Recent Advances in Catalytic Pyrolysis of Municipal Plastic Waste for the Production of Hydrocarbon Fuels

Abstract

Currently, the resources of fossil fuels, such as crude oil, natural gas, and coal, are depleting day by day due to increasing energy demands. Nowadays, plastic items have witnessed a substantial surge in manufacturing due to their wide range of applications and low cost. Therefore, the amount of plastic waste is increasing rapidly. Hence, the proper management of plastic wastes for sustainable technologies is the need of the hour. Chemical recycling technologies based on pyrolysis are emerging as the best waste management approaches due to their robustness and better economics. However, research on converting plastic waste into fuels and other value-added goods has yet to be undertaken, and more R&D is required to make waste-plastic-based fuels economically viable. In this review article, the current status of the plastic waste pyrolysis process is discussed in detail. Process-controlling parameters such as temperature, pressure, residence time, reactor type, and catalyst dose are also investigated in this review paper. In addition, the application of reaction products is also described in brief. For example, plasto-oil obtained by catalytic pyrolysis may be utilized in various sectors, e.g., transportation, industrial boilers, and power generation. On the other hand, byproducts, such as solid residue (plasto-char), could be used as a road construction material or to make activated carbon or graphenes, while the non-condensable gases have a good potential to be utilized as heating/energy source.

1. Introduction

The demand of energy is continuously increasing due to increasing population and industrialization. Higher energy demand has led to rising demand for petroleum exploration as well as more environmental pollution [1]. As per the petroleum planning and analysis cell, India consumed almost 133.05 billion litres of fuels in the 2020 financial year. The trend of the fuel market from 2011 to 2019 in India shows that the fuel market increased by 152.7 billion from 2015 to 2019, which was 3.1% higher than that reported for the 2018 financial year [2]. As a result, it is reasonable to expect that it will rise in the next few years as the population’s need for energy rises. Therefore, there will be a need to place more emphasis on alternative fuels.

In recent times, there has been a strong tendency to move away from traditional fossil fuels in favor of novel and renewable energy sources that are cleaner, safer, and inexhaustible. With the supply-demand gap expanding, it is more important than ever to diversify energy sources and look into alternative ways to meet the country’s energy demands while preserving economic growth. Growing environmental concerns present a huge issue for energy companies, emphasizing the need to move to cleaner, more sustainable energy sources, such as compressed natural gas (CNG), liquefied petroleum gas (LPG), hydrogen, biodiesel, and plastic-derived oils. Several state-of-the-art studies show that these alternative fuels are a valuable source of energy, with fewer emissions, and are being used in the energy sector [3,4,5,6,7], e.g., CNG is used in cars, but there is a limit on its power output [8]. Hydrogen is a clean energy source with low toxicity, but it may not be considered the safest fuel in commercial vehicles [9,10]. Similarly, the Indian government has been focusing on biofuels as they have the potential to be used in the transportation sector. However, as compared to conventional diesel, the higher oxygen concentration in biofuels enhances nitrogen oxides (NO_x) emissions in the vehicle, but some controversial studies have suggested that it depends on the composition of biodiesel [11]. It can be 10% higher than petroleum-based compounds, which may dissolve in moisture in the atmosphere and generate acid rain. However, fuel produced from municipal plastic waste attracts the researcher due to its higher potential and availability [12].

In the current scenario, plastic is one of the essential products utilized in a huge amount by the growing population due to its durability, lightweight, and ease of production, making it more viable in utilization. Figure 1 depicts the plastic production in 2020 and the plastic production forecasted from 2025 to 2050. Plastics end up as waste after being used once. The Central Pollution Control Board (CPCB) reported that 3.3 million tons of waste plastic were generated in India in 2018–19 [13] that were dumped into the nearby municipal dumping yards.

Collection and segregation of the municipal mixed plastic waste from the dumping yard is again a challenging step, and it should be cost-effective to implement the technology for waste to energy conversion [14]. Technically, it is possible to separate municipal mixed waste into distinct streams; however, this may be more expensive due to the infrastructure requirements for efficient collection and separation. It is noted that existing technology such as tribocharging and electrostatic separation technology can separate the different compositions of waste plastic: high-density polyethylene (HDPE)/polypropylene (PP), low-density polyethylene (LDPE)/PP, and polyethylene terephthalate (PET)/ polyvinyl chloride (PVC) [15]. Other technologies, such as biological treatment to separate nanoparticles from plastic waste and a hierarchical classification strategy to separate LDPE and HDPE, are similar [16]. Thus, different technologies are available to separate municipal mixed plastic waste (MPW), but the employment of the technology to handle the waste plastic may not be economically viable. Sharuddin et al. discussed, in a review study, that different kinds of plastic (PET, PVC, HDPE, LDPE, and MPW) have the potential to be converted into energy as valuable liquid products. However, they concluded that PET and PVC both have a lower tendency to convert into valuable liquid products than other plastics. For instance, thermal degradation of the polyvinyl chloride (PVC) releases hydrochloric acid, adversely affecting liquid product quality and yield [17]. Moreover, contamination of PVC in polyethylene terephthalate (PET) will decompose the resin of PET by becoming brittle and yellowish, which further requires reprocessing. As a result, MPW treatment via the thermal degradation process is not worthwhile, with impurities necessitating some additional operation for waste plastic segregation based on colors, transparency, and resin [18].

A study conducted on the pyrolysis of mixed plastic waste contained 46 wt.% LDPE, 30 wt.% HDPE, and 24 wt.% PP at 650 °C temperature via catalytic pyrolysis (1% of Z–N catalyst), and the ratios of reaction products (gas/liquid/solid) were 6.5/89.0/4.5 wt.%, respectively [19]. On the other hand, some studies have shown that the pyrolysis of municipal plastic waste produces a lower liquid yield of less than 50%, while single plastic feedstocks yielded liquid yields of 97.0 wt.%, 93.1 wt.%, 82.12 wt.%, 80.88 wt.%, 23.1 wt.%, and 12.79 wt.%, respectively, for PS, LDPE, PP, HDPE, PET, and PVC [20,21,22,23,24,25,26]. Similarly, Cepeliogullar et al. explored the pyrolysis of PET waste by using the fixed-bed reactor at a temperature of 500 °C and a heating rate of 10 °C/min, where they observed 23.1 wt.% liquid yields and 76.9 wt.% gaseous with no solid product [24]. However, lower liquid yield indicates low availability of volatile content in PET at around 86.83% compared to the other plastics (>90%) [24]. Salem et al. performed thermal pyrolysis of the reclaimed plastic wastes from unsanitary landfill sites in an auger pyrolysis system at a temperature of 500 °C. The process mass balance was developed on a dry basis, and the yields of pyro-oil, light wax, heavy wax, and gases were 5.5, 23.8, 69.4, and 1.3 wt.%, respectively [27]. The result proves that when increasing the temperature, the liquid yield will increase up to a certain limit, but too-high temperature will reduce the liquid yield and increase the gaseous product. This may be due to the reason that lower temperature leads to volatile formation at a slow rate, which allows sufficient time for volatiles to crack into lower hydrocarbons inside the reactor before leaving to the condensation section, whereas at higher temperature, the volatile formation is rapid due to higher temperature, which does not provide the same residence time for the volatiles to stay inside the reactor to crack down into more smaller hydrocarbons [28]. Furthermore, at higher temperatures, the reaction mechanism is dominated by the inter- and intramolecular hydrogen transfer, followed by b-scission, producing more lighter hydrocarbons.

Table 1. Proximate analysis of different type of plastic wastes.

Type of Plastic	Fixed Carbon (%)	Moisture Content (%)	Ash (%)	Volatile Matter (%)	References
PET	14.5	NA	0.7	84.8	[29]
PET	13.9	NA	NA	84.1	[30]
HDPE	NA	NA	0.8	97.15	[30]
HDPE	16.85	NA	NA	83.15	[31]
HDPE	0.00	0.00	0.01	99.99	[32]
LDPE	0.68	0.30	3.37	95.61	[33]
LDPE	0.051	0.11	0.023	99.816	[34]
PP	1.62	0.16	4.45	93.77	[35]
PP	1.0	NA	NA	96.9	[30]
PP	0.5	NA	0.00	99.5	[36]
PP	0.43	0.29	0.00	99.28	[37]
PS	1.05	0.32	0.09	98.54	[38]
PS	0.22	0.00	0.00	99.78	[39]
PS	0.071	0.09	0.025	99.814	[34]
PE	0.07	NA	NA	99.93	[40]
PE	0.00	0.2	0.4	99.4	[41]
PVC	1.97	0.65	0.11	97.92	[42]
PVC	4.10	0.00	0.01	95.89	[43]
PA	0.69	0.00	0.00	99.7	[44]
ABS	0.04	0.10	0.99	98.87	[45]
ABS	9.6	0.00	13.6	76.8	[46]
PBT	2.88	0.16	0.00	97.12	[45]
MPW	5.34	0.86	1.21	93.45	[13]
MPW	3.5	0.00	3.3	93.2	[47]

shows the volatiles present in different types of plastics.

In the present study, the author focused on thermal pyrolysis and factors affecting the process. Recent development in the pyrolysis process was also investigated, including a comparative study with catalytic thermal pyrolysis with different catalysts and their effect on the solid, liquid, and gaseous hydrocarbons.

Figure 1. Worldwide production of plastic [48,49].

Figure 1. Worldwide production of plastic [48,49].

2. Thermal Decomposition of Plastic Waste

Conventional Pyrolysis Process

Conventional pyrolysis of plastic refers to the pyrolysis process that uses an electrical heater or burner as the source of heat. Many studies have been performed on conventional pyrolysis using different types of plastics, e.g., PS, PP, PS, HDPE, LDPE, and mixed waste plastic [23,50,51,52]. The operating modes for plastic pyrolysis are batch, semi-batch, or continuous reactors, and each has pros and cons of its own. High conversion can be achieved in a batch reactor, but the inconsistent nature of the end product and the high labor costs make it unsuitable for industrial production. Although fixed-bed reactors are noted for their simplicity in design, they have a smaller surface area where reactions can take place. Fluidized-bed reactors, as opposed to fixed-bed reactors, guarantee that the catalyst and fluid are thoroughly mixed, increasing the catalyst’s usable surface area and enhancing heat transmission. Large-sized particles can be handled by conical spouted-bed reactors (CSBR), although product collection and catalyst feeding provide technological hurdles. Ghodke et al. carried out the conventional pyrolysis of virgin mixed plastic and municipal mixed plastic waste (MMPW) at 500 °C and obtained a maximum yield of 62.5 wt.% with MMPW. Heating is the main cost and bottleneck in pyrolysis. The huge cost of the conventional pyrolysis method is due to inefficient heating and heat loss. Thus, researchers have recently attempted to utilize other heating sources, for example, solar [53], microwave [54], and other renewable energy sources [55,56].

3. Recent Development in the Pyrolysis Process

The pyrolysis process decomposes organic matter by heating it at an elevated temperature with no atmospheric oxygen. Heating is required to break down the hydrocarbon molecules at suitable temperatures in the pyrolysis reactor and produce solid, liquid, and gaseous fuels. The quantity of these pyrolysis products depends on feedstock, temperature, heating rate, and the type of reactor being used for the process. On the basis of reaction temperature and heating rate, the pyrolysis process is categorized in slow, fast, and flash pyrolysis. The pros and cons of different pyrolysis processes are presented in

Table 2. Recent developments in the pyrolysis process [57,58].

Name of Pyrolysis	Temperature Range	Residence Time	Heating Rate (°C/s)	Feed Stock Size (mm)	Liquid (wt.%)	Solid (wt.%)	Gases (wt.%)
Slow pyrolysis	300–700 °C	10–100 min	0.1–1	5–50	30	35	35
Fast pyrolysis	400–800 °C	0.5–2 s	10–100	<3	40–70	15–25	10–20
Flash pyrolysis	700–1200 °C or above	<0.5 s	1000	<0.2	10–20	10–15	60–80

and Figure 2.

3.1. Slow Pyrolysis Process

Slow pyrolysis is pyrolysis that takes place at a lower operating temperature, with a slower heating rate, and allows more production of coke, tar, and char [59]. The feedstock is pyrolyzed with a low heating rate of around 0.1 to 0.8 °C s⁻¹. Slow pyrolysis heating rates combined with a long feedstock residence time result in more char production. The synthesis of H₂, CO, CH₄, and C₅H₁₂ is aided by a significantly lower non-isothermal heating rate, while fast pyrolysis yields predominantly gases [60].

Das et al. demonstrated in their study that thermally degraded PP and PE wastes produce light hydrocarbon liquid fractions (C6–C20) at low temperatures (400 °C). Slow pyrolysis, on the other hand, produces higher char yields [61].

Thus, the carbonization method was introduced as a novel method for producing charcoal as a valuable product for underserved areas. The process requires a proximate feedstock analysis to analyze the moisture content, ash, fixed carbon, and volatile matters. Although process temperature plays a crucial role in product distribution particle size, heating rate and moisture content also have a big impact on pyrolysis products [56].

3.2. Fast Pyrolysis

Fast pyrolysis is a cutting-edge and promising technology that contemplates rapid thermal degradation with increased heating rates and/or high process temperatures for rapid product quenching and liquid product formation. This process is allowed to produce good-quality liquid fuel with a higher calorific value [62].

Fast pyrolysis results in high yields of gaseous components with low yields of char and oil because the decomposition of the plastic waste is rapid, allowing the molecules to break into small carbon chain compounds such as methane, ethane, propane, butane, etc. During fast heating, the dissociation reaction of the waste plastics allows the β-scission, followed by inter- and intramolecular hydrogen transfer, leading a larger quantity of short-chain hydrocarbons [62]. The process requires a higher heating rate of around 10 to 100 °C s⁻¹, and the feedstock particle size must be less than 3 mm. Mass transfer rate, heat, and chemical kinetics are the essential factors in deciding the pyrolysis product chemistry. Various researchers conducted studies on fast pyrolysis and produced higher liquid yield at a temperature range of 500–550 °C [57,62,63,64,65]. The process produces byproducts such as residue char and pyrolytic gases. The state-of-the-art shows that pyrolytic gases increase by decreasing the liquid oil and solid-product-char [45,63,65].

3.3. Superfast/Ultra-Fast Pyrolysis

Superfast pyrolysis is the most advanced pyrolysis technique. The process produces higher amount of operable liquid oil and pyrolytic gases with lower water content at a high-temperature range of 700 to 1200 °C or above. The process requires a higher heating rate of about 1000 °C s⁻¹ or above to provide a higher temperature to the feedstock. The feedstock particle size should be less than 0.2 mm because of the faster heating rate [66]. The pyrolysis process converts 80% of the energy contained in the feedstock distributed into various energy products, producing higher energy-dense products than the raw feedstock. However, the liquid oil produced contains much oxygen content, indicating that the product is unstable for further use. It also contains impurities such as heavy metals and nitrogen, and removing these pollutants necessitates a considerable amount of hydrogen, which makes the process somehow costly [67].

3.4. Catalytic Thermal Degradation (CTD)

The catalytic thermal degradation (CTD) process is more advanced than the conventional pyrolysis process because the reaction is completed in a catalyst environment, and offers improved liquid fuel quality compared to the conventional thermal process. Thus, selecting a suitable catalyst plays a vital role throughout the process. The CTD process involves a catalyst to increase the reaction rate of thermal degradation. The catalysts lower the activation energy and the process temperature required to improve the overall process and conversion efficiency. The thermal degradation process is energy-intensive as it requires a higher temperature to degrade the waste plastic and transform it into a quality end product, which might not require any upgrading process [63]. Thus, the CTD process is economically viable to produce valuable products such as liquid oil yields of low molecular weight and upgraded quality at reduced thermal and energy demand. This liquid product is obtained with 38–46 MJ kg⁻¹ higher heating value (HHV) and is almost close to conventional diesel, with 45.5 MJ kg⁻¹ [68]. The plastic waste decomposition can be completed in two stages in the presence of a catalyst. The first stage is related to the disintegration of short-length branches, whereas the second stage is associated with the degradation of the molecular backbone of the main chain.

Moreover, increasing the wt.% of the catalyst leads to increased residue char because of the non-decomposable content available in the catalyst and carbon deposition over the catalyst surface. Therefore, it can be concluded that catalyst concentration directly affects the thermal degradation process. The significance of a catalyst is that it starts the decomposition at a lower temperature, which is proportional to the concentration of the catalyst.

4. Catalyst Effect on Thermal Degradation Process

The catalyst plays a vital role in cracking the hydrocarbon chain during the thermal process, and affects the yield of the optimized product and residence time [56,69]. The yield of the product may vary which may depend on the type of plastic waste and the percentage of the catalyst introduced in the process. Muhammad, et al. [70] studied the thermal pyrolysis of post-consumer mixed plastic waste and a simulated plastic mixture using zeolite HZSM-5 catalyst in a two-stage fixed bed reactor. The highest liquid oil yield was 81–97 wt.% in the thermal degradation of plastic. The addition of the catalyst reduced the liquid yield in the range of 44–51 wt.% and produced a higher gas yield due to the cracking of oil volatiles. Researchers studied the impact of zeolite ZSM-5 on the thermal degradation of plastic. It is economically viable and has excellent thermal stability as well as selectivity [71,72,73]. In the catalytic thermal process, zeolite increases the cracking reaction and deoxygenation to produce liquid oil. Onwudili et al. investigated the effect of temperature and zeolite catalyst type on waste plastic pyrolysis [74]. The introduction of a different catalyst at temperatures ranging from 500 to 600 °C had no apparent effect on fuel range liquid products. However, higher temperatures led to an increase in the gaseous product during the process. Muneer et al. investigated the pyrolysis process on feedstock maize stalk and polypropylene at 500 °C with a 1:4 ratio of ZSM-5 catalyst to feedstock; the liquid hydrocarbon yield increased to 66.4% due to the high surface area and selectivity of ZSM-5 catalyst to form the hydrocarbon products [75]. Herein, catalysts with acidic sites on the surface and the ability to contribute hydrogen ions accelerate the rate of isomerization products, increasing the yield of hydrocarbon isomers and the quality of the fuel generated. Catalysts with more acid sites and high density are more effective in cracking polyolefin. However, the catalyst deactivates quickly due to the high acidity and large pore size. According to the literature, it is better to perform polyolefin pyrolysis in the presence of a catalyst with low acidity [76]. The efficiency of hydrocarbon cracking was improved by dissolving the catalyst in a polymer melted state. The catalysts modified the charge distribution in the carbon chain, allowing them to extract hydrocarbon hydride ions and produce carbonium ions. It improved the catalytic impact, allowing for a lower pyrolysis temperature and increased ion production for olefin and aromatic chemicals [19]. As a result, zeolite catalysts influence the degradation process, boost the generation of gaseous hydrocarbons, and result in high liquid hydrocarbons.

4.1. Factors Affecting the Pyrolysis Process

Pyrolysis of municipal plastic waste has been affected by the critical process parameters that play an essential role in optimizing the product yield and quality in the process. In the MPW thermal degradation process, critical parameters influence the final hydrocarbon products. The parameters include temperature and pressure.

4.2. Effect of Temperature

Temperature is the essential parameter for carrying out the pyrolysis of plastic waste because it regulates the cracking response of the polymer chain and influences product distribution. Temperature controls the breakdown process of the hydrocarbon molecules and alters the amount of gaseous and liquid hydrocarbons. Moreover, it also affects the char produced. The van der Waals force attracts molecules together, preventing them from collapsing [77]. The vibration of molecules inside the system increases as the temperature of the system rises, and molecules tend to evaporate away from the feedstock surface. The energy produced by the van der Waals force along polymer chains exceeds the enthalpy of the C–C bond in the chain, causing the carbon chain to break. In polymers, carbon bonds are nonpolar, stable, and highly tight, requiring a lot of energy to break. As a result, it needs a higher temperature range of 300–800 °C. Various studies demonstrate the effect of temperature on liquid yield with various feedstocks [78,79,80]. For instance, Adnan et al. performed a study on polystyrene waste material at 500 °C where they obtained more than 80 wt.% liquid hydrocarbons [81]. The lower temperature range resulted in the formation of a long hydrocarbon chain. However, higher temperature range resulted in the synthesis of a short carbon chain due to the breaking of C–C bonds and the production of aromatic hydrocarbons.

4.3. Effect of Pressure

The pyrolysis of plastic waste is usually performed at atmospheric pressure. As a result, the impact of pressure on pyrolysis is rarely explored in the literature and needs to be addressed in detail, leaving a substantial research gap in the field. However, some researchers have performed pyrolysis under vacuum (lower than atmospheric pressure) or higher pressure conditions (higher than 1 atm). Low pressures (under vacuum or in the presence of an inert diluent) favor the synthesis of primary products, such as monomers, whereas high pressures favor complex liquid fractions. The boiling point of pyrolytic compounds increases when pressure is increased. This raises the pressure in the atmosphere, causing heavy hydrocarbons to be pyrolyzed rather than vaporized at a given working temperature [82].

Lopez et al. studied the waste tire pyrolysis process under vacuum at 25 kPa and 50 kPa under atmospheric pressure at a temperature 500 °C, where they obtained a higher diesel fraction similar to liquid hydrocarbons above the atmospheric pressure [83]. The principal impact of a vacuum pressure over an atmospheric pressure is an increase in the liquid product. Furthermore, a reduction in pore blockage improved the surface areas of the carbon blacks formed, which had a good influence on the porous structural features of the residual carbon black. The vacuum impact on feedstock was introduced to devolatilization and diffused the volatiles inside the particle. Increasing pressure resulted in more gaseous products and a higher yield of lower molecular liquid hydrocarbons [84].

4.4. Residence Time

The residence time of the feedstock material plays a crucial role in the pyrolysis process. It is defined as the amount of time a particle spends in a pyrolysis reactor as long as the products continue to produce, and has a high impact on product quality as well as quantity [85,86]. Higher residence times favor higher liquid production due to the secondary reaction in the pyrolysis reactor. Al-Salem et al. performed a study on residence time, and the results showed that a higher liquid yield was obtained with increasing the residence time. Studies have shown that residence time influences liquid hydrocarbon yield at a given temperature and that increasing residence time may reduce liquid hydrocarbon yield [87]. Similarly, Mastral et al. studied the thermal cracking of HDPE in a fluidized-bed reactor to obtain the effect of residence time. The experiments were performed at temperatures ranging from 650 °C to 850 °C, with residence time ranging from 0.64 s to 2.6 s. At 650 °C, it was observed that the liquid hydrocarbon yields 79.7% at 0.8 s and 68.5% at 1.5 s [79].

Moreover, the increase in the gas yields were 11.4 wt.% and 31.5 wt.% at a residence time of 1 s and 1.5 s, respectively. The lower liquid yield of 9.6 wt.% was obtained at a temperature of 780 °C and 86.4 wt.% of gaseous hydrocarbons by increasing the yield of methane, ethane, and hydrogen [87].

Therefore, increasing residence times resulted in higher gaseous products. However, along with residence time, other factors, such as temperature, heating rate, catalyst, and reactor design, also simultaneously highly influence the hydrocarbon fuel quality [52].

4.5. Type of Reactor

Fluidized-bed (bubbling and circulation), rotary kiln, ablative, auger, and screw reactors are among the most typical pyrolysis reactors used to produce liquid hydrocarbons from MPW. To achieve successful pyrolysis, two essential design characteristics must be addressed, where the reactor should blend the MPW and catalyst effectively, deliver a large amount of heat to the feedstock, and have a short residence period [88]. Large volumes of solid and gaseous hydrocarbons can be generated if the heat transfer rate is low and the residence time is prolonged.

4.5.1. Fixed-Bed Reactor

A fixed-bed reactor is used in the pyrolysis process and is often operated in batch mode. The reactor’s design is simple to construct and suitable for the uniform size of the feedstock, as shown in Figure 3a. However, larger size of the particles or polymers leads to feeding issue into reactors. Heat is provided to this reactor for the thermal breakdown of MPW either externally or by minimal combustion. The products can flow out of the pyrolizer reactor due to expanding the volume of gaseous hydrocarbons. At the same time, the char remains in the reactor. However, a sweep gas is often used to remove the volatiles from the reactor effectively [89].

In a few cases, the solid catalyst was used as a packed bed where the volatiles pass over the bed and improve the efficiency. On the other hand, fixed-bed reactors are often linked to the problems of feeding irregularly shaped polymers with poor heat conductivity. Moreover, researchers have used the fixed-bed reactor for secondary pyrolysis so that the primary product may be easily transferred to the secondary reactor [90]. The catalytic process of polyethylene with Y-zeolite produced 85 wt.% liquid hydrocarbons. However, in the absence of a catalyst, 95 wt.% liquid hydrocarbons were produced at 500 °C. Furthermore, they concluded that increasing the temperature decreased the liquid hydrocarbons with higher gaseous products [91].

When large volumes of material are processed in the traditional fixed-bed reactor, it shows some limitations regarding a slow heating rate, a long residence period, and a non-uniform temperature of the sample within the reactor. Furthermore, the fixed-bed reactor is uneconomic due to its batch-scale nature. As a consequence, the reactor might be used for research as well as small-scale heating and power generation.

4.5.2. Fluidized-Bed Reactor

A fluidized-bed reactor is the most viable reactor, as presented in Figure 3b, and is being used on the industrial scale to process crude oil. The reactor has comprehensive utilization in the industries due to the continuous provision of the feedstock to produce the liquid hydrocarbons. The upward motion of the co-reactant feed in the reactor suspends tiny catalyst particles in suspension. A gas is often used as the fluid in a fluidized-bed reactor, which facilitates particle mixing by maintaining a high flow rate. The particle size is much lower than in a fixed-bed reactor, ranging from 10 to 300 µm with a constant temperature. Thermal and catalytic thermal processes can be carried out at 290–850 °C in the reactor [91]. Luo et al. conducted a comparative study on LDPE on a fluidized-bed reactor and produced a higher liquid yield [92]. Furthermore, the fluidized-bed reactor provides considerably more significant mass enhancement and heat transmission, reducing temperature gradients in the reactor [93,94]. It shows that a reactor with a fluidized bed would be better suited for a large-scale operation in terms of economic sustainability, even though batch reactors are equally efficient at the laboratory scale of the experiment.

4.5.3. Conical Spouted-Bed Reactor (CSBR)

In the pyrolysis of plastic waste and gasification, a conical spouted-bed reactor is employed. As illustrated in Figure 3c, the CSBR comprises a hopper, feeding system, gas mixture, gas preheater, reactor, and condenser. The conical spouted reactor is conical from the end and has a cylindrical part at the top [56]. The CSBR might be used to process the large particle size distribution with adequate mixing. The principal advantages of the CSBR over the fluidized-bed reactor include fast heat transfer rates and negligible defluidization issues when handling highly sticky inputs such as PE and PP at a particular temperature. The reactor works on a slow pyrolysis process at low temperatures and results in higher wax yield. The results of a study using HZSM-5, HY, and Hβ zeolite-based catalysts in the pyrolysis of high-density polyethylene (HDPE) continuously fed into a conical spouted-bed reactor (CSBR) at 500 °C and atmospheric pressure revealed that the HZSM-5 zeolite-based catalyst is very selective to light olefins, yielding 58 wt.%, whereas Hβ and HY result in high yields of non-aromatic C5–C11 products (around 45 wt.%). Due to catalyst deactivation by coke generation, wax yield increases as reactions proceed, notably with HY and Hβ zeolite-based catalysts [94]. Fernandez et al. conducted a study on the biomass steam pyrolysis in a CSBR and achieved 75.4 wt.% bio-oil at a 500–800 °C temperature range [95]. Arabiourrutia et al., on the other hand, employed CSBR to obtain wax products from polyolefins plastics. They discovered that the spouted bed’s design was mainly suited for extracting wax by pyrolysis at low temperatures since the wax yield decreased with rising temperature [96]. This might be because as temperature rises, more waxes are broken into liquid or gaseous products.

Hence, it may be concluded that CSBR provides efficient heat transmission between phases and proper mixing, and the ability to handle large particles of varying densities. However, feeding and entrapment of the catalyst are challenging.

4.5.4. Rotary Kiln Reactor

Figure 3d shows the schematic diagram of the rotary kiln reactor. A rotary kiln reactor is more efficient due to its proper mixing. The reactor efficiently mixes municipal solid waste (MSW) very well compared to other reactors. However, a slow pyrolysis process is performed in a reactor, suggesting that the heating rates are modest. The heating rate is limited to 100 °C/min, and the residence duration is one hour [97]. Rotary drums, furnaces, and kilns are composed of steel drums walled internally with high-resistance and anti-corrosive refractory material. Reactors are intended for prolonged residence durations of 20 min or more, with inclined low-speed rotational movements. The product in rotary kilns varies significantly due to the huge temperature differential in the system. The rotation of the kiln aids in the appropriate mixing of the MPW. Then, it is progressively heated as pyrolysis gases are produced as it moves down the cylinder and carbonizes. The rotary kiln reactor is the most often described for MPW pyrolysis due to distinct advantages [73]. Some of the benefits include proper MPW mixing, flexibility in residence time modification, the ability to feed heterogeneous materials, the absence of the need for MPW pre-treatment, and ease of maintenance.

4.5.5. Auger Reactor

The auger type reactor is also known as the screw type reactor because of having a screw, as shown in Figure 3e. The reactor consists of a hopper, tubular shape, and having a screw that carries the continuous feedstock in operational mode. The screw’s spinning aids in transferring feedstock into the reactor, and the heat required for pyrolysis is carried through the reactor’s tubular wall. The heat from the outside has been observed to be sufficient to heat reactor tubes with lower diameters [98]. If higher production quantities are envisaged, a hot solid carrier filled with feedstock particles is required. This allows feedstock particles to interact more closely as they move through the reactor tubes. Steel and ceramic pellets may be used to make the solid carrier. The liquid hydrocarbons are produced from the vapors obtained by the pyrolysis process in a condenser and compressing. An auger reactor can be constructed to be extremely small and even portable in certain situations. It may also be used near the dumping yard of MPW.

4.6. Type of Catalyst

Catalysts play a crucial role in enhancing product quality while lowering process temperature and retention time, resulting in overall process optimization. Catalytic cracking is more enticing than thermal cracking alone since it is quicker and needs lower temperatures, lowering the energy requirement substantially. Furthermore, catalytic cracking using catalysts such as zeolites produces high-quality products in the spectrum of motor engine fuels, decreasing the need for additional upgrading. Because its products need additional upgrading, thermal pyrolysis is limited to regions of existing oil refineries [68]. Catalysts decrease the desired pyrolysis temperature, shorten reaction time, create diesel components in the appropriate boiling point range of 390–425 °C, improve gasoline selectivity, and accelerate the occurrence of isomerization [99].

Catalysts are widely employed in refineries and in the pyrolysis of plastics to recover hydrocarbons from waste, both of which follow a similar process. The global refinery catalysts market has been significantly rising despite strict rules and regulations and increased demand for petroleum and its derivatives. The industry’s rapid rise is fueled by the growing demand for transportation fuel. Furthermore, strict emission control regulations throughout the world have demonstrated the necessity for efficient, conservative, and high-yield techniques, which is one of the key drivers behind market advancement. As a consequence of this environmental concern, demand for refinery catalysts has increased, enhancing the efficiency of the process. The type of catalyst impacts the process outcome since a large surface area of the catalyst may change the nature of the pyrolysis and, consequently, have a substantial impact on the pyrolysis products [100,101].

Generally, two types of catalysts such as heterogeneous and homogeneous, have been employed in the thermal degradation of plastic waste. The former uses a single phase (reactant and catalyst in the same phase), while heterogenous catalyst refers to the reaction where the catalyst and reactant are in different phases. Catalysts made of Fe/Al₂O₃ ratio are the most prevalent form of homogeneous catalyst used in plastic waste pyrolysis [101]. Nonetheless, heterogeneous catalysts are the most often utilized in plastic waste pyrolysis because the fluid product can be readily separated from the solid catalyst, easily regenerated, and reused. Nanocrystalline zeolites, the most widely used [102], ordinary acid solids, mesostructured catalysts, for example, metal-supported carbon, and basic oxides. Heterogeneous catalysts have also been reported to endure extreme reaction conditions of up to 1300 °C and 35 MPa and are readily separated from the gas and liquid reactants and products. Heterogeneous catalysts include acid solids, mesostructured, and nanocrystalline zeolites catalysts [103]. Various researchers have shown interest in nanocrystalline zeolites due to their significant role in product distribution [104,105,106]. In addition, various catalysts, such as fly ash, bentonite, silicate, silica-alumina, ceramic, activated char, and MVM-41, have recently attracted researchers’ attention.

Table 3. Catalytic effect on the liquid, solid, and gaseous hydrocarbons.

Feedstock	Reactor Type	Heating Rate (°C min−1)	Reaction Temperature (°C)	Concentration of Catalyst	Type of Catalyst	Reaction Products			References
						Liquid Hydrocarbons	Solid Residue	Gaseous Hydrocarbons
PS	Batch Reactor	10	500	Pelletized	MgO	88	10	2	[107]
Plastic waste (PS+PE)	TL- 200 Tube furnace	10	480	10%	Organic vermiculite	80.6	0.1	19.4	[108]
PS+PE	TL-200 Tube furnace	10	480	10%	Co/Vermiculites	73.2	0.1	26.7	[108]
PS+PE (Plastic waste)	TL-200 Tube furnace	10	480	10%	Ni/Vermiculites	70.7	1.3	28.0	[108]
PS+PE (Plastic waste)	TL-200 Tube furnace	10	480	10%	Co-Ni/Vermiculites	73.9	0.1	26.0	[108]
PP+LDPE+ HDPE	Batch reactor	20	500	33.3%	Calcium bentonite	81.3	12.7	6	[109]
PP+PE+PS+PET	Vertical tube reactor		600	100%	Ca(OH)2	52.2	9.0	27.7	[110]
PP+PE+PS+PET	Vertical tube reactor		600	100%	Cao	46.7	31.8	17.6	[110]
Mixed plastic waste	Semi-batch reactor	20	440 20 °C/min	10%	ZSM-5	56.9	3.2	40.4	[111]
Mixed plastic	Batch reactor	-	240	-	Activated carbon	82.43	15.22	2.35	[112]
Mixed plastic	Batch reactor	-	240	-	Charcoal	95.54	2.33	2.13	[112]
Mixed plastic	Vertical reactor	-	500	10%	USY-Zeolite	70–80			[113]
Mixed plastic waste	Batch glass reactor	5	350	10%	Al–Si	93.11	9.27	0.36	[114]
Mixed plastic waste	Stirred tank reactor	10	450	1.65 kg/h 60 g catalyst (36.36%)	Fe-restructured clay	83.73			[115]
Mixed plastic waste	Vertical tube reactor	20	500	-	CaCO3	68	5.7	26.3	[52]
Mixed plastic waste	Fixed-batch reactor	5.5	400	-	-	86	8	6	[116]
Mixed plastic waste	Fixed-bed reactor	10	600	10%	FCC	69	28	3	[74]
Mixed plastic waste	Batch reactor	15	510	5% 1:1:2	Red mud: Ca(OH): Ni/SAPO	64.2–71.9	-	-	[117]
Mixed plastic waste	Continuous microwave-assisted pyrolysis system	-	620	10% pelletized form	ZSM-5	48.9	-	49.0	[118]

summarizes the catalytic effect on the solid, liquid, and gaseous hydrocarbons with different heating rate and reactors.

4.6.1. Zeolite Catalyst

In the thermocatalytic process, zeolite has been extensively documented as an efficient and selective material catalyst for creating biofuels. ZSM-5 is considered a relatively inexpensive catalyst for transforming plastic waste into liquid hydrocarbons, in addition to its catalyst base. The catalyst also has higher thermal stability, selectivity, activity, and coke deactivation [119]. Zeolites, also known as aluminosilicate crystalline sieves, are solid crystalline structures made up of a three-dimensional framework in which oxygen atoms join tetrahedral sides with open pores which have ion exchange capabilities. The reactivity and efficacy of zeolite are influenced by the SiO₂/Al₂O₃ ratio, which influences the type and quality of the pyrolysis process ultimate yield product. The proportion of silica–alumina in various zeolites varies, and the particular ratio has a distinct reactivity. Ates et al. proved this with varied SiO₂/Al₂O₃ ratios of 30, 80, and 280, utilizing the catalyst HZSM-5 zeolite. SiO₂/Al₂O₃ (30) has the highest acid strength and the largest BET surface area and average pore diameter. Higher acidity of the catalyst is obtained with a SiO₂/Al₂O₃ ratio of 30, sufficient to crack the wax and lead to a higher yield of the light olefins and lower the heavy fraction (C12–C20). However, decreasing the SiO₂/Al₂O₃ from 280 to 30 results in an increase in light olefins output from 35.5 to 58 wt.% and a reduction in C12–C20 and C5–C11 fraction yields from 28 to 5.3 wt.% and 28.8 to 15.2 wt.%, respectively [120]. The pore size and acidity of various zeolite base catalysts are varied, affecting the final product. For example, a larger pore size makes it simpler to convert plastic into polyalkylaromatics, but a smaller pore size converts only to aromatic compounds with small dynamic diameters. According to their findings, choosing the right zeolite to use as a catalyst in the pyrolysis reaction is crucial in deciding the end product formed. The pore size and acidity of various kinds of catalyst base zeolite varies, affecting the end product [121]. Wong et al. investigated LDPE pyrolysis over ZSM-5 zeolite and found that catalytic cracking produced the C1–C8 hydrocarbons due to the efficient heat exchange between catalyst and polymer chain [122].

Sivagami et al. found that LDPE pyrolysis with synthesized ZSM 5 catalyst at 500 °C temperature produced 70% liquid yield with 16 wt.% gas and 14 wt.% solid char. The synthesized ZSM-5 catalyst’s strong acidic characteristics and microporous crystalline structure allow greater cracking and isomerization, resulting in higher breakdown of larger molecules into smaller molecules and more oil production in pyrolysis studies [123].

Susastriawan et al. investigated the influence of zeolite size on the low-temperature pyrolysis of LDPE [80]. The scientists observed that reducing zeolite size might improve reaction rate, pyrolysis temperature, heat transfer rate, and oil products during the pyrolysis process due to larger surface-active area with a smaller size. The oil yields were maximum when the zeolite particle size was 1 mm; however, the results of oil produced were not significant when the particle size was 1–3 mm. According to prior studies published by Kim et al., the presence of the phenolic functional group on the lignin may increase the 39% aromatic hydrocarbon in the final products over zeolite catalysts [124]. Condensation is caused by the presence of a hydroxyl group on the surface of a zeolite-type catalyst that speeds up the formation of aromatic hydrocarbon molecules by dihydroxylation, aromatization, isomerization, and oligomerization pathways [125].

However, researchers have also proven that heat activation and acid activation may improve the catalytic capabilities of natural zeolite. Rashid et al. used natural zeolite that had been modified by thermal and acid activation in the pyrolysis of plastic waste (PS, PE, PP, and PET) and found that acid-activated zeolite has better catalytic activity than thermally activated zeolite [126]. Another advantage of this catalyst is that the spent zeolite catalyst may be recovered and utilized in a similar reaction with a similar goal with more or less efficacy, as well as reduce impurities of sulfur in the liquid product generated [127].

4.6.2. Fluid Catalytic Cracking (FCC) Catalyst

The fluid catalytic cracking catalyst plays a vital role in cracking the polymer during pyrolysis reaction for petroleum-based products. It has been used in the petrochemical industries to convert the high boiling and molecular weight products to the valuable fuel products in gasoline and LPG fractions [128]. Some progress in FCC catalysts has been investigated in order to improve the effectiveness, cracking activity, coke selectivity, and stability of this catalyst, which focused primarily on modifying zeolite using extra aluminum framework where ion exchange takes place [129]. Aisien et al. evaluated that pyrolysis of polyethylene in the presence of an FCC catalyst reduced the liquid yield from 83.3 wt.% to 77.6 wt.%; however, it increased the gaseous hydrocarbons from 13.2 wt.% to 19.7 wt.% and decreased the solid char formation from 3.0 wt.% to 2.7 wt.% [130].

Similarly, Abbas et al. examined the effect of the FCC catalyst in the range of 10 wt.% to 60 wt.% at a temperature range of 450 °C in a stirrer semi-batch reactor and found no major impact on the non-condensable gaseous hydrocarbons, but it seems that there was an increase in the gaseous products [131]. However, it increases the solid hydrocarbons by increasing the catalyst-to-polymer ratio concentration. This might be attributed to greater dehydrogenation and aromatization on the catalyst surfaces, resulting in an increase in solid hydrocarbons on the catalyst surface. Coke causes the catalyst to deactivate, making it ineffective for breaking waxes. Catalyst activity monitoring revealed that an equilibrated catalyst is an excellent choice for plastic waste pyrolysis. Abadi et al. suggested that the FCC catalyst is excellent in the PP pyrolysis to boost the output of liquid hydrocarbons beyond 90 wt.% [132]. Sharuddin et al. [17] examined the spent FCC catalyst over the waste plastics and obtained the 80 wt.% liquid hydrocarbons. With the increasing development in pyrolysis to recycle plastic waste, FCC is a widely used catalyst in the pyrolysis of plastic waste. The fluid catalytic cracking unit (FCCU) is used to supply high-octane gasoline to the refinery gasoline pool, which indicates FCC’s potential in the pyrolysis of plastic waste in the near future [133].

4.6.3. Bimetallic Catalyst

Bimetallic catalysts have been used in the pyrolysis reaction to produce liquid hydrocarbons from the plastic waste. Wen et al. conducted a study on the Ni-loaded CNT for polyolefin and found an appropriate result because of the higher concentration of the carbon available in the catalyst [134]. Bimetallic catalysts provide a synergic effect in the pyrolysis process due to high surface area and stability. Chen et al. conducted a study on the Fe–Ni bimetallic modified MCM-41 with 10 wt.% each, resulting in the 49.9 wt.% liquid hydrocarbon and 65.9% single styrene hydrocarbons [135]. A wide surface area helps the plastic to enter the pore structure directly for cracking. Fe metal acts as a foundation for transforming raw materials to styrene, whereas nickel–metal oxides raise the acidity of the catalyst, allowing multi-ring compounds to be processed into a single hydrocarbon structure. Chemisorption is determined by the external surface character and pore size, which may be designed using the synthesis process. In this scenario, the larger the pore width of the FeNi catalyst, the greater the hydrogen desorption. The large pore size of Co/SBA-15 follows the trend of long-chain products [136].

Furthermore, Mo and Ni have various benefits as pyrolysis catalysts, including cheap cost absorbent, excellent stability and performance, large surface area, and simplicity of manufacture [137]. However, the sulfurization procedure influences the bimetallic catalyst’s reactivity. The improved sulfurization process may consistently improve the proportion of crude oil conversion to fuel oil, which now stands at 86.9%. Zhou et al. conducted the study on the Ni–Fe/ZrO₂ catalyst, resulting in the excellent degradation of the PS at a temperature of 500 °C [138]. Hence, the bimetallic catalyst has good selective conversion in the pyrolysis process and may improve the catalytic activity. The combined characteristics of Ni–Fe might lower the water gas shift reaction and other reforming reaction activation energies [139].

5. Physicochemical Properties of the Plastic Oil with and without Catalyst

Table 4 presents the physicochemical properties of the liquid hydrocarbons produced from municipal plastic waste with and without a catalyst. The physicochemical properties of the fuel play a vital role in its utilization in the transport sector. In recent years, conventional fuels such as gasoline and diesel have been used in engine vehicles. However, continuous work is also being carried out on alternative energy. The conventional fuel density for diesel and gasoline is 0.807 gcm⁻³ and 0.780 gcm^−3,, respectively, which is in line with fuels produced with zeolite and kaolin catalysts of 0.871 and 0.800 gcm⁻³, respectively [140]. Herein, in a combustion chamber, the kinematic viscosity of the fuel appears as a spray pattern and atomization behavior. Too-high viscous oil result in poor fuel atomization and leakage at the fuel injector, respectively, resulting in poor engine performance [141,142]. Therefore, the kinematic viscosity of the fuel should be lower for proper atomization and complete combustion of the fuel in a cylinder engine. Table 4 presents that pyrolysis with a zeolite catalyst obtained a 1.99 cSt lower than other catalysts and close to the diesel viscosity.

Calorific value is the essential characteristic of the obtained liquid hydrocarbons. The calorific value of the liquid hydrocarbon obtained with zeolite and kaolin is 46.67 MJ/kg and 46.47 MJ kg⁻¹, close to the conventional fuel shown in Table 4. Moreover, studies have shown that higher calorific values consume less fuel than lower calorific values, leading to complete combustion in the cylinder and improving the power efficiency of the engine [143,144]. However, the higher pour point of the plastic liquid hydrocarbons lowers the flow characteristic because it may cause the wax to form in the cylinder and make it troublesome in the engine. However, plastic pyrolysis with kaolin clay obtained the pour point at a very low temperature of 18 °C. The flashpoint of the liquid hydrocarbon is very low compared to conventional fuel, indicating the safety hazards of liquid hydrocarbons in terms of explosion and fire during fuel transportation [145,146].

Hence, it is concluded from the table that liquid hydrocarbons have the same characteristics as conventional fuel so liquid hydrocarbons may be used with the blending of diesel and some additives for use in transportation.

Table 4. Physicochemical properties of the liquid hydrocarbons.

Physical Properties	No Catalyst [13]	FCC [118]	Natural Zeolite [147]	Kaolin [44]	Silica–Alumina [148]	Fly Ash [149]	Calcium Bentonite [130]	Gasoline [23]	Diesel [23]
Density @ 15 °C (g/cm3)	0.860	0.752	0.868	0.800	0.770	0.800	na	0.780	0.807
Viscosity (cSt)	2.48	2.273	2.191	2.72	2.21	0.145	2.32	1.17	1.94.1
Calorific Value (MJ/kg)	40.42	43.28	45.58	46.479	na	41.93	43.28	42.5	43.5
Octane Number	na	na	na	na	na	na		81–85	na
Pour Point (°C)	18	−11	24	−18	na	na		na	6
Flash Point (°C)	35	29	<10	40	na	na		42	52
Aniline Point (°C)	60	na	na	na	na	na		71	77.5
API Gravity@ 60 °F	38.1	na	na	na	na	na		55	38
Diesel Index		na	na	na	na	na		na	na
Moisture % vol	2.4	na	na	na	na	na		0.4–0.5	0.1–0.3

Table 4. Physicochemical properties of the liquid hydrocarbons.

Physical Properties	No Catalyst [13]	FCC [118]	Natural Zeolite [147]	Kaolin [44]	Silica–Alumina [148]	Fly Ash [149]	Calcium Bentonite [130]	Gasoline [23]	Diesel [23]
Density @ 15 °C (g/cm3)	0.860	0.752	0.868	0.800	0.770	0.800	na	0.780	0.807
Viscosity (cSt)	2.48	2.273	2.191	2.72	2.21	0.145	2.32	1.17	1.94.1
Calorific Value (MJ/kg)	40.42	43.28	45.58	46.479	na	41.93	43.28	42.5	43.5
Octane Number	na	na	na	na	na	na		81–85	na
Pour Point (°C)	18	−11	24	−18	na	na		na	6
Flash Point (°C)	35	29	<10	40	na	na		42	52
Aniline Point (°C)	60	na	na	na	na	na		71	77.5
API Gravity@ 60 °F	38.1	na	na	na	na	na		55	38
Diesel Index		na	na	na	na	na		na	na
Moisture % vol	2.4	na	na	na	na	na		0.4–0.5	0.1–0.3

6. Byproducts of the Catalytic Pyrolysis Process

The pyrolysis process of municipal solid plastic waste contains byproducts such as solid and gaseous hydrocarbons. Effects of catalyst on byproducts are described in the following.

6.1. Catalyst Effect on Solid Residue and Its Potential

Solid residue as char is unburnt plastic leftover in the pyrolysis reactor. The char formation depends on the temperature, heating rate, and residence time: the lower the value, the more significant the proportion of char in the process [150]. The state-of-the-art has reported that char is comparatively lower than the liquid and gaseous hydrocarbons produced in the process [112,113,114]. The presence of a catalyst increases the hydrocarbon ratio in the char residue. However, char deposition on the catalyst reduces the effectivity of the reaction, which may cause a lower pyrolysis reaction and higher production of ash content as byproducts [151,152,153]. Jamradloedluk et al. [154] performed the pyrolysis of HDPE 400–450 °C, and char (solid residues) obtained were collected and analyzed. Proximate and ultimate analyses showed that pyrolysis char had a large amount of volatile matter (51.40%) and fixed carbon (46.03%), a small amount of moisture (2.41%), and little amount of ash (61.0%). Char-derived briquette showed good potential as a solid fuel for heating application. Some studies showed that it can also be used as a heavy metal absorbent in industrial wastewater and toxic gases or graphene production [56,154].

6.2. Catalytic Effect on Gaseous Hydrocarbons and Its Potential

Gaseous hydrocarbons are the byproducts of the pyrolysis process (Figure 4). Production of the gaseous hydrocarbon depends on the type of plastic and temperature. Polyolefin and polystyrene yielded gaseous hydrocarbons in the 5–20 wt.% range. Pyrolysis of the mixed plastic waste at temperature of 350 °C produced more gaseous products than the individual waste plastics and increased by 8.5 wt.% upon increasing the temperature. It was found that 1 kg of feedstock produced 13–26 wt.% of gaseous products. In addition, the catalyst has a higher impact on the production of gaseous hydrocarbons than liquid hydrocarbons. It increases the gaseous product due to the higher cracking process but lowers the yield of the liquid hydrocarbons. It improves the quality of the liquid hydrocarbons [55]. In conventional pyrolysis of municipal plastic waste, CO, CO₂, hydrogen, methane, ethane, propane, butane, and other gaseous hydrocarbons are a source of gaseous products from the pyrolysis process [13,155]. The presence of catalyst leads to the decarboxylation route, which may reduce the concentration of carbon monoxide and increase the carbon dioxide in the gaseous products. However, the state-of-the-art shows that the most significant impact of kaolin and zeolite on the produced gases raises the hydrogen concentration and carbon frame isomerization, which increases i-butane concentration [74,93].

The gases produced by plastic garbage have a significant calorific value. Gases generated by the pyrolysis of agricultural plastic waste include 45.9 and 46.6 MJ kg⁻¹ HHV, respectively [108]. Moreover, the HHV of gases generated by PP and PE pyrolysis is 50 and 42 MJ kg⁻¹, respectively [156]. In addition, there are comparable findings for HHV of waste tires pyrolysis (45 MJ kg⁻¹) [157]. As a result, the generated gases can be used in boilers for heating or in gas turbines for power generation. Moreover, due to their composition, 1-butene and isoprene may be recovered by condensation and employed in the manufacturing of tires. Following separation from other gases, propane and ethane may be used as a chemical feedstock to make polyolefin.

7. Future Challenges

Thermal pyrolysis of municipal mixed plastic waste is a common technology to convert it into liquid, solid, and gaseous hydrocarbons having good potential applications in various fuel industries. However, to achieve more energy and viable products from the pyrolysis process, there is a need for additional research and development for further commercialization.

These are some points that need to be focused on for development:

• Design and development of the cost-effective and highly efficient pyrolysis reactor.
• Acquaint the main waste plastic pyrolysis reactor and its process.
• Understand the constraints and opportunities for improving product quality and yield via plastic pyrolysis.
• Development in the synthesis of the low-cost catalyst to enhance the plastic oil yield with upgradation.
• Development of both rapid pyrolysis and plastic-oil upgrading as long as both are aimed at producing useful and valuable products.
• Post-pyrolysis processing to increase the plastic-oil characteristics of the product.

At present, plastic waste technology is being adopted or is in the initial stage of implementation in various countries such as India, Taiwan, Japan, Thailand, and Malaysia. However, advancements in pyrolysis research are needed for scaling up and commercialization. Therefore, private companies can come forward to take initiatives and provide proficient services of pyrolysis to their customers. Hence, there is a possible chance that the plastic pyrolysis technology program may come under the national-level waste management program in developed and developing countries in the forthcoming years due to the evident benefits.

8. Conclusions

Pyrolysis can convert municipal mixed plastic waste into liquid, solid, and gaseous hydrocarbons. However, produced products have significant calorific value and are likely to be used by recycling and energy industries. The quality of the liquid fuel is affected greatly by the catalyst utilized. To produce liquid products suitable for replacing fossil fuels, particularly gasoline and diesel, low-acidity catalysts with high conversion rates are required. As a result, a bimetallic catalyst such as Ni, Fe, or a Fe–Ni combination may be a supported catalyst to improve the yield and quality of liquid hydrocarbons. Variables including plastic-type, contamination, reactors, pyrolysis, and residence time can also affect liquid hydrocarbon output. They may influence the catalytic pyrolysis mechanism, product yield, and distribution. These influences must be properly identified and handled to ensure process sustainability. Although fast pyrolysis and auger reactors may address product distribution issues, auger pyrolyzers have a simple design and alleviate some of the challenges associated with transferring heat for pyrolyzing plastic waste. The mechanical forces associated with auger reactors improve particle mixing and heat transport, both of which are essential for pyrolysis performance, although they require more investigation to determine their practicality in municipal plastic waste recycling. This review focuses on the pyrolysis of municipal mixed plastic waste in several reactors. However, there are other factors to consider when studying MPW pyrolysis. We should consider using inexpensive catalysts such as clay, red mud, cement, sand, and pyrolysis reactors for fast pyrolysis. Studying MPW pyrolysis kinetics is also important for process design. More study is required to fully understand polymer pyrolysis. More research is needed to fully comprehend MPW pyrolysis.