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    The partitioning of trace elements during pulverized coal combustion

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    Author
    Seames, Wayne Stewart
    Issue Date
    2000
    Keywords
    Engineering, Chemical.
    Environmental Sciences.
    Energy.
    Advisor
    Wendt, Jost O. L.
    
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    Show full item record
    Publisher
    The University of Arizona.
    Rights
    Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
    Abstract
    The environmental impact resulting from the release of trace elements during coal combustion is an important issue for the coal-fired electric utility industry. Trace elements exit the combustor by partitioning between the flue gas and the fly ash particles. A comprehensive study has been conducted to investigate the mechanisms governing the partitioning of trace elements during pulverized coal combustion. The behavior of seven trace elements (arsenic, selenium, antimony, cobalt, cesium, thorium, and cerium) in six pulverized coals were studied under commercially relevant conditions in a well-described laboratory combustion environment. The partitioning of trace elements is governed by the extent of volatilization during combustion, the form of occurrence in the flue gas, and the mechanisms controlling vapor-to-solid phase transformation to fly ash particle surfaces. The most common vapor-to-solid phase partitioning mechanism for semi-volatile trace elements is reaction with active fly ash surfaces. Trace elements that form oxy-anions upon volatilization (e.g. arsenic, selenium, antimony) will react with active calcium and iron cation fly ash surface sites. Trace elements that form simple oxides upon volatilization (e.g. cobalt, cesium) will react with active aluminum oxy-anion fly ash surface sites. The maximum combustion temperature affects the availability of active calcium and iron surface sites but not aluminum sites. Sulfur inhibits the reactivity of oxy-anions with iron surface sites. For coals with high sulfur contents (>1 wt % as SO₂), volatilized trace elements that form oxy-anions will partition by reaction with calcium surface sites if sufficient sites are available. For coals with low sulfur contents, volatilized trace elements that form oxy-anions, will partition by reaction with iron surface sites. Volatilized trace elements that form oxy-anions will not partition by reaction if the coal sulfur content is high and the calcium content is low (<3 wt% as CaO). Transition metals (e.g. cobalt) may form simple oxides, oxy-anions or both upon volatilization. An appreciable fraction of trace elements with limited volatility (e.g. cobalt, thorium, cerium, cesium) will volatilize. These will partition back to the solid phase by homogeneous nucleation or surface reaction depending upon the post-combustion conditions present.
    Type
    text
    Dissertation-Reproduction (electronic)
    Degree Name
    Ph.D.
    Degree Level
    doctoral
    Degree Program
    Graduate College
    Chemical and Environmental Engineering
    Degree Grantor
    University of Arizona
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