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Scientific Reports
2045-2322
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10.1038/s41598-024-71958-3
Article
Continuous process design of the microwave chemical recycling of waste plastics using microwave-absorbing heating elements
Kondo Hikaru 1
Sawai Anna 2
Hirabayashi Kazuki 2
Nakanishi Ayami 2
Fukumoto Kazuki 1
Tanaka Yutaro 1
Horikoshi Satoshi horikosi@sophia.ac.jp

2
1 grid.418306.8 0000 0004 1808 2657 Science and Innovation Center, Mitsubishi Chemical Corporation, 1000 Kamoshida-cho, Aoba-ku, Yokohama-shi, Kanagawa, 227-8502 Japan
2 https://ror.org/01nckkm68 grid.412681.8 0000 0001 2324 7186 Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554 Japan
20 9 2024
20 9 2024
2024
14 2195216 6 2024
2 9 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
Conventional pyrolysis methods to chemically recycle plastic waste require significant energy input owing to inefficient energy transfer from the heat sources to plastics and the generation of excess CO2. Accordingly, there is an urgent need to develop alternative methods for the chemical recycling of waste plastics to limit global warming. In this study, the use of microwaves (MWs) as an efficient energy source for pyrolysis and chemical recycling. However, because plastic is transparent to MW energy, a method that utilizes carbon materials as MW-absorbing heating elements (MWAHEs) was developed. This method directly converted high-density polyethylene (HDPE) into light chemicals in up to 94% yield with 45% ethylene selectivity. A two-stage pyrolysis system incorporating MWAHE-assisted MW pyrolysis was also developed to produce light chemicals in 95% yield with 49% ethylene selectivity. From a chemical engineering perspective, this two-step pyrolysis system is an efficient and feasible method for producing valuable light olefins. This study also demonstrates that MWAHE assisted MW pyrolysis is effective for the chemical recycling of plastic waste. This study offers potential solutions for the environmental problems posed by plastic waste by developing efficient and scalable methods for its chemical recycling.

Subject terms

Chemical engineering
Microwave chemistry
Sustainability
issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

Plastic waste poses a global environmental issue, with hundreds of millions of tons of plastic entering landfills or accumulating in the oceans annually1,2. Conventional waste management methods, such as incineration have proven to be mediocre, because they produce toxic emissions and large amounts of carbon dioxide (CO2), a greenhouse gas that exacerbates the environmental impact of plastic waste3,4. An estimated 25% of plastic waste is incinerated2, emitting polluting gases and CO2 into the atmosphere in exchange for heat energy. Conventional thermal energy recycling methods using incineration should be avoided due to their environmental pollution impacts and global warming effects. The renewability of one-time-use plastics compounds the problem. Fossil fuels, which are a finite resource, are the most commonly used raw materials for producing plastics. Although the use of biomass as a plastic feedstock has attracted attention for its potential sustainability, feedstock procurement and life cycle assessment (LCA) are still challenging5. Chemists have the responsibility to provide solutions for global warming and resource depletion problems by designing innovative chemical recycling methods6–11.

Pyrolysis is widely considered one of the most promising methods for converting plastic waste back into feedstock7,12. However, the pyrolysis process often requires significant energy to break down the polymer chains of common plastics, such as polyethylene (PE) and polypropylene (PP), typically requiring temperatures in the 450–800 °C range7,13. Although conventional heat sources derived from fossil fuels, such as burners and steam, are commonly used as heat source to operate chemical processes, energy-efficient electric heat sources are vital for developing an effective recycling process6.

Our group previously explored the use of MW as an alternative heat source to conventional convection heating14. By MW heating, energy is transferred directly to MW-absorbing materials that self-heat, and efficiently increases their temperature beyond 1000 °C in a very short period without relying on heat conduction. This MW heating feature is suitable for developing a novel energy-saving chemical recycling process. Furthermore, MW heating may be capable of improving temperature uniformity, slowing thermal conduction from the heat source to the plastic waste, and suppressing the generation of difficult-to-decompose byproducts like tar and char, thereby overcoming the disadvantages associated with conventional-pyrolysis-based chemical recycling systems. However, concerns persist regarding the validity of this hypothesis. The effective use of MWs to heat low-dielectric materials is one issue requiring resolution. Many common plastics, such as PE, have low dielectric loss values, and are therefore, transparent to most MW radiation; consequently, the direct heating of polyolefins using MWs is difficult to achieve.

Previous studies have demonstrated that carbon materials can be used as MW-absorbing heating elements (MWAHEs) to rapidly heat materials that are difficult to heat to high temperatures using MW heating. Our group demonstrated that activated carbon (AC) in a container into which seawater is continuously fed heats up to 600 °C immediately after MW irradiation, decomposing the water and generating hydrogen gas effectively, even though the temperature of the process is generally considered excessively low for steam reforming and the water–gas shift reaction. The use of MWs in this process exhibits a 59% lower power consumption rate than conventional heating15. Additionally, several pyrolysis methods have been reported for polyolefins containing MW-absorbing substances16–18. For example, Ludlow-Palafox and Chase reported that MW-induced PE pyrolysis using carbon coke provided pyrolyzed gas in up to 20.9%, yield, along with oil and wax in 79.1% yield17. Jing et al. also recently reported that PE pyrolysis using AC and MW irradiation provided an up to 64% gas yield18. Although these results demonstrate success in obtaining oil from polyolefins, the oil obtained from these plastics is mainly used as a fuel and requires several extra processes for formal chemical recycling, including distilling the oil to obtain naphtha fractions and additional processes like cracking, dehydrogenation, and/or reduction. Unfortunately, these additional processes lead to low product yields, high production costs, and high energy requirements. Directly converting waste plastics into basic chemicals, such as ethylene and propylene, rather than producing fuel is important for achieving a simple and sustainable chemical recycling method. Scaling up of the process using MWs is another problem that remains unsolved. Although industrial processes using MWs have been previously reported19–22, there is a need to design a robust process for scaling up MWs, starting with laboratory-scale experiments.

This paper reports the direct gasification of polyolefins using MWAHE-assisted MW-induced pyrolysis, contributing to the development of such a MW heating process for recycling plastic waste. The proposed method enables the direct production of ethylene and propylene from waste polyolefins via MW-assisted pyrolysis. Furthermore, the MW-based plastic waste recycling process is tolerable and scalable for future applications.

Results and discussions

Effect of the pyrolysis temperature on the chemical recycling process

The conversion and pyrolysis efficacy of HDPE were evaluated using AC particles as MWAHEs to examine the feasibility of using the MWs and MWAHE hybrid method. The HDPE pellets were pyrolyzed by heating the pellets from 850 to 1200 °C in increments of 50 °C under proportional-integral-differential (PID) control (Fig. 1) to determine the optimum reaction temperature and heating costs. AC particles and PE were packed into a quartz tube reactor under a flowing nitrogen atmosphere, and the reactor was irradiated with MWs for 3 min.Fig. 1 Gas yield and composition of light chemicals originating from the microwave (MW) pyrolysis of high-density polyethylene (HDPE) using various MW temperatures and activated carbon (AC) as the MW-absorbing heating element (MWAHE).

The AC rapidly reached the intended temperature within several seconds of MW irradiation, immediately decompositing the HDPE pellets. The HDPE pellets were no longer visible after 3 min of MW irradiation, and the obtained gas yields were 61–94 wt.%. Increasing the pyrolysis temperature improved the gas yield, and the pyrolysis progressed by > 80% when the reaction temperature exceeded 1000 °C. The detection of C1–C25 compounds using a gas chromatograph with a flame ionization detector (GC-FID) analysis revealed that 99.9 vol.% of the products were C1–C6 compounds. Additionally, the selectivity for ethylene in the light chemical stream was 39–47 vol.% at each pyrolysis temperature, and ethylene was the most abundant product under each condition. Light chemicals other than ethylene gas were present in the following descending order of abundance: methane, propylene, ethane, propane, and butanes. These results are intriguing because they suggest that HDPE can be directly chemically recycled into ethylene gas. Interestingly, almost no tar or CO2 was generated during this process. Thus, the MWAHE-assisted MW method can directly produce ethylene from HDPE. The pyrolysis process also produces other light chemicals that can be used as raw materials.

Effect of the particle size of the AC MWAHEs on the chemical recycling process

The effects of various AC particle sizes on the HDPE gasification process were further evaluated. AC derived from the same batch was used to conduct the MW-assisted pyrolysis of HDPE. The AC was mechanically crushed and sieved to obtain particles of five different size ranges (Fig. 2). The particle sizes of AC1, AC2, AC3, AC4, AC5 were 0.045–0.60, 0.60–0.71, 0.71–1.18, 1.18–1.7, and 1.7–2.0 mm, respectively (Table S1). The MW power was fixed at 80 W to investigate the effect of the particle size on the MW heating efficiency of AC with varying particle sizes. HDPE was pyrolyzed for 3 min with AC1. A small amount of PE residue not in contact with the AC1 particles was observed and a gas yield of 77% with 30% ethylene selectivity, along with 22% of methane and 4% of acetylene, was observed. The use of AC2 increased the gas yield to 93% with 35% ethylene selectivity without generating acetylene. These results indicate that using a smaller AC particle size with a larger surface area may decompose ethylene excessively. The selectivity for the decomposition gas remained almost unchanged when AC3, AC4, and AC5, which had larger particle sizes than AC2, were used; however, the yield gradually decreased with increasing particle size. These results suggest that the gas yield is affected by the AC particle size, owing to the contact efficiency between PE and AC.Fig. 2 Gas yield and composition of light chemicals of the HDPE (0.10 g) pyrolyzed using MWs and AC (0.05 g) having different particle sizes.

Effect of the MWAHE type on the chemical recycling process

Other carbon-based materials were also evaluated as MWAHEs. Silicon carbide (SiC) can also rapidly generate heat when exposed to MWs23. The MWAHEs were compared under two different output conditions (40 and 80 W) for 3 min of MW irradiation. The experiments were conducted at room temperature (23 °C) to ensure that the heat release efficiency from the heated MWAHEs to the atmosphere remained constant throughout the experiment (Fig. 3).Fig. 3 (a) Gas yield and composition of products from the chemical recycling pyrolysis of HDPE (0.10 g) using MWs and various MWAHEs (AC1: 0.05 g, SiC: 0.20 g) under fixed MW powers of 40 and 80W. (b) Temperature profiles for each of the conditions.

Pyrolyzing PE with AC1 at 40W provided a gasification yield of 16% with 16 vol.% ethylene selectivity. Increasing the MW power from 40 W to 80 W markedly improved both the gas yield and ethylene selectivity, affording an 84% gas yield with 33% ethylene selectivity. The maximum temperatures reached at 40 W and 80 W were 651 and 1471 °C, respectively (Fig. 3b). These results suggest that a threshold MW output energy for the effective thermal decomposition of PE exists as high-temperature fields can directly gasify PE in a very short time.

The gasification yield for the MW-irradiated pyrolysis using SiC at 40W was low (9%) despite the marginally higher temperature of the reaction system, as compared with that using AC1 at 40W. The maximum temperature achieved when SiC was heated at 80W was 1,356℃, which is similar to the temperature achieved when using  AC1 at 80W. In contrast to AC1, although the gasification yield was 66%, the ethylene selectivity improved to 64 vol.%. Even under similar temperature conditions, changing the MWAHE type significantly changed the selectivity.

The use of AC1 as the MWAHE, as compared with using SiC as the MWAHE, improved the gasification yield of PE, decreased the selectivity for ethylene, and increased the production of alkane compounds. The temperature profiles of AC1 and SiC at 80W were compared. The heating trends for both MWAHEs were similar; however, AC1 exhibited temperature fluctuations after reaching the terminal temperature (Fig. 3b).

To understand the causes of the fluctuating temperature, the reactor was visually observed during pyrolysis (Fig. 4a). Light emission was observed after 0.5 s of MW irradiation (Fig. 4b). The light emission was not uniform, but occurred intermittently, with the color ranging from white to orange. AC emits discharge plasmas when irradiated with MWs, increasing the microscopic surface temperatures to 750 °C or more24,25. Brighter incandescence was observed after 1.0 s of MW irradiation (Fig. 4c). PE and AC1 were fluidized at 5.0 s (Fig. 4d). Gases were observed to rapidly form at 15.0 s (Fig. 4e). Gas formation further fluidized the reactor contents, and particulate matter was observed to migrate to previously unpacked areas within the tube reactor. This observation suggests that the temperature fluctuated because of the radiation thermometer used to measure the dynamic material. In contrast, this fluctuation was not observed for SiC, possibly because the electrical properties of SiC make it less prone to fluidization. Eq. 1, in which the first, second, and third terms are the Joule, dielectric, and magnetic heating components, defines MW heating14.1 P=12σE2+πfε0εr"E2+πfμ0μr"H2

where E and H denote the strength of the electric field (E field) and magnetic field (H field) of the MWs, respectively; σ is the electrical conductivity; f is the frequency of the MWs; ε0 is the permittivity in vacuum; εr" is the relative dielectric loss factor; μ0 is the magnetic permeability in vacuum; and μr" is the relative magnetic loss factor. When the applied MW energy enters the heated object, the energy losses (i.e., Joule, dielectric, and magnetic losses) are converted into heat energy.Fig. 4 Dynamic incandescence changes from the AC and polyethylene (PE) sample under continuous MW irradiation at 80W in a quartz flow reactor at (a) 0, (b) 0.5, (c) 1.0, (d) 5.0, (e) 15.0, and (f) 20.0 s.

It was reported in our previous paper that the MW heating of AC in seawater using E-field irradiation was faster than that using H-field irradiation15. Since AC and SiC have no magnetic properties, the heating of MWAHEs prioritizes the Joule and dielectric losses over magnetic losses. The electrical conductivity of AC (1.69 S cm−1) is approximately 250-fold larger than that of SiC (0.0069 S cm−1), and it is assumed that the contribution of Joule heating for AC is very significant, causing temperature fluctuation via discharge emission.

Mechanism of the MWAHE-assisted MW pyrolysis of HDPE

The pyrolysis of PE occurs via free radical reactions7, and the selectivity for ethylene formation can be attributed to the bond dissociation energies (Fig. 5)26,27. Bond cleavage reactions involving the homolytic cleavage of C–C bonds favorably occur at the tertiary carbons, which occur at the branching points of the main chain26. The reaction in the MWAHE-assisted MW pyrolysis of HDPE occurs at nearly 1000 ºC. Bond dissociation can also simultaneously occur at nearly 1000 °C at secondary C–C bonds owing to the entropically favorable conditions, followed by radical-induced C–C bond cleavage at the β-position, which forms smaller gaseous hydrocarbons. Ethylene is preferentially formed via the chain cleavage reaction of alkyl radicals and subsequent C–H cleavage of ethyl radicals. The pathway for the further decomposition of ethylene to acetylene is inhibited by the high energy barrier, which results in the selectivity for ethylene being significantly greater than that for acetylene. The formation of ethane via ethylene hydrogenation requires the addition of a hydrogen radical to ethylene and a subsequent hydrogen abstraction step. The ethyl radical-to-ethylene reaction occurs easily because it has an overwhelming energy advantage over other reaction pathways at 149 kJ mol−1. Methane is formed by hydrogen atom abstraction between the hydrocarbon and the methyl radical derived from the degradation of odd-numbered alkyl radicals and the homolytic C–C cleavage of ethane. This process leads to the regeneration of the alkyl radical, which in turn causes further decomposition and generates ethylene-rich gaseous products.Fig. 5 Proposed mechanism of PE pyrolysis. Values are in kJ mol−1 and represent the energy required to break only the bond indicated.

Considering the above-proposed mechanism, controlling the temperature of the heating materials and the contact time between the decomposed material and the heat source are the most critical factors contributing to the selectivity of the pyrolysis products. MWAHE-assisted MW heating is valuable because of its agility and flexiblity.

Design for the continuous chemical recycling of HDPE using MWAHE-assisted MW pyrolysis

Although the one-step MWAHE-assisted MW pyrolysis of HDPE was examined and its potential application for chemical recycling was demonstrated, operation with continuous feedstock and the uniform transfer of heat from the MWAHE to the feedstock are synthetic issues that hinder its practical implementation. Pyrolysis decreases the molecular weight of waste plastics from several millions to less than 100, and changes their physical form from solid to liquid and gas. A scalable process is required to accommodate this rapid phase change. PE decomposes randomly at the main chains following a statistical distribution7. Therefore, light gases, naphtha, light oils, and heavy oils with different molecular weight distributions are formed during decomposition. High temperatures and short residence times are required to maximize the light gas content28. However, in addition to the energy required to cleave the main chain during the pyrolysis of solid polyolefins, energy is required to melt and vaporize the polyolefins. Controlling the heat transfer during rapid volume changes originating from the transition from the liquid phase to the gas phase is challenging. Dong et al. reported the use of electrified spatiotemporal heating as an effective temperature management method for selectively degrading PP beyond equilibrium29. However, the scalability of the method for large-scale chemical manufacturing remains a concern. Plastic waste streams also contain contaminants. Plastics are commonly compounded with other materials like talc fillers and pigments. Metal fastening can cause arc discharges. Therefore, it is necessary to filter these materials from MW pyrolysis feedstock29.

A two-step pyrolysis system was designed to address these issues (Fig. 6a)30,31. Since residence time significantly affects the decomposition product composition28, combining the first-step crude decomposition/vaporization and subsequent feeding of the gaseous decomposition products into the second MW cracking step is logical. Della Zassa et al. obtained ethylene as the main product by pyrolyzing the feedstock at temperatures below 500 °C and further cracking at temperatures above 800 °C, with the reaction time for PE cracking being < 1.3 s30. Artetxe et al. also reported that pyrolysis at 500 °C combined with cracking at 800–950 °C for 0.02–0.03 s yielded C2–C4 olefins at 77 wt.%31. This system can remove impurities in the waste stream in the first step and reduce the risk of overheating due to arc discharge in the second MW step Furthermore, a compact MW furnace can be realized by increasing the heat-transfer area in fixed-bed reactors.Fig. 6 (a) Photograph of the two-step continuous pyrolysis system in the HDPE chemical recycling system using a conventional electric furnace and MWs with SiC-MWAHEs (0.20 g) in a quartz flow reactor; (b) Composition of the gasification yield of the products from the chemical recycling system for the pyrolysis of HDPE using a hybrid system consisting of electric furnace and MW heating with SiC-MWAHEs. All yields and selectivities represent the average of two experiments. (c) Temperature profile of the electric furnace (620 °C) and temperature of the SiC-MWAHEs under MW irradiation (950 °C). See supplementary information S-II for detailed experimental setup.

To demonstrate the effectiveness of the two-step thermal MWAHE-assisted MW pyrolysis system for the chemical recycling of plastics, a setup with a ceramic electric tube furnace and MW device were constructed as the first- and second-stage heat sources, respectively, to provide two stages of independent temperature control. The quartz flow reactor was operated in two stages. The crude decomposition stage was operated at 450–650 °C using DIXON packing, while the cracking section was operated at 900–1200 °C using the MWAHEs. SiC, which exhibits superior ethylene selectivity for HDPE pyrolysis (vide supra), was used as the MWAHE in this setup. The decomposition gas yield was determined by measuring the weight decrease in the reaction tube from start to end of the process relative to the weight of the polyolefin sample. A gas chromatograph with a thermal conductivity detector (GC-TCD) was used to quantify the vol.% of carbon monoxide (CO), methane, CO2, acetylene, ethylene, ethane, propylene, propane, n-butane, and i-butane.

Operating the first and second stages of the two-step continuous pyrolysis system at 620 °C for and 950 °C, respectively, with a 0.2 g HDPE sample, resulted in an impressive 95% gasification yield with 50% selectivity for ethylene formation. Surprisingly, tar or char were not observed in the reaction tube after the first and second stages, which is expected when using conventional heating. Operating the respective stages at 450 and 1200 °C resulted in the desired light gas in up to an 84% yield. The composition shifted slightly to lighter hydrocarbons and a higher acetylene content (Fig. 6b). This result demonstrates that acetylene might be derived from ethylene and that extremely high temperatures trigger a high-energy-barrier reaction route26,27.

Figure 6c shows the temperature profiles of the two stages. The electric furnace and MW system heating were controlled using PID because good temperature responsiveness may be advantageous in recycling waste plastics, where the composition of the waste stream varies, and agile temperature control is necessary.

Pyrolysis of actual waste plastics

The practicality of HDPE MWAHE-assisted MW pyrolysis for treating residential plastic waste containing impurities such as biomass and metal nanoparticles was subsequently investigated. Pyrolysis tests were conducted on thin plastic films originating from household waste in Tokyo. The thin plastic films were cut into 0.5–2.0 cm squares and were not mechanically or chemically purified (Fig. 7a). See supplementary information S-III for detailed experimental setup (Fig. S1).Fig. 7 (a) Photographic image of the u-shaped quartz flow reactor used for the chemical recycling of plastic waste with AC1 (0.5 g) in the MW applicator. (b) Incandescence of the reactor under MW irradiation. (c) Gasification yield and composition of the light chemicals.

Pyrolysis was conducted using a MW input power of 500 W for 4 min, and the pyrolysis process proceeded smoothly (Fig. 7b), resulting in 53% gas yield and 52% ethylene selectivity, while effectively minimizing CO2 formation (Fig. 7c). These findings suggest that PE is the primary constituent of residential plastic waste, which corresponds with the industrial plastic production statistics in Japan32. GC-TCD was employed to quantify the selectivity ratio of light gases (31% methane, 2% CO2, 6% acetylene, 52% ethylene, 3% ethane, 5% propylene, 1% propane, and 0% i-butane). In addition to the pyrolysis gases yellow-to-brown oil and black residue also formed. Subsequent a gas chromatograph with mass spectrometry detector (GC-MS) analyses of the tetrahydrofuran (THF) soluble oils and residues revealed the presence of polyaromatic hydrocarbons, such as naphthalene, biphenyl, and anthracenes, indicating that the formation of tar and char was unavoidable, as observed in conventional pyrolysis methods. However, this process is promising because it produces fewer low-value byproducts13.

Conclusion

The feasibility of flash pyrolysis using MW as an efficient heat source for the rapid deconstruction of common bulk polyolefins into monomers and valuable light gases was investigated in this study. Mixing HDPE with a carbon material as the MWAHE and applying MW irradiation directly decomposes HDPE to light olefins owing to the rapidly generated heat. The MW input energy in the single-step pyrolysis process was more influential than selection of carbon materials for the pyrolysis results.

The two-step continuous pyrolysis system designed in this study, in which conventional heating is combined with MWAHE-assisted MW pyrolysis, is suitable for processing continuous feedstock while maintaining the pyrolysis temperature and product composition. The two-step cracking of PE, in which crude cracking was performed at 620 °C and second-stage cracking was performed at 950 °C, resulting in a gasification yield > 95% with good selectivity toward ethylene and without tar and char generation, was demonstrated. This result shows that the two-step cracking system is potentially suitable for scaling up owing to the ease of maintaining the cracking temperature for chemical engineering. Using a higher pyrolysis temperature of 1200 °C for the second-stage affords ethylene and acetylene as valuable products, demonstrating that the product composition ratio can be altered by tuning the pyrolysis temperature, which in turn demonstrates the importance of uniform heating in this process. Additionally, this study demonstrates that MW flash pyrolysis can recover more than 50% of light gases from residential plastic waste with high selectivity toward ethylene.

Such a rapid and highly selective pyrolysis process with excellent temperature responsiveness to MW heating offers a chemical recycling method for complex waste plastic mixtures. The excellent pyrolysis results obtained using a potentially sustainable electric heating source such as MWs could become a valuable tool for tackling the difficulties in the development of a next-generation chemical recycling process. Waste plastics can be used as feedstock to production new plastics by diverting them from landfills or oceans to appropriate industries.

Experimental setup

MW system

The setup of the single-mode MW applicator (SM2; Ryowa Electronics Co., Ltd., MR-2G-100) is shown in Fig. 8. The system consists of a variable frequency semiconductor MW generator (2.45 ± 0.05 GHz, maximum power of 100 W), coaxial cable, slug tuner, and cylindrical resonator (TM010 single mode). The E field intensity reaches its maximum value on the central axis of the resonator33. Furthermore, the computer control algorithm can tune the resonance frequency and MW output to maintain the agile temperature of the MWAHE in the reaction tube, which is adjusted by feedback from a thermometer33. The internal temperature of the reaction tube was measured through a viewport using a quartz transmission-type radiation thermometer (Japan Sensor FLHX-TNE022) with a temperature measurement range of 220–2000 ℃. See supplementary information S-IV for detailed experimental setup.Fig. 8 Schematic of the quartz flow reactor filled with sample materials (left) and the experimental equipment configuration including the MW equipment (right).

General PE pyrolysis procedure

A 180-mm-long and 4-mm-inner diameter quartz tube was used as the flow reactor. A mixture of PE particles and MWAHE (~100 mg) was placed inside the reaction tube, with glass wool packed above and below the mixture. Nitrogen gas was introduced from the bottom of the tube at 20 mL min−1 to create a nitrogen atmosphere inside the reaction tube, and the tube was purged for 1 min. A gas bag (vinyl alcohol polymer film bag; GL Sciences Co. Ltd. Smart Bag PA CEK-0.5) was connected to the top of the reaction tube. Simultaneously, the double-slug tuner was adjusted to maximize the difference between the travelling and the reflected waves). The reaction was allowed to proceed for 3 min, during which time the polyolefin decomposition gas was collected in the gas bag. The yield of the obtained decomposition gas was determined from the weight decrease of the reaction tube at the end of the reaction relative to the original weight of the PE. GC-TCD was used to quantify the vol% of CO, methane, CO2, acetylene, ethylene, ethane, propylene, propane, and butane in the gas components. See refer to the supplementary information S-V for detailed calculation methods.

Supplementary Information

Supplementary Information.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-024-71958-3.

Acknowledgements

We wish to express our gratitude for the financial support provided by the Mitsubishi Chemical Corporation and for the helpful discussions with Mr. Ken Oyachi of the Mitsubishi Chemical Corporation.

Author contributions

H.K., K.F. and S.H. prepared and designed the study. H.K., A.S., K.H., A.N., K.F., Y.T., and S.H. designed the detailed experiments, and evaluated and analyzed the performance data. H.K. drafted the original manuscript which was subsequently re-examined following various discussions regarding data interpretation by H.K., K.F. and S.H.

Data availability

All raw data used in the Figures are included in the supplementary information files. No other generated data is provided in this paper.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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