GRAPHITIZATION SYNERGIES BETWEEN COAL TAR PITCH, WASTE PLASTICS AND GRAPHENE FORMS: A STUDY OF MIXTURE COMPOSITIONS, PROCESS CONDITIONS AND CHEMICAL COMPATIBILITY
Open Access
- Author:
- Gharpure, Akshay Pradip
- Graduate Program:
- Energy and Mineral Engineering (PHD)
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 11, 2023
- Committee Members:
- Jeremy Gernand, Program Head/Chair
Sarma Pisupati, Major Field Member
Randy Vander Wal, Chair & Dissertation Advisor
Ramakrishnan Rajagopalan, Outside Unit & Field Member
Barbara Arnold, Major Field Member - Keywords:
- Graphitization
Carbon Materials
Characterization
Pitch
Waste Plastics
Upcycling
Graphene
Graphene Oxide
TEM
XRD
Raman Spectroscopy
FTIR
Crystallite Size
Nanostructure
TGA DSC
Mesophase - Abstract:
- Carbon materials are critical components of the modern economy with uses in batteries, capacitors, carbon fibers, refractories, lubricants, C-C composites, to name a few. Demand for graphitic carbons for energy storage applications is rapidly rising and domestic sources of high-quality precursors are declining. The main focus of this study is to investigate strategies for obtaining graphitic carbon materials through synergies between pitches, waste plastics and graphene forms. This dissertation investigates plasma-assisted precursor upgradation, the effect of precursor upgradation on graphitic quality, additive/subtractive strategies to improve graphitic quality and synthetic alternatives from waste plastics. In this work, the term "upgradation" is used to indicate an increase in the hydrogen-to-carbon ratio (H/C). Chapter 2 focuses on a comparison between conventional fast thermal pyrolysis versus microwave (MW) plasma pyrolysis of coals in inert and reactive environments. Coals featuring a range of ranks were pyrolyzed in inert and reactive (H2, CH4 and H2 + CH4) gas environments to assess upgrading (increasing the hydrogen-to-carbon ratio) of the devolatilized products. Thermal pyrolysis in reactive environments produced little to no upgrading of devolatilized products indicating a clear need for plasma-assistance to the process. CHEMKIN simulations were in general agreement with the thermal pyrolysis results validating the applicability of the model for thermal pyrolysis. Chemical kinetic simulations reveal that very little dissociation of H2 and CH4 occurs under fast thermal reaction conditions. Major devolatilized products such as benzene, toluene and xylene, undergo minimal upgrading reactions in H2 and CH4 environments while undergoing cracking at longer residence times. The primary devolatilization product, benzene, was chosen as the model compound for evaluating MW plasma pyrolysis in both inert and reactive environments. The assessment was conducted using a custom modified in-house plasma reactor system using a conventional kitchen MW unit. The graphitizability of poorly graphitizable benzene precursor improved remarkably after exposure to MW plasma containing H2 and CH4. Products with large crystallite sizes and nanostructure comparable with model graphitizable material were obtained from this upgraded precursor, and the underlying mechanisms were discussed based on the analysis of compositional changes as a result of reactive plasma treatment. To gather supporting evidence, a set of non-methylated versus methylated and hydrogenated model compounds have been used as surrogates for upgraded precursors to evaluate differences in crystallite parameters upon graphitization. Methylated and hydrogenated compounds exhibited a noteworthy enhancement in graphitic quality when compared to their non-methylated counterparts. Chapter 3 extends the study of MW plasma pyrolysis on real coals at pilot scale. In collaboration with H Quest Vanguard Inc., bituminous and sub-bituminous coals were pyrolyzed in their pilot-scale MW reactor under inert and reactive plasma environments. The structural characteristics and graphitizability of the tars from different thermal and reactive MW plasma environments were compared. The study showed that the composition of the tars can be tailored by changing gaseous environment in the MW plasma. Tars derived from MW plasma processing exhibit increased aromaticity, decreased condensation, and a lower concentration of oxygenated molecules when compared to tars obtained through thermal processes. These features led to substantially larger crystallite sizes and improved nanostructure in the graphitized products. Chapter 4 investigates an alternative approach of obtaining high-quality pitches from solvent extraction of coal. The method outlined in this chapter presents notable benefits, including reduced temperature, pressure, and hydrogen addition prerequisites. This results in the production of an enhanced aromatic pitch with higher hydrogen-to-carbon ratio (H/C), accompanied by lower quinoline insoluble (QI) and ash content. The synthetic pitch obtained through solvent extraction exhibited superior nanostructure characteristics, with mean crystallite sizes exceeding those of traditional commercial pitches by more than 50%. This chapter further investigates if improvement in graphitic quality can also be achieved by simple physical addition of components with higher H/C ratio such as plastics. Two recycled waste plastics – polystyrene (rPS) and polypropylene (rPE), were used as low-cost additives in an aromatic coal tar pitch, and bulk crystallinity of the graphitized composites was evaluated using XRD. Simple addition of both plastics improved graphitizability of the pitch significantly and produced a uniform highly graphitic phase. Aromatic polymers were found to be better pitch extenders than aliphatic polymers. Chapter 5 focuses on improving graphitic quality of the pitches by templated growth. The main purpose is to “catalyze” the emergence of graphitic structure through addition of a structure directing agent. Graphene and graphene oxide (GO) have been used as templating agents in pitches. The ensuing graphitic quality improvement is assessed using transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. Pitches with different chemistries have very different nanostructure upon graphitization. Aliphatic pitches develop more glassy structure while aromatic pitches develop a graphitic structure characterized by long contiguous and well-stacked lamellae. Graphene catalyzes graphitic growth by engendering higher graphitic quality compared to pure pitches but at lower temperatures. The templating effects are even more impactful in aliphatic pitches. Thermogravimetric Analysis/Differential Scanning Calorimetry unveiled that graphene serves as a catalyst, effectively lowering the onset temperature of polymerization reactions for stabilization, decreasing devolatilization, and enhancing concatenation reactions during carbonization. To qualitatively assess the mesophase, scanning electron microscopy imaging was employed. TEM with in-situ heating disclosed three mechanisms in the graphene-pitch composite: chemical templating, physical templating and self-templating. Building upon the insights gained in Chapter 5, Chapter 6 delves into an in-depth examination of the influence of structural parameters – loading, lateral size, stacking height and composition, of graphenic additives on the crystallite sizes within graphitized pitch composites. Based on the observations, the mechanisms of interaction between pitch and graphenic additives are proposed and discussed. In contrast to the additive approach explored in Chapter 5 and 6, Chapter 7 focuses on the removal of constraining crosslinks as a strategy to enhance the graphitic quality. The study utilizes a model non-graphitizable carbon, sucrose char, and two commercially available carbon blacks with distinct nanostructures to investigate improvements in crystallinity at two levels of oxidative burnout. Comparative analyses were conducted between the pristine materials and their partially oxidized counterparts at various heat treatment temperatures. Bulk crystallinity was analyzed using X-ray diffraction, while high-resolution transmission electron microscopy was employed for nanostructural characterization. Notably, sucrose char exhibited remarkable improvements, transforming from a non-graphitizable, sponge-like structure to highly ordered graphenic flakes through the mechanism of oxidative crosslink liberation. After the removal of restrictive crosslinks, the in-plane crystallite size and stacking height tripled. Raman spectroscopy assessed molecular-level changes after partial oxidation, while X-ray photoelectron spectroscopy was employed to evaluate alterations in elemental composition. The carbon blacks also demonstrated notable improvements, manifested in the development of longer, less tortuous lamellae accompanied by expanded particle sizes. Focused fringe analysis algorithms were applied to extract lamellae statistics, revealing nuanced changes in nanostructure after partial oxidation and heat treatment. Chapter 8 investigates waste plastics as the main precursor and alternative to pitches for obtaining graphitic carbon materials. This chapter introduces innovative research focused on the conversion of diverse waste plastics into high-quality synthetic graphite with elevated purity. Six types of recycled plastics, sourced in various forms, were acquired, including reprocessed polypropylene, high-density polyethylene flakes, shredded polyethylene films, and reprocessed polyethylene from the Pennsylvania Recycling Markets Center. Additionally, polystyrene foams and polyethylene terephthalate bottles were sourced from a local recycling bin. The waste plastics underwent carbonization in sealed tubing reactors. The study demonstrates the adaptability of this process, showcasing its effectiveness on a mixture of waste plastics in different recycled forms, thereby addressing challenges related to separation and transportation within the plastic recycling industry. Furthermore, the incorporation of graphene oxide (GO) additives led to a notable improvement of up to 250% in the conversion yield to elemental carbon for recycled plastics. To elucidate the interaction mechanisms between plastics and GO during pyrolysis, five distinct grades of GO and graphene were employed. This investigation involved thermogravimetric analysis/differential scanning calorimetry and ReaxFF-based reactive molecular dynamics simulations to analyze the impact of GO additives on carbonization. Subsequent graphitization at 2500 ℃ revealed that the synthetic graphites derived from plastic waste exhibited remarkable graphitic quality, with mean crystallite sizes comparable to a model graphitizable material, anthracene coke. The thin, flake-like morphology and nanostructure featuring well-stacked contiguous lamellae position these graphitic carbons as highly promising candidates for energy storage applications. Based on comprehensive experiments and atomistic-scale simulations, interaction mechanisms between plastic polymers and graphenic additives are proposed, offering insights into the chemical conversion pathways for GO-assisted waste plastic carbonization and graphitization. Chapter 9 provides the conclusions from this research work.