the oil's composition. Thermal and catalytic pyrolysis produced similar proportions of hydrocarbons in the C8-C24 range associated with gasoline- and diesel-range compounds. The catalyst helped convert more of the feedstock into liquid but did not substantially shift that liquid toward a higher proportion of diesel-range molecules. Temperature had a greater effect: lower pyrolysis temperatures favoured diesel-range organics, while temperatures above 450°C produced a heavier, waxier oil.
The resulting oil showed several properties comparable to commercial diesel. The researchers reported 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, however. 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 needed before the oil could be used as a diesel-range fuel; the team suggested distillation and centrifugation, as well as optimising reaction conditions through lower temperatures and longer residence times.
Next steps
The study positions pyrolysis as a technically promising route for recovering value from automotive plastics while highlighting the development still required before broader commercial application. Future work could investigate 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 end-of-life vehicles, reliable data on the volume and composition of available automotive plastic waste remain limited, and better information would help researchers and recyclers assess feedstock availability and plan potential industrial-scale pyrolysis operations.