1. Introduction
Since the emergence of coronavirus disease 2019 (COVID-19), face masks have become an essential line of defense for public health protection. According to the global daily mask model proposed by Nzediegwu and Chang
[1], approximately 2064 tonnes (t) of face masks are discarded every day. It is anticipated that wearing face masks will become the new norm
[2]. Face masks are primarily made of plastics. Improper disposal of these masks can pose significant ecological threats, including the spread of viruses
[3] and the exacerbation of microplastic pollution (i.e., plastic fragments and particles with a diameter of less than 5 mm)
[4],
[5]. Studies
[4],
[6],
[7] have demonstrated that the infectious virus can survive on face masks for several days
[3], potentially endangering wildlife, marine life, and even humans. Furthermore, face masks discarded by medical professionals may carry numerous pathogenic microorganisms and require careful handling.
The current technologies used for treating medical waste, such as high-temperature incineration and sterilization followed by landfilling, present environmental, and economic challenges. For example, high-temperature incineration generates hazardous gases, and the slag produced from waste incineration can be a potential source of microplastics
[8]. These environmental concerns require the installation of expensive pollution-control devices
[9]. The medical waste treatment industry—particularly in developing countries—faces significant challenges due to high costs and a lack of valuable product output. Therefore, current disposal methods for face masks not only fail to add value but may also cause substantial environmental impacts and resource consumption.
Studies
[10],
[11],
[12] have explored sustainable valorization methods for discarded face masks. Thermochemical conversion technologies, mainly pyrolysis, are promising methods for valorizing polymer waste. Researchers
[13],
[14] have proposed using these methods to convert discarded face masks into high-value products. Several studies have investigated the pyrolysis properties
[15],
[16],
[17] and kinetics
[18],
[19] of face masks when used to produce combustible hydrocarbon gas. Other researchers have used zeolite (HZSM-5, H-Beta, and HY) in catalytic fast pyrolysis to prepare aromatic hydrocarbons
[20]. However, research into the valorization of discarded face masks is still in its infancy. This study presents a new technology for converting discarded face masks into value-added green products, such as carbon nanotubes (CNTs), hydrogen, tar, and recycled iron, through the catalytic pyrolysis process. This technology was inspired by the plastic refining process
[21] and aims to meet the demand for electric and hydrogen vehicles in the context of global carbon neutrality.
Numerous new catalysts have been synthesized, and efficient reactors have been designed to enhance the yield of CNTs and hydrogen by promoting the cleavage of C–H bonds. However, these research efforts often overlook the tradeoff among technological, economic, and environmental benefits
[22],
[23]. Integrated assessment of the technical performance, economics, and environmental impacts is crucial for technology deployment. This approach has been extended to emerging material
[24] and energy technologies
[25],
[26]. Traditional integrated assessments are mainly carried out in the application stage of technologies (e.g., for demonstration projects when techno–economic data for industrial-scale system are available), which delays the observation of adverse effects and may result in unsatisfactory technology developments
[27]. Conducting a proper pre-assessment before demonstration can minimize unforeseen impacts related to material selection and system design decisions. However, it is challenging to conduct a pre-assessment of catalytic pyrolysis technology at the research and development (R&D) stage due to the absence of techno–economic data for assumed industrial-scale systems and a robust analytical capability for evaluating the impact of data uncertainties
[28].
This study presents an economic–environmental hybrid pre-assessment method based on a coupling model that considers the tradeoffs among technical design, systemic economic, and life-cycle environmental analysis in order to achieve system optimization in the R&D stage of technologies. Key data for industrial-scale technologies are predicted using process simulation software (i.e., Aspen Plus (Aspen Technology, Inc., USA)), linking laboratory-scale mechanisms and performance. A life-cycle assessment (LCA) and sensitivity analysis (SA) are used to overcome the challenge of the lack of techno–economic data and to reduce the impact of data uncertainty based on experimental and simulation results. In this study, we compare two widely recognized catalysts (Fe/Al
2O
3 and FeNi/Al
2O
3)
[29],
[30] and two reaction methods (conventional heating and microwave-assisted heating) using the proposed pre-assessment method. System performance (i.e., reaction performance, product properties, and service life of catalyst), systemic economic benefits (e.g., the product yields, catalyst price, and product price based on product performance), and environmental performance (e.g., the availability of the resource and the life-cycle environmental impacts associated with the technology design) are evaluated using the coupled model. The aim of this study is to identify the economic and environmental caveats of new and conventional catalytic pyrolysis technologies in order to provide guidance for future research.
2. Methods and materials
2.1. Preparation of the catalysts
The catalysts were prepared as follows:
Fe/Al
2O
3 catalyst: a total of 7.214 g of Fe(NO
3)
3·9H
2O was dissolved in 200 mL of water, followed by the addition of 10 g of γ-Al
2O
3. The mixture was stirred for 2 h and dried in an oven at 105 °C under an air atmosphere for 24 h to obtain a solid. The resulting solid was calcined for 2 h at 800 °C under an air atmosphere
[31],
[32]. The iron loading was set at 10 wt% for the catalyst.
FeNi/Al2O3 catalyst: the preparation process was the same as that of the single metal catalyst, except for the use of 5.411 g of Fe(NO3)3·9H2O and 1.376 g of Ni(NO3)2·6H2O. The metal loading was set at 10 wt%, and the iron (Fe) to nickel (Ni) ratio was 3:1.
2.2. Catalysis system
A schematic diagram of the pyrolysis–catalysis system with microwave-assisted heating is provided in Fig. S1 in Appendix A. The system consisted of a feeding unit, a pyrolysis–catalysis unit in a vertical tube (600 mm height, 45 mm inner diameter), an electric furnace, a microwave furnace, a condensing unit, a gas cleaning unit, and gas offline measurement. Prior to each trial, a mask weighing 3.23–3.31 g was placed in the pyrolysis zone, and a catalyst of the same mass was placed in the catalysis zone. High-purity nitrogen (N2; 99.99%) was supplied as an inert gas at 100 mL·min–1. Based on the results of a thermogravimetric analysis (TGA), the pyrolysis furnace was set at 500 °C to ensure the full decomposition and thorough sterilization of the face masks. The catalytic zone was heated to a preset reactor temperature of 380 °C with a free power range of 200–1000 W. Following the pyrolysis–catalysis process, carbon residue and waste metal were obtained in the pyrolysis zone, and a mixture of the catalyst and CNTs was collected in the catalytic zone. Condensable volatiles (tar) were liquefied in an ice–water condenser, while non-condensable volatiles (gas) were purified by being passed through water, dried over silica gel, and finally collected in a gas sampling bag. Each experiment was repeated three times. The cycling experiments were repeated using a spent catalyst instead of a fresh one. The spent catalyst was not separated from the CNTs and was used directly in subsequent experiments.
The experimental procedure for the conventional heating catalysis system, as shown in Fig. S2 in Appendix A, was identical to that of the microwave-assisted catalysis, except for the catalytic zone. An electric furnace was used instead of a microwave oven, and the reactor temperature was kept at 800 °C.
2.3. Temperature measurement and characterization
The temperature of the reactor in the microwave oven and electro–furnace was measured using an infrared pyrometer. It should be noted that the pyrometer can only measure the external surface temperature of the reactor. The surface functional groups of the components of the face mask were studied using Fourier-transform infrared spectrometry (FTIR; VERTEX 70, Bruker, Germany) with 32 scans and a 4 cm–1 resolution between 400 and 4000 cm–1. The pyrolytic behavior was investigated through TGA (STA-449F3, NETZSCH, Germany). Approximately 10 mg of the sample was heated at 10 °C·min–1 from 50 °C to 800 °C in a nitrogen atmosphere. The crystal structure was determined by X-ray diffraction (XRD; X’Pert PRO, Malvern PANalytical B.V., the Netherlands). Temperature-programmed reduction was performed using TGA. Approximately 20 mg of the sample was pre-treated at 150 °C for 20 min and then heated to 900 °C at 10 °C·min–1 under a reduction atmosphere (5 v% H2 and 95 v% N2). The catalysts’ size and surface morphology were imaged using a scanning electron microscopy (SEM; Nova NanoSEM 450, FEI, USA) at an acceleration voltage of 10 kV. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) images were obtained using a JEM-2100F (JOEL, Japan) with Super-X energy dispersive X-ray spectroscopy (EDX; Bruker, USA) at an acceleration voltage of 200 kV. Raman spectroscopy was performed using a LabRAM HR800 (Jobin-Yvon, Horiba, France) instrument with a Nd: yttrium aluminum garnet crystal (YAG) laser at 532 nm. Temperature-programmed oxidation (TPO) was performed in a TGA. Approximately 20 mg of the sample was heated to 800 °C at 10 °C·min–1 with air purging. The composition of the gas product was analyzed using a dual-channel micro-gas chromatography (GC) system (A91 GC, Panna, China) equipped with a thermal conductivity detector (TCD) and flame ionization detectors (FIDs). The hydrogen yield can be determined by the ratio of the concentrations of nitrogen and hydrogen when a known amount of nitrogen is present.
2.4. The characteristics of face masks
Surgical masks were selected as a typical raw material in this study due to their widespread use. Experimental samples of surgical masks with ear loops (HYNAUT, China) measuring 17.5 cm × 9.5 cm were used. The main effective component of the surgical masks was a filter, consisting of two layers of spunbonded fabric and one layer of melt-blown fabric. Auxiliary accessories include ear straps and nose clips. After mechanical dismantling, as shown in
Fig. 1(a), a mask consisted of a 64.31 wt% filter, 15.07 wt% ear strap, 12.51 wt% nose clip, and 8.11 wt% metal wires. Considering that the pollution potential of face masks is mainly related to the polymer components, while the metal wires can be easily recycled, the polymer fraction was the main target. FTIR was used to determine the surface functional groups of each part (
Fig. 1(b)). The filter and the nose clip exhibited similar infrared responses, with two main vibration ranges at 2750–3000 cm
–1 and 750–1500 cm
–1, which were attributed to some of the tensile and flexural vibrations of alkanes. The nose clip and filter conformed to the infrared spectrum of polypropylene (PP)
[33]. The ear straps showed more complex functional groups, including C=O/C=N (1615–1750 cm
–1), C–O/C–N (980–1300 cm
–1), and the C–H (810–920 cm
–1) in arene.
A TGA was performed to assess the basic thermal decomposition profiles of the face mask (
Fig. 1(b)). The pyrolysis of the plastic waste was divided into three main stages: the dry stage (100–300 °C), pyrolysis (300–500 °C), and the curing stage (> 500 °C). This is consistent with other findings on mask pyrolysis
[15]. During the first stage, there was a small mass loss for the nose clip, but no obvious mass loss for the filter and ear strap. The second stage was the main decomposition range at which rapid depolymerization occurred, releasing large volumes of volatile matter (consisting of short-chain polymers and monomers) and gases (including H
2). The remaining solid residue slowly decomposed further in the third stage. When the individual components of a face mask were pyrolyzed separately, the filter and nose clip rapidly decomposed to less than 2 wt% of the residue in the second stage. However, the ear strap displayed a different thermal decomposition profile, with solid residue after the second phase of more than 20 wt%. According to the differential thermal gravity (DTG) curves, the temperature of the maximum decomposition rate for the three components decreased in the following order: filter (449 °C) > nose clip (440 °C) > ear straps (429 °C). In general, all the volatiles had been released by the time the samples reached 500 °C.
2.5. Economic–environmental hybrid pre-assessment method
Traditional economic–environmental assessments typically focus on industrial-scale systems, using data (e.g., costs and energy/material flows) from demonstration projects
[34],
[35]. Limited by data availability, these traditional assessments are not applicable to novel technologies in the R&D phase. The data available determine the reliability of an economic–environmental assessment. Compared with the use of empirical data or the direct scaling up of experimental data, the application of a process simulation (e.g. Aspen Plus) could be a viable option to obtain data such as equipment costs, operating costs, investment costs, and energy/material flows in an assumed large-scale system for a novel technology
[24]. However, such a method cannot provide some important economic data on the micro-components and microstructures of products, which may lead to misleading assessments and hinder the sustainable development of related research areas. The proposed hybrid economic–environmental pre-assessment (
Fig. 2) integrates research on experimental studies, process simulations, and economic–environmental assessments. Experimental studies provide essential data for building process simulation models and assessing economic–environmental impacts. These simulation models, informed by experiments, also provide a complete dataset for understanding the economic costs and energy/material flows needed in the assessment. Therefore, this approach allows for a thorough hybrid pre-assessment, which is critical in minimizing unanticipated impacts in the development of innovative technologies such as catalytic pyrolysis.
This study simulated the catalytic pyrolysis system for face mask treatment using Aspen Plus V11 software (Aspen Technology, Inc., USA). Table S1 in Appendix A provides detailed information on the modules, parameters, material inputs and outputs, and energy and water consumption for the catalytic pyrolysis model.
An LCA model for the environmental analysis was built using Gabi Version 5.0 (PE-international, Germany) with the impact assessment method of TRACI 2.1 (USA). The functional unit was 1 kg of face mask production. The environmental impact indicators considered in the model were the ozone depletion potential (ODP), acidification potential (AP), eutrophication potential (EP), human toxicity (HT), ecotoxicity (EC), fossil fuel depletion (FFP), and greenhouse warming potential (GWP). Among these, we especially focused on the GWP profile (expressed as greenhouse gas (GHG) emissions intensity; unit: kilogram CO2 equivalent per kilogram of face masks (kgCO2e·kg−1face mask) for the whole system, including the emissions of carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4). A detailed input and output inventory and the calculation methods are shown in Sections S1–S2 in Appendix A. The life-cycle reduction potential of environmental impact () for the system was calculated by accounting for two components: the net life-cycle environmental impact () and the traditional products offset ():
where i indicates the environmental impact categories, including ODP, GWP, AP, EP, HT, EC, and FFP; and considers the net life-cycle environmental impact (i) associated with the PP fabric, electret-treated melt-blown fabric, spunbond fabric, face mask processing and treatment, and carbon fixed by CNTs (related to GWP). The waste treatment included discarded face mask incineration and landfilling, conventional pyrolysis with an Fe/Fe–Ni catalyst, and microwave-assisted pyrolysis with an Fe/Fe–Ni catalyst. refers to the i environmental impact saved from catalytic pyrolysis products compared with the traditional products from fossil fuel and ore resources.
For the economic analysis, the equipment, installation, and utility costs were collected from the simulation results of the economic analysis module in Aspen Plus (i.e., Aspen process economic analyzer). The system generated revenue from various sources including the production of CNTs, pyrolysis gas, tar, scrap iron, and carbon reduction. In addition, the system earned income from processing discarded face masks, which are considered medical waste, for a fee paid by hospitals. The revenue from carbon reduction was calculated based on the environmental analysis (i.e., GWP). The price of CNTs was primarily determined by their quality, such as diameter, in this study. CNTs with a diameter of 10–30 nm were priced at 5.80 × 10
4 USD·t
–1, while those with a diameter of 30–80 nm were priced at 4.35 × 10
4 USD·t
–1. These prices were assumed based on TIMENANO’s product offerings in 2022
[36]. Tables S2–S3 in Appendix A provide detailed economic evaluation parameters and financial indicators.
3. Results
3.1. A pyrolytic–catalytic system for the conversion of face masks into CNTs and hydrogen
The XRD patterns of FeNi/Al
2O
3 and Fe/Al
2O
3 are shown in Fig. S3 in Appendix A. The weak and wide diffraction peaks of the two types of catalysts represent good dispersion and small particle size. Fe
2O
3 and Al
2O
3 phases were observed in Fe/Al
2O
3. New phases of NiO, Ni–Fe–Al, and Fe–Al were observed for FeNi/Al
2O
3. The presence of cubic crystalline Ni–Fe and Ni–Fe–Al detected at 35.77° and 36.40° indicated a strong interaction between the active components Ni, Fe, and the carrier. This interaction is essential for the formation of hydrogen
[32]. The temperature-programmed reduction (TPR) of FeNi/Al
2O
3 and Fe/Al
2O
3 is shown in Fig. S4 in Appendix A. The reduction process of FeNi/Al
2O
3 was concentrated at a higher temperature range, suggesting a stronger interaction between the bimetallic catalyst and the support in comparison with that of the mono-metal catalyst, which can be attributed to the presence of eutectic metal states (Ni–Fe–Al) in the bimetallic catalyst. The interaction between the metal and support is beneficial to the dispersion of the active metal components
[37].
Two catalytic methods were selected: conventional heating catalysis (i.e., Fe-CH and FeNi-CH) and microwave-assisted catalysis (i.e., Fe-MW and FeNi-MW). Unlike a conventional heating setup that relies on heat transfer from a heating source by convection, conduction, and radiation, with heat first heating the material surface, microwave-assisted heating offers volumetric heating resulting from the interaction of microwaves (electromagnetic radiation) with the target material. As a microwave absorber, the catalyst heats up throughout the whole volume, and the whole process has a higher thermal efficiency
[38] and more uniform heating. After successive cycle tests under the same conditions as those shown in Figs. S5–S7 in Appendix A, the consumption of the catalysts and the average yield of the pyrolysis–catalytic system were obtained, as shown in
Table 1. The yield of hydrogen and CNTs ranged from 0.486 to 0.715 m
3·kg
–1, with a weight percentage of 28.09–38.49 wt%.
Compared with the single-metal catalyst, the bimetal catalyst exhibited higher catalytic activity and better stability. FeNi-MW had the highest catalytic activity for converting organics to CNTs and hydrogen, with the highest yields of 38.49 wt% and 0.715 m
3·kg
–1, respectively. This finding suggests that the catalyst promoted the cracking of hydrocarbons by activating the C–H bonds during the pyrolysis of the intermediate products of the masks (e.g., long-chain hydrocarbons and aromatics). This resulted in more carbon deposition and hydrogen formation at the expense of tar yield. In particular, the initial catalytic activity of FeNi-MW resulted in a CNT yield of 48.76 wt% and a hydrogen yield of 0.080 m
3·kg
–1 from the face masks (Table S3). Regarding the consumption of the catalysts, the amount of catalyst used ranged from 2.14 to 30.63 kilogram per 100 kilograms face masks. Here, the minimum amount of catalyst was needed for FeNi-CH, which had the best stability, still maintaining 75% of its initial activity after 43 cycles (Fig. S5). FeNi-CH was also better in comparison with the commercial catalyst Ni/La
2O
3, which consumed 11.2 kg of catalyst when 100 kg of natural gas was reacted to produce CNTs
[39]. In general, the four studied catalytic conditions showed good catalytic activity for hydrogen and CNT production. FeNi-MW had higher initial catalytic activity, while FeNi-CH showed better catalytic stability and required the least amount of catalyst.
3.2. Properties of the CNTs
CNTs were the main solid target products of these processes. This material has a wide range of commercial applications, such as aerospace materials, energy storage, and separation technology
[40],
[41]. This work presents an inexpensive technology for the large-scale preparation of multi-walled CNTs using cheap discarded face masks and the abundant reserves of iron and alumina.
Typical SEM, TEM, and HR-TEM images of the CNTs (including the used catalysts) are provided in
Fig. 3, showing the dense, long, and curly filamentous carbon covering the surface of the catalysts. The filamentous carbons are several microns in length. Among them, the filamentous carbon on FeNi-MW had a smoother surface, while the fiber-like carbon on FeNi-CH and Fe-CH presented a more bamboo-like morphology (Figs. S8 and S9 in Appendix A). The outer diameter of the CNTs was mainly concentrated between 20–30 nm, except for those on the sample Fe-MW, which exhibited a coarser average outer diameter (> 50 nm). In the TEM picture, a tube wall with 22 carbon layers is 7.35 nm, corresponding to a layer spacing of 0.34 nm, which is consistent with the spacing of a graphite layer. Complete and incomplete bamboo knots were observed in the CNTs on FeNi-CH and Fe-CH. In general, the CNTs on FeNi-MW and FeNi-CH showed a more uniform outer diameter of approximately 26 nm and an inner diameter of around 10 nm.
The Raman spectra of the CNTs showed three distinct peaks at D, G, and G', as shown in Fig. S10 in Appendix A. The D peak indicated defects and represented a disordered structure in the graphite. The G peak was attributed to the stretching vibration mode with E2g symmetry in graphite
[42],
[43]. Ratio of the intensity of D-Raman peak (
ID) and G-Raman peak (
IG) was used to reflect the degree of graphitization and purity. FeNi-MW, FeNi-CH, and Fe-CH all exhibited a low
ID/
IG value, with FeNi-CH having the lowest value of 0.65. This indicated a high degree of graphitization and purity. The values of
ID/
IG decreased in the following order: Fe-MW > Fe-CH ≈ FeNi-MW > FeNi-CH. The G' peak represented second-order phonon resonance
[22], which is related to the graphene layer and can be used to reflect the purity of carbon deposition on catalysts
[32]. Among the samples, FeNi-CH showed the most prominent and sharp G' peak, which was consistent with the observations from the
ID/
IG ratio.
TPO was carried out in a TGA, as shown in Figs. S11 in Appendix A. The mass loss observed after reaching 450 °C was attributed to the combustion of solid carbon materials. The thermal weight loss indicated the carbon content of the product. For crude CNTs, the FeNi-MW and FeNi-MW showed the maximum weight loss, indicating high carbon contents. The thermogravimetric curves showed that weight loss occurred mainly above 600 °C, which is the ignition temperature of graphitic carbon
[30]. Different samples showed distinct TPO peaks for differential mass loss. All oxidation peaks were observed to be higher than 650 °C, indicating high thermal stability. These results, along with the Raman spectra, provide evidence of the high purity of the CNTs.
3.3. Economic–environmental analysis of catalytic pyrolysis technologies for discarded face masks
Based on the experimental results, a process simulation model for preparing CNTs via the catalytic pyrolysis of discarded face masks was designed. The model comprised three parts: catalyst preparation, the catalytic pyrolysis of face masks, and the purification of CNTs (Fig. S12 in Appendix A).
Fig. 4 provide an evaluation of the economic benefits of the catalytic pyrolysis systems and the environmental benefits of face mask production and treatment based on the simulated models.
Fig. 4(a) shows the life-cycle environmental profiles for the four catalytic pyrolysis scenarios, including the reduced environmental impacts resulting from substituting for the traditional method to produce pyrolysis products. The results indicate that performing catalytic pyrolysis on the discarded face masks reduced the environmental impacts, except for EP. The significant impact of the catalytic pyrolysis on EP was mainly caused by the emission of nitrogen, which was used as a protective gas in the purification process of the crude CNTs. In addition, the microwave-assisted pyrolysis with the Fe–Ni catalyst system (Scenario 4) and the conventional pyrolysis with the Fe–Ni catalyst system (Scenario 2) had a better environmental performance than the other two scenarios (Scenario 1: conventional pyrolysis with the Fe catalyst system; Scenario 3: microwave-assisted pyrolysis with the Fe catalyst system).
The analysis of the GWP result (life-cycle GHG emissions) shown in
Fig. 4(b) reveals that Scenario 2 has the lowest
(3.47 kgCO
2e·kg
−1face mask) owing to the high stability of the Fe–Ni catalyst in the conventional heating system. However, it should be noted that the current production of CNTs using fossil fuels is a carbon-intensive process. The
of applying the catalytic pyrolysis systems (Scenarios 1–4) ranged from 39.50 to 53.45 kgCO
2e·kg
−1face mask , with over 90.00% of the emissions resulting from the substitution of fossil-fuel-based CNT products. Thus, Scenario 4 (–49.52 kgCO
2e·kg
−1face mask) performed better than Scenario 2 (–45.27 kgCO
2e·kg
−1face mask) in terms of the life-cycle reduction potential of GWP impact (
), taking into account both
and
.
Fig. 4(c) shows the economic performance of Scenarios 1–4. It was found that the Scenario 2 system reduced the operating costs by 42.39% by improving the stability of the Fe–Ni catalyst compared with Scenario 4. However, the Scenario 4 system increased the system revenue by 24.88% compared with Scenario 2. These results are closely related to the increase in product yield, particularly for the CNTs. Thus, based on the economic performance, Scenario 4 is the best option, with a net present value (NPV) of 8.429 × 10
7 USD, followed by Scenario 2 with an NPV of 6.893 × 10
7 USD.
Moreover, the sensitive economic and GHG emissions effects of the four scenarios were explored, as shown in Section S2 in Appendix A.
Fig. 4(d) displays the potential range in the net present value per kilogram of mask production (NPV
unit) and GHG emissions reduction. Scenario 4 has greater potential for economic benefits and GHG reduction; Scenario 2 would bring larger economic benefits compared with Scenario 3, while the carbon reduction potential of these two scenarios is not significantly different. However, both Scenarios 1 and 3 have their own disadvantages in terms of economic performance and environmental impact. Generally, in the current context, catalytic pyrolysis technologies for face masks could provide great economic benefits and result in GHG reduction, especially compared with the GHG emissions resulting from the incineration and landfilling of discarded face masks.
A sensitivity analysis (SA) was also conducted to examine the impact of key parameters (E1–E5 and L1–L4, as shown in
Table 2) on the system’s NPV and GHG emissions reduction. From an economic perspective, the annual operation time and the price of CNTs are crucial factors. When the price of CNTs is lower than 1.49 × 10
4 USD·t
−1, Scenario 2 would be a better choice in the market compared with Scenario 4 (Fig. S13 in Appendix A). In addition, increasing the system efficiency and annual working time significantly boosted the systems’ economic profits. From an environmental perspective, offsetting the traditional products would be the key factor. With a reduction in the carbon emissions intensity of traditional products, the GHG reduction from the proposed catalytic pyrolysis of discarded face masks would decrease dramatically. However, if the face masks were made from bio-based plastics, the proposed technology could achieve carbon removal regardless of traditional product offsetting in the future.
4. Conclusions
Ensuring sustainable social development with net-zero carbon emissions has become a prerequisite for new technology development. This study focuses on the treatment and reuse of face masks, converting discarded face masks into high-value products (i.e., CNTs, hydrogen, tar, and recycled iron). Compared with traditional incineration and landfill technologies for managing hazardous waste, the catalytic pyrolysis technology developed in this study for producing CNTs from discarded face masks offers several advantages. From a circular economy perspective, high-temperature pyrolysis not only effectively eliminates bacteria to meet the requirements of medical hazardous waste treatment but also transforms discarded face masks into high-value products. From an atomic economy perspective, this technology aims to separate the C and H elements in face masks to obtain high-value CNTs and high-grade pyrolysis gas (mainly hydrogen and methane). Catalysts are used to activate C–H bonds to produce hydrogen gas, while the carbon atoms are arranged into CNTs via vapor–liquid–solid. In comparison with conventional chemical vapor deposition (CVD) methods for preparing CNTs, catalytic pyrolysis does not require additional methane or propylene. Moreover, the energy from the pyrolytic volatiles (> 300 °C) can partially offset some of the high energy costs of the deposition process. Furthermore, hydrogen is often used in conventional CVD processes to produce a reducing atmosphere, introducing industrial operational risks and additional costs.
Comparing microwave-assisted heating with a conventional heating method, it was found that microwave-assisted heating had higher thermal efficiency and the reactor temperature was lower, at 380 °C, greatly reducing energy consumption. Furthermore, microwave-assisted catalysis resulted in higher initial catalytic activity, leading to the production of more CNTs and hydrogen, even if this meant sacrificing tar. However, the disadvantage was that more bimetallic catalyst was consumed because of the low stability of the catalyst in the microwave atmosphere. Compared with the monometallic catalyst, despite the introduction of the more expensive nickel element, the bimetallic catalyst exhibited higher activity and stability.
This study also proposed an economic–environmental hybrid pre-assessment method based on the experimental results. The technological design was expanded to a system model through chemical process simulation and was combined with a systemic comprehensive evaluation. This approach optimizes the technological, economic, and environmental analysis during the basic process of technological design. Based on the results, microwave-assisted pyrolysis with an Fe–Ni catalyst is the most economically viable and environmentally sustainable option in the current market and technological background. However, the price of CNTs is expected to gradually decrease as the market demand for them continues to grow and catalytic pyrolysis technology becomes more mature. This makes conventional catalytic pyrolysis technology with an Fe–Ni catalyst an attractive option, especially when CNTs become more affordable.
Future research endeavors should expand the application of catalytic pyrolysis to include a broader range of separated plastic wastes, such as those originating from the industrial sector and packaging materials, to produce CNTs that are technically and economically viable for various applications, including integrated circuits, hard disk drives, and conductive plastics. Efforts should concentrate on enhancing the cost-effectiveness of catalyst preparation, improving the efficiency of catalyst and CNT separation, and advancing the development of low-energy pyrolysis technologies. Innovative heating methods, such as microwave, electromagnetic, and Joule heating, show promise for reducing the energy requirements for carbon deposition and thereby further lowering the environmental impact and increasing the sustainability of this process. By addressing these challenges, this field can move closer to realizing the full potential of catalytic pyrolysis in transforming plastic waste into valuable resources, thereby contributing to a circular economy and mitigating the environmental burden of plastic waste.
CRediT authorship contribution statement
Hewen Zhou: Writing – review & editing, Writing – original draft, Software, Methodology, Formal analysis. Sunwen Xia: Writing – review & editing, Writing – original draft, Resources, Methodology, Conceptualization. Qing Yang: Supervision, Funding acquisition, Conceptualization. Huamei Zhong: Software, Methodology. Wang Lu: Investigation, Data curation. Ning Cai: Investigation, Data curation. Ondřej Mašek: Writing – original draft. Pietro Bartocci: Writing – original draft. Francesco Fantozzi: Supervision. Chao Liu: Data curation. Bo Miao: Data curation. Qie Sun: Data curation. Haiping Yang: Supervision, Funding acquisition, Conceptualization. Hanping Chen: Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (52076099, 52306257, and 72293601). We also would like to thank members of the Harvard-China Project on Energy, Economy and Environment for useful comments and suggestions, and the Harvard Global Institute for an award to the Harvard-China Project on Energy, Economy, and Environment.
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.eng.2024.11.016.
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(1971KB)
