Laboratory Measurements of Alkali Metal Containing Vapors Released during Biomass Combustion

  • David C. Dayton
  • Thomas A. Milne

Abstract

Alkali metals, in particular 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, the deposits formed reduce efficiency, and in the worst case lead to unscheduled plant downtime. Blending biomass with other fuels is often used as a strategy to control fouling and slagging problems. Depending on the combustor, sorbents can be added 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 detailed understanding of the mechanisms of alkali release during biomass combustion. Identifying these alkali vapor species and understanding how these vapors enhance deposit formation would also be beneficial.

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 (MBMS) system. We have successfully used this experimental technique to identify alkali species released during the combustion of selected biomass feedstocks used in larger scale combustion facilities. Multiple combustion conditions have been investigated to target those 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. Four mechanisms 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 or decomposition of alkali sulfates. Alkali metal chlorides are the primary alkali metal species released during combustion of herbaceous feedstocks, grasses, and straws 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 an important parameter that facilitates alkali release.

Keywords

Wheat Straw Corn Stover Biomass Combustion Wood Waste Almond Shell 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Preview

Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.

References

  1. Bain, R.L., (1993). “Electricity From Biomass in the United States: Status and Future Direction,” Bioresource Technology, 46, 86–93.Google Scholar
  2. Baxter, L.L. (1993). “Ash Deposition During Biomass and Coal Combustion: A Mechanistic Approach,” Biomass and Bioenergy 4, pp. 85–102.CrossRefGoogle Scholar
  3. Benson, S.A., Ed. (1992). Inorganic Transformations and Ash Deposition During Combustion, Engineering Foundation Conference March 10–15, 1991, Palm Beach, FL Published on behalf of the Engineering Foundation by the American Society of Mechanical Engineers, New York, NY.Google Scholar
  4. Benson, S.A., Jones, M.L., and Harb, J.N. (1993). “Ash Formation and Deposition.” In L.D. Smoot, Ed. Fundamentals of Coal Combustion for Clean and Efficient Use Elsevier, New York, NY.Google Scholar
  5. Bryers, R.W., (1978). Ash Deposits and Corrosion Due to Impurities in Combustion Gases. Hemisphere Publishing Corporation, New York, NY.Google Scholar
  6. Bryers, R.W. and Vorres, K.S., Eds. (1990). Mineral Matter and Ash Deposition From Coal, Engineering Foundation Conference February 22–26, 1988 Santa Barbara, CAGoogle Scholar
  7. Dayton, D.C., French, R.J., and Milne, T.A. (1995). “The Direct Observation of Alkali Vapor Release During Biomass Combustion and Gasification. I. The Application of Molecular Beam/Mass Spectrometry to Switchgrass Combustion.” EnergyandFuels, 9, pp. 855–865.Google Scholar
  8. Dayton, D.C. and Wang, D., (1995). “CID Studies of Inorganic Species Released During Biomass Combustion.” 43rd ASMS Conference of Mass Spectrometry and Allied Topics, May 21–26, 1995, Atlanta, GA. Paper TPA 034.Google Scholar
  9. Evans, R.J. and Milne, T.A. (1987a). “Molecular Characterization of the Pyrolysis of Biomass: I. Fundamentals,” Energy and Fuels 1, pp. 123–127.CrossRefGoogle Scholar
  10. Evans, R.J. and Milne, T.A. (1987b). “Molecular Characterization of the Pyrolysis of Biomass: II. Applications,” Energy and Fuels 1, pp. 311–319.CrossRefGoogle Scholar
  11. French, R.J., Dayton, D.C., and Milne, T.A. (1994). “The Direct Observation of Alkali Vapor Species in Biomass Combustion and Gasification,” NREL Technical Report (NREL/TP-430–5597). January 1994.Google Scholar
  12. Hastie, J.W.; Plante, E.R.; Bonnell, D.W. “Alkali Vapor Transport in Coal Conversion and Combustion Systems,” in Metal Bonding and hlteraction in High Temperature Systems. Gole; Stwalley; Eds.; ACS Symposium Series #179, 1982. Chapter 34.Google Scholar
  13. Hastie, J.W., Zmbov, K.F., and Bonnell, D.W. (1984). “Transpiration Mass Spectrometric Analysis of Liquid KCl and KOH Vaporization.” High Temperature Science, 17, 333–364.CrossRefGoogle Scholar
  14. Levin, E., Robbins, C.R., and McMurdie, H.F. (1964). Phase Diagrams for Ceramists. Columbus, OH: American Chemical Society.Google Scholar
  15. Miller, M. and Skudlarski, K. (1983). “Mass Spectrometric Study of Potassium Cyanate and Potassium Cyanate-Potassium Cyanide System at High Temperatures.” Int. Journal of Mass Spec. and Ion Phys., 47, 243–246.CrossRefGoogle Scholar
  16. Milne, T.A. and Klein, H.M., (1960). “Mass Spectrometric Study of Heats of Formation of Alkali Chlorides,” J. Chem. Phys., 33, 1628–1637.CrossRefGoogle Scholar
  17. Moses, C.A. and Bernstein, H. (1994). “Impact Study on the Use of Biomass-Derived Fuels in Gas Turbines for Power Generation,” NREL Technical Report (NREL/TP430–6085). January 1994.Google Scholar
  18. Raask, E. (1985). Mineral Impurities in Coal Combustion Hemisphere Publishing Corporation, Washington, DC.Google Scholar
  19. Reid, W.T., (1981). “Coal Ash-Its Effect on Combustion Systems,” Chapter 21 in Chemistry of Coal Utilization Elliot, M.A.; Ed; New York, NY.Google Scholar
  20. Scandrett, L.A. (1984). “The Thermodynamics of Alkali Removal From Coal-Derived Gases,” Journal of the Institute of Energy, December 1984, 391–397.Google Scholar
  21. Turnbull, J.H., (1993). “Use of Biomass in Electric Power Generation: The California Experience.” Biomass and Bioenergy, 4, 75–84.CrossRefGoogle Scholar
  22. Vorres, K.S., Ed. (1986). Mineral Matter and Ash in Coal ACS Symposium Series 301 American Chemical Society, Washington, DC.Google Scholar
  23. Weast, R.C., Ed. (1985). “66`h Edition of the CRC Handbook of Chemistry and Physics,” CRC Press, Inc., Boca Raton, FL.Google Scholar

Copyright information

© Springer Science+Business Media New York 1996

Authors and Affiliations

  • David C. Dayton
    • 1
  • Thomas A. Milne
    • 1
  1. 1.National Renewable Energy LaboratoryIndustrial Technologies DivisionGoldenUSA

Personalised recommendations