Theses and Dissertations from UMD
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Item CATALYST DEVELOPMENT FOR NON-OXIDATIVE METHANE UPGRADING TOWARDS HYDROCARBONS AND HYDROGEN PRODUCTION(2024) LIU, ZIXIAO; Liu, Dongxia; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Methane, the main constituent of natural gas and biogas is deemed to be an alternative source to replace crude oil in the production of chemicals and fuel. Non-oxidative methane conversion enables methane coupling or splitting to produce hydrogen and more significant hydrocarbons, but catalyst deactivation has been a challenge in past research. This dissertation addresses catalyst deactivation issues in non-oxidative methane conversion by inventing novel catalyst systems. For direct non-oxidative methane coupling, a pathway for methane upgrading into hydrogen, olefin, and aromatic products, the silica-supported catalysts were synthesized by flame fusion of a mixture of quartz silica and metal silicate precursors. Compared to the cristobalite silica-supported catalysts reported previously, vitreous silica-supported catalysts have disordered Si-O bonds and structural defects, enabling better metal dispersion and more vital metal-support interaction. The as-prepared vitreous silica-supported iron catalyst had a shorter induction period in methane activation and lower coke yield. The increase in iron concentration elongated the catalyst induction period and promoted aromatics and coke formation. Among different transition metal catalysts, the cobalt supported by vitreous silica had the best methane conversion, hydrocarbon product yield, and catalyst stability. For catalytic methane pyrolysis, a pathway producing COx-free hydrogen and valuable carbon product, a siliceous zeolite-supported cobalt catalyst was invented. In comparison to the methane pyrolysis catalysts in literature, the siliceous zeolite support in the invented catalyst has limited Brönsted acidity and increased mesoporosity, which limited the acid-catalyzed deactivation mechanism and facilitated the mass transport, and thus significantly increased the catalyst lifetime. The cobalt sites change the cluster sizes and coordination structures with the loading concentrations in the zeolite support, which leads to carbon products with different properties.Item Exploiting Causal Reasoning to Improve the Quantitative Risk Assessment of Engineering Systems Through Interventions and Counterfactuals(2023) Ruiz-Tagle, Andres; Groth, Katrina; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The main strength of quantitative risk assessment (QRA) is to enable risk management by providing causal insights into the risk of an engineering system or process. Bayesian Networks (BNs) have become popular in QRA because they offer an improved causal structure that represents analysts’ knowledge of a system and enable reasoning under uncertainty. Currently, the use of BNs for risk-informed decisions is based solely on associative reasoning, answering questions of the form "If we observe X=x, how likely is it to observe Y=y?” However, risk management in the industry relies on understanding how a system could change in response to external influences (e.g., interventions and decisions) and identifying the causes and mechanisms that could explain the outcome of past events (e.g., accident investigations and lessons learned). This dissertation shows that associative reasoning alone is insufficient to provide these insights, and it provides a framework for obtaining more complex causal insight using BNs with intervention and counterfactual reasoning. Intervention and counterfactual reasoning must be implemented along with BNs to provide more complex insights about the risk of a system. Intervention reasoning answers queries of the form "How does doing X=x change the likelihood of observing Y=y?” and can be used to inform the causal effect of interventions and decisions on the risk and reliability of a system. Counterfactual reasoning answers queries of the form "Had X been X=x' in an event, instead of the observed X=x, could Y have been Y=y', instead of the observed Y=y?” and can be used to learn from past events and improve safety management activities. BNs present a unique opportunity as a risk modeling approach that incorporates the complex causal dependencies present in a system’s variables and allows reasoning under uncertainty. Therefore, exploiting the causal reasoning capabilities of BNs in QRAs can be highly beneficial to improve modern risk analysis. The goal of this work is to define how to exploit the causal reasoning capabilities of BNs to support intervention and counterfactual reasoning in the QRA of complex systems and processes. To achieve this goal, this research first establishes the mathematical background and methods required to model interventions and counterfactuals within a BN approach. Then, we demonstrate the proposed methods with two case studies concerning the risk of third-party excavation damage to natural gas pipelines in the U.S. The first case study showed that the intervention reasoning methods developed in this work produce unbiased causal insights into the effectiveness of implementing new excavation practices. The second case study showed how the counterfactual reasoning methods developed in this work can expand on the lessons learned from an accident investigation on the Sun Prairie 2018 gas explosion by providing new insights into the effectiveness of current damage prevention practices. Finally, associative, intervention, and counterfactual reasoning methods with BNs were integrated into a single model and used to assess the risk of a highly complex challenge for the future of clean energy: excavation damages to natural gas pipelines transporting hydrogen. The impact of this research is a first-of-its-kind approach and a novel set of QRA methods that provide expanded causal insights for understanding failures and accidents in complex engineering systems and processes.Item MECHANISMS CONTROLLING VOLATILE FATTY ACID AND FERMENTATION GAS PRODUCTION IN THE RUMEN(2022) Scott, Jarvis G; Kohn, Richard A; Animal Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Atmospheric methane accounts for less than approximately 16% of global anthropogenic greenhouse gas emission. However, it is significantly greater and trapping heat when compared to atmospheric CO2 on a molar lever and any reduction in atmospheric abundance in warranted. Enteric methane from ruminant species accounts for a fraction (< 30%) of the total atmospheric methane however its production also accounts for major dietary energy loss in ruminant species and affects feed efficiency and overall production. Major studies have investigated numerous feed additives and supplements with highly variable finding on the antimethanogenic property of these compounds or feeding strategies, however the findings have raised other questions regarding shifts in VFA profiles accompanying methane inhibition. Higher inclusion levels of concentrate and other nonstructural starch in the diet of ruminants have been shown to decrease methane production and shift volatile fatty acid (VFA) profiles in the rumen. Additionally, many studies have suggested that inhibiting methane production avails as a reducing equivalent to fuel the propionate producing pathway and therefore results in shift in VFA profiles in the rumen. However, very little is understood regarding how these VFA shifts come about. Microbial Kinetics and thermodynamics are physiochemical principles that can be used to study how concentrate inclusion in ruminant diets can change the substrate concentrations and ultimately lead to shifts in fermentation profile in the rumen. Substrate availability supports and/or limits the growth of microbial population in the rumen, while the accumulation of the products or reactants for major fermentation reactions dictate the profile of the VFA. Understanding the role of these physiochemical principles and ultimately the mechanisms involved with changes the profile of VFA and fermentations gas in the rumen would help researchers understand how VFA profiles are shifting during methane inhibition as well as possibly identifying a more targeted approach for inhibiting enteric methane production. Therefore, the objectives of this projects are: to develop an in vitro method to understand the basal kinetic parameters of metabolism in the rumen, to evaluate the effects of increasing forage-to-concentrate ratio on performance and change in in VFA and fermentation gas in vivo, and to test the effect of various perturbations (fermentation metabolites e.g. sodium acetate, sodium lactate etc.) on the fermentation profile of rumen fluid adjusted to different forage-to-concentrate ration. The results indicate that rumen fluid from cows on a high-concentrate diet have a greater capacity to make propionate compared to the high forages diet. The higher propionate production limits the availability of which is necessary for the synthesis of CH4. The finding also suggests that methanogenesis is process limited by substrate concentration. Finally, our studies indicate that feeding strategies targeting enzymatic activity favoring propionate production may be more beneficial than targeting methanogens in a high forage diet.Item Metal hydrides as a platform for reconfigurable photonic and plasmonic elements(2021) Palm, Kevin James; Munday, Jeremy N; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Metal hydrides often display dramatic changes in optical properties upon hydrogenation. These shifts make them prime candidates for many tunable optical devices from optical hydrogen sensors and switchable mirrors to physical encryption schemes. In order to design and fabricate optimized devices for any of these applications, we need to determine the optical and structural properties of these materials. In this dissertation, we design and implement an apparatus that dynamically measures the gravimetric, stress, calorimetric, and optical properties of metal hydrides as they are exposed to H2. We use this apparatus to measure the properties of 5 different pure metal hydrides (Pd, Mg, Ti, V, and Zr) and then use these properties to design tunable color filters and switchable perfect absorbers, among other devices. To widen our parameter space and to combine desirable characteristics of different metal systems, we use the same apparatus to investigate the properties of different metal alloy hydride systems including Pd-Au, Mg-Ni, Mg-Ti, and Mg-Al. We demonstrate many improved nanophotonic designs with these materials, including a thin-film physical encryption scheme with Pd-Au and a switchable solar absorber with Mg-Ti. Many of these photonic devices can be further enhanced by tailoring the substrate of the device along with the metal hydride. In this dissertation, we also investigate combining the switchable optical properties of metal hydrides with near-zero-index substrates to further enhance the optical device changes. Near-zero-index materials are ones where the refractive index is below 1 and can lead to a variety of interesting optical effects, including high absorption in surrounding materials and enhanced non-linear effects. By combining an ITO substrate with a near-zero-index resonance at ~1250 nm with a thin Pd capped Mg film, we demonstrate a switchable absorption device with >76% absorption change at 1335 nm illumination. To further explore the possibility of large-scale fabrication of these devices, we survey the properties of commercially available near-zero-index materials and report the range of attainable optical properties, showing its feasibility.Item STRUCTURAL EVOLUTION DURING THERMAL TREATMENTS AND THE RESULTANT MECHANICAL BEHAVIOR OF HIGH STRENGTH LOW ALLOY STEELS(2018) Draper, Matthew Charles; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)HY steels were designed as a solid solution strengthened grade for both high yield strength and high impact toughness in structural applications for Naval vessels. These alloys are susceptible to both hydrogen and temper embrittlement which necessitates high expense manufacturing processes to preclude these conditions. Successful implementation of lower cost and higher reliability treatments requires an improved understanding of the structural evolution and corresponding changes in mechanical behavior for the alloy. This research combines mechanical and microstructural characterization methods along with thermodynamic and kinetic models to build a comprehensive understanding of the effects of thermal treatments on the structure-property relationship of the alloy system. The embrittlement rate was studied between 315°C and 565°C at varied logarithmic time intervals up to 40,000 minutes. The embrittlement recovery rate was studied between 593°C and 704°C at logarithmic time intervals up to 10,000 minutes. Finally, hydrogen aging was studied between 315°C and 565°C at varied thermodynamically equivalent time intervals. A variety of test methods were employed for characterization including: traditional metallographic techniques, mechanical testing, computational modeling, and a novel image analysis technique for carbide analysis. Metallographic along with computational work supports a conclusion that temper embrittlement and subsequent recovery cannot be solely explained by the segregation of phosphorus and other embrittling elements to grain boundaries. Rather it is shown for the first time that alloy carbides play a key role in embrittlement for this system. The evolution of these carbides serves both to create initiation sites for cleavage fracture and deplete the matrix of Mo, which is a P scavenger. Recovery from embrittlement is thus proposed to be related to both the removal of P from the boundary and the dissolution of carbides. From these results a series of kinetic models have been developed for the nucleation, dissolution, and coarsening of alloy carbides. Models developed for the mitigation of monatomic hydrogen show a novel treatment for hydrogen aging via performing the aging within the embrittlement range with follow on treatments designed to recover from embrittlement. This new treatment has the potential to reduce hydrogen aging times by up to 90% in industrial manufacture.Item Non-Catalytic Thermal Reforming of JP-8 in a Distributed Reactor(2017) Scenna, Richard Michael; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This investigation focuses on developing a fundamental understanding of the thermochemical behavior of the application of the advanced combustion technique of Colorless Distributed Combustion to the thermal partial oxidation of a hydrocarbon fuel. Distributed Reaction Regime is achieved through internal entrainment and dilution to enlarge the “reaction zone” to encompass the entire reactor. The expanded reaction zone results in a uniform thermal field and product distribution. This in turn increases the local availability of water and carbon dioxide, which promotes steam and dry reforming reactions to a lesser extent, enhancing syngas yields. It was observed that the more distributed conditions (greater entrainment) yielded higher reformate quality. In the high temperature reactor, this resulted in higher hydrogen yields. In lower temperature reactor, the more distributed conditions shifted the hydrocarbon carbon distribution to favor ethylene and methane over acetylene. Middle distillate fuels are very challenging to reform. The high sulfur, aromatic, and carbon content inherent in these fuels will often deactivate conventional reforming catalysts. To compensate for the lack of catalyst, non-catalytic reformers employ high reactor temperatures, but this promotes soot formation and reduces reforming efficiency. Reforming under Distributed Reaction Regime avoids the issues associated with catalysts, while avoiding the issues associated with operating at higher reactor temperatures. The middle distillate fuel, Jet Propellant 8 (JP-8) is of particular interest to the military for small fuel cell applications and was determined to be a good representative for middle distillate fuels. This novel approach to reforming is undocumented in literature for a non-catalytic approach. This investigation studies the thermochemical behavior of a middle distillate fuel under reforming conditions. Chemical time and length scales are controlled through variations in injection temperature, oxidizer concentration, and steam addition. Two reactors were developed to study two different temperature ranges (700-800°C and 900-1100°C). These reactors will allow systematic means to enhance favorable hydrogen and carbon monoxide yields. Through the course of investigation it was observed that conditions that promoted a more distributed reactor were found to yield higher quality reformate. On multiple instances, the improvement to reforming efficiencies was greater than could be accounted for by varying the reactants alone. Reforming efficiency was demonstrated as high as 80%, rivaling that of catalytic reforming (85%)[1]. The Distributed Reaction Regime suppressed soot formation from occurring within reactor. No soot formation within the reactor was observed while operating within the Distributed Reaction Regime.Item Temperature and Pressure Effects on Hydrogen Permeation in Palladium Based Membranes(2010) James, Ryan T.; Gupta, Ashwani K.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Palladium based membranes offer a promising method for extracting hydrogen from multi-component synthetic gas (syngas) mixtures. Thin palladium and palladium alloy membranes supported on porous media combine both enhanced strength and durability with increased permeation. The syngas produced from waste and biomass contains several gases of different concentrations. The availability of clean hydrogen from syngas is novel since the hydrogen storage and transportation are amongst the major issues for the utilization of hydrogen. A lab scale experimental facility has been designed and built that allows one to examine different types of membranes for efficient and effective separation of hydrogen from syngas. Experimental results have been obtained from this facility using palladium membranes. The results show hydrogen permeation increased with both temperature and pressure, with the greatest increase occurring with rising temperature. Determination of the pressure exponent revealed that the reaction was limited by both the surface reaction and diffusion process.Item Numerical Investigations of Gaseous Spherical Diffusion Flames(2009) Lecoustre, Vivien Renaud Francis; Sunderland, Peter B.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Spherical diffusion flames have several unique characteristics that make them attractive from experimental and theoretical perspectives. They can be modeled with one spatial dimension, which frees computational resources for detailed chemistry, transport, and radiative loss models. This dissertation is a numerical study of two classes of spherical diffusion flames: hydrogen micro-diffusion flames, emphasizing kinetic extinction, and ethylene diffusion flames, emphasizing sooting limits. The flames were modeled using a one-dimensional, time-accurate diffusion flame code with detailed chemistry and transport. Radiative losses from products were modeled using a detailed absorption/emission statistical narrow band model and the discrete ordinates method. During this work the code has been enhanced by the implementation of a soot formation/oxidation model using the method of moments. Hydrogen micro-diffusion flames were studied experimentally and numerically. The experiments involved gas jets of hydrogen. At their quenching limits, these flames had heat release rates of 0.46 and 0.25 W in air and in oxygen, respectively. These are the weakest flames ever observed. The modeling results confirmed the quenching limits and revealed high rates of reactant leakage near the limits. The effects of the burner size and mass flow rate were predicted to have a significant impact on the flame chemistry and species distribution profiles, favoring kinetic extinction. Spherical ethylene diffusion flames at their sooting limits were also examined. Seventeen normal and inverse spherical flames were considered. Initially sooty, these flames were experimentally observed to reach their sooting limits 2 s after ignition. Structure of the flames at 2 s was considered, with an emphasis on the relationships among local temperature, carbon to oxygen atom ratio (C/O), and scalar dissipation rate. A critical C/O ratio was identified, along with two different sooting limit regimes. Diffusion flames with local scalar dissipation rates below 2 1/s were found to have temperatures near 1410 K at the location of the critical C/O ratio, whereas flames with greater local scalar dissipation rate exhibited increased temperatures. The present work sheds light on important combustion phenomenon related to flame extinction and soot formation. Applications to energy efficiency, pollutant reduction, and fire safety are expected.Item Fire Safety of Today's and Tomorrow's Vehicles(2008-05-02) Levy, Kevin Martin; Sunderland, Peter B; Milke, James A; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis considers fire hazards in the existing vehicle fleet and uses failure modes and effects analyses of three generic designs to identify and rank potential fire hazards in the Emerging Fuel Vehicle (EFV) fleet. A statistics based predictive quantitative risk assessment framework and estimated uncertainty analysis is presented to predict risk of EFV fleets. The analysis also determines that the frequency of fire occurrence is the greatest factor that contributes to risk of death in fire. These preliminary results predict 420±14 fire related deaths per year for a fleet composed entirely of gasoline-electric hybrid vehicles, 910±340 for compressed natural gas vehicles, and 1300±570 for hydrogen fuel-cell vehicles relative to the statistical record of 350 for traditional fuel vehicles. The results are intended to provide vital fire safety information to the traveling public as well as to emergency response personnel to increase safety when responding to EFV fire hazards.Item Flame Quenching Limits of Hydrogen Leaks(2008-05-01) Moran, Christopher; Sunderland, Peter; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study examines flame quenching limits of hydrogen leaks in compression fittings and tube burners. Experimental work is presented. Measurements included ignition limits for leaking compression fittings on tubes of 3.16-12.66 mm in diameter and the ignition and quenching limits of tube burners with diameters of 0.15 - 0.56 mm. Minimum ignition flow rates of 0.028 mg/s for hydrogen, 0.378 mg/s for methane, and 0.336 mg/s for propane were found in the compression fitting experiments. The upstream pressure does not play a role in the ignition flowrate limit. The minimum quenching limits of hydrogen found in tube burners were 2.1 and 3.85 μg/s in oxygen and air, respectively. These correspond to heat release rates of 0.252 and 0.463 W, respectively, the former being the weakest observed flame ever.