Review
High-value utilization of waste tires: A review with focus on modified carbon black from pyrolysis

https://doi.org/10.1016/j.scitotenv.2020.140235Get rights and content

Highlights

  • The policy changes of the Chinese government about waste tires are introduced.

  • Compared with previous reviews, this paper focuses more on the solid phase products.

  • High-value utilization forms of pyrolytic carbon black are summarized and prospected.

  • The development direction of policy and technology on waste tires are pointed out.

Abstract

Recently, the recycling of waste tires has caused widespread concern for its environmental issues. The experience of the producer responsibility and tax system is of great beneficial to developing countries. The article also elaborates on the efforts of Chinese government to focus on establishing and perfecting waste tire treatment system by strengthen legislation. The main reasons such as immature market, non-uniform policy and repeated taxation for the survival difficulties of waste tire recycling enterprises in China are summarized. Among numerous resource methods, pyrolysis has been considered as a promising thermochemical process to deal with the waste tires. Unlike other similar reviews that mainly focus on its liquid phase, special attention has been given to solid char, pyrolysis carbon black, due to its wide application and high-value utilization in the future. We summarize the available research on application of pyrolysis carbon black as an alternative to commercial carbon black in rubber manufacture, as activated carbon in pollution control and as biochar for soil improvement.

Analysis of the available data revealed that 1) the influence of temperature and time has been basically established; 2) catalyst type, dosage and reactor selection should be adjusted according to product demand; 3) pickling has become the primary means of improving pyrolysis carbon black; 4) the type of modifier and modification method must be adjusted according to the specific characteristics of the raw materials and needs to be combined with the experimental results to realize resource utilization and give full play to its economic value.

Introduction

Every year millions of tires are discarded, thrown away or buried all over the world, representing a very serious threat to the ecology. By the year 2030, there would be 5000 million tires to be discarded on a regular basis (Thomas et al., 2016). Discarded tires often lead to “black pollution” because they are nonbiodegradable and pose a potential threat to the environment (Nehdi and Khan, 2001). For instance, tires without proper treatment cause land occupation, fire risk and contaminate (Brandsma et al., 2019). In landfills, rubber tires are not degraded easily but float to the top over time due to trapped gases (Juma et al., 2006). Combustion of tires can recycle part of energy but produce toxic gases that require expensive air emissions control systems. According to the report of Xinhua News Agency, the authoritative media in China, the annual amount of waste tires in China is over 13 million tons with a growth rate of 6% to 8% per year (Net, 2019).

In the waste tires processing industry, the waste tires from trucks, all-steel tires and natural rubber are mostly processed by producing retreaded tires, recycled rubber or rubber powder. At the same time, the waste passenger car tires of semi-steel tires and synthetic rubber are flowed into unqualified small workshops, causing irreparable damage to the ecological environment. This requires a more workable way to treat waste tires and recover valuable products from them.

Through the replacement of tread rubber, the production of retreaded tires is widely used in engineering tires from truck tires. It can significantly improve the tire life cycle, reducing the cost of enterprises and the wastage of resources (Simic and Dabic-Ostojic, 2017). However, this is not applied in some conditions requiring higher quality of tires (such as passenger car tire). By improving their performance, such as recycling the scrap waste tires into civil engineering materials (Siddique and Naik, 2004), it can reduce the environmental impact caused by the accumulated waste tires (Ortiz-Rodriguez et al., 2017). However, there is no recycling method for such products mentioned above. Therefore, this is just postponing the final solution. Waste rubber products still require an ultimate disposal method (Lonca et al., 2018).

The reclaimed rubber made by crumb rubber possesses the advantages of low price and good processing performance. It can be partially replaced or separately manufactured as a usable rubber product (Hassan et al., 2019). Its shortcomings are poor elasticity, flex crack resistance and tear resistance. Concerns have also been raised about exposure to carcinogenic chemicals resulting from contact with synthetic turf fields made by reclaimed rubber (Perkins et al., 2019).

Rubber powder is a powdery substance obtained from vulcanized rubber by mechanical attrition. It is a kind of special elastic powder material with the basic characteristics of powder material. It has a wide range of applications: can be utilized as a brake pad, waterproof roofing coil, mechanical gasket, cushion, sound-absorbing material and so on (Silva et al., 2019). The disadvantage is that the tensile strength of the mixture is reduced, and the fatigue resistance is poor. The mixing process requires high requirements on the formula.

Heat energy utilization is to treat waste tires as fuel, alone or mixed with combustible waste. Although combustion to recover heat energy is simple, it will lead to air pollution. However, new thermochemical utilization such as thermal cracking technology can make waste tire block undergo negative/atmospheric pressure reaction to get carbon black, steel wire, fuel oil and a small amount of non-condensable gas. It was found that people can use the products from pyrolysis in different applications, realizing the complete utilization of waste tire resources (Kommineni et al., 2018).

From the overview, it is clear that the development of technology makes the disposal of waste tires not simply pursue the reduction of quantity, but pays more attention to the high-value utilization of its recycled products (Bockstal et al., 2019). Pyrolysis has been considered as an efficient recycling method (Benedetti et al., 2017; Debnath et al., 2018; Vouvoudi et al., 2017). As a process of heating feedstock materials like biomass, plastics and tires, pyrolysis is generally done under inert atmosphere in a specially designed reactor. There are two main advantages (Arabiourrutia et al., 2008): ① it enables the subsequent individual valorization of three-phase product, which has great potential in the economy; ② it has a higher efficiency for energy recovering and a lower impact on the environment.

Three main products are produced from the pyrolysis process including gas, oil (TPO), and solid char (CBp). There are many researches on tire pyrolysis, most of which focus on the utilization of gas and TPO. The gases are used to generate energy, which is similar to the application of combustion. Laresgoiti et al. (Laresgoiti et al., 2000) reported that CHs were a major fraction of gas yields. Furthermore, Leung et al. (Leung et al., 2002) found that CH4 and H2 accounted for 44.5% and 20.7% of the gas products at 800 °C, respectively. Kaewluan et al. (Kaewluan and Pipatmanomai, 2011) stated that tire pyrolysis gas could be used in gas turbines to produce power. TPO contains a wide variety of compounds. It is pointed that TPO cannot be used as fuel directly but can be an alternative to diesel fuel with proper upgrading (Ahoor and Zandi-Atashbar, 2014; Martinez et al., 2014; Wang et al., 2017). Aydin and Ilkilic (2012) used automobile tires as raw materials to prepare alternative fuel for diesel engines by pyrolysis, proving that CaO, Ca(OH)2, NaOH and other catalysts could reduce the high sulphur content of the fuel. This alternative fuel produced by TPO has slightly higher density and sulphur content than diesel fuel, but other characteristics and distillation curves are very similar to diesel fuel. Researchers reported that blends of TPO and diesel can be used in diesel engines, but the usage of 100% TPO in the engine was a failure (Frigo et al., 2014; Murugan et al., 2014). However, blends of tire oil have longer ignition delays compared to pure diesel engine performance due to its low cetane number. Thus, researches worked to enhance the cetane number of TPO with cetane improvers and achieved good results (Hariharan et al., 2014; Sharma and Murugan, 2013; Tudu et al., 2016). This proves that the application of TPO as a partial substitute for diesel as fuel can effectively save the consumption of fossil fuels in the future. Moreover, Rambau et al. (Rambau et al., 2018) also demonstrated that it is possible to utilize waste tyres pyrolysis oil vapor as a substitute for some expensive commercial carbonaceous gases.

CBp mainly comes from the carbon black and other inorganic fillers added in the process of tire production, as well as the coke material partially deposited on the surface of CBp for the coking reaction caused by a secondary reaction in the pyrolysis process.

In contrast, the raw materials of industrial carbon black are natural gas and heavy oil rich in aromatics, such as catalytic cracking clarified oil, ethylene tar, coal tar and its distillate. This determines the formation mechanism of CBp to differ from that of industrial carbon black. It is not the growth process from small molecules to aggregates, but the pyrolysis process from solid tires to aggregates. Therefore, there is a certain difference between the performance of industrial carbon black added in the pyrolysis of waste tires and rubber, but the composition and structure of CBp are basically similar to the phase of industrial carbon black (Lee et al., 1999).

Compared with commercial carbon black, CBp is mainly composed of carbon in aliphatic and small aromatic ring compounds, which are derived from deposits generated in pyrolysis and partial pyrolysis products adsorbed on the surface of pyrolytic carbon (Darmstadt et al., 2000).

Similar to TPO, in recent years, many studies have attempted to clarify the process control of waste tire pyrolysis to form CBp, and modify CBp to replace commercial carbon black partially in some fields.

This paper presents the current situation of waste tires management in some countries and reviews several waste tires recycling and utilization technologies. Taking the utilization of modified carbon black from pyrolysis as an example, this work can provide a valuable and promising reference for the management and industrial implementation on high-value utilization of waste tires.

Section snippets

The situation of waste tires management

The management and recycling of waste tires is the fundamental solution to the problem of disposal of waste tires. Because of the huge amount of waste tires, it has attracted extensive attention from social, economic and environmental fields.

Pyrolysis mechanism

Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen (or any halogen). It involves the simultaneous change of chemical composition and physical state, and is irreversible. The pyrolysis reaction process begins with weak points in the polymer or molecule, especially at the tertiary and secondary carbon atoms as shown in Fig. 3. The cracking of natural rubber mainly starts from the carbon atoms of the main chain. This is mainly because

Conclusions

Waste tyres can be a source of health and environmental concerns, but they also represents a loss of potentially valuable resource. Due to the progress of society, all countries have adopted a variety of models for the recycling and utilization of used tires through policy guidance and financial support.

In terms of policy and management, developed regions started early and mainly focused on establishment of the specific systems. Through learning advanced experience from some developed

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors greatly appreciate the support and funding of Shanghai Institute of Pollution Control and Ecological Security and the support of State Key Laboratory of Pollution Control and Resource Reuse (SKL) devices.

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