The Influence of Heterogeneity of Polyolefin Waste and Alu-PEX Laminates on the Composition and Yield of Pyrolysis Gas: A Comparative Analysis with RDF

Abstract

The composition and type of polymers used as feedstocks in the pyrolysis of plastic waste determine the decomposition process and the proportions of the final products. This paper examines the effect of feedstock heterogeneity on pyrolysis efficiency and pyrolysis gas composition. Four types of plastic waste were considered: real polyolefin waste of municipal origin, LDPE, Alu-PEX laminates, and an alternative refuse-derived fuel (RDF). Low-temperature pyrolysis (450 °C) was conducted in a laboratory reactor, and the gas composition was analyzed using GC-TCD/FID gas chromatography, which allowed for the determination of light hydrocarbons, oxygenates, and sulfur content. Compared to RDF, both municipal and LDPE polyolefin wastes produced gas with a higher calorific value and a predominance of light C₁–C₄ hydrocarbons, while Alu-PEX laminates produced gas rich in C₁–C₂ and low in sulfur, suitable for direct use. RDF was characterized by increased CO₂ and non-flammable gas production and significantly higher sulfur content, requiring advanced purification. The results emphasize the importance of feedstock segregation and standardization and demonstrate that pyrolysis of polyolefins and Alu-PEX laminates can provide higher-quality energy gas than RDF, supporting the circular economy and energy self-sufficiency of industrial installations.

1. Introduction

The average annual increase in plastics production is approximately 9% [1]. The last ten years have seen a significant increase in demand for plastics, which in 2022 reached a record level of 400.3 million Mg, representing the highest volume of global plastic production ever recorded [2]. Global annual municipal waste production currently amounts to approximately 2.01 billion Mg, of which plastics account for nearly 12% of this mass [3]. Their low production costs and very good physicochemical properties, such as low density, high mechanical strength, and resistance to atmospheric factors, have led to these materials being used in almost every sector of the economy—in construction, the automotive industry, packaging, the electrical and electronics sector, and many others [4].

Polymer synthesis is primarily based on natural gas and lower gaseous fractions obtained during the crude oil refining process. The production of single-use plastics consumes approximately 3.5 million barrels of oil per day, accounting for nearly 4% of global oil demand [5]. Currently, the plastics market comprises approximately 30 basic types of polymers, which, when combined with a wide range of additives that modify polymer properties, enable the production of products with a wide range of properties [6].

Plastic waste (PW) is highly resistant to environmental factors, so its complete decomposition can take from several dozen to even several hundred years, depending on the polymer type and environmental conditions [7]. Given the rapidly increasing environmental pollution with PW and the growing requirements related to the implementation of circular economy principles, technologies enabling its effective processing and recovery of valuable raw materials are particularly important. To date, the most common method for managing PW has been energy recovery through incineration. However, this process does not solve the problem of environmental pollution, because it involves the emission of toxic and harmful substances into the atmosphere [8]. Another method is mechanical recycling, which is only effective for homogeneous plastics. It is worth emphasizing that PW is characterized by a wide variety of types and significant heterogeneity in the composition of the waste stream. When disposing of this type of waste, thermochemical recycling is necessary, which allows for the recovery of valuable raw materials from PW, such as gas and pyrolysis oil. It is estimated that this method can transform up to 50% of global PW into valuable products, in line with the assumptions of the circular economy [9]. This means the potential utilization of over 150 million Mg of polymers per year, without the need for landfilling or incineration.

One of the key thermochemical recycling processes for polymer waste is pyrolysis, which enables its transformation into valuable gaseous and liquid products. Unlike homogeneous polymers such as LDPE, municipal polymer waste is characterized by heterogeneity, diverse composition, and the presence of additives and impurities, which can significantly affect the pyrolysis process and the quality of the resulting products [10]. In practice, this waste is a mixture of various polyolefins (LDPE, HDPE, PP) with added dyes, fillers, UV stabilizers, pigments, flame retardants, impact modifiers, and mechanical impurities [11]. Flame retardants, for example, limit fire development through physical and chemical mechanisms [12]. External and internal plasticizers are used in plastics to increase the materials’ flexibility and processability. External plasticizers are not permanently chemically bonded to the polymer structure and may migrate into the environment during recycling [13]. Studies in Ref. [14] have shown that the new polyphosphazene releases toxic NH₃ and SO₂ gases in PET recycling, which significantly affects the thermochemical degradation mechanisms and the quality of the final products [15].

The pyrolysis process breaks down long polymer chains into smaller, less complex molecules under anaerobic conditions and at elevated temperatures. This process produces three main product fractions: gaseous, liquid, and solid (char) [15,16]. Unlike the partial oxidation of plastics, which produces syngas, pyrolysis is conducted under strictly controlled conditions, without oxygen, allowing for the production of a gaseous hydrocarbon mixture with significant calorific value. This process requires high temperatures and a sufficient residence time of the material in the reactor [17]. The pyrolysis process can also be carried out with the use of catalysts, which modify the course of chemical reactions, enabling the process to be carried out at lower temperatures [18]. The use of catalysts in the pyrolysis process also enables the selective production of gaseous and liquid fractions with precisely defined chemical compositions. Apart from temperature and catalysts presence, the course of the pyrolysis process is largely affected by heating rate and residence time of the material. Based on this, slow (conventional) and fast pyrolysis are distinguished [19]. Typically, the heating rate is in the range of 0.2–5.0 °C/s for slow pyrolysis [20], whereas from 10 to 200 °C/s for fast pyrolysis [21].

Studies conducted so far on obtaining pyrolysis gas from PW have shown significant differences in its efficiency. These differences result from many factors, e.g., different pyrolysis process conditions (temperature, raw material residence time in the reactor), type of raw material and type of catalysts used in the process, their chemical composition, and structural properties [22]. Furthermore, the configuration of the pyrolysis process is important, as it can be carried out in a single- or two-stage system. Marchetti et al. [23] demonstrated that in the two-stage pyrolysis process, the gas fraction yield was in the range of 61–65 wt%, depending on the raw material residence time in the reactor, which ranged from 2 to 4 s. Park et al. [24] conducted studies on the pyrolysis of polyethylene (PE), which were conducted in a two-stage apparatus consisting of a screw reactor and a fluidized bed reactor. The studies have shown that in a screw reactor, it is possible to obtain a gas yield of about 75 wt% at a temperature of 300 °C. In the case of using a fluidized bed reactor, a similar yield was achieved only at a much higher process temperature of 736 °C. Aminu et al. [25] confirmed the effectiveness of a two-stage configuration by combining pyrolysis at 500 °C with non-thermal plasma reforming at 250 °C and in the presence of a Ni/MCM-41 catalyst. In both cases, it was shown that appropriate process configuration and catalyst selection significantly increase the yield and quality of the pyrolysis gas. In turn, Zhang et al. [26] showed that in the two-stage pyrolysis and catalytic steam reforming process, the highest gas fraction yield of 95 wt% was obtained for a 1:1 Fe/Ni catalyst. According to the study by Singh et al. [27], conducting the process under fast pyrolysis conditions and elevated temperature favors obtaining high yields of pyrolysis gases while limiting the amount of formed char. Slow pyrolysis, on the other hand, leads to an increase in char share in the products [28]. Serrano et al. [29] carried out the pyrolysis process of municipal waste in a tubular reactor at a temperature of 520 °C, obtaining an average gas yield of 16.5 ± 4.6 wt%. Kordoghli et al. [30] carried out the pyrolysis of polyethylene at a temperature of 500 °C in the presence of an Al₂O₃ catalyst, obtaining a gas fraction with an efficiency of about 31 wt%.

Analysis of raw material heterogeneity, including blends of various polyolefins and aluminum–polymer layers, represents a significant research gap. Previous studies have focused primarily on the efficiency of metal fraction separation and the assessment of aluminum recovery [31,32]. The influence of the heterogeneity of municipal polyolefin waste and Alu-PEX laminates on the composition and properties of pyrolysis gas remains poorly understood, as confirmed by the analysis of the available literature. Issues related to the balancing of pyrolysis products and the detailed characterization of the gas fraction, including chemical composition, efficiency, and calorific value, are still insufficiently investigated. Consequently, there is a lack of comprehensive data enabling the assessment of the efficiency of the pyrolysis process not only in terms of materials, but also in terms of energy and the environment. This constitutes a significant research gap requiring further analysis, especially since pyrolysis gases can be used as a heat source in industrial installations, in gas turbines for electricity generation, and for direct combustion in boilers without the need for flue gas cleaning [33].

The aim of this study is to assess the impact of heterogeneity of actual municipal polyolefin waste (MPW) and multilayer Alu-PEX laminates on the chemical composition and energy properties of the obtained pyrolysis gas. The properties of the obtained gas were compared with those for gas obtained in the pyrolysis process from reference materials, i.e., from low-density polyethylene (LDPE) and from the overflow fraction of alternative fuel (RDF) separated from the combustible fraction of municipal waste. Comparative analysis of pyrolysis gases obtained from various fractions of polymer waste enables the informed design of feedstock streams targeted at specific energy or chemical applications. An additional goal of the research is to determine the relationship between key parameters of the pyrolysis process and the characteristics of the resulting products, with particular emphasis on the composition of the pyrolysis gas and its calorific value. The obtained results can provide a basis for developing strategies for the effective management of pyrolysis gas, both in terms of its energy use and as a potential chemical feedstock in further processing.

2. Materials and Methods

2.1. Materials

Four types of polymer waste were used in this study:

• mixed municipal plastic waste (MPW), containing polyolefin waste from the packaging fraction after waste sorting process—polyethylene (PE) (low-density polyethylene (LDPE) and high-density polyethylene (HDPE)) and polypropylene (PP), coded in Europe as 15 01 02 (Figure 1a);
• RDF (Refuse-Derived Fuel)—a coarse fraction of an alternative fuel (code 19 12 10) and the following morphological composition: plastics (approx. 45 wt%), paper and cardboard (approx. 20 wt%), biomass and wood (approx. 25 wt%), textiles and rubber (approx. 5 wt%) and other flammable components and impurities (<5 wt%) (Figure 1b);
• low-density polyethylene (LDPE) isolated as a homogeneous fraction from municipal waste—a polymer with a wide range of packaging applications (Figure 1c);
• cross-linked polyethylene with an aluminum layer (Alu-PEX), a post-industrial, multi-material waste with a high degree of structural complexity (Figure 1d).

Municipal polymer waste, as well as other waste from sorting processes, was obtained from a municipal services company located in the European Union. The collected polymer waste samples were subjected to preliminary preparation. First, the waste was cleaned of surface and residual contaminants by rinsing in distilled water. The material was then dried at 105 °C for 1 h to reduce residual moisture content. The samples were then ground in a two-shaft shredder, obtaining a fraction in the form of flakes with dimensions ranging from 0.5 to 1.0 cm.

2.2. Methods

2.2.1. Analyses of the Physicochemical Properties of Waste and Pyrolysis Gas

Analyses of the physicochemical properties of the waste and the resulting pyrolysis gas were conducted in accordance with applicable standards as follows:

• True density (ρ) was determined in accordance with [34], using a liquid pycnometer placed in a thermostat for 30 min with a 2 g sample;
• Bulk density (ρ_n) was determined in accordance with [35], as the ratio of the sample mass to the volume it occupied in a 5 dm³ measuring container;
• Total moisture content (W^a) was determined in accordance with [36], by recording the sample’s mass loss during heating at 105 ± 2 °C for 60 min until a constant mass was reached;
• Volatile matter content (VM^daf) was determined according to [37] by calcining the sample in a porcelain crucible at 850 °C in the absence of air for 7 min;
• Ash content (Ash^d) was determined according to [38] by calcining the sample in an FCF22S muffle furnace (CZYLOK, Jastrzębie-Zdrój, Poland), heated to 600 °C for 30 min;
• Elemental analysis (C^d, H^d, N^d) was performed according to [39], using a CHN 828 analyzer (Leco, St. Joseph, MI, USA); nitrogen content was determined using a thermal conductivity (TC) detector (Leco, St. Joseph, MI, USA);
• Total sulfur content (S^d) was determined according to [40]; sulfur was determined in an oxygen atmosphere using high-temperature combustion with IR detection on a CS 580 analyzer, Eltra, Bydgoszcz, Poland;
• Total chlorine content (Cl^d) was determined according to [40]; the determination involved high-temperature combustion of the sample in a HF-210 horizontal furnace (Mitsubishi Chemical Analytech, Tokyo, Japan) coupled to a Dionex Aquion ion chromatograph (Thermo Scientific, Waltham, MA, USA);
• The gross calorific value ($Q_s$, q_v,gr, HHV—Higher Heating Value) and net calorific value (W_u, q_p,net, LHV—Lower Heating Value) were determined in accordance with [41] using a Parr 6400 Calorimeter (Parr Instrument Company, Moline, IL, USA). The gross calorific value was measured under constant volume conditions and at a temperature of 25 °C. The calorimeter was calibrated using certified benzoic acid. The net calorific value was determined based on the gross calorific value, taking into account the heat of vaporization of water formed during combustion.

Based on the gas’s molar composition, its physicochemical properties were calculated in accordance with [42]. The calculations were performed under assumed reference conditions: a temperature of 0 °C and a pressure of 101.325 kPa for the gas volume and 25 °C and 101.325 kPa for the combustion process. Net calorific value (NCV) calculations were based on the actual (measured) gas composition, taking into account the presence of nitrogen and other inert components. The results were not converted to a nitrogen-free composition.

• The average molar mass ($M$), density (ρ), and relative density of the gas ($d$) were calculated as the sum of the products of the component molar fractions and their molar masses. Based on this, the relative density was determined as the ratio of the mixture’s molar mass to the molar mass of air. The gas density was calculated under reference conditions in accordance with the standard.
• The gross calorific value ($Q_s$) of the gas mixture was determined as the sum of the products of the component molar fractions and their molar heats of combustion:

  $$Q_s=\sum x_j·Q_{s,j},$$

  (1)

  where $Q_s$—gross calorific value of the gas mixture,${ Q}_{s,j}$—molar gross calorific value of the component j, and $x_j$—molar fraction of the component j. The reference heats of combustion of the individual components, as specified in the standard, were used for the calculations.
• The net calorific value ($Q_i$) of the gas mixture was determined as the sum of the molar fractions of the individual components and their molar net calorific values. This value does not take into account the heat of condensation of water vapor generated during the combustion process:

  $$Q_i=\sum x_j·Q_{i,j}, Q_i= Q_s-r_{{\mathrm{H}}_2\mathrm{O}},$$

  (2)

  where $Q_i$—net calorific value of the gas mixture, $Q_{i,j}$—molar calorific value of the component $j$, $x_j$—molar fraction of the component, and $r_{{\mathrm{H}}_2\mathrm{O}}$—energy term related to water vapor condensation.
• The gross Wobbe index ($W_s$) was determined based on the gross calorific value of the gas and the relative density to air, according to the relationship used in the analysis of gaseous fuels. This parameter determines the energy capacity of the gas supplied to the burner at constant pressure and is commonly used to assess the interchangeability of gaseous fuels.

  $$W_s={\displaystyle \frac{Q_s}{\sqrt{d}}}$$

  (3)

  where $W_s$—gross Wobbe index, $Q_s$—gross calorific value, and $d$—relative density.
• The compressibility factor ($Z$) was determined based on the gas molar composition and reference conditions. This parameter accounts for the deviation of the real gas from the behavior of an ideal gas.

The physicochemical analyses were performed in triplicate, and the results are presented as the arithmetic mean of the obtained values. Standard deviation was used to assess uncertainty.

2.2.2. Low-Temperature Pyrolysis

Low-temperature pyrolysis was conducted in a laboratory batch reactor with a tubular structure, an internal diameter of 450 mm, and a maximum batch capacity of 10 kg. The entire system was designed to enable both stable process operation under batch conditions and safe sampling of gaseous and liquid samples for further analysis. A schematic diagram of the reactor used in the pyrolysis experiments is shown in Figure 2.

The pyrolysis reactor was equipped with an insulating jacket made of high-temperature ceramic wool, enabling the process to be conducted in thermally stable conditions (up to 700 °C) and minimizing heat losses. Flanged connections with expanded vermiculite-based seals were used to ensure system tightness. This solution effectively prevented atmospheric air from entering the reaction zone, enabling the process to be conducted in strictly controlled, anaerobic conditions.

The reactor interior was equipped with a mechanical stirrer, whose purpose was to homogenize the charge and ensure uniform temperature distribution throughout the entire volume of the device. It should be noted, however, that due to the relatively large scale of the process and the heterogeneous nature of the tested materials (polyolefin waste, Alu-PEX laminates and RDF), the occurrence of local temperature gradients during heating cannot be completely ruled out, which is a phenomenon widely described in the literature on pyrolysis processes [43,44]. Nevertheless, the applied design solutions (including mixing and thermal insulation) help to limit these effects, especially as conditions approaching steady state are achieved.

The feedstock was prepared by grinding the polymer waste using a two-shaft shredder mill, obtaining a flaked fraction measuring 0.5–1.0 cm as described in Section 2.1. Each time, 8000 g of one of the previously prepared polymer materials was introduced into the reactor chamber. The process was conducted in an inert atmosphere using nitrogen as the shielding gas at a flow rate of 5 dm³/min. Also before starting the process, the reactor was inertized with technical nitrogen to remove atmospheric air from the reaction space. Since the aim of this work was to comparatively analyze the effect of feed heterogeneity on gas composition and yield, and not to optimize process parameters, it was decided to use one representative set of experimental conditions. Therefore, pyrolysis was carried out under identical process conditions for each of the four waste types analyzed.

The temperature was increased at a constant heating rate of 10 °C/min until the target temperature of 450 °C was reached, which was then maintained for 30 min. The pyrolysis temperature was selected based on preliminary laboratory studies and previous studies [15,43,45], which demonstrated that this range enables effective decomposition of polyolefin waste and its mixtures. This temperature was also chosen to ensure comparable analytical conditions for materials with varying degrees of heterogeneity (polyolefins, Alu-PEX laminates, and RDF), while simultaneously limiting secondary decomposition reactions that could interfere with the interpretation of the pyrolysis gas composition. The heating rate used (10 °C/min) corresponds to values commonly used in pyrolysis studies and represents a compromise between process control and experimental time.

The volatile pyrolysis products (hydrocarbon mixtures in the gas phase) were directed through the outlet tube to the distillation column (C-3), filled with Pall rings and equipped with top and bottom collection. The gas stream was passed through a heat exchanger (E-4) cooled by ice water (12 °C) and a gas separator (S-5). Then, after passing through a gas flowmeter (FI-7), the gas was directed to Tedlar bags and glass pipettes used as gas sampling containers for chromatographic analyses (GC-FID, GC-TCD).

After the process was completed, the reactor was flushed with nitrogen and cooled, and the solid residue—the char—was collected. The liquid fraction, the so-called broad hydrocarbon fraction, was collected from the bottom of the distillation column (Figure 2). This fraction was the subject of earlier studies, including [45], while in this experiment the focus was on the analysis of volatile components of pyrolysis products present in the gas stream. The obtained products were weighed, and their yield was calculated based on a mass balance as follows [46,47]:

$$liquid\left(\mathrm{w}\mathrm{t}\%\right)={\displaystyle \frac{liquid mass}{total waste mass}}·100\%,$$

(4)

$$char\left(\mathrm{w}\mathrm{t}\%\right)={\displaystyle \frac{char mass}{total waste mass}}·100\%,$$

(5)

$$gas\left(\mathrm{w}\mathrm{t}\%\right)=100\%-\left(liquid+char\right).$$

(6)

2.2.3. Composition Analysis of Gas Mixtures

Physicochemical characterization of gas samples was performed in accordance with applicable standards and internal procedures. The content of the following components: O₂, N₂, CO₂, CO, C₁, C₂, C₃, i-C₄, n-C₄, and the sum of C₅, C₆, C₇, C₈, C₉, and C₁₀ hydrocarbons, ethylene, and butadiene was determined using a dual-channel AGILENT 7890A valve gas chromatograph (Agilent Technologies, Santa Clara, CA, USA) equipped with a suitable column and detector system. The composition of the gas mixtures was determined using gas chromatography with a thermal conductivity detector (GC-TCD) (Agilent Technologies, Santa Clara, CA, USA) for non-flammable gases, while the presence of the lightest hydrocarbons (C₁ to C₆) was detected using a flame ionization detector (FID) (Agilent Technologies, Santa Clara, CA, USA). The concentration of aliphatic hydrocarbons in the C₆ to C₁₂ range was determined by gas chromatography with a flame ionization detector (GC-FID). Argon was used as the carrier gas during the analyses at a flow gradient from 3 mL/min to 6 mL/min. Column temperatures ranged from 35 °C to 260 °C. The nickel catalyst temperature was 375 °C, the FID temperature was 300 °C, and the injector and TCD temperatures were 200 °C. The analytical system used HP-PLOT/Q (15 m × 0.53 mm and 30 m × 0.53 mm), HP-MOLESIEVE 5A (30 m × 0.53 mm), and HP-PONA (50 m × 0.20 mm) columns, adapted for the separation of inorganic gases and light and heavier hydrocarbons.

CO and CO₂ content was determined using an additional nickel catalyst tube connected to a flame ionization detector. The gaseous sample was separated on the column and passed with added hydrogen over the catalyst, which converted CO and CO₂ to CH₄.

Hydrogen (H₂) content was determined using gas chromatography with a thermal conductivity detector (TCD) (Agilent Technologies, Santa Clara, CA, USA) and a packed column Molecular Sieve 5A Ultimetal (9 ft × 1/8 in × 2.00 mm). Argon was used as the carrier gas at a constant flow rate of 3 mL/min. A column temperature gradient was applied from 30 °C to 240 °C, while the injector and TCD temperature was 200 °C.

The sulfur (S) content in the gas was determined using gas chromatography with a flame photometric detector (GC-FPD) (Agilent Technologies, Santa Clara, CA, USA) using a capillary column (60 m × 0.32 mm) according to [48]. Argon was used as the carrier gas at a constant flow rate of 3 mL/min. A column temperature gradient was applied ranging from 30 °C to 240 °C, the injector temperature was 200 °C, and the FPD detector temperature was 250 °C.

The calorific values, relative density, Wobbe index, and gas compressibility coefficient were determined according to [42].

3. Results and Discussion

3.1. Material Characteristics

The physicochemical composition of PWs has a significant impact on the quality of the final products obtained in thermochemical recycling processes [49]. The physicochemical composition of the tested PWs is given in

Table 1. Physicochemical properties of the tested plastic waste.

Parameter	Unit	LDPE	MPW	Alu-PEX	RDF
Wa	wt%	0.11 ± 0.1	0.12 ± 0.1	0.99 ± 0.1	5.4 ± 0.1
Ashd	wt%	1.08 ± 0.1	0.99 ± 0.1	24.18 ± 0.1	11.50 ±0.1
VMdaf	wt%	98.92	99.01	75.82	88.50
ρ or ρn *	kg/m3	910.0	940.0	1450.0	181.0 *
$Q_s$	MJ/kg	44.85 ± 1.64	45.96 ± 1.39	34.76 ± 1.57	17.61 ± 1.53
Wu	MJ/kg	43.25 ± 1.72	43.12 ± 1.43	32.18 ± 1.59	16.42 ± 1.54
C	wt%	85.2 ± 2.5	84.4 ± 2.6	61.3 ± 2.5	51.6 ± 2.5
H	wt%	13.25 ± 0.61	12.85 ± 0.37	9.76 ± 0.56	6.50 ± 0.45
N	wt%	0.02 ± 0.01	0.01 ± 0.01	0.00 ± 0.07	0.95 ± 0.07
S	wt%	0.01 ± 0.01	0.01 ± 0.01	0.01 ± 0.01	0.30 ± 0.01
O	wt%	1.52 ± 0.01	2.73 ± 0.01	28.93 ± 0.01	40.65 ± 0.01
Cl	wt%	0.606 ± 0.015	0.124 ± 0.015	0.119 ± 0.071	0.805 ± 0.052
H/C	-	1.85	1.81	1.9	1.5
O/C	-	0.013	0.024	0.35	0.59

W^a—moisture content, Ash^d—ash content, VM^daf—volatile matter content, ρ—true density, ρ_n—bulk density, $Q_s$—gross calorific value, W_u—net calorific value, ^a—analytical, ^d—dry, ^daf—dry and ash free, *—ρ_n for RDF, ρ for others.

.

LDPE and MPW are characterized by low moisture content, which amounts to 0.11 and 0.12 wt%, respectively. The low moisture content of LDPE and MPW results from the high content of polyolefin waste, which is a hydrophobic material and does not absorb water. Furthermore, these wastes come from the packaging fraction, which typically does not contain high moisture content. Low moisture content is beneficial for thermal waste treatment processes because it reduces energy losses associated with water evaporation from the waste. The measured moisture content is similar to that reported in literature. For example, in [50], the moisture content in LDPE was determined to be 0.3 wt%, and in [22] the authors reported that the moisture content in PP was 0.15 wt%, while no moisture was detected in HDPE. In contrast, Alu-PEX waste is characterized by a clearly higher moisture content (0.99 wt%), which is likely due to its multilayer structure. The highest moisture content (5.4 wt%) was found in RDF waste, which is typical for alternative fuels produced from municipal waste. Excessive moisture significantly reduces its net calorific value.

The highest ash content was found in RDF (11.5 wt%), and the lowest in Alu-PEX (0.06 wt%). After sieving the ash fraction from Alu-PEX waste, the presence of metallic aluminum was shown in the amount of about 24 wt%. In turn, LDPE and MPW were characterized by an ash content of about 1 wt%. For comparison, in [50], the ash content for LDPE, HDPE and PP plastics was determined to be 0.4, 1.4 and 2.0 wt%, respectively. As for RDF, the ash content was 11.5 wt%, which is close to the value of 11.8 wt% reported in [51], or the range 5.97–10.02 wt% given in [52]. The presence of ash in the waste material significantly affects the pyrolysis process [53]. An increase in its content leads to a significant reduction in the yield of liquid products, which is most likely related to the intensification of secondary reactions and the catalytic effect of mineral components.

Analysis of the volatile matter content showed the highest values in MPW and LDPE, reaching 99.01 wt% and 98.92 wt%, respectively, and indicating their high susceptibility to thermal degradation. These results are consistent with the literature data—Ref. [22] reports the volatile matter content in LDPE, HDPE, and PP at 99.70 wt%, 99.75 wt%, and 95.08 wt%, confirming their high reactivity during pyrolysis. According to [54], polymers with a high volatile matter content and low ash and sulfur content favor the recovery of a larger amount of liquid hydrocarbons during pyrolysis, which results from the easier disintegration of polymer chains into liquid fractions. This is confirmed by further research presented in the next section (

Table 2. Mass yield of individual products obtained in the pyrolysis process.

Sample	Mass Share, g
	Gas	Liquid	Char
LDPE	1140	6600	260
MPW	2200	5700	100
Alu-PEX	1640	4600	1760
RDF	3800	3100	1100

), where it is shown that materials such as Alu-PEX (75.8 wt%) and RDF (88.5 wt%) exhibit a lower proportion of volatile substances, which indicates the presence of fillers and/or aluminum layers and results in a higher amount of solid residues after the thermal process. RDF, being a mixture of various wastes, is characterized by moderate reactivity and thus the potential to produce a higher amount of pyrolysis gas. This allows for a moderate proportion of liquid hydrocarbons.

The obtained true density values showed significant differences between the individual materials. For LDPE, the measured value was 910 kg/m³, while MPW was characterized by a slightly higher value of 940 kg/m³. The higher density of MPW compared to LDPE results from the presence of additives and possible organic impurities in this material, which increase its volumetric density [55,56,57]. The highest true density was found for Alu-PEX laminates (1450 kg/m³), which is consistent with the literature reporting that multilayer composite materials containing aluminum foils (aluminum density approx. 2700 kg/m³) and polymers filled with additives have a higher density than pure polymers [31,58]. In contrast, RDF has the lowest density (181 kg/m³ for bulk density). This value is typical for alternative fuels, which are characterized by heterogeneous composition and loose material structure. Literature data [52] indicate that the bulk density for RDF is in the range of 118–259 kg/m³, which encompasses the result obtained in this study.

The highest net calorific value was found for LDPE, which was 43.25 MJ/kg. A similar value was also obtained for MPW—43.12 MJ/kg. Such high net calorific values are typical for polyolefins, which consist mainly of carbon and hydrogen (Table 1), and their calorific value is comparable to liquid fuels such as diesel or heating oil [45,59]. This is due to the high share of C-C and C-H bonds in the polymer structure and the low content of oxygen (1.52 wt%) and other non-flammable elements, which favors high energy released during combustion. The lowest net calorific value among the analyzed materials was obtained for RDF, which was 16.42 MJ/kg. The calorific value of RDF determined in the conducted studies is lower than the range reported in the literature. The study [52] showed that the calorific value of RDF was in the range of 28.34–30.93 MJ/kg. These differences may be primarily due to the different morphological composition of the prepared RDF fraction, in particular the lower share of high-energy plastics and the higher share of biodegradable and mineral fractions. The high moisture content also significantly reduced the net calorific value of this material, which causes additional energy consumption for water evaporation during the pyrolysis process (Table 1).

The highest variability in elemental composition was observed in the case of RDF, which results from its heterogeneous nature (Table 1). This sample is characterized by the lowest C and H content (51.6 and 6.5 wt%, respectively), which translates into a lower calorific value. The increased Cl content indicates the participation of chlorinated plastics in this material, such as PVC. The high oxygen content (ca. 40 wt%) results mainly from the presence of cellulosic materials, while N and S are associated with organic additives and pollutants present in the waste stream [60]. Similar content of the elements C, H, N, and O was obtained for RDF in [61] concerning the overflow fraction of municipal waste, containing 60% of plastic waste (including gloves and protective masks), as well as paper, cardboard, and foil. The highest C and H content was found in LDPE waste (85.2 and 13.25 wt%, respectively). The low content of other elements (N, S, and O) is typical for polyolefins and results mainly from the presence of technological additives or trace impurities [15]. The obtained results are similar to the literature data. In [50] they reported about 81 wt% of C and 13.2 wt% of H for LDPE. The presence of Cl at a level of about 0.6 wt% in LDPE is not typical for pure polyethylene and most often results from technological additives or admixtures of other chlorine-containing impurities. In the case of Alu-PEX, the elemental composition is more diverse due to its multilayer structure. The higher O content results from the presence of additives, stabilizers, and adhesive layers, while the presence of inorganic components, such as aluminum, increases the content of mineral residue after combustion and reduces the C content in the sample [62].

Based on elemental analysis, the atomic H/C and O/C ratios were calculated, which ranged from 1.5 to 1.9 and from 0.013 to 0.59, respectively. The highest H/C ratios were obtained for polyolefin materials (LDPE, MPW and Alu-PEX), which indicates a high hydrogen content in the material structure and favors the formation of hydrocarbon pyrolysis products. In turn, RDF was characterized by the lowest H/C ratio and the highest O/C ratio, which indicates a higher share of oxidized components and non-polymeric materials [63,64,65].

3.2. Pyrolysis

3.2.1. Process Efficiency

The mass yield and percentage share of individual pyrolysis products of the tested wastes are presented in Table 2 and Figure 3.

Data analysis showed that the distribution of pyrolysis products is strongly dependent on the type of waste processed and its chemical composition. For LDPE, a dominant liquid fraction of 82.5 wt% was obtained, with a relatively low share of the gaseous fraction (14.25 wt%) and minimal char (3.25 wt%). LDPE is a simple, almost entirely organic polymer, with a high H/C ratio of 1.85 and a low O/C ratio of 0.013, which favors effective depolymerization in the pyrolysis process, yielding mainly liquid products [19]. In turn, for MPW, the share of the gaseous fraction increased to 27.5 wt%, with a slightly lower liquid share (71.25 wt%) and minimal char (1.25 wt%). The higher gas percentage can be explained by the presence of a mixture of polyolefins (LDPE, HDPE, PP) and minor impurities present in actual municipal waste. Furthermore, the H/C and O/C ratios of ca. 1.81 and 0.024, respectively, indicate the presence of oxygen and trace amounts of chlorine, which favor the formation of gaseous decomposition products such as CO, CO₂, and HCl. As a result, the gas percentage is higher than for LDPE, while liquid remains the dominant fraction. As for Alu-PEX, 1640 g (20.5 wt%) of gas, 4600 g (57.5 wt%) of liquid, and 1760 g (22 wt%) of solid residue were obtained. The material is characterized by H/C ratios of ≈1.9 and O/C ratios of ≈0.35, indicating partially oxidized organic components (Table 1). The presence of aluminum in Alu-PEX limits gaseous decomposition and reduces the share of combustible gases, which results in a lower energy efficiency of this gas compared to MPW and RDF. The highest share of the gaseous fraction was obtained for RDF, which amounted to 13.75 wt%. The H/C and O/C ratios of ca. 1.5 and 0.59, respectively, indicates that RDF is a highly oxidized and heterogeneous material, containing both organic and inorganic substances. Low H/C and high O/C favor the formation of more gaseous products and partially unreacted carbon in the form of char [66,67]. Due to the diverse decomposition pathways and undesirable by-products, pyrolysis of mixed plastics usually produces a smaller amount of the liquid fraction (often below 50 wt%) and higher gas emissions than in the case of pyrolysis of single polymers under similar conditions [68]. For example, in [10] it was shown that adding PP and LDPE to PS increased the gas yield in pyrolysis at 500 °C from 2.5% (PS alone) to 11.3% (blend), thus increasing the total gas production and the profitability of the process.

According to [69] pyrolysis carried out in a moderate temperature range of 500–550 °C allows for obtaining gaseous products in the amount of 35–40 wt% and liquid products up to 90 wt%. The product shares obtained in this study were lower: 14–48 and 38–83 wt% for gaseous and liquid products, respectively. In the work [70], the pyrolysis process of HDPE carried out at a temperature of 430 °C allowed for obtaining gaseous, liquid and solid products in the amounts of 15.11, 74.89 and 10.00 wt%, respectively. In turn, research [71] on the pyrolysis of mixed plastic waste (PP:PS:LDPE:HDPE in a weight ratio of 45.5:20:20.5:14) at 530 °C showed efficiencies of 36.69, 56.7 and 6.61 wt% for the gaseous, liquid and solid fractions, respectively. Analysis of these data indicates that both the composition of the processed material and the pyrolysis conditions significantly influence the material decomposition and product yield. Generally, optimization of pyrolysis requires matching the temperature and the type of processed material to the desired product profile—gaseous, liquid, or solid.

3.2.2. Characteristics of Pyrolysis Gas

Energy and Physical Properties

Table 3. Basic energy and physical properties of pyrolysis gas.

Quantity	Unit	LDPE	MPW	Alu-PEX	RDF
gross calorific value	MJ/m3	69.95 ± 2.86	81.16 ± 2.76	67.65 ± 1.19	24.68 ± 0.76
net calorific value	MJ/m3	64.72 ± 2.60	75.32 ± 2.48	62.35 ± 1.10	22.76 ± 0.70
gross Wobbe index	MJ/m3	60.38 ± 1.46	68.59 ± 1.28	62.56 ± 0.63	21.85 ± 0.58
relative density	-	1.342 ± 0.046	1.4 ± 0.044	1.169 ± 0.019	1.2756 ± 0.022
density	kg/m3	1.735 ± 0.056	1.811 ± 0.057	1.512 ± 0.025	1.6494 ± 0.029
compressibility coefficient	-	0.988 ± 0.004	0.984 ± 0.004	0.989 ± 0.004	0.993 ± 0.004

Reference conditions: 25 °C and 101.325 kPa for combustion, 0 °C and 101.325 kPa for volume measurement.

presents selected physical properties of gases obtained from the pyrolysis process for the considered waste streams.

The obtained results indicate a clear differentiation in the fuel quality of the gases depending on the type of feedstock used in the pyrolysis process. The highest net calorific value was observed for gas obtained from MPW (75.32 MJ/m³), while the lowest was for gas obtained from RDF (22.76 MJ/m³). This indicates that this gas has a significantly lower energy potential compared to gases obtained from polymer fractions. Gases obtained from LDPE and Alu-PEX showed similar net calorific values of 64.72 and 62.35 MJ/m³, respectively, suggesting similar energy efficiency of these feedstocks in the pyrolysis process. A similar relationship was observed for the gross calorific values. The obtained results confirm the significant effect of feedstock on the energy properties of the pyrolysis gas—polymeric materials and MPW exhibit a significantly higher energy potential compared to RDF. Polymer fractions are the main source of energy in the pyrolysis gas, while the lower calorific value of RDF gas results from its heterogeneous composition.

One of the most important parameters in assessing the quality of gaseous fuel is the Wobbe index, which enables the interchangeability of various gases in combustion processes. The highest value of this index was recorded for gas from MPW (68.59 MJ/m³), similar for Alu-PEX (66.35 MJ/m³), and a little less for LDPE (60.38 MJ/m³), which indicates comparable combustion properties of these gases. A significantly lower Wobbe index was recorded for gas from RDF (21.85 MJ/m³), which excludes its use as an alternative fuel. A low value of this parameter can lead to deterioration of flame stability and a reduction in combustion temperature [72].

The density of all obtained gases is higher than that of air, indicating the presence of hydrocarbons (C₂+). The highest density was recorded for the gas from MPW, which directly translates into its calorific value and the Wobbe index—the highest among the analyzed samples. The dominant component of the gas obtained from MPW was propene (20.06 mol%), which most likely results from the presence of polypropylene in the feed stream (

Table 4. Composition of volatile pyrolysis products detected in the gas stream.

Group	Compound Name	Formula	Content, % mol/mol
			LDPE	MPW	Alu-PEX	RDF
Inorganic gases	hydrogen	H2	6.53 ± 0.096	6.88 ± 0.113	3.20 ± 0.192	11.45 ± 0.415
	carbon dioxide	CO2	7.67 ± 0.166	5.79 ± 0.140	2.21 ± 0.133	54.48 ± 1.011
	oxygen	O2	0.63 ± 0.032	0.55 ± 0.031	0.34 ± 0.031	0.61 ± 0.032
	nitrogen	N2	12.14 ± 0.211	2.54 ± 0.123	9.47 ± 0.473	2.87 ± 0.127
	carbon monoxide	CO	2.53 ± 0.018	2.74 ± 0.021	0.75 ± 0.060	5.31 ± 0.038
C1–C2 hydrocarbons	methane	CH4	7.67 ± 0.359	8.64 ± 0.451	18.34 ± 0.734	6.55 ± 0.313
	ethane	C2H6	10.56 ± 0.358	8.68 ± 0.318	14.51 ± 0.581	3.75 ± 0.126
	ethene	C2H4	16.76 ± 0.567	13.43 ± 0.506	17.63 ± 0.7057	4.92 ± 0.165
C3–C4 hydrocarbons	propane	C3H8	9.00 ± 0.422	5.67 ± 0.295	21.32 ± 0.853	1.57 ± 0.075
	propene	C3H6	<LOD	20.06 ± 0.329	<LOD	<LOD
	isobutane	C4H10	0.10 ± 0.003	0.21 ± 0.005	0.11 ± 0.011	0.11 ± 0.012
	n-butane	C4H10	3.73 ± 0.175	2.27 ± 0.119	1.20 ± 0.072	0.47 ± 0.010
	1-butene	C4H8	5.65 ± 0.265	2.16 ± 0.113	<LOD	0.65 ± 0.013
	2-methylpropene	C4H8	0.71 ± 0.073	<LOD	<LOD	<LOD
	isobutene	C4H8	<LOD	4.49 ± 0.234	<LOD	1.85 ± 0.088
	trans-2-butene	C4H8	0.52 ± 0.053	0.64 ± 0.014	0.21 ± 0.022	0.31 ± 0.006
	cis-2-butene	C4H8	1.93 ± 0.090	0.43 ± 0.010	0.11 ± 0.011	0.20 ± 0.004
	1,3-butadiene	C4H6	<LOD	1.34 ± 0.070	5.66 ± 0.281	0.48 ± 0.010
C5–C12 hydrocarbons	isopentane	C5H12	0.06 ±0.004	0.03 ± 0.001	0.03 ± 0.00	0.02 ± 0.002
	n-pentane	C5H12	3.39 ± 0.155	11.08 ± 0.481	4.36 ± 0.216	0.50 ± 0.010
	neopentane	C5H12	9.75 ± 0.457	<LOD	<LOD	2.60 ± 0.126
	n-hexane	C6H14	0.11 ± 0.005	0.63 ± 0.028	0.20 ± 0.020	0.80 ± 0.034
	n-heptane	C7H16	0.07 ± 0.003	1.39 ± 0.064	0.03 ± 0.005	0.15 ± 0.007
	n-octane	C8H18	0.03 ± 0.002	0.16 ± 0.010	<LOD	<LOD
	n-nonane	C9H20	0.04 ± 0.007	0.13 ± 0.022	0.01 ± 0.003	0.04 ± 0.013
	n-decane	C10H22	0.21 ± 0.047	0.06 ± 0.015	0.31 ± 0.031	0.31 ± 0.031
	n-undecane	C11H24	0.18 ± 0.048	0.00 ± 0.001	<LOD	<LOD
	n-dodecane	C12H26	0.03 ± 0.005	<LOD	<LOD	<LOD

LOD—limit of detection.

). Propene is a characteristic primary product of PP pyrolysis, formed as a result of polymer chain cracking in both primary, secondary, or tertiary reactions, which is confirmed by literature studies [73]. The obtained results clearly indicate that the feed composition has a key impact on the physicochemical properties of the pyrolysis gas.

Chemical Composition

The chemical composition of the volatile pyrolysis products is presented in Table 4 and Figure 4 in % mol/mol units, which enabled direct comparison of the molar fractions of the individual components. It should be emphasized that the analyzed gas stream contained both permanent gases and partially condensable hydrocarbons (C5–C12), which were not completely condensed under the cooling conditions used in the system (12 °C). These compounds should be considered components of the volatile pyrolysis products associated with the liquid fraction, present in the gas stream before full condensation.

The composition of pyrolysis gases varies significantly depending on the feedstock type. The content of inorganic gases such as H₂, CO₂, O₂, N₂ and CO is relatively low in gas samples after pyrolysis of MPW, LDPE, and Alu-PEX waste (approx. 15.97–29.50% mol/mol), but significantly higher in RDF (approx. 74.72%), which reduces the fuel quality of this gas. These differences result primarily from the nature of the feedstock—polymeric materials and MPW contain a significant proportion of organic hydrocarbons (Table 1), which decompose during the pyrolysis process into hydrocarbon gases (C₁–C₄) with high energy value (Figure 4). Consequently, gases obtained from polymeric materials (LDPE, Alu-PEX) and MPW are characterized by a significantly higher content of hydrocarbons in the C₂–C₄ range, which are the main energy carriers. Figure 4 indicates that C₃–C₄ hydrocarbons are the dominant group in the gas from MPW, whereas the gases from LDPE and Alu-PEX contain a higher proportion of C₁–C₂ hydrocarbons, i.e., light hydrocarbons responsible for good combustion properties. Indeed, in the case of MPW, propene (approx. 20.06%) and ethylene (13.43%) have a significant share. In turn, LDPE and Alu-PEX show increased contents of ethene (16.76% and 17.63%, respectively), ethane (10.56% and 14.51%, respectively), and propane (9% and 21.32%, respectively) (Table 4).

In the case of the MPW sample, the high propene content (20.06 %) may be due to the more complex composition of the material, which includes various types of polyolefins, including PP, whose thermal degradation promotes the formation of unsaturated light hydrocarbons, particularly propene. Similar results were obtained in [74]. In the case of LDPE, on the other hand, a greater proportion of reactions leading to the formation of propane and other saturated gaseous products, as well as heavier liquid and paraffinic products, may have limited the amount of propene in the gas phase. In addition, the presence of other components in Alu-PEX and RDF may have influenced the course of secondary cracking reactions and hydrogen transfer reactions, leading to further conversion of propene to other gaseous products.

The presence of heavier hydrocarbon fractions (C₅–C₁₂) in the analyzed gas samples is most likely due to the partial non-condensation of the most volatile components of the liquid fraction at the cooling temperature of 12 °C, as well as to the possible entrainment of heavier hydrocarbon vapors (included in the pyrolysis oil fraction) by the gas stream. However, these compounds were present in all samples in relatively small amounts; their highest share was observed in the gas from MPW, mainly in the form of n-pentane (approx. 11.08%). The presence of these compounds additionally increases the calorific value of the gas (Table 3).

RDF, on the other hand, is a heterogeneous material rich in inorganic components, which are not converted into fuel hydrocarbons during pyrolysis but remain as inorganic gases or dust [75]. The gas produced after RDF pyrolysis contained the highest proportions of CO₂ (54.48 ± 1.01% mol/mol), H₂ (11.45% mol/mol), and CO (5.31% mol/mol), which results from the presence of fractions such as organic residues, paper, and cardboard in the feed stream. These materials contain significant amounts of oxygen in their structure, which is a precursor to the formation of CO₂ and CO. Additionally, decarbonization processes and C–O bond cleavage, occurring even at relatively low pyrolysis temperatures, favor the intensive release of these gases. In [29], a similar CO₂ content (ca. 60.5 ± 2.1% mol/mol) was obtained in the gas generated by pyrolysis of mixed non-recyclable municipal waste. The presence of nitrogen (N₂) in the gases results mainly from the use of nitrogen as an inert gas in the pyrolysis process, where it acts as a carrier gas. Additionally, the nitrogen content may result from small amounts of air introduced into the process with the raw material. Its share is the highest in LDPE (ca. 12% mol/mol) and Alu-PEX (ca. 9.5% mol/mol), and lower in MPW (2.54% mol/mol) and RDF (2.87% mol/mol). Furthermore, differences in the N₂ content between samples may result from different characteristics of the pyrolysis process for individual materials, including different gas evolution dynamics, volume of the generated gas phase, and degree of process gas dilution. This effect was particularly visible for heterogeneous materials, such as RDF and MPW, for which the gas yield was higher than for LDPE.

Sulfur Content

Table 5. Sulfur compounds concentration in pyrolysis gas.

Compound	Concentration, mg/dm3
	LDPE	MPW	Alu-PEX	RDF
Hydrogen sulfide	<0.15	0.67 ± 0.12	<0.15	599.1 ± 106.6
Carbonyl sulfide	<0.27	<0.27	<0.27	330.2 ± 58.8
Methyl mercaptan	<0.21	283.2 ± 50.4	<0.21	628.4 ± 111.9
Ethyl mercaptan	<0.28	6.03 ± 1.07	<0.28	64.3 ± 11.5
Dimethyl sulfide	0.9 ± 0.16	58.4 ± 10.39	<0.28	62.3 ± 11.1
Carbon disulfide	10.9 ± 1.93	3.1 ± 0.55	2.8 ± 0.49	29.9 ± 5.32
i-Propyl mercaptan	<0.34	1.2 ± 0.21	<0.34	1.8 ± 0.32
tert-Butyl mercaptan	<0.40	0.9 ± 0.15	<0.40	1.3 ± 0.23
n-Propyl mercaptan	<0.34	0.99 ± 0.18	<0.34	24.8 ± 4.41
Methyl ethyl sulfide	<0.34	<0.34	<0.34	<0.34
s-Butyl mercaptan	0.85 ± 0.15	4.58 ± 0.82	0.55 ± 0.1	59.6 ± 10.61
i-Butyl mercaptan	<0.40	3.25 ± 0.58	<0.40	21.6 ± 3.85
Diethyl sulfide	<0.40	<0.40	<0.40	<0.40
n-Butyl mercaptan	<0.40	<0.40	<0.40	1.94 ± 0.35
Dimethyl disulfide	0.97 ± 0.17	48.1 ± 8.6	<0.42	<0.42
Dipropyl sulfide	<0.53	<0.53	0.54 ± 0.1	<0.53
Diethyl disulfide	<0.54	<0.54	<0.54	<0.54
Total sulfur	10.6	261.9	2.7	1290.6
Mercaptan sulfur	0.3	195.9	0.2	493.3

presents the results for sulfur content in pyrolysis gas from the considered wastes.

The obtained results indicate significant variation in sulfur content in pyrolysis gases, which affects their usability and compliance with emission requirements. The highest total sulfur content was found in RDF gas (1290.6 mg/dm³), with dominant contents of hydrogen sulfide (599.1 ± 106.6 mg/dm³), methyl mercaptan (628.4 mg/dm³), and carbonyl sulfide (330.2 ± 58.8 mg/dm³). According to the Industrial Emissions Directive (IED), the permissible SO₂ concentration in emissions from industrial installations should not exceed 200–400 mg/Nm³. High sulfur concentrations in RDF pyrolysis gases would lead to SO₂ emissions significantly exceeding these limits. Moreover, according to the standard [76], which specifies permissible H₂S concentrations in fuel gases, the concentration of this compound exceeds the values accepted for technical applications (H₂S < 50–200 mg/Nm³ for engines). These concentrations significantly exceed the gas permissible values for energy and industrial applications. Furthermore, in [77], attention was drawn to the high potential for sulfur migration from waste that was subjected to the pyrolysis process. In the case of plastics recycled from end-of-life vehicles, sulfur migration to the pyrolysis gas is approximately 5.87%.

Sulfur concentrations in the gas from the MPW were moderate (total sulfur 261.9 mg/dm³), while in the gas from the pyrolysis of LDPE and Alu-PEX, the content of sulfur compounds was the lowest (10.6 mg/dm³ and 2.7 mg/dm³, respectively), often below the limit of detection, indicating their more favorable fuel properties and lower environmental impact. Pyrolysis gases from LDPE and Alu-PEX are within the limits of permissible European standards and can be used practically without additional purification.

Additionally, in accordance with national regulations specified in the Regulation of the Minister of Climate and Environment on the levels of certain substances in the air [78], the permissible concentrations of SO₂ in the air are 0.350 mg/m³ (1-h average) and 0.125 mg/m³ (24-h average), which indicates that even a small content of sulfur compounds in pyrolysis gases may lead to exceeding emission standards after their combustion.

4. Conclusions

Using plastic waste in chemical recycling via pyrolysis, which yields gas, diesel fuel, and char, is a promising and increasingly important strategy for reducing environmental pollution from this waste while contributing to meeting the growing demand for clean energy. However, currently, polymer waste processing plants using this method require further process optimization, improved product quality, and effective pyrolysis gas purification.

A comparative analysis of various wastes showed that the heterogeneity of the feedstock—resulting from both the type of polyolefin waste (LDPE, MPW) and the presence of Alu-PEX laminates—significantly affects the physicochemical composition and yield of pyrolysis products. LDPE polyolefins are characterized by a high yield of the liquid fraction (approximately 82 wt%) and a gas with a high calorific value, with a high proportion of light hydrocarbons C₁–C₄. Mixed and composite wastes (MPW, RDF) generate larger amounts of gas and char, with a simultaneous increase in the proportion of non-flammable gases, primarily CO₂, which reduces the energy value of the gas. Alu-PEX laminates produce gas with a high C₁–C₂ content and very low sulfur content, making it suitable for use as an effective gaseous fuel, although in this case, more solid residues are produced.

Compared to RDF, which is characterized by a more complex structure, high inorganic content (approx. 70–75%), and high sulfur content (nearly 1300 mg/dm³), gases from polyolefins and laminates exhibit more favorable fuel properties, higher calorific value, and lower sulfur content (LDPE—10.6 mg/dm³, Alu-PEX—2.7 mg/dm³), which allows their use without additional purification and reduces the environmental impact of the process. RDF gas, on the other hand, requires advanced purification, while MPW gas requires partial purification.

Analysis indicates that feedstock selection and initial segregation are crucial for process stability, product quality, and optimized energy recovery. Pyrolysis can be targeted to specific products: the liquid fraction from polyolefins and the energy gas from MPW and laminates (Alu-PEX). Removing inorganic fractions from mixed waste improves the energy value of the gas and increases process efficiency. Furthermore, pyrolysis gas can be used both as a fuel and as a chemical feedstock for synthesis, and the entire process can support the plant’s energy self-sufficiency and the implementation of circular economy principles.