Mechanical recycling involves two processes: physical methods to homogenize the waste (i.e., storage, shredding, washing, and sorting) and melt processing (i.e., re-granulation and reprocessing) (Vannessa, 2007). A generalized mechanical recycling process for plastic recovery is depicted in Figure 6. The flakes are further processed by compounding into a more turgid material (Ragaert, 2019).

The recyclates can effectively replace the original plastics. The process after re-melting could involve injection molding, extrusion, rotational molding, and heat pressing (Lettieri and Baeyens, 2009; Ignatyev et al., 2014).

Generally, the mechanical recycling method is only applicable to thermoplastic materials. Examples of mechanical recycling of post-consumer plastics waste include 1) collecting, cleaning, sorting, pulverizing of PP crates, followed by blending with plain polymer for molding new crates (Coulier et al., 2007), 2) collecting, washing, pulverizing, re-washing, sorting, drying, re-granulation, and converting of low-density polyethylene (LDPE) films into refuse bags (Ragaert et al., 2017), and 3) collecting, sorting, pulverizing, washing, sorting, drying, and processing PET bottles into polyester fibers, used to make diverse forms of sheets and containers (López-Fonseca et al., 2011; European Environmental Agency, 2019).

Chemical recycling occurs by chemically reducing a polymer to its original monomeric form for reprocessing (re-polymerization) into brand new plastics (Figure 7) (Andrady, 2003; Karayannidis and Achilias, 2007; Al-Salem et al., 2009; Francis, 2016; Das and Tiwari, 2018). Here, the thermal and catalytic depolymerization of long polymer chains into oligomers can either be deployed solely or used to complement mechanical recycling (Grigore, 2017). The option of reproducing the original polymer or a different kind suffices because the derived monomers, oligomers, or mixtures of other hydrocarbons are suitable feedstock materials (Olah et al., 2008; Francis, 2016).

For example, through microwave irradiation in the presence of (di)ethylene glycol and metal salt catalyst, PET monomerizes into the intermediate monomer bis(2-hydroxyethyl)terephthalate (BHET) (Pingale and Shukla, 2008; Achilias et al., 2010). The BHET can then be used to reproduce PET via polymerization with the release of ethylene glycol (Scheirs, 1998; Scheirs and Long, 2003).

The chemical processes for depolymerizing plastics include glycolysis, gasification, methanolysis, and pyrolysis. This review focuses on the pyrolysis route.

During the chemical recycling of PET via glycolysis, ethylene glycol molecule is incorporated into the PET chains, forming bis(hydroxyethyl) terephthalate (BHET), which is a substrate for PET synthesis and other oligomers (Colomines et al., 2005; Karayannidis and Achilias, 2007; López et al., 2011). Generally, BHET catalyzes PET glycolysis of several ionic liquids. For instance, with basic ionic liquid, 1-butyl-3-methylimidazolium hydroxyl ([Bmim]OH), it exhibits higher catalytic activity than 1-butyl-3-methylimidazolium bicarbonate ([Bmim]HCO3), 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), 1-butyl-3-methylimidazolium bromide ([Bmim]Br) (Yue et al., 2011). Elsewhere, the purification of glycolysis products was catalyzed by ionic liquids, a process simpler and more efficient than the use of traditional compounds (such as metal acetate) (Wang et al., 2009).

Gasification involves partial combustion, initially developed for coal and oil industries. There are several system variations, depending on the gas used, such as pure oxygen, air, steam, oxygen-enriched air, or CO₂ (Mishra et al., 2018). In turn, the temperature required depends on the fuel type, usually within 800–160°C. Gasification is beneficial for fuel gas production because one gaseous product is formed. However, the process is too energy intensive, cost-ineffective, and potentially unsafe (Sikarwar et al., 2017). The synthetic gas generated is graded based on its composition, heat capacity, and applicability. Usually, more than 73% of the waste's carbon content is converted to gas, leaving behind benign ash residue for subsequent and terminal disposal (Vannessa, 2007).

Elsewhere, a typical example of methanolysis is one whereby polyethylene terephthalate (PET) is degraded by methanol at 180–280°C and 2–4 MPa, yielding dimethyl terephthalate (DMT) and ethylene glycol (EG), among other minor by-products (Al-Sabagh et al., 2016; Geyer et al., 2016). The chemical process of PET methanolysis is presented in Figure 8.

Energy recovery usually refers to the recovery of the inherent energy of a material (Eriksson and Finnveden, 2017). Because most polymers oil-based materials, it is understandable that they are excellent sources of liquid fuel. The quantity of energy recoverable from a plastic when incinerated largely depends on the calorific value of the plastic. The approximated average calorific value of mixed plastic waste is 35 MJykg, superior to those of paper (16 MJykg) and organic waste (3 MJykg) (Hopewell et al., 2009). Aside from incineration, waste-derived fuel and recovery of methane from landfill are other energy recovery approaches.

Comparatively, Japan and the United States are nations on the frontline of implementing energy recovery technologies. For instance, Japan subjects about 78% of its municipal waste to energy recovery mechanisms, while Denmark and the United Kingdom only use 58 and 9%, respectively (Goodship, 2007). These wide differences among the nations can be attributed to the varying degree of acceptance of landfilling as a waste disposal option. However, with the growing cost and strict restrictions on landfilling, the prospect of energy recovery is expected to improve.

Many European countries have adopted routine use of municipal solid waste combustors with modern energy recovery and fiue gas cleaning technologies to improve meeting the high domestic electricity demands. It is more energy efficient when the combustor is coupled with a local municipal heating system, thereby producing and supplying hot water and steam to homes and other structures where needed (Ryu and Shin, 2013). In some areas in Paris, France, residential buildings are equipped with combustors, enabling domestic waste incineration that also provides readily available and low-cost heating systems for the residents (Larané, 2000). The first ever electricity generating plant that operates solely on waste plastics as fuel was built in Japan. The plant can manage 700 tonnes of plastic wastes a day, generating enough power for 30,000 Japanese homes (Vannessa, 2007).

Al-Salem et al. (2020) pyrolyzed plastic waste in an Auger Pyrolysis Reactor at 500°C. Their research gathered that plastic waste reclamation from landfills and subsequent conversion into hydrocarbon products was efficient and commercialized, judging by the mass balance and product characterization. Further analysis confirmed that the liquid fractions shared similar properties with petrol and diesel, while the wax products were viable and could be used for coating, covering, and lubrication (Gabbar et al., 2017). Before then, Demirbas (2004) pyrolyzed conducted three plastics waste types viz. polystyrene, polyethylene, and polypropylene, retrieved from landfill. The pyrolyzed polystyrene yielded higher liquid (especially styrene, with minor quantities of naphtha, gasoline, and light gas oil). In contrast, the others were more efficient in generating gaseous fuels (mainly paraffinic hydrocarbons and olefins).

Onwudili et al. (2009) carried out Co-pyrolysis of PS and PE at 300–500°C in a closed batch reactor. Specifically, PE degraded to oil at 425°C. Beyond this temperature, the amount of fuel oil (mainly aliphatic hydrocarbons) product reduced as more char and fuel gas were produced. On the other hand, PS degraded at ≈350°C into a viscous dark-colored aromatic oil, mainly toluene and ethylbenzene, often used as raw materials in the paint industry. Thus, a closed batch system can effectively degrade LDPE and PS into high-grade liquid fuels, new industrial raw materials, and/or chemical feedstock for the petroleum refinery.

About a decade ago, Adrados et al. (2012) collected waste plastics (of various types, such as PE, PP, PS, etc.) from a local material recovery facility. They pyrolyzed the waste plastics in a non-stirred, semi-batch reactor, at a heating rate 20°C/min to 500°C, and held for 30 min. The final products consisted of 40.9 wt% of oils, 25.6 wt% gases, and 5.3 wt% of char, with 28.2 wt% of inorganic residue, derived from the non-plastic packaging materials in the waste. The oils consisted of C5–C9, C10–C13, and >C13 compounds, while the pyrogas comprised of light hydrocarbons (such as methane, ethane, ethene, and <C6 molecule), CO2, CO, and hydrogen. The char, with 29.3% carbon content, is potentially an excellent solid fuel.

Elsewhere, Chanashetty and Patil (2015) carried out pyrolysis of waste LDPE to derive liquid fuel similar to petrol or diesel. The team found that the properties of the fuel obtained were similar to those from petrol. Also obtained from the thermal cracking were some light gases, such as methane, ethane, propane, and butane.

More recently, Budsaereechai et al. (2019) pyrolyzed four waste plastic types viz. PS, PP, LDPE, and HDPE in the presence of pelletized bentonite clay as a catalyst. They reported that the oils from PS were primarily aromatics in the gasoline range (C5–C9). At the same time, PP, LDPE, and HDPE yielded aliphatic hydrocarbons with longer chains, making them suitable fuels for diesel engines.

Due to the incessant increase in the production, use, and abuse of plastics on a global scale, the rate and magnitude at which plastic wastes are generated have also increased astronomically in the past 3 decades. This scenario has led to severe environmental problems arising from the need to dispose of such huge quantities of waste daily (Miteva et al., 2016). Because plastics are non-biodegradable, eliminating them from the ecosystem is cumbersome. Their common treatment via indiscriminate burning has been indicted to contributing to greenhouse gas emissions. Thus, incineration has sufficed as a major treatment option for plastic waste in reducing greenhouse gas emissions (Oasmaa et al., 2020).

Of the three main plastic waste maintenance methods (i.e., incineration, mechanical recycling, and chemical recycling (pyrolysis)), landfilling is more responsible for severe environmental issues because of the environmental load involved (Miskolczi et al., 2009). Plastic waste disposal via landfills provides habitats for insects and rodents, causing various diseases among nearby residents (Alexandra, 2012). Because landfilling is time-consuming before noticeable results can be observed, the method has been found to induce environmental burdens, acting as stressors to the soil, aquifers and promoting air pollution in the form of odor release (Al-Salem et al., 2020).

Alternatively, the incineration of plastics produces a large amount of thermal energy, which is often accompanied by some air pollutants. When not properly operation, incinerators release carcinogens in the form of dioxin, furan, and toxic metals in the smoke, causing secondary environmental problems (Chanashetty and Patil, 2015). Based on these facts, researchers have agreed that the most prospective route for plastic waste management (i.e., control and utilization) is via pyrolysis (Miskolczi et al., 2009; Syamsiroa et al., 2014). Moreover, the oil produced from biomass pyrolysis is a “green” fuel because it is combustion contributes to lowering CO2 emissions. Overall, pyrolysis is a more efficient, economical, and cleaner plastic waste management technology (Abnisa et al., 2004).

To improve the energy recovery efficiency of plastic waste pyrolysis and shorten the recovery time, numerous catalysts (such as zeolite, silica-alumina, and fluid-cracking catalyst) have been used.

Catalytic pyrolysis of plastic waste with binder-free bentonite clay pellets was carried out by Budsaereechai et al. (2019). The clay pellets successfully ensured the production of liquid fuels with improved calorific values and lowered viscosity. Also, the study demonstrated that using the catalyst in a powdery form induced unwanted pressure drop in the catalyst column, a scenario avoided by using pellets. Also, with the pellets, the operation time was reduced to only 10 min/kg of plastic waste. Moreover, no wax was formed in this particular system.

Elsewhere, Suhartono et al. (2018) reported plastic pyrolysis using natural zeolite as a catalyst in the search for alternative fuel. The catalyst achieved the highest fuel oil yield derivation. The researchers obtained 650 ml (65% vol/w) and 550 ml (55% vol/w) of fuel oil from 1 kg of HDPE and LDPE, respectively. However, without catalysis, the respective yields were 250 ml (25% vol/w) and 300 ml.

In another study, Miteva et al. (2016) performed catalytic (Al2O3-SiO2) pyrolysis on waste plastic. The researchers reported that SiO2 exhibited the most significant effect on the quality and quantity of liquid fuel produced. Another research on fuel production from municipal plastics wastes using Y-zeolite and natural zeolite as catalysts was reported (Syamsiroa et al., 2014). Here, the catalyst reduced the liquid fraction at the expense of the gaseous fraction. The authors attributed this result to the presence of impurities in the waste. When the two catalysts were later compared against each other, it was found that the natural zeolite catalyst was more efficient of the two materials. Elsewhere, zeolite catalyst has been credited with lowering the impurities' levels derived from the oil of pyrolyzed municipal plastic waste (Miskolczi et al., 2009).

Apart from using zeolite, bentonite, and silica/aluminum, spent Fluid Cracking Catalyst (FCC) has been adopted as a catalyst to improve the plastic conversion during pyrolysis (Lee et al., 2003). The efficiency of FCC was compared with that of thermal degradation (without catalyst). FCC was observed to have lowered the degradation temperature, hastened the liquid fuel product rate, and reduced the residue content than the conventional thermal degradation.

Although the environmental benefit of plastic pyrolysis has been established, there is a need to evaluate the cost of operating a plastic pyrolytic process for commercial purposes (Enerkem, 2018). In this scope, the production cost is the consumption of Liquid Petroleum Gas (LPG) for the pyrolysis of plastic waste, excluding equipment investment cost. Suhartono et al. (2018) evaluated production cost and the price of oil fuel produced per unit 2 kg of plastic waste raw. In their research, they concluded that the operational cost of pyrolysis using LPG as a thermal source was IDR 12,300/L (≈$170/L) of fuel oil, while the present market price of conventional kerosene (as at the time of their research) was IDR 13,600/L (≈$180/L).

Last year, Kulkarni and Shastri (2020) also conducted an economic analysis of waste plastic pyrolysis in Mumbai, India. In their evaluation, the factors considered include the costs of transporting sorted waste (over 40 km from the plant site), shredding, and the power required by the pyrolyzer. The ignored factors include the labor cost. The heat energy generated from the plant was used to generate the electricity needed for the operation. Considering these factors, they estimated the total expenditure as INR 8,189.8/ton (≈$112.6/ton) of plastic waste, with a total earning of INR 15,236.9/ton (≈$210/ton) of the trash. Hence, a profit of INR 7,047.1 per ton ($97/ton) was achieved.