Unlocking value from waste: Thermochemical strategies for end-of-life tyre transformation

The global surge in automotive industry growth presents an escalating challenge: the disposal of billions of end-of-life tyres (ELTs) annually. These durable, complex materials resist natural degradation, posing significant environmental and societal burdens. To address this mounting problem, a recent comprehensive review meticulously examines cutting-edge thermochemical processes as a viable pathway to transform ELTs into valuable products, thereby fostering a more circular economy.

Published in Carbon Research, the article meticulously synthesizes advancements in thermochemical techniques, specifically focusing on gasification, pyrolysis, and incineration. Researchers delved into the primary by-products of these processes, including oil, gas, and char, assessing their energy efficiency, product yield, and overall environmental footprint. The study clarifies the intricate correlations between diverse process parameters and the resulting composition, yield, and quality of these recovered materials, providing a robust foundation for future applications.

Among the various approaches, pyrolysis and gasification stand out for their potential. Pyrolysis facilitates both energy and material co-recovery, producing valuable streams of char, oil, and light hydrocarbons. This method minimizes air pollution significantly when compared to direct combustion. Gasification, conversely, excels in generating high quantities of gaseous products, notably syngas rich in hydrogen, making it attractive for power generation or chemical synthesis, and showing potential for higher revenue generation per ton of ELT processed.

The recovered products exhibit diverse functionalities. The solid char, primarily derived from carbon black, possesses a high carbon content and energy density, making it suitable as a solid fuel or, after activation, as an effective adsorbent for environmental remediation. The pyrolysis oil, a complex mixture of hydrocarbons, offers potential for producing valuable chemicals such as limonene and BTX (benzene, toluene, xylene), or for refining into diesel or marine-grade fuel. The gaseous fraction, composed of H2, CO, and CH₄, serves as a versatile energy carrier or as a fuel source for the thermochemical reactors themselves.

Navigating the Technical Landscape

Despite promising advancements, challenges persist in the widespread industrial adoption of these technologies. Pyrolysis, being an endothermic and relatively slow process, demands higher temperatures and extended residence times, coupled with stringent process control to ensure consistent product quality. The presence of high sulfur content in pyrolysis oil necessitates costly desulfurization steps before direct fuel application, and the unavoidable formation of polycyclic aromatic hydrocarbons (PAHs) requires robust gas control systems. Furthermore, catalyst deactivation due to sulfur poisoning and coke deposition presents a hurdle in optimizing hydrogen production.

Charting Future Directions

The investigation extends to innovative solutions, particularly emphasizing low-temperature approaches for modifying ELT char. This involves introducing acidic oxygen-containing functional groups onto the carbonaceous surfaces, enhancing the material's adsorption properties for pollutants like heavy metal cations and organic dyes. This environmentally friendly method, which is exothermic and utilizes readily available O₂ as an oxidizing agent, offers a compelling alternative to conventional high-temperature activation.

Catalytic Innovations and Sustainable Integration

Crucially, the review highlights the role of catalysts in overcoming kinetic inhibitions at lower temperatures. Future research will explore the efficacy of catalysts like Pt/TiO₂ in facilitating O₂ activation, thereby reducing activation energy and expediting reaction kinetics. Such catalytic innovations are poised to enhance the mass yield, lower reaction temperatures, and significantly improve the adsorption capacity of modified ELT char materials. Integrating these advanced thermochemical processes into existing industrial plants presents a significant opportunity for decarbonization and resource optimization, converting waste into alternative fuels and raw materials, and minimizing reliance on fossil resources.

Abbas Z. Kouzani, corresponding author and researcher at Deakin University, emphasizes the strategic imperative behind these efforts. "Transforming end-of-life tyres from a persistent environmental nuisance into a valuable resource is a cornerstone of sustainable development. While we have identified promising pathways through pyrolysis and gasification, refining our processes, particularly through catalytic enhancements and low-temperature modifications, remains key to achieving broad commercial viability and truly closing the loop on tyre waste."

Corresponding Author: Abbas Z. Kouzani

Original Source: https://doi.org/10.1007/s44246-024-00167-4

 Contributions: All authors contributed to the discussion of the manuscript. The first draft of the manuscript was written by Bing Han and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.