Chemical upcycling of waste plastics via non-thermal plasma and advanced pyrolysis
Date
2024-12
Authors
Radhakrishnan, Harish
Major Professor
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Bai, Xianglan
Mba-Wright, Mark
Hu, Shan
Hu, Hui
Li, Wenzhen
Committee Member
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Abstract
Escalating waste plastic disposal and increasing CO2 emissions together have caused great environmental problems. So far, various chemical recycling technologies have been explored to convert plastics into platform and commodity chemicals. Polyolefins account for nearly two-thirds of global plastic production dominating single-use plastic waste streams. However, both their mechanical and chemical recycling are challenging compared to other common plastics due to low-quality products and costly multistep conversion processes. Chemical upcycling via conventional thermochemistry in the short-term while developing more energy-efficient and unconventional techniques for the long-term, such as non-equilibrium plasma, offers the most promise to convert polyolefin waste into valuable chemicals and fuels. Till now, non-thermal plasma, an upcoming technology, was considered a suitable technology only for biomass conversion under a chemically reactive atmosphere produced by plasma at mild conditions. This dissertation expands the scope of plasma-based conversion technologies to the conversion of waste plastics and greenhouse CO2 to produce valuable platform and commodity chemicals. This dissertation also explored pathways to improve and better understand the current conventional thermochemical technologies, such as thermal pyrolysis and catalytic pyrolysis, to produce high-value platform chemicals and monomers.
The first study pioneers an electrified approach to upcycle waste plastics into carbon-negative commodity chemicals using greenhouse gas CO2 as the oxidant and additional carbon source. Herein, we report a selective and energy-efficient co-conversion of waste polyolefins and CO2 for oleochemicals and syngas CO using a single-step low-temperature plasma approach. We obtained up to 97.6 wt% of fatty alcohols from polyolefins within minutes under ambient pressure. It showed that CO2 plasma is efficient in oxidatively depolymerizing plastics under mild conditions at high reaction rates, while plastics can serve as carbon sinks to improve CO2 conversion. Furthermore, mixing CO2 with a small amount of O2 resulted in a facile approach to control plasma reactions and tailor product selectivity toward fatty alcohols. This catalyst- and solvent-free process requires electricity only, providing a promising waste upcycling route to enable carbon-negative chemicals.
In the second study, a new molten-phase unsaturation strategy was studied to enhance the chemical upcycling of waste plastics by fast pyrolysis. Fast pyrolysis is a robust deconstruction technology for chemically upcycling waste plastics. However, highly viscous molten polyolefins and large amounts of waxy hydrocarbons can negatively impact the reactor operation. The products with broad molecular weight distributions can also affect the processibility of pyrolysis products during downstream separation and upgrading. We found that coupling a molten-phase thermal preheating and subsequent fast pyrolysis is a facile approach to improve polyolefin pyrolysis and catalytic upgrading compared to direct pyrolysis of polyolefins. It shows that the molten phase thermal treatment increased unsaturated carbon-carbon bonds in the treated polyolefins. During subsequent pyrolysis, the preheated polyolefins significantly reduced wax range hydrocarbons in the condensable products without increasing gas formation. The wax yield from pyrolysis of high-density polyethylene preheated to 295 ℃ and low-density polyethylene preheated to 275 ℃ was 20.5% and 26.5%, respectively, compared to 38.6% and 46% produced from pyrolyzing untreated polyolefins. When catalytically pyrolyzed using a zeolite catalyst, the preheated polyolefins produced more olefins during ex-situ catalytic pyrolysis and more aromatic hydrocarbons during in-situ catalytic pyrolysis. During ex-situ catalytic pyrolysis, ethylene yield increased to 23.3% and 24.7% for the preheated HDPE and LDPE compared to 16.7% and 9.3% for untreated HDPE and LDPE, respectively.
Chemical upcycling offers a promising method to convert polyolefin waste into valuable chemicals and fuels, and understanding the influence of additives is crucial for optimizing this process. The third study in this dissertation investigated the pyrolysis behavior of polyolefins in the presence of various common polymer additives, including antioxidants, stabilizers, pigments, fillers, and slip agents. The additives studied include hindered phenol and phosphite antioxidants, hindered amine light stabilizers (HALS), titanium dioxide (TiO2) pigment, carbon black pigment, and fillers such as calcium carbonate (CaCO3), kaolin, talc, and barium sulfate (BaSO4). Thermogravimetric analysis (TGA) and pyrolysis-gas chromatography-mass spectrometry-flame ionization detector (Py-GC/MS-FID) were employed to analyze the thermal stability and pyrolysis products of HDPE samples co-extruded with these additives. The results indicate that antioxidants and stabilizers delayed pyrolysis initiation, while fillers and slip agents significantly affect the yield and composition of pyrolysis products. Specifically, kaolin and zinc stearate increased char formation and gas yields. The effect of these additives on in-situ and ex-situ catalytic pyrolysis of polyolefins was also studied, and the product analyses shine the light on the reaction chemistry in the presence of additives. This research enhances understanding of additive effects in pyrolysis, supporting the development of efficient recycling technologies for heterogeneous plastic wastes in a circular economy.
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