Theses and Dissertations at Montana State University (MSU)
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Item Condensation of chromium vapor, generated in high-temperature (>800°C) environments, and interactions with aluminosilicate surfaces(Montana State University - Bozeman, College of Engineering, 2024) van Leeuwen, Travis Kent; Chairperson, Graduate Committee: Paul E. Gannon; This is a manuscript style paper that includes co-authored chapters.This work represents a collection of research and reporting with the goal of improving fundamental understanding of chromium (Cr) vapor reactive condensation, relevant in many high-temperature (>800°C) process environments where Cr-containing alloys are used. While reactive evaporation of Cr from stainless steels used in high-temperature solid oxide electrochemical systems is well-documented, the dynamics of Cr condensation onto surrounding interfaces during complex and dynamic system operation is less understood. Understanding these interactions during operation is critical for improving system performance and safeguarding environmental, health and safety, as some condensed species contain hexavalent chromium (Cr(VI)), a known carcinogen. A series of studies were designed and conducted to investigate the condensation pathways of Cr vapors within representative high-temperature system environments, simulating extreme conditions for Cr evaporation and downstream aluminosilicate fibers used in high-temperature insulation. The first study focuses on the influence of water vapor concentration in the gaseous environment on reactive Cr condensation and speciation onto aluminosilicate fibers. The second study explores the effects of alkaline oxide additives in aluminosilicate fibers on Cr condensation and speciation. The third study investigates the effects of presence of alkaline oxides within the Cr vapor source on reactive evaporation and condensation of Cr vapors onto downstream aluminosilicate fibers. To accomplish the specific objectives of these studies, Cr vapors, produced by high-temperature (>800°C) air exposures of trivalent chromium (Cr(III)) oxide (Cr 2O 3) (chromia) powder with variable moisture content, were condensed onto various ceramic materials (aluminosilicate fibers) downstream at lower temperatures (100-500°C). Total condensed Cr and ratios of oxidation states were measured using inductively coupled plasma optical emission spectroscopy (ICP-OES) and diphenyl carbazide (DPC) colorimetric/direct UV-VIS spectrophotometric analyses, respectively. Results indicate presence of both Cr(III) and Cr(VI) species condensed on all samples investigated. Total Cr and ratio of Cr(VI) to total Cr detected was significantly more on those containing alkaline oxides and at higher atmospheric water vapor concentration, while the presence of alkaline oxides in the Cr vapor source (Cr 2O 3) decreased the evaporation and amount of Cr/Cr(VI) condensed on the samples downstream. Computational thermodynamic equilibrium modelling helps explain experimental results showing the relative stability of alkaline-chromate compounds.Item Investigation of aluminosilicate refractory for solid oxide fuel cell applications(Montana State University - Bozeman, College of Engineering, 2010) Gentile, Paul Steven; Chairperson, Graduate Committee: Stephen W. Sofie; Paolo R. Zafred and Stephen W. Sofie were co-authors of the article, 'Progress in understanding silica transport process and effects in solid oxide fuel cell performance' in the journal 'Proceedings of the ASME eight international fuel cell science, engineering & technology conference in Brooklyn, New York, USA'. Its abstract is contained within this thesis.; Stephen W. Sofie, Camas F. Key, and Richard J. Smith were co-authors of the article, 'Silicon volatility from alumina and aluminosilicates under solid oxide fuel cell operating conditions' in the journal 'International Journal of Applied Ceramic Technology' which is contained within this thesis.; Stephen W. Sofie was a co-author of the article, 'Investigation of aluminosilicate as a solid oxide fuel cell refractory' in the journal 'Journal of Power Sources' which is contained within this thesis.Stationary solid oxide fuel cells (SOFCs) have been demonstrated to provide clean and reliable electricity through electro-chemical conversion of various fuel sources (CH 4 and other light hydrocarbons). To become a competitive conversion technology the costs of SOFCs must be reduced to less than $400/kW. Aluminosilicate represents a potential low cost alternative to high purity alumina for SOFC refractory applications. The objectives of this investigation are to: (1) study changes of aluminosilicate chemistry and morphology under SOFC conditions, (2) identify volatile silicon species released by aluminosilicates, (3) identify the mechanisms of aluminosilicate vapor deposition on SOFC materials, and (4) determine the effects of aluminosilicate vapors on SOFC electrochemical performance. It is shown thermodynamically and empirically that low cost aluminosilicate refractory remains chemically and thermally unstable under SOFC operating conditions between 800°C and 1000°C. Energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) of the aluminosilicate bulk and surface identified increased concentrations of silicon at the surface after exposure to SOFC gases at 1000°C for 100 hours. The presence of water vapor accelerated surface diffusion of silicon, creating a more uniform distribution. Thermodynamic equilibrium modeling showed aluminosilicate remains stable in dry air, but the introduction of water vapor indicative of actual SOFC gas streams creates low temperature (<1000°C) silicon instability due to the release of Si(OH) 4 and SiO(OH) 2. Thermal gravimetric analysis and transpiration studies identified a discrete drop in the rate of silicon volatility before reaching steady state conditions after 100-200 hours. Electron microscopy observed the preferential deposition of vapors released from aluminosilicate on yttria stabilized zirconia (YSZ) over nickel. The adsorbent consisted of alumina rich clusters enclosed in an amorphous siliceous layer. Silicon penetrated the YSZ along grain boundaries, isolating grains in an insulating glassy phase. XPS did not detect spectra shifts or peak broadening associated with formation of new Si-Zr-Y-O phases. SOFC electrochemical performance testing at 800-1000°C attributed rapid degradation (0.1% per hour) of cells exposed to aluminosilicate vapors in the fuel stream predominately to ohmic polarization. EDS identified silicon concentrations above impurity levels at the electrolyte/active anode interface.