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.

Typical Fly Ash Mount Shows a Wide Range of Particle Types

Magnetite Particles Are Oxidized Remains of Framboidal Pyrite

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.

Dendritic Spinel Crystals in Fly Ash Particle

SEM Shows Diversity of Particle Sizes and Types

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  m 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.

TEM Shows Ash Particle with Spinel Crystallites in Glass Matrix

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.

Partitioning of Fe between Fly Ash and Bottom Ash

Partitioning of Mo between Fly Ash and Bottom Ash

Partitioning of Cu between Fly Ash and Bottom Ash

Cenospheres Are a Very Useful Ash Component  Believed to form when an ash droplet is inflated by CO 2 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[12], , 1981).

Cenospheres Correlate Well with Slag Viscosity Y Y Y

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.