Wilson and others

—Mineral and Energy Resource Assessment of the Gallatin National Forest, Montana

—U.S. Geological Survey Professional Paper 1654

Characterization and 

Occurrence of Eleme

and Coal Combustion 

Power Plant Utilizing 

from the Powder Rive

U.S. Department of the Interior

U.S. Geological Survey

 Scientific Investigations Rep

 FOLD

 FOLD
Characterization and Modes of

Occurrence of Elements in 

Feed Coal and Coal Combustion 

Products from a Power Plant Utilizing 

Low-Sulfur Coal from the Powder River 

Basin, Wyoming

By Michael E. Brownfield, James D. Cathcart, Ronald H. Affolter, 

Isabelle K. Brownfield, Cynthia A. Rice, Joseph T. O’Connor, 

Robert A. Zielinski, John H. Bullock, Jr., James C. Hower, and 

Gregory P. Meeker 

Scientific Investigations Report 2004–5271

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

This publication is available only online at: http://pubs.usgs.gov/sir/2004/5271/

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Posted online July 2005, Version 1.0 

Although this report is in the public domain, permission must be secured from the individual copyright owners 

to reproduce any copyrighted materials contained within this report.

Suggested citation:

Brownfield, M.E., Cathcart, J.D., Affolter, R.H., Brownfield, I.K., Rice, C.A., O’Connor, J.T., Zielinski, R.A.,

Bullock, J.H., Jr., Hower, J.C., and Meeker, G.P., 2005, Characterization and modes of occurrence of elements

in feed coal and coal combustion products from a power plant utilizing low-sulfur coal from the

Powder River Basin, Wyoming: U.S. Geological Survey Scientific Investigations Report 2004-5271, 36 p.

Available at URL: http://pubs.usgs.gov/sir/2004/5271/
iii

Contents

Abstract.......................................................................................................................................................... 1

Introduction .................................................................................................................................................. 1

Acknowledgments........................................................................................................................................ 2

Methodology ................................................................................................................................................. 2

Sample Collection................................................................................................................................ 2

Chemical Analysis ............................................................................................................................... 3

Radiographic Analysis ........................................................................................................................ 4

Petrographic Analysis......................................................................................................................... 5

X-Ray Diffraction Analysis ................................................................................................................. 5

Microbeam Analysis .......................................................................................................................... 5

Leaching Analysis ............................................................................................................................... 6

Results of Analysis ...................................................................................................................................... 6

Feed Coal Characterization ............................................................................................................... 6

Leaching and Modes of Occurrence of Selected Elements in Feed Coal.................................. 7

Coal Combustion Products ................................................................................................................ 8

X-Ray Element Mapping .................................................................................................................. 12

Discussion and Interpretation ................................................................................................................. 12

Summary ..................................................................................................................................................... 31

References Cited ........................................................................................................................................ 33

Figures

1. Diagram of sample and data flow ............................................................................................. 3

2. Diagram showing data sources and studies........................................................................... 4

3. Photograph showing Wyodak-Anderson coal zone, Campbell County, Wyo.................... 4 

4. X-ray diffractogram of low-temperature ash from feed coal............................................. 10 

5–16. Scanning electron microscope secondary electron images of power plant feed 

 coal samples from the Wyodak-Anderson coal zone, showing: 

 5. Crandallite............................................................................................................................ 11 

 6. Gorceixite............................................................................................................................. 11 

 7. Beta-form quartz................................................................................................................. 12 

 8. Zircon.................................................................................................................................... 13 

 9. Beta-form quartz showing dissolution and authigenic overgrowths ........................ 13

10. Potassium feldspar............................................................................................................. 14 

11. Plagioclase feldspar .......................................................................................................... 14 

12. Calcite................................................................................................................................... 15

13. Dolomite ............................................................................................................................... 15 

 14. Authigenic anatase ............................................................................................................ 16 

15. Authigenic quartz that replaced plant matter................................................................ 16 

16. Iron oxide sphere................................................................................................................ 17 

17. X-ray diffractogram of fly ash.................................................................................................. 20 
iv

18. Scanning electron microscope backscattered electron image of lime and 

periclase crystals in a fly ash grain ....................................................................................... 22

19. X-ray diffractogram of bottom ash.......................................................................................... 23

20. Scanning electron microscope backscattered electron image showing intergrowth of feldspar and pyroxene in a bottom ash grain ..................................................... 23 

21. Element intensity maps showing chromium, nickel, and silica in a ferrite-rich fly 

ash grain...................................................................................................................................... 24

22. Element intensity maps showing calcium, magnesium, and sulfur .................................. 24

23. Box plot showing barium content of the feed coal and coal combustion products....... 25 

24. Box plot showing phosphorus content of the feed coal and coal combustion 

products ..................................................................................................................................... 25

25. Graphs showing temporal variation of trace elements from feed coal and fly ash

at a Kentucky power plant ....................................................................................................... 26

26–30. Scanning electron microscope backscattered electron images, showing:

26. Exsolution dendrites of magnetite in a bottom ash grain ............................................ 27

27. Periclase and merwinite in a fly ash grain..................................................................... 27

28. Periclase, bredigite?, and 3:2 Ca aluminate fly ash grain............................................ 28 

29. Corundum crystals in a phosphate-silicate glass......................................................... 28 

30. Rare earth element-bearing fly ash grain ...................................................................... 29 

31. Photograph and fission track radiograph of a Ca-P rich fly ash grain ............................. 30 

32. Photograph and fission track radiograph of fly ash grains ................................................ 30 

33. Graph showing cumulative percentage of arsenic leached from a 

Wyodak-Anderson fly ash........................................................................................................ 31 

34. Graph showing percentage of arsenic leached from a Kentucky fly ash........................ 32 

35. Graphs showing leachate concentration of aluminum, copper, iron, molybdenum, 

and zinc during the leaching of a Wyodak-Anderson fly ash ........................................... 32

36. Graphs showing leachate concentration of aluminum, copper, iron, molybdenum, 

and zinc during the leaching of a Kentucky fly ash ............................................................. 32

37. Scanning electron microscope backscattered electron image of steklite coating 

on spinel-rich Kentucky fly ash grain..................................................................................... 33

Tables

1. Mean values for proximate and ultimate analysis, caloric value, forms of sulfur, 

and ash-fusion temperatures of 10 samples of Indiana power plant feed coal mined 

from the Wyodak-Anderson coal zone, Powder River Basin, Wyo. ................................... 8

2. Petrology and vitrinite reflectance of an Indiana power plant feed coal from the 

Wyodak-Anderson coal zone, Powder River Basin, Campbell County, Wyo. ................... 9

3. Results of X-ray diffraction, SEM, and microprobe analysis of feed coal, fly ash, and 

bottom ash from an Indiana power plant utilizing coal from the Wyodak-Anderson 

coal zone, Powder River Basin, Wyo. .................................................................................... 10

4. Concentrations of major and minor oxides for feed coal, fly ash, and bottom ash 

for the first 15 days of sampling (n=15) from an Indiana power plant utilizing coal

from the Wyodak-Anderson coal zone, Powder River Basin, Wyo. ................................. 17 
v

 5. Comparison of concentrations of selected elements for feed coal (n=15), fly ash 

 (n=25), and bottom ash (n=14) from an Indiana power plant utilizing coal from the 

Wyodak-Anderson coal zone, Powder River Basin, Wyo. ................................................. 18

6. Element concentration data for selected elements, sequential leaching

experiments from Wyodak-Anderson coal samples, Powder River Basin, Wyo. .......... 19 

7. Semiquantitative mineralogy of fly-ash samples from an Indiana power plant 

using feed coal from the Wyodak-Anderson coal zone, Powder River Basin, 

Wyo., determined by Rietveld X-ray diffraction analysis.................................................... 21

8. Fly ash petrography of selected samples from an Indiana power plant using feed 

coal from the Wyodak-Anderson coal zone, Powder River Basin, Wyo. ........................ 21 

9. Modes of occurrence data for selected elements for feed coal samples from 

Wyodak-Anderson coal zone, Powder River Basin, Wyo. ................................................. 26 
Abstract

The U.S. Geological Survey and the University of Kentucky Center for Applied Energy Research are collaborating 

with an Indiana utility company to determine the physical and 

chemical properties of feed coal and coal combustion products from a coal-fired power plant. The Indiana power plant 

utilizes a low-sulfur (0.23 to 0.47 weight percent S) and lowash (4.9 to 6.3 weight percent ash) subbituminous coal from 

the Wyodak-Anderson coal zone in the Tongue River Member 

of the Paleocene Fort Union Formation, Powder River Basin, 

Wyoming.

Based on scanning electron microscope and X-ray diffraction analyses of feed coal samples, two mineral suites were 

identified: (1) a primary or detrital suite consisting of quartz 

(including beta-form grains), biotite, feldspar, and minor 

zircon; and (2) a secondary authigenic mineral suite containing alumino-phosphates (crandallite and gorceixite), kaolinite, 

carbonates (calcite and dolomite), quartz, anatase, barite, and 

pyrite. The primary mineral suite is interpreted, in part, to 

be of volcanic origin, whereas the authigenic mineral suite 

is interpreted, in part, to be the result of the alteration of the 

volcanic minerals. The mineral suites have contributed to the 

higher amounts of barium, calcium, magnesium, phosphorus, 

sodium, strontium, and titanium in the Powder River Basin 

feed coals in comparison to eastern coals. 

X-ray diffraction analysis indicates that (1) fly ash is 

mostly aluminate glass, perovskite, lime, gehlenite, quartz, 

and phosphates with minor amounts of periclase, anhydrite, 

hematite, and spinel group minerals; and (2) bottom ash is 

predominantly quartz, plagioclase (albite and anorthite), 

pyroxene (augite and fassaite), rhodonite, and akermanite, 

and spinel group minerals. Microprobe and scanning electron 

microscope analyses of fly ash samples revealed quartz, zircon, and monazite, euhedral laths of corundum with merrillite, 

hematite, dendritic spinels/ferrites, wollastonite, and periclase. 

The abundant calcium and magnesium mineral phases in the 

fly ash are attributed to the presence of carbonate, clay, and 

phosphate minerals in the feed coal and their alteration to new 

phases during combustion.

The amorphous diffraction-scattering maxima or glass 

“hump” appears to reflect differences in chemical composition 

of fly ash and bottom ash glasses. In Wyodak-Anderson fly 

and bottom ashes, the center point of scattering maxima is due 

to calcium and magnesium content, whereas the glass “hump” 

of eastern fly ash reflects variation in aluminum content.

The calcium- and magnesium-rich and alumino-phosphate mineral phases in the coal combustion products can 

be attributed to volcanic minerals deposited in peat-forming 

mires. Dissolution and alteration of these detrital volcanic 

minerals occurred either in the peat-forming stage or during coalification and diagenesis, resulting in the authigenic 

mineral suite.

The presence of free lime (CaO) in fly ash produced 

from Wyodak-Anderson coal acts as a self-contained “scrubber” for SO3

, where CaO + SO3 form anhydrite either during 

combustion or in the upper parts of the boiler. Considering 

the high lime content in the fly ash and the resulting hydration 

reactions after its contact with water, there is little evidence 

that major amounts of leachable metals are mobilized in the 

disposal or utilization of this fly ash.

Introduction

Coal is likely to remain an important part of the United 

States energy supply, largely because it is the most abundant, 

domestically available fossil fuel. One of the major concerns 

related to the use of coal for electricity production is the 

release of elements during combustion and the resulting coal 

combustion products (CCPs)—fly ash and bottom ash—to 

the environment. This concern and the recent inclusion of 

Characterization and Modes of Occurrence of Elements 

in Feed Coal and Coal Combustion Products from a Power 

Plant Utilizing Low-Sulfur Coal from the Powder River 

Basin, Wyoming

By Michael E. Brownfield, James D. Cathcart, Ronald H. Affolter, Isabelle K. Brownfield, Cynthia A. Rice, 

Joseph T. O’Connor, Robert A. Zielinski, John H. Bullock, Jr., James C. Hower,1 and Gregory P. Meeker 

1

University of Kentucky, Center for Applied Energy Research, Lexington, 

KY 40511.
coal-fired power plants in the U.S. Environmental Protection 

Agency (EPA) Toxic Release Inventory (TRI) have prompted 

the need for accurate, reliable, and quantitative information 

on the concentration and modes of occurrence of chemical 

elements in feed coal and CCPs. The TRI is a publicly available EPA database that contains information on toxic chemical releases and other waste management activities reported 

annually by certain covered industry groups as well as Federal 

facilities. This inventory was established under the Emergency Planning and Community Right-to-Know Act of 1986 

(EPCRA) and expanded by the Pollution Prevention Act of 

1990. Many factors contribute to, and control, how coal and 

coal combustion products will affect the environment and the 

aqueous solutions that they contact. 

The U.S. Geological Survey (USGS), the Kentucky 

Center for Applied Energy Research, and the Kentucky Geological Survey are collaborating with several electric utilities 

to determine physical and chemical properties of a variety 

of feed coals and CCPs from coal-fired electric power plants 

in Indiana and Kentucky. The long-range goal of the collaborative study is to collect analytical data on the feed coals 

that will provide a basis for predicting the properties of fly 

ash produced by the power plants. To accomplish this goal, 

a team of geologists, geochemists, chemists, coal technologists, and engineers from the USGS, State geological surveys, 

universities, and electric utility companies was organized to 

(1) determine the content and modes of occurrence of selected 

trace elements of the feed coal and the high-volume CCPs; (2) 

observe variations through time in the properties of CCPs with 

respect to concurrent variations in the chemical and mineralogical properties of the feed coal; (3) determine the chemistry, mineralogy, magnetic properties, organic properties, and 

radionuclide content of selected representative samples; and 

(4) model potential landfill disposal scenarios using batch 

and flow-through column leaching methods on feed coal and 

combustion product samples. (See fig. 1.)

In the first phase of the study, the team analyzed coal 

and CCPs from a Kentucky utility that burns bituminous coal 

from the Appalachian and Illinois Basins in two power plants. 

Respective sulfur contents in the coal ranged from 0.6 to 0.9 

percent and from 2.5 to 3.5 percent. Much of the data has 

been released in several reports, including Affolter and others 

(1997), Breit and others (1996), Brownfield and others (1997), 

Cathcart and others (1997), Finkelman and others (1996), 

Finkelman and others (1997), Hower and others (1995), 

O’Connor (1997a), O’Connor (1997b), Rice and others 

(1997), Rice and others (1999), and Zielinski and Finkelman 

(1997). In the second phase of the study, the results of which 

are the focus of our report, coal and CCPs from an Indiana 

power plant utilizing low-sulfur (0.23 to 0.47 percent) subbituminous coal from the Wyodak-Anderson zone in the Powder 

River Basin, Wyo., were similarly analyzed; these analyses 

provide quantitative data characterizing the physical and 

chemical properties of the feed coal and combustion products, 

and give us the means to accurately predict the properties of 

the fly ash and modes of occurrence of selected trace elements 

in the fly ash.

Acknowledgments

We are very grateful to Gregory H. Kennan of American 

Electric Power and Carl Scaggs of the Indiana power plant for 

providing access to the power plant, providing plant coal and 

coal combustion product data, and facilitating the sampling at 

the plant. We have benefited from discussions with American 

Electric Power and power plant personnel. The authors thank 

Lorna Carter, Richard Keefer, Richard Pollastro, and Christopher Schenk for their editorial review of the report. 

Methodology

Coal is a complex combustible rock made up of organic 

and inorganic mineral components, each containing many elements. Many factors contribute to, and control, how coal and 

coal combustion products will affect the environment and the 

aqueous solutions that they contact. During combustion, elements present in the organic and mineral components of coal 

are redistributed, as a result of high temperatures, into gaseous 

and solid-phase reaction products. In the solid-phase reaction 

products (CCPs), the elements may be uniformly distributed 

throughout a fly ash grain, enriched in certain grains or areas 

of grains, or present as coatings, or adsorbed onto fly ash grain 

surfaces. Particle size, coal rank, amount of ash, coal mineralogy, and trace-element content are important variables controlling the combustion and mobility of elements in coal. For fly 

ash, the distribution and mobility of elements are influenced 

by the original composition of the feed coal, the combustion 

conditions, the size of the fly ash particles, and the fly ash 

mineralogy. Figure 2 shows various stages in the extraction 

and utilization of coal that are subject to analytical study.

To characterize the chemistry, mineralogy, and temporal 

variation of the feed coals and their combustion products, an 

analytical approach was applied that involved (1) quantitative 

chemical analysis of feed coal and fly ash; (2) radiographic 

techniques that quantify the distribution and abundance of 

radioactive particles; (3) petrographic analysis, X-ray diffraction analysis, and microbeam analysis, including element concentration mapping of small fly ash grains; and (4) analysis of 

leaching extracts that selectively dissolve certain components 

of coal or fly ash to simulate environmental conditions (fig. 1).

Sample Collection

The Indiana power plant utilizes a low-sulfur (0.23 to 

0.47 weight percent S) and low-ash (5.3 to 6.9 weight percent 

ash) subbituminous coal from the Wyodak-Anderson coal zone 

in the Tongue River Member of the Paleocene Fort Union 

2 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
Formation, Powder River Basin, Wyo. (fig. 3). To determine the temporal variability of feed coal and coal combustion products at this plant, samples were collected daily (15 

samples), weekly (14 samples), and monthly (9 samples). 

Feed coal samples were collected over a 24-hour period from 

an automated coal collector located before the coal pulverizer after which a representative sample split was prepared 

at the power plant. Fly ash samples were also collected from 

automated collectors (old and new samplers) located near the 

electrostatic fly ash precipitators. The “old sampler” consisted 

of a 1-in. (inch) pipe, under vacuum, placed in a silo fly ash 

feed pipe, whereas the “new sampler” consisted of a funnelshaped pipe, under vacuum, placed in a silo feed pipe. Both 

devices were designed to collect the fly ash systematically 

through time. For each sampling, fly ash was collected at four 

sites in the power plant—one from the economizer (closest 

to the furnace), two from automated fly ash collectors, and 

one from a truck storage silo. Power plant personnel collected 

representative samples of economizer ash, truck fly ash, and 

bottom ash. Fly-ash sizing data were obtained from the utility 

operator. During the sampling period, a total of 38 feed coal, 

76 fly ash, 38 economizer ash, and 38 bottom ash samples 

were collected.

Chemical Analysis

Ash yields and major-, minor-, and trace-element contents of feed coal, fly ash, and bottom ash were analyzed in 

USGS laboratories in Denver, Colo. Coal samples were ashed 

prior to analysis at 525°C. The presence of most elements was 

determined by inductively coupled plasma–atomic emission 

Methodology 3

Sample collection Participating

power

plant

Proximate/Ultimate

Chemical analysis

-Proximate/Ultimate

-Elements

Modes of occurrence

-SEM

-Electron microprobe

-Selective leaching

-Other techniques

Mineralogy

-XRD

-Petrography

Magnetic properties

Radionuclides

Database creation and

Data access

Environmental impact studies

-Leaching

-Modeling

-Hydrology

-Thermodynamics

Kentucky Center for Applied

Energy Research, University

of Kentucky

-Coal petrography

-Fly ash petrography

Sample selection

Sample splitting

Leaching data

Fly ash sizing data

Sample flow

Data flow

Figure 1. Diagram of sample data flow and sample analysis for the characterization of feed coal, fly 

ash, economizer ash, and bottom ash.
spectrometry (ICP-AES) and inductively coupled plasma–

mass spectrometry (ICP-MS). Mercury was analyzed by cold 

vapor atomic absorption spectroscopy (CV-AAS), whereas 

chlorine content was determined by ion chromatography 

and selenium was determined by hydride generation atomic 

absorption. The contents of mercury and selenium in feed coal 

were determined on sample splits that were not ashed prior to 

analysis. Proximate, ultimate, and forms-of-sulfur analyses 

following American Society of Testing and Materials (ASTM) 

procedures (American Society for Testing and Materials, 

1999a) were determined by a commercial laboratory (Geochemical Testing, Inc., Pittsburgh, Pa.). 

Radiographic Analysis

The distribution of uranium in individual particles of 

fly ash and bottom ash was directly observed through use 

Coal mine

Mine product

Product trace-element chemistry

Modes of occurrence

Coal deposit

Depositional environment

Bed trace-element chemistry

Modes of occurrence

Power plant

Feed coal

Trace-element chemistry

Modes of occurrence

Temporal variation

CCPs

Fly ash / Bottom ash

Trace-element chemistry

Modes of occurrence

Temporal variation

Figure 2. Diagram showing the different data sources with the types of studies and data 

required to characterize coal and coal combustion products (CCPs).

Figure 3. Upper part of the Wyodak-Anderson coal zone in the Antelope mine, 

Powder River Basin, Campbell County, Wyo. The Wyodak-Anderson coal zone (as 

much as 140 ft thick) is the major source of coal in the central Powder River Basin.

4 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
Methodology 5

of fission-track radiography. Polished thin sections of grains 

of fly ash and bottom ash were prepared and irradiated with 

neutrons in a research reactor, which induced fission of 235U in 

the samples. During irradiation, fission fragments recoil from 

the surface of the thin section and pass into an overlying sheet 

of muscovite (mica) detector material. The passage of fission 

fragments causes linear paths of damage or tracks in the mica 

that are made visible by subsequent etching of the recovered 

mica in hydrofluoric acid. Areas of high fission-track density 

in the mica can be related to areas of high uranium concentration in the original sample. Depending upon the angle of 

incidence, fission-track lengths in the mica are variable, ranging from less than 1 μm (micrometers) to as much as 5 μm. 

Uraniferous grains or rinds <1 μm thick are magnified by this 

technique because resulting fission tracks are commonly as 

much as 5 μm long. Subtler gradients in uranium concentration 

(less than × 2) can be detected by observing changes in fissiontrack density across particles greater than 10 μm in diameter. 

Petrographic Analysis

Sample splits of feed coal and fly ash were analyzed by 

the Kentucky Center for Applied Energy Research, which used 

petrographic microscopy to determine the abundance of major 

coal macerals in the feed coal and the mineral and amorphous 

phases in the fly ash. Microscopic analysis of feed coal and fly 

ash was conducted on ground and polished epoxy-bound pellets (American Society for Testing and Materials, 1999c). The 

final polishing step used a 0.05-μm alumina slurry. Examination was conducted with reflected-light, oil-immersion optics 

at a final magnification of × 500–625. Petrographic nomenclature follows the outline of Hower and others (1995) and 

American Society for Testing and Materials (American Society 

for Testing and Materials, 1999e). 

X-Ray Diffraction Analysis

Coal ash for mineralogical studies was derived by two 

methods: (1) low-temperature ashing (LTA), and (2) treating 

the coal with aqueous sodium hypochlorite (NaOCl). Coal 

samples were ground to –60 mesh and then ashed in a lowtemperature plasma asher with radio frequency activated compressed oxygen. During low-temperature ashing, coals were 

subjected to a maximum ashing temperature of approximately 

150°C following procedures of Gluskoter (1965). The NaOCl 

procedure was modified by Brownfield and others (1995) from 

Anderson (1963) and Starkey (1984). In this latter procedure, 

which involved separation of mineral matter from the organic 

fraction, the coal was first ground to –60 mesh in a ceramic 

mortar and pestle. The mineral-poor portion of the sample was 

then separated by flotation methods with carbon tetrachloride 

(tetrachloromethane, density 1.59 g/cm3). Approximately 2.5-g 

units of each mineral-rich sample were placed in 50-mL centrifuge tubes with 5 mL of NaOCl and the pH adjusted to 9.5 by 

addition of HCl or NaOH. The tubes were heated in a boiling 

water bath for 10 minutes, then centrifuged at 600 rpm for 8 

minutes, and the liquid decanted. Successive treatments were 

performed (as many as 20) until the supernatant liquid became 

clear to pale yellow. Distilled water was used to flush the 

residue into a 500-mL plastic beaker, after which the residue 

was disaggregated in an ultrasonic generator, and washed and 

wet-sieved through a 200-mesh screen. Both LTA and NaOCl 

procedures oxidize and remove organic matter with only 

minor alteration of the minerals.

Mineralogy of the coal ash, fly ash, and bottom ash 

samples was determined by X-ray diffraction (XRD) methods. 

XRD of selected samples was performed on a Phillips XRD 

3000 Diffractometer in Bragg-Brentano geometry with Cu Kα

radiation (1.5418 Å wave length) scanned at 4°–64° 2θ with a 

0.02° stepping increment and a 2-second count scan rate. XRD 

data were collected digitally and analyzed with Materials Data 

Inc. (MDI), Livermore, Calif., software and the International 

Centre for Diffraction Data Power Diffraction File (1999). 

Magnetic separations (Cathcart and others, 1997) were conducted on some fly and bottom ash samples and the separates 

were later analyzed by XRD. Relative abundance of minerals 

was determined using reference intensity methods.

Quantitative mineral data were determined on selected fly 

ash samples with Rietveld refinement quantitative XRD methods (Larson and Von Dreele, 1994). An internal rutile (TiO2

) 

standard was added to each sample (0.1 g rutile to 0.9 g of the 

sample). Rutile was selected because it has a linear absorption coefficient in the mid-range of the phases expected in the 

sample, therefore minimizing intensity error due to microabsorption (Brindley, 1945). Samples were analyzed on a Phillips Diffractometer using Cu Kα radiation scanned at 20°–80° 

2θ with a 0.03° stepping increment and a 2-second count scan 

rate. XRD data were collected digitally with MDI software, 

and the publicly available General Structure Analysis System (GSAS) software was used for the Rietveld refinements 

(Larson and Von Dreele, 1994). Dr. Ryan S. Winburn at Minot 

State University completed the Rietveld refinements on the fly 

ash XRD data and reported the quantitative mineral data.

Microbeam Analysis

Samples were prepared and analyzed by microbeam 

methods using the electron probe microanalyzer (EPMA) 

and scanning electron microscope (SEM) with energy dispersive X-ray analysis (EDS). Microbeam methods support 

the mineralogical identifications obtained by X-ray diffraction methods. Feed coal and CCP samples were prepared as 

polished epoxy-bound sections of the bulk material, whereas 

low-temperature ash and sodium hypochlorite (NaOCl) treated 

samples were mounted on conductive carbon tape. Images 

of the mineral grains were obtained with secondary electron 

imaging (SEI) and the backscattered electron imaging (BEI) 

that is responsive to the average atomic number of the imaged 

object. The SEM analysis was assisted by semiquantitative 

energy dispersive spectral (EDS) analysis, which allowed the 

calculation of cation ratios and element contents for many of 
the observed phases in the feed coal CCPs. Highly precise 

chemical compositions (accuracy generally within ±1 percent) were obtained for macrocrystalline phases greater than 

5 μm in diameter by means of EPMA wavelength dispersive 

spectral (WDS) analysis. X-ray intensity mapping was used 

to collect data on modes of occurrence of selected elements in 

fly ash and bottom ash grains. Electron microprobe and SEM 

scans of fly ash grains allowed analysis of samples for relative 

intensity of individual elements at each counting point. Use of 

quantitative EPMA with WDS analysis allows as many as five 

elements per sample to be mapped at one time; SEM element 

mapping can analyze more elements at one time, but the data 

are less precise. 

Leaching Analysis

Leaching experiments range in duration and severity of 

chemical treatment. Different types of leaching tests provide 

different types of information on the leaching behavior of 

coal and CCPs. Experiments are designed either to simulate 

a disposal scenario or to selectively dissolve mineral phases. 

Batch leaching tests combine a solution and a fly ash or coal 

in a known ratio of solution to material for a selected period 

of time (seconds to minutes). Solution compositions from 

the batch tests represent the initial, and commonly highest, 

concentrations of elements expected to be leached from a 

material; batch tests are used for regulatory purposes in tests 

such as the Environmental Protection Agency’s Toxicity Characteristic Leaching Protocol (TCLP). Several leaching solution 

compositions may be used sequentially in these batch tests 

to dissolve different solid phases. In column leaching experiments, vertical cylinders are filled with fly ash and a solution, 

such as distilled water, and the solution is allowed to flow 

through the column. Solutions are collected over the course of 

the test, which may last several months, and the cumulative 

amounts of leached elements are determined. Column tests are 

used to determine long-term (days to months) leaching behaviors of fly ash, simulating the flow of ground or surface water 

through a material in the natural environment. The material 

that was leached can also be characterized using XRD or SEM 

methods to determine any alteration that took place. 

Four leaching methods were used in this study for the 

coal and fly ash samples: 18-hour batch leaches, <1-hour batch 

leaches, flow-through column leaches, and sequential leaching. A more comprehensive presentation of leaching results 

from experiments conducted on the Wyodak-Anderson fly ash 

is underway (C.A. Rice and others, work in progress).

Developing models to predict the behavior of elements 

during combustion of coal and the potential leaching of CCPs 

requires quantitative data on the modes of occurrence of those 

elements in the feed coal. Such data can also be beneficial 

for evaluation of the environmental impacts, technological 

impacts, and economic byproduct potential of coal and CCPs. 

Sequential leaching experiments provide data that determine 

modes of occurrence of trace elements in feed coal, fly ash, 

and bottom ash; these data are included herein.

Sequential selective leaching can be used to rapidly determine the modes of occurrence of some inorganic elements in 

coal (Finkelman and others, 1990) and to support microprobe, 

SEM, and X-ray diffraction data on these elements. Previous 

methods to determine the modes of occurrence of elements in 

coal were labor intensive and provided only limited quantitative data. The most common of these methods included the 

analysis of density separates of coal (Gluskoter and others, 

1977; Querol and others, 1995) and the analysis of size and 

density separates in low-temperature ash (Palmer and Filby, 

1984). Direct methods such as SEM, microprobe, and X-ray 

diffraction were also used, but were effective only if the element (or mineral) content was above the detection limits.

The sequential leaching procedure used in our study is 

similar to that described by Finkelman and others (1990) and 

Palmer and others (2000). For sequential leaching, duplicate 

10-g coal samples, ground to –60 mesh, were combined with 

a solution and agitated for 18 hours, then centrifuged and the 

leachate separated by filtration. The samples were first leached 

with 1N ammonium acetate (NH4

C2H3O2). This procedure was 

repeated in subsequent leaching steps using 3N hydrochloric 

acid (HCl), concentrated (48–51 percent) hydrofluoric acid 

(HF), and 2N nitric acid (HNO3

). This sequence of solvents 

was selected so that (1) exchangeable cations, water-soluble 

compounds, and some carbonates would be removed by 

NH4

C2H3O2; (2) cations associated with carbonates, monosulfides, iron oxides, and chelated organic compounds would 

be removed by HCl; (3) elements associated with silicates 

would be removed by HF; and (4) elements associated with 

disulfides, especially pyrite, would be removed using HNO3

. 

The sample residue present following the sequential leaching 

procedure contains the insoluble elements associated with the 

organic matter and the insoluble or shield minerals. 

Trace-element and cation concentrations of the leachates and the final sample residue were determined by inductively coupled atomic emission spectroscopy (ICP-AES) and 

inductively coupled mass spectroscopy (ICP-MS); anions 

were determined by ion chromatography (IC). Mercury was 

analyzed by cold vapor atomic absorption spectroscopy (CVAAS), whereas selenium was determined by hydride generation atomic absorption.

Results of Analysis

Feed Coal Characterization

Thirty-eight representative feed coal samples were collected at the Indiana power plant. Ten of the feed coal samples 

collected at the power plant underwent proximate and ultimate 

analysis, as well as analysis for caloric value, forms of sulfur, 

and ash-fusion temperatures, following American Society for 

6 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
Testing and Materials (ASTM) standards methods (American Society for Testing and Materials, 1999a). The feed coal, 

which was mined from the Wyodak-Anderson coal zone in 

the Powder River Basin, Campbell County, Wyo., has a mean 

ash yield of 5.5 percent and a low sulfur (0.3 percent) content (table 1). The apparent rank of the feed coal is within the 

subbituminous B range (10,395 Btu/lb) using the Parr formula 

(American Society for Testing and Materials, 1999c), which 

compares favorably with data reported in the study by Ellis 

and others (1999) on the Wyodak-Anderson coal.

Coal petrography and vitrinite reflectance data (American 

Society for Testing and Materials, 1999d) were determined on 

six sample splits prepared from the feed coal samples collected at the plant by James C. Hower at the Center of Applied 

Energy Research, University of Kentucky (written commun., 

2001) and are shown in table 2. The estimated rank, as determined by the maximum vitrinite reflectance (Ro max; table 2), is 

not indicative of the calculated subbituminous apparent rank 

and ranges from lignite to subbituminous (Stach and others, 

1982). The feed coal vitrinite reflectance data show a slight 

distribution of v3 and v4 values (table 2), ranging from 100 

percent v3 (0.30–0.39 Ro max) to 65 percent v4 (0.40–0.49 

Ro max). 

About 40 sample splits from the feed coal samples from 

the Wyodak-Anderson coal collected from the coal-fired 

Indiana power plant were prepared by low-temperature ashing 

(LTA) methods and analyzed by XRD. X-ray diffraction analyses of the coal LTA ash (fig. 4) reveal a predominance of wellcrystallized kaolinite (Al2

Si2O5(OH)4) and quartz (SiO2). Scanning electron microscope and XRD analysis of the feed coal, 

coal ash (table 3), and NaOCl-treated coal revealed minor 

amounts of hydrated alumino-phosphate minerals (figs. 5 and 

6) containing Ba, Ca, and Sr, and trace amounts of beta-form 

quartz (fig. 7), apatite (Ca5

(PO4)3F), zircon (ZrSiO4; fig. 8), 

plagioclase (albite, NaAlSi3

O8), and potassium feldspar (sanidine, (K,Na)AlSi3

O8). The hydrated aluminum-phosphates 

were grouped by Palache and others (1951) as the plumbogummite group and later regrouped by Fleischer (1987) 

as the crandallite group. Fleischer’s classification is used 

in this report; the general formula for the crandallite group 

is XAl3

(PO4)2(OH)5

.H2

O. The minerals identified include 

crandallite (X=calcium) and gorceixite (X=barium), which 

are commonly in a solid solution state rather than representing 

pure end members, and they may contain trace amounts of Sr, 

Ce, and Pb. Other minerals identified in the NaOCl-treated 

feed coal include rounded grains of detrital quartz, calcite 

(CaCO3

), dolomite (CaMg(CO3)2), siderite (FeCO3), ankerite 

(Ca(Fe,Mg,Mn)(CO3

)), iron oxide (hematite, α-Fe2O3), biotite 

(KX3

AlSi3O10(OH)2, where X=Fe and (or) Mg), muscovite 

(KAl2

(Si3Al)O10(OH,F)2), anatase (TiO2), barite (BaSO4), 

pyrite (FeS2

), rutile (TiO2), chromite (FeCr2O4), monazite 

((RE,Th)PO4

, where RE=rare earth elements Ce, La, Nd), 

ilmenite (FeTiO3

), and cerussite (PbCO3).

Two mineral suites were identified in the feed coal from 

the Wyodak-Anderson coal: a primary detrital suite and an 

authigenic suite. The primary detrital suite consisted of quartz 

(detrital and volcanic beta-form grains), biotite, apatite, potassium feldspar (sanidine?), plagioclase (albite?), zircon, rutile, 

ilmenite, chromite, and monazite. Quartz grains in this suite 

are primarily clear, colorless, and angular; flake and blade 

shapes are common with minor frosted and rounded grains, 

which may be of windblown origin. Sharp euhedral (fig. 7) 

to rounded beta-form quartz bipyramids are also present, a 

few of which display pitting along the hexagonal bipyramids 

and authigenic quartz overgrowth on the prism faces (fig. 9). 

Biotite and muscovite are generally in the form of irregular 

flakes some of which display dissolution along their edges. 

Pseudo-hexagonal biotite crystals were also observed. Zircon 

is represented by euhedral prismatic crystals terminated by 

pyramids in NaOCl-treated coal samples; both doubly terminated and broken crystals with only one or no terminations 

were observed. Potassium feldspar (fig. 10) and plagioclase 

(fig. 11) are mostly angular cleavage fragments in the NaOCldigested coal samples.

An authigenic mineral suite consisted of kaolinite, alumino-phosphates (crandallite, fig. 5; gorceixite, fig. 6), calcite 

(fig. 12), dolomite (fig. 13), siderite, ankerite, anatase (fig. 

14), quartz (fig. 15), barite, iron oxide (hematite, fig. 16), and 

pyrite. In polished coal mounts, kaolinite occurs in plant-cell 

fillings and irregular masses within thin zones of mineral matter. Crystallized kaolinite was observed in the low-temperature ash samples. The alumino-phosphate minerals were also 

observed, filling plant cells (fig. 5) and as subhedral crystals 

(fig. 6) in the NaOCl-digested coal samples. Authigenic quartz 

was observed as quartz overgrowths on the beta-form quartz 

grains (fig. 9) and as quartz casts replacing plant material (fig. 

15) in the digested coal samples. Carbonate minerals (calcite, 

dolomite, siderite, ankerite) are mostly subhedral grains and 

rhombohedral cleavage fragments (figs. 12 and 13). Barite 

was noted in polished mounts as cell fillings and as subhedral cleavage fragments in the digested coal samples. Iron 

oxide, possibly hematite, was abundant in the NaOCl-treated 

samples, forming irregular masses and spheres (fig. 16) that 

most likely represent fecal pellets. Pyrite was observed as 

irregular flat grains, probably representing cleat fillings, and 

as small framboids.

 Major and minor oxide contents for the feed coal from 

the Wyodak-Anderson coal are shown in table 4. Trace elements of environmental interest as defined by the 1990 Clean 

Air Act Amendments (U.S. Statutes at Large, 1990) for the 

Wyodak-Anderson feed coal are shown in table 5. Results of 

chemical analyses show minor differences in the compositions 

of the feed-coal stocks. A more complete report of temporal 

variation of the Wyodak-Anderson feed coal is underway 

(R.H. Affolter and others, work in progress). 

Leaching and Modes of Occurrence of Selected 

Elements in Feed Coal

Three feed-coal samples were sequentially leached with 

1N ammonium acetate (NH4

C2H3O2), 3N hydrochloric acid 

Results of Analysis 7
(HCl), concentrated hydrofluoric acid (HF), and 2N nitric acid 

(HNO3). The leachates were analyzed and the chemical data 

were processed to obtain the percentages for each element 

leached by the four solvents. The calculated percentages were 

used as a semiquantitative measure of the modes of occurrence 

of the elements. A comparison of quality control data from 

standards with data from the leachates indicated an estimated 

error of plus or minus 20 percent for this study. Table 6 shows 

concentrations of selected elements resulting from the sequential leaching experiments on Wyodak-Anderson coal samples. 

(See table 9 for modes of occurrence for selected elements.)

The selective leaching results indicate that 40 percent 

or more of the elements Ba, Co, Fe, Mn, P, Pb, Sr, Th, and U 

are associated with HCl-soluble compounds (table 6) such as 

carbonates, monosulfides, and oxides, whereas 40 percent or 

more of the elements As, Co, Cr, Ni, Se, and Zn are associated with insoluble phases or with organic matter. All of the 

mercury in the Wyodak-Anderson coal samples appears to be 

organically associated. Portions of the elements Be, Cr, Se, 

Th, and U are associated with HF and HNO3

 soluble compounds, including clays and other silicates and pyrite and 

other disulfides.

Coal Combustion Products

More than 150 fly ash samples, produced from combustion of feed coal from the Wyodak-Anderson coal zone, 

Powder River Basin, Wyo., were collected and analyzed by 

X-ray diffraction (fig. 17; table 3). X-ray diffraction data 

revealed a predominance of aluminate glass, merwinite 

(Ca3

Mg(SiO4)2), lime (CaO), and gehlenite (Ca2Al(Al,Si)O7) 

with minor and trace amounts of quartz, apatite, anhydrite 

(CaSO4), hematite (α-Fe2O3), periclase (MgO), perovskite 

(CaTiO3), magnesioferrite (MgFe2

3+O4

), and magnetite 

(Fe2+Fe2

3+O4

). Periclase and lime were detected in all samples 

except the economizer fly ash (fig. 18). Gehlenite forms a 

solid solution series with akermanite (Ca2MgSi2O7) and melilite ((Ca,Na)2

(Al,Mg)(Si,Al)2O7), which were found in trace 

amounts. A comparison among the fly ash samples showed 

gehlenite to be more abundant in the economizer ash, whereas 

perovskite was more abundant in the fly ash samples collected from the precipitators. Anhydrite forms thin coatings on 

many of the fly ash grains. Rietveld X-ray diffraction analysis 

conducted on three samples (table 7) confirmed that glass, 

merwinite, gehlenite, and quartz were the predominant phases 

with minor and trace amounts of C3A (a concrete industry 

abbreviation for a calcium aluminate, Ca3

Al2O6), anhydrite, 

periclase, hematite, lime, and magnetite. Petrographic analysis 

confirmed the presence of minor amounts of mullite, quartz, 

and spinel/ferrite (table 8) with traces of coke and inertinite 

(James C. Hower, written commun., 2001). 

Microprobe and SEM analysis of fly ash samples 

detected mineral grains that were consistent with quartz, 

zircon, monazite, euhedral laths of corundum, merwinite, 

hematite, dendritic spinels and ferrites, and rounded grains of 

Range Standard

Mean Median Minimum Maximum deviation

Moisture 17.1 17.4 8.3 23.3 5.4

Volatile matter 36.7 36.5 34.2 39.1 1.6

Fixed carbon 40.6 40.8 36.3 46.7 3.4

Ash 5.5 5.4 4.9 6.3 0.4

Hydrogen 6.0 5.9 5.4 6.5 0.4

Carbon 57.4 57.0 52.8 64.0 3.7

Nitrogen 0.7 0.7 0.6 0.9 0.1

Oxygen 30.1 30.7 23.0 35.0 3.9

Sulfur 0.34 0.33 0.23 0.47 0.1

Caloric value

Btu/lb 9,790 9,700 9,060 10,780 600

Forms of sulfur(percent)

Sulfate 0.02 0.02 0.01 0.03 0.01

Pyritic 0.04 0.04 0.04 0.05 0.0

Organic 0.28 0.26 0.17 0.39 0.09

Ash-fusion temperatures (°F)

Initial deformation 2,060 2,065 2,020 2,080 19

Softening temperature 2,090 2,090 2,060 2,150 26

Hemispherical temperature 2,110 2,105 2,090 2,160 21

Fluid temperature 2,130 2,125 2,100 2,170 23

Proximate and ultimate analysis (percent)

Table 1. Mean values for proximate and ultimate analysis, caloric value, forms of sulfur, and ash-fusion 

temperatures of 10 samples of Indiana power plant feed coal mined from the Wyodak-Anderson coal zone, 

Powder River Basin, Wyo.

[All samples on an as-received basis]

8 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
wollastonite (CaSiO3

) with periclase. The composition of the 

associated glass was somewhat variable but generally consisted of calcium and aluminum with minor and trace amounts 

of Si, Fe, Mg, Ti, Na, K, P, and As (O’Connor and Meeker, 

1999).

A small percentage of fly ash grains (<5 percent) are 

composed of cryptocrystalline phosphorus phases with whitlockite (Ca9

(Mg, Fe+2)(PO4)6(PO3(OH)) and its anhydrous 

analog merrillite, or α-Ca3(PO4)2 , as the most likely mineral 

phases (O’Connor and Meeker, 1999). 

nce

nce

Table 2. Petrology and vitrinite reflectance of an Indiana power plant feed coal from the Wyodak-Anderson coal zone, Powder River Basin,Campbell County, Wyo.

[Data from James C. Hower, Center for Applied Energy Research, University of Kentucky, Lexington, Ky., written commun., 2001. t, trace amount; Rmax, mean maximum reflecta

measured in oil; s.d., standard deviation; v3, percent of values from 0.30-0.39; v4, percent of values from 0.40-0.49; n.d., not determined, humic macerals not suitable for reflecta

measurements]

Sample Macerals and petroleum coke (Pet coke), in percent

Vitrinite Liptinite Inertinite Pet coke

Ulminite Textinite Humodetrinite Corpocollinite Exinite Resinite Fusinite Semifusinite Micrinite Macrinite

RP1 26.0 26.0 22.4 4.0 4.2 0.8 11.8 4.8 0.0 0.0 0.0RP4 30.2 20.8 27.2 5.6 5.6 2.4 5.2 3.0 t t 0.0RP14 23.4 23.6 29.4 7.2 2.2 0.6 9.8 3.6 0.2 0.0 0.0RP17 33.0 18.4 25.4 6.2 4.6 0.6 8.0 3.8 0.0 t 0.0RP20 30.8 16.8 29.4 2.8 2.6 1.4 11.2 4.4 0.0 0.6 0.0RP29 25.8 24.3 30.6 6.9 0.6 0.2 8.5 2.7 0.2 0.0 0.2Sample Vitrinite reflectance (oil-immersion, 500X)

R

max

s.d. v3 v4

RP1 0.39 0.03 57 43RP4 0.38 0.03 76 24RP14 n.d. n.d. n.d. n.d.RP17 0.40 0.03 35 65RP20 0.40 0.02 36 64RP29 0.35 0.02 100 0.0

Results of Analysis 9
1,500

1,000

500

0

10 20 30 40 50 60

INTENSITY (COUNTS)

DEGREES 2θ

Q 

K Kaolinite

B Bassanite

C Calcite

Q

Q

Q

Q

Q

Q

Q

Q

K Q

K

K

K K

K

K

K

B

B

B

B

B

C C

C

C

B

EXPLANATION

4

Quartz

Figure 4. X-ray diffractogram of low-temperature ash (LTA) from a power plant feed coal mined from the WyodakAnderson coal zone, Powder River Basin, Wyo.

Feed coal Fly ash Bottom ash 

Quartz SiO2 alpha form – Ma Glass – Ma Glass – Ma

Kaolinite Al2Si2O5(OH)4 – Ma Merwinite Ca3Mg(SiO4)2 – Ma Quartz – Mi

Calcite CaCO3

 – T Gehlenite Ca2Al(Al,Si)O7 – Mi Anorthite CaAl2Si2O8 – Mi

Biotite and muscovite(?) – T Quartz – Mi Augite (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2

O6 – Mi

Crandallite group – T Anhydrite CaSO4 – T Fassaite Fe-Al diopside or augite – Mi

Plagioclase albite – T 1

C3A Ca3Al2O6 – T Rhodonite (Mn2+,Fe2+,Mg,Ca)SiO3 – Mi

Potassium feldspar – T Apatite Ca5

(PO4)3F – T Akermanite Ca2MgSi2O7 – T

Hematite D-Fe2

O3 – T Periclase MgO – T Melilite (Ca,Na)2

(Al,Mg)(Si,Al)2O7 – T

Quartz SiO2

 beta-form – T Mullite Al6Si2O13 – T Albite (NaAlSi3O8) – T

Dolomite CaMg(CO3

)2 – T Lime CaO – T Magnetite Fe2+Fe2

3+O4 – T

Anatase TiO2 – T Magnesioferrite MgFe2

3+O4 – T Magnesioferrite MgFe23+O4 – T

Barite BaSO4 – T Hematite D-Fe2

O3 – T Pyrite – T

Pyrite FeS2 – T Perovskite CaTiO3 – T Anhydrite – T

Zircon ZrSiO4 – T Akermanite Ca2

MgSi2O7 – T Zircon – T

Apatite Ca5

(PO4)3F – T Melilite (Ca,Na)2(Al,Mg)(Si,Al)2O7 – T

Siderite FeCO3 – T Whitlockite Ca9

(Mg, Fe+2)(PO4)6(PO3,OH) – T

Ankerite (Ca(Fe,Mg,Mn)(CO3

)2)–T Merrillite D-Ca3

(PO4)2 – T

Cerussite (PbCO3

) Magnetite Fe2+Fe2

3+O4 – T

Monazite (RE,Th)PO4 – T

Ilmenite FeTiO3 – T 1

C3A is a concrete industry abbreviation for a calcium aluminate, Ca3Al2O6.

Table 3. Results of X-ray diffraction, SEM, and microprobe analysis of feed coal, fly ash, and bottom ash from an Indiana 

power plant utilizing coal from the Wyodak-Anderson coal zone, Powder River Basin, Wyo. 

[Feed coal mineralogy determined from low-temperature ash, NaOCl-treated samples, and polished pellets. Mineral formulas from Fleischer (1999). 

Minerals are listed in order of relative abundance. Ma, major (> 10 percent); Mi, minor (5-10 percent); T, trace (< 5 percent)]

10 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
Figure 6. Scanning electron microscope (SEM) secondary electron image of a subhedral 

crystal of gorceixite (BaAl3

(PO4)

2

(OH)5·H2O), from an Indiana power plant feed coal from the 

Wyodak-Anderson coal zone, Powder River Basin, Wyo.

Figure 5. Scanning electron microscope (SEM) secondary electron image of hydrated 

alumino-phosphate (crandallite) filling plant cells, from an Indiana power plant feed coal from

the Wyodak-Anderson coal zone, Powder River Basin, Wyo.

Results of Analysis 11
Fly ash samples were also collected and analyzed from an 

abandoned surface disposal site from which the ash was being 

processed as soil amendment. The material had been subjected 

to natural precipitation as well as water used for dust suppression. X-ray diffraction analysis revealed major amounts (> 10 

percent) of chabazite (Ca,K,Na)4Al4Si8O24·12H2O), ettringite 

(Ca6Al2(SO4)3(OH)12·26H2O), and strätlingite (Ca8Al4(Al4Si4)

O8(OH)40·10H2O) and lesser amounts (<10 percent) of calcite, 

C3A, gehlenite, merwinite, mullite, quartz, thaumasite (Ca6Si2

(CO3)(SO4)2(OH)12·24H2O), and hematite. Chabazite, ettringite, strätlingite, thaumasite, and calcite are authigenic mineral 

phases resulting from alteration of the original fly ash.

The magnetic fraction recovered from fly ash samples 

from combusted Wyodak-Anderson coal in our study ranges 

from 1.0 to 2.0 percent; it consists of dendritic magnetite and 

magnesioferrite and subhedral to euhedral hematite (table 3). 

The small percentage of magnetic minerals is the result of the 

low pyrite content of the feed coal when compared to Appalachian Basin and Illinois Basin feed coals (Brownfield and 

others, 1997; Brownfield, Affolter, Cathcart, O’Connor, and 

Brownfield, 1999). Magnetic minerals in fly ash have been 

described as magnetite, ferrites, and hematite (Lauf and others, 1982; Brownfield and others, 1997; Cathcart and others, 

1997). 

X-ray diffraction analysis of 38 samples of the WyodakAnderson bottom ash (fig. 19) revealed quartz, plagioclase 

(anorthite), and pyroxene (augite and fassaite) with minor and 

trace amounts of rhodonite, akermanite, melilite, pyrite, and 

anhydrite (table 3). All samples contained some amorphous 

aluminum silicate glasses, char (unburned coal), sulfate minerals, and illite and (or) muscovite. Analysis of samples by SEM 

detected detrital grains of quartz and zircon and high-aluminum pyroxene (fassaite) laths with anorthite (fig. 20). The 

composition of the associated bottom ash glass was variable in 

composition but generally consisted of Ca and Al with minor 

and trace amounts of Si, Fe, Mg, Ti, and Na. The bottom ash 

contains less Ca and Mg when compared to the fly ash 

(table 4).

X-Ray Element Mapping

Quantitative, element intensity mapping via the electron 

microprobe provides relative intensities of individual elements at each counting point and helps to develop a better 

understanding of modes of occurrence and potential release of 

selected elements to the environment. For example, figure 21 

shows a backscattered electron image and three element maps 

(Cr, Ni, and Si) of a fly ash grain from a Kentucky power 

plant. The images reveal a distinct enrichment of nickel but 

a lesser amount of chromium in both the glassy and ferrite 

portions of the grain (Robert B. Finkelman, written commun., 

2002). Figure 22 shows a second example of a backscattered 

electron image and three elements maps (Ca, Mg, and S) of a 

fly ash grain with a distinct rim of calcium and sulfur resulting 

from a thin anhydrite (CaSO4) coating on a power plant fly ash 

grain (Brownfield, Affolter, Cathcart, O’Connor, and Brownfield, 1999). The bright areas in the center of the Mg element 

map are small periclase crystals (MgO) in a Ca-aluminate. 

Electron microprobe element intensity mapping is limited to 

elements that are present at or above the detection limits of 

the microprobe (approximately 0.1 weight percent). Attempts 

to map elements of environmental interest (U.S. Statutes at 

Large, 1990) in Wyodak-Anderson fly ash grains (table 5) 

were not successful because the elements were all near or 

below detection limits of the microbeam instruments. Element 

intensity mapping is still a valuable tool in determining modes 

of occurrence of other elements in coal combustion products.

Discussion and Interpretation

Determination of elements in feed coal is important 

because the content, distribution, and behavior of elements 

during and after combustion largely reflect the content and 

distribution of trace elements in the feed coal. Elements of 

environmental interest as defined by the 1990 Clean Air Act 

Amendments (U.S. Statutes at Large, 1990) are doubly important because they can be potentially released into the environment during coal utilization.

In our study, samples of the Wyodak-Anderson coal 

used as feed coal contained higher Ba, Ca, Mg, Na, P, Sr, and 

Ti contents than the eastern feed coals that were studied by 

Figure 7. Scanning electron microscope (SEM) secondary 

electron image of a euhedral crystal of beta-form quartz, from an 

Indiana power plant feed coal from the Wyodak-Anderson coal 

zone, Powder River Basin, Wyo.

12 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
Brownfield, Affolter, Cathcart, O’Connor, and Brownfield 

(1999). These elements are associated with the minerals 

crandallite (Ba, Ca, P, and Sr), gorceixite (Ba, Ca, P, and Sr), 

biotite (Ba and Ca), calcite (Ba, Ca, and Sr), and clay minerals (Ca and Mg). They are indicative of coals containing 

altered airfall volcanic ash (Affolter and Brownfield, 1987; 

Brownfield and Affolter, 1988; Affolter and others, 1997). In 

low-rank Powder River Basin coals, major portions of the total 

Ca and Mg contents are associated with the organic matter as 

a result of cation exchange. Additionally, detrital mineral input 

and mineral enrichment as a result of epigenetic ground-water 

flow have contributed to the geochemical makeup of the feed 

coal.

The Wyodak-Anderson coal, although mined in several 

mines in the eastern Powder River Basin, displayed little 

variation in element content during its first 15-day sampling 

Figure 9. Scanning electron microscope 

(SEM) secondary electron image of a 

euhedral crystal of beta-form quartz, from 

an Indiana power plant feed coal from the 

Wyodak-Anderson coal zone, Powder River 

Basin, Wyo. The crystal has been partially 

etched on its hexagonal pyramids and 

shows authigenic quartz overgrowth on its 

prism faces.

Figure 8. Scanning electron microscope (SEM) 

secondary electron image of a euhedral crystal of 

zircon, from an Indiana power plant feed coal from 

the Wyodak-Anderson coal zone, Powder River 

Basin, Wyo.

Discussion and Interpretation 13
period as feed coal at the Indiana power plant (Affolter and 

others, 1999). In general, the Wyodak-Anderson coal displays 

little variation in the standard deviation with respect to the 

proximate and ultimate analysis (table 1) of ash and sulfur 

during the sample period. However, minor trace-element 

variation can occur within the Wyodak-Anderson coal zone, 

which is as much as 140 ft thick, as the result of both vertical and lateral changes in mineral content and the amount of 

individual mineral phases within the coal bed at the different 

mines. Examples of minor variations in trace-element content 

are shown in figures 23 and 24, which display the maximum, 

minimum, and mean contents of Ba and P as determined for 

Figure 10. Scanning electron microscope 

(SEM) secondary electron image of an irregular 

cleavage fragment of potassium feldspar (sanidine?), from an Indiana power plant feed coal 

from the Wyodak-Anderson coal zone, Powder 

River Basin, Wyo.

Figure 11. Scanning electron microscope 

(SEM) secondary electron image of an irregular 

cleavage fragment of plagioclase feldspar 

(albite?), from an Indiana power plant feed coal 

from the Wyodak-Anderson coal zone, Powder 

River Basin, Wyo.

14 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
the first 15 feed coal samples. These element variations 

result from the variation in the barite, crandallite, and gorceixite mineral content in the different coal samples.

In contrast, analysis of the trace-element content of 

the feed coal and CCPs at a Kentucky power plant, which 

utilized coal from several different mines in the Illinois and 

Appalachian Basins, displayed considerable variability (Affolter 

and others, 1997). Figure 25 shows such variations for selected 

elements at that power plant during a 16-month sampling period. 

These variations reflect the multiple mine sources in two coal 

Figure 12. Scanning electron microscope 

(SEM) secondary electron image of a subhedral crystal of calcite (CaCO3

), from an Indiana 

power plant feed coal from the Wyodak-Anderson coal zone, Powder River Basin, Wyo.

Figure 13. Scanning electron microscope 

(SEM) secondary electron image of a subhedral 

crystal of dolomite (CaMg(CO3

)

2

), from an Indiana power plant feed coal from the WyodakAnderson coal zone, Powder River Basin, Wyo.

Discussion and Interpretation 15
basins, and the variability in mineral content and amount of 

the individual phases in the different feed coals.

In general, the trace-element contents in the different fly 

ashes and bottom ashes collected at the Indiana power plant 

during the sample period appear to be similar; the composite truck fly ash shows somewhat higher variability and the 

economizer fly ash slightly lower variability for some elements (Affolter and others, 1999). (Variations in barium and 

phosphorus contents are shown in figures 23 and 24.) The 

economizer ash is coarser grained than the truck fly ash: it has 

a wide range in particle sizes with different adsorption characteristics, whereas the truck fly ash is a composite sample 

Figure 14. Scanning electron microscope 

(SEM) secondary electron image of a 

euhedral crystal of authigenic anatase 

(TiO2

), from an Indiana power plant feed 

coal from the Wyodak-Anderson coal zone, 

Powder River Basin, Wyo.

Figure 15. Scanning electron microscope (SEM) secondary electron 

image of authigenic quartz (SiO2

) that 

replaced plant matter, from an Indiana 

power plant feed coal from the 

Wyodak-Anderson coal zone, Powder 

River Basin, Wyo.

16 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
old sampler, which was a pipe shape (Gregory H. Keenan, 

American Electric Power, written commun., 1999). A more 

complete discussion of temporal variation of the WyodakAnderson feed coal is underway (R.H. Affolter and others, 

work in progress). 

Feed coal samples from the Wyodak-Anderson coal contain higher amounts of Ba, Ca, Mg, Na, Sr, and P than eastern 

feed coal (Affolter and others, 1997; Brownfield and others, 

1997; Brownfield, Affolter, Cathcart, O’Connor, and Brownfield, 1999). These elements are associated with hydrated 

alumino-phosphate (crandallite and gorceixite) and clay 

minerals, as well as apatite, biotite, calcite, and barite. Except 

for calcite, apatite, and biotite, the minerals associated with 

these elements are derived from dissolution and devitrification 

of volcanic ash. The original ash probably consisted of silica 

glass, beta-form quartz phenocrysts, biotite, zircon, potassium feldspar, and plagioclase. Pyroxenes and amphiboles, 

which are common in volcanic airfall ash and are relatively 

unstable minerals, were not detected in the coal. Pyroxenes 

and amphiboles may have been present in the original ash; if 

so, they were most likely dissolved in the acid environment of 

the peat-forming mires. 

The elemental and minor minerals associations in the 

Wyodak-Anderson coal samples indicate that the coals contain 

reworked and (or) altered volcanic ash, based on the presence 

of (1) clear, sharp angular quartz grains, some of which are 

blade and flake shaped and may be broken phenocrysts; (2) 

beta-form quartz (high temperature) phenocrysts; (3) euhedral zircon and biotite crystals; and (4) alumino-phosphate 

(crandallite and gorceixite) minerals (Brownfield and others, 

1987; Triplehorn and others, 1991; Crowley and others, 1993). 

Dissolution and alteration of the volcanic mineral suite most 

likely occurred in the early diagenetic peat-forming stage 

(strong leaching environment) and (or) possibly during the late 

diagenetic coalification stage, and contributed to the authigenic formation of kaolinite, quartz, calcite, anatase, crandallite, gorceixite, and barite. 

Major, minor, and trace elements in coal can occur in 

three phases: (1) included in, or attached to, organic 

with adsorption characteristics common to both new and old 

fly ash samples. This lack of variation in trace-element content 

is in strong contrast to results from other studies (Affolter and 

others, 1997) in which feed coal was utilized from numerous 

sources and showed considerable variation in element chemistry. Because the Indiana power plant utilizes coal from the 

Wyodak-Anderson coal zone, particle-size distribution and 

shape of the fly ash grains in the samples collected from old 

and new automated fly ash samplers are similar. The new fly 

ash sampler contained greater quantities of very fine particles 

because of the sampler’s funnel shape when compared to the 

Figure 16. Scanning electron microscope (SEM) secondary 

electron image of an iron oxide sphere, probably a fecal pellet, 

from an Indiana power plant feed coal from the Wyodak-Anderson 

coal zone, Powder River Basin, Wyo.

Table 4. Concentrations of major and minor oxides for feed coal, fly ash, and bottom ash for the

first 15 days of sampling (n=15) from an Indiana power plant utilizing coal from the WyodakAnderson coal zone, Powder River Basin, Wyo.

[All values are in percent and are presented on an as-determined basis for the feed coal, fly ash and bottom ash; n.d., not

determined]

Oxide Feed coal mean Fly ash mean Bottom ash mean

(Ash) 6.1 n.d. n.d.

SiO2 23.6 32.3 27.8

Al2

O3 14.7 18.1 17.5

CaO 20.8 26.4 20.2

MgO 4.9 5.8 4.8

Na2

O 1.6 2.4 1.7

K2

O .30 .33 .31

Fe2

O3 5.3 6.5 6.4

TiO2 1.1 1.4 1.4

P2

O5 .88 1.0 .81

Discussion and Interpretation 17
compounds as organic complexes or exchangeable cations; (2) 

in mineral matter as major or trace components of minerals; 

and (3) only rarely as dissolved constituents of fluids in the 

coal. Sequential leaching experiments were used to quantify 

the modes of occurrence of inorganic elements in the feed coal 

from Wyodak-Anderson coal; results with respect to selected 

elements are summarized in table 9. 

Leaching experiments indicate that at least 40 percent 

of the Ba, Co, Fe, Mn, P, Pb, Sr, Th, and U is associated 

with HCl-soluble compounds (table 9), such as carbonates, 

monosulfides, phosphates, and oxides, whereas at least 50 

percent of As, Co, Ni, and Se is associated with insoluble 

phases or organic matter. About 20 percent of the arsenic is 

associated with HCl-soluble minerals such as crandallite and 

gorceixite, where arsenic can substitute for phosphorus or 

arsenates. Chromium is contained in both clay minerals and 

organic matter. Beryllium is evenly distributed among silicates 

(clays, micas, and albite), organic matter, and HCl-soluble 

compounds such as monazite and beryllium-bearing oxides, 

hydroxides, or sulfates. Some of the Se, Th, and U is associated with HF- and HNO3

-soluble compounds, including clays 

and other silicates, as well as pyrite and other disulfides. All of 

the mercury in the Wyodak-Anderson coal samples appears to 

be organically associated.

The minerals identified in the feed coal samples contain 

leachable elements, and these minerals are important modes 

of occurrence for elements in the Wyodak-Anderson coal. 

Crandallite and gorceixite (table 3) were found in the feed coal 

and are aluminum phosphate minerals that can contain Ba, Ca, 

and Sr. Barite (BaSO4

), potassium feldspar, and biotite were 

also identified in the feed coal and are known to contain trace 

amounts of barium. HCl-soluble calcite (CaCO3

) was detected 

in trace amounts and is known to contain trace amounts of 

Ba, Co, Fe, Mn, Ni, Sr, and Pb. Trace amounts of HCl-soluble 

dolomite (Ca,Mg(CO3

)2), ankerite (Ca(Fe,Mg,Mn)(CO3)2), 

siderite (FeCO3), and cerussite (PbCO3) were found in the 

NaOCl-treated coal samples. Lead can substitute in trace 

amounts for magnesium in ankerite and dolomite, which may 

account for the high amounts of lead in the HCl leachate. 

At least 50 percent of the iron (Fe) is attributed to iron-bearing carbonates, iron oxides (hematite?, fig. 16), iron-bearing 

silicates (biotite and muscovite); shielded (very small mineral 

grains surrounded by organic matter) or insoluble minerals 

(ilmenite), and trace amounts of pyrite account for the 

remainder. 

Palmer and others (1998, 2000) reported selective leaching results for Wyodak-Anderson coal samples and indicated 

that more than 40 percent of the Co, U, Be, Ca, Fe, Mn, Ba, 

Zn, Cd, and Pb is associated with HCl-soluble carbonates, 

iron oxides, or monosulfides, whereas greater than 40 percent 

of the Al, Cr, and Mo is associated with HF-soluble silicates. 

Other leaching experiments indicate that more than 40 percent 

of the As, Cu, Ni, and Sb is associated with acid-insoluble 

phases and (or) organic matter. Fifty percent or more of the 

mercury is associated with HNO3

-soluble pyrite or other sulfides. This is in marked contrast to our findings where pyrite 

was only noted as a trace phase by SEM analysis. Thirty percent of the thorium content was leached by HCl, and another 

30 percent was leached by HF.

The abundant Ca-, Mg-, and less abundant P-mineral 

phases (merrillite, anhydrite, C3A, gehlenite, periclase, lime, 

perovskite, and whitlockite) in the fly ash and anorthite, 

augite, and melilite in the bottom ash are from high-temperature alteration of carbonate, clay, alumino-phosphate, biotite, 

feldspar, and anatase minerals in the feed coal. The feed coal 

minerals, in part, formed during dissolution and alteration of 

airfall volcanic ash in the coal (Brownfield, Affolter, Cathcart, 

and others, 1999). Therefore, the Ca-, Mg-, and P-rich mineral 

phases in the CCPs are from the high-temperature transformation of volcanic minerals deposited in the peat-forming mire 

Table 5. Comparison of concentrations of selected elements of environmental concern for feed

coal (n=15), fly ash (n=25), and bottom ash (n=14) from an Indiana power plant utilizing coal from

the Wyodak-Anderson coal zone, Powder River Basin, Wyo.

[All values are in parts per million and are presented on an as-determined basis for the feed coal, fly ash, and bottom

ash except for Hg and Se, which have been recalculated to an ash basis. Mean ash yield of the feed coal is 6.1 percent]

Element Feed coal mean Fly ash mean Bottom ash mean

As 14 18.7 7.5

Be 3.1 5.1 .81

Cd .82 .93 .29

Co 27 28 31

Cr 53 62 53

Hg 0.08 .01 0.02

Mn 170 200 200

Ni 32 50 60

Pb 23 38 14

Sb 1.7 2.7 1.1

Se 0.72 29 22

Th 20 30 24

U 6.5 9.4 8.4

18 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
Element As Ba Be Co Cr Fe Hg Mn Ni P Pb Se Sr Th 

Original concentration (ppm)

Feed coal (RP14)1 0.86 240 0.24 1.5 2.8 1,900 0.07 7.4 1.5 230 1.2 0.43 210 1.9 Feed coal (RP17)1 1.1 0.20 330 2.0 3.6 2,500 0.06 2.2 9.8 260 1.5 0.64 190 1.4 

Feed coal (RP29)1 0.92 320 0.20 1.9 3.6 2,500 0.07 13 2.4 280 1.4 0.36 210 1.2 

Feed coal average1 0.96 300 0.21 1.8 3.3 2,300 0.07 10 2.0 260 1.4 0.48 200 1.5 As-mined2 0.7 0.19 390 2.5 4.1 2,500 0.07 4.0 12.0 490 1.9 0.3 220 1.1 

R23 1.2 1.5 id 0.40 2,500 0.08 6.0 8.7 5.1 id 1.6 1.3 id 1.8 

Percent leached by CH3COONH4

Feed coal 5 10 0 1 1 0 0 10 5 1 0 0 25 0 As-mined 15 20 0 0 0 0 10 5 0 id 0 id id 0 R2 0 id id 0 0 0 0 10 10 id id 0 id 5 

Percent leached by HCl

Feed coal 20 80 35 40 10 40 0 90 20 90 90 0 50 60 As-mined 10 80 55 40 15 60 0 65 20 id 70 id id 30 R2 35 id id 40 10 60 0 75 10 id id 5 id 5 

Percent leached by HF

Feed coal 5 5 30 5 40 10 0 5 5 10 15 0 5 30 As-mined 10 0 30 20 45 10 5 5 0 id 20 id id 30 R2 10 id id 20 45 10 0 15 15 id id 0 id 10 

Percent leached by HNO3

Feed coal 5 1 1 5 10 1 0 1 5 1 10 30 1 15 As-mined 15 0 0 10 10 20 50 5 15 id 10 id id 10 R2 0 id id 5 10 20 0 0 10 id id 5 id 40 

Percent unleached

Feed coal 65 5 35 50 40 50 100 0 65 1 0 70 20 0 As-mined 50 0 15 20 30 25 35 20 65 id 0 id id 30 R2 55 id id 35 35 10 100 0 55 id id 90 id 40 

1 Feed coal source is Wyodak-Anderson coal zone, Campbell County, Powder River Basin, Wyo., sample collected at power plant (n=3). 2 Sample from 

Anderson coal zone, Powder River Basin, Wyo. (n=4) from Palmer and others (2000). 3 Wyodak-Anderson coal sample data (n=4) reported by Palmer a

Table 6. Element concentration data for selected elements from sequential leaching experiments from Wyodak-Anderson coal samples, Powder 

Basin, Wyo.

[Original concentrations (ppm, parts per million) are presented on a whole-coal basis; solvent data presented in percent leached by each solvent. Totals can exceed 100 percrounding but are within the expected error of plus or minus 20 percent. id, insufficient data]
and the authigenic mineral suite derived from the altered volcanic ash. In lower rank coals, major amounts of Ca and Mg 

may be associated with the organic matter by cation exchange, 

inasmuch as sequential leaching experiments conducted on 

feed coal samples indicate that major amounts of Ca and Mg 

as well as Na are associated with exchangeable cations and 

organics (table 9). Additionally, detrital mineral input and the 

epigenetic ground-water flow have affected the final geochemistry of the feed coal.

Two fly ash classes, Class C and Class F, are defined by 

ASTM (American Society for Testing and Materials, 2003) 

based on chemical and physical characteristics. Minimum 

percent of silicon dioxide (SiO2) plus aluminum oxide (Al2O3) 

plus iron (Fe2O3) for Class C fly ash must be at least 70 percent, whereas this sum for Class F fly ash must be at least 50 

percent. Class C fly ash is also defined as containing from 15 

to 30 percent CaO, with some of the CaO in the form of lime, 

whereas Class F fly ash CaO content ranges from 1 to 12 percent. Because the feed coal from the Wyodak-Anderson coal 

zone is rich in Ca (20.8 percent CaO, table 4) the resulting fly 

ash (26.4 percent CaO, table 4), will be classified as 

Class C. 

The Class C fly ash and bottom ash from the WyodakAnderson coal are higher in CaO and MgO and lower in 

Al2

O3, Fe2O3, and SiO2 when compared to the eastern United 

States bituminous Class F fly ash (Brownfield, Affolter, 

Cathcart, O’Connor, and Brownfield, 1999). The WyodakAnderson fly ash contains about 60 percent amorphous 

calcium-aluminum-silicate glass (table 7), which is highly 

variable in composition. The associated fly ash- and bottom 

ash-glass composition is dependent on the rate of quenching 

and on the amount of substitution by other elements in the 

glass lattice such as Ca, Fe, K, Mg, and Na. More than 190 fly 

ash and bottom ash samples (including splits and duplicates) 

were analyzed by X-ray diffraction (XRD), and the position 

of the amorphous diffraction (scattering) maxima or glass 

“hump” was determined. The center point of the scattering 

maxima reflects the position of the diffraction peak for the 

crystalline phase that would have exsolved from the incandescent ash had it been allowed to cool slowly. The scattering 

maxima for the fly ash glass forms an asymmetrical hump 

centered around 32°–33° 2θ, which is near the major reflection of C3A (fig. 17) and merwinite, whereas the bottom ash 

glass scattering maxima is centered around 28° 2θ near the 

major reflection of anorthite (fig. 19). Anorthite (figs. 20, 

26), C3A, and merwinite (fig. 27) were identified in analyzed 

samples. X-ray diffraction analysis of more than 70 eastern 

fly ash and bottom ash samples resulted in a glass scattering 

maxima centered around 25°–26° 2θ, which is closer to the 

position for the major reflection of mullite that is also present 

Figure 17. X-ray diffractogram of Class C fly ash from an Indiana power plant burning low-sulfur subbituminous coal from the Wyodak-Anderson coal zone, Powder River Basin, Wyo.

Q

Q

Q

Q

Q

Pr

Me

Mw

L

L

L,H

G

G G

G

G G G

G

M

Pc

H

H A

A A

A

A

A

A

A

Me

Me

Me Me

Ap Ap

DEGREES 2θ

INTENSITY (COUNTS)

750

500

250

0

4 20 30 40 50 60

H,Mw

H

Mw

Mw

Mw Merwinite

L Lime

G Gehlenite

Q Quartz

Pc Periclase

Me Melilite

A Anhydrite

H Hematite

C C3A Ca Al oxide

 EXPLANATION

C

C

C

20 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
in fly ash samples (Brownfield and others, 1997; Brownfield, 

Affolter, Cathcart, O’Connor, and Brownfield, 1999). As mullite or aluminum contents decreased, the glass “hump” shifts 

to a lower 2θ value (22°–24° 2θ), indicating a more siliceous 

glass. Fly ash from Wyodak-Anderson coal contains more 

CaO than does bottom ash, 26.4 versus 20.2 percent (table 

4), whereas the CaO content in the eastern fly ash and bottom 

ash ranges from 1.1 to 3.8 percent. Therefore, the scattering 

maxima appears to reflect differences in chemical composition of fly ash and bottom ash glasses. In Wyodak-Anderson 

Sample No. --- RP1FAT RP15FAT RP22FAT

Mineral phase Average

Quartz SiO2 8.7 6.8 6.7 7.4

Hematite α-Fe 2O3 2.6 1.5 2.0 2.0

Lime CaO 1.2 0.8 0.6 0.9

Anhydrite CaSO4 3.2 4.3 3.8 3.8

Magnetite Fe2+Fe2

3+O4 nd 0.9 0.5 0.7

Merwinite Ca3

Mg(SiO4)2 13.6 11.7 7.1 10.8 1

C3A Ca3Al2O6 4.0 4.1 4.2 4.1

Periclase MgO 3.1 2.6 1.8 2.5

Gehlenite Ca2

Al(Al, Si)O7 7.4 10.2 7.8 8.5

Amorphous 56.1 59.2 66.0 60.4

Total 99.9 102.1 100.5 101.1 1

C3A is a concrete industry abbreviation for a calcium aluminate, Ca3Al2O6.

Table 8. Fly ash petrography of selected samples from an Indiana power plant using feed coal from the WyodakAnderson coal zone, Powder River Basin, Wyo.

[James C. Hower, written commun., 2001. FAE, economizer fly ash; FAN, new automated fly ash sampler; FAT, silo fly ash sample; FAO, old

automated fly ash sampler; t, trace amount. All values in percent]

Sample Glass Mullite Spinel Quartz Isotropic coke Anisotropic coke Inertinite

RP1- FAE 92.5 4.0 0.5 2.0 0.0 0.0 0.0

RP1- FAN 97.6 1.6 0.8 0.0 0.0 0.0 0.0

RP1- FAT 95.6 1.2 1.2 2.0 0.0 0.0 t

RP4- FAE 91.2 5.6 1.0 2.2 0.0 0.0 0.0

RP4- FAN 95.0 1.4 0.3 3.0 0.0 0.0 t

RP4- FAO 95.0 2.0 0.6 2.4 t 0.0 t

RP4- FAT 97.4 0.2 0.2 2.0 0.0 0.0 0.2

RP14- FAE 88.8 8.6 1.0 1.6 0.0 0.0 0.0

RP14- FAN 95.2 2.2 0.6 2.0 0.0 0.0 t

RP14- FAO 96.8 0.2 0.8 2.0 t 0.0 t

RP14- FAT 97.6 0.2 0.6 1.4 0.0 0.0 0.2

RP17- FAE 90.0 7.6 t 2.4 0.0 0.0 0.0

RP17- FAO 95.8 0.4 1.0 2.6 0.0 0.2 t

RP17-FAT 98.8 0.4 0.2 0.6 t t t

RP20-FAE 87.2 8.8 0.8 3.2 0.0 0.0 0.0

RP20-FAO 97.8 t 0.8 1.4 t 0.0 t

RP20-FAT 97.2 0.4 t 2.2 0.0 0.0 0.2

RP29-FAE 86.8 11.2 t 2.0 0.0 0.0 0.0

RP29-FAO 97.6 0.6 0.4 1.4 0.0 t 0.0

RP29-FAT 96.6 0.2 0.4 2.8 0.0 t 0.0

Table 7. Semiquantitative mineralogy of fly-ash samples from an Indiana power plant using feed 

coal from the Wyodak-Anderson coal zone, Powder River Basin, Wyo., determined by Rietveld X-ray 

diffraction analysis.

[Values in percent. Due to rounding, total can equal greater than 100 percent; nd, not detected]

Discussion and Interpretation 21
ashes, the center point of scattering maxima is due to calcium 

and magnesium content, whereas the glass “hump” of eastern 

ashes reflects a variation in aluminum content.

Three general chemical types of fly ash (characterized by 

mineral plus glass assemblages) were noted in the Ca/Mg-rich 

Wyodak-Anderson fly ash samples examined as part of our 

overall study by O’Connor and Meeker (1999) using optical petrographic microscope, scanning electron microscope 

(SEM), electron microprobe analyzer (EPMA), and Gandolfi, 

bulk-powder, and Rietveld X-ray diffraction techniques. That 

these characteristic assemblages or types commonly appear 

with distinctive and easily recognized mineral phases and 

crystallization patterns nearly eliminates the need for further 

analysis. These assemblages are as follows: Type 1, (Ca±Mg) 

+Al (silica (Si) poor) assemblages (figs. 19 and 27); Type 2, 

(Ca±Mg)+Si (moderate aluminum (Al) content) assemblages 

(figs. 20, 27, 28); and Type 3, (Ca±Mg)+Si+P (variable Al) 

assemblages (figs. 29 and 30). In addition to the three major 

assemblages, minor iron (Fe)-bearing minerals (pyrite, for 

example) in the feed coal resulted in complex Fe-spinel, ferrite, and Fe-silicate phases being present (figs. 20, 26) in some 

fly ash grains.

Among the diverse mineral phases identified in our study, 

oxides, low-Si silicates, and aluminates commonly occur as 

relatively large (>10 μm) unzoned subhedral to euhedral or 

graphically intergrown crystals within glass, some of which 

approach ideal stoichiometry. The crystal morphologies and 

chemical compositions indicate that mineral growth probably 

occurred near or at equilibrium with the glass phase.

One of the silica-poor fly ash mineral assemblages, 

shown in figure 18, consists of euhedral crystals of

periclase and euhedral to rounded subhedral crystals of 

lime observed growing in a matrix of approximately 3:2 Ca 

aluminate (Type 1; see preceding). Small subhedral crystals 

of an unknown Ca-Mg-aluminate compound were also noted 

by energy dispersive spectral (EDS) analysis with a possible 

3CaO·MgO·2Al2

O3 composition. Fly ash grains with this 

composition likely would have exsolved lime and periclase, 

changing in composition along the liquid stability field of 

aluminate (O’Connor and Meeker, 1999).

Commonly, the silica-poor assemblages (Type 1) contain euhedral octagons or cubes of periclase and possibly 

minor melilite (O’Connor and Meeker, 1999), which crystallized in a glass matrix of 3:2 calcium and 12:14 (Ca) aluminate, gehlenite (Ca2

Al(AlSi)O7), or mayenite (Ca12Al14O33). 

Other silica-poor mineral assemblages include bredigite 

(Ca7Mg(SiO4)4, fig. 28) or larnite (β-Ca2SiO4) and a 3:2 Ca 

aluminate (C3A?, Ca3Al2O6). Other Type 1 fly ash grains 

contain melilite ((Ca,Na)2(Al,Mg)(Si,Al)2O7) and larnite (βCa2

SiO4) with 3Ca aluminate glass.

Some Si+Al-bearing fly ash grains retain enough element 

mobility to form nearly stoichiometric crystals. High magnesium content within the fly ash grain apparently aids in the 

mobility of elements (O’Connor and Meeker, 1999). One such 

Type 1 grain is shown in figure 27 where euhedral crystals of 

periclase (MgO) and merwinite (Ca2Mg(SiO4)2) exsolved from 

a glass consisting of 3Ca aluminate. 

The increase of silica content in the fly ash grains 

increases viscosity and interferes with the ability to form wellcrystallized minerals in the short time available for crystallization during combustion. Bottom ash samples, however, remain 

at elevated temperatures for a longer period, and more complete ionic mobility is possible. A bottom ash sample (Type 

2; see preceding) showing a coarse intergrowth of plagioclase 

(anorthite, CaAl2Si2O8) and fassaitic pyroxene (ferrian-aluminian diopside, CaMgSi2

O6) or augite (Ca,Na)(Mg,Fe,Al,Ti)(Si,

Al)2O6) is shown in figure 20. The anorthite is relatively pure 

calcic plagioclase (An96Ab4). The pyroxene has a high aluminum and titanium content and is distinctly compositionally 

zoned, illustrating the restricted element mobility in the richer 

silica-glasses. 

As much as 5 percent of the fly ash grains examined in 

our study contains phosphorus or phosphorus-bearing minerals 

(Type 3; see preceding), representative of the Class 5 type fly 

ash of O’Connor (1997a). The phosphorus, which was likely 

derived from apatite (Ca5(PO4)3), crandallite group minerals, 

or amorphous hydrous aluminum silicate material in the feed 

coal, is responsible for the assemblage of cryptocrystalline 

phases of which whitlockite (Ca9(Mg,Fe)(PO4)6(PO3,OH)), 

its anhydrous analog merrillite, and (or) α-Ca3(PO4)2 are the 

probable constituents (O’Connor, 1997b). Figure 29 shows a 

Type 3 fly ash grain containing euhedral crystals of corundum 

(Al2O3) in a cryptocrystalline phosphate/silicate glass. A rare 

phosphorus-bearing fly ash grain (Type 3) with rare earth 

element content is shown in figure 30. The small size of the 

crystals in the glass precludes meaningful electron microprobe 

analysis, but a semiquantitative analysis of the 

Figure 18. Scanning electron microscope (SEM) backscattered electron image of a fly ash grain with euhedral light-gray 

lime and dark-gray periclase crystals and subhedral Ca-Mg 

aluminate crystals in a Ca-aluminate matrix. Sample is from an 

Indiana power plant fly ash, coal from the Wyodak-Anderson 

coal zone, Powder River Basin, Wyo.

22 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
F Feldspar

G Gehlenite

P Pyroxene

Q Quartz

H Hematite

DEGREES 2θ

INTESITY (COUNTS)

Q

Q Q

P P

Q,H

Q

F,H,F

H,P

P

G

F

F

G

Q

F

F

P

P

G

G

Q

F

P

P G

P

G

Q

Q

Q F F,H

F

P

P,G,P

4 10 20 30 40 50 60

0

250

500

750

1,000

1,250

EXPLANATION

Figure 19. X-ray diffractogram of bottom ash from an Indiana power plant burning feed coal from the WyodakAnderson coal zone, Powder River Basin, Wyo.

Figure 20. Scanning electron microscope (SEM) backscattered electron image of a 

(Ca+Mg)-rich bottom ash grain showing coarse intergrowth of feldspar (anorthite?) and 

pyroxene (fassaite?). Bright inclusion in lower right contains magnetite dendrites (bright 

white) that exsolved from an iron-rich glass (light gray). Sample is from an Indiana power 

plant bottom ash, coal from the Wyodak-Anderson coal zone, Powder River Basin, Wyo.

Discussion and Interpretation 23
cryptocrystalline material shows a rare earth element content 

greater than 26 percent, dominated by the heavy rare earth 

elements. Phosphorus-bearing fly ash grains contain anomalously high uranium, arsenic, and fluorine contents (O’Connor, 

1997b). In our study, the rare earth element anomaly was the 

major variation in the fly ash chemistry detected among the 

analyzed phosphorus-bearing fly ash grains.

The fly ash grains containing relatively more iron exsolve 

an iron spinel (fig. 26), generally magnetite and magnesioferrite (MgFe2

O3), with many of the same crystallization features 

Figure 21. Electron microprobe 

backscattered electron image 

(BSE) and element X-ray intensity 

maps showing relative abundances 

and distribution of chromium (Cr), 

nickel (Ni), and silica (Si) in a 

Kentucky power plant ferrite-rich 

fly ash grain. Image from Robert B. 

Finkelman.

Figure 22. Scanning electron 

microscope (SEM) backscattered 

electron image (BEI) showing a 

bright anhydrite coating and element X-ray intensity maps showing 

relative abundances and distribution of sulfur (S), calcium (Ca), and 

magnesium (Mg) in one Indiana 

power plant fly ash grain. The 

higher abundances of Ca and S are 

the result of a thin anhydrite (CaSO4

) 

coating on the fly ash grain. The 

bright areas in the lower left image 

are periclase crystals (MgO) in a 

Ca-aluminate glass. Sample is from 

an Indiana power plant fly ash, coal 

from the Wyodak-Anderson coal 

zone, Powder River Basin, Wyo.

24 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
3,600 4,400 5,200 6,000 6,800

BARIUM, IN PARTS PER MILLION ON ASH BASIS

BA

FAT

FAN

FAO

FAE

FC

TYPE OF SAMPLES

 Mean 

 Min Max 

Figure 23. Variation of barium content during the first 15 days of sampling (n=15) of the feed coal from the 

Wyodak-Anderson coal zone, Powder River Basin, Wyo., and combustion products at the Indiana power 

plant. Range plot shows mean, minimum, and maximum values from feed coal (FC), economizer fly ash (FAE), 

old fly ash sampler (FAO), new fly ash sampler (FAN), truck fly ash (FAT), and bottom ash (BA) samples.

Figure 24. Variation of phosphorus content during the first 15 days of sampling (n=15) of the 

feed coal from the Wyodak-Anderson coal zone, Powder River Basin, Wyo., and combustion 

products at the Indiana power plant. Range plot shows mean, minimum, and maximum values 

from feed coal (FC), economizer fly ash (FAE), old fly ash sampler (FAO), new fly ash sampler 

(FAN), truck fly ash (FAT), and bottom ash (BA) samples.

0.6 0.8 1.0 1.2 1.4

P2O5, IN PERCENT ON ASH BASIS

BA

FAT

FAN

FAO

FAE

FC

TYPE OF SAMPLES

 Mean 

Min Max 

Discussion and Interpretation 25
As

Co

Cr

Ni

Pb

JULY 1994–OCTOBER 1995

10

50

100

500

Jul Aug Nov Dec Jan Feb Apr Jun Jul Aug Sep Oct

As

Co

Cr

Ni

Pb

JULY 1994–OCTOBER 1995

PARTS PER MILLION, AS-DETERMINED

50

100

500

Jul Aug Nov Dec Jan Feb Apr Jun Jul Aug Sep Oct

A

B

Figure 25. Temporal variation of selected elements in A, feed coal, and B, fly ash at a Kentucky power plant during 

16-month sampling period.

Table 9. Modes of occurrence data for selected elements for feed coal samples from Wyodak-Anderson coal zone, Powder

River Basin, Wyo.

[Mode of occurrence data presented in percent leached by each solvent's compounds and (or) mineral associations. NH4=water-soluble compounds,

some carbonates, and elements bonded onto exchangeable sites; HCl=carbonates, iron oxides, monosulfides, phosphates, and chelated organic

compounds; HF= silicates; HNO3= disulfides, especially pyrite. Feed coal source is the Wyodak-Anderson coal (n=3), Campbell County, Powder River

Basin, Wyo., sample collected at power plant. *indicates an element of environmental interest (U.S. Statutes at Large, 1990). Totals can exceed 100

percent because of rounding but are within the expected error of plus or minus 20 percent]

Element Exchangeable cations,

water-soluble compounds

Pyrite and other

disulfides

Silicates Carbonates, arsenates, iron

oxides, monosulfides,

phosphates and sulfates

Organic associations or

insoluble and shielded

minerals

As* 5 5 5 20 65

Ba 10 1 5 80 5

Be* 0 1 30 35 35

Ca 40 1 1 55 5

Co* 1 5 5 40 50

Cr* 1 10 40 10 40

Fe 0 1 10 40 50

Hg* 0 0 0 0 100

Mg 60 0 1 25 15

Mn* 10 1 5 90 0

Na 90 5 2 5 0

Ni* 5 5 5 20 65

P 1 1 10 90 1

Pb 0 10 15 90 0

Se* 0 30 0 0 70

Sr 25 1 5 50 20

Th* 0 15 30 60 0

U* 0 5 30 60 5

Zn 1 15 10 30 45

26 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
Figure 26. Scanning electron microscope (SEM) backscattered electron image of a (Ca+Mg+Fe+Al+Si)-rich bottom 

ash grain showing exsolution dendrites of magnetite (Fe2+Fe2

3+O4

) in a matrix of anorthite (CaAl2Si2O6) and fassaitic 

pyroxene (ferrian aluminian diopside, CaMgSi2

O6) or augite, (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6). Highly variable conditions 

in the boiler allowed chemical zonation to develop, causing formation of a band of magnetite surrounding the silicate 

core and a rim of silicate glass with a pyroxene composition. Sample is from an Indiana power plant bottom ash, 

coal from the Wyodak-Anderson coal zone, Powder River Basin, Wyo.

Figure 27. Scanning electron microscope (SEM) backscattered electron image of a 

Ca+Mg-rich fly ash grain showing euhedral crystals of periclase (MgO) and merwinite 

(Ca3

Mg(SiO4)

2

) in a 3Ca aluminate-silicate glass. Sample is from an Indiana power plant 

fly ash, coal from the Wyodak-Anderson coal zone, Powder River Basin, Wyo. 

Discussion and Interpretation 27
noted in fly ash from other eastern feed coals (Brownfield and 

others, 1997; O’Connor, 1997a). 

Anhydrite (CaSO4

) was detected in all the fly ash samples 

by XRD (fig. 17), microprobe, and SEM analyses (element 

X-ray intensity mapping, fig. 22), forming as a result of the 

high contents of CaO and the presence of SO3. The lime (CaO) 

in the fly ash acted as a self-contained “scrubber” for SO3, 

where CaO + SO3 formed anhydrite during combustion or as a 

precipitate on fly ash grains in the upper parts of the boiler.

Fly ash is an aluminous and siliceous material that, in the 

presence of water, will react with the calcium in the lime to 

produce calcium silicate hydrates with cementitious properties. This is known as a pozzolanic reaction. The Class C fly 

ash in this study contains free lime (CaO) and has pozzolanic 

properties. 

Amorphous calcium-aluminate glass, anhydrite, and C3A 

(Ca3

Al2O6) in the Indiana power plant fly ash can enhance 

the pozzolanic reactions causing stored or disposed fly ash 

Figure 29. Scanning electron microscope (SEM) backscattered electron image of a 

(Ca+Si+Al+P)-rich fly ash grain showing euhedral crystals of corundum (Al2

O3) in a phosphatesilicate glass. Sample is from an Indiana power plant fly ash, coal from the Wyodak-Anderson 

coal zone, Powder River Basin, Wyo.

Figure 28. Scanning 

electron microscope (SEM) 

backscattered electron 

image of a (Ca+Mg)-rich 

fly ash grain showing small 

dark euhedral periclase 

(MgO) crystals, subhedral 

gray crystals of bredigite? 

(β-Ca2

SiO4), and bright 3:

2 Ca aluminate (3:2 CaAl, 

C3

A?, Ca3Al2O6)). Sample is 

from an Indiana power plant 

fly ash, coal from the Wyodak-Anderson coal zone, 

Powder River Basin, Wyo.

28 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
to set-up and harden after contact with water. This enhanced 

pozzolanic reaction property in Class C fly ashes is known as 

self-cementing. This reaction forms ettringite and strätlingite 

in the fly ash at the Indiana power plant disposal site. Analysis of samples from two flow-through column tests by X-ray 

diffraction revealed a predominance of ettringite, strätlingite, 

and calcite with a minor amounts of monosulfoaluminate 

(Ca4

Al2(SO4)(OH)·10H2O) and portlandite (Ca(OH)2). Hydration of the column fly ash formed these secondary minerals, 

indicating that these minerals can form rapidly. A possible 

explanation for the absence of portlandite in the disposal 

site samples is that it may have reacted with atmospheric 

CO2

 to form calcite. This reaction was also indicated by a 

long-term leaching study conducted by McCarthy and others (1993) on fly ashes similar to those at the Indiana power 

plant. Schlorholtz and others (1988) suggested that strätlingite enhances the self-cementing of high-calcium fly ashes, 

whereas McCarthy and others (1993) believed that ettringite 

is a strengthening factor in the utilization of high-calcium fly 

ashes.

Class C fly ashes—with their pozzolanic and self-cementing properties—are widely utilized as raw material in concrete, 

as a partial replacement (generally by weight) for Portland 

cement. These fly ashes are also utilized to reduce or stop 

water drainage at abandoned mine sites, where the fly ash 

reacts with mine water, causing the fly ash to self-cement, thus 

reducing or stopping the mine drainage. 

Uranium content in the Kentucky and Indiana power plant 

feed coals is 16 and 8.9 ppm, respectively, and the uranium 

content of the fly ashes is 19 and 9.0 ppm, respectively. The 

uranium is organically bound, with lesser amounts associated 

with uranium-bearing minerals such as apatite, monazite, and 

zircon. The distribution and form of radioactive elements in 

fly ash determine the availability of these elements for release 

to the environment during ash utilization or disposal. The 

distribution of uranium, as determined by fission-track radiography, was observed to be uniform throughout the glassy 

grains of the fly ash (figs. 31 and 32). The low amount of 

surface-bound, potentially leachable uranium suggests that the 

rate of release of uranium will be slow and controlled by the 

very slow rate at which the ash grains dissolve (Zielinski and 

Finkelman, 1997).

Other elements of environmental concern detected by 

chemical analysis of the Wyodak-Anderson fly ash were difficult to characterize because the elemental contents in fly ash 

particles were at or below the detection limit of the microbeam

instruments. Arsenic is present in larger amounts in the fly ash 

than in the bottom ash (table 5), and it is uniformly distributed 

in the Ca-P and glassy fly ash grains when observed by SEM 

energy dispersive X-ray analysis. Contents of Co, Cr, and Ni 

are generally similar in both fly ash and bottom ash, and are 

enriched in the magnetic fraction. Magnetite and magnesioferrite may contain trace amounts of Co, Cr, and Ni. Contents of 

Sb, Be, Pb, and Se are greater in fly ash compared to bottom 

ash, and leaching data indicate that these elements were most 

likely volatilized (table 9) during combustion and are distributed throughout the glassy grains.

Coal and coal combustion products (CCPs) can alter the 

composition of aqueous solutions through water-rock interactions. These water-rock interactions can be an important 

environmental consideration in the use of coal for energy 

production, especially because CCPs can be used in building materials and as soil amendments, and also are disposed 

Figure 30. Scanning electron microscope (SEM) backscattered electron image of a 

rare earth element (REE)-bearing phosphate-silicate fly ash grain. The heavy rare earth 

elements dominate as determined by semiquantitative, normalized energy dispersive 

spectral methods. Sample is from an Indiana power plant fly ash, coal from the WyodakAnderson coal zone, Powder River Basin, Wyo.

Discussion and Interpretation 29
in surface impoundments. The water-rock interactions can 

be determined by laboratory techniques that allow mixing 

of solids and leaching solutions that simulate environmental 

conditions where leachable metals may be mobilized from the 

disposal or utilization of fly ash.

Column tests are used to determine long-term (days to 

months) leaching behaviors of fly ash, simulating the flow 

of ground or surface water through a material in the natural 

environment. Figure 33 shows one example in which the percentage of arsenic leached from the Wyodak-Anderson fly ash 

(24.9 ppm initial As) was relatively small, whereas the cumulative amount of arsenic leached from the fly ash was small—

but still increasing—after 70 days. Only about 1.1 percent of 

the initial arsenic (or 0.27 ppm As) was leached from the fly 

ash after 70 days. Another example (fig. 34) shows the percentage of arsenic leached from the Kentucky plant fly ash (40 

ppm initial As) in 18-hour batch tests to have been relatively 

small, but the cumulative amount of arsenic leached from the 

fly ash was significant during a column test. In the Kentucky 

fly ash, the amount of arsenic being leached seemed to be 

decreasing during the later part of the column test, although 

the total amount of arsenic leached from the fly ash was nearly 

25 percent or 10 ppm in 64 days (Rice and others, 1999).

Reactions between the solid phase, such as fly ash, and 

the solution caused a change in the solution composition, 

which affects the pH and the mobility of trace elements. For 

fly ash, the original composition of the feed coal, mineral 

composition, the size of the fly ash particles, and the combustion conditions influence the extent to which trace metals are 

leached from the fly ash and substantially change solution 

composition. As an example, figure 35 shows pH and concentration of five selected elements in a solution of deionized 

Figure 31. Photograph (left) of a Ca-P 

rich fly ash grain and its fission track 

radiograph (right). Uranium distribution 

and concentration are indicated by the 

location and density of the fission tracks 

in the radiograph. Size of photograph 

and radiograph is approximately 0.25 by 

0.15 millimeters. Photograph and radiograph by Robert A. Zielinski. Sample is 

from an Indiana power plant fly ash, coal 

from the Wyodak-Anderson coal zone, 

Powder River Basin, Wyo.

Figure 32. Photograph (left) of fly ash grains with mineral inclusions and its fission track radiograph (right). 

Uranium distribution and concentration are indicated 

by the location and density of the fission tracks in the 

radiograph. Bright mineral inclusions are quartz and 

contain very low concentrations of uranium. Size of 

photograph and radiograph is approximately 1.0 by 0.6 

millimeters. Photograph and radiograph by Robert A. 

Zielinski. Sample is from an Indiana power plant fly ash, 

coal from the Wyodak-Anderson coal zone, Powder 

River Basin, Wyo.

30 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
water in contact with the Wyodak-Anderson fly ash. The pH 

of the solution is initially acidic (pH=5.5) from the deionized 

water, and the trace metals have relatively low concentrations 

in solution. Almost instantly, lime (CaO) in the fly ash reacts 

with the solution, causing a rise in the pH, slowing the mobilization of the elements, especially Al, Fe, and Cu. A second 

example, figure 36, shows the pH and concentration of the 

same elements in a solution of deionized water in contact with 

a fly ash from a Kentucky power plant utilizing a high-sulfur 

feed coal. The pH of the solution is initially acidic (pH<4), 

and most of the trace metals have high concentrations in 

solution because of the rapid dissolution of a sulfate (steklite, 

KAl(SO4)2) surface coating (fig. 37) on the fly ash grains (Rice 

and others, 1999; Fishman and others, 1999). After a few 

minutes, however, the reaction between lime in the fly ash and 

the solution begins to raise the pH but at a slower rate because 

of the lesser amounts of lime present in the Kentucky fly ash. 

Secondary phases (Al and Fe oxyhydroxides) form from the 

pH increase resulting in the co-precipitation of some trace 

metals (for example, Fe, Zn, Cu; fig. 36), which lowers their 

concentrations. In contrast, the concentration of molybdenum 

(Mo) increases as the pH increases (fig. 36). The pH dependency of trace metal mobility in batch experiments emphasizes 

the importance of solution-solid phase interaction studies. A 

more comprehensive discussion of leaching experiments on 

the Wyodak-Anderson fly ash is underway (C.A. Rice and others, work in progress).

Leaching experiments range in duration and severity of 

chemical treatment. Batch leaching tests combine a solution 

and a fly ash or coal for a selected period of time (seconds to 

minutes). Several solution compositions may be used sequentially in these batch tests to dissolve different solid phases. 

Solution compositions of batch tests represent the initial, and 

commonly highest, concentrations of elements expected to be 

leached from a material and are used for regulatory purposes 

in tests such as the U.S. Environmental Protection Agency’s 

toxicity characteristic leaching protocol (TCLP, example, 

fig. 34). 

Summary

Many factors influence feed coal reactions during combustion to form fly ash and bottom ash. Particle size, coal 

rank, ash content, coal mineralogy, and the trace element composition are important variables that control the combustion of 

coal-forming gaseous and solid combustion products. Original 

composition of the feed coal, the combustion conditions, the 

size of the fly ash particles, and the fly ash mineralogy influence the distribution and mobility of trace metals in fly ash.

Feed coal from the Wyodak-Anderson coal zone studied 

displayed little variation in element content despite coming 

from several different mines in Campbell County, Wyo.; small 

variations are to be expected, however, because of both vertical and lateral changes in mineral content and the amount of 

individual mineral phases within the coal bed from place to 

place. The subbituminous Wyodak-Anderson coal has higher 

Ba, Ca, Mg, Na, Sr, and P contents when compared to eastern 

bituminous feed coal. Higher contents of these elements are 

due to the presence of hydrated alumino-phosphates, clays, 

carbonates, apatite, biotite, and barite. The elemental content 

and mineral composition of the Wyodak-Anderson coal are 

indicative of coals having a large influx of volcanic ash.

Analytical analyses using multiple methods indicate that 

40 percent or more of Co, Mn, Th, and U contents are from 

carbonates, phosphates, and oxides, whereas 50 percent or 

0

0 10 20 30 40 50 60 70 80

FLOW DAYS

CUMULATIVE PERCENT OF TOTAL ARSENIC

0.2

0.4

0.6

0.8

1

1.2

Column

Deionized

 water batch

EXPLANATION

Figure 33. Percentage of arsenic (As) 

leached from an Indiana power plant 

fly ash (24.9 ppm initial As) by deionized 

water in a flow-through column test 

compared to the percentage leached 

by an 18-hour batch test (0.02 percent 

leached) using deionized water. Sample 

is from an Indiana power plant fly ash, 

coal from the Wyodak-Anderson coal 

zone, Powder River Basin, Wyo.

Discussion and Interpretation 31
more of As, Co, Ni, and Se contents are related to organic 

matter. About 20 percent of the arsenic is associated with 

arsenates or has substituted for phosphorus in crandallite or 

gorceixite. Chromium is evenly distributed among the philosilicates (clay minerals) and organic matter whereas beryllium 

is equally shared among the silicates (clays, micas, and albite), 

organic matter, and unidentified Be-bearing oxides, hydroxides, or sulfates. Portions of the Se, Th, and U are in clays, 

other silicates, and pyrite. All of the mercury in the WyodakAnderson coal samples appears to be associated with organic 

matter. 

Ca- and Mg-rich mineral phases in the Wyodak-Anderson 

fly and bottom ashes can be attributed to volcanic minerals 

deposited in the peat-forming mires and their influence on the 

authigenic mineral suite in the coal. Spinel-ferrite minerals 

present in the fly ash are related to the presence of Fe-bearing 

pyroxenes, carbonates, clays, biotite, and pyrite found in the 

feed coal.

On the basis of XRD analysis, the amorphous diffraction scattering maxima, or glass “hump,” appears to reflect 

differences in chemical composition of fly ash and bottom ash 

Figure 34. Percentage of arsenic (As) leached from a Kentucky 

power plant fly ash (40 ppm initial As) by deionized water in a 

flow-through column test compared to the percentage leached 

by two 18-hour batch tests. TCLP, U.S. Environmental Protection 

Agency’s toxicity characteristic leaching protocol. 

Figure 35. Leachate concentration of A, aluminum (Al), iron 

(Fe), and pH; and B, copper (Cu), molybdenum (Mo), and zinc (Zn) 

during the leaching of a power plant fly ash with deionized water. 

Sample is from an Indiana power plant fly ash, coal from the Wyodak-Anderson coal zone, Powder River Basin, Wyo.

Figure 36. Leachate concentration of A, aluminum (Al), iron 

(Fe), and pH; and B, copper (Cu), molybdenum (Mo), and zinc 

(Zn) during the leaching of a Kentucky power plant fly ash with 

deionized water.

Column

TCLP batch

Deionized

 water batch

EXPLANATION

0

0 10 20 30 40 50 60 70

FLOWDAYS

25

20

15

10

5

CUMULATIVE PERCENT OF 

TOTAL ARSENIC

1

A TIME, IN MINUTES

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

0 10 20 30 40 50 60 70 80

0.0

0.1

10

100

1,000

pH

pH

Al

PA

Fe

RTS PER BILLION

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70

Mo

Zn

Cu

TIME, IN MINUTES

PARTS PER BILLION

B

PARTS PER MILLION

1,000

100

10

10

8

6

4

2

0

1

0.1

0 5 10 15 20 25

TIME, IN MINUTES

Al

Fe

pH

pH

A

1.2

0.8

0.4

0

0 5 15 20 10 25

Zn

Mo

Cu

TIME, IN MINUTES

PARTS PER MILLION

B

32 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
glasses. In Wyodak-Anderson fly and bottom ashes, the center 

point of scattering maxima is due to calcium and magnesium 

content, whereas the glass “hump” of eastern fly ash reflects 

variation in aluminum content.

Wyodak-Anderson Class C fly ash and bottom ash are 

higher in CaO and MgO and lower in Al2

O3 and SiO2 when 

compared to the eastern United States bituminous Class F fly 

ash and bottom ash. The scattering maxima related to glass 

in the Class C fly ash and bottom ash reflect variable calcium 

and magnesium content, whereas the eastern Class F fly ash 

and bottom ash glass reflect low calcium and variable aluminum content.

Free lime (CaO) in the fly ash acts as a self-contained 

“scrubber” for SO3

, forming anhydrite coatings on the fly ash 

grains either during combustion or as a precipitate before the 

electrostatic fly-ash collectors. Anhydrite, C3A, and Ca-aluminate glass enhance the self-pozzolanic reactions in the fly ash, 

causing the fly ash to set up after contact with water-forming 

ettringite, strätlingite, and calcite. X-ray diffraction analysis of 

fly ash samples from column tests identified ettringite, strätlingite, monosulfoaluminate, and portlandite, indicating rapid 

formation during disposal. The lack of portlandite in disposal 

site samples reflects a reaction of portlandite with atmospheric 

CO2

, forming calcite.

Arsenic leached from the Wyodak-Anderson fly ash (24.9 

ppm initial As) during an 18-hour batch test is minor (0.02 

percent leached), whereas the cumulative amount of arsenic 

leached from the fly ash, although initially small, increased 

slightly after 70 days to about 1.1 percent (0.27 ppm) of the 

initial arsenic.

A reaction between lime in the fly ash and water causes 

a rapid rise in the pH, slowing the mobilization of elements. 

Considering the high lime content and the resulting hydration 

of the fly ash, there is little evidence that major amounts of 

leachable metals are mobilized from the disposal or utilization 

of Wyodak-Anderson fly ash.

References Cited

Affolter, R.H., and Brownfield, M.E., 1987, Characterization 

of Lower Williams Fork Formation coals from the eastern 

Yampa coal field, Routt County, Colorado: Geological 

Society of America Abstracts with Programs, v. 19, no. 7, p. 

567.

Affolter, R.H., Brownfield, M.E., and Breit, G.N., 1997, Temporal variations in the chemistry of feed coal, fly ash, and 

bottom ash from a coal-fired power plant: Proceedings of 

the 1997 International Ash Utilization Symposium, Center 

for Applied Energy Research, Lexington, Ky., October 

20–22, 1997, p. 757–764.

Affolter, R.H., Brownfield, M.E., and Cathcart, J.D., 1999, 

Chemical variation of feed coal and coal combustion products from an Indiana power plant utilizing low sulfur Powder River Basin coal: Proceedings of the 1999 International 

Ash Utilization Symposium, Center for Applied Energy 

Research, Lexington, Ky., 7 p. On CD-ROM.

American Society for Testing and Materials (ASTM), 1999a, 

1999 Annual Book of ASTM Standards; Petroleum products, lubricants, and fossil fuels, sect. 5, v. 05.05—Gaseous 

fuels, coal, and coke: Philadelphia, Pa., ASTM, 522 p.

American Society for Testing and Materials (ASTM), 1999b, 

Standard classification of coals by rank, D388-98a, in 1999 

Annual Book of ASTM Standards; Petroleum products, 

lubricants, and fossil fuels, sect. 5, v.05.05—Gaseous fuels, 

coal, and coke: Philadelphia, Pa., ASTM, p. 188–193.

American Society for Testing and Materials (ASTM), 1999c, 

Standard practice for preparing coal samples for microscopical analysis by reflected light, D2797-85, in 1999 Annual 

Book of ASTM Standards; Petroleum products, lubricants, 

and fossil fuels, sect. 5, v. 05.05—Gaseous fuels, coal, and 

coke: Philadelphia, Pa., ASTM, p. 295–298.

Figure 37. Scanning electron microscope (SEM) 

backscattered electron image of a spinel-rich Kentucky power plant fly ash grain with a sulfate (steklite, 

KAl(SO4

)

2

) enriched coating, which probably controls 

initial pH (<4) and leaching of some trace metals—see 

figure 35.

References Cited 33
American Society for Testing and Materials (ASTM), 1999d, 

Standard test method for microscopical determination of 

the reflectance of vitrinite in a polished specimen of coal, 

D2798-97, in 1999 Annual Book of ASTM Standards; 

Petroleum products, lubricants, and fossil fuels, sect. 5, v. 

05.05—Gaseous fuels, coal, and coke: Philadelphia, Pa., 

ASTM, p. 299–303.

American Society for Testing and Materials (ASTM), 1999e, 

Standard test method for microscopical determination of 

volume percent of physical components of coal, D2799-98a, 

in 1999 Annual Book of ASTM Standards; Petroleum products, lubricants, and fossil fuels, sect. 5, v. 05.05—Gaseous 

fuels, coal, and coke: Philadelphia, Pa., ASTM, p. 304–307.

American Society for Testing and Materials (ASTM), 2003, 

Standard specifications for fly ash and raw or calcined 

natural pozzolan for use as mineral admixture in portland 

cement concrete, C618-03, in 2003 Annual Book of ASTM 

Standards; Construction, sect. 4, v. 04.02—Concrete and 

aggregates: Philadelphia, Pa., ASTM, 3 p.

Anderson, J.U., 1963, An improved pretreatment for mineralogical analysis of samples containing organic matter: Clays 

and Clay Minerals, v. 10, p. 380–388.

Breit, G.N., Finkelman, R.B., Eble, C., Affolter, R.H., Belkin, H.E., Brownfield, M.E., Cathcart, J.D., Crowley, S.S., 

Hower, J.C., Leventhal, J.S., McGee, J., Palmer, C.A., 

Reynolds, R.L., Rice, C.A., and Zielinski, R.A., 1996, 

Systematic investigation of the compositional variations 

in solid coal combustion waste products: 13th Annual 

Pittsburgh Coal Conference Proceedings, Pittsburgh, Pa., 

September 3–7, 1996, p. 1356–1361.

Brindley, G.W., 1945, The effect of grain or particle size on Xray reflections from mixed powders and alloys considered 

in relation to the quantitative determination of crystalline 

substances by X-ray methods: Philosophical Magazine, v. 

36, p. 347–369.

Brownfield, M.E., and Affolter, R.H., 1988, Characterization 

of coals in the lower part of the Williams Fork Formation, 

Twentymile Park district, eastern Yampa coal field, Routt 

County, Colorado, in Carter, L.M.H., ed., U.S. Geological 

Survey Research on Energy Resources—1988, Program and 

Abstracts (V.E. McKelvey Forum on Mineral and Energy 

Resources): U.S. Geological Survey Circular 1025, p. 6.

Brownfield, M.E., Affolter, R.H., Cathcart, J.D., Brownfield, 

I.K., Hower, J.C., and Stricker, G.D., 1999, Dispersed volcanic ash in feed coal and its influence on coal combustion 

products: Proceedings of the 1999 International Ash Utilization Symposium, Center for Applied Energy Research, 

Lexington, Kentucky, 8 p. On CD-ROM.

Brownfield, M.E., Affolter, R.H., Cathcart, J.D., O’Connor, 

J.T., and Brownfield, I.K., 1999, Characterization of feed 

coal and coal combustion products from power plants in 

Indiana and Kentucky: Proceedings Volume of the 24th 

International Technical Conference on Coal Utilization 

and Fuel Systems, Clearwater, Fla., March 8–11, 1999, p. 

989–1000.

Brownfield, M.E., Affolter, R.H., and Stricker, G.D., 1987, 

Crandallite group minerals in the Caps and Q coal beds, 

Tyonek Formation, Beluga Energy Resource Area, upper 

Cook Inlet, south-central Alaska, in Rao, P.D., ed., Proceedings Focus on Alaska’s coal ’86: Fairbanks, Alaska, University of Alaska, MIRL Report No. 72, p. 142–149.

Brownfield, M.E., Affolter, R.H., Stricker, G.D., and Hildebrand, R.T., 1995, High chromium contents in Tertiary 

coal deposits of northwestern Washington—A key to their 

depositional history: International Journal of Coal Geology, 

v. 27, p. 153–169.

Brownfield, M.E., Cathcart, J.D., and Affolter, R.H., 1997, 

Characterization of feed coals and waste products from a 

coal-burning power plant in Kentucky: Proceedings Volume 

of 1997 International Ash Utilization Symposium, Lexington, Ky., October 20–22, 1997, p. 780–784.

Cathcart, J.D., Reynolds, R.L., Brownfield, M.E., and Hower, 

J.C., 1997, Chemical, mineralogical, and magnetic characterization of sized fly ash from a coal-fired power plant in 

Kentucky: Proceedings of the 1997 International Ash Utilization Symposium, Center for Applied Energy Research, 

Lexington, Ky., October 20–22, 1997, p. 785–792.

Crowley, S.S., Ruppert, L.F., Belkin, H.E., Stanton, R.W., and 

Moore, T.A., 1993, Factors affecting the geochemistry of a 

thick, subbituminous coal bed in the Powder River Basin—

Volcanic, detrital, and peat-forming processes: Organic 

Geochemistry, v. 20, no. 6, p. 843–853.

34 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.
Ellis, M.S., Flores, R.M., Ochs, A.M., Stricker, G.D., Gunther, 

G.L., Rossi, G.S., Bader, L.R., Schuenemeyer, J.H., and 

Power, H.C., 1999, Gillette coalfield, Powder River Basin; 

Geology, coal quality, and coal resources, Chapter PG of 

the 1999 resource assessment of selected Tertiary coal beds 

and zones in the northern Rocky Mountains and Great 

Plains Region: U.S. Geological Survey Professional Paper 

1625–A, p. PG1-PG84. On CD-ROM.

Finkelman, R.B., Breit, G.N., Eble, C., Affolter, R.H., Belkin, H.E., Brownfield, M.E., Cathcart, J.D., Crowley, S.S., 

Hower, J.C., Leventhal, J.S., McGee, J., Palmer, C.A., 

Reynolds, R.L., Rice, C.A., and Zielinski, R.A., 1996, 

Systematic investigation of the compositional variations in 

solid waste products from coal combustion: American Association of Petroleum Geologists, Eastern Section, October 

12–16, 1996, p. 39–40.

Finkelman, R.B., Colker, Allan, and Palmer, C.A., 1997, 

Modes of occurrence of arsenic and other trace elements in 

fly ash from a Kentucky power plant: Proceedings of the 

1997 International Ash Utilization Symposium, Center for 

Applied Energy Research, Lexington, Ky., October 20–22, 

1997, p. 239–246.

Finkelman, R.B., Palmer, C.A., Krasnow, M.R., Aruscavage, 

P.S., and Sellers, G.A., 1990, Heating and leaching behavior 

of elements in the eight Argonne Premium Coal Samples: 

Energy and Fuels, v. 4, no. 6, p. 755–767.

Fishman, N.S., Rice, C.A., Breit, G.N., and Johnson, R.D., 

1999, Sulfur-bearing coatings on fly ash from a coal-fired 

power plant—Composition, origin, and influence on ash 

alteration: Fuel, v. 58, p. 187–196.

Fleischer, Michael, 1987, Glossary of mineral species: Tucson, 

Ariz., The Mineralogical Record, Inc., 227 p.

Gluskoter, H.J., 1965, Electric low-temperature ashing of 

bituminous coal: Fuel, v. 44, p. 285–291.

Gluskoter, H.J., Ruch, R.R., Miller, W.G., Cahill, R.A., Dreher, G.B., and Kuhn, J.K., 1977, Trace elements in coal—

Occurrence and distribution: Illinois Geological Survey 

Circular 499, 154 p.

Hower, J.C., Trerice, D.R., Graham, U.M., Groppo, J.G., 

Brooks, S.M., Robl, T.L., and Medina, S.S., 1995, 

Approaches to the petrographic characterization of fly ash: 

Proceedings of the 11th International Coal Testing Conference, Center for Applied Energy Research, Lexington, Ky., 

May 1995, p. 49–54.

International Centre for Diffraction Data, 1999, Powder diffraction file (PDF-2): International Centre for Diffraction 

Data, 12 Campus Boulevard, Newtown Square, PA 19073 

U.S.A.

Larson, A.C., and Von Dreele, R.B., 1994, General structure 

analysis system (GSAS): Los Alamos National Laboratory 

Report LAUR 86-748, 231 p.

Lauf, R.J., Harris, L.A., and Rawlson, S.S., 1982, Pyrite framboids as the source of magnetic spheres in fly ash: Environmental Science and Technology, v. 16, p. 218–220.

McCarthy, G.J., Solem, J.K., Bender, J.A., and Eylands, K.E., 

1993, Mineralogical analysis of advanced coal conversion 

residuals by X-ray diffraction: Proceedings of the Tenth 

International Ash Use Symposium, American Coal Association, EPRI TR-101774, p. 58-1–58-12.

O’Connor, J.T., 1997a, Silicate fly ash classification based on 

exsolution mineralogy: Proceedings of the 1997 International Ash Utilization Symposium, Lexington, Ky., October 

20–22, 1997, p. 692–699.

O’Connor, J.T., 1997b, U- and F-bearing phosphate-silicate fly 

ash grains: Proceedings of the 1997 International Ash Utilization Symposium, Center for Applied Energy Research, 

Lexington, Ky., October 20–22, 1997, p. 716–721.

O’Connor, J.T., and Meeker, G.M., 1999, Liquidus (Ca+Mg)-

rich exsolution phases in low-sulfur fly ash: Proceedings 

Volume of the 24th International Technical Conference on 

Coal Utilization and Fuel Systems, Clearwater, Fla., March 

8–11, 1999, p. 333–342.

Palache, Charles, Berman, Harry, and Frondel, Clifford, 1951, 

Dana’s System of Mineralogy, Seventh Edition: New York, 

John Wiley, 1124 p.

Palmer, C.A., and Filby, R.H., 1984, Distribution of trace elements in coal from the Powhatan No. 6 Mine, Ohio: Fuel, v. 

63, p. 318–328.

Palmer, C.A., Mroczkowski, S.J., Kolker, Allan, Finkelman, 

R.B., and Bullock, J.H., Jr., 2000, Chemical analysis and 

modes of occurrence of selected trace elements in a Powder 

River Basin coal and its corresponding simulated cleaned 

coal: U.S. Geological Survey Open-File Report 00-323, 

52 p.

References Cited 35
Palmer, C.A., Mroczkowski, S.J., Kolker, Allan, Finkelman, 

R.B., Crowley, S.S., and Bullock, J.H., Jr., 1998, The use of 

sequential leaching to quantify the modes of occurrence of 

elements in coal: Fifteenth Annual International Pittsburgh 

Coal Conference Proceedings, Pittsburgh, Pa., September 

15–17, 1998, CD-ROM, PDF 166, 28 p.

Querol, X., Fernadez-Turiel, J.L., and Lopez-Soler, A., 1995, 

Trace elements in coal and their behavior during combustion in a large power station: Fuel, v. 74, no. 3, p. 331–343.

Rice, C.A., Breit, G.N., Fishman, N.S., and Bullock, J.H., Jr., 

1999, Leachability of trace elements in coal and coal combustion products: The Proceedings of the 24th International 

Technical Conference on Coal Utilization and Fuel Systems, 

Clearwater, Fla., March 8–11, 1999, p. 355–366.

Rice, C.A., Breit, G.N., Fishman, N.S., Bullock, J.H., Jr., 

and Motooka, J., 1997, Geochemical analysis and modeling of coal combustion waste leaches: Proceedings of the 

1997 International Ash Utilization Symposium, Center for 

Applied Energy Research, Lexington, Ky., October 20–22, 

1997, p. 231–238.

Stach, E., Mackowsky, M.-Th., Teichmüller, M., Taylor, G.H., 

Chandra, D., and Teichmüller, R., 1982, Stach’s textbook of 

coal petrology: Berlin, Gebrüder Borntraeger, 535 p.

Starkey, H.C., 1984, Sodium hypochlorite as an aid to the 

extraction of clay minerals from black shales, in Bush, A.L., 

ed., Contributions to mineral resources research, 1984: U.S. 

Geological Survey Bulletin 1694, p. 75–80.

Schlorholtz, S., Bergeson, K., and Derirel, T., 1988, Monitoring of fluctuations in the physical and chemical properties 

of a high-calcium fly ash, in Fly ash and coal conversion 

by-products—Characterization, utilization and disposal IV: 

Materials Research Society Symposium Proceedings, v. 

113, p. 99–106.

Triplehorn, D.M., Stanton, R.W., Ruppert, L.F., and Crowley, 

S.S., 1991, Volcanic ash dispersed in the Wyodak-Anderson 

coal zone, Powder River Basin, Wyoming: Organic Geochemistry, v. 17, no. 4, p. 567–575.

U.S. Statutes at Large, 1990, Provisions for attainment and 

maintenance of National ambient air quality standards: 

Public Law 101-549, 101st Congress, 2nd Session, v. 104, 

pt. 4, p. 2353–3358.

Zielinski, R.A., and Finkelman, R.B., 1997, Radioactive elements in coal and fly ash—Abundance, forms, and environmental significance: U.S. Geological Survey Fact Sheet 

FS-163-97, 4 p.

36 Elements and Combustion Products in Feed Coal from Powder River Basin, Wyo.