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Characterization of Coal Ash by Materials Science Techniques. R. J. Lauf Metals & Ceramics Division Oak Ridge National Laboratory. Acknowledgment. Work supported by the ORNL Exploratory Studies Program
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Characterization of Coal Ash by Materials Science Techniques R. J. Lauf Metals & Ceramics Division Oak Ridge National Laboratory
Acknowledgment • Work supported by the ORNL Exploratory Studies Program • Oak Ridge National Laboratory is operated by UT-Battelle, LLC for the U.S. Department of Energy under contract number DE-AC05-00OR22725
Outline • Original Goals of the Program • Techniques and Methods • Major Findings and Conclusions
Original Goals of the Program • Develop a better understanding of the structure of ash on a microscopic scale • Crystalline phases, size, and distribution • Particle-to-particle homogeneity • Elemental partitioning • Relate these results to coal mineralogy • Use this knowledge to improve disposal and utilization schemes.
Why Use the Materials Science Approach? • Ash is a complex mixture of many phases coexisting on many size scales. • “Wet chemistry” work had suggested surface segregations of some elements, particularly heavy metals. • Clearly the health impact and resource values of ash both depend on mineralogy and microchemistry.
Techniques and Methods • Characterization techniques • Optical Microscopy • Scanning Electron Microscopy (SEM) • Transmission Electron Microscopy (TEM) • X-Ray Diffraction (XRD) • Other studies • Chemical partitioning between fly ash and bottom ash • Vacuum heating to assess volatility of metals • Rationale for variability of cenosphere formation
Optical Microscopy Revealed Surprising Details • Samples were mixed with epoxy and polished for reflected-light microscopy (to 1000X). • Many microstructural types were catalogued. • Glass • Glass with distributed crystallites • Magnetite with aluminosilicates • Hollow balloons (cenospheres and plerospheres) • Inhomogeneity parallels the mineral distribution within the source coal.
Large, Inhomogeneous Particle Formed by Agglomeration or Encapsulation
Implications of Optical Microscopy • Individual mineral particles pass through the boiler, retaining some degree of individuality. • Some ash particles can be linked to recognizable coal minerals. • Clearly bulk chemical analysis cannot tell the whole story.
TEM Revealed Even More Surprising Details • Ash was dispersed in aluminum and hot pressed into a pellet, sliced, and thinned by ion milling. • Particles as small as 0.5 mm were found to contain crystallites (usually a mixed ferrite material). • Conventional wisdom would have suggested that such small particles would cool so fast that they would be all glass.
X-Ray Diffraction Complements Microscopic Observations • The main crystalline phase is an aluminum-rich ferrite (“dirty magnetite”). • Ferrites are thermodynamically stable relative to iron silicates at the oxygen potential present in the boiler. • Very little crystalline silicates are seen, other than a small amount of mullite. • Most silicate material is amorphous.
Metals Partitioning between Fly Ash and Bottom Ash • Does volatility play a role? • Large vs small particles; evaporation and condensation. • Are particular metals lost after heating in vacuum? • What role, if any, does coal mineralogy play?
Fly Ash from New Mexico Lost 0.6 wt% on Heating in Vacuum to 750 C • 6000 ppm is more than all trace elements combined, so much of this weight loss is water. • Cu, Cr, Th, As, Mo, Be, and Se did not decrease after heating to 650 C. • B, Zn, Pb, and Ni decreased very slightly. • Between 450 and 650 C, S went from about .275 to .24 wt%.
We Compared Levels of Several Elements in Fly Ash and Bottom Ash • Five plants burning a variety of coal types in a variety of boilers. • Concentration in the fly ash was plotted against concentration in the bottom ash or slag. • Fe and Mn were about equally distributed between fly ash and bottom ash. • S, As, Mo, and Pb strongly segregated to the fly ash. • Co, V, Cu, and Cd favored the fly ash only when framboidal pyrite was present.
Cenospheres Are a Very Useful Ash Component • Believed to form when an ash droplet is inflated by CO2 either as a combustion product or when liberated during decomposition of carbonate minerals. • Much more prevalent at some plants than others. • We found no correlation with Ca concentration, suggesting presence or absence of calcite has little influence. • We found a strong positive correlation with slag viscosity (Fuel 60, 1177-9, 1981).
Y-176335 Y-176331 Y-176334 Cenospheres Correlate Well with Slag Viscosity
Major Findings and Conclusions • Ash is highly inhomogeneous and variable, with structure even in the smallest particles. • The ash reflects the mineralogical inhomogeneity of the coal - i.e., the boiler is not one big “melting pot”. • Coal and ash mineralogy influence many important aspects of trace element behavior. • Principles of materials science and crystal chemistry can improve our understanding of the environmental and engineering properties of ash.