Microwave assisted pyrolysis of halogenated plastics recovered from waste computers

A Systematic Review on Biomass Treatment Using Microwave-Assisted Pyrolysis under PRISMA Guidelines

Biomass as a renewable energy resource is a major topic on a global scale. Several types of biomass heat treatment methods have been introduced to obtain useful byproducts via pyrolysis. Microwaves are a practical replacement for conventional stoves and ovens to perform pyrolysis of biomass. Their rapid heating rate and user-friendliness make them a good choice for the pyrolysis process over conventional methods. The current study reviewed research articles that used microwaves for the pyrolysis process on different types of biomass. This study primarily provides comprehensive details about the pyrolysis process, especially microwave-assisted pyrolysis (MAP) and its feasibility for treating biomass. A systematic literature review, according to the PRISMA guidelines, was performed to find research articles on biomass treatment using MAP technology. We analyzed various research studies (n = 32), retrieved from different databases, that used MAP for pyrolysis on various types of biomass, and we achieved good results. The main goal of this study was to examine the usefulness of the MAP technique, comparing its effects on distinguished types of biomass. We found MAP's effective parameters, namely, temperature, concentration of microwave absorber, moisture percentage of starting material and flow rate, microwave power and residence time of the initial sweep gas that control the pyrolysis process, and effect quality of byproducts. The catalytic agent in MAP pyrolysis was found to be useful for treating biomass, and that it has great potential to increase (nearly double) the production yield. Although MAP could not be used for all types of materials due to some challenges, it produced good results compared to conventional heating (pyrolysis) methods. We concluded that MAP is an effective method for reducing pyrolysis reaction time and improving the quality of value-added products. Also, MAP eliminates the shredding requirement for biomass and improves heating quality. Therefore, it is a viable method for reducing pyrolysis processing costs and should be applied on a larger scale than lab scale for commercialization.

Perspectives on Thermochemical Recycling of End-of-Life Plastic Wastes to Alternative Fuels

Due to its resistance to natural degradation and decomposition, plastic debris perseveres in the environment for centuries. As a lucrative material for packing industries and consumer products, plastics have become one of the major components of municipal solid waste today. The recycling of plastics is becoming difficult due to a lack of resource recovery facilities and a lack of efficient technologies to separate plastics from mixed solid waste streams. This has made oceans the hotspot for the dispersion and accumulation of plastic residues beyond landfills. This article reviews the sources, geographical occurrence, characteristics and recyclability of different types of plastic waste. This article presents a comprehensive summary of promising thermochemical technologies, such as pyrolysis, liquefaction and gasification, for the conversion of single-use plastic wastes to clean fuels. The operating principles, drivers and barriers for plastic-to-fuel technologies via pyrolysis (non-catalytic, catalytic, microwave and plasma), as well as liquefaction and gasification, are thoroughly discussed. Thermochemical co-processing of plastics with other organic waste biomass to produce high-quality fuel and energy products is also elaborated upon. Through this state-of-the-art review, it is suggested that, by investing in the research and development of thermochemical recycling technologies, one of the most pragmatic issues today, i.e., plastics waste management, can be sustainably addressed with a greater worldwide impact.

Plastic waste as pyrolysis feedstock for plastic oil production: A review

Turning plastic waste into plastic oil by pyrolysis is one of the promising techniques to eradicate plastic waste pollution and accelerate the circular economy of plastic materials. Plastic waste is an attractive pyrolysis feedstock for plastic oil production owing to its favorable chemical properties of proximate analysis, ultimate analysis, and heating value other than its abundant availability. Despite the exponential growth of scientific output from 2015 to 2022, a vast majority of the current review articles cover the pyrolysis of plastic waste into a series of fuels and value-added products, and up-to-date reviews exclusively on plastic oil production from pyrolysis are relatively scarce. In light of this void in the current review articles, this review attempts to provide an up-to-date overview of plastic waste as pyrolysis feedstock for plastic oil production. A particular emphasis is placed on the common types of plastic as primary sources of plastic pollution, the characteristics (proximate analysis, ultimate analysis, hydrogen/carbon ratio, heating value, and degradation temperature) of various plastic wastes and their potential as pyrolysis feedstock, and the pyrolysis systems (reactor type and heating method) and conditions (temperature, heating rate, residence time, pressure, particle size, reaction atmosphere, catalyst and its operation modes, and single and mixed plastic wastes) used in plastic waste pyrolysis for plastic oil production. The characteristics of plastic oil from pyrolysis in terms of physical properties and chemical composition are also outlined and discussed. The major challenges and future prospects for the large-scale production of plastic oil from pyrolysis are also addressed.

On the use of plastic precursors for preparation of activated carbons and their evaluation in CO2 capture for biogas upgrading: a review

In circular economy, useful plastic materials are kept in circulation as opposed to being landfilled, incinerated, or leaked into the natural environment. Pyrolysis is a chemical recycling technique useful for unrecyclable plastic wastes that produce gas, liquid (oil), and solid (char) products. Although the pyrolysis technique has been extensively studied and there are several installations applying it on the industrial scale, no commercial applications for the solid product have been found yet. In this scenario, the use of plastic-based char for the biogas upgrading may be a sustainable way to transform the solid product of pyrolysis into a particularly beneficial material. This paper reviews the preparation and main parameters of the processes affecting the final textural properties of the plastic-based activated carbons. Moreover, the application of those materials for the CO₂ capture in the processes of biogas upgrading is largely discussed.

Thermochemical Conversion of Plastic Waste into Fuels, Chemicals, and Value-Added Materials: A Critical Review and Outlooks

Plastic waste is an emerging environmental issue for our society. Critical action to tackle this problem is to upcycle plastic waste as valuable feedstock. Thermochemical conversion of plastic waste has received growing attention. Although thermochemical conversion is promising for handling mixed plastic waste, it typically occurs at high temperatures (300-800 °C). Catalysts can play a critical role in improving the energy efficiency of thermochemical conversion, promoting targeted reactions, and improving product selectivity. This Review aims to summarize the state-of-the-art of catalytic thermochemical conversions of various types of plastic waste. First, general trends and recent development of catalytic thermochemical conversions including pyrolysis, gasification, hydrothermal processes, and chemolysis of plastic waste into fuels, chemicals, and value-added materials were reviewed. Second, the status quo for the commercial implementation of thermochemical conversion of plastic waste was summarized. Finally, the current challenges and future perspectives of catalytic thermochemical conversion of plastic waste including the design of sustainable and robust catalysts were discussed.

Current Trends in Waste Plastics’ Liquefaction into Fuel Fraction: A Review

Polymers and plastics are crucial materials in many sectors of our economy, due to their numerous advantages. They also have some disadvantages, among the most important are problems with the recycling and disposal of used plastics. The recovery of waste plastics is increasing every year, but over 27% of plastics are landfilled. The rest is recycled, where, unfortunately, incineration is still the most common management method. From an economic perspective, waste management methods that lead to added-value products are most preferred—as in the case of material and chemical recycling. Since chemical recycling can be used for difficult wastes (poorly selected, contaminated), it seems to be the most effective way of managing these materials. Moreover, as a result this of kind of recycling, it is possible to obtain commercially valuable products, such as fractions for fuel composition and monomers for the reproduction of polymers. This review focuses on various liquefaction technologies as a prospective recycling method for three types of plastic waste: PE, PP and PS.

Effect of brominated flame retardant on the pyrolysis products of polymers originating in WEEE

Chemical recycling is an environmentally friendly method, which is often used for the recycling of plastics included in waste electric and electronic equipment (WEEE), since fuels and secondary valuable materials can be produced. Brominated flame retardants (BFRs) are usually added into these plastics to reduce their flammability; but they are toxic substances. The aim of this work is to examine the thermal behaviour and the products obtained after pyrolysis of polymer blends that consist of acrylonitrile-butadiene-styrene (ABS), high-impact polystyrene (HIPS), polycarbonate (PC) and polypropylene (PP) with composition that simulates real WEEE, in the absence and presence of a common BFR, tetrabromobisphenol A (TBBPA), in order to investigate its effect on pyrolysis products. Blends were prepared via the solvent casting method and the melt-mixing in an extruder; it was revealed that the latter method may be a better choice for blends preparation, since it did not affect the products obtained. The chemical structure of each polymeric blend was identified by Fourier transform infrared spectroscopy (FTIR). Thermal degradation of the blends was evaluated by thermogravimetric (TG) experiments performed using a thermal analyser (TGA) and a pyrolyser for evolved gas analysis (EGA). It was observed that blends had a similar behaviour during their thermal degradation; and in most cases, they followed a one-step mechanism. Pyrolysis products were identified by the pyrolyser combined with a gas chromatographer/mass spectrometer (GC/MS), and comprised various useful compounds, such as monomers, aromatic hydrocarbons and phenolic compounds that could be used as chemical feedstock. Furthermore, it was found that TBBPA affected products distribution by enhancing the formation of phenolic compounds and on the other hand by resulting in brominated compounds, such as dibromophenol.

A review of microwave pyrolysis as a sustainable plastic waste management technique

The high demand for plastic has led to plastic waste accumulation, improper disposal and environmental pollution. Even though some of this waste is recycled, most ends up in landfills or flows down rivers into the oceans. Therefore, researchers are now exploring better ways to solve the plastic waste management problem. From a socio-economic perspective, there is also a concerted effort to enable energy recovery from plastic waste and convert it into useful products to generate income for targeted segments of the population. In fact, this concept of waste-to-wealth has been adopted by the United Nations as part of its Sustainable Development Goals strategies. The current article begins by reviewing the strengths and weaknesses of plastic recycling before focusing specifically on microwave pyrolysis as an alternative to conventional technologies in plastic waste management, due to its benefit in providing fast and energy-efficient heating. The key parameters that are reviewed in this paper include different types of plastic, types of absorbent, temperatures, microwave power, residence time, and catalysts. The yield of the final product (oil, gaseous and char) varies depending on the main process parameters. Key challenges and limitations of microwave pyrolysis are also discussed in this paper.

Polyolefins and Polyethylene Terephthalate Package Wastes: Recycling and Use in Composites

Plastics are versatile materials used in a variety of sectors that have seen a rapid increase in their global production. Millions of tonnes of plastic wastes are generated each year, which puts pressure on plastic waste management methods to prevent their accumulation within the environment. Recycling is an attractive disposal method and aids the initiative of a circular plastic economy, but recycling still has challenges to overcome. This review starts with an overview of the current European recycling strategies for solid plastic waste and the challenges faced. Emphasis lies on the recycling of polyolefins (POs) and polyethylene terephthalate (PET) which are found in plastic packaging, as packaging contributes a signification proportion to solid plastic wastes. Both sections, the recycling of POs and PET, discuss the sources of wastes, chemical and mechanical recycling, effects of recycling on the material properties, strategies to improve the performance of recycled POs and PET, and finally the applications of recycled POs and PET. The review concludes with a discussion of the future potential and opportunities of recycled POs and PET.

Novel trends in the thermo-chemical recycling of plastics from WEEE containing brominated flame retardants

The amount of plastics from waste electric and electronic equipment (WEEE) has enormously increased nowadays, due to the rapid expansion and consumption of electronic devices and their short lifespan. This, in combination with their non-biodegradability, led to the need to explore environmentally friendly solutions for their safe disposal. One main obstacle when recycling plastics from WEEE is that they usually comprise harmful additives such as brominated flame retardants (BFRs) that need to be removed before or during their recycling. This paper reviews existing techniques for the recycling of plastics from WEEE and focuses specifically on the advantages, disadvantages, and challenges of pyrolysis as an environmentally friendly method for the production of value-added materials (monomers, hydrocarbons, phenols, etc.). Current technological trends available for the recycling of plastics containing brominated flame retardants are reviewed in an attempt to provide insights for future research on the sustainable management of plastics from WEEE. Emphasis is given on conventional pyrolysis, where a pretreatment step for the debromination of products is applied. This is required since brominated compounds treated at high temperatures may result in the production of harmful to health compounds such as dioxins. All current pretreatment methods (solvent extraction, supercritical fluid technology, etc.) are presented and compared in detail. Co-pyrolysis is also investigated, as it seems to be a very interesting approach, since no catalysts or solvents are used, and at the same time, more plastic wastes can be consumed as feedstock. Furthermore, catalytic pyrolysis along with key parameters, such as the type of the catalyst or pyrolysis temperature, are fully analyzed. Catalysts affect the products' distribution and enhance the removal of bromine from pyrolysis oils. Finally, an emerging technique, that of microwave-assisted pyrolysis, is also highlighted, as it offers many advantages over conventional pyrolysis. Of course, there are some impediments, such as the operational costs or other difficulties as regards the industrial implementation of the mentioned techniques that need to be overcome through future works.

Efficient recovery of Cu and Ni from WPCB via alkali leaching approach

The large generation of electronic waste (e-waste) is posing a serious threat to society. It is important to develop sustainable technology for the effective management of e-waste and the recovery of valuable metals from it. The present study employed hydrometallurgical approach for Cu and Ni extraction from waste printed circuit boards (WPCB) of mobile phones. This study demonstrates the application of ammonia-ammonium sulfate leaching for the maximum recovery of Cu and Ni. Investigations revealed that the most favourable reaction parameters for efficient metal extraction are - ammonia concentration - 90 g/L, ammonium sulfate concentration - 180 g/L, H₂O₂ concentration - 0.4 M, time - 4 h, liquid to solid ratio - 20 mL/g, temperature - 80 °C and agitation speed - 700 rpm. Under these conditions, 100% Cu and 90% Ni were extracted. Furthermore, the kinetic study was performed using the shrinking core model which revealed that the internal diffusion is the rate-controlling step for Cu and Ni extraction. The activation energies for Cu and Ni extraction were found out to be 4.5 and 5.7 kJ/mol, respectively. Finally, Cu was recovered with 98.38% purity using electrowinning at a constant DC voltage of 2.0 V at Al cathode. The present study provides a solution for concurrent extraction of Cu and Ni from the raw WPCB of mobile phones and selective recovery of Cu from metal leached solution. The process has the potential to recover the resources from WPCB while minimising the pollution caused by mismanagement of WPCB.

Achievements in pyrolysis process in E-waste management sector

Many aspects of modern life of our civilization are associated with using electrical and electronic devices (EEE). Ever-increasing demand for high-performance EEE and accelerated technological development make the replacement of EEE become frequent. This leads to the generation of a tremendous amount of electronic waste (E-waste). Challenges of the management of E-waste have recently arisen out of a dearth of proper technologies to treat E-waste. Pyrolysis process can thermochemically treat waste materials that have a complicated nature and inhomogeneity. This article gives a systematic review as an effort to tackle the challenges in the context of achievements in pyrolysis process in E-waste management sector. Pyrolysis mechanism and types of pyrolysis processes and pyrolysis reactors are first discussed. Various pyrolysis technologies applied to the E-waste treatment are then summarized and compared to each other. Points to be considered for further research and pending challenges of E-waste pyrolysis are also discussed. The pyrolysis treatment of E-waste is not yet fully industrialized mostly because of high costs. However, there should be much room for further developing the E-waste pyrolysis; hence, its industrialization and commercialization is just a matter of time.

Current Developments in the Chemical Upcycling of Waste Plastics Using Alternative Energy Sources

The management of plastics waste is one of the most urgent and significant global problems now. Historically, waste plastics have been predominantly discarded, mechanically recycled, or incinerated for energy production. However, these approaches typically relied on thermal processes like conventional pyrolysis, which are energy-intensive and unsustainable. In this Minireview, some of the latest advances and future trends in the chemical upcycling of waste plastics by photocatalytic, electrolytic, and microwave-assisted pyrolysis processes are discussed as more environmentally friendly alternatives to conventional thermal reactions. We highlight how the transformation of different types of plastics waste by exploiting alternative energy sources can generate value-added products such as fuels (H₂ and other carbon-containing small molecules), chemical feedstocks, and newly functionalized polymers, which can contribute to a more sustainable and circular economy.

Effects of oxygen vacancy defect on microwave pyrolysis of biomass to produce high-quality syngas and bio-oil: Microwave absorption and in-situ catalytic

This paper proposed to use ferric oxide (Fe₂O₃) and ferroferric oxide (Fe₃O₄) as catalysts with both microwave absorption and catalytic properties. Carbon dioxide (CO₂) was introduced as the reaction atmosphere to further improve the quality of biofuel produced by microwave pyrolysis of food waste (FW). The results showed the bio-gas yield and the syngas concentration (H₂ + CO) increased to 70.34 wt% and 61.50 mol%, respectively, using Fe₃O₄ as the catalyst. The content of aliphatic hydrocarbons in bio-oil produced with the catalyst Fe₂O₃ increased to 67.48% and the heating value reached 30.45 MJ/kg. Compared with Fe₂O₃ catalyst, Fe₃O₄ exhibited better microwave absorption properties and catalytic properties. Transmission electron microscopy (TEM) and Electron paramagnetic resonance (EPR) characterizations confirmed that the crystal surface of Fe₃O₄ formed more oxygen vacancy defects and unpaired electrons. Additionally, according to the X-ray photoelectron spectroscopy (XPS) analysis, the content of lattice oxygen in Fe₃O₄ was 14.11%, a value that was much lower than Fe₂O₃ (38.54%). The oxygen vacancy defects not only improved the efficient utilization of microwave energy but also provided the reactive sites for the reaction between the volatile organic compounds (VOCs) and CO₂ to generate CO. This paper provides a new perspective for selecting catalysts that have both microwave absorption and catalytic properties during the microwave pyrolysis of biomass.

Microwave-assisted catalytic pyrolysis of waste printed circuit boards, and migration and distribution of bromine

Microwave-assisted pyrolysis (MAP) of waste printed circuit boards (WPCB) was performed to investigate the characteristics of pyrolysis product and Br fixation. Pyrolysis conversion increased with increasing temperature, reaching 93.3 % at 650 °C. However, increasing heating time did not exhibit remarkable influence on pyrolysis conversion. At 350 °C, phenols were main compounds in the oil accounting for 91.15 %. As the temperature increased to 650 °C, polycyclic aromatic hydrocarbons and monocyclic aromatic hydrocarbons (except phenols) increased to 20.55 % and 19.03 %, respectively. Meanwhile, the total content of CO₂, CO, CH₄ and H₂ in the non-condensable gases increased significantly. Addition of ZSM-5 and kaolin promoted the recombination reaction of pyrolysis products, increased the relative percentage of monocyclic aromatic hydrocarbons (except phenols) and C₁₁-C₂₀ compounds in the oil, and reduced non-condensable gases. The oxygen bomb-ion chromatography was used to evaluate the Br content of pyrolysis residues. Higher pyrolysis temperature enhanced transfer of Br to pyrolysis gas. K₂CO₃, Na₂CO₃ and NaOH reacted with hydrogen bromide to generate KBr and NaBr, which significantly improved the Br fixation efficiency of pyrolysis residues (i.e. from 29.11%-99.80%, 96.39 % and 86.69 %, respectively) and reduced Br content in pyrolysis gas.

Effect of different ash/organics and C/H/O ratios on characteristics and reaction mechanisms of sludge microwave pyrolysis to generate bio-fuels

To study the effects of different ash/organics and C/H/O ratios on bio-fuel characteristics and energy efficiency, four kinds of sludge with different properties were used for microwave pyrolysis (800 °C). Moreover, the microwave pyrolysis reaction mechanisms of different sludge were also explored. The results showed that high-ash sludge could accelerate the frequency of polar molecule rotation in the microwave field due to the presence of oxides with dielectric properties in ash, thereby achieving faster heating rates and higher temperatures. However, compared with high-organic sludge, high-ash sludge exhibited lower bio-gas yield and higher bio-char yield. As the H/C ratio increased from 0.127 to 0.148, the bio-gas yield increased from 15.41% to 40.01%, and the content of H₂ in bio-gas and aliphatics in bio-oil increased to 36.69 vol% and 26.54 wt%, respectively. When the O/C ratio was reduced to 1.31, the content of CO and oxygenated compound in bio-oil increased to 31.25 vol% and 40.04 wt%, which lowered the quality of the bio-oil. Those consequences also determined that a mixture of sludge with different ash/organic ratios could be pyrolyzed to obtain high-quality bio-fuels and high energy efficiency. Differences in C/H/O ratios in the mixed sludge greatly affected the microwave pyrolysis heating process, which affected the pyrolysis reactions and the quality of the bio-fuels. Therefore, this study provides a theoretical basis to elevate the quality of bio-fuels and reduce microwave pyrolysis costs.

Microwave-Assisted Pyrolysis of Biomass Waste: A Mini Review

The utilization of biomass waste as a raw material for renewable energy is a global concern. Pyrolysis is one of the thermal treatments for biomass wastes that results in the production of liquid, solid and gaseous products. Unfortunately, the complex structure of the biomass materials matrix needs elevated heating to convert these materials into useful products. Microwave heating is a promising alternative to conventional heating approaches. Recently, it has been widely used in pyrolysis due to easy operation and its high heating rate. This review tries to identify the microwave-assisted pyrolysis treatment process fundamentals and discusses various key operating parameters which have an effect on product yield. It was found that several operating parameters govern this process such as microwave power and the degree of temperature, microwave absorber addition and its concentration, initial moisture content, initial sweep gas flow rate/residence time. Moreover, this study highlighted the most attractive products of the microwave pyrolysis process. These products include synthesis gas, bio-char, and bio-oil. The benefits and challenges of microwave heating are discussed.

Valorization of the plastic residue from a WEEE treatment plant by pyrolysis

The possibility of a pyrolysis process as a mean of recycling the residual plastic rich fraction (WEEE residue) derived from of a material recovery facility has been evaluated. The unknown product composition of WEEE residue has been supposed through coupled thermal - infrared analysis and ultimate analysis and resulted as PP 3 wt%, PBT 3 wt%, PVC 4 wt%, styrene-based polymers (principally ABS) 50 wt%, thermosetting resins (principally, epoxy/phenolic resins) 24 wt%, inorganic fraction (principally fiber glass) 16 wt%. DSC experiments showed that the overall energy, defined as the degradation heat, needed in order to completely degrade WEEE residue was about 4% of the exploitable energy of the input material. The effect of temperature and different zeolite catalysts were investigated, in particular in terms of yield and quality of the produced oils during the pyrolysis process. Produced oils were potentially exploitable as fuels and almost all catalysts improved their quality. The best performance was reached by NaUSY(5.7) with the second highest production of light oil and the greatest total monoaromatics yield, plus 12 wt% in comparison to thermal pyrolysis experiments. Furthermore, light oil produced by NaUSY(5.7) has one of the best LHV (36 MJ/kg) and no halogenated compounds were detected by GC-MS analysis. Char or pyrolytic gas combustion could supply the energy required for the thermal degradation of WEEE Residue.

Characterization of bio-oil and bio-char produced by low-temperature microwave-assisted pyrolysis of olive pruning residue using various absorbers

Olive pruning residue is largely formed during cultivation, and is usually disposed through open-air combustion directly in the field, but this habit is a possible source of pollution. The pyrolytic conversion of olive pruning residue has been run in a new and very appealing way using microwave as a heating source and different microwave absorbers in a multimode batch reactor. In this way, olive residue is converted into interesting bio-chemical products with a short pyrolysis time, ranging from 15 to 36 min, and with a peak temperature ranging from 450 K to 705 K according to the different microwave absorber. Thus, a very efficient and selective system was realized, which was able to address the process towards the formation of a large amount of bio-char (up to 61.2%) or a high formation of bio-oil (56.2%) and gas (41.7%) with a very low formation of bio-char (2.1%). However, when carbon and iron were used as microwave absorbers, it was possible to obtain an intermediate amount of bio-char (26-30%) and bio-oil (40 wt%). Bio-oils were collected as dark-brown liquids with low viscosity and density. A bio-oil with a low water concentration was obtained using carbon or iron as the microwave absorber. The bio-oils formed in all experiments contained a very large amount of acetic acid, even when NaOH was the microwave absorber. Furthermore, a large amount of aromatics were present in the bio-oil obtained using carbon as the microwave absorber.

An Overview of Temperature Issues in Microwave-Assisted Pyrolysis

Microwave-assisted pyrolysis is a promising thermochemical technique to convert waste polymers and biomass into raw chemicals and fuels. However, this process involves several issues related to the interactions between materials and microwaves. Consequently, the control of temperature during microwave-assisted pyrolysis is a hard task both for measurement and uniformity during the overall pyrolytic run. In this review, we introduce some of the main theoretical aspects of the microwaves–materials interactions alongside the issues related to microwave pyrolytic processability of materials.