Microwave-Assisted Pyrolysis of Waste Plastics: History
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Contributor:
Changze Yang
,
Hui Shang
,
Jun Li
,
Xiayu Fan
,
Jianchen Sun
,
Aijun Duan

Microwave treatment, owning specific interactions between the electric field and the molecules of treated materials, presents potential advantages in the application of plastic pyrolysis. High value-added liquid oil, gas and solid carbon can be obtained from microwave-assisted pyrolysis (MAP) of plastics. Factors that affect the distribution of pyrolysis products of plastics include the properties of plastics, microwave treatment parameters, microwave absorber, co-pyrolysis, catalysts and reactor design. MAP of plastics has broad application prospects, and large-scale pyrolysis processing devices need to be developed. At the same time, the research platform of MAP simulation of plastics still needs to be developed.

  • pyrolysis
  • waste plastics
  • microwave-assisted pyrolysis

1. Introduction

The energy of microwave-assisted pyrolysis (MAP) comes from microwave radiation, the heat transfer mode is gradually diffused from the internal heating of the substance to the external, and the heat source and raw materials are heated without contact. By contrast, conventional pyrolysis uses an electric heater or a burner, and the plastic is heated from outside to reach the desired temperature through conduction. This method has uneven temperature distribution, serious heating inertia, and high energy consumption. The disadvantage of solar pyrolysis is that it is greatly affected by the weather. Plasma pyrolysis [1] gasifies and decomposes plastics in a short time by ionizing gas to generate a high temperature as the heat source. This method has a high energy density and an extremely high working temperature, which greatly limits its popularization and application. These four types of pyrolysis schematic diagrams are shown in Figure 1.
Processes 11 01487 g001 550
Figure 1. Schematic diagram of (a) MAP, (b) conventional pyrolysis, (c) solar pyrolysis, and (d) plasma pyrolysis.
In the traditional plastic pyrolysis, due to uneven heating, the excessive local temperature will lead to excessive pyrolysis of local plastics to produce non-condensable small molecules and coking, while an insufficient internal pyrolysis temperature will lead to more long-chain hydrocarbons, resulting in lower recovery values. The MAP technology can rapidly heat the inside of materials and improve the heating efficiency. The products of MAP of plastics tend to be medium and short chain hydrocarbons, providing maximum possibility for the replacement of transport fuels with bio-oil/methanol blends in a CRDI engine. Microwave heating has the advantages of environmental protection, safety, controllability, rapidity, and strong operability in the application of plastic pyrolysis. Therefore, MAP is one of the bright ways for the green recycling of plastic and sustainable development. All of these should definitely attract the researcher’s attention to use MAP as an important prospect.

2. Influencing Factors, Pyrolysis Residue, and Energy Consumption of MAP of Plastics

2.1. Plastic Types

In the review of Abbas-Abadi et al. [2], the pyrolysis properties of polyolefin plastics were summarized. It is not difficult to find that their structures and densities are different due to the different types of synthetic monomers and additives. Therefore, when studying the pyrolysis process, the influence of plastic types should be considered. The products from HDPE and LDPE tend to generate wax rather than oil because of fewer branched chains. The primary cracking of free radicals is inhibited during pyrolysis. Therefore, in the experiment of the microwave pyrolysis of HDPE by Undri et al. [3], the main products of pyrolysis were short-chain hydrocarbons (C1–C4) in the gas phase, long-chain waxy hydrocarbons (>C20) in the solid phase, and a less liquid phase. However, there are many methyl branches in PP, which is easy to trigger the free radical cracking mechanism of β-elimination in the pyrolysis process. Therefore, the main product of PP pyrolysis is a low viscosity liquid mainly composed of methyl-branched hydrocarbons. In the pyrolysis process of PS, because it contains thermally unstable vinyl functional groups, it is beneficial to produce styrene monomer through the mechanism of end-chain breakage; thus, the pyrolysis of PS can often obtain higher pyrolysis oil [4]. Meanwhile, PS’s main products should be aromatics and low-molecular-weight olefins [5]. Moreover, the pyrolysis products of PET and PVC are corrosive to the reactor, making the relevant research limited [6]. In addition, researchers [7] have also studied the co-pyrolysis of waste polystyrene (PSW) and waste polypropylene (PPW), and found that the properties of the oil achieved were similar to those of gasoline, with a density and viscosity of 0.76 g/mL and 2.4 cSt, respectively. The experiments show that the microwave pyrolysis of mixed plastics presents great energy-recovery potential.

2.2. Microwave Power

Microwave power directly determines the heating rate of the sample and is one of the key parameters in the pyrolysis process. It is obvious that the higher the microwave power, the greater the heating rate, which was mainly due to the increase in energy density, beneficial to the thermal effect. In studying the influence of microwave power and the number of carbon absorbers on pyrolytic polypropylene, Suriapparao et al. [8] found that the experimental results follow these rules: when the microwave power is fixed, the heating rate decreases with the increasing amount of carbon absorbers, and the time required for heating to the target temperature is longer; when the number of carbon absorbers is fixed, the heating rate increases with the increase in the power, and the time required for heating to the target temperature is shorter. In addition, the composition analysis of pyrolysis oil shows that different microwave power has different selectivities for product formation. The selectivity of cycloalkane is the highest under a microwave power of 450 W, and the highest yield of liquid oil is 63.4%. Jing et al. [9] also found that the yield of a target product can be adjusted by microwave power. Jing et al. [10] used a commercial spherical activated carbon (SAC) with metal cations (Mg, Ca, Na, K, Ba, Sr) for the pyrolysis of polypropylene to obtain value-added products. The effect of input power on pyrolysis products was studied. It was found that the combination of low power and high power could improve the yield of light oil during pyrolysis. At the same time, the liquid yield was increased to more than 70%. Therefore, the design of the power employed in pyrolysis process is also a good strategy to improve liquid products.

2.3. Microwave Absorbers

Plastics have a low dielectric constant and weak microwave absorption capacity, being transparent to microwaves. Without a microwave absorber, these types of plastics may not reach 200 °C. Therefore, microwave absorbers are normally required to improve the system temperature. Different microwave absorbers have various properties, which may greatly influence the process of MAP of plastics. Microwave absorbers include carbon-based materials, such as activated carbon, graphite, SiC, metals and their derivatives, aluminosilicate molecular sieves, and so on [11][12][13][14][15]. The addition of the microwave absorber can reduce energy loss and greatly improve heat transfer efficiency. The intrinsic structures and valence bands determine the heating rates of the reacting system, resulting in different pyrolysis degrees and product distributions. Researchers also compared the efficiency of various absorbers, which not only act as microwave absorbers but assist pyrolysis reactions.

2.4. Biomass Co-Pyrolysis

As a renewable resource, biomass is an important node of the carbon cycle and the only source of renewable carbon [16][17][18]. Biomass consists of cellulose, hemicellulose, and lignin with C, H, O, N, and S as the main elements. Pyrolysis to make oil is one of the efficient utilizations of biomass. The application and development of biomass pyrolysis products containing oxygen are limited to some extent due to their high acidity and low calorific value [19][20][21][22]. In recent years, people have had strong interests in the co-pyrolysis of plastics and biomass. Many reports have demonstrated that there is a synergistic effect in the co-pyrolysis process of plastics and biomass, which increases the yield of pyrolysis products and improves the quality of products [23][24][25].
Sridevi et al. [26] studied the synergistic effect of co-pyrolysis of rice hull (RH) and polystyrene (PS) and found that different proportions of PS and RH had different synergistic effects. Oxyfuran in RH pyrolysis can react with hydrocarbons with a high carbon–hydrogen ratio produced by PS pyrolysis to generate a Diels–Alder reaction, which improves the yield of hydrocarbons, thus obtaining a higher liquid yield. The results showed that the higher the content of rice husk, the higher the liquid yield and the more obvious the positive synergism in the co-pyrolysis process. Similarly, Zhang et al. [19] further conducted microwave-assisted co-pyrolysis of pepper straw (CS) and polypropylene (PP). The study also proved an obvious synergistic effect. When the ratio is 1:1, the oxygen-containing compounds in pyrolysis oil are reduced by 76.69% compared with CS. Compared with direct co-pyrolysis, the oxygen content in the oil products of CS and PP co-pyrolysis pretreated by microwave decreased by 4.32%. Microwave pretreatment damaged the lignin structure, cracked the CS particles, increased the contact area between PP and CS, and promoted the interaction between CS and PP during pyrolysis.
Beneš et al. [27] used a rather novel method to depolymerize the glyceride in coconut oil to produce polyols. Firstly, coconut oil was subject to transesterification with glycerol to obtain a glycerol monoester with hydroxyl terminal group. Then, the co-pyrolysis of polycarbonate and glycerol was performed at 200–220 °C under microwave-assisted heating, and the polycarbonate was completely transformed into polyol. The reason is that excessive hydroxyl groups promote the fracture of carbonate bonds, so that PC is converted to bisphenol A(BPA) and aromatic carbonate polyols. They also found that BPA will decompose into phenol and iso-allylphenol above 220 °C, which will affect the purity of the product polyols.
 
In the study of Suriapparao Group [28], the synergistic effect of co-pyrolysis of algae (FA) with PP, PE, and EPS was investigated. In the co-pyrolysis combination of FA+PP and FA+PE, the pyrolytic volatile oxygen compounds of PA are easy to react with the volatile hydrocarbons of polymer pyrolysis, resulting in an increased gas yield in the product, showing a positive synergistic effect of the gas. However, the oil in FA+EPS has a higher yield, and the content of single aromatics is about 70%. This is due to the higher activation energy of the ring-opening reaction of aromatic compounds, which provides hydrogen-free radical deoxidation for oxygen-containing organic matter in FA pyrolysis through hydrogen transfer, thus increasing the oil yield of the product and showing positive oil production synergy, which is consistent with the research work of Mahari et al. [29]. It is also found that these synergies make a lower energy demand of co-pyrolysis than that of single pyrolysis.
Zhao et al. [30] studied the difference in bamboo/polypropylene co-pyrolysis products in different proportions with microwave assistance. After testing, when the catalytic temperature is 250 °C, and the bamboo/PP ratio is 1:2, the oil yield is 61.62 wt.%. The contents of aliphatic hydrocarbons and aromatic hydrocarbons in bamboo pyrolysis products are extremely low, but the hydrocarbon content in the products is significantly increased, because PP can be used as a hydrogen donor to provide hydrogen for the dehydration and dehydrogenation of bamboo pyrolysis steam on the catalyst, reducing the formation of coke; thus, forming a synergistic effect with bamboo pyrolysis.
It is clear from the aforementioned information that the products are highly dependent on the types of the pyrolyzed materials and the operating conditions; in the same way as the pyrolysis of plastics, the application of MAP in waste tires also presents similar characters. Due to the unique compositions, the solid yield from the pyrolysis of waste tires was found to be high with high HHV carbon black [31], which can act as a good microwave absorber. The obvious synergistic effect of the co-pyrolysis of waste tires and plastics would undoubtedly benefit in improving the products’ values, giving this technique a high development and utilization value.
In the case of co-pyrolysis of biomass with PE, PP, PS, etc., the different compositions and content of the organic matter in biomass have different effects on the distribution of co-pyrolysis products with plastics. In the process of co-pyrolysis, the behavior that biomass promotes the pyrolysis of plastics lies in the Diels–Alder reaction between the oxygen-containing free radicals generated by biomass pyrolysis and the hydrogen-rich hydrocarbons generated by polymer pyrolysis, which reduces the oxides in the pyrolysis oil and increases the hydrocarbon content through dehydration. In biomass co-pyrolysis, the pyrolysis products of PE or PP are mainly aliphatic hydrocarbons, partial cyclic aliphatic hydrocarbons, and polycyclic aromatic hydrocarbons, among which aliphatic hydrocarbons are mainly olefin. The monomer of PS is styrene, so the product oil in the process of pyrolysis is mainly aromatic hydrocarbons containing benzene rings. 

2.5. Catalyst

In the process of MAP of plastics, the addition of a catalyst can improve the selectivity of pyrolysis products and increase the output of certain products. Common catalysts used in the plastic catalytic pyrolysis can be divided into molecular sieve catalysts, metal compounds, etc. [32][33][34][35][36].
In the process of catalytic cracking of plastic macromolecules with catalysts, the cracking of plastics mainly includes thermal transformation and catalytic transformation. The direct pyrolysis process of plastic macromolecules generally follows the free radical mechanism, including initiation, pyrolysis propagation, and radical coupling [37]. In the presence of a catalyst, the catalyst can reduce the initial cracking temperature of plastics and participate in the process of free radical coupling, thus improving the selectivity of products. In addition, the catalyst can generate more active sites under the action of the microwave [38][39], which increases the contact between raw materials and the catalyst, thus improving the pyrolysis rate of plastics.

2.6. Pyrolysis Temperature

Appropriate pyrolysis temperature can improve the selectivity of the target product. When the pyrolysis temperature is low, the plastic cannot be completely pyrolyzed or generate wax with a large carbon number. When the pyrolysis temperature is too high, the excessive pyrolysis of the plastic will generate more non-condensable small molecules. Therefore, in order to obtain the target product, researchers need to reveal the operating temperatures.
From the study by Fan et al. [40], that the above 460 °C for the pyrolysis of PS, secondary pyrolysis of volatiles into low molecular weight gaseous hydrocarbons would be enhanced. For PET, a report from Liu et al. [41] shows that PET cannot be fully pyrolyzed below 550 °C. According to Zhang et al. [19], the pyrolysis range of PP was found to be 425–510 °C. Influenced by density, the pyrolysis range of polyethylene is wide, ranging from 300 °C to 520 °C [42][43]. The optimum pyrolysis temperature range of PVC is 250–350 °C [44]. In a word, the control of plastic pyrolysis temperature is influenced by the material type, and the best pyrolysis oil can be obtained by choosing the best pyrolysis temperature range.

2.7. The Device Used for MAP

The product distribution of the MAP of plastics is also affected by the design of experimental equipment. The products required by the experimental device generally include microwave ovens, reaction vessels, temperature detectors, gas condensers, liquid collection bottles, gas collection bags, and insulation materials. The successful design of the experimental device depends on the matching of each component and the tightness of the whole system. In the reported literature, most experimental devices for MAP of plastics use batch reactors [45], and there are continuous reactors [46] in the expansion devices. The biggest feature of an intermittent pyrolysis device is that the sample is added at one time, and a new sample can only be replaced after pyrolysis. The continuous reaction device can continuously feed and make the system work. 
Small MAP devices in the laboratory can be divided into two types: in-situ catalytic pyrolysis [25] and ex-situ catalytic pyrolysis [47]. In the in-situ catalytic pyrolysis device, the reaction container is placed in the center of the microwave reactor, and the reaction container is connected to a steam condensation device, a temperature sensor, and a gas purge inlet. Terapalli et al. [5] used an in-situ catalytic device and a borosilicate flask as the reaction vessel. In the in-situ catalytic experiments, the first microwave oven is often used for the direct pyrolysis of samples, and the second microwave oven is used as the heat source for the catalytic reforming of volatiles. This way is more conducive to the regeneration of the catalyst and the separation from the reactants, and to some extent slows down the deactivation of the catalyst by carbon deposition. The ex-situ catalytic pyrolysis device designed by Suriapparao et al. [48] is an example.
Liang et al. [47] added a continuous stirring device in the microwave pyrolysis reactor. Compared with the pyrolysis experiment without stirring, it was found that the long carbon chain of C14-C20 had higher selectivity under continuous stirring, while the experiment without stirring produced more methane gas because the rotation increased the temperature uniformity of the system and prevented the long-chain molecules from overheating and cracking into non-condensable small molecules due to hot spot effect. In addition, in the experimental laboratory device, supplying energy to the pyrolysis system without interruption is called continuous heating pyrolysis, and alternately supplying energy through power supply and power failure at a fixed time interval is called intermittent heating pyrolysis. Jing et al. [49] observed that reasonable control of the size of the container and the amount of absorber is helpful to the formation of wax in continuous heating mode, while intermittent heating can obtain more liquid products.
In the pilot systems, Zhang et al. [50] developed a set of continuous microwave radiation dual-mode spiral crackers. They designed the reaction vessel to be cylindrical and horizontal and pushed the feed through the screw rod, which realized the high recovery of organic matter in waste-printed circuit boards (WPCB) (88.03–92.79%). Zhou et al. [46] developed a continuous downdraft microwave-assisted pyrolysis system (CMAP). The reaction vessel is designed to be cylindrical and vertical, with a stirring rod in the middle and raw materials supplied by the upper airtight hopper. The device realizes a material handling capacity of 10 kg/h, possessing advantages of fast material handling and small heat loss, and having great potential for commercial application.
The insulation part is the key to the microwave pyrolysis device. In the process of high-temperature pyrolysis, heat loss should be prevented. The temperature in the reaction container is ensured to reach the standard evenly, and the reaction is carried out smoothly without distortion. Thermal insulation materials used for microwave pyrolysis need to have high microwave transparency at working temperature, and improper selection of materials can easily cause the thermal insulation materials to absorb microwaves and reduce the energy utilization rate. Silica cotton [5][51], glass wool [52], and ceramic fiber [29][53] are commonly used in microwave pyrolysis devices.
Accurate temperature control is very important in the pyrolysis process, and it also plays a decisive role in the success of the experiment. At present, there are still differences in temperature measurement systems, including K-type thermocouples, infrared (IR), and fiber optic (FO) temperature measurements. Kappe [54] explained the measurement of the chemical reaction temperature in the process of microwave heating in detail. Ordinary PT100 thermocouples will be coupled with a microwave to generate heat, which makes the results inaccurate. Suriapparao [55] separated the two wires by adding four insulation layers to the Cr-Al thermocouple to reduce the interference of microwave coupling and improve temperature measurement accuracy. A considerable number of microwave pyrolysis researchers use microwave-compatible K-type thermocouples, which are directly inserted into reactants to monitor the reaction temperature in real-time. This type of thermocouple is less affected by microwaves and is widely used [5][40][41][48][56]. Wang et al. [9] made a blank control test with the K-type thermocouple and found that there was no obvious difference between the temperature measured continuously under microwave working conditions and the temperature measured within three seconds after the microwave working was suspended. In addition, an IR is also a means of measuring the temperature, but it obtains the apparent temperature of the reaction system and cannot reflex the real temperature of the internal reaction, so it is used cautiously [57][58][59]. In addition, FO is considered to be the best choice because its probe posses microwave transparency and is directly inserted into the reaction system. However, for the system with high viscosity, there will still be errors in the measurement results using FO due to the hot spot effect [60].

2.8. Residues from MAP of Plastics

The pyrolysis products of plastics generally include gas, liquid, and solid, and the solid component is generally coke deposits. The quantity of the pyrolysis products would highly depend on the operating conditions, the types of plastics, as well as the catalysts types. Some unwanted products would form during the process, which may mainly refer to the polymerization of olefins and aromatics, and finally generate coke. Table 3 lists the proportion of residues produced. In the study of Potnuri et al. [61], the amount of coke produced increases with the increase in KOH, and it is thought that KOH accelerates the co-pyrolysis rate of plastics and biomass to produce coke. The research on the used frying oil (UFO) and plastic (PW) co-pyrolysis by Mahari et al. [62] found that at a high UFO/PW ratio, the hydrogen supply of PW was insufficient, which reduced the depolymerization and dehydrogenation of polymer, increased carbonization, and led to the formation of coke. Suriapparao et al. [63] found that the yield of coke in plastic pyrolysis was related to the types of co-pyrolysis biomass. It was found that higher coke was obtained in the co-pyrolysis products of RH and PS. The research of Saifuddin et al. [64] shows that the increase in plastic composition is helpful to reduce the formation of coke during the co-pyrolysis of bamboo and LDPE, since the increase in plastic provides hydrogen for the pyrolysis of polymer, which makes the pyrolysis of polymer produce hydrocarbons as much as possible instead of coke. Temperature is also an important factor affecting the pyrolysis products. In the study of PS MAP by Fan et al. [40], SiC was used as the microwave absorbent, and almost all PS converted to liquid oil at 460 °C, while 36.44 wt.% wax and 56.00 wt.% liquid oil were produced at 340 °C. In addition, selecting suitable catalysts, such as HZSM-5, HY, Hβ, SAPO-34, etc. [65][66], can obviously reduce coke formation.

2.9. Energy Consumption of MAP of Plastics

Low energy consumption and high energy efficiency are the outstanding advantages of MAP. Suriapparao et al. [63] conducted the MAP of rice husk and plastic, and reported that the highest efficiency of the microwave co-pyrolysis process could reach 68%. In the MAP study of PP by Kamireddi et al. [67], the microwave conversion efficiency calculated by experiments reached 84.7%, and the pyrolysis oil with a calorific value of 45.4 MJ/kg was obtained. In the study of MAP of HDPE, Zhou et al. [46] calculated the energy balance and found that the energy efficiency of pyrolysis of HDPE can reach 89.6%, better than that of traditional pyrolysis. Zhang et al. [68] analyzed the energy of pyrolysis of mixed plastics in a rotary kiln with a filling degree of 20%, and found that the total energy efficiency was 65.8%. In addition, Rex et al. [7] estimated the cost of PS and PP microwave pyrolysis oil and the price of commercial gasoline, and found that the price of pyrolysis oil was much lower than that of commercial gasoline (2019). Therefore, MAP has high energy conversion efficiency and high application value.

3. Modeling and Simulation Research

The development of computer skills makes it possible to transfer experiments to simulation, which can not only free people from time-consuming and laborious experiments but also save manpower and material costs. The simulation study can verify and predict the synergistic law between product yield and process parameters in the reaction, provide optimization scheme and guidance for experimental or commercial amplification, improve development efficiency, and help to understand the temperature distribution law in the reaction system and the mechanism of catalytic reaction under microwave pyrolysis. Machine learning (ML), based on a large number of experimental studies, can analyze the data by computer software, and predict the results of amplification experiments. In addition, there are a few reports on the catalytic process of MAP based on the FDTD model and molecular dynamics calculation.

4. Future Development and Challenges

Under the long-term vision of carbon neutrality in the global peak carbon-dioxide emissions, it has become a sustainable development strategy for human beings to improve the efficient utilization of plastic recycling and reduce white pollution. Further optimization in the laboratory can improve the selectivity of the target products in a regulated way, and the research results will be actively transformed into the process of commercial mass production, which will be the future research interest.

With the development of technology, microwave technology has gradually matured and high-power microwave technology has been developed into an industrial capability. At present, microwave-assisted heating has been widely used in the research area of plastic pyrolysis in the laboratory and will become the development trend for replacing conventional heating to supply energy for plastic pyrolysis. Most of the research focuses on the influence of using only one microwave absorber, but research on the influence of using different microwave absorbers at the same time is rare. In the co-pyrolysis of biomass and plastics, hydrogen-rich plastics provide hydrogen atoms for the deoxidation of the biomass, which makes the co-pyrolysis of both raw materials show obvious synergy and improves the quality of biomass pyrolysis oil. It is a trend that the deep development of co-pyrolysis has advantages.

The selectivity of products can be controlled by changing the composition, morphology, structure, and proportion of catalysts through reasonable design. Acidity is the decisive factor of catalyst activity. The higher the acidity, the easier it is to pyrolyze macromolecules into small gaseous molecules, but at the same time, it is accompanied by coking. Therefore, it is also very important to accurately regulate the acidity of the catalyst. In molecular sieve catalysts, the pore structure greatly influences the types of products, and the cooperation and utilization of different molecular sieves will be the research focus in the future. Metal catalysts can significantly pyrolyze polyolefin into hydrogen and high-quality carbon fiber because of their unique plasma effect, but its large-scale experiment remains to be carried out.

The arrangement of the pyrolysis reactor affects the distribution of products. In the in-situ catalytic pyrolysis reactor, because the catalyst is in direct contact with pyrolysis raw materials, its performance often decreases due to carbon deposition, and the regeneration of the catalyst is also challenging. However, the ex-situ catalytic pyrolysis device adopts the form of separation of materials and catalysts, which reduces the formation of carbon deposits to some extent, but the pyrolysis process is not controlled. At present, the combination of in-situ catalytic agitation pyrolysis and ectopic catalysis has great development space to improve the selectivity of target products. Although the intermittent microwave pyrolysis device is mostly used in the laboratory, it can only be used as a research platform for researchers to analyze the pyrolysis products of raw materials, develop catalysts, and adjust process-operating parameters in small experiments. The limitations of this operating platform in handling raw materials are obvious. Therefore, after the initial effect is achieved in the laboratory, it is necessary to further diversify the development of pilot plants to expand the scale of processing raw materials. At the same time, it is necessary to realize commercial applications and develop MAP devices that can continuously process plastics in large quantities.

In the research of MAP of plastics, the application of simulation is still less. In the application of machine learning technology, the mathematical model established by researchers corresponds to a specific experimental system and cannot reflect the general law. Therefore, it is of great significance to develop a generalized model that can be used to predict pyrolysis. Although the mathematical model developed by machine learning can predict unknown experiments, it is based on a certain database of experiments. In addition, the research reports using FDTD and ReaxFF methods are very limited, and numerical simulations and molecular dynamics’ calculation are effective means for researchers to understand the pyrolysis state and mechanism of plastics, which will be very helpful for the design of pyrolysis devices and the development of catalysts.

This entry is adapted from the peer-reviewed paper 10.3390/pr11051487

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