Weibold Academy article series discusses periodically the practical developments and scientific research findings in the end-of-life tire (ELT) recycling and pyrolysis industry.

These articles are reviews by Claus Lamer – the senior pyrolysis consultant at Weibold. One of the goals of the review is to give entrepreneurs in this industry, project initiators, investors and the public, a better insight into a rapidly growing circular economy. At the same time, this article series should also be a stimulus for discussion.

For completeness, we would like to emphasize that these articles are no legal advice from Weibold or the author. Please refer to the responsible authorities and specialist lawyers for legally binding statements.

Introduction

A research team from the School of Chemical & Process Engineering at the University of Leeds, UK, has published a comprehensive study in the Journal of the Energy Institute (Volume 118, 2025) on the thermal decomposition of end-of-life tires (ELT) and plastics. The study investigates the potential of co-pyrolysis, a process that thermally decomposes mixed materials under oxygen-free conditions, to enhance the production of valuable products such as oil (TPO), gas, and char (raw rCB). Utilizing a laboratory-scale batch reactor, the researchers focused on a 1:1 mass ratio of tire and plastic feedstocks, examining the synergistic effects of decomposition and their impact on product quality and yield.

Thermal Decomposition Behavior: TGA Analysis

The thermal decomposition behavior of the materials was evaluated using thermogravimetric analysis (TGA), revealing distinct degradation characteristics for each feedstock. Tire decomposition is a multi-phase process spanning a temperature range from 230°C to 510°C. This includes two primary degradation peaks at 387°C and 445°C, corresponding to the breakdown of natural rubber and synthetic rubber components such as styrene-butadiene rubber.

Plastics exhibit thermal degradation profiles that vary according to polymer type:

  • HDPE and LDPE demonstrate high thermal stability, with degradation peaks at 495°C and 491°C, respectively.
  • Polypropylene (PP) decomposes at 478°C, with a rapid degradation phase due to its branched polymer structure.
  • Polystyrene (PS) peaks at 434°C, with its aromatic backbone contributing to secondary thermal reactions.
  • Polyethylene terephthalate (PET) undergoes decomposition at 441°C, generating significant amounts of CO and CO₂ due to the breakdown of its ester groups.

During co-pyrolysis, tire volatiles reduce plastic decomposition's onset temperature by 30–40°C, indicating strong thermal interactions. Polyalkene plastics (HDPE, LDPE, PP) exhibit two distinct stages of mass loss, reflecting the sequential breakdown of tire and plastic components. Aromatic polymers such as PS and PET demonstrate a more integrated decomposition pattern, characterized by a single, broad thermal peak around 430–450°C.

Oil Production and Composition

Oil (TPO) is a primary product of pyrolysis, with its yield and composition highly dependent on feedstock characteristics and co-pyrolysis interactions. The pyrolysis of individual materials produces oils with distinct chemical profiles.

Oil Yields from Individual Pyrolysis

  • Tires: Pyrolysis yields 54.83 wt% oil (TPO), enriched in aromatic hydrocarbons, including toluene, xylene, and naphthalene. Limonene, derived from the depolymerization of natural rubber, constitutes 39.7% of the oil.
  • HDPE/LDPE: High oil yields (85–90 wt%) are observed, with long-chain alkanes and alkenes as the dominant components.
  • PP: Oil yield is 89 wt%, characterized by a mixture of linear and branched unsaturated hydrocarbons.
  • PS: Nearly complete conversion to oil (99.2 wt%) is achieved, with styrene accounting for 70.08% of the product.
  • PET: The oil yield is 53 wt%, with a high concentration of oxygenated compounds such as benzoic acid (41.1 wt%).

Oil Yields from Co-Pyrolysis (1:1 Mass Ratio)

Co-pyrolysis significantly modifies the oil composition, indicating the presence of synergistic interactions between tire and plastic volatiles.

  • Tire-HDPE and Tire-LDPE: Oil yields remain high, but the concentration of long-chain aliphatic hydrocarbons decreases by 7–10%, while aromatic hydrocarbons (e.g., toluene, xylene) increase by 12–18%.
  • Tire-PP: The proportion of BTEX compounds rises by approximately 20%, with a corresponding reduction in aliphatic hydrocarbons.
  • Tire-PS: Styrene concentration decreases by 15–25% as secondary reactions convert it into more stable aromatic compounds, including polycyclic aromatic hydrocarbons (PAHs).
  • Tire-PET: Oxygenated compounds are reduced by 25%, while aromatic hydrocarbons significantly increase due to catalytic interactions with tire-derived volatiles.

Gas Production and Composition

Pyrolysis gas primarily comprises light hydrocarbons (C1–C4), carbon oxides, and hydrogen. The feedstock and the degree of co-pyrolysis interactions influence the gas yield and composition.

Gas Yields from Individual Pyrolysis

  • Tires: Gas yield is 8.69 wt%, with methane, ethene, and CO₂ being predominant.
  • HDPE/LDPE/PP: Gas yields range between 4.34–6.45 wt%, mainly light hydrocarbons (methane, ethene, propene) generated through random chain scission.
  • PS: Gas formation is minimal (1.68 wt%), with most decomposition products forming liquid aromatics.
  • PET: PET produces a high gas yield (33.60 wt%), dominated by CO and CO₂ from ester cleavage.

Gas Yields from Co-Pyrolysis (1:1 Mass Ratio)

Co-pyrolysis enhances gas production, with yields and composition deviating from additive predictions.

  • Tire-HDPE and Tire-LDPE: Gas yields increase to 12–14 wt%, with CO₂ production rising by 30–40% and methane output increasing by 10–15%.
  • Tire-PP: Gas yield reaches 12.77 wt%, with hydrocarbon gases such as ethene and propene increasing by 20–25%.
  • Tire-PET: The gas yield rises to 24.58 wt%, driven by a 35% increase in CO₂ emissions.

Char Properties and Yield

Char (raw rCB) is a solid pyrolysis product of fixed carbon and inorganic fillers. Its formation is influenced by the structural stability of the feedstock polymers and the extent of co-pyrolysis interactions.

Char Yields from Individual Pyrolysis

  • Tires: High char (raw rCB) yield (37.33 wt%) is observed, primarily due to carbon black and other non-volatile fillers.
  • HDPE, LDPE, PP: Minimal char formation (0–5.33 wt%) occurs, reflecting near-complete volatilization.
  • PET: PET yields 10.17 wt% char, consisting of aromatic residues and ash.

Char Yields from Co-Pyrolysis (1:1 Mass Ratio)

  • Tire-HDPE and Tire-LDPE: Char yields decrease by 4–5 wt%, indicating enhanced degradation through volatile-phase interactions.
  • Tire-PP: Similar reductions are observed, with improved conversion of polymer residues.
  • Tire-PS: Char yield increases slightly by 1.0 wt%, suggesting the formation of stabilized aromatic structures.
  • Tire-PET: The char remains relatively stable, with slight variation in yield.

Conclusion

This research demonstrates the potential of co-pyrolysis as an advanced waste management solution, particularly for end-of-life tires and plastics. By processing these materials in a 1:1 mass ratio, the study highlights significant synergies that enhance product yields and quality through complex radical interactions and volatile-phase reactions. Plastic decomposition onset is lowered by 30–40°C during co-pyrolysis, improving thermal efficiency and more complete material breakdown.

Oil yields show a shift towards increased aromatic hydrocarbons, such as BTEX, and reduced long-chain aliphatic hydrocarbons, enhancing their suitability for fuel and chemical applications. Oxygenated compounds from PET decrease by 25%, while aromatic hydrocarbons increase. Gas yields also rise substantially, with a 20–25% increase in light hydrocarbons and a 30–40% rise in CO₂ emissions, particularly in oxygen-rich PET mixtures. Char production decreases in Polyalkene mixtures but stabilizes for aromatic polymers due to residue condensation and carbon fixation.

These findings suggest that co-pyrolysis could serve as a promising technology for sustainable resource recovery, with potential flexibility to control product yields and composition through adjustments in feedstock ratios and process conditions. Promoting increased hydrocarbon production and lowering oxygenated byproducts may meet industrial demands for efficient fuel and chemical generation. Further research could optimize reactor designs, implement continuous processing, and incorporate catalytic upgrading to enhance scalability and product refinement. If these improvements are achieved, co-pyrolysis has the potential to significantly reduce environmental impact while maximizing the conversion of waste into valuable resources.

Source

Noof Alzahrani, Mohamad A. Nahil, Paul T. Williams, "Co-pyrolysis of waste plastics and tires: Influence of interaction on product oil and gas composition," Journal of the Energy Institute, Volume 118, 2025, 101908, ISSN 1743-9671, https://doi.org/10.1016/j.joei.2024.101908.

Access the complete publication here: ScienceDirect Article Link.

According to Elsevier's CC BY (Creative Commons Attribution) policy, this research is published under an Open Access license. This license allows users to read, download, distribute, and reuse the article with appropriate attribution to the original authors and source.