Recent Progresses in Pyrolysis of Plastic Packaging Wastes and Biomass Materials for Conversion of High-Value Carbons: A Review

The recycling of plastic packaging wastes helps to alleviate the problems of white pollution and resource shortage. It is very necessary to develop high-value conversion technologies for plastic packaging wastes. To our knowledge, carbon materials with excellent properties have been widely used in energy storage, adsorption, water treatment, aerospace and functional packaging, and so on. Waste plastic packaging and biomass materials are excellent precursor materials of carbon materials due to their rich sources and high carbon content. Thus, the conversion from waste plastic packaging and biomass materials to carbon materials attracts much attention. However, closely related reviews are lacking up to now. In this work, the pyrolysis routes of the pyrolysis of plastic packaging wastes and biomass materials for conversion to high-value carbons and the influence factors were analyzed. Additionally, the applications of these obtained carbons were summarized. Furthermore, the limitations of the current pyrolysis technology are put forward and the research prospects are forecasted. Therefore, this review can provide a useful reference and guide for the research on the pyrolysis of plastic packaging wastes and biomass materials and the conversion to high-value carbon.

Primary Products from Fast Co-Pyrolysis of Palm Kernel Shell and Sawdust

Co-pyrolysis is one possible method to handle different biomass leftovers. The success of the implementation depends on several factors, of which the quality of the produced bio-oil is of the highest importance, together with the throughput and constraints of the feedstock. In this study, the fast co-pyrolysis of palm kernel shell (PKS) and woody biomass was conducted in a micro-pyrolyser connected to a Gas Chromatograph-Mass Spectrometer/Flame Ionisation Detector (GC-MS/FID) at 600 °C and 5 s. Different blend ratios were studied to reveal interactions on the primary products formed from the co-pyrolysis, specifically PKS and two woody biomasses. A comparison of the experimental and predicted yields showed that the co-pyrolysis of the binary blends in equal proportions, PKS with mahogany (MAH) or iroko (IRO) sawdust, resulted in a decrease in the relative yield of the phenols by 19%, while HAA was promoted by 43% for the PKS:IRO-1:1 pyrolysis blend, and the saccharides were strongly inhibited for the PKS:MAH-1:1 pyrolysis blend. However, no difference was observed in the yields for the different groups of compounds when the two woody biomasses (MAH:IRO-1:1) were co-pyrolysed. In contrast to the binary blend, the pyrolysis of the ternary blends showed that the yield of the saccharides was promoted to a large extent, while the acids were inhibited for the PKS:MAH:IRO-1:1:1 pyrolysis blend. However, the relative yield of the saccharides was inhibited to a large extent for the PKS:MAH:IRO-1:2:2 pyrolysis blend, while no major difference was observed in the yields across the different groups of compounds when PKS and the woody biomass were blended in equal amounts and pyrolysed (PKS:MAH:IRO-2:1:1). This study showed evidence of a synergistic interaction when co-pyrolysing different biomasses. It also shows that it is possible to enhance the production of a valuable group of compounds with the right biomass composition and blend ratio.

Investigation of gauze and medical bottle co-pyrolysis on the product formation, reactivity, and reaction pathway of char, liquid oil, and gas

Effective in-site treatment of medical waste has become a weak link in hospitals. Pyrolysis technology is a treatment method for medical waste that can enable rapid disposal in hospital settings and relieve environmental pressure, while also producing high-value products and reducing disposal costs. In this work, the effects of feedstock ratio and temperature on product yield and components of gauze (GA) and medical bottles (MB) co-pyrolysis have been investigated. The higher yield of solid products was obtained by co-pyrolysis of GA and MB at 400 ℃. With the addition of MB and an increase in temperature for the co-pyrolysis of GA and MB in a similar ratio, the pyrolysis oil and gas yields gradually increased. According to GC-MS analysis, co-feeding 75% MB to GA improved the alcohol content from 33.21% to a maximum yield of 59.8% at a pyrolysis temperature of 700 ℃. The content of aliphatic hydrocarbon reached 38.68% when the pyrolysis temperature and MB addition ratio were 700 °C and 75%, respectively. The GC data shows that the main gas components of co-pyrolysis of GA/MB were CH4 and H2, while the pyrolysis of pure GA or MB resulted in CO or CO2. Additionally, the solid carbon products obtained have an excellent pore structure. This strategy can benefit medical waste control and resource utilization for the low-cost disposal of medical waste and the acquisition of high-value resource products.

Graphical Abstract

Upcycling of Plastic Wastes and Biomass for Sustainable Graphitic Carbon Production: A Critical Review

Upcycling of waste plastics diverts plastics from landfill, which helps in reducing greenhouse gas emissions. Graphitic carbon is an interesting material with a wide range of applications in electronics, energy storage, fuel cells, and even as advanced fillers for polymer composites. It is a very strong and highly conductive material consisting of weakly bound graphene layers arranged in a hexagonal structure. There are different ways of synthesizing graphitic carbons, of which the co-pyrolysis of biomass and plastic wastes is a promising approach for large-scale production. Highly graphitized carbon with surface areas in the range of 201 m²/g was produced from the co-pyrolysis of polyethylene and pinewood at 600 °C. Similarly, porous carbon having a superior discharge capacity (290 mAh/g) was developed from the co-pyrolysis of sugar cane and plastic polymers with catalysts. The addition of plastic wastes including polyethylene and high-density polyethylene to the pyrolysis of biomass tends to increase the surface area and improve the discharge capacity of the produced graphitic carbons. Likewise, temperature plays an important role in enhancing the carbon content and thereby the quality of the graphitic carbon during the co-pyrolysis process. The application of metal catalysts can reduce the graphitization temperature while at the same time improve the quality of the graphitic carbon by increasing the carbon contents. This work reports some typical graphitic carbon preparation methods from the co-pyrolysis of biomass and plastic wastes for the first time including thermochemical methods, exfoliation methods, template-based production methods, and salt-based methods. The factors affecting the graphitic char quality during the conversion processes are reviewed critically. Moreover, the current state-of-the-art characterization technologies such as Raman, scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy are discussed in detail, and finally, an overview on the applications, scalability, and future trends of graphitic-like carbons is highlighted.

Pyrolysis of polypropylene plastic waste into carbonaceous char: Priority of plastic waste management amidst COVID-19 pandemic

COVID-19 global pandemic, originated from Wuhan, resulted in a massive increase in the output of polypropylene (PP)-based personal protective equipment (PPE) for healthcare workers. The continuous demand of PPE across the world caused the PP based plastic wastes accumulation. Some alternative approaches that have been practiced apart from collecting the plastic waste in the landfills are incineration approach and open burning. However, there were many drawbacks of these practices, which promote the release of chemical additives and greenhouse gases into the environment. Therefore, a proper approach in treating the plastic wastes, which introduces conversion of plastic wastes into renewable energy is paramount. Along the way of extensive research and studies, the recovery of PP plastic to fuel-like liquid oil and solid char through thermal decomposition of pyrolysis process, helps in reducing the number of PP plastic wastes and produces good quality pyrolysis liquid oil and solid char to be used in fuel applications. This paper summarizes the pyrolysis process for massively produced PP plastic wastes, type of pyrolysis used and the main pyrolysis parameters affecting the product yields. Literature studies of pyrolysis of PP plastic and several key points to optimize solid char production for PP were thoroughly elaborated in this review paper.

Co-pyrolysis performances, synergistic mechanisms, and products of textile dyeing sludge and medical plastic wastes

This study aimed to quantify the co-pyrolysis of textile dyeing sludge (TDS) and the two medical plastic wastes of syringes (SY) and medical bottles (MB) in terms of their performances, synergistic mechanisms, and products. The pyrolysis of polyolefin plastics with its high calorific value and low ash content can offset the poor mono-pyrolytic performance of TDS. The synergistic mechanisms occurred mainly in the range of 400-550 °C. The addition of 10% SY or MB achieved the best co-pyrolysis performance with the lowest activation energy. The co-pyrolysis increased the contents of CH₄ and CH but reduced CO₂ emission. The co-pyrolysis released more fatty hydrocarbons, alcohols, and cyclic hydrocarbon during but reduced the yields of ethers and furans, through the synergistic mechanisms. The addition of the polyolefin plastics made the micro surface particles of chars smaller and looser. Our results can benefit energy utilization, pollution control, and optimal operational conditions for the industrial thermochemical conversions of hazardous wastes.

Investigation of a Ni-Modified MCM-41 Catalyst for the Reduction of Oxygenates and Carbon Deposits during the Co-Pyrolysis of Cellulose and Polypropylene

Catalytic fast co-pyrolysis of biomass and plastic is an effective method to achieve high-quality bio-oil production. In this work, (Ni)-MCM-41 catalysts with different Ni loadings were prepared and characterized in detail by using a variety of advanced analytical techniques, and the effects on the catalytic performance were analyzed by micropyrolysis with gas chromatography mass spectrometry (Py-GC/MS) and thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR) methods. The results showed that an appropriate amount of Ni addition can effectively modulate the physicochemical properties of MCM-41. For a Ni loading of 25.1 wt % (Cat-C), the catalyst showed an optimal catalytic performance, a decrease in the proportion of oxygenated compounds in the product from 35.6 (MCM-41) to 13.4%, and an increase in the relative total amount of olefins plus aromatics from 62.2 (MCM-41) to 84.6%. The excellent catalytic performance of Cat-C can be ascribed to a balancing of its proper physical structural properties, appropriate acidity, strong metal-carrier interaction, high metal dispersion, and excellent compatibility balance between active and acidic sites.

Effects of Different Conditions on Co-Pyrolysis Behavior of Corn Stover and Polypropylene

The pyrolysis behavior of corn stover and polypropylene during co-pyrolysis was studied using a tube furnace reactor. The effects of mixing ratio of corn stover and polypropylene, pyrolysis temperature, addition amount of catalyst (HZSM-5) and reaction atmosphere (N₂ and CO₂) on the properties of pyrolysis products were studied. The results showed that co-pyrolysis of corn stover and polypropylene can increase the yield of pyrolysis oil. When corn stover:polypropylene = 1:3, the yield of pyrolysis oil was as high as 52.1%, which was 4.5% higher than the theoretical value. With the increase of pyrolysis temperature, the yield of pyrolysis oil increased first and then decreased, and reached the optimal yield at 550 °C. The addition of catalyst (HZSM-5) reduced the proportion of oxygenates and promoted the generation of aromatic hydrocarbons. CO₂ has a certain oxidation effect on the components of pyrolysis oil, which promoted the increase of oxygen-containing aromatics and the reduction of deoxy-aromatic hydrocarbons. This study identified the theoretical basis for the comprehensive utilization of plastic and biomass energy.

Effective deoxygenation for the production of liquid biofuels via microwave assisted co-pyrolysis of agro residues and waste plastics combined with catalytic upgradation

Rice straw and sugarcane bagasse were co-pyrolyzed with polypropylene and polystyrene using microwaves, and the pyrolysis vapors were catalytically upgraded using HZSM-5 catalyst. The product yields, composition and properties of bio-oil from pyrolysis of individual feedstocks and equal composition mixtures before and after catalytic upgradation were thoroughly investigated. The pyrolysis oil yields from polypropylene (82 wt%) and polystyrene (98 wt%) were high compared to that from rice straw (26 wt%) and bagasse (29 wt%). Catalytic upgradation at weight hourly space velocity of 11 h^-1 resulted in higher selectivity to unsaturated aliphatics and aromatic hydrocarbons. Properties of upgraded bio-oil from biomass-polypropylene mixtures were similar to that of light fuel oil with high calorific value (43 MJ/kg), low viscosity (1 cP), optimum density (0.850 g/cm³) and flash point (70 °C). Oxygen content in catalytically upgraded co-pyrolysis bio-oil was low (<5%) as compared to upgraded pyrolysis bio-oil (14-18%), and pyrolysis bio-oil without upgradation (20-24%).

Synergistic Effect on the Non-Oxygenated Fraction of Bio-Oil in Thermal Co-Pyrolysis of Biomass and Polypropylene at Low Heating Rate

Biomass pyrolysis and polypropylene (PP) pyrolysis in a stirred tank reactor exhibited different heat transfer phenomena whereby heat transfer in biomass pyrolysis was driven predominantly by heat radiation and PP pyrolysis by heat convection. Therefore, co-pyrolysis could exhibit be expected to display various heat transfer phenomena depending on the feed composition. The objective of the present work was to determine how heat transfer, which was affected by feed composition, affected the yield and composition of the non-polar fraction. Analysis of heat transfer phenomena was based on the existence of two regimes in the previous research in which in regime 1 (the range of PP composition in the feeds is 0–40%), mass ejection from biomass particles occurred without biomass particle swelling, while in regime 2 (the range of PP composition in the feeds is 40–100%), mass ejection was preceded by biomass particle swelling. The co-pyrolysis was carried out in a stirred tank reactor with heating rate of 5 °C/min until 500 °C and using N2 gas as carrier gas. Temperature measurement was applied to pyrolysis fluid at the lower part of the reactor and small biomass spheres of 6 mm diameter to simulate heat transfer to biomass particles. The results indicate that in regime 1 convective and radiative heat transfers sparingly occurred and synergistic effect on the yield of non-oxygenated phase increased with increasing convective heat transfer at increasing %PP in feed. On the other hand, in regime 2, convective heat transfer was predominant with decreasing synergistic effect at increasing %PP in feed. The optimum PP composition in feed to reach maximum synergistic effect was 50%. Non-oxygenated phase portion in the reactor leading to the wax formation acted as donor of methyl and hydrogen radicals in the removal of oxygen to improve synergistic effect. Non-oxygenated fraction of bio-oil contained mostly methyl comprising about 53% by mole fraction, while commercial diesel contained mostly methylene comprising about 59% by mole fraction

Reaction engineering and kinetics of algae conversion to biofuels and chemicals via pyrolysis and hydrothermal liquefaction

A state-of-the-art review on pyrolysis and hydrothermal liquefaction of algae to fuels and chemicals with emphasis on reaction chemistry and kinetics.

Evaluation of pressure and temperature effects on hydropyrolysis of pine sawdust: pyrolysate composition and kinetics studies

Kinetics and product distribution from high pressure hydropyrolysis of biomass using Py-GC/MS and Py-FTIR.

Unraveling the interactions in fast co-pyrolysis of microalgae model compounds via pyrolysis-GC/MS and pyrolysis-FTIR techniques

The pyrolysate composition, product time evolution and kinetics of fast co-pyrolysis of protein, carbohydrate and lipid surrogates are investigated to unravel the interactions among microalgae components.

Co-pyrolysis of lignin and plastics using red clay as catalyst in a micro-pyrolyzer

In the current study, low-density polyethylene and polystyrene were co-pyrolyzed with dealkaline lignin in a micro-reactor at 500 °C with and without low-cost red clay catalyst. The products were analyzed with GC-MS/FID to quantify phenolic compounds, alkanes and alkenes. The synergistic effect between plastics and lignin was studied by comparing the carbon yield of compounds from co-pyrolysis with that from individual pyrolysis. The co-pyrolysis of lignin and polystyrene was also performed at 600, 700 and 800 °C to examine the effect of pyrolysis temperature. The study explores a novel approach to enhance lignin depolymerization with red clay catalyst while utilizing waste plastics.

Fast pyrolysis kinetics of alkali lignin: Evaluation of apparent rate parameters and product time evolution

In this study, the apparent kinetics of fast pyrolysis of alkali lignin was evaluated by obtaining isothermal mass loss data in the timescale of 2-30s at 400-700°C in an analytical pyrolyzer. The data were analyzed using different reaction models to determine the rate constants and apparent rate parameters. First order and one dimensional diffusion models resulted in good fits with experimental data with apparent activation energy of 23kJmol^-1. Kinetic compensation effect was established using a large number of kinetic parameters reported in the literature for pyrolysis of different lignins. The time evolution of the major functional groups in the pyrolysate was analyzed using in situ Fourier transform infrared spectroscopy. Maximum production of the volatiles occurred around 10-12s. A clear transformation of guaiacols to phenol, catechol and their derivatives, and aromatic hydrocarbons was observed with increasing temperature. The plausible reaction steps involved in various transformations are discussed.

Fast co-pyrolysis of waste newspaper with high-density polyethylene for high yields of alcohols and hydrocarbons

Waste newspaper (WP) was first co-pyrolyzed with high-density polyethylene (HDPE) using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) to enhance the yields of alcohols and hydrocarbons. The effects of WP: HDPE feed ratio (100:0, 75:25, 50:50, 25:75, 0:100) and temperature (500-800°C) on products distribution were investigated and the interaction mechanism during co-pyrolysis was also proposed. Maximum yields of alcohols and hydrocarbons reached 85.88% (feed ratio 50:50wt.%, 600°C). Hydrogen supplements and deoxidation by HDPE and subsequently fragments recombination result in the conversion of aldehydes and ketones into branched hydrocarbons. Radicals from WP degradation favor the secondary crack for HDPE products resulting in the formation of linear hydrocarbons with low carbon number. Hydrocarbons with activated radical site from HDPE degradation were interacted with hydroxyl from WP degradation promoting the formation of linear long chain alcohols. Moreover, co-pyrolysis significantly enhanced condensable oil qualities, which were close to commercial diesel No. 0.