2.1. Permits and field-site access

Waste plastic samples used in this study were collected from publicly accessible municipal solid waste collection points in Dhaka, Bangladesh. The samples consisted only of discarded post-consumer plastic materials, including PET, PVC, PP, and HDPE, and did not involve any protected areas, privately restricted sites, human participants, animals, endangered species, or biological specimens. Therefore, no formal environmental, ethical, wildlife, or field-collection permit was required for this work. Access to the waste collection sites was obtained with verbal permission from the relevant local waste-management personnel/site supervisors responsible for the municipal waste collection points. No personally identifiable information or sensitive data were collected during sample acquisition. The collected plastic wastes were manually sorted, washed, dried, and processed solely for laboratory-scale pyrolysis experiments.

2.2. Sample Collection and Feed Preparation

The plastics were collected from the campus of Dhaka University and Old Dhaka (illustrated in Fig 1). The collected plastic waste was sorted into different categories such as PP, HDPE, PET and PVC before further processing. Mixed plastic waste was first manually sorted according to resin identification codes, product labels, physical characteristics, and visual appearance. To improve separation accuracy, density-based flotation techniques were applied where appropriate. Polyolefin plastics such as PP and HDPE, having densities lower than water (<1.0 g cm ⁻ ³), floated during water separation, whereas PET and PVC, possessing densities greater than water (>1.3 g cm ⁻ ³), sank. This density-based approach is widely used for preliminary segregation of mixed plastic waste and improves feedstock homogeneity prior to pyrolysis.

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Fig 1. Collected waste plastics (a) PET, (b) PP, (c) HDPE, and (d) PVC.

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2.3. Experimental setup

2.3.1. Pyrolysis plant.

A stainless-steel batch reactor, measuring 30 cm in both diameter and length, was employed for the pyrolysis process. This reactor was externally heated using an electrical system, equipped with an automatic temperature controller to ensure precise control of the heating elements. The reactor’s temperature was constantly monitored by a digital temperature controller. Integral components of the fixed-bed reactor include electrical heating elements to maintain the desired internal temperature, a water-cooled condensing coil that converts condensable vapor into pyrolytic liquid, and various other elements such as a temperature sensor, a heating coil, thermal insulation, a storage tank, a valve, and a gas exit line. Notably, the temperature sensor was strategically placed to gauge the temperature through the walls of the stainless-steel pyrolysis chamber.

The pyrolyzer is designed to operate within a temperature range of 400–600 °C, adjustable in 50 °C intervals. Thermocouple sensors, protruding inside the reactor chamber, facilitated continuous temperature monitoring. Each experimental run began by manually loading 100 grams of the sample into the reactor. Prior to initiating the experiment, nitrogen gas (N₂) was flowed through the reactor for two minutes to purge any residual air, ensuring a controlled and oxygen-free environment. The preheater was also initially filled with N₂ gas. Subsequently, the reactor heater was activated, gradually raising the temperature to the target of 400 °C, as indicated by the temperature display.

During the experiments, temperatures were meticulously recorded using the digital display. A steady supply of nitrogen gas was maintained to preserve the inert atmosphere within the reactor and to efficiently sweep the pyrolyzed vapor towards the condensers. This vapor was then condensed into pyrolytic liquid, collected in bottles positioned beneath the condensers. Any gases that did not condense were safely flared into the atmosphere. The collection bottles were removed once they were filled with liquid, ensuring they were free from any air bubbles. Upon completion of the decomposition process, a colorless gas was emitted, signaling that the thermal decomposition of the plastic sample was complete. The absence of color in the flare indicated thorough pyrolysis.

Once the pyrolysis was completed, the reactor heater was switched off, the vapor exit was sealed, and the nitrogen supply was discontinued. The char byproduct was then removed from the reactor chamber after it had cooled down and was subsequently weighed. The weight of uncondensed gases was calculated by subtracting the total weight of the feedstock from the combined weight of the liquid and char. Finally, the system was reset and prepared for the next run, ensuring a seamless continuation of the pyrolysis process.This meticulous setup and process underline the controlled environment necessary for efficient pyrolysis, aiming to maximize the yield and quality of the bio-oil and char, thereby contributing effectively to waste management and resource recovery.

Fig 2 illustrates the experimental pyrolysis setup used to convert waste plastics into bio-oil, biochar, and gaseous products. The setup features a stainless-steel batch reactor with a 30 cm diameter and length, equipped with electrical heating elements, a digital temperature controller, and a nitrogen gas purging system to maintain an oxygen-free environment. The pyrolysis vapors generated within the reactor are directed to a water-cooled condenser, where they are converted into liquid bio-oil. Non-condensable gases are safely flared, and the solid char is collected from the reactor after cooling. This setup ensures precise temperature regulation and efficient collection of pyrolysis products.

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Fig 2. Experimental setup of the pyrolysis plant, showing reactor, temperature control, nitrogen purging, and bio-oil condensation.

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2.4. Pyrolysis Process

Before the pyrolysis process commenced, the plastic chips were meticulously cleaned and dried. These chips were then heated outside the reactor in an oxygen-free environment to prevent combustion. During pyrolysis, the large molecules of the liquefied polymers were transformed into steam, rising out of the reactor at high temperatures. This vapor was then directed to a reactor condenser, specifically designed to cool and condense the vapor back into a liquid form. The process was maintained for 60 minutes within a temperature range of 400–550 °C, ensuring optimal conversion of the polymers. As the pyrolysis byproducts emerged as vapor, they were channeled into a water-cooled condenser where they condensed into a liquid. This liquid, still unrefined, was collected and could potentially be used as fuel. The system was designed to release any gases that did not condense into the atmosphere, minimizing pressure build-up within the apparatus. The operation of this process is illustrated in Fig 3, which provides a schematic overview of the waste plastics pyrolysis setup. The oil collected from this process was derived from the described pyrolysis plant, capturing the essence of transforming waste into valuable resources through controlled thermal degradation.

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Fig 3. Schematic of the proposed process flow diagram.

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2.5. Physiochemical and elemental analysis

The physicochemical and elemental analyses of the pyrolytic oil derived from waste plastics were conducted to comprehensively characterize its properties and potential as a biofuel. These analyses involved the determination of key properties such as density, viscosity index, flash point, pour point, and ash content. The tests were carried out using a variety of specialized instruments while adhering strictly to the protocols outlined by the Institute of Petroleum (IP) and the American Society for Testing and Materials (ASTM) standards. These standards ensure that the results are reliable and comparable to conventional fuels.

Density and viscosity measurements are critical for understanding the flow characteristics and energy content of the pyrolytic oil, influencing its suitability for various combustion applications. The flash point and pour point assessments provide insights into the safety and usability of the oil under different temperature conditions, particularly in cold climates. The ash content analysis is vital for evaluating the cleanliness of the fuel during combustion, with lower ash content indicating fewer residues and emissions. These analyses collectively provide a detailed understanding of the pyrolytic oil’s properties, allowing for a comparison with conventional fossil fuels. The results from these evaluations support the feasibility of using pyrolytic oil as a renewable energy source, contributing to the broader goal of sustainable waste management and energy production.

2.6. Characterization techniques

The compositional analysis of the plastic pyrolytic oil was carried out in the form of functional group and the characteristics of the molecule were measured from its absorbance spectrum. The functional group compositions of the PPO (plastic pyrolysis oil) were analyzed by Fourier transform infra-red spectroscopy (FTIR) to identify the basic compositional groups. The IR-spectra of the plastic oil was produced by a standard FTIR instrument (model SHIMADZU FTIR 4500) and an online pen plotter. Information about the composition and type of organic compounds of bio-oil and its distillate product was obtained by Gas Chromatography/Mass-Spectroscopy (GC/MS) analysis by GC-MS-QP 2010 Ultra using a capillary column (30 × 0.25 mm × 0.25 μm). The XRD patterns of the materials were examined using an X-ray diffractometer (Ultima IV, Rigaku Corporation, Akishima, Japan) fitted with Cu K radiation (= 0.154 nm). Scanning electron micrographs were acquired from JEOL-JSM 5600 LV microscope with a 6587 EDS (energy dispersive X-ray spectrometry) detector.

3.1. Product analysis

In the plastic pyrolysis system, three primary products are obtained: liquid pyrolytic oil, char, and non-condensable gases. Among these, pyrolytic oil is the main product of interest in the present study because of its potential application as a renewable fuel. Char and non-condensable gases may also be used as supplementary energy sources for process heating, thereby improving the overall energy efficiency of the pyrolysis system. Multiple experimental runs were conducted using prepared plastic waste samples under controlled pyrolysis conditions. The pyrolysis process involves the thermal decomposition of long-chain plastic polymers into smaller hydrocarbon compounds through complex cracking and free-radical reactions. The product distribution is strongly influenced by plastic type, temperature, reactor design, residence time, and heating conditions.

The overall decomposition pathway of plastic waste during pyrolysis can be represented as follows:

Decomposition of polymers:

Cracking of heavy hydrocarbons:

Expected gas-phase products:

These equations represent simplified pyrolysis pathways based on typical thermal cracking behavior of plastic polymers. In the present study, the total gaseous product yield was estimated by mass balance; however, individual gaseous species were not quantified experimentally.

The gas phase generated during plastic pyrolysis is expected to contain light gases such as H₂, CH₄, C₂H₆, C₂H₄, C₂H₂, CO, and CO₂, depending on the feedstock type and operating conditions. The pyrolytic oil contains a mixture of light to heavy hydrocarbons, and its composition varies according to the plastic material processed. Notably, HDPE and PP produced a substantial amount of liquid hydrocarbons within the C₆–C₁₆ range, making these oils potentially suitable for refining into fuel-range products. The char produced during pyrolysis is a carbon-rich solid residue and may have potential applications as a solid fuel or as a precursor for activated carbon after suitable post-treatment. Table 1 highlights the distinct product yields and characteristics obtained from the pyrolysis of different plastic materials. The optimized conditions show that PP and HDPE provide the highest yields of pyrolytic oil, supporting their potential use in renewable fuel production.

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Table 1. Pyrolysis product mass balance and characteristics of different plastic materials.

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The char yields from PP and HDPE were 5.21% and 6.72%, respectively, indicating that both polyolefin plastics generated relatively low solid residues compared with PET and PVC. However, the XRD patterns suggest that the structural and inorganic residue characteristics of these chars were not identical.

Hydrogen formation in the non-condensable gas fraction:

Hydrogen may be produced during plastic pyrolysis through secondary cracking, dehydrogenation, and free-radical reactions of hydrocarbon fragments at elevated temperatures. In the present study, H₂ is considered a possible component of the non-condensable gas fraction together with CH₄, C₂H₆, C₂H₄, C₂H₂, CO, and CO₂. However, the gas fraction was not collected for detailed compositional analysis using GC-TCD, GC-FID, or other gas analyzers. Therefore, the exact concentration or yield of H₂ could not be quantified in this work. The total gaseous product yield was estimated by mass balance, but individual gas components were not measured separately. Accordingly, H₂ production is discussed qualitatively rather than quantitatively. Future studies should include gas-phase analysis to determine the precise H₂ content and evaluate the possible use of pyrolysis gas as a hydrogen-containing energy stream.

3.2. Thermal cracking of PP and HDPE

In the thermal cracking of PP and HDPE, these polyolefin polymers serve as representative models in pyrolysis research to explore the impact of temperature on the development of various cracking products. The pyrolysis process was investigated in a nitrogen environment across a range of temperatures, including 250 °C, 300 °C, 350 °C, and 400 °C. The pyrolysis reaction occurs through three successive phases: initiation, propagation, and termination. During the initiation phase, large polymer molecules undergo thermal cleavage, resulting in the formation of free radicals. In the propagation phase, these free radicals and molecular species continue to break down into smaller radicals and molecules. Finally, in the termination phase, the unstable free radicals are converted into stable compounds.

The mechanism of the free radical route and random chain scission of thermal cracking involves several key steps. During the initiation phase, large polymer molecules undergo thermal cleavage, leading to the formation of free radicals. In the propagation phase, these free radicals react with the polymer chains, causing further breakdown into smaller molecules and additional free radicals. The termination phase involves the stabilization of these radicals into stable end products, which can include aliphatic hydrocarbons, olefins, and gases like hydrogen. The process can be described in detail as follows:

Initiation (Thermolysis):

Large polymer chains (e.g., PP, HDPE) are subjected to thermal energy, leading to the homolytic cleavage of carbon-carbon bonds, producing primary free radicals.

Propagation:

The primary free radicals react with the polymer chains, causing further breakdown. This reaction can proceed through hydrogen abstraction, where a radical abstracts a hydrogen atom from a neighboring polymer, generating a new radical and a stable molecule.

Alternatively, a radical may undergo β-scission, where it fragments to form a smaller radical and a stable molecule.

Termination:

The free radicals combine to form stable molecules. This can occur through several pathways, such as recombination or disproportionation, where two radicals combine to form a stable product.

This study reveals that the cracking of polypropylene (PP) and high-density polyethylene (HDPE) is strongly temperature-dependent, with distinct product distributions emerging at varying temperatures. As temperatures rise, there is a notable shift towards the production of lighter hydrocarbons and gaseous products, underscoring the critical role of temperature control in optimizing the yield of targeted products. By delving into free radical mechanisms and random chain scission pathways, the research provides valuable insights that enhance the efficiency and yield of the pyrolysis process, contributing significantly to plastic waste management and renewable energy generation. Building on existing research, this investigation introduces novel approaches to refining pyrolysis conditions specifically for polyolefin polymers. The study’s rigorous analysis and systematic methodology make its findings both authoritative and transformative, offering groundbreaking insights that are likely to drive future advancements in the field.

3.3. Effect of temperature on product yield

The study initially focused on temperatures of 400 °C and 550 °C, as these were identified as crucial for significant thermal degradation based on TG and DTG data. A response time of 60 minutes was selected to ensure complete thermal degradation at these temperatures. To optimize the liquid yield from the raw material conversion, subsequent tests varied the reaction time between 30 and 90 minutes. The research examined the influence of reactor temperature on pyrolysis production across a range of 400 °C to 550 °C. The choice of 400 °C as the starting temperature was driven by the observed degradation rates. However, at 400 °C for 60 minutes, raw material conversion was minimal, resulting in a solid residue of nearly 40% (39.86%). Since complete thermal degradation of all plastic materials occurred below 550 °C, this temperature was chosen as the final process temperature. Multiple TG and DTG analyses corroborate this temperature range by highlighting the peak thermal degradation rates of the polymers within this interval [19, 20].

Initial experiments were conducted at 400 °C and 550 °C due to the significant thermal deterioration velocities observed at these temperatures in TG and DTG data. A 60-minute response time was employed to achieve total thermal degradation at the indicated temperature. Further tests adjusted the reaction time between 30 and 90 minutes to maximize the liquid yield from the raw material conversion. The effects of reactor temperature on pyrolysis production were studied across temperature intervals of 400 °C to 550 °C. The increased deterioration rates observed at 400 °C justified its selection as the starting temperature. However, at 400 °C for 60 minutes, raw material conversion remained minimal, with a solid residue of 40% (39.86%). Complete thermal degradation of all plastic materials was achieved below 550 °C, validating this as the final process temperature. TG and DTG analyses of the polymers’ peak thermal breakdown rates further support this temperature interval [19, 20]. Fig 4 illustrates how reactor temperature affects the pyrolysis production of the samples studied, showing that even at 400°C, a significant amount of raw material remains unreacted, leading to the decision to not conduct studies at lower temperatures.

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Fig 4. Pyrolysis of PP and HDPE at different temperature.

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The rate of raw material conversion increased with temperature, with HDPE and PP producing the highest liquid phase (pyrolysis oil) yields at 500 °C and 450 °C, respectively. As the temperature increased, gaseous products increased while pyrolysis oils decreased due to higher temperatures favoring secondary reactions that cause chain-cracking and the formation of shorter molecules or non-condensable gases. This behavior is consistent with observations from other researchers. Ayhan Demirbas reported that the gaseous product yield of a plastic mixture (PP, PE, and PS) increased from 12.3% to 42.4% as the temperature rose from 550 to 900 K. The liquid product output increased up to 750 K (477°C) before declining significantly up to 900 K [21]. According to Achilias et al. [22], the pyrolysis of PP, LDPE, and HDPE in a fixed bed reactor at 450 °C for 17 minutes resulted in low conversion rates, even with a small sample size of 0.7 g, leaving substantial solid residues for LDPE (76.4%), HDPE (76.7%), and PP (46.6%) [22]. However, a combination analysis of PP and HDPE at the same temperature with a different reaction time of 45 minutes showed less residue (Fig 4), suggesting that reaction time significantly influences conversion. Saad & Williams [23] found that 80% of the products were condensable when synthetic and waste PP were pyrolyzed in a fixed bed reactor at 500 °C using a 20 g sample [23]. On the other hand, Adrados et al. [24] and López et al. [25] observed complete conversion at 460°C from a mixture of pure synthetic polymers (PP 40%, PE 35%, PS 18%, PVC 3%, and PET 4%) in a pyrolysis reactor [24, 25]. For superior quality products, a temperature of 500°C was recommended, yielding 65.2% liquid product. The optimal reactor temperatures for maximal liquid product yield in the tested laboratory pyrolysis setup with a fixed bed and typical heating of the plastic material combination were 500°C for HDPE and 450 °C for PP within 45 minutes.

3.4. Temperature distribution of plastic pyrolysisplant

The temperature distribution within a pyrolysis plant plays a crucial role in determining the efficiency and yield of the process. Initially, the reactor temperature is maintained between 20–25°C, which is the ambient temperature. As the reactor is heated, temperature readings are recorded at five-minute intervals. The heating process is gradual, with the reactor temperature steadily increasing during the first 30–40 minutes of operation. Following this, the reactor is maintained at a constant temperature of approximately 450 °C (±10 °C) and 500 °C (±10 °C) for the next 30–40 minutes. This consistent temperature distribution is critical to ensuring uniform pyrolysis and optimal product yield. The recorded temperature data, as depicted in Fig 5, shows that the reactor temperature stabilizes at around 450 °C and 500 °C, which are the key operational temperatures for the pyrolysis process. These temperature ranges are selected based on their effectiveness in breaking down the plastic feedstock into pyrolytic oil, gas, and char, thus maximizing the production of valuable byproducts.

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Fig 5. Temperature distribution in reactor with time.

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3.5. Effect of time on product yield

The study focused on evaluating the impact of reaction time on product yield during the plastic pyrolysis process, with a fixed temperature of 500 °C. This temperature was selected based on recommendations from previous studies by Adrados et al. and López et al., which emphasized the significance of 500 °C for investigating the influence of reaction time on plastic pyrolysis [24, 25]. The experiments varied the reaction time from 30 to 90 minutes, utilizing a nitrogen flow rate of 50 ml/min and a heating rate of 20 °C/min. The results, illustrated in Fig 6, demonstrate that a reaction time of 45 minutes achieved the maximum conversion of plastic into gaseous and liquid-condensable compounds. During the initial 45 minutes, the production of pyrolytic oil increased significantly, reaching a peak yield. However, extending the reaction time to 60 and 90 minutes resulted in a rapid decline in oil production. The highest yields of condensable products, 47.23% from HDPE and 61.30% from PP, were obtained at 45 minutes at 500 °C. Consequently, a reaction time of 45 minutes at this temperature is deemed sufficient for optimal conversion of raw materials in this particular reactor setup.

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Fig 6. Pyrolysis of PP and HDPE at different time.

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PP and HDPE were employed as representative polyolefin polymers in the pyrolysis study to explore the effects of temperature on the development of various cracking products. The pyrolysis process was investigated at temperatures of 250 °C, 300 °C, 350 °C, and 400 °C in a non-sweeping nitrogen atmosphere. The pyrolysis reaction occurs through three distinct phases: initiation, propagation, and termination. During the initiation phase, large polymer molecules undergo thermal cleavage, resulting in the formation of free radicals. In the propagation phase, these free radicals, along with molecular species, continue to break down into smaller radicals and molecules. Due to their instability, the free radicals undergo termination reactions, leading to their incorporation into stable molecules. The mechanism of the free radical pathway and random chain scission involved in thermal cracking is illustrated earlier in the manuscript.

Previous studies have cited a reaction time of 30 minutes for the pyrolysis of a mixture containing PP, PE, PS, PVC, and PET in a 3.5 L fixed bed reactor, with typical plastic mixture heating through the reactor wall [25]. In other laboratory reactors, such as the Parr Mini Bench Top Reactor (Type 4561 m), an autoclave with a volume of 300 ml and a mixer operating under 19.5 MPa pressure was used to process a waste plastic mixture (HDPE, PP, PS, PET, and PVC) at 500 °C for 60 minutes, although the effect of reaction time was not specifically examined [26]. The study highlights the differences between laboratory micro-reactors and pilot reactors, particularly regarding heat transfer mechanisms. Pilot reactors, while capable of processing small samples, have more restricted heat transfer mechanisms compared to laboratory micro-reactors. This limitation affects the efficiency of raw material conversion. Further studies are necessary, especially in pilot reactors, to better understand these heat transfer dynamics and optimize the pyrolysis process at a larger scale.

3.6. Fractional distillation of crude pyrolysis oil

The fractional distillation of crude pyrolysis oil was carried out using a distillation column, effectively separating the pyrolytic liquid into distinct components based on their boiling points. The pyrolytic liquid was fractionated into five main categories: 30–80 °C, 81–140 °C, 141–180 °C, 181–250 °C, and a non-fractionated residual oil. Multiple successful runs were conducted using raw pyrolysis liquid samples to ensure an adequate yield of each fractionated oil component. The product distributions obtained through this method are depicted in Fig 7, demonstrating the efficiency of the fractional distillation process in refining the crude pyrolysis oil. This separation technique is crucial for enhancing the quality and usability of the derived products, aligning them more closely with conventional fuels. This approach not only refines the crude oil but also enables the isolation of valuable fractions that can be further processed or used directly, thus contributing to the overall efficiency and sustainability of the pyrolysis process.

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Fig 7. Crude Pyrolysis oil and fractionated liquid products from pyrolysis crude oil.

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3.7. Fuel Properties of plastic derived pyrolysis

The fuel properties of plastic-derived pyrolysis oil can vary widely due to the complex composition of the oil, which includes a mixture of different hydrocarbons. Key parameters such as density, viscosity, calorific value, flash point, pour point, and ash content are critical in determining the suitability of pyrolysis oil for various applications. While minor variations in these properties may have little impact on some uses, they can pose significant challenges when using pyrolysis oil in devices designed for conventional hydrocarbon fuels. These challenges may make the operation of such devices difficult or, in some cases, entirely unfeasible. As a result, it is essential to thoroughly characterize any fuel oil intended for use in engines and compare its properties with those of conventional diesel and other standard oils. Table 2 presents a comparative study of the properties of pyrolysis oil alongside those of conventional oils, highlighting the differences and similarities that influence its potential applications.

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Table 2. Comparative study of pyrolysis oil obtained from PP and HDPE with other conventional oil.

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The densities of the liquid oils derived from PP and HDPE were measured at 0.81 g/cm³ and 0.84 g/cm³, respectively, aligning closely with the density range of diesel hydrocarbons (C12-C16). The viscosity of a fluid, whether Newtonian or not, reflects its resistance to shearing forces during flow and its ability to flow under gravity, which are crucial for the functionality of fuel injection systems. This characteristic also affects the fuel’s combustibility and atomization quality. The kinematic viscosity of the oil derived from PP was 4.23 mm²/s, while the HDPE liquid exhibited a viscosity of 5.48 mm²/s, both within the range required for high-quality gasoline and diesel fuels [27]. The flash point, an important safety parameter that defines the lowest temperature at which the vapors above a liquid ignite when exposed to an external flame, was recorded at 32 °C for the PP-derived liquid and 50 °C for the HDPE-derived liquid. These values are comparable to those of light petroleum distillate fuels. The pour point, which represents the lowest temperature at which a liquid ceases to flow and thereby indicates the fuel’s fluidity at low temperatures, was found to be –10 °C for the PP-derived liquid and –4°C for the HDPE-derived liquid. Although pour points for light distillate petroleum fractions vary, the pour point of naphtha-range fractions generally occurs around –6°C. The calorific values of the pyrolysis oils from PP and HDPE were determined to be 25 MJ/kg and 26.5 MJ/kg, respectively. While these values are slightly lower than those of conventional diesel fuel, they highlight the potential for using plastic-derived oils as a viable fuel source. Lastly, the ash content of each pyrolysate was negligible for both PP and HDPE-derived liquids, indicating the absence of metal contamination and high molecular weight soot, which further underscores the purity and quality of the liquid fractions.

3.8. Compositional Analysis

3.8.1. FTIR.

The FTIR spectra of raw plastic materials including PP, pyrolysis oil from PP, and HDPE, as well as pyrolysis oil from PP and HDPE, are presented in Fig 8. The functional group and composition analysis of the pyrolytic oil at different wave numbers per cm are detailed in S1–S4 Table (supporting information). The FTIR analysis revealed that the pyrolysis oil contains a variety of functional groups, predominantly alkanes, aromatics, alkenes, ketones, alcohols, and aldehydes. Specific absorbance peaks were observed: C-H stretching between 3000–2850 cm ⁻ ¹ and C-H bending between 1000–650 cm ⁻ ¹, indicating the presence of alkanes; C = C stretching between 1035–845 cm ⁻ ¹ and CH₂ bending at 725.23 cm ⁻ ¹, further confirming the presence of alkanes. Additionally, alcohol groups were identified through O-H stretching between 3200–3400 cm ⁻ ¹ and C-O stretches between 1250–1300 cm ⁻ ¹. Aldehyde groups were also detected, with an absorbance peak for C = O stretching between 1850–1650 cm ⁻ ¹. The functional groups identified in the pyrolytic oil are quite similar to those found in diesel oil, as reported in other studies. The presence of hydrocarbon groups such as C-H, C = C, and alcohols suggests that the pyrolysis liquid has significant potential for use as a fuel.

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Fig 8. FT-IR spectra of, (a) raw PP, (b) PP pyrolysis oil, (c) Raw HDPE and (d) HDPE pyrolysis oil.

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3.8.2. GC-MS.

GC-MS analysis was employed to determine the distribution of hydrocarbons with varying carbon chain lengths in the liquid products derived from PP and HDPE. The hydrocarbon distribution in the PP-derived liquid fraction, as shown in Table 3, was found to be 14.66%, 34.92%, 12.15%, 25.39%, and 7.60% across different carbon chain ranges. The significant proportions of hydrocarbons in the C13 to C16 (34.92%) and C17 to C20 (25.39%) ranges suggest that the liquid product is diesel-enriched. Nearly 50% of the product comprised light naphtha hydrocarbons (C6-C16).

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Table 3. Products distribution in liquid fractions derived from PP and HDPE.

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For the HDPE-derived liquid fraction, the hydrocarbon distribution was 33.45%, 31.28%, 15.64%, 11.93%, and 11.24% across the C6-C12, C13-C16, C17-C20, C20-C30, and >C30 ranges, respectively. This distribution indicates that the fraction is enriched with low boiling naphtha hydrocarbons. Diesel and gasoline hydrocarbons accounted for 63% of the total product, while C17 to C30 hydrocarbons constituted 34% of the liquid component. S5–S6 Tables (supporting information), and Fig 9 illustrate the relative amounts of olefins, cycloalkanes, aliphatic, and aromatic components in the liquid products derived from PP and HDPE. The PP-derived liquid products contained 25% olefins, 7% cycloalkanes (naphthenes), and 66% paraffins, whereas the HDPE-derived liquid products consisted of 31% olefins, 8% cycloalkanes, and 59% paraffins.

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Fig 9. Hydrocarbon group type distribution in liquid fractions derived from PP and HDPE.

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Notably, PP produced a higher proportion of paraffinic compounds compared to HDPE, which resulted in a greater yield of olefins and naphthenes. Since paraffinic hydrocarbons possess higher energy content than aromatic or naphthenic hydrocarbons, the PP-derived liquid holds more value as a fuel source compared to the HDPE-derived liquid.

3.9. Analysis of char from plastic pyrolysis

3.9.1. X-Ray diffraction analysis.

X-ray diffraction analysis was conducted to evaluate the crystalline and short-range ordered structures of the char samples obtained from PP and HDPE pyrolysis. As shown in Fig 10, both char samples exhibited a broad diffraction region between approximately 20° and 30° 2θ. This broad band, centered around 21°–24° 2θ, can be assigned to the (002) reflection of disordered carbon, indicating the presence of amorphous or turbostratic carbon structures rather than well-developed graphitic carbon. In addition to this region, a second weak and broad diffraction feature was observed between approximately 40° and 45° 2θ, which may be associated with the (100)/(101) planes of carbonaceous materials. These two broad diffraction regions are commonly associated with poorly ordered carbon structures formed during thermal decomposition and indicate that the produced chars contain partially ordered carbon domains with limited graphitic stacking.

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Fig 10. XRD of bio char from PP and HDPE.

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Several minor sharper peaks were also observed in the XRD patterns, particularly in the PP-derived char. These reflections may be related to residual inorganic components, ash-forming species, additives, fillers, pigments, or impurities originally present in the post-consumer plastic waste. Therefore, although the XRD patterns confirm the carbonaceous nature of the produced chars, they also indicate that the chars are not completely pure, fully crystalline, or fully graphitized. The presence of minor crystalline reflections suggests that further purification or activation, such as washing, acid treatment, or controlled thermal activation, may be required if the char is intended for high-purity activated carbon or advanced adsorbent applications.

A comparative assessment of the PP- and HDPE-derived char patterns shows that both chars are mainly carbonaceous and poorly ordered, but they differ in the intensity and sharpness of their diffraction features. The HDPE-derived char is characterized mainly by broad diffraction features in the 20°–30° and 40°–45° 2θ regions, suggesting a predominantly amorphous or turbostratic carbon structure with limited graphitic ordering. In contrast, the PP-derived char shows more pronounced sharper reflections superimposed on the carbon background, particularly in the higher 2θ region. These sharper peaks may indicate the presence of relatively higher crystalline inorganic residues, ash-forming species, fillers, pigments, or additives retained from the original plastic waste. Therefore, the difference between PP- and HDPE-derived chars is not only related to carbon structural ordering but may also be influenced by residual inorganic components associated with the feedstock. Since XRD alone cannot confirm the exact chemical identity of these minor crystalline phases, further analysis such as EDS, XRF, ICP-OES, or ash analysis would be required for precise identification. Overall, HDPE-derived char appears comparatively more amorphous, whereas PP-derived char may contain more detectable crystalline residues and may require further purification for high-value activated carbon applications.

3.9.2. Scanning electron microscopic (SEM) studies.

The surface morphology of char produced from the pyrolysis of polypropylene (PP) and high-density polyethylene (HDPE) was investigated using scanning electron microscopy (SEM), as shown in Fig 11. The SEM micrographs reveal distinct morphological characteristics for the chars derived from the two polymers, reflecting differences in their thermal degradation behavior and char formation mechanisms.

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Fig 11. SEM micrographs of char obtained from the pyrolysis of PP and HDPE.

(a) PP-derived char showing agglomerated particles (b) PP-derived char exhibiting porous regions, agglomerated particles, and fiber-like structures; (c) PP-derived char displaying large flake-like particles, (d) HDPE-derived char showing layered structures and sharp edges (e) HDPE-derived char illustrating banding features and (f) HDPE-derived char characterized by brittle flakes, porous structures.

https://doi.org/10.1371/journal.pone.0354825.g011

The PP-derived char (Fig 11a–c) exhibits a heterogeneous carbonaceous structure composed of agglomerated particles, flake-like structures, porous regions, and fiber-like features. At lower magnification (Fig 11a), clusters of agglomerated particles are distributed throughout a matrix of irregular flakes, indicating the formation of a porous carbon framework during pyrolysis. At higher magnification (Fig 11b), interconnected porous regions and fiber-like structures become more evident, suggesting the release of volatile compounds and the development of internal pore networks during thermal decomposition. The micrograph shown in Fig 11c further reveals large flake-like particles, wrinkled thin layers, and sharp-edged fragments. These features indicate localized shrinkage, polymer chain scission, and structural rearrangement during carbonization. The well-developed porous morphology observed in the PP-derived char is advantageous for the impregnation of activating agents and may facilitate its conversion into activated carbon with a high surface area. Such characteristics are desirable for applications in adsorption, water purification, gas treatment, and catalyst support materials.

In contrast, the HDPE-derived char (Fig 11d–f) exhibits a more fragmented and layered morphology. As shown in Fig 11d, the surface is characterized by layered structures with prominent sharp edges, indicating brittle fracture behavior during pyrolysis. At higher magnification (Fig 11e), distinct banding features, sharp ridges, and undulating surface textures are observed, suggesting anisotropic carbon formation and localized thermal stress effects. The micrograph in Fig 11f reveals brittle flakes, porous structures, and sharp-edged fragments, indicating extensive fragmentation and pore development during decomposition. The porous nature of the HDPE-derived char is also favorable for activation processes; however, its morphology appears more irregular and fractured than that of the PP-derived char.

The observed morphological differences between PP- and HDPE-derived chars can be attributed to variations in polymer structure, crystallinity, decomposition kinetics, and volatilization behavior during pyrolysis. While both chars possess porous structures suitable for activation, the PP-derived char exhibits a comparatively more interconnected porous framework, whereas the HDPE-derived char demonstrates greater fragmentation and layered carbon formation. These characteristics suggest that both materials have potential for value-added applications such as activated carbon production, adsorbents, catalyst supports, and carbon-based composite materials. Further optimization of pyrolysis conditions and activation processes may enhance their surface properties and expand their industrial applicability.

3.10. Preliminary economic consideration of char production

A preliminary char-yield estimation was carried out to evaluate the possible resource-recovery potential of the solid residue obtained from plastic pyrolysis. Based on the char yields obtained in this study, the amount of char produced from 1 ton of plastic waste would vary depending on the plastic type. For 1 ton of PET, PVC, HDPE, and PP waste, the estimated char production would be approximately 188.5 kg, 343.6 kg, 67.2 kg, and 52.1 kg, respectively. Based on the experimental char yields obtained in this study, a preliminary mass-balance estimation was conducted to calculate the amount of char that could be produced from 1 metric tonne of different plastic wastes. The estimated char production would be approximately 188.5 kg from PET, 343.6 kg from PVC, 67.2 kg from HDPE, and 52.1 kg from PP, while an equal-mass mixed plastic feedstock would produce approximately 162.9 kg of char per tonne (S7 Table in supporting information). Therefore, PVC and PET generated higher char yields, whereas PP and HDPE produced lower char yields but higher liquid-oil fractions.

From an economic perspective, the char can contribute to process feasibility in two ways. First, it can be used as a supplementary solid fuel or heat source, thereby reducing external energy demand during pyrolysis. Second, after appropriate purification and activation, the carbon-rich char may be upgraded into activated carbon or adsorbent material for environmental applications such as wastewater treatment, gas adsorption, or pollutant removal. However, the XRD results indicate that the char may contain residual inorganic matter, ash-forming components, additives, or impurities, especially in PP-derived char. Therefore, additional post-treatment steps such as washing, acid treatment, or thermal/chemical activation may be required before high-value applications.

The preliminary economic feasibility of char utilization depends on the plastic feedstock, char yield, purity, activation cost, and local market demand. For example, if 1 ton of mixed plastic waste containing equal proportions of PET, PVC, HDPE, and PP is considered, the average char yield would be approximately 16.29%, equivalent to about 162.9 kg of char per ton of mixed plastic waste. This indicates that char recovery can provide an additional value stream along with pyrolysis oil and gas, although the main economic product in the present study remains the liquid fuel fraction, particularly from PP and HDPE. Further techno-economic analysis, including energy balance, activation cost, purification cost, and market evaluation, is recommended for large-scale feasibility assessment.