Management of Plastic Wastes through Recent Advanced Pyrolysis Processes

Abstract

Plastics are predominant in numerous sectors like packaging, agriculture, hardware, electronics, and many others. Annual plastic demand has been rapidly growing in the last few decades because of the increasing dependency on plastics. As a consequence, massive amounts of plastic waste are being generated every year. These plastic wastes are non-biodegradable, and hence their disposal poses a serious threat to the ecosystem and causes significant environmental problems such as endangering the safety of marine life, wildlife, air, water, and soil, etc. A large portion of plastic waste ends up in landfills, and only a small fraction is recycled. The continuous dependence on landfills as the main disposal method for plastic waste is costly and ineffective. Common solutions to plastic waste management are incineration and recycling; however, incineration emits harmful pollutants and greenhouse gases that contribute to ozone layer depletion and global warming; moreover, recycling is expensive and inefficient. As an alternative to recycling and incineration, the pyrolysis process can convert plastic wastes into more valuable fuel products. Pyrolysis is a thermal process that converts raw material into pyrolysis liquid, solid wax, and non-condensable gases in the absence of oxygen. This process is attractive because it is economical and energy-efficient, and it can be used to convert various types of plastic waste into valuable products. In recent years, there have been significant developments in pyrolysis applications in liquid fuel production from plastic wastes. This work reviews recent advances in and challenges for the pyrolysis process for converting plastic wastes into a valuable alternative fuel, focusing on studies of advanced pyrolysis processes published over the last five years. The paper also highlights the numerical modeling of pyrolysis of plastic wastes and the potential impact of pyrolysis on the future of sustainable waste-management practices of plastics.

1. Introduction

In recent decades, the exponential growth in plastic production has led to a corresponding surge in plastic waste generation, presenting a significant environmental challenge globally. According to the United Nations (UN) [1], the global average plastic production stands at 430 million tons annually. Figure 1 represents the percentage of plastic and polymer waste generated by the region in 2019. Inspecting Figure 1, it can be seen that the United States of America, Europe, and China, respectively, produce more plastic waste as compared to the other regions of the world [2]. More than half of these plastics are produced for single-use purposes and are subsequently disposed of following their initial use. Plastics, specifically single-use plastics, significantly contribute to land and water pollution, degrade soil fertility, and damage aquatic ecosystems [3,4]. Moreover, as the global population grows and society standards improve, plastic waste generation also increases, causing worldwide concern.

There are hundreds of different types of plastics available on the market. However, there are six common plastics used in many industrial and daily applications. These plastics are low-density polyethylene (LDPE), high-density polyethylene (HDPE), polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC) (refer to Figure 2) [5].

Unfortunately, only PET and HDPE are routinely recycled at most recycling centers. [6]. Other plastics, namely, PVC, LDPE, PS, and PP are typically not recycled since they are difficult and expensive to process. Consequently, the extensive use of these plastics, especially the hard-to-recycle ones, caused the accumulation of much plastic waste in landfills (See Figure 3) that could take hundreds of years to completely degrade, hence contributing to significant plastic pollution in the environment [7], and causing the degradation of marine life, forests, and wildlife, and the growth of landfills.

Additionally, the escalating concerns over fossil fuel depletion have increased the interest in alternative energy sources and sustainable waste management solutions. Waste-to-energy (WTE) technology has emerged as a promising approach to address both environmental and energy challenges [8]. The most common WTE technologies in the industry are incineration, pyrolysis, hydrogenation, gasification, anaerobic digestion, and mechanical and biological treatment [9]. WTE processes rely on heat energy to decompose the plastic waste to produce gas, oil, and char. These products can then generate electrical energy or other useful products. The general steps of WTE include waste pretreatment, thermal decomposition, product formation, and energy conversion. The primary difference between the WTE processes is the oxygen supply in each process. For instance, incineration requires a complete oxygen supply, while gasification requires partial oxidation, and pyrolysis and anaerobic digestion, and torrefaction processes only need a limited or no oxygen supply [10,11]. Pyrolysis has gained attention over most of the other WTE technologies because of its potential advantages in mitigating environmental pollution and reducing the carbon footprint associated with plastic products. This is achieved by minimizing emissions of carbon monoxide and carbon dioxide, as compared to combustion and gasification processes [12].

Table 1 presents the proximate and ultimate analyses of the six common plastic wastes mentioned above. The proximate analysis determines plastic composition regarding moisture, ash, fixed carbon, and volatile matter. The volatile content is the major factor in determining the pyrolysis oil yield. The higher the volatile and lower the ash amount in plastic, the higher the yield of pyrolysis oil. All plastic wastes have higher volatile-matter content, implying that they are easy to burn and good candidates for pyrolysis. Also, the low ash content suggests that the primary products of pyrolysis of these plastics are plastic oil and gases, with a low char content. The low moisture content of the plastic wastes indicates that there is no need for thermal processing to remove the water content before processing them by pyrolysis [13,14].

The ultimate analysis provides more comprehensive data by obtaining the elemental composition of plastic waste. Alternatively, plastics with more carbon and hydrogen components instead of oxygen or chlorine result in higher-quality petroleum-based products. HDPE, LDPE, PS, and PP are composed of mainly carbon and hydrogen with low levels of oxygen and chlorine. Therefore, they can potentially produce pyrolysis oil with a higher heating value (HHV). However, the presence of chlorine in these wastes is undesirable because pyrolysis of this waste will produce hydrogen chloride (HCl) gas, and chlorinated organics, which are harmful. PET and PVC have lower carbon and hydrogen content, higher chlorine content for PVC, and higher oxygen content for PET waste. The low carbon and hydrogen contents would result in a low percentage of pyrolysis oil with a low HHV. Also, pyrolysis of PET would generate significant amounts of CO and CO₂, and pyrolysis of PVC would generate high amounts of HCl and chlorinated organic compounds [15,16,17].

Table 1. Proximate and Ultimate Analyses of Common Types of Plastic.

Type of Plastic	Chemical Structure	Proximate Analysis			Ultimate Analysis					Reference
		Fixed Carbon	Volatile Matter	Ash	C	H	O	Cl	HHV (MJ/kg)
PET		9.37	88.54	2.09	61.87	4.35	33.78	0.00	23.40	[18,19]
PP		0.030	99.97	0.00	84.61	14.96	0.43	0.00	45.3	[20,21]
PS		0.10	99.90	0.00	92.80	7.20	0.00	0.00	41.25	[22,23]
PVC		4.20	95.80	0.00	38.70	4.80	0.00	56.50	22.59	[24,25]
HDPE		0.00	100.00	0.00	81.45	12.06	6.49	0.00	51.32	[26,27]
LDPE		0.00	92.40	4.30	81.0	14.50	0.20	0.00	43.47	[28,29]

Table 1. Proximate and Ultimate Analyses of Common Types of Plastic.

Type of Plastic	Chemical Structure	Proximate Analysis			Ultimate Analysis					Reference
		Fixed Carbon	Volatile Matter	Ash	C	H	O	Cl	HHV (MJ/kg)
PET		9.37	88.54	2.09	61.87	4.35	33.78	0.00	23.40	[18,19]
PP		0.030	99.97	0.00	84.61	14.96	0.43	0.00	45.3	[20,21]
PS		0.10	99.90	0.00	92.80	7.20	0.00	0.00	41.25	[22,23]
PVC		4.20	95.80	0.00	38.70	4.80	0.00	56.50	22.59	[24,25]
HDPE		0.00	100.00	0.00	81.45	12.06	6.49	0.00	51.32	[26,27]
LDPE		0.00	92.40	4.30	81.0	14.50	0.20	0.00	43.47	[28,29]

The pyrolysis process (refer to Figure 4) is an endothermic reaction occurring in an oxygen-free atmosphere at temperatures ranging from 200 to 1300 °C, depending on the feedstock characteristics [30]. Heat must be provided into the reactor to drive the thermal decomposition of the waste into products. This process is carried out in an inert gas environment, usually at atmospheric or slightly high pressure, although vacuum conditions are sometimes employed [16]. During pyrolysis, long-chain polymers (plastic waste) are broken into smaller segments, resulting in wax solid (char), pyrolysis oil, and non-condensable gasses. Pyrolysis oil consists of several oxygenated organic compounds such as carboxylic acids, aldehydes, ketones, and water [31]. Nevertheless, the exact composition of the pyrolysis oil depends on the feedstock of the process. The non-condensable gases, consisting of CH₄, H₂, CO, and CO₂, can be recycled into the process as an energy source to make the pyrolysis process self-sustaining. Lastly, the char is usually carbon-rich and can be used to make activated carbon and adsorbents, and graphene and its derivatives [32].

The major product of pyrolysis of waste is dependent mainly on the reaction temperature, process residence time, and the heating rate used. The main product of the pyrolysis of biomass according to Bridgwater et al. [33], with low temperatures, less than 450 °C, is biochar. At higher temperatures of more than 800 °C, larger amounts of gases and ash are produced. Pyrolysis oil is primarily produced at an intermediate temperature range. The pyrolysis process can also be classified into slow, intermediate, and fast, based on the temperature, heating rate, feedstock residence time, and vapor residence time. In slow pyrolysis, the major product is char, while intermediate pyrolysis produces fractions of liquid, solid, and gases. However, the fast pyrolysis process maximizes the liquid fraction [33,34].

The main objective of this work is to review and discuss recent advances in using the pyrolysis process for converting plastic waste into fuel and their potential impact on the future of plastic waste management. This paper focuses on recent articles published over the last five years (from 2021 to 2024) employing advanced technologies to improve the pyrolysis process of plastic waste.

2. Methodology

This paper carried out a systematic review of the most recent available studies from 2021 to 2024, on the pyrolysis of plastic waste into alternative fuels as a sustainable plastic waste management method. Science Direct, Scopus, and MDPI have been scrutinized using the keywords “pyrolysis” AND “plastic” AND “fuel’’. Based on the keywords, 5723 references were identified. The references were carefully screened by browsing through the titles and abstracts to focus only on (i) recent research articles that are published between 2021 and 2024, (ii) those covering advanced pyrolysis processes of plastic wastes employed in liquid fuel production, and (iii) also those discussing mathematical modeling of advanced processes used in liquid fuel production. Although the remaining articles screened are important, in our opinion they did not meet all the criteria (i.e., advanced pyrolysis, recent, liquid fuel production, etc.) of our research scope. The selected references have been further analyzed and summarized in this study. A detailed summary of all the articles can be found in Table 2.

3. Recent Advances in Fuel Production from Plastic Pyrolysis

Currently, research is focusing on utilizing the pyrolysis process to convert plastic wastes into fuels and alternatives to petroleum-based products. Recent advances in the use of pyrolysis to treat plastic waste are discussed below.

Microwave-assisted catalytic pyrolysis: Zhou et al. [35] investigated the feasibility of producing fuel from the catalytic pyrolysis of plastic waste. The pyrolysis process was conducted in a novel continuous microwave-assisted pyrolysis (CMAP) system constructed in the lab. The details of the CMAP unit are illustrated in Figure 5a. In their study, the effect of plastic composition, temperature, and catalysis on the yield and composition of the products were investigated. The main types of plastic used for the pyrolysis process were HDPE, PP, and PP pellets. The catalyst utilized in this study was ZSM-5 (Zeolite Socony Mobil–5). The temperature inside the ball bed reactor was maintained at set levels between a range of 500 °C and 740 °C, using a PLC control system. It was found that the highest plastic liquid yield of HDPE 47.4 wt%, was found at 560 °C. Also, as the temperature increased from 500 to 740 °C, more non-condensable gas was produced, increasing their yield from 17.6 to 74.7 wt%, while decreasing the wax yield from 40.5 wt% to 1.3 wt%. The higher heating values (HHVs) of the gas products were also calculated to be in the range of 45.3 to 46.9 MJ/kg, compared to that of common gas fuels such as propane (50 MJ/kg) and natural gas (45 MJ/kg).

Moreover, the effect of plastic composition was investigated for three different feedstocks at 600 °C. It is observed that pyrolysis of pure PP chip produced higher yields of gases (48.6%) and liquid (44.8%), and less wax (5.2%) compared to HDPE pyrolysis. The yields of gas, liquid, and wax products of HDPE pyrolysis were 37.6%, 40.2%, and 15.7%, respectively. The results suggested that PP is more prone to thermal degradation than HDPE. This finding was also reflected in the composition difference of the liquid products. The liquid product from the pyrolysis of PP had more aromatic content and a larger C5-C12 fraction, and less n-alkene content. Finally, the effect of catalysis on the process was studied, by packing 200 g of ZSM-5 pellets into a catalyst bed and heating to 620 °C. It was found that in feedstocks the catalytic activity of ZSM-5 was diminished after 30 min of the pyrolysis process. However, the catalytic pyrolysis for 30 min resulted in higher liquid yield and minimal wax yield in comparison to the thermal pyrolysis process. Also, the liquid product has considerably higher aromatic content, higher gasoline fraction, higher isomerized aliphatic content, and lower n-alkene content. The energy balance analysis indicated that 5 MJ of electrical energy was needed to process 1 kg of HDPE with this system, with energy efficiency as high as 89.6%. Moreover, 6.1 MJ of electrical energy can be generated from the gas products, making this pyrolysis process self-sufficient in terms of energy.

Fast-pyrolysis in auger reactor: Hasan et al. [36] used an auger reactor (shown in Figure 5b) to generate fuel from the fast pyrolysis of organic and plastic waste. This study aimed to measure and analyze the density, pH, kinematic viscosity, water content, and calorific value of plastic oil to assess the applicability of the oil as a fuel for transportation. This assessment was achieved by comparing the properties of the pyrolysis oil with those of typical engine fuel. Different types of organic waste were collected and studied; these were macadamia nutshell, beauty leaf fruit husk (BLFH), and municipal green waste. The plastic waste used in the study was HDPE. The pyrolysis was performed at varied temperatures in the range of 400 °C to 550 °C, at intervals of 25 °C.

Each type of waste generated the maximum pyrolysis oil yield at different temperatures. For BLFH, the maximum yield, 42.75%, was obtained at 475 °C and at 500 °C for the macadamia nutshell (45.09%) and municipal green waste (44.72%). The pyrolysis of HDPE at 525 °C resulted in the highest pyrolysis oil yield (61.29%). It is interesting to note that below 425 °C, HDPE was converted primarily to wax, which caused the auger to stop rotating, resulting in incomplete pyrolysis. However, increasing the temperature up to 550 °C decreased the oil yield for all the samples, because of the secondary cracking of vapor at high temperatures.

Different analytical techniques such as FT-IR, TGA, GC-MS, and elemental and physicochemical-properties analyses were used to analyze the chemical and physical properties of the pyrolysis oils. The results showed that the oils produced from BLFH, municipal green waste, and macadamia nutshell are rich in aromatic, phenolic, and oxygenated compounds, while the oil obtained from HDPE is made up of mostly aromatics and hydrocarbons. Moreover, the HDPE-derived oil had lower density and viscosity, and a lower calorific value compared to that of the other oils. Based on these results, the pyrolysis oils generated from organic wastes are not appropriate for engine application and require further refinement if used. However, they can still be utilized as a heating source for boilers and furnaces. On the other hand, the characteristics of HDPE-derived oil are very similar to that of light fuel, and hence have a potential for engine application, showing the superiority of plastic waste over organic waste to produce better alternative fuels. The oil derived from HDPE can also be used to produce various fine chemicals, while the other obtained oils can be refined and used in the biofuel industry. In the future, new catalytic methods and separation approaches could be used to selectively enhance and extract valuable components from the pyrolysis oils.

Microwave-assisted pyrolysis quartz reactor: In another recent study [37], microwave-assisted pyrolysis (MAP) of different plastic wastes was conducted to produce cleaner aviation oil. A schematic of the MAP experimental setup can be found in Figure 5c. Five different types of plastic waste were used in this study, which are polypropylene (PP), high-density polyethylene, low-density polyethylene, polystyrene (PS), and polycarbonate (PC). The reaction system consisted of a quartz reactor filled with plastic waste and microwave absorbents. Different reaction parameters such as microwave power, microwave absorbent loads, and reactor temperatures were investigated. At a temperature of 450 °C and microwave power of 650 W, the plastic oil generated from polystyrene had the highest yield (98.78 wt%) at a microwave absorbent load of 60 g SiC (Silicon carbide), which is currently the highest oil yield worldwide. Fixing the microwave absorbent load of 60 g, and the microwave power at 650 W, the maximum oil yield was obtained at a temperature of 460 °C. The plastic oil yield at these conditions for PP, PS, and PC were 78.22 wt%, 98.78 wt%, and 39.78 wt%, respectively. The low yield of PC was denoted by the presence of oxygen, which causes a moisture content in the product, resulting in low oil yield. However, for the pyrolysis of HDPE and LDPE, the mixture of oil and wax was difficult to separate, and hence the pyrolysis oil could not be extracted.

The different pyrolysis oils of the plastic wastes had different components. The oils obtained from pyrolysis PP, LDPE, and HDPE were rich in aliphatic hydrocarbons, which are paraffin, cycloalkanes, cycloalkenes, and olefins. Plastic oil from PS consists of aromatic hydrocarbons, which are monocyclic aromatic hydrocarbons and polycyclic aromatic hydrocarbons. The main oil components from the pyrolysis of PC were phenols. Other components present in the oils were MAHs (Monocyclic Aromatic Hydrocarbons) and PAHs (Polycyclic Aromatic Hydrocarbons), which contribute to the production of aviation oil. The aviation oil hydrocarbon molecular chains are normally between C8 and C16. Despite PC, the amounts of C8–C16 hydrocarbons found in PP, HDPE, PS, and LDPE were high, and were 91.02 area%, 77.52 area%, 64.79 area%, and 59.73 area%. The low-carbon hydrocarbons in pyrolysis oils from PS were much higher than high-carbon hydrocarbons. This resulted in pyrolysis oil with low viscosity and higher heating value, and hence the pyrolysis oils from PS are very similar to aviation oils. Pyrolysis oils derived from HDPE and LDPE were rich in alkanes and alkenes, which are also ideal sources for oil production because of the high conversion of alkenes and alkanes. Microwave-assisted pyrolysis of plastic waste can not only efficiently solve the environmental problem of plastic pollution but also contribute to the production of valuable products, illustrating a simple and cleaner production approach.

Pyrolysis Plasma Reactor: A very recent study [38] investigated the viability of a novel plasma pyrolysis reactor, for converting plastic waste into hydrogen-rich fuel gas. A schematic of the experimental set-up for the plasma pyrolysis reactor can be found in Figure 5d. The plastic waste tested in the study consisted of PP, PET, LDPE, and HDPE. Also, the study evaluated the effect of feed flow rate and reactor temperature on the yield and efficiency of gas, the yield of solid residue, and the ratio of H₂/CO using the response surface method. The pyrolysis process was carried out at different temperatures in the range of 700–1000 °C, and the feed flow rate of 2.5, 5, 9, and 10 kg/h was tested. The plasma in the reactor increases the reactivity by heating the plastic feed molecules rapidly. This rapid heating leads to fast thermal cracking of the plastic, converting it to CO, CH₄, H_2, and other lighter hydrocarbons.

It was observed that the plasma pyrolysis of plastic generated combustible hydrogen-rich gas and solid residue, but no liquid products were observed, unlike conventional pyrolysis. The results of the study revealed that increasing the reaction temperature increases the gas product and reduces the solid residue yield for all the plastic samples. However, for some samples like PET, the gas yield did not vary significantly with temperature. Therefore, the optimum temperature for the samples was in the range of 700–900 °C. The gas yield and efficiency were also found to increase with an increase in feed flow rate. An overall mass balance showed that 79.4–90.8% of the mass of the feed has been converted to product gas, with PP having the highest % yield. The technology proposed in this study is relatively fast and produces negligible amounts of CO₂ and tar compared to other methods. Therefore, this technique presents a novel, fast, and clean method to convert plastic waste into syngas/hydrogen fuels.

Pyrolysis natural-zeolite catalyst: Another recent study [39] reports the conversion of HDPE and PP from plastic packaging waste into fuel oil by a pyrolysis process assisted by a natural zeolite. Plastic packaging of instant noodles was chosen as a source of PP, while clear plastic bags were used as a source of HDPE. The pyrolysis was carried out in a batch reactor, heating up to 400 °C. The evaluation of the catalytic properties of natural zeolite was also carried out with various reaction conditions and different concentrations of zeolite catalysts of 1%, 3%, 5%, 7%, and 9% w/w. The optimum pyrolysis conditions were obtained with catalyst concentrations of 7% for PP and 3% for HDPE (w/w), yielding 65.60% and 69.69% liquid fractions from HDPE and PP, respectively. When pyrolysis was carried out without the catalyst, only a small yield of 29.40% and 12.02% liquid fractions was obtained from PP and HDPE, respectively. Moreover, the use of the zeolite catalyst reduced the reaction temperature and time of the process. The reaction time needed for the process without a catalyst was 149 min for the PP sample and 192 min for the HDPE sample. However, when the catalyst was used, the reaction time decreased to 105 min for the PP sample and 132 min for the HDPE sample.

The composition of the liquid product from the PP sample is different from the HDPE sample. The distribution of pyrolysis liquid of PP was rich in hydrocarbons of C5–C12, followed by C17–C20, and the lowest was C13–C16. This distribution indicates that the pyrolysis liquid consists mainly of a light (gasoline) fraction and a small fraction of diesel (C18–C28). In contrast, the pyrolysis liquid of HDPE has a distribution of 30.36% for C5-C12, 40.39% for C13–C16, 24.69% for C17–C20, and 4.5% for longer chains (>C20). The analysis showed that approximately 70% of the hydrocarbons of HDPE pyrolysis liquid are distributed in the gasoline and diesel range. Natural zeolite demonstrated reliable catalytic performance in producing hydrocarbon liquid from the pyrolysis of HDPE and PP plastic samples with high yields and shorter reaction times.

Pyrolysis-MIL-53 (Cu)-loaded Zeolite Y catalyst: A more recent study [40] aimed to improve the limitations of zeolites in catalytic pyrolysis by loading MIL-53 (Cu) onto the zeolite’s surface. The plastic feed used in this study was a mixture of HDPE, LDPE, PS, and PP. MIL-53 and zeolite Y were synthesized, mixed, and pyrolyzed in a reactor to obtain the catalyst. The pyrolysis process took place in a fixed bed reactor under laboratory pressure conditions. The effect of reactor temperature, catalyst loading, and crystallinity on the production of liquid oil was investigated using a response surface approach. The results revealed that the modified zeolite catalyst behaves similarly to expensive and noble metals because of its high electron-transfer density. Moreover, the obtained liquid oil was separated into gasoline, diesel fractions, and jet fuel, and the effect of the tested parameters on the production of each type was determined.

The overall percentage yield of the liquid oil was found to be 80.8% at a temperature of 450 °C. The liquid oil contained valuable light aromatics, making it suitable for several applications including transportation, pharmaceutical, chemical, and painting applications. Based on the experimental results, the highest gasoline yield of 41.4% was obtained at a temperature of 381 °C, with a catalyst crystallinity of 82%, and catalyst loading of 2.5%. However, a much higher yield of 73.1% for jet fuel was achieved at a temperature of 525 °C, with an amorphous catalyst loading of 2.5%. Finally, the highest yield of diesel of 36.7% was obtained when the temperature was at 523 °C, with a fully crystalline catalyst, and catalyst loading of 10.5%. This novel composite catalyst showed high performance in improving the pyrolysis process to produce high-quality fuel.

Pyrolysis Natural-Mineral Catalyst: Luna et al. [41] proposed a sustainable approach for the pyrolysis of plastic waste using a natural mineral catalyst without modification. Bentonite was used as the natural mineral catalyst while HZSM-5 zeolite was selected for comparison of performance. The feedstock consisted of LDPE, PS, HDPE, and PP. The plastic mixture was prepared to ratios determined from non-recycled plastic (NRP) data from Malaysia, the US, the UK, and globally. The pyrolysis was conducted in a fixed bed reactor at a temperature between 400 °C and 500 °C. The pyrolysis process was repeated twice for each plastic mixture with and without the catalyst.

Thermal pyrolysis of the plastic mixture without a catalyst for Malaysia, the UK, the US, and globally was carried out at 500 °C. The highest liquid oil yield of 49.94% was obtained for the global composition. The effect of zeolite and bentonite on the oil yield was carried out using the plastic composition of Malaysia. When the zeolite catalyst was used, it was observed that the highest liquid yield of 55.99% was obtained at a temperature of 450 °C and using 1 g of HZSM-5. However, when the bentonite catalyst was used, a higher liquid yield of 60.06% was obtained using 1 g of catalyst at 500 °C. Based on the results, bentonite was found to be a better catalyst for producing a high liquid yield and the least tar residue. The optimal catalytic process conditions were then applied to the UK, US, and global plastic-waste composition. The highest liquid yield of 63.01% was obtained when the global plastic composition was used. Moreover, analyzing the liquid oil, aromatic compounds were higher in oils produced by both catalysts. Also, oil produced using bentonite had higher yields, produced more alkanes and alkenes, and had better physical properties, than oil produced using zeolite. This study showed the potential of pyrolysis to convert mixed plastic waste to an alternative sustainable fuel.

Pyrolysis Effect of Type of Catalyst: Also, another study [42] investigated the effect of different catalysts on the pyrolysis of polyethylene film waste to produce an alternative fuel. Four different catalysts were utilized in this study; they were Zeolite Ammonium, Zeolite HUSY, Zeolite HY, and Beta (HBEA), and the pyrolysis was conducted in a fixed-bed batch reactor at an operating temperature of 500 °C. The obtained plastic liquid yield was 37% for zeolite HY, 39% for zeolite HUSY, and 43% for Beta HBEA. All three oils had a good calorific HHV in the range of 45.8 to 48.0 MJ/kg, while the oil obtained using Beta HBEA had the lowest viscosity of 89.0 cSt (centistokes). Also, when the oils were further analyzed, they were found to be similar to conventional fuels.

The pyrolysis oil produced in this study consisted mainly of naphthene and aromatic compounds. It was found that the catalytic pyrolysis of PE reduced the high paraffin content and increased the naphthene and aromatic contents that were found in oils produced from the thermal pyrolysis of PE waste by 13% to 68% [43,44]. Moreover, analyzing the oil using TGA, FT-IR, and GC-MS, the carbon-number compounds found in the oil changed to C7–C32, which present kerosene, heavy naphtha, and diesel fractions in different ratios based on the type of catalyst used. For instance, HBEA produced the largest amount of transportation fuels because of its high ability for cracking heavy hydrocarbons. This study showed the important role of catalysts in converting plastic waste, namely PE, to produce valuable oils with potential industrial applications.

Table 2. Summary of recent studies published over last five years (from 2021 to 2024) on the conversion of plastic waste into alternative fuel using the pyrolysis process.

Type of Plastic Waste Used	Type of Reactor Used	Pyrolysis Oil Yield	Optimum Process Conditions	Type of Catalyst (If Used)	Pyrolysis Oil Application	References
HDPE Pure PP PP with fillers	Continuous microwave-assisted pyrolysis	48.90% 44.80% 23.70%	620 °C and the space velocity of 10 h−1 for the catalyst	Zeolite Socony Mobil–5	Fuel oil	[35]
BLFH MNS MGW HDPE	Auger reactor	42.75% 45.09% 44.72% 61.29%	475 °C 500 °C 500 °C 525 °C Residence time of 2 min.	----	Heating source for boilers and furnaces Engine fuel	[36]
HDPE LDPE PC PP PS	Microwave-assisted pyrolysis in a quartz reactor	-- -- 39.78% 78.22% 98.78%	460 °C, 650 W, and microwave absorbent load of 60 g	----	Aviation oil	[37]
PP PET LDPE HDPE	Continuous plasma pyrolysis reactor	90.80% 79.40% 85.50% 84.40%	800 °C and 10 kg/h feed rate 800 °C and 5 kg/h feed rate 800 °C and 10 kg/h feed rate 800 °C and 2.5 kg/h feed rate	----	Hydrogen-rich fuel gas	[38]
HDPEPP	Batch reactor	65.60% 69.69%	3% catalyst and 224 °C 7% catalyst and 197 °C	Natural zeolite	Fuel oil	[39]
Mixed plastic waste (HDPE, LDPE, PS, PP)	Fixed-Bed reactor	41.40% (Gasoline) 73.10% (Jet fuel) 36.70% (Diesel)	381 °C, 82% catalyst crystallinity 2.5% catalyst 525 °C, amorphous catalyst, 2.5% catalyst loading 523 °C, fully crystalline catalyst, and catalyst loading of 10.5%.	Cu-loaded Zeolite Y	Gasoline Jet fuel Diesel	[40]
A mixture of HDPE, LDPE, PP, and PS, mixed in different ratios	Fixed-bed reactor	63.01%	500 °C, 1 g of bentonite, global composition	HZSM-5 zeolite And bentonite	Fuel oil	[41]
PE film	Fixed-bed batch reactor	37.00% for zeolite HY 39.00% for zeolite HUSY 43.00% for Beta HBEA	500 °C and 120 min residence time	Zeolite HUSY Zeolite HY Beta (HBEA	Kerosene, heavy naphtha, and diesel	[42]
LDPE	Micropyrolysis reactor	79.20%	600 °C and split flow of 60 mL/min	HZSM-5 and SAPO-34.	Propylene and ethylene	[45]
PS, PE, HDPE, and LDPE	MAP system	----	360 °C and 50 μm catalyst size	ZSM-5 catalyst	olefins and paraffins	[46]
PS, PE, PP, PVC, and PET	-----	----	DT algorithm prediction	----	diesel and gasoline	[47]
PS 25 wt% PP +75 wt% HDPE	Batch unstirred tank reactor	85.20% 83.00%	430 °C and 60 min residence time	----	Fuel oil	[48]
PP	Semi-batch pyrolytic fixed-bed reactor	89.34%	481 °C, 13 mL/min nitrogen flow rate, and 0.61 kg/h feed rate.	----	Kerosene, diesel, and gasoline	[49]
Plastic bags	Fixed-bed reactor having two condensers	70.60%	30 °C for the first condenser	----	Diesel and gasoline	[50]
Mixed plastic waste (glasses, polyethylene bags, plastic bottle folders, etc.)	Batch reactor	80.00%	425 °C	----	Source for heat and energy production	[51]
LDPE and HDPE	Batch catalytic reactor	73.00%	280 °C	Catalyst	Alkyl aromatics	[52]
LDPE	Batch reactor	67.30%	300 °C and 0.6 g of catalyst	Zinc oxide	Transportation oil	[53]
Plastic grocery bags	Batch reactor	44.10%	403 °C, 17.3 °C/min heating rate, and 96.3 min residence time	Silicon dioxide	Fuel oil	[54]
PE	Plasma reactor	70.00%	900 °C and 3 kg/s feed rate	ZSM-5 catalyst	Hydrogen, electricity, hot water heating	[55]
HDPE bottle caps	Laboratory-scale fixed-bed reactor	---	450 °C and 30 min residence time	---	Diesel	[56]

Table 2. Summary of recent studies published over last five years (from 2021 to 2024) on the conversion of plastic waste into alternative fuel using the pyrolysis process.

Type of Plastic Waste Used	Type of Reactor Used	Pyrolysis Oil Yield	Optimum Process Conditions	Type of Catalyst (If Used)	Pyrolysis Oil Application	References
HDPE Pure PP PP with fillers	Continuous microwave-assisted pyrolysis	48.90% 44.80% 23.70%	620 °C and the space velocity of 10 h−1 for the catalyst	Zeolite Socony Mobil–5	Fuel oil	[35]
BLFH MNS MGW HDPE	Auger reactor	42.75% 45.09% 44.72% 61.29%	475 °C 500 °C 500 °C 525 °C Residence time of 2 min.	----	Heating source for boilers and furnaces Engine fuel	[36]
HDPE LDPE PC PP PS	Microwave-assisted pyrolysis in a quartz reactor	-- -- 39.78% 78.22% 98.78%	460 °C, 650 W, and microwave absorbent load of 60 g	----	Aviation oil	[37]
PP PET LDPE HDPE	Continuous plasma pyrolysis reactor	90.80% 79.40% 85.50% 84.40%	800 °C and 10 kg/h feed rate 800 °C and 5 kg/h feed rate 800 °C and 10 kg/h feed rate 800 °C and 2.5 kg/h feed rate	----	Hydrogen-rich fuel gas	[38]
HDPEPP	Batch reactor	65.60% 69.69%	3% catalyst and 224 °C 7% catalyst and 197 °C	Natural zeolite	Fuel oil	[39]
Mixed plastic waste (HDPE, LDPE, PS, PP)	Fixed-Bed reactor	41.40% (Gasoline) 73.10% (Jet fuel) 36.70% (Diesel)	381 °C, 82% catalyst crystallinity 2.5% catalyst 525 °C, amorphous catalyst, 2.5% catalyst loading 523 °C, fully crystalline catalyst, and catalyst loading of 10.5%.	Cu-loaded Zeolite Y	Gasoline Jet fuel Diesel	[40]
A mixture of HDPE, LDPE, PP, and PS, mixed in different ratios	Fixed-bed reactor	63.01%	500 °C, 1 g of bentonite, global composition	HZSM-5 zeolite And bentonite	Fuel oil	[41]
PE film	Fixed-bed batch reactor	37.00% for zeolite HY 39.00% for zeolite HUSY 43.00% for Beta HBEA	500 °C and 120 min residence time	Zeolite HUSY Zeolite HY Beta (HBEA	Kerosene, heavy naphtha, and diesel	[42]
LDPE	Micropyrolysis reactor	79.20%	600 °C and split flow of 60 mL/min	HZSM-5 and SAPO-34.	Propylene and ethylene	[45]
PS, PE, HDPE, and LDPE	MAP system	----	360 °C and 50 μm catalyst size	ZSM-5 catalyst	olefins and paraffins	[46]
PS, PE, PP, PVC, and PET	-----	----	DT algorithm prediction	----	diesel and gasoline	[47]
PS 25 wt% PP +75 wt% HDPE	Batch unstirred tank reactor	85.20% 83.00%	430 °C and 60 min residence time	----	Fuel oil	[48]
PP	Semi-batch pyrolytic fixed-bed reactor	89.34%	481 °C, 13 mL/min nitrogen flow rate, and 0.61 kg/h feed rate.	----	Kerosene, diesel, and gasoline	[49]
Plastic bags	Fixed-bed reactor having two condensers	70.60%	30 °C for the first condenser	----	Diesel and gasoline	[50]
Mixed plastic waste (glasses, polyethylene bags, plastic bottle folders, etc.)	Batch reactor	80.00%	425 °C	----	Source for heat and energy production	[51]
LDPE and HDPE	Batch catalytic reactor	73.00%	280 °C	Catalyst	Alkyl aromatics	[52]
LDPE	Batch reactor	67.30%	300 °C and 0.6 g of catalyst	Zinc oxide	Transportation oil	[53]
Plastic grocery bags	Batch reactor	44.10%	403 °C, 17.3 °C/min heating rate, and 96.3 min residence time	Silicon dioxide	Fuel oil	[54]
PE	Plasma reactor	70.00%	900 °C and 3 kg/s feed rate	ZSM-5 catalyst	Hydrogen, electricity, hot water heating	[55]
HDPE bottle caps	Laboratory-scale fixed-bed reactor	---	450 °C and 30 min residence time	---	Diesel	[56]

Pyrolysis Active Learning: This work [45] presented and validated a novel Gaussian N-Dimensional Active Learning Framework (GandALF) for predicting the yield of light olefins from the catalytic pyrolysis of LDPE. The GandALF works by combining the Expected Model Output Change (EMOC) acquisition function with k-means clustering optimized for Gaussian Processes (GPs). The system was used to simulate the hydroconversion hydrocracking and catalytic pyrolysis of the feed. The focus of the study was to model the yield of C2–C4 olefins as a function of catalyst temperature, space-time, and catalyst type. The two types of catalysts examined in the study were HZSM-5 and SAPO-34. To validate the ability of GandALF to facilitate an improved prediction, it must be validated against an experimental study of the catalytic pyrolysis of LDPE, by running 18 experiments with both GandALF and a full-factorial design.

The maximum reported yield of the C2–C4 olefins was found to be 79.2 wt%, with an experimental standard deviation of 1.29 wt%. The GandALF was able to achieve a similar model accuracy only after ten experiments compared to the regular design of experiments (DoE) after 18 experiments. The reduction in the number of experiments corresponds to a large reduction in cost, time, and effort. Investigating the effect of the catalyst showed that using HZSM-5 achieved higher yields between 7.2 wt% and 41.6 wt%, as opposed to using SAPO-34 (0.5−21.8 wt%). The results also showed that increasing the catalyst temperature and space-time elevates the ethylene and propylene production for both catalysts. The GandALF has proven to be more efficient than regular DoE methods for the development of a model for catalytic pyrolysis of LDPE.

Pyrolysis Modeling and Simulation: Bandi et al. [46] also developed a comprehensive mathematical model to predict the catalytic pyrolysis of healthcare plastic waste for fuel production. The pyrolysis process is conducted in a fluidized-bed reactor using a ZSM-5 catalyst in conjunction with a microwave-assisted pyrolysis (MAP) system. A transient numerical analysis of the process was performed by using a lumped kinetic model that considers three lumped pyrolysis products, namely paraffin, olefins, and aromatics, coupled with the governing equations describing the process. The study examined the effect of different factors on the rate of consumption and production in the pyrolysis reactions using MAP. The factors of interest included the fluidizing-gas inlet velocity, the microwave power input, and the mass and particle size of the ZSM-5 catalyst.

The accuracy of the MAP model was validated by comparing the numerically simulated temperature results against the experimental data. There was a close correlation between experimental temperature outcomes and the numerical predictions, with a difference of less than 5%. The mass and particle size of the catalyst used also have a significant impact on the yield of olefins, paraffins, and aromatics. The results showed that decreasing the catalyst’s particle size generally increases the reaction rate, facilitating the production of paraffins, olefins, aromatics, and coke. However, using a catalyst with a particle size of less than 50 μm was not applicable in the fluidized bed because of increased intermolecular forces. On the other hand, increasing the mass of the catalyst over 5 g did not have a significant influence on the reaction rate. Finally, when the inlet velocity of the fluidizing gas was increased, this resulted in an incomplete consumption of the intermediates and a low yield of liquid products. The MAP system presented a highly efficient design to model the pyrolysis of plastic waste to produce pyrolysis oil as a source of energy. This system provides a more energy-efficient and lower-temperature process compared to conventional fluidized-bed reactors.

Pyrolysis Machine Learning: This study [47] modeled the effect of input variables or parameters on the non-catalytic pyrolysis process of plastic waste into valuable fuel and chemicals. The algorithms of the Artificial Neural Network (ANN), Decision Tree (DT), Support Vector Machine (SVM), and Gaussian process (GP) were compared to identify the most accurate model for this process. The experimental data needed to develop the model were collected from 93 reviewed studies related to the scope of the study. The plastics included in this study were PS, PE, PP, PVC, and PET. The variables tested were the feedstock properties, the feed intake capacity, reaction temperature, and vapor residence time.

The results showed that the DT algorithm predicted the response with the best accuracy among the other algorithms, with an R² value of 0.984. The ANN algorithm also showed a relatively high accuracy, with an R² value of 0.926. However, the GP and SVM algorithms were not applicable for predicting the liquid product yield in this study. The fitrensemble (i.e., applying the formula to fit the model to the predictor and response data) mode of DT also performed excellent results in predicting the mass and energy balances of the process. The DT model was also able to generalize the process, making it highly efficient for this type of predictive modeling. It was also observed that the data set with the elemental composition of plastic achieved higher accuracy in predicting the liquid yield than the sample with the plastic. This observation allows for accurate predictions even when specific plastic-type data are unavailable, relying instead on easily obtainable elemental analysis. This study highlights the potential of machine learning in improving the predictability and control of the pyrolysis process of plastic waste into useful products, thereby addressing its environmental challenges.

Pyrolysis-effect of temperature and residence time: Asueta et al. [48] studied the effect of reactor temperature and residence time on the quality of liquid oil produced from low-temperature pyrolysis of a plastic mixture. Two types of samples were tested in this study: a virgin mixture of PP, HDPE, and PS, and real plastic waste collected from a recycling facility. The real-waste sample was pyrolyzed in a batch reactor conducting a 2² factorial design to optimize the production of liquid oil using values in the range of 430–460 °C, and 45–60 min for temperature and residence time, respectively.

The highest liquid yield of 36.8% was achieved at the central point of the experimental design at 445 °C and 53 min. The results of the experiments also showed the importance of the temperature variable over residence time in the pyrolysis process. Furthermore, three mixtures of virgin plastics in different ratios were also pyrolyzed to determine which type of plastic generates the highest pyrolysis oil. The experiments were carried out at a temperature of 430 °C and a residence time of 60 min. The PS sample produced the highest oil with a yield of 85.2%. Also, a mixture of 25 wt.% PP + 75 wt.% HDPE produced a high yield of 83%. Analyzing the obtained oil, it was found to contain high amounts of aromatic hydrocarbons and phenol and its derivatives. Moreover, the oil obtained from different experiments had an HHV of around 40 MJ kg⁻¹, matching that of commercial fuel. The oil was also less viscous and lighter, with a similar toxicity profile to commercial fuels.

Pyrolysis Box–Behnken Design: Another study [49] proposed an optimization method for the pyrolysis process parameters using the Box–Behnken design (BBD) technique for the first time. Pyrolysis of polypropylene (PP) grocery bags was conducted using a semi-batch pyrolytic fixed-bed reactor to produce fuel oil. The effect of three major process parameters, namely temperature, nitrogen flow rate, and substrate feed rate, on fuel oil yield was investigated using the response surface methodology (RSM) with the BBD. A BBD-based 3³ factor design was employed in this study; the values of the parameters tested were in the range of 400–550 °C, 5–20 mL min⁻¹ nitrogen flow rate, and 0.25–1.5 kg h⁻¹ feed rate, and their effects on the fuel oil yield were observed.

The highest oil yield of 89.34% was obtained at 481 °C, 13 mL min⁻¹ nitrogen flow rate, and 0.61 kg h⁻¹ feed rate. The quadratic model obtained for the response was also very reliable, with R² = 0.99. The statistical test showed the strong influence of these process parameters on the yield of fuel oil. Moreover, the fuel oil was further analyzed to determine its properties. The oil consisted of high contents of carbon and hydrogen with a calorific value of 45.42 MJ kg⁻¹, resembling values of those of commercial petroleum fuels. The fuel oil had higher Research Octane Number (RON) and Motor Octane Number (MON) values than that of conventional gasoline, indicating better antiknock properties. A further analysis of the oil revealed that the oil contained around 30 compounds and was enriched with paraffin and olefins in the carbon range of C4–C20. This carbon range indicates the presence of diesel, kerosene, and gasoline in the fuel oil, which shows its potential as an alternative fuel for different industrial applications.

Pyrolysis-effect of condenser temperature. Pannucharoenwong et al. [50] aimed to study the effect of condenser temperature on the quality of liquid oil obtained from the pyrolysis of plastic bags. The feedstock consisted of plastic bags collected from different stores in Kalasin province, Thailand. The pyrolysis process took place in a fixed-bed reactor at 300 °C, having two condensers, and the liquid oil was collected at both condensers. The second condenser is set at a temperature of −40 °C, while the first condenser’s temperature was set at five different levels, which are 10, 20, 30, 40, and 50 °C. It was observed that increasing the temperature of the first condenser from 10 to 30 °C increased the yield of the first oil from 53.3 to 70.6%. However, when the temperature was further increased to 50 °C, the % yield dropped to 37.3%. Also, the yield of the second oil increased from 2.7 to 7.3% when the temperature was increased from 10 to 40 °C, but decreased at a temperature of 50 °C.

Analyzing the first fuel showed that it exhibited the highest heating value (HHV) of 42.4 MJ/kg at 10 °C, while the lowest value of 37.1 MJ/kg was observed at 50 °C. Also, the composition of the first fuel was very similar to the chemical diesel. However, increasing the temperature of the first condenser from 10 to 50 °C decreased the density and calorific value of the first fuel. On the other side, analyzing the second fuel, it was found that increasing the temperature from 10 to 50 °C increased the HHV from 36.2 to 38.1 MJ/kg, but the highest HHV was still lower than that of the first fuel. However, increasing the temperature increased the density and calorific value of the second fuel. The second fuel composition consisted mainly of benzene, cyclononane, and toluene, giving it the physical appearance of gasoline. This study has demonstrated the potential use of plastic oils as alternative fuels for diesel and gasoline.

Pyrolysis with mixed-plastic wastes: Rajendran et al. [51] conducted a study to characterize and evaluate plastic oil produced from the pyrolysis of mixed plastic waste as an alternative to diesel oil. The mixed plastic waste consisted of bread bags, glasses, polyethylene carry bags, plastic bottle folders, plastic file spoons, cable covers, and pet bottles. The pyrolysis reactor was designed to handle high temperatures, between 340 °C and 440 °C, and corrosive chemicals. Thermal breakdown of the mixed plastic waste at an optimal temperature produces volatile matter which, when condensed, results in plastic oil. The maximum yield of the pyrolysis process was obtained at 425 °C, which was nearly 80%. The heating value of the sample’s fuel was found to be 42.58 MJ kg⁻¹, which is very similar to that of diesel (44.27 MJ kg⁻¹), which supports the use of plastic oil in several industries.

The plastic oil produced consisted of around 15 different hydrocarbons and had nearly the same physical characteristics as diesel. However, commercial fuel had lower viscosity than plastic oil obtained in this study. The reason for this higher viscosity might be because of the presence of light and heavy naphtha in the oil sample. Also, the average density of the study’s different oil samples was 783 kg/m³, which is similar to that of diesel (837 kg/m³). The plastic oil was composed mainly of aliphatic hydrocarbons, ranging from C9 to C17. 1-decene and 1-hexadecene had the highest weight fraction in the liquid fuel. The study stated that fuel produced from plastic waste has high combustion capabilities. Also, the fuel includes hydrocarbons found mainly in gasoline instead of the diesel-range composition. Based on the analysis of the plastic oil, it was concluded that this oil may be used as a source of heat and energy production.

The above discussion reveals the diverse pyrolysis methodologies and outcomes in converting plastic waste into valuable fuel oil. It was observed that the type of plastic feedstock significantly affects the liquid yield. Among the different types of plastic, PS and PP produced the highest liquid oil yield, followed by HDPE and LDEP, while PET showed the lowest yield. Also, it was observed that PP, HDPE, and LDPE are frequently studied due to their prevalence in waste streams. Moreover, recent advances in pyrolysis systems such as continuous microwave-assisted pyrolysis and plasma reactors achieve higher yields but require more complex equipment. In contrast, batch and fixed-bed reactors, while simpler, offer varied results based on the feedstock and operating conditions. Analysis of the operating factors like feed rate, temperature, and residence time showed that regarding the type of plastic, the optimum conditions to achieve the highest liquid oil yield would be changed accordingly. Catalysts, especially zeolites, significantly enhance the liquid oil yield and quality. Studies also showed the diverse applications for pyrolysis oil, including fuel oil, aviation oil, and hydrogen-rich fuel gas. However, economic and regulatory challenges persist, with high operational costs and strict compliance requirements. Addressing these challenges by improving reactor design, catalyst quality, and operating conditions would advance plastic pyrolysis as a sustainable waste management solution

4. Challenges Associated with the Pyrolysis of Plastics

Although pyrolysis of plastic waste is a promising waste-management method, there are still various challenges associated with this process that need to be addressed. One of the major challenges that is slowing the scale-up of plastic waste pyrolysis is the complexity of plastic waste feedstock. Plastic waste usually contains dyes, pigments, papers, coatings, glasses, metal foils, etc., which can affect the purity and quality of fuel produced. Moreover, the percentage yields of liquid fuel from the pyrolysis process are relatively low. Another problem with plastic pyrolysis is the high fraction of olefins in liquid oil. The excess amounts of olefins limit the economic feasibility of pyrolysis oil, especially when it is intended to be used as a transportation fuel because high amounts of olefins can result in the formation of unwanted gums in the engine of the vehicles. Another important challenge that faces plastic pyrolysis is the complexity of its legislative framework involving policies on waste management, economy, product safety, and fuels. Currently, EU legislation does not recognize pyrolysis as a recycling technology if the end product is used to generate energy. This status needs to be changed for pyrolysis to become a feasible solution for plastic waste recycling. Establishing end-of-waste (EoW) criteria for pyrolysis oil would help classify it as a product, promoting it for commercial use and regulatory alignment with REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) registration [57,58]. To address these challenges, recent research is focused on developing efficient plastic-waste pretreatment and impurity-removal technologies to produce valuable plastic feedstock. Also, research efforts are targeted toward developing more promising catalysts that can improve the efficiency and quality of the fuel produced from plastic waste.

5. Conclusions

In conclusion, producing liquid fuel from plastic waste through pyrolysis has gained significant interest in recent years as a possible solution to the problem of plastic waste. Pyrolysis is an energy-efficient, cost-effective, and environmentally friendly process that can convert a wide range of plastic wastes such as HDPE, LDPE, and PP into valuable liquid fuels. The production of liquid fuels from the pyrolysis of plastic waste is a promising technology. This technology has significant potential in improving the global economy and reducing the dependence on fossil fuels. However, further research is still needed to improve the process parameters and tackle the challenges related to scaling up the pyrolysis process to commercial levels. With continuous innovation and investment, the pyrolysis of plastic waste into liquid fuels can significantly contribute to more sustainable waste management systems and the transition to greener energy sources.