Laboratory Measurements of Alkali Metal Containing Vapors Released During Biomass Combustion

    Research output: Contribution to conferencePaper

    Abstract

    Alkali metals, especially potassium, have been implicated as key ingredients for enhancing fouling and slagging of heat transfer surfaces in power generating facilities that convert biomass to electricity. When biomass is used as a fuel in boilers, deposits form that reduce efficiency, and in the worst case lead to unscheduled plant downtime. Biomass is often blended with other fuels to controlfouling and slagging problems. Another approach, depending on the combustor, is to add sorbents to the fuel mixture to sequester alkali metals. Another possibility is to develop methods of hot gas cleanup that reduce the amount of alkali vapor to acceptable levels. These solutions to fouling and slagging, however, would greatly benefit from (a) a detailed understanding the mechanisms of alkalirelease during biomass combustion; and (b) identifying these alkali vapor species and how these vapors enhance deposit formation. Our approach is to directly sample the hot gases liberated from the combustion of small biomass samples in a variable-temperature quartz tube reactor employing a molecular beam mass spectrometer system. We have successfully used this experimental technique to identifyalkali species released during the combustion of selected biomass feedstocks used in larger-scale combustion facilities. Multiple combustion conditions have been investigated to target conditions that minimize alkali metal release. The results of these laboratory studies indicate that initial feedstock composition has the most pronounced effect on alkali metal released during combustion. Fourmechanisms of alkali metal release have been identified, depending on the feedstock composition. Primary alkali metal release in the combustion of relatively low alkali metal containing woody feedstocks is through the vaporization and/or decomposition of alkali sulfates. Alkali metal chlorides are the primary alkali metal species released during combustion of herbaceous feedstocks, grasses, andstraws with high alkali metal and chlorine contents. For feedstocks with high alkali metal and low chlorine content, alkali metal hydroxides are the most abundant alkali vapor released. If a high alkali content is coupled with high levels of fuel-bound nitrogen, the dominant form of alkali metal vapor is the alkali cyanate. In general, the chlorine content of biomass has been identified as animportant parameter that facilitates alkali release. Contact (e-mail): david_dayton@nrel.gov
    Original languageAmerican English
    Pages161-185
    Number of pages25
    DOIs
    StatePublished - 1996
    EventEngineering Foundation Conference - Waterville Valley, New Hampshire
    Duration: 1 Jul 19951 Jul 1995

    Conference

    ConferenceEngineering Foundation Conference
    CityWaterville Valley, New Hampshire
    Period1/07/951/07/95

    NREL Publication Number

    • NREL/CP-433-8132

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