ALKALI DEPOSITS FOUND IN BIOMASS POWER PLANTS
A PRELIMINARY INVESTIGATION OF THEIR EXTENT AND NATURE
for National Renewable Energy Laboratory 1617 Cole Boulevard Golden, CO 80401-3393 NREL Subcontract TZ-2-11226-1 Thomas R. Miles, P.E. Thomas R. Miles, Jr. Consulting Design Engineer 5475 SW Arrowwood Lane Portland, OR 97225 Larry L. Baxter Combustion Research Facility Sandia National Laboratories, Livermore, CA Richard W. Bryers emeritus Foster Wheeler Development Corporation Livingston, NJ Bryan M. Jenkins Biological and Agricultural Engineering University of California, Davis, CA Laurance L. Oden Albany Research Facility, Bureau of Mines, U.S. Department of the Interior, Albany, OR APRIL 15, 1995 The complete report can be obtained from: www.trmiles.com: Alkali Deposits Found in Biomass Power Plants (4 MB) Volume 1 http://www.nrel.gov/docs/legosti/fy96/8142v1.pdf
If those links fail search for "Miles" and "Alkali" at http://www.nrel.gov/publications/
Deposits were evaluated using elemental analysis, X-ray diffraction and other mineralogical techniques. These analyses have advanced the understanding of the role of minerals in the combustion of biomass, and their occurrence in biofuels. Deposits occur as a result of the boiler design, fuel properties and boiler operation. The limited furnace volume and high furnace exit gas temperatures of most biomass boilers promote slagging or deposits from those biofuels that contain significant amounts of potassium or sodium, sulfur, chlorine and silica. All annual growth, whether from urban tree trimmings, annual crops or their residues, or from energy crops, contains sufficient volatile alkali (0.34 kg/GJ or 0.8 lb./MMBtu) to sufficiently lower the fusion temperature of the ash so that it melts in combustion, or the elements vaporize and condense on boiler tubes and refractories.
Deposits observed in this project are consistent with all known mechanisms for deposit formation: particle impaction, condensation, thermophoresis and chemical reaction. Particle impaction was the dominant mechanism, especially on cross flow convection tubes, followed by condensation on waterwalls and chemical reaction. Analysis by scanning electron microscopy (SEM) showed that compounds containing potassium, sulfur and chlorine were the principal bonding agents in most deposits and were usually associated with fuel blends containing annual growth materials such as agricultural crops and residues. Most deposits occur during post-combustion and cannot be predicted solely by analysis of the fuel composition.
Several techniques were used to analyze deposits. Scanning electron microscopy (SEM) helped to determine the composition and mineralogy of deposits. DTA (differential thermal analysis) and TGA (thermogravimetric analysis) were used to evaluate volatilization and fusion of ash constituents. Ash fusion temperatures using the pyrometric cone tests were of little value in predicting deposits, the alkali having been lost during ashing and calcining.
Pilot scale combustor simulations demonstrated deposit formation on combustor refractory and on heat transfer surfaces, confirming mechanisms observed in the field. Alkali volatilization, condensation and enrichment on flyash and in deposits were observed in the Sandia Multi-Fuel Combustor (MFC).
Limestone was the principal additive used in test boilers
to maintain bed fluidization. While limestone improved operation the calcium appears as a constituent of deposits on convection surfaces (as CaCO3, CaSO4) and may reduce but does not prevent deposition. High alumina sand also reduced agglomeration in a circulating fluidized bed (CFB) but did not change the composition of deposits on the superheater tubes.
Boiler Design and Operation
Conventionally designed boilers are not suitable for burning high alkali fuels. Special boiler designs with low furnace exit gas temperatures,
<1500° F, are required for annual crops or residues, including grasses and straws. Designs should include: adequate waterwall surface area or parallel heat exchange surfaces, and combustion air control to control gas temperatures, grates suitable for removing large quantities of ash, and sootblowing to clean tenacious deposits.
Limestone was the principal additive used in test boilers to maintain bed fluidization. While limestone improved operation the calcium appears as a constituent of deposits on convection surfaces (as CaCO3, CaSO4) and may reduce but does not prevent deposition. High alumina sand also reduced agglomeration in a circulating fluidized bed (CFB) but did not change the composition of deposits on the superheater tubes.
Gasification at low temperatures,
<1400° F, with additives, may be necessary to inhibit alkali volatilization in order to burn large quantities of these biofuels. A review of straw experiences in Europe shows some success with straw pyrolysis. Additional research is needed.
The Bureau of Mines Research Laboratory in Albany, Oregon provided X-ray diffraction (XRD), scanning electron microscopy (SEM) and mineralogical analysis courtesy of Larry Oden. Sandia National Laboratories contributed pilot scale combustion tests and analyzed fuels and deposits as part of a parallel NREL project conducted by Larry Baxter. Bryan Jenkins, professor of Biological and Agricultural Engineering, participated through the courtesy of the University of California, Davis.
Results of the project were reported in presentations to nine conferences and will be published in forthcoming journals. Four project meetings were held with participants at Sandia National Laboratories. The suite of fuels and fuel samples developed during the tests were archived at Hazen Laboratories and were used in independent investigations by NREL and by Richard Bryers at Foster Wheeler Development Corporation. The conclusions from the project have changed the way participants evaluate deposits, and the way they buy and prepare fuels. Project data helped in the startup of a new biomass power plant in Florida. Through more than twenty five inquiries from biomass plants around the country it has been possible to assist other interested parties including: an industrial boiler manufacturer in the northeast, a power plant boiler manufacturer, other power plants, including a 20 MW power plant in New York and a plant cofiring wood and refuse derived fuel, many of the state and regional biomass energy agencies, and developers of short rotation woody crops and non-wood crops for fuel.
APPENDIXA. Project Participants A.1 Investigators and Advisors A.2 Industry Sponsors and Participants B. Methods of Sampling and Analysis B.1 Analytical Request Form B.2 Standards for Biomass and Coal B.3 Summary of Analytical Methods Used B.4 Microwave Dissolution and Atomic Absorption Method B.5 Comparison of Microwave and Thermal Ashing B.6 Determination of Water-Soluble Alkali B.7 Chemical Fractionation Procedure B.8 Fuel Sampling Procedure C. Fuel Characteristics: Data Summary C.1 Wood Fuel Blends: Alkali Deposit Investigation C.2 Wood Fuels: Oak, Fir, Pine, Poplar, Forest Residues, Christmas Trees C.3 Urban Waste Fuels and Residues: Demolition, Land Clearing, Waste Paper, RDF C.4 Wood Fuels - Energy Crops: Willow C.5 Grasses and Straws: Bagasse, Bana Grass, Switchgrass C.6 Grasses and Straws: Energy Crops, Miscanthus and Reeds C.7 Grasses and Straws: Residues, Alfalfa, Mint, Wheat Straw, Rice Straw C.8 Nuts, Pits and Shells: Almond, Pistachio, Walnut, Olive, Prune Pits D. Deposits D1. Melting Temperatures of Potential Low-Melting Minerals Found in Biomass (Bryers, 1994) D2. Grate-1. Wood and 95% Wood/5% Imperial Wheat Straw Blend. D3. Grate-1. 80% Wood/20% Oregon Wheat Straw D4. Grate-1 type boiler. Wood and Landscape Residues. D5. Low Temperature Straw Boilers: Grate-2 (Bale), Grate-3 (Stoker) D6. FBC-1 Wood and Agricultural Prunings D7. CFB-1 and CFB-2 Wood and Agricultural Residues. D8. CFB-3 Wood and Pits