Maximising the Circularity and Sustainability: An End-of-Life Tyre Recycling Outlets Selection Model

    Student thesis: Doctoral ThesisDoctor of Philosophy

    Abstract

    An end-of-life product may re-enter a supply chain via different end-of-life strategies, with priority given to Reduce, followed by Reuse, Repair, Refurbish, Remanufacture, Repurpose, Recycle and Energy Recovery. Collectively, they are known as R-hierarchies or imperatives, which utilise different methods to treat an end-of-life product, transforming it into a useful product before it is reintroduced back to forward supply chain. Today, the introduction of open loop supply chain in a supply network has increased the number of alternatives to treat and consume the end-of-life product within each end-of-life strategy, making the selection of suitable alternative more challenging. For example, within the recycling strategy of an end-of-life tyre (ELT), it can either be recycled into a new tyre again or into the making of artificial turf, moulded product, sports surface, cement, pyrolysis char, asphalt, foundries reducing agent and civil engineering aggregates. The selection of alternatives is even more convoluted and complex when different criteria, comprising of various dimensions and items of different unit of measurements are used for alternatives selection purposes.

    Tyre is selected as the subject of this study because of the high volume of tyre reaching end of-life tyre annually, coupled with the elevated demand for the rubber within it. It is also one of the end-of-life products with the highest number of recycling alternatives, making the selection of alternatives bearing high research value.

    Nevertheless, a gap is observed pertaining to the alternatives ranking and selection in tyre recycling industry, where conflicting ranking and suggestions are discovered in the literature. This gap exists because of different measurements are used in the ranking of alternatives, which render the accuracy of the results and reliability of the evaluation. Besides, the criteria used are also discovered to be contradicting, representing another gap in the literature that should be investigated.

    Hence, this study focuses on examining the selection of recycling alternatives of end-of-life tyre, particularly under the effect of circularity and sustainability. Circularity is an economic system aimed at eliminating waste and the continual use of resources; while sustainability is the ability for resources to be maintained at a certain rate or level, such that the future generation can meet their own needs for resources. Circularity and sustainability are both significant criteria that received increasing concern in the literature, with immense importance within society, especially in the end-of-life product research. This study aims to propose a comprehensive list of evaluation criteria under circularity and sustainability to rank recycling alternatives, and subsequently develop an end-of-life tyre alternatives selection matrix. To achieve the aim, a hybrid method, which integrates fuzzy Analytical Hierarchy Process, fuzzy Technique of Order Preference Similarity to the Ideal Solution, the multi-objective linear programming, and semi-structured interviews, are proposed in this study.

    A Delphi study, which consists of 18 subject experts, was conducted to evaluate the relative importance of each dimension and item within circularity and sustainability criteria, via pairwise comparison in a self-completed online questionnaire. The responses from all respondents are aggregated and analysed using fuzzy Analytical Hierarchy Process to reveal the relative importance. It is discovered that within circularity criteria, monetary dimension represents the most important dimension while recyclability of the product being the most important item. Within the sustainability criteria, environmental sustainability and resources consumption are the most important dimension and item respectively.

    To further analyse the finding from Delphi study, an end-of-life tyre collector company based in France was invited to take part in this research and provided data for this study. The company supplies the end-of-life tyre collected to five recycling alternatives, which utilise the recycled rubber to make synthetic turf, sports surface, moulded objects, cement and foundries. The managers from the company were invited to provide rating for the five recycling alternatives via a survey. The responses from the managers were aggregated and analysed using fuzzy Technique of Order Preference Similarity to the Ideal Solution (TOPSIS), in obtaining the score and ranking of each recycling alternative. By analysing the empirical data collected, this study reveals that cement manufacturing, which is the primary recycling alternative, ranked the lowest among the five recycling alternatives in terms of the circularity and sustainability. Nevertheless, synthetic turf manufacturing and moulded objects manufacturing ranked the highest in the circularity and sustainability, respectively.

    In this study, a multi-objective linear programming model is developed to determine the best allocation of tyre in the five recycling alternatives, by considering the score of the alternatives as well as all the resources constraint of the company. The model is solved using a non-sorting genetic algorithm-II to reveal the optimum resources allocation among the five alternatives that will provide highest circularity and sustainability. A qualitative interview is then conducted to further support and explain the barriers towards integrating the model output into the business. It is proved that the key drivers for recycling alternative selection are cost and profit, subjected to ease of end-of-life tyre processing as well as end consumers’ perceptions towards the recycled products. Furthermore, the model output also reveals a competitive relationship between the circularity and sustainability, as excelling in the circularity would trade-off sustainability.

    This research contributes to knowledge by uncovering the comprehensive list of criteria and items to measure circularity and sustainability of the ELT recycling supply chain, which is crucial in directing the construction of a more effective framework or strategy in future research. This research also introduces a hybrid method that is able to, more effectively, determine the allocation of resources among contending alternatives because of its ability to incorporate various dimensions and items that consist of different units and nature (i.e., quantitative or qualitative), which can be applied directly in relevant research problems. Practitioners can make use of the contributions as aforementioned to device a scorecard to achieve more circular and sustainable business and supply chain. A framework is also being proposed to aid managers select the most desirable alternatives by visualising it in a 2 x 2 circularity-sustainability matrix. Further, managers can follow the proposed hybrid methods to more effectively allocate the resources among the recycling alternatives.
    Date of Award2022
    Original languageEnglish
    Awarding Institution
    • Coventry University
    SupervisorMing Lim (Supervisor), Senmao Xia (Supervisor) & Jiayao Hu (Supervisor)

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