Browsing by Subject "Hydrogen"

Now showing 1 - 6 of 6
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    Component terminal dynamics in weakly and strongly interacting blends.
    (2009-12) Ozair, Sehban N.
    Miscible blend dynamics have been long been a subject of interest and are not as well understood as dynamics of homopolymer melts. Their anomalous behavior, such as time-temperature superposition failure, broadening of calorimetric glass transition, etc., makes these systems very intriguing and challenges our understanding of miscible blend dynamics. In this work we investigated temperature and composition dependence of two different, dynamically heterogeneous blend systems using rheology and forced Rayleigh scattering (FRS). The first blend investigated was a weakly interacting one comprising poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA). Monomeric friction factors of PEO and PMMA were reported for a wide range of temperature and composition. PEO terminal dynamics were found to have strong composition dependence unlike that of PEO segmental dynamics previously reported. Also, PEO maintained its rapid relaxation mechanisms even in stiffer surroundings. The PEO hydroxyl end groups were found to have no significant impact on component chain dynamics. The FRS and rheology results agreed remarkably well for this system. The Lodge-McLeish model failed to describe the experimental results. In order to understand the role of hydrogen bonding on chain dynamics, a strongly interacting system of PEO/poly(vinyl phenol) (PVPh) was investigated using rheology. The blends consisted of a high molecular polymer tracer dispersed in low molecular weight matrix to extract relevant dynamic information from tracer contribution to material properties. Monomeric friction factors were reported for a wide temperature and composition range. Time-temperature superposition failure was observed in PEO tracer blends at high PVPh concentration. The shape of tracer relaxation spectra for PVPh tracer blends had a strong composition dependence while those for PEO tracer blends were independent of composition. The tracer contribution to blend viscosity had a strong temperature dependence at high PVPh composition. Across the composition range, single and narrow glass transitions were observed for these blends. PVPh chain conformations were investigated using SANS and contradictory conclusions were reached. Therefore, no conclusive remarks can be made regarding PVPh chain conformations in dilute solution.
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    Effects of homogeneous charge compression ignition (HCCI) control strategies on particulate emissions of ethanol fuel.
    (2010-12) Franklin, Luke
    This thesis presents a systematic investigation into the formation of particulate matter in homogeneous charge compression ignition (HCCI) engines. These engines are representative of the emerging generation of low sooting engine technology. Early research in the field concluded that engines operating with this combustion strategy could offer Diesel like efficiency while simultaneously reducing emissions of particulate matter and the oxides of nitrogen to nearly negligible levels. While quantification of gas phase emissions has changed little through modern regulatory history, the metrics defining particulate emissions and the state of understanding of the research community are rapidly evolving. Advances in technology for characterizing particulate emissions from spark ignition and compression ignition engines have been applied to HCCI emissions and the results indicate the production of significant quantities, by both number and mass, of particles from the HCCI combustion strategy. A relationship has been identified between in-cylinder behavior, and both gaseous and particulate emissions. It has been shown to be valid for 2 different fuels and multiple engine loads. Characteristics of the particulate matter suggest it is formed via gas to particle conversion, or nucleation, of the lighter distillates from the engines lubricating oil.
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    The oxidation of zinc vapor and non-stoichiometric ceria by water and carbon dioxide to produce hydrogen and carbon monoxide.
    (2012-06) Venstrom, Luke J.
    Experimental studies of two pathways for solar thermochemical metal oxide cycles to split water and carbon dioxide are presented. The heterogeneous oxidation of Zn(g) is investigated in Part I, and the oxidation of porous ceria is investigated in Part II. The heterogeneous oxidation of Zn(g) is proposed as an improved approach for rapid and complete oxidation of Zn. Reaction rates are measured gravimetrically in a quartz tube flow reactor at atmospheric pressure for conditions in which Zn is the limiting reactant, at temperatures between 800 and 1150 K, and for Zn(g), H2O(g), and CO2 partial pressures between 10-5 and 0.25 atm. The rate of Zn(g) oxidation by CO2 is between 0.3×10-8 and 6.5×10-6 mol cm-2 s-1, permitting conversions of Zn to ZnO greater than 84% in one second. The rate of Zn(g) oxidation by H2O is between 0.8×10-7 and 1.5×10-5-2 s-1 permitting conversions greater than ~80% in one second. A finite volume based numerical model decouples mass transfer and surface kinetics from the reaction rate data. The CO2-splitting kinetics are second-order, proportional to the Zn(g) and CO2 concentrations. The kinetic parameter is expressed in Arrhenius form, and the activation energy and pre-exponential factor are 44±3 kJ mol-1 and 92±6 mol m-2 s-1 atm-2, respectively. When expressed in second-order form, the apparent activation energy and pre-exponential factor of H2O-splitting are -110 kJ mol-1 and 1.8×10-5 mol m-2 s-1 atm-2 between 800 and 1050 K. At 1100 K, the activation energy becomes positive. A precursor mechanism, where the apparent activation energy is the sum of the heat of adsorption of H2O and the activation energy of the rate-limiting kinetic step is postulated to explain this behavior. The benefit of completely converting Zn via the heterogeneous oxidation of Zn(g) is an increase in the Zn/ZnO cycle efficiency from ~6% for polydisperse aerosol reactors, which have been limited to Zn conversions of 20% for reaction times on the order of a minute, to 27% and 31% for H2O- and CO2-splitting, respectively. In Part II, the effect of material morphology on the reduction and oxidation of ceria is investigated. The oxidation by H2O and CO2 of three-dimensionally ordered macroporous ceria (3DOM CeO2), which features an interconnected, ordered pore network, solid feature sizes between 80 and 200 nm, and a moderate specific surface area of 10 m2 g-1, is compared to the oxidation of non-ordered mesoporous ceria and sintered, low porosity ceria at 1100 K in 6 isothermal chemical cycles. The 3DOM CeO2 increases the maximum H2 and CO production rates over the low porosity CeO2 by 125 and 260%, and increases the maximum H2 and CO production rates over the non-ordered mesoporous CeO2 by 75 and 175%. 3DOM CeO2, non-ordered macroporous ceria (NOM CeO2), and aggregates of ceria nanoparticles are also cyclically reduced at ~1500 K under pO2 = 10-5 atm and oxidized at ~1100 K by 25 mol% CO2. The 3DOM and NOM CeO2 retain an interconnected, disordered pore network and achieve maximum CO production rates of 6.4 and 4.0 mL min-1 g-1, respectively, an order of magnitude increase over the ~0.1 mL min-1 g-1 rate of CO production of the sintered ceria nanoparticles and low porosity ceria. The present study demonstrates the importance of engineering ceria with interconnected porosity and solid feature sizes on the order of hundreds of nm.
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    Synthesis gas use in internal combustion engines.
    (2010-12) Bika, Anil Singh
    The objective of this dissertation was to investigate the combustion characteristics of a compression ignition, spark ignition, and homogeneous charge compression ignition engine operating on various blends of synthesis gas. To fully investigate the three ICE operating regimes, experimental investigations were carried out to focus on: 1.) A CI engine operating on ethanol and hydrogen fuel 2.) A CI engine operating on diesel fuel with varying blends of synthesis gas 3.) A SI engine operating on varying blends of synthesis gas 4.) An HCCI engine operating on hydrogen fuel 5.) An HCCI engine operating on varying blends of synthesis gas The three operating modes (CI, SI, and HCCI) were selected because it is unlikely that an engine will be able to operate solely in an HCCI regime throughout the complete load range. The more common CI and SI regimes will likely be necessary for high load engine operation. The results from this doctoral work sheds light into the fundamental aspects of syngas combustion and also provides a foundation for future gasification plant designers and synthesis gas producers, regarding the fuel composition needs of a syngas powered internal combustion engine. The first 3 chapters of this dissertation provide an introduction and background for this doctoral work. The remaining chapters present the results and conclusions
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    Thermochemical Recuperation and Catalytic Strategies for Anhydrous Ammonia in Combustion Systems
    (2021-06) Kane, Seamus
    The carbon intensity of combustion engines poses a major challenge to worldwide efforts to minimize climate change. Anthropogenic carbon dioxide (CO2) is greatest source of atmospheric warming and its emission must be curtailed to affect climate forcing in a meaningful way. Carbon-neutral alternatives such as ethanol and biodiesel recycle atmospheric carbon under ideal conditions yet result in net carbon emissions due to process inefficiencies. Fuels decoupled from chemical carbon are necessary to reduce carbon intensity and halt climate change. Anhydrous ammonia is one such fuel as it can be produced entirely by renewable means and contains no carbon. This body of work investigates combustion applications of anhydrous ammonia in compression ignition (CI) engines, methods of catalytically enhancing ammonia for more efficient combustion and use of the endothermic ammonia decomposition reaction for waste-heat recovery.This thesis presents the applications of catalytic ammonia decomposition, specifically pertaining to, but not limited to internal combustion engines. Ammonia has proved a suitable replacement fuel in spark-ignition (SI) engines and as a secondary fumigated fuel for CI engines. Stability of the ammonia molecule results in poor flame propagation and low ignition reactivity. Using ammonia in a dual-fuel arrangement overcomes these issues in existing engines and combustors designed for hydrocarbon fuels. Alternatively, ammonia can be converted to hydrogen using catalysis, which in turn enhances ammonia combustion without the need for a secondary high reactivity fuel. This work explores hydrogen-enhanced ammonia and diesel combustion in CI engines equipped with catalytic waste-heat recovery. Engines were operated over their full range under various levels of ammonia fuel replacement to determine the effects on engine and combustion efficiency as well as emissions and stability. Thermochemical recuperation and thermal recovery were analyzed across the operability range towards identifying optimal system parameters. The primary finding in the first part of this work is that ammonia effectively recovers waste energy using low-temperature high-active catalysts. Activity is demonstrated as low as 200 °C for Ruthenium-based catalysts, and full conversion to hydrogen results in a net lower heating value increase of 15%. Heat transfer to sustain the decomposing ammonia proved difficult however, as the experimental catalyst unit was undersized for engine operation. Despite low conversion, sufficient hydrogen was generated to enhance flame speed, combustion efficiency and engine thermal efficiency as compared to pure ammonia fumigation. Fuel-bound nitrogen in ammonia generated high oxides of nitrogen (NOx) and N2O emissions upon combustion. However, unburned ammonia present in exhaust was measured to be ideal for passive elimination of these species using selective catalytic reduction (SCR). Emissions and efficiencies measured suggest that future implementation of ammonia dual fuel requires higher rates of heat recovery and higher ammonia replacement rates than those demonstrated in this study. Both conditions can be met using a modified catalyst design and higher flow ammonia fueling system, respectively. Ammonia decomposition catalysis is thoroughly described in literature but heat transfer inside a supporting monolith structure is not. This work presents a computationally efficiency quasi-2D modeling procedure for understanding heat and mass transfer in metal monoliths. A finite difference model was developed to simulate thermal behavior of the decomposition catalyst used in experimental studies. The heat transfer model was calibrated against inert gas experiments and showed excellent agreement with convective and conductive values from literature. Argon, air and CO2 were used under identical thermal conditions to demonstrate robustness in simulating generic flows through the reactor. Agreement of the model against the entire experimental dataset demonstrated robustness in predicting metallic support thermal behavior while the simplicity of the approach presented a computationally inexpensive alternative to CFD or physical prototype design screening. Design screening was then conducted using varied input conditions and showed the relative importance of each parameter on heat exchange effectiveness and wall-average heat transfer coefficient. Optimal performance was quantified, and the effects of design parameters on heat exchange was discussed. High catalyst activity and reaction residence time are needed to achieve high hydrogen yield, promoting efficient combustion of ammonia-hydrogen mixtures. To overcome thermal limitations posed by waste-heat driven decomposition, ammonia partial oxidation can be used to create an endogenous heat source and increase yield. Oxidation and decomposition were combined in an autothermal process and were shown to increase both hydrogen fraction and the hydrogen-to-ammonia ratio of the reformate stream. Autothermal ammonia decomposition (ATD) resulted in fuel heating value decrease, which was comparable to heating value losses expected from poor combustion efficiency in engines. A comprehensive reactor model was developed using two global reaction rates and the previous monolith heat transfer model. Rates were determined through non-linear regression and showed excellent fit across thermal and autothermal regimes. Deficiencies in experimental reactor design were identified using the model, and potential design changes were discussed. The model and experiment both suggest that ATD is a promising alternative to waste-heat recovery approaches alone when a high reactivity reformate mixture is needed. The research shows that ammonia ATD reformate is of sufficient reactivity to enable drop-in replacement of hydrocarbon fuels in unmodified engines and combustors.
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    Zinc aerosol hydrolysis in a transverse jet reactor.
    (2011-12) Haltiwanger, Julia Frances
    Some of the major challenges---both technical and economic---of the Zn/ZnO two-step thermochemical hydrogen production cycle are investigated in this study. Technically, complete hydrolysis of Zn in the hydrogen production step remains a major barrier to implementation, and much attention has been given to Zn nano-scale reacting aerosols as a solution. Smaller particles favor faster reaction kinetics, and because they can be entrained and reacted in a gas flow, a continuous controllable process is possible. However, success of this continuous process depends on achieving high particle yields and high conversions in the aerosol, neither of which have yet been achieved in laboratory reactors. The ability of a new reactor concept based on transverse jet fluid dynamics to control the flow field and rapidly cool the Zn vapor is investigated. In the transverse jet reactor, evaporated Zn entrained in an Ar carrier gas issues vertically into the horizontal tubular reactor through which cooler H2O and Ar flow. Particles are formed in the presence of steam at ~450 K. The objective of controlling the flow field is to keep Zn away from the walls, thereby reducing particle deposition in the reactor and increasing particle yields on the filter. A computational fluid dynamics (CFD) model indicates that the trajectory of the jet can be controlled so that the majority of the Zn mass is directed down the center of the reactor, not near the reactor walls. Furthermore, the model shows that quench rates of 2x10^4 K/s are achieved and reactants are well mixed. Experimentally, maximum particle yields of 93% of the mass entering the reactor are obtained. Hydrolysis experiments are conducted in the transverse jet reactor at 418 K, 573 K, 603 K, and 713 K to assess the mechanisms of particle growth and hydrolysis. Experiments are conducted with and without steam to assess the effect of the reacting gas on particle morphology. SEM images of particles collected on a filter downstream from the reaction zone indicate that particle growth is dominated by condensation, resulting in hexagonal particles generally with lengths across their hexagonal face of 300 nm to 1micron in experiments with stream, and 1 to 3 micron in experiments without steam. Furthermore, the SEM images indicate that in hydrolysis experiments, ZnO forms on the surface of particles early on, protecting them from re-evaporation. Particle yield on the filter, Y, is defined as the fraction of the total mass entering the reactor that is collected on a filter placed downstream of the reaction zone. Overall conversion, X, is measured by monitoring the H2 content of the effluent gas throughout experiments with a gas chromatograph. Conversion of aerosol particles, Z, is the ZnO content (by mole) of particles collected from the downstream filter; it is measured by x-ray diffractometry with the internal standard calibration method. At all temperatures, particle yield remains high---generally 70 to 80% in hydrolysis experiments---and particle deposition on the walls of the reaction zone is eliminated for temperatures of 573 K and above. However, the conversion in the aerosol is <7% and decreases with reaction zone temperature. The overall conversion ranges from 11% at 418 K to 49% at 713 K. The higher overall conversion than conversion in the aerosol is attributed to heterogeneous Zn vapor hydrolysis. Visual observation proves heterogeneous hydrolysis occurs on the reactor walls; it is inferred that the heterogeneous Zn vapor reaction also occurs on the surface of aerosol particles. In this study, high particle yields are achieved for the first time---an important step forward for the continuous aerosol process. However, complete conversion of the aerosol particles remains a major challenge. In an economic and policy study of the Zn/ZnO cycle, the time frame for economic viability is assessed through the use of experience curves under minimal input, mid-range, and aggressive incentive policy scenarios. For the technology to become cost competitive, incentive policies that lead to early implementation of solar hydrogen plants will be necessary to allow the experience effect to draw down the price. Under such policies, a learning curve analysis suggests that hydrogen produced via the Zn/ZnO cycle could become economically viable between 2032 and 2069, depending on how aggressively the policies encourage the emerging technology. Thus, if the technical challenges are resolved, the Zn/ZnO cycle has the potential to be economically viable by mid-century if incentive policies--such as direct financial support, purchase guarantees, low interest rate loans, and tax breaks--are used to support initial projects.