Pyrolysis is a technique of converting high molecular weight waste plastics into gasoline, kerosene, and diesel by emerging technological solutions to the vast amount of plastic that cannot be economically recovered by conventional mechanical recycling. Pyrolysis is a tertiary recycling technique in which higher molecular weight organic polymers are converted into liquid oil, char, and gases at high temperatures through thermal or catalytic decomposition without burning the polymer waste.34, 35 The main advantage of the pyrolysis technology is that it can convert both thermoplastic and thermoset waste plastics to high-quality oils and chemicals. Furthermore, it can be employed to treat any mixed, unwashed and unsorted waste without releasing toxic substances into the atmosphere. It is environmentally friendly and solves the complex problem of municipal waste management.36 The pyrolysis of miscellaneous waste plastics yields an average of 45–50 % of oil, 35–40 % of gases and 10–20 % of char, depending on the pyrolysis technology. Previous research reports indicated that more than 80 wt % of oil could be recovered from the pyrolysis of individual plastic, which is higher than the pyrolysis of wood-based biomasses.37

Waste minimization through pyrolysis is an auspicious method that involves the thermochemical decomposition of the plastics at an elevated temperature (usually at 300–900 °C). It is carried in the absence of oxygen to ensure that no oxidation reaction is taking place to extract the fuels. Basically, four different mechanisms may occur during the plastic waste pyrolysis, namely, random-chain scission, end-chain scission or depolymerization, cross-linking, and chain stripping.38, 39 It is important to conduct proximate analyses of the waste plastic compositions based on their moisture content, fixed carbon, volatile matter, and ash content. Thus, the volatile matter and ash contents are the major factors influencing pyrolysis yields. The amount of volatile matter favored oil production while high ash content decreased the amount of liquid oil, and, consequently, increased the gas yield and char formation.40

In thermal or catalytic pyrolysis, the feedstocks are allowed to melt at a high temperature, and the polymer macromolecules are broken down into fragments and small molecules, mainly aliphatic and aromatic hydrocarbons. Finally, the pyrolytic products are separated into oil, gases, and chars.10, 41 The composition of the obtained products depends on the type of the waste plastic; more CO and CO2 are obtained if the feedstock is PET; benzene-rich (aromatic) yields are obtained if PS and PET are pyrolyzed, and aliphatic hydrocarbon-based waxes are obtained if the waste materials are HDPE, LDPE, and PP.42 Among the polymer recycling methods, thermal and/or catalytic degradation of waste plastics to fuels show the highest potential for successful future commercialization because plastic wastes are available everywhere.43

Thermal pyrolysis takes place by employing high temperatures to decompose the waste materials under inert atmospheric pressure. Polyolefins-based waste plastics are broken down through a random-chain scission mechanism to produce heterogeneous products; a wide range of products such as linear paraffin and olefin are formed that may need further improvement and upgrading of their quality.9, 44-46 On the other hand, more core/waxes are formed from thermal degradation of the polymers that may jam the apparatus due to the high viscosity and low heat transfer rate.47 However, catalyst-assisted pyrolysis breaks down the polymers at lower temperatures and shorter times by lowering the activation energy and boiling temperature. Thus, catalytic pyrolysis has more benefits than its thermal counterpart due to the decreasing consumption energy, forming a narrow range distribution of hydrocarbon products depending on the carbon number atoms directed to high-quality products such as aromatic, branched or cyclic hydrocarbons.9, 44-46, 48

A comparative study of thermal and catalytic pyrolysis of LDPE has been carried out using a two-stage reactor (pyrolizer and reformer). Iron-modified ZSM-5 catalysts were used in this catalytic pyrolysis process. In both cases, an increase in the pyrolysis temperature from 400–500 °C results in decreasing the amount of condensable gases. However, increasing the pyrolysis temperature showed less effect on the aromatization content of liquids in the thermal pyrolysis, but the liquid product‘s aromatic content was higher (70 % in the liquid fraction) applying the catalyst.49 The produced liquids or waxes are highly viscous and composed of alkanes and alkenes with high boiling points. Wax is an intermediate product which was dominantly obtained under fast pyrolysis conditions, and further fluid catalytic cracking (FCC) is applied to convert it into liquid oils. Fast pyrolysis, which is performed in a continuous setup, produces more waxes than the slow pyrolysis, which is performed in a batch setup due to the short vapor residence time and reduced cracking reaction that minimizes secondary reactions.50

A comparative analysis of the thermal and catalytic pyrolysis of waste plastics under certain operational conditions is demonstrated in Table 2. High pyrolytic oil (wax) is obtained in the non-catalytic process; however, the oil yield decreases in the catalytic pyrolysis as the gas yield increases, which might impact the quality of the resulting fuel oil from the catalytic process. The pyrolytic oil yield of PE and PP under hydrogen and nitrogen carrier gases/fluidizing gases leads to a high conversion but lower yields of gas and no solid residue products.51 For example, the pyrolysis of HDPE in a packed-bed reactor using the thermal process gave 100 % wax of a dark yellow color and high viscosity. Similarly, using a silica sand bed, 54, 40, and 9 % yields of gas, liquid, and wax were produced, respectively. Using a cement powder bed, 82 % liquid, 18 % gas yields, and no wax was obtained. However, under catalytic pyrolysis conditions, the silica sand bed with NaOH yields 81 % liquid, 19 % gas, and no wax, the cement and white clay beds under catalyst hierarchical H-style ultra-stable Y (HUSY) gave the highest yield of gas (54 and 45 %) over that of the liquid (40 and 40 %), respectively.52 Mostly, the liquid yield is when thermal pyrolysis is considered as a single step. However, those liquid products can followingly be further cracked by the action of a catalyst to lower the amount of hydrocarbon products (gas and liquid) which reduces the liquid yield in the catalytic pyrolysis.53

Additionally, different valuable products such as fine chemicals, hydrogen, petrochemicals, carbon black (as a source for carbon nanotubes (CNTs)) and others can be generated from the waste plastics as shown in Figure 2.

Saturated hydrocarbon gases are formed using metal-containing catalysts when carried out at higher temperatures.54 A wide range of hydrocarbon gases (methane, acetylene, natural gas) and liquids (benzene) which are obtained during the pyrolysis can be used as a feedstock to produce CNTs, by interaction with a suitable catalyst like nickel plates along with the recovery of value-added fuels.55 Nickel-based/hybrid catalysts are promising for the production of CNTs through catalytic pyrolysis of plastics due to their excellent ability to cleave C−C and C−H bonds.56 Waste plastic pyrolysis using a two-stage bed reactor was carried out for the production of hydrogen and CNTs using a bimetallic NiFe (1 : 3) catalyst. CNTs with good thermal quality and a high yield of H2 (8.47 g g−1 plastic and 73.93 vol %) was obtained with the NiFe catalyst.57 Similarly, a Ni/Mo/MgO catalytic system (4/0.2/1) has been reported for the fabrication of CNTs using polypropylene waste as a precursor. A maximum yield of 150 mg CNTs out of 5 g PE waste was obtained at a combustion temperature of 800 °C.58

In another study, Wu et al. reported the production of both high-value CNTs and hydrogen using a trimetallic Ni−Mn−Al catalyst system and applying the pyrolysis-reforming technology in a two-stage reaction system, in the presence of steam and waste HDPE/PVC and waste plastics consisting of plastics from a motor oil container (MOC).59 The presence of PVC played a significant role in reducing the quality of the CNTs; however, the presence of sulfur showed less influence. Nearly 94.4 mmol g−1 plastic of H2 were obtained in the presence of steam at a reforming temperature of 800 °C using this catalyst. Further increasing the amount of steam results in increased hydrogen production while the CNT yield and quality are reduced. Generally, the pyrolysis-reforming and steam-gasification of waste plastic are among the latest research interests of forming hydrogen/syngas and high-value CNTs. A two-state pyrolysis process is applied to produce a good yield of CNTs and hydrogen gas simultaneously.60 For example, the in-line catalytic steam reforming pyrolysis using a conical spouted bed reactor-fluidized bed reactor (CSBR-FBR) configuration generates 34.8–37.3, 29.1, and 18.2 wt % of H2 from polyolefins, PS, and PET waste plastics, respectively.61

2.1 Factors Affecting the Pyrolysis Process

The pyrolysis of waste polymers can be impacted by several parameters, including temperature,72 retention time, feedstock composition, moisture content, particle size, catalyst choice,73 reactor type, pressure, and carrier gases,27 among others. These factors play a significant role in the quality, quantity, reaction time, and distribution of the products. In this section, we highlight the impacts of the aforementioned parameters on the pyrolysis process during plastic waste conversions.

2.1.1 Temperature

Temperature is considered as the most determinant factor that affects both the quality and quantity of the pyrolytic results because it affects the cracking reactions to various oil fuel, gases with little influence on char production. The effect of temperature is strongly seen in the fast or flash pyrolysis process due to the rapid heating rate and short residence time (<3 s), which vaporizes feedstocks to gases at high temperature to form liquid after condensing, thereby reducing the amount of chars.74 The effect of temperature mainly depends on the heating rate. Under fast pyrolysis, the heating rate is high (1000 °C min−1), which enhances the production of liquid fuels from the condensed gases in various reactors such as fluidized-bed reactors. However, under slow pyrolysis conditions, solid/residue products are predominantly formed, and no significant changes of product distributions are observed due to the low heating rates (1–10 °C min−1) and prolonged residence times. Char yields are decreased if a continual increase of temperature takes place while condensable gas yield increases.75 Slow pyrolysis is performed at temperatures between 350 and 550 °C, whereas fast pyrolysis is performed at 500–700 °C. Flash pyrolysis takes place above 700 °C.

Long-chain hydrocarbon oil fuels are produced at low temperatures whereas short carbon chain compounds are produced at higher temperatures due to the rapid cracking of C−C bonds. Similarly, aromatic compounds are formed at higher temperatures as a result of the triggering secondary process reactions.76, 77 Sogancioglu and coworkers investigated the pyrolysis of unwashed HDPE and LDPE carried out at 300, 400 500, 600, and 700 °C to yield 88.39, 87.87, 87.62, 87.55, 83.86 and 78.39, 76.58, 69.19, 73.20, 72.85 % of oils corresponding to HDPE and LDPE, respectively.78 A study by Ahmad et al. on the pyrolysis of PP showed an overall conversion of 86.32 % (250 °C) and 98.66 % (300 °C), which corresponds to 57.27 to 69.82 % liquid oil.79 This is related to the easily degradable PP polyolefin resulting from its branched structure. Further raising the temperature from 300 to 350 °C and then from 350 to 400 °C results in decreasing the liquid oil yield to 67.74 and 63.23 %, respectively. Thus, the yield of the liquid product reaches a maximum at the optimum temperature and then begins to decrease with a further increase in temperature. In another study, Miandad et al. investigated the effect of time and temperature on PS waste transformation to fuel oils at temperatures of 400, 450, and 500 °C, and 60, 75, and 70 min reaction times.26 At 400 °C, the char yield was maximal (16 %), whereas the gas yield was only 8 %, while the liquid oil yield amounted to 76 %. At 500 °C, the gas yield was doubled to 16.8 %, and the char yield was lowered to 4.5 %. A maximum liquid oil yield (80.8 %) was obtained at 450 °C and this was therefore considered the optimum temperature as confirmed by TGA. Therefore, the pyrolytic liquid content has been increased upon raising the reaction temperature and time but further increasing the temperature and reaction time does not show any further enhancement until it starts to slow down the yield of the fuel oil. Liu et al. reported that the pyrolysis of PS using a fluidized-bed reactor and nitrogen fluidizing gas yields 97.6 % of crude oil at 450 °C, but it decreases to 90.2 % as the temperature is raised to 700 °C.80 On the other hand, the amount of gas increases from non-detectable to 3.54 %. A recent study by Panda et al. on the pyrolysis of PP, HDPE, LDPE, and mixed plastics by employing a sulphated zirconium catalyst at a temperature range of 400–500 °C showed a production of low yield of condensates that have low viscosity products at a minimum temperature of 400 °C.81 But when the temperature was increased to 450 and 475 °C the yield and viscosity of the products increased gradually, too. 500 °C was considered as an optimum temperature since a high yield of the condensed product oils of 82.5, 76.6, 78.9, and 77.2 wt % had been obtained corresponding to PP, LDPE, HDPE, and mixed plastics, respectively.

The temperature not only affects the yield products; it also affects the compositions of the fuel oils. Jung et al. studied the pyrolysis of PP and PE under different temperatures, and PP pyrolysis provides 53 wt % oil, mainly benzene toluene and xylene (BTX), at 746 °C while PE pyrolysis at 728 °C gives 32 wt % of BTX fractions.82 The formation of aromatics in the pyrolysis of polyolefins takes place through Diels-Alder reactions followed by dehydrogenation. PP undergoes a random chain scission mechanism to generate, in a first step, primary and secondary radicals, followed by intramolecular radical transfer reactions that produce tertiary radicals. The β-cleavage of the tertiary radicals finally leads to the formation of propene. Benzene, among the BTX aromatics, has been formed in significant yield in both the PP and PE fractions.

2.1.2 Retention Time and Feedstock Composition

Retention time and feedstock compositions also affect the pyrolysis process and products. However, their impact is lower compared to temperature. As reviewed by Miandad et al., at shorter retention times, aromatic hydrocarbons are produced, especially when the feedstock consists of PS plastic.27 60 % of aromatic hydrocarbons are formed from the pyrolysis of mixed plastics of PS, PP, and PE at 350 °C.83 However, as nearly similar carbon chain-containing fractions were obtained at each temperature even though the retention time was varied, an insignificant effect of the retention time on carbon chain fractions was deduced. On the other hand, fractions which have >C13 were observed with increasing temperature. This is due to the long retention time in the reactors, as plastic and its derivatives decomposed to generate high carbon chain compounds in comparison to light carbon chain compounds that are formed at low retention time.27 The type of feedstock composition also affects the pyrolysis process. For instance, PE- and PP-based plastic required higher temperatures for their complete degradation as compared to PS plastic due to their complex structures.77 The capacity of PS to produce a monomer is superior compared to PE and PP. For the case of PS, liquid evolution started at much lower temperatures.84 Jan et al. reported the pyrolysis of HDPE at different time intervals using a batch reactor.85 5 g of waste HDPE was allowed to degrade at 450 °C using a catalyst at an optimum catalyst/polymer ratio of 0.1. The degradation reaction was conducted for 0.5 h, 1.0 h, 1.5 h, 2.0 h, 2.5 h, and 3 h, keeping the other reaction conditions constant. The total conversion was found to have increased when the reaction time was raised from 0.5 h to 2 h with a subsequent increase in oil yield and wax. About 96 % total conversion was achieved, resulting in a 41.33 % oil yield at 2 h. Beyond 2 h reaction time, no significant change in the quantity of any of the reaction products was observed which means the reaction has been completed, and therefore, a reaction time of 2 h reaction time was considered optimal. Miandad and coworkers studied the pyrolysis of PS at an optimum temperature of 450 °C at 60, 75, and 120 min reaction times.77 Varying the reaction time between 75 and 120 min does not show a significant difference in the yield of the fuel oil. 80.8 % and 80.7 % oil yield has been obtained in 75 and 120 min reaction times respectively. A comparable amount of char was produced at 75 min reaction time as compared to 120 min (6.1 % versus 5.3 %). Thus, the formation of a similar yield of oil at 75 min and the extra timed indicated that 75 min is the optimum reaction time. However, more chars are produced at 60 min reaction which indicated that 60 min reaction time is not enough for the PS to yield the maximum amount of oil. Similarly, a research team led by Motawie reported the pyrolysis of HDPE at 450 °C in 0.5–3 h time intervals.86 Only 52.2 wt % oil and 41.2 wt % of gas yields were obtained at 3 h reaction time. Conducting the pyrolysis reaction for about 0.5 h, the oil yield was found at a higher 72 wt %, while the gas yield amounted to only 12 wt %. When the reactor is operated for a longer time (residence time), a secondary reaction may take place in which longer carbon chain oils may crack and be consumed towards the gas formation. At 30 and 60 min, the oil contained 23 wt % paraffin within the range C5−C9 as the main compounds. However, >C9 alkanes are formed in less than 23 wt %. C5−C9 alkenes gave a total concentration of 30 wt %. The composition of the oil produced at 0.5 h residence time was similar to the oil produced at 1 h residence time and was dominated by light alkanes. The oil products at higher residence times (2 and 3 h) gave higher alkene and smaller alkane yields. At a residence time of 2 and 3 h, 19 and 13 wt % paraffinic compounds (C5−C9) were formed, respectively. The effect of residence time was much more pronounced on the alkenes than alkanes. Adnan et al. studied the effect of added PET on the catalytic pyrolysis of PS oil products.87 They designed the experiment as PS and 10 wt % PET+PS (500 °C, 60 min, sample to catalyst ratio of 1:0.2), 20 wt % PET+PS (450 °C, 60 min, sample to catalyst ratio of 1 : 0.2) and for 30 wt % PET+PS (450 °C, 90 min, sample to catalyst ratio of 1 : 0.2) using an Al-Al2O3 catalyst. The pyrolysis of only PS yielded 92.69 % liquid, 7.31 % gas, and no residue; the 10 wt % PET+PS add mixture yielded 76.40 % liquid, 21.30 % gas, and 2.30 % residue, 20 wt % PET+PS yielded 44.60 % liquid, 50.51 % gas and 3.89 % residues and 30 wt % PET+PS gave 22.50 % liquid, 71.80 % gas, and 5.70 % solid. Accordingly, excess addition of PET results in the liquid yield decline and gas yield increase.

2.1.3 Use of Catalysts

Catalytic pyrolysis has an advantage over conventional, purely thermal pyrolytic processes in fuel recovery from waste plastics. Various heterogeneous catalysts have been used; the most common conventionally used ones are natural and synthetic zeolites like the catalysts employed in the cracking of heavy petroleum fractions.73 Catalysts play a vital role in improving the quality of pyrolysis oil as well as reducing, mostly, temperature and retention time of the process.88 Catalysts such as Fe2O3,89 Ca(OH)2,89 FCC,90 Al2O3,91 natural17, 55 and synthetic zeolite,92 and sawdust93 are commonly used in the pyrolysis technology. The use of catalysts increases the rate of the cracking reactions, leading to an increase in the gas yield of but reducing the yield of liquids. However, the quality of the liquid oil is improved, as some of the larger carbon chain compounds are either adsorbed in the catalyst or further broken down into smaller carbon chain compounds.77

Reactor design, residence time, and contact of the fussed plastics with catalyst as well as the contact time of the volatiles with the catalyst can strongly affect the efficiency of the catalytic process and the respective catalyst. The cracking of waste polymers such as HDPE is carried out as follow i) the polymers melt inside the reactor; ii) coating of the catalyst on the surface of the fused plastics; iii) pyrolysis of the fused plastics; iv) catalytic cracking of the fused plastics and vaporizations.94 The acidity, pore size, surface area to volume ratio, and thermal stability of the catalyst are important in the catalytic pyrolysis mechanism and product distributions. The reactions such as cracking, isomerization, oligomerization, cyclization, and aromatization take place on the surface of the catalyst. Zeolite catalysts were selected since their surface feature Lewis- and Brønsted-acidic sites: the abstraction of a hydrogen ion from the waste polymer is initiated by the Lewis acid site, while the addition of a proton to the C−C bond is carried out by the Brønsted acid sites. Therefore, if the catalytic surface contains more Brønsted acid sites, more hydrogen is provided for the double bond.95, 96

The catalysts employed for the pyrolysis of waste plastics can be applied either directly in the reaction system (in situ) or through a second reactor for the actual catalytic process (ex situ).97 A recent study by Fan et al. used continuous-stirred microwave pyrolysis (CSMP) and a batch microwave system for the pyrolysis of linear LDPE (LLDPE) in the presence and absence of HZSM-5.98 Using CSMP, long hydrocarbons (C14−C20) are obtained selectively. In contrast, using the batch system, more gaseous products (CH4) are produced. An ex situ catalytic bed with HZSM-5 was applied to increase the amount of gasoline-range hydrocarbons. Both the catalytic continuous-stirred and batch process showed almost similar yields but differed in chemical selectivity. Mono-aromatics with 72.3 % are obtained in the CSMP.

A two-stage fixed-bed reactor catalytic pyrolysis has been reported by Akubo et al., using transition-metal-impregnated Zeolite catalysts for HDPE pyrolysis.99 The first stainless steel reactor holds the waste plastics and heats to 600 °C at a heating rate of 10 °C min−1. The second reactor holds the catalyst, which receives the volatiles from the first reactor. Non-catalytic pyrolysis of HDPE using the two-stage fixed bed reactor leads to a high oil yield (≈70 wt %) of 100 % aliphatic hydrocarbons. Introducing a Y-zeolite catalyst results in a decrease of the oil yield, but >80 % of the oil consisted of mono- or bicyclic aromatic compounds). The connection of two reactors in line with the catalytic pyrolysis and reforming steps can also be applied to improve the yield of hydrogen production. Continuous plastic pyrolysis and catalytic reforming can be conducted in in-line fluidized-bed-fixed-bed, fixed-bed-fixed-bed, spouted-bed-fixed-bed, spouted-bed-fluidized-beds, and screw-kiln-fixed-bed two-stage reactor combinations as shown in Figures 3a-f.100

Different reactor configurations used in the pyrolysis and in-line reforming process: (a) in-line fluidized bed and fixed bed, (b) spouted bed and fixed bed, (c) spouted bed and fluidized bed, (d) fixed bed and fixed bed, (e) screw kiln-fixed bed reactor (f) fluidized bed, entrained flow and fixed bed reactor. Reproduced with permission from Refs. [100] and [101]. Copyright 2018, Elsevier.

Pinto and coworkers used unsorted municipal plastics composed of PE, PP, and PS to study the effect of the catalyst on pyrolysis.102 More than 90 % of the total conversion to oil occurred without any catalyst, producing very little gaseous products. However, in the presence of ZnCl2 and NH4-Y-zeolite (NH4Y), a decrease in the liquid yield to lower than 90 % was noted, whilst gas yields increased. Homogeneous catalysts which have a Lewis-acidic nature, such as AlCl3, are also used for polyolefin plastic pyrolysis.91 The use of heterogeneous catalysts is, however, preferred due to the ease of separating and recycling them from the reacting mixtures. Nanocrystalline zeolites, aluminum pillared clays, conventional acidic solids, mesostructured catalysts, superacidic solids, gallosilicates, metals supported on carbon, and basic oxides are among the heterogeneous catalysts used.45 The pyrolysis of PE in the absence of any catalyst results in 95 wt % oil yields, no char, and low gas yield, but with various zeolite Y beds and temperatures of 500 °C, the oil yield was reduced to 85 wt %. However, when the zeolite bed temperature was raised, the oil yield was decreased with a consequent increase in the gas yield.103 In another study by Zeaiter from 2014,104 non-catalytic pyrolysis of HDPE generates 78.7 %, 17.8 %, and 3.5 % of liquid wax yield, gas, and residue, respectively, using a tubular reactor. However, when zeolite catalysts were added to the pyrolysis process, the waste HDPE generated high gas yields; H-Beta zeolite produces the highest gas yield of 95.7 %, 2.4 % liquid, and 1.9 % residue, followed by HUSY (93.2 % of gas, 4.9 % liquid and 1.9 % residue) at 450–470 °C.

Kumar et al. studied PS, PP, PE, and PET waste pyrolysis in a batch reactor individually and by mixing them without and in the presence of charcoal, activated carbon, and CaO catalysts.105 They obtained 80 %, 60.7 %, 75 %, and 66.86 % of liquid yields from PS, PP, PP, and mixed PS+PP+PE, respectively, without the use of catalysts. On the other hand, using activated carbon, 82.43 % liquid yield from PS+PP+PE, using charcoal, 95.54 % from PS+PP+PE, and applying a mixture of activated carbon and CaO as a catalyst provides 75.50 % of liquid yield from PE+PP+PS PET.

The type of the catalysts also affects the product oil composition. Ratnasari et al. reported an oil obtained from the zeolite catalytic pyrolysis of HDPE using a two-stage reactor (pyrolysis and catalytic reactor).1 Mesoporous MCM-41 and zeolite socony mobil–5 (ZSM-5) catalysts were used, and aliphatic hydrocarbon-based oils were obtained by the first catalyst, whereas by using the microporous zeolite ZSM-5 catalyst, a mostly aromatic-based oil was produced.

During PP degradation employing NiO catalyst in the form of solution yields more oil fuel when compared to its counter catalyst in solid form, and the composition of the oil is reported to be rich in 1-olefins and poor in aromatics and branched isomers.106 Pyrolysis of PS was carried out with zeolite and Ni/Si catalysts at 460 °C; 130 g of PS foam was pyrolyzed in the presence of zeolite and nickel/silica catalysts to give yields of 86.69 % oil or 112.70 mL and 91.65 % liquid or 119.15 mL, respectively.107 Similarly, fluidized bed reactor-assisted pyrolysis of PS results in the production of 90 wt % liquid fractions through thermal pyrolysis at 580 °C, while by employing a BaO catalyst, a 93.4 wt % yield was obtained at only 350 °C. A 91 % yield of oil was observed when zeolite ZSM-5 was applied at 500 °C.108 Singh et al. studied the conversion of HDPE to energy fuels using 0, 2, 5, and 8 wt % loading of a CuCO3 catalyst.109 A maximum of HDPE-derived liquid (94 %) was obtained at a 5 wt % loading of the catalyst. Similarly, 85 %, 90 %, and 92 % liquid yields were obtained applying 0, 2 and 8 wt % loading. 14.67 % 9.66 %, 5.64 %, 7.45 % light gases and 0.33 %, 034 %, 0.36 %, 0.55 % residues have additionally been reported from this experiment using 0, 2, 5 and 8 wt % loading of the catalyst, respectively.

Catalytic pyrolysis of PET was investigated by Park et al., using a carbon-supported Pd nanocatalyst in a tube furnace.110 More solids and gases than pyrolytic liquid oil have been produced at 400 °C, but as the temperature was raised to 800 °C, pyrolytic oils gave way to pyrolytic gases (CO, CH4, and H2) due to the operation of the free radical mechanism and thermal cracking with the help of the Pd catalyst that, on its surface, accelerates ring-opening reactions. The mass balance indicates that 16 % char, 42 % liquid, and 42 % gas products had been obtained without any catalyst; using Pd:PET (0.01 w/w), a product composition of 18 % char, 39 % liquid, and 43 % gas was obtained. Further increasing the catalyst-to-PET ratio to 0.05 w/w yielded 19 % char, 33 % liquid, and 49 % gas at 800 °C.

Kassargy et al. reported a comparative study on the thermal and catalytic pyrolysis of PE and PP applying a batch reactor.111 80 wt % of wax and 85.5 wt % of liquid yields were obtained from the thermal pyrolysis at 450 °C. In contrast, applying USY zeolite in the catalytic pyrolysis showed 71 and 82 wt % yields of liquid with a mixture of C5−C39 and C5−C30 corresponding to PE and PP, respectively. Further separation of the products by distillation results in the production of 60.6 % and 57 % of gasoline with a high octane number of 96 and 97 corresponding to PP and PE. 36.5 % and 35.3 % yields of diesel with cetane numbers of 52 and 53 have been reported from PP and PE, respectively.

2.1.4 Reactor Types

The reactor is considered as the heart of pyrolysis which is used to control the quality of heat transfer, mixing, gas and liquid phase, residence times, and the escape of main products. The design and setup of the reactors is grouped under one of the following categories as shown in Figure 4: Batch, semi-batch, continuous-flow reactors such as fluidized-bed, fixed-bed, and conical spouted bed reactors (CSBR).9, 27

Different types of reactors (CSBR – Conical spouted bed reactor)

2.1.4.1 Batch and Semi-Batch Reactor

The batch reactor operates as a closed system with no inflow or outflow of inputs or outputs when the reaction is being carried out. High conversions of the reactor can be achieved by keeping the reactant in the reactor for a prolonged time which is one of its advantages. However, batch reactors provide inconsistent products from batch to batch, high labor costs per batch, and are difficult to adapt for large-scale production.112

The flexibility of adding reactants over time is an advantage of the semi-batch reactor in terms of reaction selectivity. However, the semi-batch reactor is similar to the batch reactor in terms of labor cost and difficulty of large-scale production.113, 114 Pyrolysis in a batch reactor or semi-batch reactor is normally performed at a temperature range of 300–900 °C and reaction time of 30–90 min for both thermal and catalytic pyrolysis.32 Catalysts are added with the plastics to improve hydrocarbon yield and to upgrade products inside the reactor. Such reactors are, however, not preferable for catalytic pyrolysis due to the formation of coke on the surface of the catalyst, which reduces the catalyst efficiency over time. It is furthermore a challenge to separate the catalyst from the residue at the end of the reaction. Both these reactors are suitable for thermal pyrolysis to obtain high oil yield due to their easily controlled parameters, but, as previously mentioned, are difficult to scale up.115

Thermal and catalytic pyrolysis of HDPE was carried out in a pyrex batch reactor at a temperature of 400–450 °C. Increasing the temperature increases the yield of the liquid products in both the catalytic and the noncatalytic system. Using the thermal process, 74.5, 5.8, and 19.5 wt % yields of liquid, gas, and residue were achieved at 450 °C.

However, using FCC and HZSM-5 results in the production of 78.5 and 81.0 wt % yields of liquids, 6.5 and 15.1 wt % gas, and 11.2 and 3.9 wt % residue, respectively.116 A semi-batch reactor was also designed for the pyrolysis of HDPE at a higher temperature range of 400–550 °C. 7.86 wt % liquid, 71.22 wt % of viscus wax, 18.42 wt % gas, and 2.5 wt % residues were observed at 550 °C and 54 min residence time. More wax was formed at a higher temperature.117 Thermo-catalytic pyrolysis of PS have been conducted using batch and semi-batch reactors. The oil yield was higher in the semi-batch reactor. Applying MgO as a catalyst, the amount of styrene monomer was increased in both reactors. However, the quantities of dimers and trimers were higher in the semi-batch than in the batch reactor. On the other hand, the gas yield was higher in both the catalytic and noncatalytic pyrolysis using the batch reactor.118

2.1.4.2 Fixed and Fluidized Bed Reactors

The technology of the fixed bed reactor is simple, reliable, and proven for fuels that are relatively uniform in size and have a low content of fines.119 The reactor contains a gas cooling and cleaning system.

In a fixed-bed reactor, the catalyst is usually present in palletized form and packed in a fixed bed. The design is relatively easy. The feedstock is placed in the reactor (often made from stainless steel), which is heated externally. There are some constraints, such as the irregular particle size and shape of feedstocks, that need to be considered or would otherwise cause a problem during the feeding process. Besides, the accessibility of the surface area of the catalyst during the reaction is limited. In certain conditions, fixed-bed reactors are merely used as a secondary pyrolysis reactor because the product from primary pyrolysis can be easily fed into the fixed-bed reactor, which generally consists of a liquid and a gaseous phase.112 The fixed-bed reactor is characterized by a low heating rate, and as a result of its low heat transfer coefficient, the temperature is not uniform inside the sample, and the feedstock is decomposed at different temperatures simultaneously.33

On the other hand, the fluidized bed reactor solves some of the problems that occur in a fixed-bed reactor. In contrast to a fixed-bed reactor, the catalyst in a fluidized bed reactor sits on a distributor plate where the fluidizing gas passes through it, and the particles are carried in a fluid state. Therefore, there is better access to the catalyst to be well-mixed with the fluid, thus providing a very high surface area for the reaction to occur in uniform temperature distribution. This reduces the variability of the process conditions with a high heat transfer coefficient. Besides, it is also more flexible than the batch reactor, and frequent feedstock charging can be avoided, avoiding the need to pause the process too often. A fluidized bed reactor is considered to be the best reactor to perform catalytic plastic pyrolysis since the catalyst can be regenerated many times without the need of discharging, especially worth considering if the catalyst is a very expensive substance. It is also the most suitable reactor for a large-scale operation in terms of the economic point of view.115, 119 There are some difficulties in using fluidized-bed reactors, however: the raw material provided to the reactor must be tiny, so it can float in the fluid, and separating the char from the bed material is difficult. Thus, this type of reactor is seldom used in large-scale projects.120

A recent study by Al-Salem et al. reported the recovery of wax from virgin LDPE, HDPE, and plastic solid waste (PSW) using a fixed-bed reactor.121 The highest wax yield (64.5 wt %) was obtained from LDPE at 500 °C and 32 wt % from HDPE at the same temperature; however, a low wax yield (9.25 wt %) was achieved using the PSW at 700 °C. Similarly, pyrolysis of HDPE employing a fixed bed reactor at 500 °C with a heating rate of 10 °C min−1, 20 min residence time using nitrogen gas as a carrier gas, gives the highest liquid yield of 95 wt %, low gas yield and no residue without catalyst, while 85 wt % have been achieved using the Y-zeolite catalyst.103 Li et al. recently published an investigation into the influence of the thickness of the fixed bed reactor during the pyrolysis of HDPE.122 More wax yield was observed in a thin bed (11.4 %) than for the thick bed (5.6 %) at 425 °C. As the temperature increased from 450–550 °C, the wax yield increased in both beds, but again stronger so in the thick bed.

2.1.4.3 Rotary Kiln Reactors

The rotary kiln reactor is more efficient than the fixed-bed reactor in heating the feedstocks. Its slow rotation of an inclined kiln enables good mixing of wastes and yields uniform pyrolytic products.27 Rotary kiln reactors are widely used, typically for conventional pyrolysis (slow pyrolysis), usually performed at 500 °C with a residence time of 1 h. Proceeding with a slow heating rate, significant product portions of char, liquid, and gas are the result.123 Rotary kiln reactors have many unique advantages over other reactor types such as good mixing of wastes, flexible adjustment of residence time, larger channel for the waste stream allowing feeding of heterogeneous materials, thus, preventing extensive pre-treatment of wastes, and simple maintenance.33, 120

2.1.4.4 Stirred Tank Reactors (STRs)

STRs are the most frequently applied reactors for the pyrolysis of waste plastic and biomasses. They are designed featuring a heat transfer medium like hot oil (Nano fuel process) with good temperature control and are easy to construct and operate. Catalysts are frequently added directly to the plastic waste, or upgrading can take place in a separate vapor upgrading tower (Thermofuel). The stirrer facilitates better heat transfer to the melt, uniform heat distribution, and scrapes char deposits from the reactor walls, which would otherwise act as heat insulators.124 Char, spent catalysts, and/or contaminants are generally removed from the bottom of the reactor (Nano fuel, Thermofuel, Royco), except in the Hitachi process, which vacuums char from the bottom through a vertical vacuum line. One of the main disadvantages of stirred tank reactors is that they require frequent maintenance and so require a large infrastructure. They also h