Valorisation of End of Life Tyres (ELTs) in a Newly Developed Pyrolysis Fixed-Bed Batch Process

Highlights

• A patented technology for pyrolysis was employed to treat end of life tyres.

• The pyrolysis reaction was conducted between 500 to 700 °C and has yielded a maximum pyrolysis oil at 500 °C.

• Maximum non-condensable gaseous fraction was obtained at 700 °C as 18.7%.

• Oil obtained showed diesel fuel range hydrocarbons.

• A proportional relationship was observed between operating temperature and diesel range hydrocarbons.

Abstract

This article shows a preliminary study of an end of life tyres (ELTs) use as a feedstock in a newly developed and patented reactor system that utilises a three zone heating element set-up along its fixed bed. The pyrolysis reaction was conducted between 500 to 700 °C and has yielded a maximum pyrolysis oil at 500 °C which was attributed to the promotion of secondary cracking of the oil into permanent (non-condensable) gaseous products. The oil yield and properties including estimated mass balance, sulphur content and higher heating value (HHV) shows that the pyro-oil is within fuel standards of the market making it a green and renewable resource off of waste. In addition, The hydrocarbon (HC) range of the products obtained from the oil shows that it is within the diesel range of typical fuels. The analysis of the gaseous products from the pyrolysis showed that an increase in the average reactor bed temperature promotes the decomposition of primary HC and pyrolysis oil into secondary products. This results in the detection of tetradecanoic acid, limonene and eicosane, among other major chemical groups at temperatures above 550 °C which is the end-set temperature of feedstock examined. It can be concluded that by using such a reactor set-up and upgrading the fuels extracted from its downstream stigmatic features of fixed bed reactors might be overcome with a lucrative economical value and rate of return in a circular economy perspective.

Introduction

End of life tyres (ELTs) represent a major component of the solid waste stream (SW). It is estimated that 1.62 billion tyre units were produced in 2018 to cover consumers demand which will eventually be discarded as ELTs (Statistia, 2019). In a European Union (EU) context, about 3.3 million ELTs are generated annually and stockpiles of which are still posing a major concern within the EU-27 (Williams, 2013). On the other had, EU has issued a number of directives to combat this issue. ELTs are regulated under the end of life vehicle (ELV) directive of the European Commission (EC) 53/2000 (EC et al., 2000). This directive encourages the separate collection of tyres from scrap and worn-out vehicles. In 2018, the EC has also adopted a new regulation proposal for labelling tyres with respect to their fuel efficiency and noise reduction with the aim of increasing end-users’ awareness (EC et al., 2018). Other parts of the world suffer similar problems when it comes to ELTs management. In China, about 200 million ELT units equalling to 5.2 million tonnes are disposed of in open dumps annually (Wang et al., 2019a). The small Middle-Eastern nation of Kuwait hosts the world’s largest ELTs stockpiles where over 7 million units are compiled on top of each other within its Western desert (KN, 2018).

Furthermore, ELTs have been recognised as a major concern to local environments when disposed off in an unsanitary manner or landfilled. ELTs can cause damage to groundwater aquifers; and can also lead to harmful pollutants emissions such as volatile organic compounds (VOCs) and sulphur oxides (SOx) (Costa and dos Santos, 2019). Residues from tyres have also been proven to be resistant to biological and chemical treatment (Ouyang et al., 2018). Therefore, ELTs landfilling has been banned under EC Directives and are recognised as a major heavy metal leachate medium that occupies major land space (EC et al., 1999, Han et al., 2018). Reuse and recycling ELTs also present a pathway for their valorisation. However, major shortcomings arise when such methods are considered namely loss of quality and residue accumulation (Murugan et al., 2009; Labaki and Jeguirim, 2016; Han et al., 2018). Incineration via direct combustion can also be conducted to recovery heat for steam turbines with ELTs as a feedstock. However, secondary pollutants are an imminent fate when such techniques are undertaken to valorise tyres. ELTs carcasses are comprised of natural and synthetic rubbers, carbon black and additives. Tyres are also considered to be a thermoset polymer which renders their melting and re-compounding an impossibility (Zeatier et al., 2018). ELTs encompasses a higher heating value (HHV) ranging between 29 to 39 MJ kg⁻¹ with an organic matter of up to 90%, which renders their thermochemical conversion (TCC) a very lucrative prospect (Williams, 2013; Mohan et al., 2019; Wang et al., 2019a, 2019b; Lewandowski et al., 2019). The process of pyrolysis (e.g. TCC under inert atmospheres of carbonous matter in temperature ranges between 400 to 900 °C to cleave molecules) has been proven to be a sustainable method of utilizing ELTs for chemicals recovery. It can also close the loop for a wholesome circular economy, merging the chemicals industry with SW management as a sustainable solution.

ELTs have been studied in the past using various reactor set-ups stretching a broad range from fixed beds (Kordoghli et al., 2017; Xu et al., 2018; Abdul Aziz et al., 2018), fluidized bed reactors (FBRs) (Zang et al., 2019) to rotary and microreactors (Olazar et al., 2008a; Williams, 2013). These studies are typically coupled with thermogravimetric analysis (TGA) to report on the tyres decomposition mechanism and degressive reaction kinetics (Al-Salem et al., 2009; Danon et al., 2015a, 2015b; Danon and Görgens, 2015; Chen et al., 2019). The type of experimental set-up and reactor configuration has immense influence on the products yields in pyrolysis experiments (Al-Salem et al., 2017a). However, fixed bed pyrolysis presents a number of advantages when compared to other types of reactors. Ease in operation and maintenance by comparison to other configurations, showcase their superiority coupled with their ability to conduct analytical pyrolysis procedures to study the product yields in a detailed manner. On the other hand, its main disadvantage lies within its lack of economic value in conducting processes by a singular batch that require additional cost in loading and packing the reactors system. On the other hand, the distribution of heat along the main unit of reactors system (e.g. reaction chamber furnace) represents a major obstacle in ELTs pyrolysis which is typically transferred through external sources such as electrical resistance (Zeaiter et al., 2018). The design of the apparatus also presents a challenge for product yields analysis and collection in fixed beds. Wang et al. (2020) studied the pyrolysis of tyre tread rubber and side walls in the amount of 3.5 g using a two stage fixed bed reactor with a carrier gas flow rate of 40 ml min^-1. The side walls were treated to produce hot char in the two fixed beds. The tyre tread was pyrolysis was used to derive hot char used as a catalyst. It was noted that under pyrolytic conditions and an operating temperature range between 500 to 550 °C, a high yield of single ring aromatics were obtained in the pyrolysis oil (pyro-oil) which was estimated at a total of 50% conversion rate. Khalil et al. (2020) have reported the results of using microporous zeolite catalysts and mesoporous zeolite catalysts in a two-stage fixed bed reactor to produce liquid fuels from ELTs. There was a significant reduction in the coke deposition over Ce-zeolite Y catalyst reduced from 8 wt% to 5.7 wt%. The process of fixed bed was proven to be effective to overcome conventional problems associated with this type of ELTs treatment. Xu et al. (2018) used a fixed bed reactor with internals to treat ELTs in pyrolysis model. The results have clearly indicated that employing internals produced more light oil at externally heating temperatures (<700 °C). This was attributed to the inhabitation of the secondary cracking reactions within the reactor. This presents a new strategy for treating ELTs in overcoming certain problems associated with fixed beds and promotes the production of more valuable light oils from ELTs.

Fixed bed reactor’s popularity is also supported by the fact that it allows slow pyrolysis process with long residence time for evolved products promoting cracking to valuable chemicals (Alsaleh and Sattler, 2014). However, to scale-up the operation successfully the economical return of the products must overcome the costs associated with operation and time spent in loading and cleaning the set-ups. The thermal lag and heat transfer limitations between the furnace and the actual reactor bed, is one of the most common problems associated with the fixed bed operation due to the configuration of the set-up. Therefore, overcoming this issue will be of interest for scale-up operation especially if a uniform distribution of the heat is achieved along the reactor’s bed. This is one of the advantages that this current work provides and will be discussed at later stages of this communication. Furthermore, FBRs have also been employed in various studies in the past to overcome catalytic ELTs pyrolysis problems associated with fixed bed units. Due to the fact that the catalysts is placed on the distributor, access to catalysts is easier and the operation provides high mixing with the fluids to yield higher surface area to host the cracking reactions (Sharuddin et al., 2016). Raj et al. (2013) utilized slow feeding rates (10 g min^-1) to treat ELTs in an FBR to maximise oil yields. A low pyrolysis temperature was used (440 °C) which resulted in determining the optimal operating conditions for the process after varying the operating temperature and feeding rates by developing a mathematical model based on the yield of each product (oil, char and gases). Kaminsky et al. (2009) used FBR to produce about 31% of pyro-oil from the treatment of shredded ELTs under a temperature range between 750 to 780 °C. However, it was also reported that the use of whole tyres reduces the oil yield to some 26% with an operating temperature of 700 °C. It is also commonly accepted that FBR units are optimal to conduct fast catalytic pyrolysis with small particle sizes to produce high quantity and quality products from ELTs pyrolysis (Alsaleh and Sattler, 2014).

Another popular choice for the pyrolysis of ELTs is rotary kilns which have used in past research as a means to produce high quality products from solid waste and ELTs (Zajec, 2009). Typically rotary kilns are lined with refractory materials, in order to, protect steel surfaces from high temperatures. Rotary kilns are also inclined (1o to 10o) to ease the waste feedstock throughput, and have been optimised for operation against rotation speed and particle sizes of feedstock (Alsaleh and Sattler, 2014). Ayanoglu and Yumrutas (2016) pyrolysed ELTs in a rotary kiln reactor (between 400 to 500 °C) with a maximum yield of light and heavy oils in the amount of 12% and 25%, respectively. Physical and chemical analysis of the products revealed that they were in close proximity to standard petroleum fuel. Yazdani et al. (2019) performed ELTs pyrolysis with a wide range of temperatures between 400 to 1050 °C to determine the effect of temperature on product yields. The maximum pyo-oil was obtained at 550 °C with gases increasing as temperature rises. Acevedo et al. (2013) performed the pyrolysis of scrap tyres with a coal of 36 wt.% volatile matter content in a rotary oven and compared it to a fixed one. The oils obtained from the rotary oven were higher in aromatic content with lower amounts of oxygenated functional groups. This was attributed to their higher residence time in the hot zone of the rotary reactor. Another popular choice typically used for its ability to have good mixing for large particle size materials, is conical spouted bed reactors (Artetxe et al., 2013; Olazar et al., 2009; Al-Salem et al., 2017b). Amutio et al. (2012) used commercial FCC catalyst to pyrolyse ELTs in a conical spouted bed reactor. The catalyst was steamed to maximise the obtained diesel fraction (50.5%) at 450 °C. Lopez et al. (2017) performed flash pyrolysis in a conical spouted bed for ELTs originating from truck tyres under operating temperatures between 425 to 575 °C. It deemed that 475 °C was the appropriate temperature for ELTs pyrolysis using this experimental set-up given that it ensures total devolatilisation of tyre rubber based on the chemical analysis of the obtained products. Olazar et al. (2008b) also used a similar set-up with a moderate temperature for ELTs pyrolysis. The amount of 62% conversion to pyro-oil was achieved at 500 °C. Other non-conventional reactor set-ups have also been reported in literature. Microreactors are one such prime example that will typically eliminate heat transfer limitations between feedstock and surrounding media whilst promoting good catalyst contact (Hafeez et al., 2018). Zabaniotou and Stavropoulos (2003) performed pyrolysis on ELTs using wire mesh microreactor on a 200 g feedstock. The temperature of the mesh was kept between 390 to 890 °C and the experiment resulted in oil by the amount of 5% only. This was attributed to the high heating rate due to the electrical current supplied through the mesh (70 to 90 °C s^-1) representing one of its drawbacks. A plasma reactor was used by Huang and Tang (2009) with operating temperatures reaching up to 1500 °C. The set-up resulted in high hydrogen gas and carbon monoxide (CO) production in the product gas stream (78%). Vacuum reactors have also been previously reported in literature in various set-ups and operating modes (batch and continuous) operating at around 500 °C (Zhang et al., 2008; Pakdel et al., 2001). The oil yield from ELTs pyrolysis is typically within the range of 47 to 56% with gas evolved gas yield. In this work, a newly developed reactor system that is patented with the United States Patent & Trademark Office (USPTO) has been employed for the pyrolysis of real life waste ELTs. The reactor system overcomes various disadvantages faced with conventional fixed bed reactors namely in heat distribution around the furnace casing surrounding the feedstock material to result in better deterioration of feedstock and yield higher pyrolysis oil fraction in a fully automated system that is compatible for scale-up and commercial use. The main objectives of the current study are to report on the total products yields, determine the pyrolysis oil components and compatibility as a fuel range product and to study the degradation behaviour of the ELTs valorised.