Browsing by Subject "pyrolysis"

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    High-Temperature Chemistry of Polypropylene Pyrolysis: Millisecond Reaction Kinetics and Visualization
    (2023-06) Sidhu, Nathan
    The ubiquity of plastics in modern life is evident by the rapid and continual growth of global plastic production. Polypropylene is one of the most widely produced and used plastic materials, accounting for approximately 20% of global polymer production. Billions of tons of plastic waste have been produced as a byproduct of the widespread use of plastics and are insufficiently managed under the current linear plastic economy, with the majority of plastic waste accumulating in landfills or the environment. To allow for the continued use of plastics in a sustainable fashion, a transition must be made towards a circular plastic economy, wherein end-of-life plastics are recycled in a closed loop, fully regenerating the original polymers. To realize a circular plastic economy, new recycling techniques must be developed. Pyrolysis, the thermal conversion of a material in an inert atmosphere, is a high-potential technology to help enable a circular plastic economy. Currently, the fundamental understanding of plastic pyrolysis is limited but will be essential for the development of industrially relevant waste management solutions. The quantification of the intrinsic reaction kinetics of plastic pyrolysis is an ongoing challenge, owing to the complexity of pyrolysis chemistry and the limitations of existing analytical techniques. In this work, a new Pulse-Heated Analysis of Solid Reactions (PHASR) technique was developed that is uniquely capable of operation under reaction-controlled conditions absent transport limitations to measure the millisecond intrinsic kinetics of polyolefin pyrolysis. The capabilities of this reactor to pyrolyze polyolefins under kinetically limited, isothermal conditions with millisecond scale control were extensively validated. A second, Visual PHASR reactor system was developed that enables in situ observation of reaction polyolefins via high-speed photography. Observations of reacting polyolefins revealed the presence of reaction phenomena, including a potential Leidenfrost effect. The intrinsic millisecond reaction kinetics of polypropylene pyrolysis were successfully quantified. The overall reaction kinetics were described by a lumped first-order consumption model with an activation energy of 242.0 ± 2.9 kJ mol-1 and a pre-exponential factor of 35.5 ± 0.6 ln(s-1). Additionally, the production of the solid residues formed during polypropylene pyrolysis was investigated, revealing a secondary kinetic regime.
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    Methods and Mechanisms of Pyrolysis: Modeling Polymer Decomposition for a Circular Economy
    (2022-01) Wang, Ziwei
    Fast pyrolysis offers one of the most accessible ways to convert macromolecular feedstocks, such as biomass or plastic waste, into small molecules that can subsequently be used to produce liquid fuels and chemicals. The complexity of the feedstock and the dynamic changes in condensed phase chemical environments make it challenging to elucidate elementary kinetics and reaction mechanisms and control the selectivity to products. These effects also manifest in macroscopic phenomena such as measurable differences in kinetics and product distribution with changes in reaction temperature, feedstock composition, and even pellet size of the feedstock. These complexities are ultimately dictated by molecular transformations and controlled by the molecular structure and local chemical environments. Understanding the molecular processes in pyrolysis is crucial for the development of large-scale and economical biomass conversion or plastic upcycling facilities. This dissertation presents the development and applications of an ab initio-based kinetic Monte Carlo and Molecular Dynamics (KMC+MD) simulation approach that can model the kinetics of the fast pyrolysis and thermal reaction networks for biomass and polyolefin plastic feedstocks. This approach tracks detailed atomic structural information, including 3D coordinates of atoms and connectivity of chemical bonds in the feedstock molecules. It uses a stochastic simulation algorithm (SSA) to track and carry out the elementary reaction steps and uses classical MD simulations to follow the dynamics of the feedstock and the reaction environment as reactions proceed. The elementary step kinetics for the KMC simulations were established from detailed first-principle density functional theory (DFT) calculations. The detailed atomic-structural information is retained throughout the simulation, thus allowing the simulations to follow molecular transformations and the local environment during the reaction. As such, the simulations capture the unique kinetic manifestations that would otherwise be lost in composition-based deterministic models. This KMC+MD simulation approach is first used to model the pyrolysis reaction pathways of cellulose, including the paths to form levoglucosan as well as light oxygenates. The simulation results are able to reproduce temporal experimental product distributions and, in addition, gain molecular-level insights into the unique catalytic features that control the kinetics. The generality of the KMC simulation framework allows it to readily be adapted and used to simulate the kinetics and product distributions of polyolefin pyrolysis, including polyethylene and polypropylene feedstocks. The simulated polyolefin pyrolysis results are extensively compared with experimental data reported in the literature and those obtained by experimental collaborators in Prof. Paul Dauenhauer’s group. More generally, the simulation framework presented in this dissertation provides a powerful tool to study the thermal degradation of different polymeric feedstocks in pyrolysis systems that challenges the capability of deterministic models. The simulation approach offers molecular-level mechanistic insights and has direct applications in reactor modeling and techno-economic analyses of large-scale pyrolysis facilities.
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    New technologies for the complete rendering and economic conversion of waste oils to biofuels
    (2017-12) Anderson, Erik
    A novel process was developed for the biorefining of floatable wastewater scum and other waste oils from water treatment facilities into biodiesel and other value-added bio-products. To test the scalability and commercial potential of the technology, a 7,000 liter/year pilot-scale system was designed and built. Scum from a waste water treatment facility, located in St. Paul, Mn, was collected and converted into methyl esters (biodiesel) according to the process chemistry. All the incoming and outgoing process streams were sampled, tested, weighed and recorded to calculate both the process efficiency and product quality. Data from the pilot-scale systems operation was compared to laboratory results and the theoretically expected values for each individual unit operation. The product quality was tested using a third-party laboratory and confirmed the biodiesel produced during a single batch process met all of EPA’s test requirements for commercial-grade biodiesel. As a substrate for biodiesel, scum derived oil requires more pretreatment consideration than standard waste oils like used vegetable oil or brown grease. Combining acid hydrolysis and solvent extraction, a free fatty acid and acyl-glycerol rich product was produced from a highly impure source. Free fatty acids (FFA) present were converted to acyl-glycols via a high temperature (238°C) glycerin esterification process known as glycerolysis. The inorganic catalysts zinc aluminum oxide and sodium sulfate was tested during glycerolysis to compare the reaction kinetics of converting FFA to acyl-glycerols. It was concluded that the zinc-based catalyst increased the reaction rate significantly, from a “k” value of 2.57 (uncatalyzed) to 5.63, completing the reaction in 60 minutes, half the time it took the uncatalyzed reaction (120 min). Sodium sulfate’s presence however slowed the reaction, resulting in a “k” value of 1.45, completing the reaction in 180 minutes. Use of the external catalyst Zn-Al2O3 showed the greatest catalytic potential, but also assumes additional costs. In the U.S., the total amount of municipal solid waste is continuously rising each year. Millions of tons of solid waste and scum are produced annually that require safe and environmentally sound disposal. The availability of a zero-cost energy source like municipal waste scum is ideal for several types of renewable energy technologies. However, the way the energy is produced, distributed and valued also contributes to the overall process sustainability. An economic screening method was developed to compare the potential energy and economic value of three waste-to-energy technologies; incineration, anaerobic digestion, and biodiesel. A St. Paul, MN wastewater treatment facility producing 3,175 “wet” kilograms of scum per day was used as a basis of the comparison. After applying all theoretically available subsidies, scum to biodiesel was shown to have the greatest economic potential, valued between $491,949-$610,624/year. The incineration of scum yielded the greatest reclaimed energy potential at 29 billion kilojoules/year. The use of vacuum distillation for biodiesel production has become a reliable post-treatment method for removing multiple impurities, to consistently produce commercial-grade biodiesel. The waste produced from biodiesel distillation, vacuum distillation bottoms (VDB), is a mixture of higher molecular weight methyl esters (84%) and derivatives. Microwave-assisted pyrolysis (MAP) has been researched as a methyl ester recovery process for VDBs leaving vacuum distillation. Two types of MAP processing, dMAP and fMAP, were developed and tested to determine the optimal reaction conditions for producing a biodiesel analogue. The results indicate that after dMAP, 85.9% wt/wt of the VDBs were recovered as a transparent bio-oil then blended back into B100 biodiesel and certified for sale using ASTM D6751. Blending dMAP bio-oil (10% wt/wt) with B100 biodiesel met all certification requirements and demonstrated that MAP processing could be a significant yield improvement technology for any commercial biodiesel producer utilizing vacuum distillation.
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    On the Intrinsic Kinetics of Polyethylene Pyrolysis
    (2023-05) Mastalski, Isaac
    Global plastic use has grown exponentially over the past several decades, and this has led to a concomitant increase in plastic waste. Because current plastics, and polyolefins such as polyethylene in particular, have become a necessity for modern life, it is unlikely that more sustainable, alternative plastics can displace them anytime soon, so one of the best ways to mitigate plastic waste is to develop more sustainable, alternative recycling methods. Pyrolysis, or thermal degradation under an inert atmosphere, shows great promise in this regard, since it is capable of chemically recycling plastics back to their constituent monomers or to value-added chemicals. However, knowledge of the mechanisms and reaction kinetics underlying polyethylene pyrolysis remains extremely lacking, hindering development of large-scale plastic recycling capabilities. Therefore, the primary objective of this thesis was to investigate those kinetics and shed new light on the reasons behind the vast discrepancies reported in the literature. Fundamental understanding of polyethylene pyrolysis has previously been limited due to an inability to obtain intrinsic reaction kinetics; instead, the literature presently reports only apparent kinetics, which are a combination of intrinsic kinetics and a variety of other transport and system design limitations. In this thesis, an extensive summary of these limitations in other works is presented, and a new system, known as the Pulse-Heated Analysis of Solid Reactions, or PHASR, system was developed to overcome these limitations. The PHASR system is uniquely capable of operating under “isothermal, reaction-controlled” conditions, at which intrinsic kinetics can reliably be measured. The PHASR system was validated extensively to ensure operation in this desired regime, and detailed descriptions of the reactor setup and experimental methodologies are presented. Alongside this system, a second, Visual PHASR system was developed as well, to enable visualization of polyethylene pyrolysis reaction phenomena for the first time, via integrated high-speed photographic equipment. The method of PHASR was then used to study the intrinsic kinetics of polyethylene pyrolysis. Conversion of low-density polyethylene to pyrolysis products was measured over a range of reaction temperatures (550 to 650 °C) and reaction durations (20 ms to 2.0 s), and three distinct product lumps were characterized via integrated gas chromatography and a microgram-resolution balance. Lumped intrinsic reaction kinetics were calculated using these product fractions. The results were further validated by applying a generalized Rice-Herzfeld radical reaction model to the polyethylene pyrolysis system; good agreement was found between this first principles approach and the PHASR experimental data. Additionally, extensive characterization was performed on the residues left behind in PHASR post-pyrolysis, and this helped elucidate new insights into the different reaction timescale regimes that are present during polyethylene pyrolysis.
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    Production of Energy and Chemicals by Thermochemical Conversion from Recycled and Renewable Biomass
    (2016-08) Zhu, Cheng
    Recovery energy from municipal solid waste and biomass are one of the leading methods to achieve maximum energy efficiency and environmental sustainability. My research mainly includes two projects, novel biomass-supported sorbent for coal combustion emission control and fundamental study of biomass fast pyrolysis in the presence of alkaline earth metals. The first research project was collaborated with Accordant Energy LLC. on the development of ReEngineered Feedstock (ReEF), consisting of sorbent containing post-recycled paper and plastics. ReEF was evaluated in a laboratory-scale fluidized bed combustor system. The results indicate that co-firing ReEF with coal provides SO2 reduction in flue gas up to 85% as well as higher carbon conversion than pure coal combustion. Sulfation kinetics of ReEF combustion were evaluated in a drop-tube reactor. Sulfation of calcium hydroxide in ReEF was delayed due to RDF combustion when compared with pure calcium hydroxide sorbent. The second project investigated the catalytic effect of alkaline earth metals on cellulose pyrolysis primary (transport-free) and secondary (diffusion-limited) reaction pathways. Catalytic materials included homogeneous metal ions from their inorganic salts, and their corresponding heterogeneous metal oxides. While oxides were shown to have limited impact on cellulose pyrolysis chemistry, metal ions were found to significantly alter the secondary reaction pathways of cellulose under diffusion-limited conditions. The initial breakdown kinetics of cellulose were examined using a millisecond, thin-film reactor called PHASR (Pulse-Heated Analysis of Solid Reactions). Using the cellulose surrogate, α-cyclodextrin, the energetics of cyclodextrin decomposition were characterized. An interesting finding is that cellulose undergoes two distinct kinetic regimes with a distinct transition at 467 °C, which is interpreted as a reactive melting point.
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    Understanding the role of local condensed phase environments in pyrolytic and catalytic biomass conversion
    (2021-05) Maliekkal, Vineet
    Biomass conversion generally involves two major sets of chemical transformations – (1) thermal breakdown of macromolecules in the feedstock, such as cellulose, to smaller sugars and oxygenates via fast pyrolysis followed by (2) catalytic upgrading to the desired fuels or precursor chemicals. These reactions of biomass conversion usually occur in the condensed phase – either in the melt phase for pyrolytic reactions or in the solvent phase for catalytic upgrading reactions. The work in this thesis sheds light on the molecular complexity of such condensed phase environments. Explicit molecular modeling of these condensed phase environments coupled with first-principles simulation techniques such as density functional theory (DFT) and ab initio molecular dynamics (AIMD) are used to elucidate the influence of such environments on the kinetics of biomass conversion reactions. Examples from cellulose pyrolysis and hydrogenation chemistry are studied to demonstrate the critical importance of considering the role of condensed phase environments in biomass conversion.Using DFT calculations, constrained AIMD and experimental kinetics from the Pulsed Heated Analysis of Solid Reactions (PHASR) set-up, it is shown that vicinal hydroxyl groups which are present in the cellulose matrix in abundance can directly participate in the activation of cellulose by promoting facile proton transfer as well as stabilizing transition states through hydrogen bonding. The kinetic influence of calcium ions, naturally present in such feedstocks, is also examined in this thesis. It is shown that calcium interacts with cellulosic melt environment such that the native hydrogen bonding is disrupted. Such disruption of the hydrogen bonding network coupled with Lewis acid stabilization of the transition states leads to dual catalytic cycles for cellulose activation and second order rate dependence on calcium. Explicit modeling of the cellulosic environment is critical towards capturing such kinetic behavior. Furthermore, the influence of hydroxyl groups, calcium ions and more generally the cellulosic condensed phase environment, is examined more broadly and extended to other ring opening and fragmentation pathways that lead to glycolaldehyde, a side product of pyrolysis. The work from this part of the thesis helps establish the ubiquitous involvement of the local condensed phase environment in mediating biomass pyrolysis reactions. Finally, aqueous phase hydrogenation of C=C bonds in phenol over Pt particles inside zeolites is studied as a model reaction to demonstrate the importance of solvent environment in catalytic upgrading. Through explicit modeling of local water clusters around the reaction centers, it is shown that increasing the acidity of the zeolite supports can alter the local acidity of the water clusters. This in turn is shown to not just open up proton coupled electron transfer (PCET) pathways but also improve the efficacy of such mechanisms for hydrogenation. Thus, this study helps demonstrate that one can alter the solvent environment to enhance reactions of biomass conversion, especially those that involve proton transfer. More generally, the collective body of work in this thesis could act as a framework for future studies that seek to understand the role of condensed phase environments in biomass conversion as well as to develop strategies that use such environments for improved reactivity and selective chemical transformations.