**1. Introduction**

The fast growth of industrialisation around the world has resulted in an expansion in vehicle production as a main mode of transportation to mobilise the population and expand economies. At the same time, oil consumption in the transportation sector is fast increasing, resulting in a rapid depletion of non-renewable petroleum-based fuel [1–3]. Alternative renewable and environmentally friendly sources of car fuel, such as biodiesel [4–8], oxygenated fuel [6,9,10], and blends with petroleum-based fuels [11,12], have received increased attention in recent decades. However, due to economic and environmental concerns, waste-to-fuel technology has received increased attention from researchers around the world in recent years [13]. Solid waste disposal in landfills is both expensive and damaging to the environment [14,15]. As a result, waste-to-fuel technology offers enormous potential to reduce global waste while also replacing petroleum-based gasoline.

The increasing use of transportation vehicles results in a global stockpile of waste tyres, which is one of the biggest sources of pollution [5,16–19]. Around 1.5 billion tyres are produced worldwide each year, which implies the same number of tyres end up as waste tyres, amounting to nearly 17 million tons [20–22]. About 15–20 per cent of tyres are considered for recycling or reuse once they have reached the end of their useful

**Citation:** Jahirul, M.I.; Hossain, F.M.; Rasul, M.G.; Chowdhury, A.A. A Review on the Thermochemical Recycling of Waste Tyres to Oil for Automobile Engine Application. *Energies* **2021**, *14*, 3837. https:// doi.org/10.3390/en14133837

Academic Editor: Idiano D'Adamo

Received: 10 May 2021 Accepted: 21 June 2021 Published: 25 June 2021

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life, while the remaining 70–80 per cent are disposed of in landfills and remain in the environment [23]. Every year, one billion WT are disposed of in landfills around the world, and one car per person is disposed of each year in industrialised countries [6]. Due to the high likelihood of hazardous fumes from fire, these landfills are a severe hazard for the environment and human health [24], and they provide ideal conditions for rats, snakes, and mosquito breeding.

Due to their highly complicated structure, the variable composition of the raw material, and the chemical structure of the rubber from which the tyres are formed, recycling waste tyres is exceedingly challenging [18]. Tyres are made up of 45–47% rubber, 21.5–22% carbon black, 16.5–25% steel belts, and 4.5–5.5% textile overlays, which give the tyre its ultimate form and practical features. In addition, depending on the production method and specification, numerous different materials can be added to the tyre [25,26]. The cross-linkages formed between the elastomer and various components throughout the production process produce a three-dimensional chemical network, resulting in excellent elasticity and strength. Tyres are difficult to break down due to their complicated chemical composition [25,27,28]. As a result, decomposition in the landfill will take more than a century [29]. Furthermore, landfilling ignores the enormous energy potential of waste tyres while also posing a fire risk, resulting in dangerous gas emissions as well as the poisoning of water and soil. Several investigations have been undertaken in the last few decades to create effective technology for converting used tyres to energy [30–32]. Pyrolysis [33–36], gasification [37,38], and hydrothermal liquefaction [39] are the most prevalent methods for turning waste tyres into energy in the form of fuels. Pyrolysis, in particular, has received a lot of interest for scraping tyre waste treatment because of its efficiency compared to other methods. Pyrolysis can be used to turn waste tyres into petrol and diesel, as well as fuel oil, without harming the environment. It is the mechanism of thermally degrading long-chain molecules into smaller molecules by heat and pressure in an oxygen-free environment, which results in the production of liquid hydrocarbons (oil), gases, and char [35,40,41]. During pyrolysis, the tyres are cracked in a medium temperature range between 400 and 700 ◦C, which produces char, tar, and gaseous fuels as well as steel [16]. This technique produces oil that can be utilised directly in industrial applications and diesel engines, or it can be refined further. In comparison to petroleum-derived fuel oils, the most essential feature of this oil is its low exhaust pollution. There has been a lot of research on the performance and emissions of diesel engines utilising tyre pyrolysis oil [21,42–44].

In recent decades, waste tyre pyrolysis technology has shown to be an effective wastemanagement strategy. This technology's ultimate goal is to manufacture high-quality fuels from scraping tyres that can compete with and eventually replace non-renewable fossil fuels. Despite extensive study and great advancements, waste tyre to vehicle fuel technology has not yet reached its full potential. This technology will need more development before it can be scaled up to an industrial level. However, in order to advance waste tyre to energy technology and upgrade the technology on an industrial scale, it is critical to thoroughly comprehend the current development stage. This paper review over 100 up-to-date papers from the literature and discussed the key findings, the current status, and the development of this technology. The information only considered from the peerreviewed literature published in reputed international journals, conference proceedings, and reports. More emphasis was given to the recently published literature on the related topic. For the analysis, data were only taken from the literature where the experiments were carried out by the authors themselves in accordance with internationally recognised testing standards. Certain extreme information was removed from the database due to the unanticipated nature of the outcomes. The novelty of this article is to elaborate the way to utilise tyre pyrolysis oil as a substitution for conventional petroleum-based automobile fuel. Additionally, limitations of current waste tyre to automobile fuel technology have been identified and based on the observation of literature research; the future direction of research for commercialising the technology has been indicated. It has been expected that

the findings of this literature review will serve as a basis on which the industrial production of waste tyre pyrolysis automobile engine oil will be possible.

#### **2. Waste Tyre Management Practice**

The goal of waste tyre management is to identify the most efficient approach to limit the waste's environmental impact. Reduction in consumption, reuse/recycling, and energy recovery are all strategies for solving the WT problem. The primary reason for developing those methods was the restrictions imposed by the government for collecting tyres for landfills. In recent years, the methods that are used for waste tyre management includes: reuse and rethreading, product recycling, and recovery of energy [45]. Figure 1 depicts the process of a typical waste tyre management system.

**Figure 1.** A typical waste tire management system.
