Maximizing olefin production via steam cracking of distilled pyrolysis oils from difficult-to-recycle municipal plastic waste and marine litter

Pyrolysis of mixed contaminated plastic wastes: Assessing the influence of polymers composition, temperature and residence time

The European Union's recycling objectives of 50 % for packaging by 2025 and 55 % by 2030 are still far from the current recycling rate of 37.8 %. Thermochemical recycling via pyrolysis might improve recovery rates, especially for challenging post-consumer dirty and contaminated plastics. Many studies focus on the pyrolysis of virgin plastics and artificial mixtures of these plastics to simulate real-world mixed plastic waste scenarios. This study examines a sample of pre-sorting waste from an Italian composting plant to represent real plastic mixtures within the collection chain. This waste contains plastics, organics, paper, and others, with plastics making up 72.18 % of the sample, primarily identified as LDPE, PP, and HDPE. The plastics were tested individually and as a mixture, mirroring the concentrations observed within the original sample, in a bench-scale semi-batch reactor at varying temperatures (420 °C, 470 °C, and 520 °C) and residence times. The analysis indicated that temperature and residence time considerably influence product yields and composition, although temperature has a more dominant effect. Wax analysis showed paraffins and olefins as the primary components, with a peak carbon number distribution in the C14-C19 range at elevated temperatures. Contaminant analysis of wax identified substantial levels of halogens and metals, posing challenges for downstream processes. Results from the mixture tests revealed that the product yields of individual polymers could be predicted as a linear combination of their behaviors, with an accuracy within a 10 % margin of error across the examined temperature range and lower residence times.

Assessing the feasibility of ocean plastic waste as secondary feedstock for the production of base chemicals

Plastic pollution in the marine environment is a growing concern, with around 10 % of globally produced plastics ending up in oceans annually. Most ocean plastics are incinerated for energy recovery if harvested, since harvesting remains a key challenge. This study evaluated the feasibility of recovering base chemicals from the polyethylene (PE) and polypropylene (PP) fraction of ocean plastic waste through a single-step olefin production method. The approach employed a micropyrolyzer unit coupled with comprehensive two-dimensional gas chromatography (µP-GC × GC) and dual detectors to analyze gaseous product yields. Elemental and matrix analyses of the waste were performed using CHNS/O elemental analysis, Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), and Combustion Ion Chromatography (CIC) to identify potentially harmful components. We present here the yields of critical light olefins such as ethylene (13 wt% from PE samples, 9 wt% from PP samples) and propylene (10 wt% from PE samples, 17 wt% from PP samples) at 700 °C. Pyrolysis products detected in PP samples included 24 wt% of branched olefins, whereas 54 wt% of linear olefins were detected in PE samples. The aromatics detected in the samples ranged between 1-3 wt%, with naphthene levels ranging between 4-7 wt%. Furthermore, metal contaminants, such asnickel, silicon, copper, iron, sodium, calcium, and potassium, were detected from the waste via ICP-OES, and chlorine levels via CIC. The results suggest that ocean plastic waste could serve as feedstock for production of light olefins, provided pre- and post-treatment procedures are implemented to mitigate contamination.

Steam gasification of char derived from refuse-derived fuel pyrolysis: adsorption behaviour in phenol solutions

The increasing waste generation trends resulted in growing attention to the technologies that aim to reduce or prevent landfilling. The pyrolysis and gasification of refuse-derived fuel (RDF) allow waste to be turned into new raw materials, like pyrolysis gas and syngas. However, the wet gas cleaning processes result in the production of highly contaminated liquid waste. Phenolic compounds are common constituents of this wastewater and often appear in the wastewater of other industries as well. In this research, the laboratory-scale steam gasification of an RDF char was performed to produce syngas and adsorbent simultaneously. The RDF was previously pyrolyzed at 700 °C maximum temperature in a Hungarian pyrolysis pilot plant with approximately 120 kg h-1 capacity. In this thermal waste processing plant, the pyrolysis gas is already utilised by burning, but currently, the char ends up in landfills. The gasification of char samples was examined with different steam-to-carbon ratios (0.56, 0.84, and 1.12) and duration (30, 60, and 120 min) at 900 °C. Following gasification, the phenol removal capability of the solid by-products was investigated. The results show that its composition and energetic properties make the produced syngas more suitable to use as a raw material in the chemical industry rather than a fuel. At lower concentrations, the effectiveness of the solid by-product for phenol removal was comparable to commercial activated carbon. These are promising results about producing activated carbon from waste without any chemical treatment.

Sustainable ethylene production: Recovery from plastic waste via thermochemical processes

The concept of monomer recovery from plastic waste has recently gained broad interest in industry as a powerful strategy to reduce the environmental impacts of chemical production and plastic waste pollution. Herein, we focus on the ethylene recovery from plastic waste via thermochemical pathways, such as pyrolysis, gasification, and steam cracking of pyrolysis oil derived from plastic waste. Ethylene recovery performance of different thermochemical conversion processes is evaluated and compared with respect to plastic waste types, process types, ethylene recovery yields, and process operating conditions. Based on the analysis of available data in earlier literature, future research is recommended to further enhance the viability of the thermochemical ethylene recovery technologies. This review is expected to offer a meaningful guideline on developing efficient platforms for the value-added monomer recovery from plastic waste through thermochemical conversion routes. It is also hoped that this review serves as a preliminary step to encourage the widespread adoption of thermochemical conversion-based ethylene recovery from plastic waste by industries.

Marine plastics, circular economy, and artificial intelligence: A comprehensive review of challenges, solutions, and policies

Global plastic production is rapidly increasing, resulting in significant amounts of plastic entering the marine environment. This makes marine litter one of the most critical environmental concerns. Determining the effects of this waste on marine animals, particularly endangered organisms, and the health of the oceans is now one of the top environmental priorities. This article reviews the sources of plastic production, its entry into the oceans and the food chain, the potential threat to aquatic animals and humans, the challenges of plastic waste in the oceans, the existing laws and regulations in this field, and strategies. Using conceptual models, this study looks at a circular economy framework for energy recovery from ocean plastic wastes. It does this by drawing on debates about AI-based systems for smart management. In the last sections of the present research, a novel soft sensor is designed for the prediction of accumulated ocean plastic waste based on social development features and the application of machine learning computations. Plus, the best scenario of ocean plastic waste management with a concentration on both energy consumption and greenhouse gas emissions is discussed using USEPA-WARM modeling. Finally, a circular economy concept and ocean plastic waste management policies are modeled based on the strategies of different countries. We deal with green chemistry and the replacement of plastics derived from fossil sources.

Thermal pyrolysis of waste versus virgin polyolefin feedstocks: The role of pressure, temperature and waste composition

Due to the complexity and diversity of polyolefinic plastic waste streams and the inherent non-selective nature of the pyrolysis chemistry, the chemical decomposition of plastic waste is still not fully understood. Accurate data of feedstock and products that also consider impurities is, in this context, quite scarce. Therefore this work focuses on the thermochemical recycling via pyrolysis of different virgin and contaminated waste-derived polyolefin feedstocks (i.e., low-density polyethylene (LDPE), polypropylene (PP) as main components), along with an investigation of the decomposition mechanisms based on the detailed composition of the pyrolysis oils. Crucial in this work is the detailed chemical analysis of the resulting pyrolysis oils by comprehensive two-dimensional gas chromatography (GC × GC) and ICP-OES, among others. Different feedstocks were pyrolyzed at a temperature range of 430-490 °C and at pressures between 0.1 and 2 bar in a continuous pilot-scale pyrolysis unit. At the lowest pressure, the pyrolysis oil yield of the studied polyolefins reached up to 95 wt%. The pyrolysis oil consists of primarily α-olefins (37-42 %) and n-paraffins (32-35 %) for LDPE pyrolysis, while isoolefins (mostly C₉ and C₁₅) and diolefins accounted for 84-91 % of the PP-based pyrolysis oils. The post-consumer waste feedstocks led to significantly less pyrolysis oil yields and more char formation compared to their virgin equivalents. It was found that plastic aging, polyvinyl chloride (PVC) (3 wt%), and metal contamination were the main causes of char formation during the pyrolysis of polyolefin waste (4.9 wt%).

Defossilization and decarbonization of hydrogen production using plastic waste: Temperature and feedstock effects during thermolysis stage

The replacement of natural gas with plastic-derived pyrolysis gas can defossilize H₂ production, while subsequent capture, utilization and storage of carbon in a solid form can decarbonize the process. The objective of this study was to investigate H₂ production from three types of plastics using a process comprising pyrolysis (600 °C) and thermolysis stages (1200-1500 °C). Depending on the plastic feedstock and thermolysis temperature, the laboratory-scale setup generated 1000-1350 mL/min product gas with H₂ purity of 74.3-94.2 vol%. The recovery of 5-9 wt% molecular H₂ per mass of plastics was achieved. Other products included solid residue (0.1-12 wt%) and oil (8-52 wt%) from the pyrolysis reactor, solid carbon (36-53 wt%) and gas impurities (2-16 wt%) from the thermolysis reactor. The purity of H₂ gas was detrimentally influenced by polyethylene terephthalate in the feedstock due to the dilution of gas by CO. The decomposition of methane containing in the pyrolysis gas was the limiting reaction step during H₂ production and improved at higher thermolysis temperature. Three solid carbon structures were formed during the thermolysis stage regardless of the plastic type: carbon black aggregates, carbon black aggregates coated with a layer of pyrolytic carbon and a carbon film on the inner reactor wall. Among the three types of carbon, the highest valorization potential was identified for carbon black aggregates. Plastic feedstock composition had little if any effect on carbon black properties, while high thermolysis temperature (1500 °C) reduced the particle sizes and increased the surface area of aggregates.

The contribution of high-resolution GC separations in plastic recycling research

One convenient strategy to reduce environmental impact and pollution involves the reuse and revalorization of waste produced by modern society. Nowadays, global plastic production has reached 367 million tons per year and because of their durable nature, their recycling is fundamental for the achievement of the circular economy objective. In closing the loop of plastics, advanced recycling, i.e., the breakdown of plastics into their building blocks and their transformation into valuable secondary raw materials, is a promising management option for post-consumer plastic waste. The most valuable product from advanced recycling is a fluid hydrocarbon stream (or pyrolysis oil) which represents the feedstock for further refinement and processing into new plastics. In this context, gas chromatography is currently playing an important role since it is being used to study the pyrolysis oils, as well as any organic contaminants, and it can be considered a high-resolution separation technique, able to provide the molecular composition of such complex samples. This information significantly helps to tailor the pyrolysis process to produce high-quality feedstocks. In addition, the detection of contaminants (i.e., heteroatom-containing compounds) is crucial to avoid catalytic deterioration and to implement and design further purification processes. The current review highlights the importance of molecular characterization of waste stream products, and particularly the pyrolysis oils obtained from waste plastics. An overview of relevant applications published recently will be provided, and the potential of comprehensive two-dimensional gas chromatography, which represents the natural evolution of gas chromatography into a higher-resolution technique, will be underlined.

Can Pyrolysis Oil Be Used as a Feedstock to Close the Gap in the Circular Economy of Polyolefins?

Plastics are engineering marvels that have found widespread use in all aspects of modern life. However, poor waste management practices and inefficient recycling technologies, along with their extremely high durability, have caused one of the major environmental problems facing humankind: waste plastic pollution. The upcycling of waste plastics to chemical feedstock to produce virgin plastics has emerged as a viable option to mitigate the adverse effects of plastic pollution and close the gap in the circular economy of plastics. Pyrolysis is considered a chemical recycling technology to upcycle waste plastics. Yet, whether pyrolysis as a stand-alone technology can achieve true circularity or not requires further investigation. In this study, we analyzed and critically evaluated whether oil obtained from the non-catalytic pyrolysis of virgin polypropylene (PP) can be used as a feedstock for naphtha crackers to produce olefins, and subsequently polyolefins, without undermining the circular economy and resource efficiency. Two different pyrolysis oils were obtained from a pyrolysis plant and compared with light and heavy naphtha by a combination of physical and chromatographic methods, in accordance with established standards. The results demonstrate that pyrolysis oil consists of mostly cyclic olefins with a bromine number of 85 to 304, whereas light naphtha consists of mostly paraffinic hydrocarbons with a very low olefinic content and a bromine number around 1. Owing to the compositional differences, pyrolysis oil studied herein is completely different than naphtha in terms of hydrocarbon composition and cannot be used as a feedstock for commercial naphtha crackers to produce olefins. The findings are of particular importance to evaluating different chemical recycling opportunities with respect to true circularity and may serve as a benchmark to determine whether liquids obtained from different polyolefin recycling technologies are compatible with existing industrial steam crackers' feedstock.