Pyrolysis of polystyrene waste in the presence of activated carbon in conventional and microwave heating using modified thermocouple

Microwave-Assisted Catalytic Deconstruction of Plastics Waste into Nanostructured Carbon and Hydrogen Fuel Using Composite Magnetic Ferrite Catalysts

Finding new catalysts and pyrolysis technologies for efficiently recycling wasted plastics into fuels and structured solid materials of high selectivity is the need of time. Catalytic pyrolysis is a thermochemical process that cracks the feedstock in an inert gas environment into gaseous and liquid fuels and a residue. This study is conducted on microwave-assisted catalytic recycling of wasted plastics into nanostructured carbon and hydrogen fuel using composite magnetic ferrite catalysts. The composite ferrite catalysts, namely, NiZnFe2O4, NiMgFe2O4, and MgZnFe2O4 were produced through the coprecipitation method and characterized for onward use in the microwave-assisted valorization of wasted plastics. The ferrite nanoparticles worked as a catalyst and heat susceptor for uniformly distributed energy transfer from microwaves to the feedstock at a moderate temperature of 450°C. The type of catalyst and the working parameters significantly impacted the process efficiency, gas yield, and structural properties of the carbonaceous residue. The tested process took 2-8 minutes to pulverize feedstock into gas and carbon nanotubes (CNTs), depending on the catalyst type. The NiZnFe2O4-catalyzed process produced CNTs with good structural properties and fewer impurities compared to other catalysts. The NiMgFe2O4 catalyst performed better in terms of hydrogen evolution by showing 87.5% hydrogen (H2) composition in the evolved gases. Almost 90% of extractable hydrogen from the feedstock evolved during the first 2 minutes of the reaction.

Conversion of waste polystyrene into valuable aromatic hydrocarbons via microwave-assisted pyrolysis

The prime objective of the current research work was to understand the role of microwave-assisted pyrolysis for the upgradation of expanded polystyrene (EPS) waste into valuable aromatic hydrocarbons. Ethyl acetate solvent was used to dissolve the EPS to enhance the homogeneous dispersion of EPS with susceptor particles. Biochar obtained from the pyrolysis was used as a susceptor. The design of experiments method was used to understand the role of microwave power (300 W, 450 W, and 600 W) and susceptor quantity (5 g, 10 g, and 15 g) in the pyrolysis process. The pyrolysis was conducted till the temperature reached up to 600 °C, and this temperature was achieved in the time interval of 14-38 min based on the experimental conditions. The obtained average heating rates varied in the range of 15 to 41 °C/min to attain the pyrolysis temperature. The EPS feed was converted into char (~ 2.5 wt.%), oil (51 to 60 wt.%), and gaseous (37 to 47 wt.%) products. The specific microwave energy (J/g) was calculated to know the energy requirement; it increased with an increase in susceptor quantity and microwave power, whereas specific microwave power (W/g) was a function of microwave power and increased from 15 to 30 W/g. The predicted values calculated using the model equations closely matched the actual values showing that the developed model equations via optimization had a good fit. The obtained pyrolysis oil physicochemical properties including viscosity (1 to 1.4 cP), density (990 to 1030 kg/m³), heating value (39 to 42 MJ/kg), and flash point (98 to 101 °C) were thoroughly analyzed. The pyrolysis oil was rich in aromatic hydrocarbons and it was predominantly composed of styrene, cyclopropyl methylbenzene, and alkylbenzene derivates.

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.

Breakdown of biomass for energy applications using microwave pyrolysis: A technological review

The agricultural industry faces a permanent increase in waste generation, which is associated with the fast-growing population. Due to the environmental hazards, there is a paramount demand for generating electricity and value-added products from renewable sources. The selection of the conversion method is crucial to develop an eco-friendly, efficient and economically viable energy application. This manuscript investigates the influencing factors that affect the quality and yield of the biochar, bio-oil and biogas during the microwave pyrolysis process, evaluating the biomass nature and diverse combinations of operating conditions. The by-product yield depends on the intrinsic physicochemical properties of biomass. Feedstock with high lignin content is favourable for biochar production, and the breakdown of cellulose and hemicellulose leads to higher syngas formation. Biomass with high volatile matter concentration promotes the generation of bio-oil and biogas. The pyrolysis system's conditions of input power, microwave heating suspector, vacuum, reaction temperature, and the processing chamber geometry were influence factors for optimising the energy recovery. Increased input power and microwave susceptor addition lead to high heating rates, which were beneficial for biogas production, but the excess pyrolysis temperature induce a reduction of bio-oil yield.

Investigation of zeolite H-β effect on pyrolysis of polystyrene by multiple kinetic analysis methods

For studying the effects of H-β zeolite on the pyrolysis of polystyrene (PS), non-isothermal thermogravimetric measurements were conducted in N₂ under 5, 10, 15, and 20 K/min. The results show that the addition of 10 ~ 30 wt.% H-β zeolite can significantly decrease the initial pyrolysis temperature of PS, indicative of the catalytic effect of zeolite used. Through kinetic analysis of the pyrolysis of PS blends, the isoconversional activation energies are calculated to be 121.8 ~ 191.9, 92.1 ~ 173.8, and 116.7 ~ 192.4 kJ/mol for the PS blends with zeolite loading of 10, 20, and 30 wt.%, respectively. Meanwhile, the pyrolysis degradation functions are determined through the Master-plots method integrated with a recently developed compensation-effect method to follow chemical reaction mechanism with the reaction order of 0.9, 1.0, and 0.6 for PS/zeolite blends of 10, 20, and 30 wt.% loading, and their pre-exponential factors are respectively calculated to be 6.18 × 10⁸ ~ 5.71 × 10¹¹, 2.36 × 10⁶ ~ 9.23 × 10¹¹, and 8.38 × 10⁷ ~ 1.11 × 10¹² min^-1. Our work may provide some insights for how to better describe experimental results with theoretical predications and necessary information for performing any potential pyrolysis designs.

Progress and perspective for conversion of plastic wastes into valuable chemicals

Today, discarded plastics in nature have caused serious "white pollution", however these plastic wastes contain abundant carbon resources that could serve as the feedstock to produce commodities. Because of this, it is requisite to convert these plastic wastes into valuable chemicals. Herein, the state-of-the-art techniques for plastic conversion are divided into two categories, those performed under violent conditions and mild conditions, in which the conversion mechanisms are discussed. The strategies under violent conditions are closer to practical application thanks to their excellent conversion efficiencies, while the strategies under mild conditions are more environmentally friendly, showing enormous development potential in the future. We summarize in detail the pyrolysis, hydropyrolysis, solvolysis and microwave-initiated catalysis for bond cleavage in plastic wastes at temperatures ranging from 448 to 973 K. Also, we overview the photocatalysis, electrocatalysis and biocatalysis for bond cleavage in plastic wastes at near and even normal temperature and pressure. Finally, we present some suggestions and outlooks concerning the improvement of current techniques and in-depth mechanisms of investigation for conversion of plastics into valuable chemicals.

Hydrogen-Rich and Clean Fuel Gas Production from Co-pyrolysis of Biomass and Plastic Blends with CaO Additive

The treatment and disposal of waste biomass and plastics are of great importance to achieve both waste management and resource recycling. In this work, pyrolysis of biomass and plastic blends were investigated to identify the influence of temperature and in situ CaO addition on the production of hydrogen-rich, HCl-free, and low tar content fuel gases. The results show that the increase in temperature and the use of CaO significantly improved both the quantity and quality of the fuel gas and mitigated the formation of tar compounds and HCl. Moreover, H₂ yield was significantly improved from 0.30 to 3.68 mmol/g with the increase in temperature from 550 to 850 °C. Also, the use of in situ CaO significantly increased the H₂ yield by 28-88%. The H₂/CO ratio was also enhanced from 0.35 to 1.50 with the temperature increase and CaO addition. Tar removal efficiency reached approximately 70.09% with the use of CaO at 850 °C. The produced HCl gas could be effectively absorbed by CaO through dechlorination reactions to form CaClOH at a highest mitigation efficiency of 92.37%. The results could be used to develop clean and efficient treatment technologies of waste biomass and plastics.

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.

Catalytic cracking of polystyrene pyrolysis oil: Effect of Nb2O5 and NiO/Nb2O5 catalyst on the liquid product composition

The catalytic cracking of polystyrene pyrolysis oil was investigated over a Nb2O5 and a NiO/Nb2O5 catalyst in a fixed bed reactor. First, the pyrolysis of two different polystyrene feedstock (polystyrene foam and polystyrene pellet) was carried out in a semi-batch reactor, and the resulting polystyrene pellets pyrolysis oil was selected for catalytic cracking reaction because of its high liquid yield (85%). Catalytic cracking experiments were then performed at different temperatures (350-500 °C) using Nb2O5 or NiO/Nb2O5 catalyst. Gas chromatography-mass spectrometry analysis of liquid product obtained from the catalytic cracking process showed that the dimers in the pyrolysis oil were converted to monomers during the catalytic cracking process. The catalytic cracking results also showed that the NiO/Nb2O5 catalyst (having slightly higher acidic sites) had slightly higher activity for monomer conversion than the Nb2O5 catalyst (having less acidic sites). X-ray diffraction, transmission electron microscopy, pyridine Fourier transform infrared spectroscopy, NH3 Temperature Programmed Desorption and X-ray photoelectron spectroscopy were used to characterize the catalyst. The highest catalytic cracking activity was observed at 400 °C with the Nb2O5 catalyst with 4% toluene, 6% ethylbenzene, approximately 50% styrene, 13% α-methyl styrene, and only 6% of dimers in the liquid oil. The increase in temperature positively affected the yield of gases during catalytic cracking process.

Conventional pyrolysis of Plastic waste for Product recovery and utilization of pyrolytic gases for carbon nanotubes production

Inevitably increase in plastic demand has resulted in an overgrowing production on a global scale. The utilization of plastics has been applied to a number of industries as it is a durable, moldable, and inexpensive material. High exploitation of plastic had resulted in a hefty amount of waste production, which is not easy to recycle due to its non-degradable nature and results in landfills. Nowadays, waste to energy processes such as pyrolysis has emerged as a superlative process for the management of plastic waste by converting it into useful products. On the other hand, the employment of carbon nanotubes (CNT's) has shown high growth in their production. CNT's were generally synthesized from conventional gases like methane, ethane, and ethylene. Plastic waste can be utilized to substitute the feed material for the CNT synthesis via pyrolysis method. In this study, a two-step pyrolysis process was investigated for product recovery and CNT's production. The first steps consisted of catalytic and non-catalytic degradation of mixed plastic waste in a vertical fixed bed reactor at 500 °C with a heating rate of 20 °C/min for the production of pyrolytic oil and gases and were analyzed. The second step consists of the employment of catalytic pyrolysis gases in a horizontal tube reactor maintained at a temperature of 800 °C over a bed of catalyst for the synthesis of CNT's via catalytic vapor deposition (CVD) technique. It was established that the use of catalyst decreases the oil phase production from 80.5 to 64%, char from 9 to 6.5% while an increase in gas phase production from 10.5 to 29.5% was reported. The alteration of hydrocarbons to CNT's was investigated via pre- and post-GC analysis of the gas samples. Post gas investigation indicates an increased concentration of hydrogen in the sample. Also, the decline of hydrocarbon gases concentration was observed in post sample analysis. Also, transmission electron microscopy (TEM) analysis confirms the synthesis of CNT's.

Pyrolysis of waste surgical masks into liquid fuel and its life-cycle assessment

Pyrolysis of the middle layer of a surgical mask (MLM) and inner and outer layers of a surgical mask (IOM) was performed to assess their potential valorization as waste-to-energy feedstocks, and the characteristics of the resulting products were investigated. Pyrolysis of the main organics in waste surgical masks occurred at a very narrow temperature range of 456-466 °C. The main product was carbon-rich and oxygen-deficient liquid oil with a high heating value (HHV) of 43.5 MJ/kg. From the life-cycle perspective, environmental benefits and advantages of this upcycling approach were verified compared with conventional waste management approaches. This study advocated the potential application of waste surgical masks as feedstocks for fuels and energy, which is beneficial to mitigate plastic pollution and achieve sustainable plastic waste-to-energy upcycling, simultaneously.

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.

Simulation of heat transfer characteristics of tire particles in rotary kiln reactor

In this study, we examined and strengthened the motion and heat transfer characteristics of tire particles in a rotary kiln. To overcome the difficulty of controlling the residence time of tire pellets and poor heat transfer, a study was conducted on heat transfer enhancement owing to lifting flight in a rotary kiln with spiral grooves. We developed a particle mechanics model using the discrete element method, and then simulated tire particle motion and obtained results using the EDEM software platform. The obtained motion results were programmed on a MATLAB platform for performing a heat transfer simulation study on the improvement in heat transfer. The coupling model of motion and heat transfer of waste tire particles was established. Through the analysis of particle movement and heat transfer, the following conclusions are obtained: with the increase of the lifting flight, the particle swarm heats up faster and more uniformly; with the increase of the rotation speed, the temperature of the particles rises faster and more uniformly, which has only the same effect as the lifting flight; to a certain extent, the smaller particle size can also improve the heat transfer effect. The maximum error of the relatively accurate coupling model of tire particle motion and heat transfer in a rotary kiln was only 12%, which provides a certain guiding significance for the improved design of the reactor.

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.

Slow pyrolysis of municipal solid waste (MSW): A review

In recent years, extensive studies have been carried out to improve our knowledge of the reactor operations and system performance in thermal pyrolysis of municipal solid wastes (MSW). However, the fundamentals of MSW pyrolysis and their engineering applications remain unsatisfactorily explored. This paper is a review of the pyrolysis of MSW and synergistic co-pyrolysis of the constituents of MSW with reference to pyrolytic performance, the distribution and energy content of the end products, and the mechanisms of the synergistic effects. The prospects for, and challenges of, the MSW pyrolysis process are provided. A MSW pyrolytic process with maximal energy recovery and minimal carbon footprint is proposed.

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.

Effects of red mud on emission control of NOx precursors during sludge pyrolysis: A protein model compound study

The nitrogen-containing gases pyrolyzed from sewage sludge can be converted into NO_x compounds, which would cause severe environmental pollution. This study developed a new strategy to reduce the emission of NO_x precursors such as ammonia (NH₃) and hydrogen cyanide (HCN) using red mud. The highest reduction efficiencies (15.10% for NH₃ and 24.72% for HCN) were achieved at 900 °C while compared with those pyrolyzed from raw sludge without the addition of red mud. The transformation and distribution of nitrogenous compounds in three-phase pyrolysates were studied at 400-800 °C for pyrolysis process of a model soybean protein compound. The nitrogenous compounds, i.e., amine-N, heterocyclic-N, and nitrile-N, were identified as the three main intermediates related with the production of NO_x precursors. Ferric oxide (Fe₂O₃) and calcium oxide (CaO) presented in red mud were identified as the driving force which facilitated nitrogen stabilization in char (e.g., at 800 °C, 21.63% increase of char-N after addition of Fe₂O₃, and 41.54% increase of char-N after addition of CaO). These metal oxides possibly reacted with protein-N to form Fe_xN and CaC_xN_y, inhibited the secondary cracking of amine-N compounds in tar (e.g., at 800 °C, 2.33% increase of amine-N after addition of Fe₂O₃, and 0.38% increase of amine-N after addition of CaO), and reduced the production of nitrile-N (e.g., at 800 °C, 30.41% reduction of nitrile-N after addition of Fe₂O₃, and 27.40% reduction of nitrile-N after addition of CaO) and heterocyclic-N compounds (e.g., at 800 °C, 21.60% reduction of heterocyclic-N after addition of Fe₂O₃, and 13.98% reduction of heterocyclic-N after addition of CaO). Hence, the emission of NH₃ and HCN in gas phase can be controlled. Moreover, Fe₂O₃ showed better capability in controlling the emission of NO_x precursors than CaO (higher reduction of NH₃-N and higher reduction of HCN-N). These results indicate that red mud is an efficient catalyst to reduce emission of NO_x precursors through controlling intermediates at 400-800 °C.

Chemical and microscopic investigation of co-pyrolysis of crumb tire rubber with waste cooking oil at mild temperature

Approximate rubber/bitumen homogeneous system formed by desulfurization and degradation of crumb tire rubber in bitumen under high temperature is beneficial to enhance the storage stability of rubberized bitumen. However, the main problems during the processing of desulfurized and degraded rubberized bitumen are aging caused by volatilization of light components, and burning or explosion due to the direct utilization of low flash point bitumen. Therefore, waste cooking oil was proposed as a safer medium to desulfurize and degrade crumb rubber prior to production of rubberized bitumen. This study focused on the feasibility and effectiveness of the application of waste cooking oil in desulfurizing and degrading rubber particles through co-pyrolysis of them at mild temperature (240-280 °C). Chemical and microscopic analyses were performed to investigate the structural changes of vulcanized rubber. Results showed that solubility of rubber powder reached above 60 wt% after pyrolysis in waste cooking oil, which increased with higher temperatures and more of oil, while increased to a maximum at 2 h and then decreased with the extension of time. The rubber hydrocarbon content decreased greatly, and dramatic reduction of carbon, hydrogen and sulfur elements happened according to component and elemental analyses. The surface of pyrolysis product was even and smooth without obvious rubber particles. The grooves and cavities of rubber residues in scanning electron microscopy micrographs proved that shedding of degraded polymer molecules occurred. Fourier transform infrared spectra revealed that breakage of carbon-sulfur, carbon=carbon and sulfur=oxygen bonds took place during pyrolysis, with appearance of natural rubber characteristic peak.