Pyrolysis procedures comprise diverse methods, such as slow, rapid, flash, catalytic, and vacuum pyrolysis, each having specific functioning circumstances and processes. Three primary kinds of pyrolysis procedures for biomass or plastic waste are distinguished by their combustion percentage, temperature, and vapor retention duration.

Also called non-isothermal, it happens at moderate temperatures (350 to 550 Celsius), with one to 10˚C/min warming charges and a more extended air residence period [65] . The primary outcome of the slow burn is firm leftover, dubbed tar, as a slow temperature rise that favors solid formation among several parallel-competitive reactions [66] . It usually produces oily liquid outputs of over 95 wt% from plastic trash [64] . Oil combustion has a thermal conductivity equal to standard diesel fuel (approximately 45 MJ/kg) and may be employed in diesel engines with simple modifications [65] .

Isothermal fast pyrolysis typically occurs between 500˚C and 700˚C. The raw material’s heat intensity may reach above 1000˚C/min, and the duration of the gas residence can sometimes be several seconds [67] . Rapid pyrolysis promotes fluid generation, and depending on the kind of substrate, fluid production might unexpectedly reach up to 90% of the polymeric material’s breakdown [68] .

Flash pyrolysis, or Ultra-fast, involves heating the input to temperatures over 700˚C, with high heating rates and gas stay times within the millisecond interval [66] . Flash pyrolysis may create more huge fuel outputs than rapid pyrolysis for biomass feedstocks. However, it varies from plastic garbage since it generates more gas than other items [69] . The byproducts derived during the pyrolysis of recyclable materials can be divided into fluid, solid residue, or gas [70] .

Thermal cracking or Pyrolysis is a thermal breakdown technique that occurs when polymers are subjected to high temperatures (often around 350˚C and 900˚C). This breaks down the components and produces fluid, gas, and charred char. The fuel is usually derived via the condensable component of the unstable item [71] . Thermal pyrolysis is a thermo-chemical treatment (TCT) process including the decrease of paraffin, isoparaffins, olefins, naphthenes, and aromatics in aqueous solutions, requiring temperatures up to 900˚C owing to bond breakage and weakening [72] . This technique’s physical parameters of the liquid fuel generated principally vary with the plastic category [73] . Table 1 offers an overview of a few of the characteristics of fluid fuel originating through the pyrolysis of multiple polymers.

NA: are not accessible for research.

Microwave pyrolysis (microwave-assisted pyrolysis) involves microwave heating of dielectric material. Microwaves interact with diverse materials in three ways: via conductors, perfect barriers, or absorbed and decaying according to their dielectric properties. In semiconductors, heat is produced via magnet-induced molecular rupture. Plastic microwave pyrolysis absorbs microwave radiation and delivers heat to plastic [71] . The uniformity of heat dispersion in microwave-induced pyrolysis is determined by physical features and absorbent volume ratio. Compared to thermal and enzymatic pyrolysis, it may be utilized to create value-added chemicals and fuels [78] . The method tries to heat polymer compounds effectively and correctly. However, due to weak dielectric factor loss, polymers cannot absorb microwave radiation. Absorbent materials like tire shreds and silicon carbide [79] , carbon atoms [80] , and iron fit [81] are required to assist in plastic pyrolysis. Table 2 offers statistics on the yield, absorption-to-polymer proportion, operation duration, and electromagnetic efficacy for five plastic kinds employing tires instead of lignite as an electromagnetic absorbent.

Catalytic pyrolysis is a method that includes heating polymeric substances without the presence of oxygen and utilizing a catalyst to break down them. Typical catalysts used in plastic material pyrolysis include silica-alumina [83] , natural zeolites [84] , fluid cracking catalytic (FCC) [85] , and mobile categorization of materials (MCM-41) [86] . Zeolite-based catalysts are powerful owing to their excellent acid resistance. Two-phase catalytic breakdown employing silica-alumina and HZSM-5 zeolite may enhance gas output and octane number, but the reverse arrangement doesn’t help. Staged catalysis may turn waste plastic into gasoline-range chemicals, yielding C2 (mostly ethene), C3 (usually propene), and C4 (mostly butene, butadiene) gases [87] . Clays were found to be inactive but could degrade polyethylene at greater temperatures, giving 70% liquid products and heavier ones. Catalysts such as zeolites (ZSM-5, Y-zeolite), FCC, and MCM-41 are utilized to increase the pyrolysis process efficiency, target particular reactions, minimize temperature and time requirements, as well as improve the nutritional value of pyrolysis items [22] .

Within vacuum pyrolysis, the procedure occurs under a controlled environment where the pressure is dropped below the level of the atmosphere. By working under vacuum circumstances, the boiling point of the reaction outcomes decreases, resulting in more effective isolation and gathering. This approach provides various benefits over traditional pyrolysis methods, including lower energy usage, increased output production, and the capacity to treat a more extensive variety of raw materials.

The process parameters in plastic pyrolysis, including temperature, power plants, strain, residence duration, catalyst, lubricating gas, and pace, may affect a combination of final products such as fluid oil, gases, and ashes. Adjusting these variables could give the desired outcome.

Temperature is a critical aspect of decomposition due to the controls of the breaking reaction of polymer chains. The Van der Waals attr pulls molecules together, preventing collapse. As temperature rises, vibrations become more evident, and molecules evaporate off the surface. This happens once the heat generated by the force produced by Van der Waals overcomes the resistance of the C-C connect, resulting in the breakdown of the corresponding carbon strand [88] .

The thermogravimetry analyzer might be employed to evaluate the heat deterioration of plastics. It includes two graph types: a thermogravimetry analysis (TG) slope and a derivative thermal gravity assessment (DTG) slope. It indicates the material’s shift in weight dependent on temps and duration, while the DTG graph delivers information on the degradation stage [89] . The following Table 3 offers a detailed description of the thermal decomposition behavior of PET, HDPE, PVC, and LDPE plastics, showing the heat ranges at which significant deterioration occurs and essential discoveries from relevant research.

Accordingly, the placement of product options, pressure, and residence time should be handled, particularly at temperatures under 450˚C [21] . The experiment by Murata et al. [98] demonstrated that pressure strongly affects the resulting gas and the carbon amount spacing among the fluid output at temperatures above freezing. Pressure directly impacts the scission rate of C-C linkages in polymers, boosting dual bond growth and residence duration at lower temperatures. However, this impact becomes less visible when temperatures near 430 C.Mastral et al. [99] observed that prolonged residence time boosts liquid yield at temperatures below 685 C but less at higher temperatures. The research discovered that pressure and residence duration in plastic pyrolysis are influenced by temperature. They impact product dispersion at temperatures below freezing but less at elevated levels, thereby raising operational costs.

The reactor type considerably impacts the combination of polymers plus catalysts, residence duration, heat transmission, and the effectiveness of the reaction in getting the intended output. Most lab-scale plastic pyrolysis is accomplished in batch, semi-batch, or continuous-flow reactors such as fluidized bed, fixed-bed reactor, and conical spouted bed reactor.

Batch reactors are enclosed devices that do not allow for introducing or removing reactants or products while a reaction occurs. They achieve excellent efficiency of conversion by maintaining reactants in the reactor for prolonged durations. Nevertheless, these methods have limitations, such as uncertain results, expensive workforce requirements, and challenges in achieving mass creation [100] . Semi-batch reactors provide concurrent reactant input and product removal, providing versatility in reaction selectivity. Nevertheless, they possess comparable labor costs to batch reactors, making them appropriate for production on a modest scale. These reactors are preferred because they are simple and allow easy operation parameter control [75] [101] - [109] . Pyrolysis is often conducted within 300 - 800 degrees Celsius for thermal and catalytic processes. Researchers incorporate catalysts into polymers to enhance hydrocarbon production and improve product refinement. Nevertheless, this process has limitations, including creating excessive coke on the enzyme’s terrain, decreased effectiveness, and difficulty removing leftovers from the catalytic after the experiment.

The stirrer in batch reactors promotes fluid oil production via the enhancing interaction of catalysts and polymers. Abbas-Abadi [110] and Kyong [111] [112] , revealed that semi-batch reactors with stirrers are ideal for the process due to parameter management. In contrast, batch reactors are inappropriate for catalytic burning owing to potential ash accumulation. Figure 1 shows the schematic batch with the stirrer components.

Fixed-bed reactors are vital for plastic pyrolysis owing to their simplicity and

capacity to manage uneven particle sizes. However, they encounter limitations, including restricted surface area and uncertain process conditions. Fluidized bed reactors alleviate difficulties by allowing greater mobility to the catalyst and more significant contact areas, minimizing unpredictability, and boosting heat transfer. The catalyst in the fixed-bed device is usually palletized and organized in a stationary bed, as shown in Figure 2 . Research investigations generally employ fixed-bed reactors for plastic pyrolysis, [90] [113] [114] [115] [116] [117] , frequently as secondary reactors owing to the ease of product feeding [118] . A fluidized bed reactor is a more flexible and cost-effective alternative to fixed-bed reactors due to its improved access to the catalyst [119] , wider surface area, and better heat transfer, as depicted in Figure 3 . Researchers recommend using fluidized bed devices in the catalytic cracking of polymers owing to their flexibility, which requires less continuous operation and avoids frequent feedstock charge compared to fixed bed reactors [120] - [127] .

The (CSBR) is a flexible device that can tolerate substantial particle changes and densities [118] . Multiple investigators were adopting CSBR for their destructive cracking of material testing [128] - [133] . Olazar et al. [128] discovered CSBR exhibited fewer losses and bed segregation compared to bursting fluidized beds and substantial heat transfer between phases. However, technical restrictions, such as catalyst being fed, stimulation, and output gathering, render it less desirable to handle sticky materials. Study [133] employed CSBR for HDPE pyrolysis with HY zeolite catalyst at 500˚C, resulting in a 68.7 wt% gasoline fraction (C5 - C10), despite the high operating expense of the complicated design that required pumps, and the method was illustrated in Figure 4 .

The microwave absorbent gathers radiation from microwaves to deliver sufficient heat to attain the degrees of heat necessary for widespread pyrolysis [135] . Microwave radiation provides benefits over classical pyrolysis, including quicker heating, more incredible output velocity, and decreased production costs. The material is directly exposed to electromagnetic radiation via chemical interaction, eliminating the need for immediate heating [136] . Microwave heating provides benefits, but needs help in industry research owing to the necessity to characterize the dielectric characteristics of the processed waste stream. The efficacy relies on material qualities, such as the tiny dielectric strength of plastics. Using carbon as an electromagnetic collector throughout pyrolysis might maximize the intake of energy and thermal conversion in a reduced timeframe [135] . The possible variance in heating efficiency for each material has been a critical worry for corporations.

Catalysts, both uniform and heterogeneous, promote product dispersion, selectivity, and yields in industries like polyolefin pyrolysis. Uniform catalysts like AlCl₃ are typically utilized [137] [138] . Nevertheless, the catalyst that is most often used is diverse because of its ability to quickly separate the fluid product mixture of the solid catalytic.

Zeolites are crystalline aluminosilicate sieves containing pore spaces and can have ion abilities [139] [140] . Zeolite, a multidimensional frame with oxygen atoms joining tetrahedral sides, is created by altering SiO₂/Al₂O₃ ratios. The ratio impacts zeolite reactivity and pyrolysis results. HZSM-5 zeolite significantly impacts HDPE pyrolysis output, with acidic catalysts more active in breaking waxes and creating light olefins. Decreases in the SiO₂/Al₂O₃ ratio improve the synthesis of simple hydrocarbons and perfumes. Other zeolite catalysts include HUSY and HMOR. Garforth, Al-Salem et al. [141] observed that HZSM-5, a better zeolite catalyst, beat HUSY and HMOR in HDPE pyrolysis, with a minimal residue left of 4.53 wt%, outperforming other materials in optimizing total product transformation.

FCC catalyst combines zeolite particles and a non-zeolite acidic substrate termed silica-alumina with binding agents [138] [142] [143] [144] . Zeolite-Y has served as a key feature of FCC catalysts for more than forty years due to its strong selectivity for products and heating equilibria [145] . FCC catalyst is extensively used in chemical treatment to convert dense oil components into lighter fuel and LPG portions [145] . The FCC catalyst is generated from commercial FCC operations in energy facilities, which is efficient and may be regenerated in pyrolysis. Kyong et al.’s [111] investigation of abandoned FCC catalysts in the decomposition of HDPE, LDPE, PP, and PS at 400˚C indicated that each of the plastics generated more than eighty-weight percent liquid oil, with PS being especially concentrated. The rejected catalyst displayed high catalytic activity and was cost-effective as a reused’ catalyst.

Silica-alumina is an irregular enzyme comprised of Bronsted acid spots plus Lewis acid locations. Its acid content is governed by the molecular fraction of SiO₂/Al₂O₃. Unlike zeolite, silica-alumina’s acidity is assessed against a high ratio of SiO₂/Al₂O₃, suggesting an enhanced degree of acidity. SA-1 and SA-2 are commercially available silica-alumina [146] . Uddin, Kasar, et al. [147] studied the impact of SA-2 on HDPE and LDPE pyrolysis, finding similar results to Sakata et al.’s findings. Uddin et al. observed that SA-2 substantially effects HDPE and LDPE pyrolysis, with HDPE being more resistant owing to its linear chain.

Fluidizing gas, including nitrogen, helium, argon, ethylene, propylene, and hydrogen, is used for plastic pyrolysis to transfer vaporized materials, with sensitivity varying based on the size of its molecules. AbbasAbadi et al. [148] discovered that carrier gas molecular size impacts product composition and temperature, and reactivity affects coke production. Ethanol, ethylene, helium, and propylene are the most prevalent, with ethylene offering greater liquid output and fewer coke forms. Nitrogen is often used in plastic pyrolysis owing to its ease of handling and safety, but helium is seldom utilized due to restricted supplies and higher prices. Fluidizing flow rate also affects product distribution, with the lowest flow rate lowering degradation rate rapidly and lengthy contact time promoting coke precursor formation.

The first step entails explicitly identifying the aim and breadth of the LCA research. The intent describes the intended use of such an assessment, such as comparing various goods or detecting environmental hotspots. The scope specifies the system limits, including the functional unit (for instance, the quantity of goods or services being assessed) and the lifespan cycle stages must be included [150] . The purpose and scope give a basis for the whole assessment.

The LCI implies generating a complete inventory of inputs, outputs, and activities related to the lifespan cycle stages indicated in the scope. It comprises data collection on substances, the extraction process, energy usage, greenhouse gases, waste creation, transportation, and other pertinent factors [150] . The LCI assesses the system’s external inputs and outputs, generating a thorough inventory record.

The inventory data is examined to identify the possible ecological consequences of the system under consideration. This entails using characterisation criteria to turn the inventory data into effect categories such as ozone depletion, global warming, toxicity to humans, etc. LCIA methodologies may vary, but they usually try to quantify and analyze the possible environmental impacts associated with the life cycle phases and emissions listed in the LCI [150] .

Interpretation entails evaluating and interpreting the findings of the LCI and LCIA to develop conclusions and make educated judgments. It involves analyzing the assessment’s strengths, weaknesses, and uncertainties and identifying the primary contributors to environmental consequences. The interpretation step generally incorporates sensitivity analysis, scenario analysis, and identification of improvement opportunities [150] . The outcomes of the interpretation influence decision-making, promote communication and inform the creation of initiatives to minimize environmental consequences. Figure 5 shows a framework of LCA modified from the ISO 14040 standard. We can see the resume of the different parts in the following figure.

LCAs of pyrolysis methods evaluate numerous impact categories to determine their environmental impacts. Table 4 offers a synopsis of some of the primary impact categories frequently evaluated in LCAs of pyrolysis.

Table 5 compiles some of the past Life Cycle Assessment investigations concerning the recycling of chemically processed polymers. The main emphasis is on pyrolysis, whereas other methods such as electromagnetic combustion, gasification, and hydrolysis are considered impractical owing to technological constraints [38] [39] [40] [43] . Neither solvent degradation nor enzyme breakdown were explored due to their unsuitability for polymers such as polyethylene and polypropylene [152] [153] . However, LCAs may enhance existing processes, and it is essential to advocate for their implementation.

Acidification potential (AP), Aquatic toxicity (FAETP), Climate change - Global Warming potential (GWP100), Climate Change (CC), Climate Change Potential (CCP), Cumulative Energy Demand (CED), Eutrophication Potential (EP), Fossil Resources scarcity (FRE), freshwater ecotoxicity (FETP), human toxicity (HT), ionizing radiation (IRP), land use (LOP), marine ecotoxicity (ME/METP), Marine Eutrophication (ME), Metal Depletion (MD), Mineral Resource scarcity (SOP), ozone depletion (ODP), particulate matter formation (PMFP), Photochemical Oxidation Potential (POP), Photochemical Ozone Formation (POFP), Photochemical Ozone Formation (HOFP), human health, abiotic resource depletion fossil fuels ((ADP-fossil)), Terrestrial Acidification (TA/TAP), Terrestrial Ecotoxicity (TETP), terrestrial eutrophication (ET), Water Consumption (WC), Heavy Vaccum Residue(HVR), Ultralow Sulfur Diesel (ULSD)