U.S. GEOLOGICAL SURVEY CIRCULAR 1143

Schweinfurth—COAL—A COMPLEX NATURAL RESOURCE U.S. Geological Survey Circular 1143 
Cover: Chunk of Texas subbituminous coal in the field. Pen for scale. Note the very thin, light-colored laminations that indicate bedding. Top-left enlargement is an example of a scanning photomicrograph showing minerals in coal. The bright-white, oval-shaped fragment is a pyrite framboid; the bright-white triangular fragment is zircon; each fragment is about 10 microns wide. The light-colored, equant blebs are quartz

grains. They are set in a light-gray matrix consisting of coal macerals and a variety of clay minerals. Topright enlargement is an example of a transmitted-light photomicrograph showing various coal macerals

(organic components) such as vitrinite (dark reddish orange); liptinite (yellow and light orange); and fusinite, inertinite, and minerals (black). View is about 200 micrometers wide.
U.S. Department of the Interior

U.S. Geological Survey

Coal—A Complex Natural Resource

An overview of factors affecting coal quality and use in the United States

By Stanley P. Schweinfurth

With a contribution on coal quality and public health by Robert B. Finkelman

U.S. GEOLOGICAL SURVEY CIRCULAR 1143

A primer on the origins and quality of coal with a brief examination of how quality

affects coal use, and a separate section on coal and public health
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

CHARLES G. GROAT, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 2003

Published in the Eastern Region, Reston, Va.

Manuscript approved for publication April 5, 2002.

Any use of trade, product, or firm names in this publication is for descriptive purposed

only and does not imply endorsement by the U.S. Government.

Free on application to 

U.S. Geological Survey

Information Services

Box 25286, Federal Center

Denver, CO 80225

or call the toll-free telephone number

1-888-ASK-USGS

Library of Congress Cataloging in Publication Data

Schweinfurth, Stanley P.

Coal—a complex natural resource : an overview of factors affecting coal quality and use in the United

States / by Stanley P. Schweinfurth ; with a contribution on coal quality and public health by Robert

B. Finkelman.

p. cm.- (U.S Geological Survey circular ; 1143)

Included bibliographical references.

1. Coal—United States. l. Title. ll. Series.

TP326.U5 S38 2001

662.6’22—dc21 00-067694
III

FOREWORD

During the 20th century, dramatic shifts took place in the types and sources of energy consumed in the United States. The future energy mix and the technologies used in fuel

combustion will be critical in determining the economic and environmental health of the

Nation as we move ahead in the 21st century. Coal was a major primary energy source

within the United States for all of the last century, and, currently, coal is used to generate

more than 50 percent of the Nation’s electric energy. Coal supplies are adequate within the

United States for this century and coal is the least expensive of the fossil fuels, making it

a reliable and economical source of energy. Recent studies, however, suggest that an

increased use of coal may contribute to the degradation of the environment and to global

warming. A thorough understanding of coal quality will lead to the development of methods to better control the harmful effects of coal use on the environment and human health.

The U.S. Geological Survey (USGS) is actively involved in research on the quality

and abundance of coal, as well as on the complex geological and geochemical factors that

govern coal quality. In addition, the U.S. Geological Survey gathers information on the

location, size, and nature of coal deposits, which also contributes to a basic understanding

of the Nation’s energy resources. This research is pertinent to discussions of future energy resources, global warming, and the dependence of the Nation on imported fossil

fuels—all of which are concerns that are being faced daily at local, State, and National

levels. Understanding the nature of coal quality is important for two critical issues facing

our Nation—the economy and the environment.

The wise use of our Nation’s resources and the careful stewardship of our public

lands depend on the dissemination of accurate and timely earth science information. The

U.S. Geological Survey is proud to present this report on one of the major issues to be

addressed in the future use of coal.

Charles G. Groat

Director

V

CONTENTS

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III

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

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

Special Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Coal in the United States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Coal Production and Consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Coal Prices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Coal Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Coal Byproducts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

What is Coal? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Origins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Minerals in Coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Macerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Coal Rank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Why is Coal so Complex? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

The Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Biological and Chemical Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Swamp Geometry, Location, and Climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Mineral Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Coalification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Determining the Quality of Coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Sampling Techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Organic Geochemistry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Mineral-Matter Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Coal Quality and Public Health, by Robert B. Finkelman . . . . . . . . . . . . . . . . . . . . . . 33

Polycyclic Aromatic Hydrocarbons (PAHs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Balkan Endemic Nephropathy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Lung Cancer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Trace Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

General Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Coal Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Coal Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Health Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Appendix 1. Federal Agencies With Regulatory or Research

Responsibilities for Coal and Coal Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
COAL—A COMPLEX NATURAL RESOURCE

FIGURES

1. Graph showing trends in U.S. coal production from 1900 to 1999 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2. Diagram showing energy consumption and production in the U.S. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Photograph of the coal-fired Navajo power plant near Page, Ariz. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4. Map showing coal fields of the U.S. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

5.–7. Photographs showing—

5. Uses for coal-combustion byproducts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

6. Okefenokee Swamp, Ga. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

7. Examples of peat and coal beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

8. Block-diagram models illustrating peat-sediment-climate relations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

9. Chart showing the periodic table of the elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

10. Diagram showing hypothetical structure of a lignin-like organic molecule in brown coal (lignite) . . . . . . . . . . . . . 13

11. Photomicrograph of pyrite filling fractures in coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

12. Photograph of a coal ball specimen from the Carbondale Formation, Illinois basin . . . . . . . . . . . . . . . . . . . . . . . . 14

13. Photograph and photomicrographs of Pennsylvanian bituminous coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

14. Scanning-electron microscope photomicrograph of a fragment of pyrite infilling plant cells in coal . . . . . . . . . . . . 16

15. Photographs of mineral deposits on boiler walls and pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

16. Photograph of cross section of calcareous coal ball from Murphysboro coal bed equivalent, Indiana . . . . . . . . . . . 16

17. Diagram showing classification of coals by rank in the U.S. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

18. Carbon-13 nuclear-magnetic-resonance spectra of coals of increasing rank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

19. Chart showing divisions of geologic time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

20. Photograph of an in-situ fossilized tree stump in the roof of a coal bed in an underground mine . . . . . . . . . . . . . . 20

21. Reconstruction of a tropical peat swamp of Middle Pennsylvanian age . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

22. Reconstruction of fluvial swamp and peripheral, bottom-land vegetation of Late Cretaceous age . . . . . . . . . . . . . . 21

23. Sketches illustrating a possible evolution from a planar peat deposit to a domed peat deposit. . . . . . . . . . . . . . . . . 22

24.–28. Photographs showing—

24. Lens-shaped parting in a coal bed in the highwall of a strip mine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

25. Face cleat in an exposed coal bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

26. Collection of a channel coal sample from a coal bed in an underground mine . . . . . . . . . . . . . . . . . . . . . . . 29

27. Core-drilling rig and a collected core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

28. Coal scientist at work identifying coal macerals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Centerfold. Illustration showing coal byproducts in tree form with basic chemicals as branches and

derivative substances as twigs and leaves

TABLES

1. Common minerals found in coal and their elemental compositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2. Procedures and purposes of ASTM International (ASTM) methods for testing coal. . . . . . . . . . . . . . . . . . . . . . . . . 26

3. Analytical methods and individual techniques used in research on coal quality and their purposes . . . . . . . . . . . . . 27

4. Results of standard ASTM International (ASTM) analyses and U.S. Geological Survey (USGS)

research analyses of a sample (USGS 212277) of Pennsylvanian bituminous coal from the Mary Lee

coal bed, Walker County, Ala. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
1

INTRODUCTION

Coal is abundant in the U.S., is relatively inexpensive,

and is an excellent source of energy and byproduct raw

materials. Because of these factors, domestic coal is the primary source of fuel for electric power plants in the U.S., and

will continue to be well into the 21st century. In addition,

other U.S. industries continue to use coal for fuel and coke

production and there is a large overseas market for highquality American coal.

Because humans have used coal for centuries, much is

known about it. The usefulness of coal as a heat source and

the myriad of byproducts that can be produced from coal are

well understood. The continued and increasingly large-scale

use of coal in the United States and in many other industrialized and developing nations has resulted in known and

anticipated hazards to environmental quality and human

health. As a result, there is still much to be learned about the

harmful attributes of coal and how they may be removed,

modified, or avoided to make coal use less harmful to

humans and nature. These issues of coal quality have not

been examined carefully until recently.

Some of the problems that accompany the mining and

use of coal are well known. Acid mine drainage results

when coal beds and surrounding strata containing medium

to high amounts of sulfur, in the form of compounds known

as sulfides, are disrupted by mining, thereby exposing the

sulfides to air and water. Atmospheric sulfur oxides (SOx)

and subsequent acid deposition (such as acid rain) result

from the burning of medium- to high-sulfur coal. The quality of surface and ground water may be affected adversely

by the disposal of the ash and sludge that result from the

burning of coal and cleaning of flue gases. These are some

of the serious problems requiring either improved or new

remedies. Other environmental problems are associated

with emissions of carbon dioxide (CO2) and nitrogen oxides

(NOx), two of the so-called “greenhouse gases.” These

emissions are often attributed to coal use only; however,

they also result from the burning of any fossil or biomass

fuel, such as wood, natural gas, gasoline, and heating oil.

The greenhouse gas problem requires a broader solution

than just reducing the use of coal. Research currently is

being conducted in the U.S. and several other countries into

the reduction and disposal of CO2 from coal combustion. An

excellent review of the results of this research and the

prospects for coal can be found in a publication of IEA Coal

Research (1999).

Fluidized-bed combustion (FBC) of coal, which is a

new method for controlling sulfur emissions, is gaining

wide acceptance. In this system, finely ground coal is mixed

with finely ground limestone. Both are fed together into a

furnace in a constant stream onto a horizontally moving

grate. Air is forced up through the grate and the entire mass

is ignited at relatively low temperatures. The forced air

causes the ground coal and limestone to be mixed with the

hot gases of combustion, which in turn promotes the conversion of any SOx to gypsum as the burning mass moves

along on the grate. According to the U.S. Department of

Energy (2000), high-sulfur coal may be burned in this way

while simultaneously capturing up to 95 percent of the SOx

and most of the NOx emitted.

The Clean Air Act Amendments of 1990 (Public Law

101-549) required the U.S. Environmental Protection

Agency (EPA) to conduct studies of 15 trace elements

released by the burning of coal to determine if they present

health hazards. These 15 elements (antimony, arsenic,

beryllium, cadmium, chlorine, chromium, cobalt, lead,

manganese, mercury, nickel, potassium, selenium, thorium,

and uranium), along with many other potentially hazardous

substances released into the air by other industries, are

termed “hazardous air pollutants” (HAPs).

On the basis of epidemiological studies, the EPA concluded that, with the possible exception of mercury, there is

no compelling evidence to indicate that trace-element emissions from coal-burning power plants cause human health

problems (U.S. Environmental Protection Agency, 1996). In

December 2000, after extensive study, the EPA announced a

Coal—A Complex Natural Resource

An overview of of factors affecting coal quality and use in the United States 

By Stanley P. Schweinfurth

Reduction and disposal of CO2

emissions is the subject of current

research in the U.S. and several 

other countries.
2 COAL—A COMPLEX NATURAL RESOURCE

finding that regulation of mercury emissions from coal-fired

power plants is necessary and appropriate because coalfired power plants are the largest unregulated source of

manmade emissions of mercury in the U.S. (U.S.

Environmental Protection Agency, 2000).

EPA will propose regulations to limit mercury emissions in 2003 and will issue final emissions rules by 2004.

Meanwhile, arsenic is still under study, not as an emissions

problem from coal combustion, but for its potential role as

a hazard in ground water if it is leached from coal-mining

waste or from fly ash in disposal sites. Additional coal-quality research on both mercury and arsenic is being conducted at the U.S. Geological Survey (USGS) and elsewhere in

order to (1) help identify their sources in coal, and (2) help

resolve any remaining potential hazards issues with regard

to these two elements.

Other problems that may arise from the use of coal are

not necessarily harmful to the environment or human health,

but affect the use and efficiency of coal-burning equipment.

For example, certain constituents in coal may cause severe

erosion and corrosion of, or the buildup of mineral deposits

on, furnace and boiler parts. These effects greatly reduce the

efficiency and life expectancy of furnaces or boilers; as a

result, costly repairs are often required.

Thus far, coal has been discussed as if it were a single

homogeneous material, but this is done only for convenience in this report. In fact, a wide variety of coal types

exists, depending on differing proportions and combinations

of organic and inorganic (mineral) components, sulfur, and

ash. For example, sulfur content may range from low (less

than 1 percent), through medium (1 to 3 percent), to high

(greater than 3 percent), and ash yields may range from a

low of about 3 percent to a high of 49 percent (if ash yields

are 50 percent or greater, the substance is no longer called

coal). Coal may produce high or low amounts of energy, or

contain high or low amounts of the substances that produce

organic chemicals and synthetic fuels, or contain higher or

lower amounts of the elements that are considered to be hazardous air pollutants (HAPs). This range in properties

results from coal’s diverse origins, including the long and

complex geologic histories of coal deposits.

The purpose of this report is to acquaint students, noncoal scientists, and the general public with some basic information on (1) the subject of coal quality and the reasons

why it is so complex, (2) methods for determining coal

quality, and (3) the need for continuing studies of this complex subject. A discussion on the origins of coal is included

in order to provide a framework for understanding coal’s

complex nature. See the section on “Further Reading” for a

bibliography on coal quality and other topics (such as coal

mining, uses for coal, or health effects of coal use).

ACKNOWLEDGMENTS

Grateful acknowledgment is extended to the following

scientists and technicians without whose contributions and

careful reviews this report would not have been possible:

Philip J. Aruscavage, Linda J. Bragg, M. Devereux Carter,

William A. DiMichele, Frank T. Dulong, Robert B.

Finkelman, Romeo M. Flores, Harold J. Gluskoter, Joseph

R. Hatch, Robert G. Johnson, Rustu S. Kalyoncu, Elizabeth

D. Koozmin, Ronald J. Litwin, Paul C. Lyons, Peter J.

McCabe, Eric A. Morrissey, Charles L. Oman, William H.

Orem, Curtis A. Palmer, Tom A. Phillips, Eleanora I.

Robbins, Leslie F. Ruppert, Ronald W. Stanton, Frank G.

Walthall, Peter D. Warwick, William D. Watson, Jason C.

Willett, Christopher Wnuk, and Chris J. Zygarlicke.

SPECIAL ACKNOWLEDGMENT

A special acknowledgment is included in remembrance

of the late Ronald W. Stanton, a U.S. Geological Survey

geologist and coal scientist. Ron's belief in the value of a

report that would make the mysteries of coal quality more

accessible to the public, and his encouragement and physical support during its preparation, were essential to the

completion of this circular.

COAL IN THE UNITED STATES

A discussion of coal use in the U.S. and of the beneficial things that come from coal is necessary in order to provide the reader with a better understanding of the importance of coal quality and the need for continuing research on

coal quality.

COAL PRODUCTION AND CONSUMPTION

Coal use in the U.S. began around two thousand years

ago. Early inhabitants of this continent probably gathered

coal from outcroppings, beaches, and streambeds. Coal

In 1996, the EPA said that, of the

potential HAPs from coal combustion,

only mercury and arsenic needed

additional study. In 2000, EPA

determined that mercury emissions

require regulation.
COAL IN THE UNITED STATES 3

mining by European-Americans began in the Richmond

basin of Virginia between 1720 and 1750. The Richmond

area is known to have furnished coal during the American

Revolution. Between 1750 and 1800, coal was discovered

in many parts of the northern and southern Appalachian

basin. As the population moved westward after the

American Revolution, coal deposits were discovered, progressively, in the midcontinent (including Texas), the Rocky

Mountains region, the Colorado Plateau, and the northern

Great Plains. The first coal in the West was found along the

Missouri River in 1804 by the explorers Meriwether Lewis

and George Rogers Clark (Collier’s Encyclopedia, 1960).

After 1830, with the beginning of the railroads, the real

growth of the coal industry began. The railroads not only

used coal, they also provided the means to transport coal,

which encouraged the growth of energy-intensive industries. The growth of the railroads also encouraged the

expansion of the iron and steel industry, which, by the

1860s, had abandoned charcoal in favor of coke for the

reduction of iron ore to pig iron. In the 1880s, after the

invention of electric turbines, coal soon became the principle energy source for steam-generated electricity. For a

much more detailed account of the early days of coal discovery and use in the U.S., see Eavenson (1942).

By 1918, industry in the U.S. depended almost entirely

on coal. During World War II, demand for coal drove coal

production to record highs, but shortly thereafter, petroleum

replaced coal as our chief source of energy. For about a

decade following World War II, most of the decline in coal

use was due to the replacement of coal-burning railroad

engines with diesel engines. In the 1960s, coal production

and consumption began to rise due to an increasing demand

for electricity, and has risen ever since. In 1961, the U.S.

produced 420.4 million tons (1 ton=2000 pounds) and used

390.4 million tons for domestic purposes; in 1998, production was 1,118.7 million tons and domestic use in 1999 was

1,045.2 million tons (figs. 1, 2) (Energy Information

Administration, 2000).

The difference between the amounts of coal produced

and used in the U.S. is accounted for mainly by exports.

Revenue from coal exports helps reduce the deficit in the

balance of payments the U.S. makes to its foreign suppliers

of consumer goods. In 1998, coal exports of 78 million tons

earned approximately $2.9 billion (Energy Information

Administration, 1999b).

In the U.S., coal is used to produce

approximately 56 percent of the

annual electricity requirement; 90

percent of the coal produced is used

for this purpose.

Figure 1. Graph showing trends in U.S. coal production from 1900 to 1999. The rapid growth in production after 1960

is attributed to (1) abundant reserves, (2) high productivity resulting in favorable prices for coal, and (3) the large

increase in demand for electricity. Data from Energy Information Administration (2000).

200

400

600

800

1000

1200

1900

1905

1910

1915

1920

1925

1930

1935

1940

1945

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

1999

MILLION SHORT TONS

YEAR
4 COAL—A COMPLEX NATURAL RESOURCE

ELECTRIC UTILITY

90%

RESIDENTIAL AND

COMMERCIAL 

1%

INDUSTRY

9%

NATURAL GAS

22.9%

COAL

22.5%

PETROLEUM

39%

NUCLEAR

8%

HYDROELECTRIC AND 

OTHER RENEWABLE

7.6%

NATURAL 

GAS

26.6%

COAL

32%

PETROLEUM

20.7%

NUCLEAR

10.6%

HYDROELECTRIC AND 

OTHER RENEWABLE

9.9%

NATURAL GAS

9%

COAL

56%

PETROLEUM 

AND OTHER

3%

NUCLEAR

23%

HYDROELECTRIC 

POWER

9%

0

200

400

600

800

1,000

1,200

1,400

EIA DRI GRI/Hill WEFA 1999 Actual

1,279

1,177

1,129

1,018

751

73

678

1,005

919

1,045.2

944.4

86

111

100.8

73

Electric Utilities

Other

MILLION SHORT TONS

102

(2000)

A B

C D

E

Figure 2. Diagrams showing energy consumption and production

in the U.S. A, Energy consumption in 1999. Of the various commodities consumed, approximately 54 percent of the oil and 16

percent of the natural gas are imported; the coal and other commodities are almost entirely domestic. B, Energy production in

1999. C, Electricity generated in 1999, by all commodities. D,

Coal consumption in 1999 by the economic sectors shown. E,

Forecasted coal consumption for 2020 by electric utilities (yellow)

and other uses (red). The basis for comparison of commodities in

diagrams A, B, and C is quadrillion Btu, or “quads.” Total quads

produced and consumed in 1999, were, respectively, 72.5 and

96.6. The basis for diagrams D and E is million short tons. EIA,

Energy Information Administration; DRI, Standard & Poor’s Data

Resources Inc.; GRI/Hill, Gas Research Institute; WEFA, The

WEFA Group. Data from Energy Information Administration

(1999a, 2000).
COAL IN THE UNITED STATES 5

According to the Energy Information Administration

(2000), approximately 1,099.1 million tons of coal were

produced in 1999 (slightly less than in 1998) and 1,045.2

million tons were used domestically (slightly more than in

1998). The differences in production and consumption

between 1998 and 1999 were due to a reduction in exports

of 20 million tons in 1999. Of the total coal used domestically in 1999, 90 percent (944.4 million tons) was used to

fuel electric power plants (fig. 3), which produced 56 percent of the electricity in the U.S. The other 10 percent was

used in industrial (coke and heating), residential, and commercial applications (fig. 2).

COAL PRICES

Steady improvements in the efficiency of coal mining

in the U.S. have kept coal prices low. In 1998, the average

price of a ton of coal at the mine was $17.67; the average

price of a ton of coal delivered to electric utilities was

$25.64 (Energy Information Administration, 2000). Prices

in 1986 of $23.99 and $33.30 per ton, respectively, demonstrate this trend. Furthermore, over the last decade, coal has

been the least expensive of the three main fuel sources—

coal, natural gas, and petroleum—in the U.S. On the basis

of a common unit of heat (million Btu) over the ten-year

period, coal prices averaged about $0.91 at the mine; natural gas averaged $1.72 at the wellhead; and crude oil averaged $2.70 at the domestic first point of purchase (Energy

Information Administration, 2000). Lower prices probably

encourage continued high and (probably) increasing levels

of coal use in the U.S.

In 1999, the Energy Information Administration (EIA)

forecasted that, by the year 2020, average coal prices would

decrease further to approximately $20.01 per ton delivered

to electric power plants (in 1998 dollars). Coal production is

forecast by the EIA (fig. 2) to grow to approximately 1,300

million tons by 2020, of which domestic use will account

for 1,279 million tons; of that amount, 1,177 million tons

will be used by electric power plants. These figures indicate

a rate of growth of approximately one percent per year for

both coal production and power plant use. Other domestic

uses of coal are forecast by the EIA to remain about the

same by 2020 (Energy Information Administration, 2000).

COAL SUPPLY

Although coal deposits are widely distributed throughout the U.S., the bulk of coal mining and consumption

remained, until recently, in the eastern part of the country,

mostly in the Appalachian basin. This was due to the concentration of population and industry in the East. Coal production recently has been moving to the Western States

because the bulk of the remaining resources of low-sulfur

coal are located in the Rocky Mountains and northern Great

Plains regions. The demand for low-sulfur coal is based on

the Clean Air Act Amendments of 1990, which began

implementation in 2000. The new regulations limit SOx

emissions from coal-fired power plants to 1.2 lbs per million Btu. Power-plant managers are finding that converting

their coal supplies from medium- and high-sulfur coal to

low-sulfur coal, rather than installing flue-gas desulfurization (FGD) systems, generally is the least expensive option

for achieving the required level of SOx emissions (U.S.

Geological Survey, 2000). There are reserves of low-sulfur

coal remaining in the Appalachian basin, but most of the

remaining reserves occur at depths requiring costlier underground mining methods to recover them.

Whether or not substantially more coal is used in the

future than presently, coal is abundant enough in the U.S. to

supply anticipated requirements for much of the 21st century. In 1999, the National Mining Association (NMA) reported that 30 of the largest coal companies in the U.S. have

Figure 3. Photograph of the Navajo power plant, which is a coalfired, electric-power-generation plant near Page, Ariz. Photograph

by J.C. Willett (U.S. Geological Survey).
6 COAL—A COMPLEX NATURAL RESOURCE 0 500 Miles EXPLANATION Current (2002) NCRDS-State cooperative projects Previous (1982-1999) NCRDS-State cooperative projects Anthracite (hardest)

Low-volatile bituminous coal

Medium- and high-volatile bituminous coal

Subbituminous coal

Lignite (softest)

COAL RANK

NCRDS-STATE COOPERATIVE PROJECTS

0 500 Miles

Figure 4. Map showing coal fields of the U.S. classified by coal rank and areas of cooperative study of coal resources and quality. For each category of coal rank, darker colorindicates area of coal outcroppings and lighter color indicates area of subsurface coal.Also shown are States with long-term, current (light blue) or previous (light

pink) cooperative relationships with the U.S. Geological Survey in the National CoalResources Data System (NCRDS) for the long-term study of coal resources and quality.Modified from Tully (1996) and M.D. Carter (U.S. Geological Survey, written commun.,2002). (Some States contain coal fields that are too small to show rank colors.)
COAL IN THE UNITED STATES 7

approximately 68 billion tons of coal in reserve (National

Mining Association, 1999). In addition, numerous small

coal-mining companies have reserves that are not included

in the NMA’s report, and there are additional coal deposits

that could be mined, when and if needed.

In order to estimate the abundance of coal in beds and

zones that will provide the largest percentage of the

Nation’s domestic energy production for the next 50 years,

the USGS, in cooperation with the geological surveys of the

major coal-producing States (Arizona, Colorado, Illinois,

Indiana, Kentucky, Maryland, Missouri, Montana, New

Mexico, North Dakota, Ohio, Pennsylvania, Tennessee,

Texas, Utah, Virginia, West Virginia, and Wyoming) is in

the midst of a basinwide reassessment of the coal resources

of the U.S. (U.S. Geological Survey, 1996, 2000). Figure 4

shows the general distribution of coal deposits in the U.S.

Results of the assessment for the Northern Rocky

Mountains and Great Plains region, which includes selected

coal beds of the Greater Green River, Hanna-Carbon,

Powder River, and Williston basins; the Colorado Plateau;

and the northern and central Appalachian Basin coal regions

are available in CD-ROM format (Fort Union Coal

Assessment Team, 1999; Kirschbaum and others, 2000;

Northern and Central Appalachian Basin Coal Regions

Assessment Team, 2001).

The USGS also is leading a cooperative program with

some of the same States to assess the actual availability and

recoverability of coal in areas smaller than basins. This

study takes into account the many restrictions on mining

imposed by environmental and land-use regulations and by

the physical constraints based on geology and mining practices (Eggleston and others, 1990). For additional information on coal resources, please see the “Coal Resources” subsection of the “Further Reading” section.

COAL BYPRODUCTS

Coal—that black, dusty rock that evokes punishment of

bad children at holiday times—is actually a very remarkable

and rewarding material. In addition to providing heat to

generate electricity (which is currently the primary use of

coal), a myriad of other beneficial substances are derived

from coal. Perhaps the best known of these substances is

coke, which is used in the steel industry to separate iron

from its ore. Coal is used in the production of other, perhaps

unexpected products such as pharmaceuticals, textile dyes,

food and wood preservatives, and other simple or highly

complex chemicals (see centerfold). These chemicals typically are produced during the production of coke by a

process called destructive distillation. In this process, coal is

packed into a closed, oxygen-restricted container (retort, or

coke oven) and then heated to a high temperature. This

process drives off the volatile matter in the coal. The

volatile matter contains the parent compounds that are used

to produce the products shown in the centerfold. The lack of

oxygen in the retort prevents the chemicals from completely burning.

In the past, large quantities of an impure gas containing

methane and carbon monoxide (called town gas or water

gas) were derived from coal in the U.S. and elsewhere.

Water gas was used for heating and cooking, but has been

replaced largely by natural gas in the U.S. since the 1940s.

In other parts of the world, notably in Germany during

World War II and in South Africa today, gasoline and heating oil have been derived from coal. Currently, research is

being conducted in the U.S. to improve the techniques and

economics of producing both gas and oil (called “synfuels”)

from coal. This work is sponsored by the U.S. Department

of Energy and involves numerous private, industrial, and

academic organizations (U.S. Department of Energy, 2000).

Coal combustion products (CCPs), consisting of fly

ash, bottom ash, boiler slag, and flue-gas-desulfurization

(FGD) material, also have become important in the economy. As coal burns, it emits sulfur in the form of sulfur oxide.

FGD is the process by which a chemical, such as calcium

carbonate (CaCO3), is injected into the flue-gas stream to

trap the sulfur by combining with it to produce gypsum

(CaSO4). Gypsum is heavy and falls to the bottom of the

FGD unit as sludge in a wet process, or as a powder in a dry

process.

According to the USGS (Kalyoncu, 1999), approximately 57.2 million tons of fly ash, 15.2 million tons of bottom ash,

2.7 million tons of boiler slag, and 22.7 million tons of FGD

material were produced in 1998. The amounts of each CCP

that were used in 1998 were (in percent) 33.6, 31.3, 80.1, and

10, respectively; the rest was disposed of in landfills, settling

ponds, or by other means. Coal fly ash is used mainly as an

additive in concrete; fly ash may also be used as structural fill,

as road base material, in waste stabilization, or in mining

applications. Bottom ash is used mainly as road base and

structural fill material as well as in concrete and as grit for

snow and ice control. Boiler slag is used mostly in blasting grit

and roofing granules. FGD gypsum is used mainly in wallboard, with small amounts being used in concrete and agricultural applications. Figure 5 shows photographs of examples of some uses for fly ash and FGD gypsum. 

There are other, perhaps less well known products that

can be derived from coal. For example, sulfur oxides can be

captured from flue gas and used to make sulfuric acid,

Coal is more than just a fuel source;

it contains complex tars, oils, and

gases that can be broken down into

many useful products such as dyes,
8 COAL—A COMPLEX NATURAL RESOURCE

which is an important industrial raw material. Other valuable elements, such as mercury and chlorine, also can be

recovered from flue gas (Finkelman and Brown, 1991).

WHAT IS COAL?

ORIGINS

Coal begins as peat, which consists of loosely consolidated layers of various mixtures of plant and mineral matter.

Peat accumulates in wetlands called “peat swamps,” “bogs,”

or “mires.” In this report, the term “peat swamp” is used to

indicate a peat-forming wetland. Figure 6 is a photograph of

the Okefenokee Swamp, which is a modern swamp where

peat is forming. Peat swamps are wetlands that have the

appropriate conditions for peat accumulation, such as abundant moisture, a stable to slowly sinking land surface, and

protection from rapidly eroding forces such as rivers and

ocean waves.

Over millions of years, burial, compression by overlying inorganic sediments, and the effects of heat (from depth

in the earth or proximity to volcanic sources) cause peat to

change to coal (fig. 7). The resultant coal is an extremely

complex, mostly organic, generally well bedded sedimentaA B

Figure 5. Photographs showing examples of the uses for coal

combustion byproducts. A, Fly-ash concrete was used in the decking and piers of the Sunshine Skyway bridge over Tampa Bay in

Florida. B, The gypsum produced by wet scrubbers was used in the

manufacture of high-quality wallboard. Photographs from

American Coal Ash Association (1998), used with permission.

Figure 6. Photograph of the Okefenokee Swamp, Ga., which is

a modern peat-forming swamp. Modern swamps are studied as

possible analogs of ancient environments in which coal beds

formed. The tall trees are mostly bald cypress with some bay and

black gum, the foreground is mostly sedge and reed-like plants,

and the open-water areas contain water lilies. Common characteristics that modern swamps share with ancient swamps include

low topography, fresh water, and varying climate. Photograph by

W.H. Orem (U.S. Geological Survey).
ry rock. To be classified as coal, the rock must contain less

than 50 percent ash-forming mineral matter. If it contains

between 30 and 50 percent mineral matter, it is classified as

impure coal.

In the U.S., individual coal beds may be as thin as a few

inches or (rarely) as thick as 200 ft and may cover areas as

small as a few square yards or as large as several counties

or even States; for example, the Pittsburgh coal bed extends

over all or parts of 25 counties in the area where Ohio,

Pennsylvania, and West Virginia join (see fig. 4). Figure 8

shows two examples from the Powder River basin in

Wyoming of peat deposits in relation to laterally and vertically bounding sediments deposited by rivers. The deposits

were subsequently buried and coverted to lignite and coal.

COMPOSITION

Coal is composed of complex mixtures of organic and

inorganic compounds. The organic compounds, inherited

from the plants that live and die in the swamps, number in

the millions. The approximately more than 120 inorganic

compounds in coal either were introduced into the swamp

from waterborne or windborne sediment, or were derived

from elements in the original vegetation; for instance, inorganic compounds containing such elements as iron and zinc

are needed by plants for healthy growth. After the plants

decompose, the inorganic compounds remain in the resulting peat. Some of those elements combine to form discrete

WHAT IS COAL? 9

A

C D

B

Figure 7. Photographs showing examples of peat and coal beds.

A, Peat bed in Indonesia. The face is approximately 3 ft high. Note

heterogeneous plant material in a finer groundmass of peat. At this

location, the peat bed is approximately 20 ft thick and 5,000 yrs

old at the base. Photograph by S.G. Neuzil (U.S. Geological

Survey). B, San Miguel lignite beds of Eocene age in a coal strip

mine in Texas. The individual beds of lignite (dark) are interspersed with beds of volcanic ash (lighter). The original peat has

been compressed into a relatively homogeneous, evenly bedded

material, but it still has a dull luster. Photograph by P.D. Warwick

(U.S. Geological Survey). C, Subbituminous coal strip mine in the

Paleocene Anderson-Wyodak coal zone, Powder River basin,

Wyoming. This bed is approximately 100 ft thick at this location,

and is divided into two 50-ft-thick sections by the light-colored

sedimentary rock parting above the power shovel. Photograph by

P.D. Warwick (U.S. Geological Survey). D, Roadcut exposing the

bituminous Pennsylvanian Pittsburgh coal bed, near Morgantown,

W. Va. The normal lustrous, black appearance of this coal has been

dulled by exposure to the weather. Photograph by C.B. Cecil (U.S.

Geological Survey).
10 COAL—A COMPLEX NATURAL RESOURCE

EXPLANATION

Swamp

Conglomerate in sand bars

Sandstone

Sandstone, siltstone and

mudstone

Peat, coal, and 

carbonaceous shale

Stream channels

Floodwater overflow

channels and related

sediments

A

B

Figure 8. Block-diagram models illustrating some of the peat-sediment-climate relationships possible in the process of peat development and burial. These models explain the environments of deposition for two large coal deposits of Paleocene age in the Powder

River basin of Wyoming. No specific scale implied, but the blocks

represent general areas of approximately 9 to 25 mi2

, and depths of

approximately 50 to 100 ft. A, Decker-Tongue River area. This

peat deposit developed continuously in the same place because

low areas, formed by faulting or compaction of underlying sediments, may have restricted the location of a meandering stream to

the edge of the peat deposit. Also, the river system may have had

a somewhat restricted sediment supply, or the river system may not

have been vigorous enough to invade the peat swamp except at the

margin. The climate probably was wet most of the year because

the peat surface in the model is shown to be higher than the surface of the stream valley. A wet climate promotes vigorous plant

growth on top of the peat, which holds water like a sponge, allowing the peat to pile up above the surrounding area. The result is a

large, thick, clean, coal deposit. B, Kaycee-Linch area. In this case,

the river system apparently had a coarser and more abundant sediment supply and was more vigorous than in part A, as shown by

the conglomerate bars in some of the stream channels, which are a

sign of stronger current flow. This river system was able to move

back and forth across the peat swamps, limiting the resulting

deposits, in part, to thin, discontinuous beds. This environment

appears also to have been wet, but a better supply of sediment and

stronger stream flows limited peat preservation. Modified from

Flores and others (1989), used with permission of the American

Geophysical Union.
WHAT IS COAL? 11

H

1

Hydrogen

Li

3

Lithium

Sc

21

Scandium

Ar

18

Argon

17

S

16

Sulfur

P

15

Phosphorus

Si

14

Silicon

Al

13

Aluminium

Ne

10

Neon

F

9

Fluorine

O

8

Oxygen

N

7

Nitrogen

C

6

Carbon

He

2

 Helium

B

5

 Boron

Ca

20

Mg

Calcium

12

Magnesium

Na

11

Sodium

4

Be

Beryllium

K

19

Potassium

Kr

36

Krypton

Xe

54

Xenon

Arsenic

Br

35 34 27

Ni

28

Ge

32

Germanium

Ga 31

Gallium

Zn

30

Zinc

Cu

29

Copper

Cr

24

V

23

Vanadium

Ti

22

Titanium

Fe

26

Iron

Mn

25

Chromium Manganese Cobalt Nickel

As

Bromine Selenium

Tc

43

Technetium

Rh

45

Rhodium

Pd

46

Palladium

Ag

47

Silver

Cd

48

Cadmium

In

49

Indium

Sn

50

Tin

Sb

51

Antimony

Te

52

Tellurium

I

53

Iodine

33

Hf

72

Hafnium

Ta

73

Tantalum

W

74

Tungsten

Re

75

Rhenium

Os

76

Osmium

Ir

77

Iridium

Pt

78

Platinum

Au

79

Gold

Hg

80

Mercury

Tl

81

Thallium

82

Lead

Bi

83

Bismuth

Po

84

Polonium

At

85

Astatine

Rn

86

Radon

Fr

87

Francium

Ra

88

Radium

Ac

89

Actinium

Th

90

Thorium

Pa

91

Protactinium

92

Ce

58

Cerium

Pr

59

Praseodymium

Nd

60

Neodymium

Pm

61

Promethium

Lu

71

Lutetium

Yb

70

Ytterbium

Tm

69

Thulium

Er

68

Erbium

Dy

66

Dysprosium

Ho

67

Holmium

Tb

65

Terbium

Gd

64

Gadolinium

Eu

63

Europium

Sm

62

Samarium

Cs

55

Rb

Caesium

37

Rubidium

Sr

38

Strontium

Y

39

Yttrium

Zr

40

Zirconium

Nb

41

Niobium

Mo

42

Molybdenum

Ba

56

Barium

La

57

Lanthanum

U

Pb

Cl

Chlorine

Uranium

Ru

44

Ruthenium

Co Se

PERIODIC TABLE OF THE NATURALLY OCCURRING ELEMENTS

RARE-EARTHELEMENTS

Figure 9. Periodic table of the elements. The 76 elements found in coal are highlightedby colors with regard to their general abundance in coal, as follows: blue, major elements(generally greater than 1.0 percent in abundance); red, minor elements (generally greaterthan or equal to 0.01 percent); and yellow, trace elements (generally less than 0.001 percent). Pursuant to the Clean Air Act Amendments of 1990 (Public Law 101-549), theU.S. Environmental Protection Agency (EPA) studied fifteen of these elements as potentially hazardous air pollutants (HAPs); green bars in their boxes indicate those fifteenelements. Thirteen of the original were cleared when the EPA found that there was no

compelling evidence that they cause human health problems; a green bar across the bottom of the box indicates those elements. Two elements from the original fifteen, mercury (Hg) and arsenic (As), indicated by a green bar across the centers of their boxes, arestill under study—mercury as a HAP and arsenic as a potential pollutant in ground waterthat flows through fly-ash and coal-mine spoil piles. Subsequently, in December 2000,EPA found that mercury emissions from coal-fired power plants require regulation; EPAwill propose regulations in 2003 and issue final rules in 2004. Modified from PeriodicTable of the Elements (Sargent-Welch Scientific Company, 1979), used with permission.
12 COAL—A COMPLEX NATURAL RESOURCE

Mineral name

Quartz SiO2

Clay Minerals:

 Kaolinite 2 2O

 Illite KAl4(AlSi 7O20 4

 Montmorillonite 0.7(Al,Mg,Fe)4 [(Si,Al)4O10 ] 2

(OH)4 nH2O

 Chlorite 12 [(Si,Al)8

O20 ] (OH)16

Pyrite FeS2

Calcite CaCO3

Siderite FeCO3 May contain Mn.

Minor Mineral Constituents

Analcime NaAlSi2

O6

.H2

O

Apatite Ca 5(PO4) 3 (OH,F,Cl)

Barite BaSO4

Chalcopyrite CuFeS2

Clausthalite PbSe

Crandallite Group

Crandallite CaAl3(PO4) 2 (OH)5 H2O

Florencite CeAl3(PO4) 2(OH)6

Gorceixite BaAl3(PO4) 2(OH)5

.

H2O

Goyazite SrAl 3(PO4 ) 2(OH)5

.

H2O

Dolomite CaMg(CO3)

2

Feldspars (Ca,K,Na)AlSi3O8

Galena PbS

Marcasite FeS2 May contain same elements 

as pyrite.

Monazite 4

Rutile/Anatase TiO2

Sphalerite ZnS May contain Cd.

Xenotime YPO4

Zircon 4 ]

Trace Mineral Constituents

Chromite FeCr2O4

Gibbsite Al(OH)

3

Gold Au

Gypsum CaSO4

. 2H2O

Halite NaCl

Magnetite Fe3O4

Chemical composition Remarks

Major Mineral Constituents

(In general order of abundance)

Si 5 (OH)4 Al

)(OH)

( Ca,Na)

(Mg,Al,Fe) May have Mn. (Clays may 

also contain Be, Cr, Ni, 

and other trace elements.)

May contain As, Cd, Co, 

Hg, Ni, Sb, and Se.

.

Muscovite KAl 2 (AlSi

3

O10)(OH) 2

(Ce,La,Y,Th,Nd)PO

Zr[SiO 

.

1

2

Table 1. Common minerals found in coal and their elemental compositions.

[Although more then 120 different minerals have been identified in coal samples, only about 33 of these occur in most coal samples; of 

these, only about 8 generally are abundant enough to be considered as major constituents. Data from R.B. Finkelman (U.S. Geological 

Survey, written commun., 2000).]
13

minerals, such as pyrite. Other sources of inorganic compounds used by the plants may be either the mud that coats

the bottom of the swamp, sediments introduced by drainage

runoff, dissolved elements in the swamp water, windborne

sand, ash, or dust.

Coals may contain as many as 76 of the 92 naturally

occurring elements of the periodic table (fig. 9); however,

most of those elements usually are present in only trace

amounts on the order of parts per million. Occasionally, some

trace elements may be concentrated in a specific coal bed,

which may make that bed a valuable resource for those elements (such as silver, zinc, or germanium) (Finkelman and

Brown, 1991). Some elements, however, have the potential to

be hazardous (for example, cadmium or selenium), particularly if they are concentrated in more than trace amounts.

Although as many as 120 different minerals have been identified in coal, only about 33 of them commonly are found in

coal, and of these, only about 8 (quartz, kaolinite, illite, montmorillonite, chlorite, pyrite, calcite, and siderite) are abundant enough to be considered major constituents (table 1).

The organic compounds in coal are composed of the

elements carbon, hydrogen, oxygen, nitrogen, sulfur, and

trace amounts of a variety of other elements. Although only

a few elements compose the organic compounds found in

coal, these compounds are extremely complex and, as a

result, they are not well understood; for example, an attempt

to define the structure of just one organic compound in a

brown coal (lignite) is shown in figure 10, but even this relatively simple structure is based on scientific conjecture.

The organic compounds in coal produce heat when coal is

burned; they also may be converted to synthetic fuels, or

may be used to produce the organic chemicals shown in the

centerfold illustration.

The affinities of trace elements for either the organic

compounds or the minerals in coal affect the quality of the

coal. If trace elements are organically bound, then it is difficult to remove them from the coal by cleaning processes,

such as crushing and washing, which remove mineral matter from coal before it is burned. Organically bound trace

elements may be released only by burning, or by deep

chemical leaching, which is both very difficult and uneconomical. In contrast, trace elements associated with clays

or pyrite may be largely removed from the coal by cleaning

processes. The trace elements then may be disposed of in

an environmentally safe manner or recovered and put to

some other use. To date, however, the recovered mineral

matter from coal has not been used for very many purposes, with a few major exceptions such as gypsum, the various forms of ash, and a small amount of germanium (used

in semiconductors).

MINERALS IN COAL

The most common minerals in coal (for example, illite

clay, pyrite, quartz, and calcite) are made up of these most

common elements (in rough order of decreasing abundance): oxygen, aluminum, silicon, iron, sulfur, and calcium. These minerals and other less common minerals usually contain the bulk of the trace elements present in coal

(table 1). Minerals in coal commonly occur as single crystals or clusters of crystals that are intermixed with organic

matter or that fill void spaces in the coal; sizes of mineral

grains range from submicroscopic to a few inches. Some

clusters of mineral grains, however, such as fracture fillings

(fig. 11) or coal balls (fig. 12), may reach sizes that range

from fractions of an inch up to several feet across. Coal

balls result when mineral matter (such as calcite, pyrite, or

siderite) infuses peat before it is compressed. Mineral grains

in coal also often occur as discrete particles, such as the

blebs of pyrite (framboids), which may be identified in the

photomicrographs of coal in figure 13. The more finely

divided the mineral grains, the higher the magnification

needed to identify them (figs. 13, 14).

WHAT IS COAL?

OHC-C-CH OH

H

H

H

2

OHCH2

HC

HC O

OCH3

CH OH

CH

HCOH

2

O

HO

CH O3

OHCH2

HCH

C O

OH

O O

Coal Coal

Benzene Ring

Figure 10. Diagram showing hypothetical structure of a ligninderived organic molecule in a brown coal (lignite). This structure

is based on studies using carbon-13 nuclear-magnetic-resonance

(

13C NMR) spectroscopy and other organic-geochemical methods

such as those described in table 2. Lignin is one of the constituents

of woody tissue found in all vascular plants, such as trees and

shrubs (W.H. Orem, U.S. Geological Survey, written commun.,

1997). Studying organic matter helps provide information about

the plant sources of coal and, subsequently, coal quality. See figure 9 for chemical elements. Coal may contain as many as 76 of the

92 naturally occurring elements of the

periodic table.
14 COAL—A COMPLEX NATURAL RESOURCE

Although much is known about the minerals in coal,

much remains to be learned about their occurrence, abundance, origin, and composition. For example, the type of clay

mineral in a coal, whether montmorillonite or illite, determines how a coal will react when burned. Montmorillonite

(table 1) may or may not break down (dissociate) into its constituent parts when coal is burned; if it does dissociate, then,

upon cooling, it may recombine with other elements or minerals to form mineral deposits on the inside surfaces of furnaces and boilers (fig. 15). This process (called “slagging” or

“fouling”) produces barriers to heat exchange in the affected

equipment, which can substantially reduce its efficiency and

require costly repairs. Illite (table 1), however, with its simpler composition, does not cause such problems under normal

furnace operating conditions. Where these two clay minerals

and others occur, their relative abundances, relationships to

other minerals, and exact compositions are subjects for continued research in coal quality.

When coal is burned, most of the mineral matter and

trace elements generally form ash; however, some minerals

break down into gaseous compounds which go out the furnace’s flue. Pyrite, for example, breaks down into the individual elements iron and sulfur. Each element then combines with oxygen to become, respectively, iron oxide and

SOx. Iron oxide, a heavy solid, becomes part of the ash and

SOx, a gas, is emitted as part of the flue gas. Some trace elements also dissociate from their organic or mineral hosts

when coal is burned and follow separate paths. Most

become part of the ash, but a few of the more volatile elements, such as mercury and selenium, may be emitted in the

flue gas.

The mineral content of coal determines what kind of

ash will be produced when it is burned. The fusion temperature (melting point) of the ash dictates the design of furnaces and boilers. In general, if the fusion temperature is

relatively low, then the molten ash is collected at the bottom

of the furnace as bottom ash, requiring one design; however, if the fusion temperature is relatively high, then the part

of the ash that doesn't melt easily, called “fly ash,” is blown

through the furnace or boiler with the flue gas and is collected in giant filter bags, or electrostatic precipitators, at

the bottom of the flue stack, requiring a different design.

Coals that are relatively rich in iron-bearing minerals (such

as pyrite or siderite) have low fusion temperatures while

coals relatively rich in aluminum-bearing minerals (such as

kaolinite or illite) tend to have high fusion temperatures. If

an electric generating or heating plant is designed to burn

one type of coal, then it must continue to be supplied with a

similar coal or undergo an extensive and costly redesign in

order to adapt to a different type of coal. Similarly, furnaces

designed to use coal that produces high amounts of heat will

suffer severe losses in efficiency if they must accept coal

that burns with substantially less heat.

The mineral content of coal

determines what kind of ash will

be produced, which in turn determines

the design of a furnace or boiler.

Figure 11. Photomicrograph of pyrite (yellow) filling fractures in the

Upper Freeport coal bed in Pennsylvania. Magnification approximately

×4000. From Atlas of Coal Geology, American Association of

Petroleum Geologists (AAPG) Studies in Geology No. 45 (Papp and

others, 1998); AAPG copyright 1998, reprinted by permission of the

AAPG, whose permission is required for further use.

Figure 12. Photograph of small coal ball specimen from the Carbondale

Formation, Illinois basin. Coal balls are concretions that occur within coal

beds and are composed largely of carbonate minerals and occasionally of

pyrite. Modified from a photograph by James Humphrey (Curragh

Queensland Mining Ltd., Australia) in Atlas of Coal Geology, American

Association of Petroleum Geologists (AAPG) Studies in Geology No. 45

(Papp and others, 1998); AAPG copyright 1998, reprinted by permission

of the AAPG, whose permission is required for further use.

1 INCH
WHAT IS COAL? 15

A

B

C

V

V

P

C

V

V

V

F

R

S

S

50μm

50μm

50μm

Figure 13. Photograph and photomicrographs of Pennsylvanian

bituminous coal. A, Photograph of a lump of bituminous coal from

Pennsylvania showing typical banding caused by layering of a

variety of preserved plant types as they accumulated in an ancestral peat swamp. Magnification ×0.55. B, Photomicrograph of a

thin section of a small piece of bituminous coal from Illinois photographed in transmitted light. Magnification approximately ×330.

C, Photomicrograph of a polished surface of a small piece of bituminous coal from Pennsylvania photographed in reflected light.

Magnification approximately ×250. Parts B and C show some typical constituents—macerals (plant remains) and minerals—found

in bituminous coals. Macerals are abbreviated as follows: V, vitrinite; F, fusinite; S, sporinite; and R, resinite. Minerals are abbreviated as follows: P, pyrite; and C, clay-sized minerals. The scale bar

on parts B and C is 50 micrometers, which is equivalent to about

0.002 inch. The lump of bituminous coal is included to provide a

comparison between what can be seen in a lump of coal and the

wealth of detail in coal that can be seen with high magnification.

The lump of coal is approximately 260 times larger than the chips

of coal represented by either of the photomicrographs.

Photographs and photomicrographs by R.W. Stanton (U.S.

Geological Survey).
16 COAL—A COMPLEX NATURAL RESOURCE

MACERALS

The particles of organic matter in coal, inherited from

the remains of plant parts, are called “macerals.” Many different types of macerals occur in coal; a few of the more

common macerals are identified in the photomicrographs of

a bituminous coal from Illinois (fig. 13) The identification

of the original plant and its parts (such as bark, roots,

spores, or seeds) that produced an individual coal maceral is

helpful in determining coal quality. These connections usually are difficult to make because the original plant material has been compressed or altered beyond recognition. Coal

balls (fig. 12) result when mineral matter (such as calcite,

pyrite, or siderite) infuses peat before it is compressed. Coal

balls often contain permineralized plant materials that have

maintained their original structures because the mineral

matter has prevented compression and degradation of the

plants (fig. 16). Coal balls, therefore, can be used as an aid

in connecting the degraded, compressed plant matter of a

coal bed to the original plants.

Macerals are grouped into three main types: (1) vitrinite, (2) liptinite, and (3) inertinite. Liptinite and inertinite

each contain additional maceral subtypes (American

Association of Petroleum Geologists, 1998); however, in

Figure 14. Scanning-electron microscope photomicrograph of a

fragment of pyrite infilling plant cells in coal. Bright areas are

pyrite, gray areas are organic cell walls, and black area is epoxy

mounting media. Sample is from a bituminous coal from

Pennsylvania. Magnification approximately ×1000. Modified from

a photomicrograph by L.F. Ruppert (U.S. Geological Survey).

B

Figure 15. Photographs of mineral deposits (“slagging and fouling”) on boiler pipes. A, Slag deposits forming on tubes in platen

area of a coal-fired boiler at Alliant Energy WP&L, Columbia

Energy Center Unit 1, Portage, Wisc. Tubes are about 1.75 inches

in diameter. B, Ash deposit removed from steam tubes in a coalfired boiler at an AmerenUE plant using subbituminous coal from

the Powder River Basin, Wyoming. The plant was originally

designed to use bituminous coal. Ash fragment is approximately

12 inches long by 5 inches wide by 3 inches tall at its tallest point.

This deposit was removed by explosive cleaning. Photographs by

C.J. Zygarlicke (University of North Dakota Energy and

Environmental Research Center, Grand Forks, N.D.); used with

permission of Alliant Energy, Portage, Wisc. (A), and AmerenUE,

St. Louis, Mo. (B).

A

Figure 16. Photograph of a cross section of a calcareous coal ball

from the Middle Pennsylvanian Murphysboro coal bed equivalent,

Indiana. The coal ball contains permineralized plant material that

enables scientists to identify the individual plants and the plant

communities (flora) that contributed to the peat, which later

became a coal bed. The coal ball contains some identifiable

remains of branches and twigs of lycopsid trees

(Diaphorodendron vasculare), labeled D on the photograph. The

structures, labeled S, are sections of lycopsid rootlets, called stigmaria. Compare these structures with the shapes of the macerals in

the photomicrographs of coal (fig. 13). Modified from a photograph by W.A. DiMichele (National Museum of Natural History,

Smithsonian Institution, Washington, D.C.).
WHAT IS COAL? 17

this report, just the principal maceral types will be discussed

with brief referral to some of the subtypes for illustrative

purposes. Vitrinite, which is the most common maceral,

results from the coalification of amorphous humic plant

material (structureless plant remains). Vitrinite, sometimes

called pure coal, is sensitive to heat and becomes denser,

tougher, and more vitreous as it is subjected to higher heat

levels, either with depth in the earth, or by proximity to an

external heat source, such as a volcano. An index of vitrinite

reflectance is used by coal scientists to determine the level

of heat, or maturity, to which coals and other organic matter, such as petroleum, have been subjected.

Liptinite develops from waxy or oily plant parts such as

spores, algae, and resin; the liptinite group contains several

subgroups based on these original plant parts—sporinite,

alginite, and resinite, respectively. Liptinite macerals are

more enriched in hydrogen than either vitrinite or fusinite;

liptinite-rich coals produce larger amounts and higher

grades of liquid fuel when subjected to destructive distillation than either vitrinite- or fusinite-rich coals. Some coals,

called “bog-head” or “cannel” coals consist almost entirely

of oil-rich algal and plant spore material. These coals tend

to yield large amounts of coal oil (kerosene) when subjected to destructive distillation because the macerals are rich in

oily material. However, both cannel and bog-head coals are

relatively rare.

Inertinite incorporates a group of common macerals

that forms from partially oxidized or burned plant cell

walls. Fusinite, or mineral charcoal, is a prominent representative of this group. Mineral charcoal is produced when,

occasionally and sometimes seasonally, the surface layers

of a peat swamp dry out and some surface peat either slowly oxidizes or catches fire, or some growing plants are partially charred. (The common and naturally occurring fires of

the Florida Everglades are modern examples of this

process.)

Coals can range in maceral composition from mostly

vitrinite to mostly fusinite, depending on the original plant

matter and the degree of preservation. Vitrinite-rich coals

are shiny black, clean, fractured conchoidally, and often

exhibit iridescence (called “peacock coal”) on fresh surfaces. Fusinite-rich coals, in contrast, are similar to charcoal

and are dull, black, friable, and dusty. For more information

on macerals and vitrinite reflectance see the subsection on

“Coal Quality” in the “Further Readings” section.

COAL RANK

A major factor in determining coal quality is coal rank.

Rank refers to steps in a slow, natural process called “coalification,” during which buried plant matter changes into an

ever denser, drier, more carbon rich, and harder material.

The major coal ranks, from lowest to highest, are lignite

(also called “brown coal” in some parts of the world),

subbituminous coal, bituminous coal, and anthracite. Each

rank may be further subdivided, as shown in figure 17. The

rank of coal is determined by the percentage of fixed carbon, moisture (water), volatile matter, and calorific value in

British thermal units (Btu) after the sulfur and mineral-matter content have been subtracted. Fixed carbon is solid,

combustible matter left in coal after the lighter, volatile,

hydrogen-rich compounds are driven off during coalification. Volatile matter is slowly removed from coal during

coalification, but may be rapidly removed during destructive distillation. Volatile matter contains the raw materials

from which the organic chemicals are obtained. In the U.S.,

the tests to determine the amounts of the above-mentioned

substances and the rank of the coal are performed using

standards published by ASTM International (2002).

In general, the higher the rank of a coal, the more

deeply it was buried, and, therefore, the higher the temperature it was subjected to during and after burial. Older coals

tend to be of higher rank because they are more likely to

have been buried more deeply for longer periods of time

than younger coals. To give a sense of the effects of increasing rank, the following comparison may be used: lignite is

Meta-anthracite

Anthracite

Semi-anthracite

Low-volatile bituminous

Medium-volatile bituminous

High-volatile A bituminous

High-volatile B bituminous

High-volatile C bituminous

or

Subbituminous A

Subbituminous B

Subbituminous C

Lignite

GROSS CALORIFIC VALUE (BTU/LB ON A MOIST, MINERAL-MATTER-FREE BASIS)

PERCENT FIXED CARBON (DRY, MINERAL-MATTER-FREE BASIS)

40

50

60

69

78

86

92

98

100

16,00014,00013,00011,50010,500

9,5008,300

7,0006,000

Figure 17. Diagram showing classification of coals by rank in the

U.S. Rank is a measure of the progressive alteration in the series

from lignite to anthracite. Higher rank coals are generally harder,

contain less moisture and volatile matter, and have higher calorific

values. Modified from Trumbull (1960). For a more detailed explanation of coal rank determination, see ASTM International (2002).
18 COAL—A COMPLEX NATURAL RESOURCE Increasing Rank

13C

240 200 160 120 80 40 0 -40

aromatic carbons

aliphatic carbons

carboxyl carbon**

phenolic carbons*

aromatic carbons*

oxygen-bonded aliphatic carbons**

methoxyl*

aliphatic carbons***

Peat

Lignite

Lignite

Subbituminous C Coal

Subbituminous B Coal

Subbituminous A Coal

High-Volatile Bituminous C Coal

ppm

Figure 18. Carbon-13 nuclear-magnetic-resonance (13C NMR)

spectra of coals of increasing rank (see figure 17); 13C is a stable

isotope of carbon. Diagram shows changes in certain organic compounds brought about by increased coalification. The peaks represent varieties of organic compounds. As rank increases, the variety of compounds decreases and the remaining compounds

become structurally tighter and denser. The compounds lost are

those that are high in hydrogen and oxygen. Peaks marked with a

single asterisk are lignin (woody tissue) indicators; those marked

with a double asterisk represent carbohydrates (sugars); and those

marked with a triple asterisk represent aliphatic substances (such as

waxes and resins). The diagram shows that the carbohydrates are

eliminated early in the coalification process, whereas the lignins,

waxes, and resins persist to become the principal constituents of

higher ranked coals. The scale at the bottom of the figure is a standard 13C NMR scale in ppm (parts per million) representing distance, in frequency units, from a standard, which is assigned a

position of zero on the scale. The position of peaks is used to interpret the chemical structure of the substance being analyzed, in this

case coal. Data from W.H. Orem (U.S. Geological Survey).
 and derivative sub

gh heating in a closed

hose basic materials.

duces approximately

6 pounds of ammonia,

ration provided by the

Schweinfurth, S.P., 2003 , Coal—A complex natural resource; an overview of factors affecting coal quality and use in the United States: U.S. Geological Survey Circular 1143, 43 p.
FOLD
 and derivative sub

gh heating in a closed

hose basic materials.

duces approximately

6 pounds of ammonia,

ration provided by the

Schweinfurth, S.P., 2003 , Coal—A complex natural resource; an overview of factors affecting coal quality and use in the United States: U.S. Geological Survey Circular 1143, 43 p.
WHAT IS COAL? 19

Eon Era Period Epoch Millions of

Years Ago

1.6

66

138

Holocene

Pleistocene Quaternary

Tertiary

Cretaceous

Cenozoic

Jurassic

205

240

290

330

Triassic

Permian

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

Late

Middle

Early

Late

Middle

Early

Late

Middle

Early

Late

Middle

Early

360

410

435

500

570

2,500

3,800?

Mesozoic

Paleozoic

Phanerozoic

Proterozoic

Archean

Late Proterozoic

Middle Proterozoic

Early Proterozoic

Late Archean

Middle Archean

Early Archean

pre-Archean

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Late

Early

Late

Middle

Early

Late

Middle

Early

Late

Early

Late

Early

Late

Middle

Early

Figure 19. Chart showing divisions of geologic time (ages are

approximate). Since the Mississippian Period, major coal deposits

have formed some place in the world except during the most recent

period (Quaternary); how ever, major peat deposits formed (and

are still forming) during the Quaternary and it is possible that there

may be Quaternary lignite deposits as well. Periods of major coal

deposits in the U.S. are shaded.
20 COAL—A COMPLEX NATURAL RESOURCE

soft, dusty, and can ignite spontaneously under the appropriate conditions, whereas anthracite is quite hard, clean to

the touch, and must reach a temperature of about 925° F

before it will ignite. Furthermore, anthracite contains about

twice the calorific value of lignite (about 15,000 Btu/lb and

7,000 Btu/lb, respectively) because lignite contains more

moisture and oxygen and less fixed carbon than anthracite.

Subbituminous and high-volatile bituminous C coals have

oxygen and moisture content and calorific values that range

between those of lignite and anthracite. Bituminous coals of

higher rank have calorific values that may exceed those of

anthracite (fig. 17). Figure 18 shows nuclear-magnetic-resonance (13C NMR) spectra of coals of increasing rank, illustrating the changes in the organic structures as coal progresses through stages of increasing rank.

WHY IS COAL SO COMPLEX?

Coal is complex because of the wide variety of factors

that determine the quality of coal. These factors include the

following: (1) the plants, plant remains, and other organisms (such as bacteria) in the peat swamp; (2) biological and

chemical processes, and the degree of preservation of the

plant matter; (3) the geometry and location of the swamp;

(4) the mineral matter that accumulated with the plant material, or was introduced at some later stage; and (5) coalification. Geologic age and history will be a recurring theme

throughout this discussion. Figure 19 shows the times in

geologic history when coal beds formed.

THE PLANTS

Coal begins as peat, which forms in swamps having

favorable conditions as explained in a previous section entitled “Origins.” These swamps, which are generally filled

with standing or slowly moving water, are ideal places for

prolific plant growth. When plants fall into the swamp, water

covers them and slows or prevents rapid decomposition.

Slow decomposition and a quiet environment allow plant

matter to accumulate and form peat. The organic matter

composing the peat may accumulate in place or, less commonly, may be carried by flowing water to accumulate elsewhere, but generally nearby. Most coal beds were formed

from plant material that accumulated in place (fig. 20).

Plant communities (flora) in peat swamps may consist

of many plant types, or of only a few. Swamp flora have

changed substantially through geologic time because of

evolution. For instance, lycopsids and tree ferns (fig. 21) are

found in older coal beds; more modern forms, such as rushes, water lilies, mangrove, and bald cypress trees (shown in

figure 6) may be found in younger coal beds. Flora of the

Cretaceous Period (fig. 22) included abundant species that

contained relatively more resinous substances than those of

the much earlier Pennsylvanian Period. Tertiary plants generally were woodier than those of either the Pennsylvanian

or the Cretaceous Periods. In addition, floral communities

varied from location to location even within the same time

period. These changes in plant species resulted in small

changes in nutrient requirements. The result is that geologically younger coals may have different mineral matter

compositions than older coals.

BIOLOGICAL AND CHEMICAL PROCESSES

Once plant debris has been deposited in a peat swamp,

it is subjected to degradation and decomposition by microCoal is a complex material formed

by a mix of substances and processes.

The substances are plant remains and

mineral matter. The processes aren’t

completely understood, but involve

chemical and biological reactions,

heat, burial, and time.

Figure 20. Photograph of an in-situ fossilized tree base and root

system in the roof of an underground coal mine. The coal bed

being mined is in the Cretaceous Blackhawk Formation, Wasatch

Plateau coal field near Huntington, Utah. Note the radiating root

network. Photograph by Alex Papp (ARCO Coal Company,

Denver, Colo.) in Atlas of Coal Geology, American Association of

Petroleum Geologists (AAPG) Studies in Geology No. 45 (Papp

and others, 1998); AAPG copyright 1998, reprinted by permission

of the AAPG, whose permission is required for further use.
WHY IS COAL SO COMPLEX? 21

Figure 21. Reconstruction of a tropical peat swamp of Middle

Pennsylvanian age (about 300 million years ago). The taller plants

(A) were tree lycopsids (Lepidophloios), some of which grew to

heights of 80 to 100 ft. Today, the lycopsids are represented by

lowly club mosses. Other plants include cordaites (B), which were

seed plants with strap-shaped leaves that are now extinct; tree

ferns (C), which still live in warm, damp areas; calamites (D),

which is a tree-size scouring rush; and pteridosperms (E), which

were seed plants with fernlike leaves that are now extinct. The

tree-like scouring rushes, which reached heights of 30 to 50 ft, are

represented today by plants that grow in damp, but not necessarily warm, areas usually to only a few feet in height. From an illustration by Alice Pricket in GSA Today (Gastaldo and others, 1996,

fig. 1); Geological Society of America (GSA), copyright 1996,

reproduced with permission of the GSA (Boulder, Colo.), whose

permission is required for further use.

Figure 22. Reconstruction

of a peat-forming, fluvial

coal swamp and peripheral,

bottom-land vegetation of

Late Cretaceous age (about

70 million years ago) in what

is now eastern Utah.

Representative plants are as

follows: A, horse-tail rush;

B, sequoia; C, palm; D,

Rhamnites, now extinct; E,

Araucaria, now extinct; F,

sycamore; G, fig; and H, cattail. Plants were growing on

the banks of the swamp.

Modified from Tidwell

(1998, figure 37), used with

permission of the author.

A

B

C

D E

F

C

G

H

A

B

C DE
22 COAL—A COMPLEX NATURAL RESOURCE

Lake with floating peat and

organic-rich mud

Low-lying peat swamp in

poorly drained area

Diverse and luxuriant flora

Extensive low-lying peat swamp

Restricted flora Slightly raised peat swamp

Very restricted and stunted flora Raised peat swamp Lakes

A

B

C

Land surface, or substrate

EXPLANATION

Earth material

Organic-rich mud

Planar (topogenous) peat

Domed (ombrogenous) peat

Water flow

Figure 23. Sketches illustrating a possible evolution

from a planar (topogenous) peat deposit to a domed

(ombrogenous) peat deposit. A, planar; B, transitional,

slightly domed; and C, domed. A planar deposit may

evolve into a transitional or domed deposit if climatic

conditions change; the two types of peat deposits also

may occur independently. No scale implied, but the

horizontal dimension could be many miles and the vertical dimension could be 500 to 1,000 ft. Individual

plant types were not identified in the source for this figure, but a typical flora might include cypress, magnolia, and willow trees; sassafras, sumac, and witch hazel

shrubs; and ferns, reeds, and lilies as low-growing and

water plants. Modified from McCabe (1987), used with

permission of The Geological Society (London).
23

bial action, oxidation, and biogeochemical processes. These

processes are pervasive and, to a large extent, determine the

character of any resultant coal. The processes, which operate on microbiological and microchemical levels, are constantly changing and, therefore, are difficult to characterize.

What is known is that (1) the degradation processes are carried out by oxygen from the atmosphere and various

microorganisms, such as bacteria and fungi; and (2) the

processes are moderated by the acidity or alkalinity of the

available water. When the oxygen in the peat has been

depleted, anaerobic bacteria (such as those responsible for

fermentation) continue the process of degradation.

Although pervasive, the process of plant decomposition is

also selective in that different plant parts degrade at different rates. The most resistant parts, such as bark, cuticle,

spores, lignin-rich wood, and charcoal (fusinized peat), are

the parts most often retained in coal as macerals.

When standing water covers fallen plant matter, atmospheric oxidation is halted; soon thereafter, biochemical

change slows to a virtual halt. Once plant matter is converted to peat and is buried, further substantial changes depend

more on increases in temperature and pressure than on oxidation and biochemical processes. The acidity or alkalinity

of peat swamp water may also control the rate and amount

of change that takes place. Usually, the more acidic the

water, the more plant matter is preserved because bacterial

activity is reduced. If their work is carried to completion,

bacteria and fungi produce CO2, water, and ash; in effect, it

is a slow-burning process.

SWAMP GEOMETRY, LOCATION, AND CLIMATE

From the late Paleozoic to the present, two principal

types of peat swamps have existed: planar (topogenous) and

domed (ombrogenous) (fig. 23). Each has unique floral

characteristics that affect the quality of the resulting coal.

The planar peat swamp generally develops in areas of

seasonal rainfall, mainly in low-lying areas, and depends on

a high ground-water table for its continued existence. The

resulting peat deposit often has an undulating bottom and a

flat top. Because it forms in low-lying areas, the planar

swamp is subject to periodic flooding, which results in the

introduction of extraneous mineral matter such as clay, silt,

and sand. This sediment brings in nutrients, but may also

add impurities to any resulting coal; in extreme cases, the

sediment may bury the swamp, which results in the termination of peat formation.

The domed peat swamp develops in areas that receive

substantial rainfall during most of the year. In this environment, new plants grow above fallen plants, which decay

very slowly because the swamp, like a sponge, remains saturated with water. This process gradually allows the top of

the swamp to rise above the surrounding topography. The

result is a peat deposit that has a raised, or domed, surface.

The dome prevents or limits the extent of flooding of the

swamp and thereby prevents the introduction of mud, silt,

and sand, except around the fringes. New plants depend on

dead ones for their nutrients, thus depleting the earlier

deposited peat of mineral matter. The flora become stunted

and restricted to plants that can survive in a mineral-poor,

acidic soil. As a result, domed peat tends to contain much

less mineral matter than planar peat and the resulting coals

are purer.

A peat deposit may have originated as either the planar

or the domed type and then, due to a change in the climate

or topography or both, may transform into the other type

and even back again (fig. 23). The resulting coal beds may

be composites of the two types. These conditions make

determining the quality of the composite coal beds difficult.

The geographic location and climate of the precursor

peat swamp also affect coal quality. At present, swamps are

located in tropical, temperate, and cool climates, near coastlines and inland, beside rivers and within river deltas, and in

upland areas. This wide variety of geographic locations and

climates in which swamps develop was most likely the case

at least as far back as the Devonian Period (fig. 19).

Peat swamps located close to rivers or within river

deltas suffer the possibility of periodic flooding and the

introduction of large amounts of mineral matter. Swamps

along coastlines, while not as easily subjected to river

flooding, may be affected by offshore or coastal storms that

wash sediment over offshore bars and beaches into the

swamps behind them. The added mineral matter may be disseminated in the peat, may occur as layers in the peat, or

may cause peat formation to stop. Discrete layers of mineral matter in coal beds are called “partings” (fig. 24).

WHY IS COAL SO COMPLEX?

Figure 24. Photograph of a coal strip-mine highwall exposure of

a thick lens-shaped parting, possibly a floodwater overflow (see

fig. 9) in the Upper Freeport coal bed, central Appalachian basin.

Photograph by Mark Scanlon (Canyon Fuel Co., LLC, Price,

Utah) in Atlas of Coal Geology, American Association of

Petroleum Geologists (AAPG) Studies in Geology No. 45 (Papp

and others, 1998); AAPG copyright 1998, reprinted by permission

of the AAPG, whose permission is required for further use.
24 COAL—A COMPLEX NATURAL RESOURCE

Climate, which to some extent depends on geography,

affects the growing season and the amount of rainfall that a

swamp receives and, therefore, helps determine the kind of

peat that will eventually form. Thick, extensive coal beds

generally are thought to have originated as large peat

swamps in warm, humid, and tropical to subtropical regions

(Cecil, 1990). For example, extensive, thick (some up to 50

ft) peat deposits have been accumulating on the coastal

areas of the islands of Indonesia (principally Sumatra; fig.

7A) and Borneo, as well as others (Neuzil and others,

1993). Those areas are in warm, ever-wet, climatic zones.

Extensive peat deposits currently are forming in the

Florida Everglades, but the climate there is much more

variable than that of Borneo. Southern Florida has periodic

rainfall and extended dry periods that allow the surface peat

to dry out and oxidize, or even burn off, resulting in a relatively thin deposit.

Other modern peat deposits form in a wide variety of

climatic conditions; two additional examples are provided.

The Vasjugankojev swamp in Siberian Russia, which is a

notoriously cold region, is described by Cameron and others (1989) as, at an area of approximately 21,000 mi2

, the

largest “peatland” in the world; however, even though the

region is cold, the location of the swamp is low, flat, and

wet, and part of the year is conducive to adequate plant

growth while the coldness of the rest of the year serves to

protect much of the plant matter from decay. No thickness

of peat in this swamp was reported.

A swamp on Great Cranberry Island, Maine, also

described by Cameron and others (1989), is another example of a swamp located in a cool temperate climate. This

swamp formed during the recent past as sea level rose and

built up a beach ridge approximately 11 to 12 ft high. A

lake, which was formed behind the rising beach ridge, slowly filled with peat as the beach rose. The resultant peat

deposit is relatively small (less than 250 acres) but it reaches thicknesses of as much as 27 ft. Whether either of these

modern peat deposits will ever become a coal bed cannot

now be determined; however, research on modern peat

deposits, wherever they are located and under whatever climatic conditions they are deposited, may help in finding

additional high-quality coal deposits in older rocks.

MINERAL MATTER

The living plants in a peat swamp absorb mineral matter from (1) the soil they grow in, (2) dissolved mineral matter in the water of the swamp, and (3) introduced mineral

matter, such as waterborne or windborne sediment, or volcanic ash. Some of the dust and ash may be dissolved in the

swamp water and serve to support plant growth, as described

in the discussion of domed peat deposits, or they may occur

as disseminated grains in the peat or as discrete layers called

partings (fig. 24). Partings may be paper thin or they may be

several feet thick. In the case of the planar swamp, dust or

ash will further dilute the organic matter composing the peat,

thus raising the mineral-matter content or creating partings.

In the case of the domed swamp, however, these sediments

may add enough new nutrients to the swamp to prolong its

life and produce a thicker peat and ultimately a thicker coal

bed. Volcanic ash and some types of dust are rich in the

nutrients required for vigorous plant growth.

Throughout geologic time, the peat swamps that were

not destroyed by erosion or oxidation were buried by sediments,

which, depending on their source, affected the mineral-matter content of the resultant coal in different ways. In general, riverborne sediments tend to be richer in some elements,

such as iron, whereas oceanborne sediments tend to be richer in other elements, such as sulfur.

In many coals, some of the mineral matter was introduced by epigenetic (low temperature) mineralization after

the bed was formed (fig. 11). In this process, the minerals

were introduced into fractures (called “cleat;” see below

under “Coalification”) and along bedding planes in the coal

bed as ions in moving water and were deposited when the

water lost its ability to maintain the mineral-forming ions in

solution. This could occur when the ion-bearing water

entered a different temperature and (or) chemical environment.

Epigenetic minerals commonly include pyrite, calcite,

siderite, and kaolinite, and in rare cases, sphalerite and galena (see table 1 for compositions of these minerals).

Epigenetic mineralization, estimated to have been introduced during the Permian Period into coal beds of older

Pennsylvanian age (see fig. 19), is particularly notable in

coals of the midcontinent region of the U.S. (Cobb, 1979;

Brannon and others, 1997).

COALIFICATION

From the time the peat is buried, it goes through a

series of chemical and physical changes called “coalification,” which is the process that produces coals of increasing

rank (fig. 17). Coalification is a continuing process involving increases in both temperature and pressure resulting

from burial in the earth. Burial is a process that may happen

very slowly or relatively rapidly depending on the speed

and magnitude of the geologic forces operating on the

region. Increasing temperature is considered more important than increasing pressure in promoting coalification.

Higher temperatures eliminate moisture and volatile elements and, therefore, help produce coals of higher rank and

higher heat (calorific) value. Higher temperatures are generally associated with deeper burial in the earth, although

proximity to an unusual source of heat, such as a volcano,

could produce similar effects.
DETERMINING THE QUALITY OF COAL 25

When increases in depth of burial or temperature no

longer occur, coalification slows and then ceases (unless

another source of higher heat affects the coal). Thereafter,

the coal will remain at the same rank even if it is raised up

again (either by tectonic uplift or erosion of overlying sediments, or both) into a region of much lower temperature or

pressure. Once the coal is exposed to weathering (oxidation), however, it is slowly reduced to ash.

Coalification affects not only the organic matter in

coal, but the mineral matter as well. As coalification proceeds, organic matter, which is relatively rich in water, oxygen, and hydrogen, gradually loses those constituents and

becomes relatively enriched in fixed carbon. Some of the

hydrogen and carbon are converted to methane gas

(CH4) in the process. Changes that occur to the mineral

matter during coalification are less well understood. During

coalification, clay minerals may become more refractory

(that is, less affected by heat and chemicals), and the elements in other minerals may become rearranged, making

them more crystalline. Although deeper burial, and therefore higher temperatures, implies greater age, coals of

approximately the same geologic age may exhibit a wide

range in rank depending on their geologic histories. For

example, Pennsylvanian coals in the eastern U.S. (figs. 4,

19) range in rank from anthracite in easternmost

Pennsylvania to high-volatile C bituminous coal in western

Illinois and Iowa. Coals beds in what is now eastern

Pennsylvania were subjected to strong mountain-building

forces during the late Paleozoic, when the Appalachian

Mountains were formed. During this time, the beds were

forced deep into the earth into zones of high heat, and then

were compressed into tight folds. In contrast, the coals to

the west, in what are now the States including western

Pennsylvania, Ohio, Indiana, Illinois and Iowa, were subjected to lesser depths of burial and little or no mountainbuilding forces.

During the process of coalification, sets of roughly parallel, closely spaced fractures (cleat) form in the coal. Cleat

tend to form in two sets at right angles to one another; one

set, the face cleat, is dominant, while the other set, the butt

cleat, may be only poorly developed (fig. 25). Cleat are generally well developed in bituminous coals; lignites and subbituminous coals, both less well coalified, generally exhibit

only incipient cleat. Anthracite in eastern Pennsylvania generally does not exhibit cleat because strong mountain-building forces welded the coal back together into solid massive

beds. (For a thorough discussion, see Damberger, 1991.)

DETERMINING THE QUALITY OF COAL

By now, it should be clear why coal is such a complex

and heterogeneous material. The interplay of the many factors that affect the peat and the resulting coal affects its

composition, but the composition is never quite static

because it is subject to continuing change. In the past, coal

was simply dug up and burned or turned into coke with little concern for its quality as long as it made a hot fire, did

not have too much ash or sulfur, and would ship long distances without turning to dust. Now we know that a complete understanding of coal quality is important in order to

address environmental or industrial concerns such as (1)

SOx and hazardous air pollutants (elements such as mercury and selenium in hot flue gases); (2) boiler slagging and

fouling; (3) the high costs of flue-gas desulfurization; (4)

the disposal of scrubber sludge, ash, and CO2; (5) the development of clean-coal technologies; and (6) the promotion of

coal as a source for synthetic oil and gas.

Many techniques currently exist for studying coal quality. Some analytical techniques (such as the ones used to

determine coal rank, moisture, ash yield, sulfur content, and

Coalification is a baking process

in the earth, under pressure. As it

proceeds, it produces coal of

increasing hardness and calorific

value and results in a reduction of

tar, oil and gas.

Figure 25. Photograph of part of an exposed coal bed showing

well-developed face cleat and poorly developed butt cleat. The

coal bed is in the Cretaceous Fruitland Formation in the San Juan

basin, near Durango, Colo. The compass is 2.75 in. wide.

Photograph by Walter Ayers (S.A. Holditch Assoc., Inc., College

Station, Tex.) in Atlas of Coal Geology, American Association of

Petroleum Geologists (AAPG) Studies in Geology No 45 (Papp

and others, 1998); AAPG copyright 1998, reprinted by permission

of the AAPG, whose permission is required for further use.
26 COAL—A COMPLEX NATURAL RESOURCE

volatile matter) are highly standardized. Coal mines, electric utilities, the coke industry, and commercial analytical

labs commonly employ these tests. Many of the standards

for these techniques, as well as the standards for precision

in their use, are published by ASTM International (2002)

and are widely followed by the coal industry in the U.S.

Research scientists studying coal quality use not only the

results of ASTM-based analyses in their work, but also

other generally accepted techniques that have not been

standardized by the ASTM; however, once research shows

that a new technique is useful and accepted by the coal

industry, standards may be developed for its application.

Research is currently underway to improve the existing

techniques used by coal scientists for analyzing coal, to

adapt some techniques used in the analysis of other materials, and to develop new analytical techniques. Procedures

approved by the ASTM and their purposes are given in table

2. A list of analytical techniques currently used in coal-quality research at the USGS and their purposes is given in table

3. The two suites of analyses serve different purposes. The

results of the ASTM suite of analyses allow coal-using

industries to directly compare coals of differing qualities,

which enables them to select a coal that is best suited to

their needs. These results also allow coal research scientists

to study the geologic history of a coal bed through levels of

rank, and ash, sulfur, and moisture contents. The suite of

analyses used by the USGS was developed to (1) aide in the

search for valuable mineral byproducts, (2) develop methods to aide in determining the origins and correlations of

individual coal beds, and (3) analyze for the locations,

amounts, and affinities of elements that are known to, or

might, pose a hazard to health. The results of both sets of

analyses performed on a sample of bituminous coal from

Alabama are provided in table 4 (the results of ASTM

analyses are given first; the results of USGS analyses follow). The ASTM analyses were performed by the U.S.

Department of Energy at the Pittsburgh, Pa., laboratory; the

USGS analyses were performed at USGS laboratories at

Reston, Va., and Denver, Colo.

The USGS has developed a large file of data on coal

quality including the results of both ASTM and

USGS procedures (Bragg and others, 1997). These data

result from analyses performed on coal samples collected

by USGS scientists or by cooperating agencies, such as

Table 2. Procedures and purposes of American Society for Testing and Materials (ASTM) methods for testing coal.

Procedure Purpose

Classify coal into categories indicating ranges of physical and chemical characteristics 

 that are useful in making broad estimates of the behavior of coal in mining, preparation, 

 and use.

Coal ash 

Equilibrium moisture

Forms of sulfur 

Free-swelling index

Fusibility of ash

Gross calorific value 1

Hardgrove grindability

Proximate analysis

Reflectance of organic matter Determine rank and how well a coal will form coke.

Total moisture

Trace elements

Ultimate analysis 

Volatile matter 

Maceral analysis

1

Identify products given off as gases or vapors.

Determine kinds and amounts of macerals in coal.

Determine amount of ash, carbon, hydrogen, nitrogen, oxygen, and sulfur.

Identify trace elements.

Determine inherent water and any other water present.

Determine amount of ash, fixed carbon, moisture, and volatile matter.

Major and minor elements Identify major and minor elements.

Determine resistance to grinding.

Determine potential for energy production, in kilocalories per kilogram.

Predict whether the coal ash will perform properly in the process for which it was chosen.

Determine how well a coal will form coke.

Determine whether sulfur is in pyrite, organically bound, or in sulfates.

Determine moisture-holding capacity of coal.

Determine amount of ash at a given temperature.

Classification of coal by rank

 One kilocalorie per kilogram = 1.798 British thermal units (Btu) per pound.
DETERMINING THE QUALITY OF COAL 27

Table 3. Analytical methods and individual techniques used in research on coal quality and their purposes.

Method Purpose

1. Analytical chemistry. Determine inorganic elements 

and compounds.

Wet chemistry.

Inductively coupled plasma 

mass spectroscopy (ICP-MS).

Inductively coupled plasma 

atomic emission spectroscopy (ICP-AES).

Neutron activation (NA).

Atomic absorption spectroscopy (AAS).

Molecular absorption spectroscopy (MAS).

Optical emission spectroscopy (OES).

Stable-isotope mass spectrometry (SINS).

2. X-ray fluorescence. Determine elemental composition.

3. X-ray diffraction. Determine mineral structure.

4. Optical microscopy. Determine distribution of coal 

particles and minerals and 

content of macerals, pollen,

and spores.

5. Accelerated particle.

Scanning-electron microscope Determine shapes, sizes, and 

(SEM) with energy dispersive distribution of mineral grains

X-ray analyzer (EDAX). at very high magnification and 

semiquantitative determination 

of some elements.

Transmission-electron Similar to SEM. Also used for 

microscope (TEM). mineral identification.

Electron microprobe (EMP). Determine structure and 

elemental composition of 

individual mineral and maceral 

grains.

Proton induced Similar to EMP, but more

X-ray excitation (PIXE) sensitive.

Laser microprobe-mass Determine trace-element presence in 

analyzer (LAMMA). coal macerals.

Ion microprobe (IM). Determine trace element 

presence in coal macerals.

6. Nuclear magnetic resonance Determine structures and

(NMR). structural changes of

organic compounds.

7. Gas chromatography/mass Determine organic compounds.

spectrometry (GC/MS).

8. Thermoluminescence (TD) Infer the origin of specific

and cathodoluminescence (CD) minerals, whether from an

detectors. igneous or sedimentary

source.

9. Fourier transform infrared Quantitatively determine clay 

analysis (FTIR). mineralogy and organic 

compounds.
28 COAL—A COMPLEX NATURAL RESOURCE

Table 4. Results of standard ASTM International (ASTM) analyses and U.S. Geological Survey (USGS) research analyses of a sample

(USGS 212277) of Pennsylvanian bituminous C coal from the Mary Lee coal bed, Walker County, Ala.

[Data from Bragg and others (1988). Btu, British thermal units; na, not available]

1

USGS ash slightly different from ASTM ash because of different temperatures used in ashing furnaces.

ASTM Standard Analyses

Proximate (in percent)

Moisture 2.6

Volatile matter 32.2

Free carbon 50.1

Ash 15.11

Ultimate (in percent, unless otherwise noted)

Hydrogen 5.0

Carbon 67.6

Nitrogen 1.5

Oxygen 9.6

Sulfur 1.2

Calorific value (Btu/lb) 12,060

Air-dried loss 0.3

Forms of sulfur

Sulfate 0.01

Pyritic 0.66

Organic 0.55

Free swelling index 2.5

Ash fusion temperature (in °F)

Initial 2,300

Softening 2,430

Fluid 2,520

Hardgrove grindability index na

Equilibrium moisture (in percent) na

USGS Research Analyses

USGS Ash (in percent) 16.01

Oxides (in weight percent of ash)

SiO2 50.0

Al2O3 26.0

CaO 3.4

MgO 0.28

MnO 0.02

Na2O 0.38

K2O 2.5

Fe2O3 7.0

TiO2 1.2

P2O5 1.3

SO3 2.3

Minor elements (whole-coal basis, in parts

per million (ppm))

Si 37,000

Al 22,000

Ca 38,000

Mg 270

Na 450

K 3,400

Fe 7,800

Ti 1,100

S 1,500

Trace elements (whole-coal basis, in parts

per million (ppm))

Ag 0.011

As 7.6

Au 1.1

B 66.0

Ba 150.0

Be 0.99

Bi 1.1

Br 11.0

Cd 0.058

Ce 24.0

Cl 540.0

Co 3.9

Cr 22.0

Cs 2.3

Cu 12.0

Dy 2.5

Er 1.1

Eu 0.47

F 140.0

Ga 5.3

Gd 1.7

Ge 0.98

Hf 1.0

Hg 0.22

Ho 0.76

In 0.76

Ir 1.7

La 13.0

Li 40.0

Lu 0.15

Mn 35.0

Mo 0.19

Nb 2.1

Nd 8.5

Ni 9.4

Os 2.5

P 900.0

Pb 4.8

Pd 0.11

Pr 7.6

Pt 0.52

Rb 32.0

Re 1.1

Rh 0.25

Ru 0.25

Sb 0.7

Sc 5.1

Se 1.5

Sm 1.9

Sn 0.34

Sr 340.0

Ta 0.36

Tb 0.33

Th 3.9

Tl 0.52

Tm 0.52

U 1.0

V 24.0

W 0.6

Y 5.9

Yb 1.1

Zn 11.0

Zr 24.0
29

State geological surveys and universities. ASTM analyses

were performed by the former U.S. Bureau of Mines

(USBM) and by independent testing laboratories; analyses

for oxides and trace elements were performed at USGS laboratories based on USGS-developed standards.

SAMPLING TECHNIQUES

Most analyses of coal, for both standard and research

purposes, are conducted on carefully collected samples of

whole coal. Coal samples are taken from a variety of

places—individual coal beds in place, coal-mine conveyor

systems, trucks, train cars, or stockpiles—depending on the

needs of the sampling and analytical programs. The goal of

this process is to collect a sample that will be as representative as possible of the coal bed or other source from which

it is taken. In order to produce reliable results from testing

programs, sampling must be done very carefully. Samples

must be taken without contamination from extraneous material, and location and orientation must be documented.

There are also a few techniques for analyzing coal beds

in place. Geophysical logging is conducted in drill holes

during exploration and mine planning programs. This technique measures electrical resistivity, transmissivity of

sound, inherent electrical properties (such as self-potential),

and the reaction of coal to bombardment by atomic particles

(such as neutrons) (Wood and others, 1983). These methods

accurately identify coal beds and their boundaries, and help

determine whether a coal bed merits additional exploration

and mining; however, they are only good for an approximate analysis of coal quality.

Channel and drill-holecore samples are two of the most

common types of samples used for characterizing a coal

bed. A channel sample is taken from a freshly exposed coal

bed (usually in an underground or surface coal mine) by

cutting a 2- to 4-in channel into the bed from top to bottom

and collecting all the coal from the channel (fig. 26). Drillhole core samples (fig. 27) are taken from subsurface coal

beds where the coring bit does the cutting. If the coal bed is

divided horizontally into benches (individual beds of coal

separated by mineral partings), or if the bed is layered by

distinct types of coal, then the benches or beds may be sampled separately, which allows for a more detailed characterization of the individual parts. The ASTM recently has published standards for the collection of channel and core coal

samples (ASTM International, 2002).

The number of channel or core samples taken depends

on how well the coal bed needs to be characterized. If any

two samples of the same bed show considerable variation in

quality between them, then more samples may be taken

from the area between the two initial samples; however,

cost may be a factor in the decision to collect more samples.

The purpose of the analysis often determines the type

of sample that is collected. If the purpose is to characterize

a stock of already mined coal, then gross samples may be

taken at regularly or randomly spaced intervals from the

stockpile, coal car, or conveyor belt. If the purpose is to

examine some specific property of a coal bed, then a sample of only a small part of the bed may be taken. If the purpose of the sample is to study some directionally controlled

characteristic, such as the orientation of mineral grains, the

sample must be labeled to show top and bottom and, in

some cases, compass directions. For some types of analyses

(for example, by either petrographic or scanning-electron

microscope, figs. 11, 13B, 13C, 14), small blocks of coal are

cut out of larger samples and one side is polished and examined. This technique is used when it is necessary to see the

microscopic details of the types, distributions, and arrangements of the macerals and minerals in a coal sample.

When a coal sample arrives at the lab, it is crushed

(unless it is to be examined in block form) and its particles

are thoroughly mixed to ensure that the sample is homogeneous. The crushed sample then is divided into appropriately sized subsamples for each analytical technique to be used.

ANALYSIS

The primary reason for analyzing coal is to determine

whether it will meet the needs of a specific application, or

to characterize the general quality of the coal for future refDETERMINING THE QUALITY OF COAL

Figure 26. Photograph of a coal bed “face” in an underground

coal mine showing personnel collecting a channel sample of fresh

coal. The collecting team is standing on a floor that has been excavated below the base of the coal bed to facilitate mining. The

white color in the background is limestone dust sprayed on the

coal face to control coal dust. The coal sample is collected on a

clean plastic sheet to minimize contamination from the floor of the

mine. This sample is from the Lower Freeport coal bed, Indiana

County, Pa., where the bed is about 56 in thick. Photograph by

R.W. Stanton (U.S. Geological Survey).
30 COAL—A COMPLEX NATURAL RESOURCE

erence. For instance, coal may be analyzed to determine

how much sulfur (or other element) is present, its form, and

how it is distributed. If the sulfur is present in discrete pyrite

grains, then much of it may be cleaned out of the coal; but,

if it is organically bound, then the sulfur may be released

only by burning, by using a solvent, or by employing a bacteriologic technique (although the latter two techniques are

still largely experimental). If the coal has a high organic sulfur content, then it may have to be mixed or blended

with a coal of lower sulfur content in order to meet sulfur

emissions standards, or the sulfur may have to be cleaned

out of the flue gas by flue-gas desulfurization (FGD), which

is an expensive procedure. Similarly, analysis may determine whether a trace element, such as arsenic, may be eliminated from a coal by washing or whether it must be trapped

in the flue gas. Finally, in extreme cases, analysis may

determine that the coal cannot be used. 

In general, the coal-mining and coal-consuming industries determine coal quality for immediate or near-term use,

whereas Federal and State governments analyze coal to (1)

characterize large areas of unmined coal (resources) for

future reference, (2) to support policy decisions related to

future coal use, and (3) to supply Federal and State regulatory

agencies with information about the quality or special characteristics of future coal supplies, such as the presence,

amounts, and modes of occurrence of sulfur and potentially

toxic trace elements. Figure 28 shows a coal scientist at

work identifying coal macerals and coal rank in a polished

sample of coal using a high-powered, reflected-light

microscope.

Coal samples may be received at the laboratory in a

variety of conditions. Accordingly, analytical results may be

reported in a variety of ways, depending on the condition of

the samples when received and the use of the analysis.

Some samples arrive in a fresh condition, not long after

sampling; these may be analyzed without further processing, except grinding and mixing. The analytical results for

these samples are reported on an “as-received” basis. Some

samples arrive dried out due to long storage, transportation

over long distances, or mishandling. The results of analyses

on dried-out samples are reported on a “dry” basis. The

opposite conditions may also occur if the sample arrives

Figure 27. Photographs of a core-drilling rig and a collected core.

A, A truck-mounted, mobile, core-drilling rig on location in the

process of collecting core of the Upper Freeport coal bed in western Pennsylvania. Photograph by R.W. Stanton (U.S. Geological

Survey). B, Coal core, 3 inches in diameter, from a coal bed in the

Powder River basin, Wyoming. The white material beneath the

core is plastic tubing used to collect the core and prevent it from

falling apart. The upper half of the tubing has been removed to

provide access to the coal. Photograph by P.D. Warwick (U.S.

Geological Survey). 

A B
31

wet because of excess moisture in the sample that may or

may not be present in the coal bed; in this case the analyses

may be reported on a “moist” basis.

Analyses also may be reported on a “mineral-matterfree” or “dry, mineral-matter-free” basis for use in calculating coal rank because rank is a function only of the maturity of the organic matter. “Mineral-matter-free” means that

the amount of mineral matter in the sample has been subtracted from the total analytical results to provide only the

amount that is organic. “Dry, mineral-matter-free” means

that the sample was received in a dry or nearly dry state, or

was dried out before an analysis was made.

An analysis on a mineral-matter-free basis also may be

used for comparison with the results of an as-received

analysis of “run-of-mine” coal (as it comes from the mine)

if there is to be further processing, such as coal cleaning.

Run-of-mine coal frequently contains (1) large amounts of

mineral matter from partings in the coal bed, (2) rocks from

above and below the coal bed that are mined along with the

coal, (3) minerals that fill veins in the coal bed, and (4) mineral matter dispersed in the coal (such as pyrite grains).

Mineral matter in run-of-mine coal may cause the coal to

exceed the ash and sulfur limits set by contract requirements

and environmental regulations. A mineral-matter-free

analysis may indicate whether or not it will be worthwhile

to clean the coal; however, the need for cleaning depends on

the intended use of the coal and normal cleaning practices

may not bring a coal to a completely mineral-free condition.

Most chemical analyses require the destruction of the

coal sample in order to determine calorific value, volatile

matter, and mineral-matter content. Destruction is achieved

by burning the coal sample in a high-temperature furnace.

Although that procedure is fine for determining the calorific value of coal, it is not suitable for retention of some of its

volatile inorganic elements, such as mercury. Subjecting the

sample to a process called “low temperature ashing,” in

which the sample is exposed to high-frequency radiation in

a high-oxygen atmosphere, permits the organic matter to be

driven off while preserving most of the mineral matter.

Nevertheless, some of the very volatile inorganic elements

(such as chlorine, mercury, or selenium) may be lost in this

process and are best measured in whole-coal samples by Xray fluorescence or other techniques. Most microscopic and

microprobe techniques require whole-coal samples because

the objective is to determine the interrelations between the

constituent parts of the coal.

ORGANIC GEOCHEMISTRY

Because coal is principally an organic substance,

organic geochemistry is an important factor in the study of

the origin, structure, quality, and utilization of coal.

Determining the organic matter in coal is necessary for

studying (1) the organic structure and the organic sulfur

compounds in coal, (2) modern peat-forming swamps as

analogues of ancient peat-forming swamps, (3) the potential

for coal gasification and liquefaction, and (4) the effects of

coal mining and coal use on surface water, ground water,

and air quality.

Recent advances in analytical instrumentation have

greatly aided organic geochemical studies of coal. Most

coals consist of a mechanical mixture of two principal

organic components: (1) resin-like (aliphatic) organic matter of uncertain origin (that is, the specific plant structures

have been destroyed during the coalification process), and

(2) benzene-like (aromatic) organic matter derived from

wood tissues (lignin). Using solid-state, 13C nuclear-magnetic-resonance (NMR) spectroscopy, coals now are routinely classified based on their aromatic content.

Other commonly used analytical techniques such as gas

chromatography/mass spectrometry, the electron microscope, and the scanning electron microscope are used to

study the organic sulfur concentration and distribution in

coals. So far, these studies seem to indicate that most of thesulfur in organic compounds in coal is present at the edges

of the organic molecules. This finding may aid the development of effective desulfurization procedures for coal containing large amounts of organically bound sulfur.

The energy produced and the reactivity of coal during

burning is determined by its chemical and physical strucDETERMINING THE QUALITY OF COAL

Figure 28. Photograph of a coal scientist at work identifying coal

macerals and rank in a polished sample of coal using a high-powered, reflected-light microscope. Photograph by R.W. Stanton

(U.S. Geological Survey).

Organic geochemical studies of

modern peat attempt to relate

processes occurring in modern

environments to those that occurred

in ancient peat swamps.
32 COAL—A COMPLEX NATURAL RESOURCE

ture. Through computer-assisted molecular design, scientists can now construct two- and three-dimensional models

of organic compounds in coal and determine the arrangements of the elements in various model structures.

Organic geochemical studies of modern peat deposits

attempt to relate processes occurring in modern environments

to those that occurred in ancient peat swamps. Such studies

serve as a mechanism for a better understanding of and an

ability to predict coal quality by providing insight into the

early stages of coal formation; for example, a geochemical

analysis of an ombrogenous peat, which is easy to sample,

greatly enables the ability to predict the quality of other coal

deposits that originated as ombrogenous peat deposits but

that are much more difficult and costly to sample.

MINERAL-MATTER CHEMISTRY

Studies of mineral matter in coal are performed by a

variety of analytical techniques. The principal techniques

are analytical chemistry, X-ray spectroscopy, scanningelectron microscopy, petrography, and mineralogical studies (table 2). Two newer techniques, (1) inductively coupled

plasma, atomic-emission spectrometry (ICP-AES); and (2)

inductively coupled plasma, mass spectrometry (ICP-MS),

have been applied to coal analysis with excellent results and

are worth a brief description. Using these techniques, a sample of coal ash is first decomposed with various acids and

put into solution. The solution is then aspirated into a flame

(plasma), which energizes the atoms of the different elements present in the solution to generate characteristic

wavelengths of radiation. The spectrum produced by the

radiation is then measured in a spectrometer. The data from

the spectrometer are fed into a computer where the results

are compared to standard samples and the concentration of

each element in the sample is calculated. The two ICP techniques simultaneously and quantitatively analyze about 50

different inorganic elements, which is a large increase in the

efficiency of coal analysis. The ASTM recently has adopted

ICP-AES and ICP-MS techniques as industry standard

methods for analyzing coals for inorganic elements (ASTM

International, 2002).

Microscopic studies of coal petrography are used to

identify the constituents in coal and how they are arranged

and to provide knowledge of the maceral composition and

distribution. Coal petrographers may assess the rank of a

coal by measuring the intensity of light reflected from the

vitrinite macerals in a polished surface of a coal sample; the

higher the reflectivity, the higher the rank. In some

instances, however, petrographic study is not adequate to

characterize all of the coal constituents. For example, if

mineral matter, such as pyrite, exists as extremely small discrete grains, or if it is located inside plant cells (which are

also extremely small), then identification becomes difficult

with an optical microscope. In these cases, a scanning-electron microscope is used to study minute particles of mineral material. Figure 14 is a photomicrograph taken with a

scanning-electron microscope of plant cells in coal containing minute grains of pyrite. In this case, chemical and mineralogic analyses had already determined the presence of

pyrite, but the photomicrograph allows the pyrite to be studied in greater detail. This information could be used to

determine whether the coal must be ground into a powder

that is fine enough to liberate such fine-grained pyrite (an

expensive process).

CONCLUSIONS

Although much is known about coal quality, much also

remains to be learned. To this end, research on coal quality

is being conducted in both the public and private sectors.

Current studies encompass all aspects of the origin, burial

history, and composition of coal in order to understand the

various factors involved in determining coal quality, such as

how and where to find the right coal for a specific use and

how to clean coal to make its use more environmentally and

industrially acceptable.

In the past, the only consideration given to coal quality

was whether coal lumps held together during shipping

burned easily to produce a hot fire and didn’t produce too

much ash. Today, however, concerns about human health,

the environment, energy demands, and the supply of certain

raw materials have broadened our concept of coal quality

and our need to know much more about the origins, nature,

and effects of coal quality. An extensive evaluation of coal

quality must be central to coal use. Whatever coal is used for,

whether for the direct production of energy, chemicals, synthetic fuels, or the recovery of useful minerals, and whatever problems and solutions accompany that use depend on the

quality of the coal. Additional, sophisticated, multidisciplinary research on coal quality is required for the future. The

results will help make it possible for society to continue to

benefit from the good qualities of coal and to make progress

in avoiding the undesirable affects of coal use.

In the near term, the most immediate advancements

related to coal-quality research appear to be improvements

in the efficiency of coal use (including more efficient production of synthetic fuels) and the development of methods

to best understand, and thus control, the less desirable components in coal that have the potential to contribute to environmental degradation and adverse health effects. In the

longer term, the recovery of useful substances from coal

(such as iron, silver, sulfur, and zinc, as well as organic

chemicals, gypsum, and ash) should be an exciting area for

research related to coal quality.
COAL QUALITY AND PUBLIC HEALTH 33

COAL QUALITY AND PUBLIC HEALTH

By Robert B. Finkelman

Prior to the widespread adoption of selective mining

procedures and pollution-control technology, human health

had been affected by sulfur and particulates emitted from

coal combustion. This section describes several examples of

how organic compounds and trace elements are still affecting the health of people in Europe, Asia, and America.

The inorganic (and in some cases, organic) constituents

in coal may have profound effects on the health of those people burning coal in their homes or living near coal deposits,

coal mines, or coal-burning power plants. Trace elements,

such as arsenic (As), emitted from coal-burning power plants

in Europe and Asia have been shown to cause severe health

problems. Water leaching of organic compounds from lignites

may be the cause of a kidney disease that has taken the lives

of more than 100,000 people in the Balkans. Widespread

health problems may be caused by domestic coal combustion

in developing countries. Millions of people suffer from fluorosis, thousands from arsenism. In the southwestern U.S.,

many members of the Navajo Nation are dependent on coal as

their primary source of energy. Many Navajo suffer from

chronic respiratory problems that are attributed to emissions

from the burning of coal in their hogans.

A better knowledge of coal-quality parameters may

help to minimize some of these health problems.

Information on the concentrations and distributions of

potentially toxic elements in coal may help us to avoid those

parts of a coal deposit having undesirably high concentrations of these elements.

Information on the modes of occurrence of these elements and the textural relations of the minerals and macerals in which they occur may help us to anticipate the behavior of the potentially toxic components during coal cleaning, combustion, weathering, and leaching. Coal quality

characterization, therefore, offers coal scientists opportunities to contribute to important societal needs such as

improved public health.

POLYCYCLIC AROMATIC HYDROCARBONS (PAHS)

BALKAN ENDEMIC NEPHROPATHY

Balkan endemic nephropathy (BEN) is a potentially

fatal degenerative kidney disease common in parts of former Yugoslavia, Bulgaria, and Romania. An article in a popular science magazine attributed more than 100,000 deaths

to BEN since it was recognized as a distinct disease in 1956

(Zimmerman, 1983). Intensive medical research had failed

to identify the cause of BEN or to explain why it occurs in

clusters of villages along river valleys.

In the early 1990s, U.S. Geological Survey (USGS)

researchers identified a link between the occurrence of BEN

and the distribution of Pliocene lignites, which are the

youngest, most chemically reactive coals in the Balkans

region (Feder and others, 1991). They suggested that BEN

is attributable to ingestion of organic compounds from well

water that had been in contact with Pliocene lignites.

Ground water in the endemic villages was found to be highly concentrated in organic chemicals such as PAHs. They

also suggested that the hydrocarbons, some of which were

known carcinogens, were the cause of BEN. Further evidence was found from laboratory experiments. Using distilled water to leach samples of Pliocene lignites from

Yugoslavia, USGS researchers extracted a suite of PAHs

similar to that found in the well water of the endemic villages (Orem and others, 1999).

BEN also may exist in Turkey and other countries.

Turkey has extensive near-surface deposits of Pliocene lignites and many Turkish villagers rely solely on well water

for their drinking needs. If these wells are drilled into the

Pliocene lignite aquifers or into aquifers in contact with the

lignites, BEN could result from drinking the water.

LUNG CANCER

PAHs released during unvented coal combustion in

homes in Xuan Wei County, China, have been cited as the

primary cause for the highly elevated incidence of lung cancer there. The PAH levels in the homes burning "smokey"

coal are at industrial levels resulting in a lung cancer mortality rate five times the national average of China

(Mumford and others, 1987).

TRACE ELEMENTS

Concerns have been expressed in the United States and

elsewhere over the release of As, Be, Cd, Cl, Co, Cr, F, Hg,

Mn, Ni, Pb, Sb, Se, and U during the burning of coal.

Recent data from the U.S. Environmental Protection

Agency (EPA) (1996) indicate that trace-element emissions

Information on the concentrations

and distributions of potentially

toxic elements in coal may help

us to avoid those parts of a coal

deposit that have undesirably high

concentrations of those elements.
34 COAL—A COMPLEX NATURAL RESOURCE

from coal-burning power plants that use coal with relatively low to moderate trace-element concentrations, or that

have efficient pollution control devices may not present a

significant health risk. Their data indicate that the risk of

cancer from trace elements inhaled by humans who live or

work near power plants is less than 1 in 1 million.

Elsewhere in the world, trace elements emitted from

coal-burning power plants have been shown to cause severe

health problems. In Czechoslovakia, children living in the

vicinity of a power plant that burned high-arsenic lignite

(about 1,000 parts per million (ppm) on a dry basis) suffered

significant hearing loss that was attributed to arsenic poisoning from the power-plant emissions (Bencko and Symon,

1977). Perhaps the most widespread health problems are

caused by domestic coal combustion in developing countries. Millions of people suffer from fluorosis and thousands

suffer from arsenosis caused by coal combustion in China.

Selenium and mercury poisoning also have been attributed

to domestic coal combustion in China. Adsorption of fluorine by corn dried over high-fluorine (>200 ppm) coal fires

is the probable cause of extensive dental and skeletal fluorosis in southwest China. The problem is compounded by the

use of clay as a binder for making briquettes; that clay typically is a high-fluorine (about 900 ppm) residue from the

intense leaching of the limestone substrate.

In Guizhou Province, China, the practice of drying

chili peppers over high-arsenic coal fires has led to thousands of cases of arsenic poisoning (Finkelman and others,

1999). The contaminated chili peppers commonly are used

for cooking and thus become the principal source of arsenic

poisoning. One coal sample from this region analyzed at the

U.S. Geological Survey laboratory contained 35,000 ppm

arsenic on an as-received basis! Chili peppers dried over the

high-arsenic coal fires contain more than 500 ppm arsenic

(fresh chili peppers contain less than 1 ppm arsenic).

Arsenosis, which results from ingestion of the chili peppers

tainted with arsenic from the mineralized coal, also has led

to severe skin cancers in the affected population (Finkelman

and others, 1999).

Nearly 500 cases of human selenosis in southwest

China have been attributed to the use of selenium-rich, carbonaceous shales known locally as “stone coal” (Zheng,

1992). The stone coals contain as much as 8,390 ppm selenium. The selenosis is attributed to the practice of using the

combustion ash as a soil amendment. This introduces large

amounts of selenium to the soil and results in selenium

uptake by the crops.

Coal combustion is considered to be the largest manmade source of mercury in the environment. Although there

are no confirmed reports that mercury from emissions

directly impacts human health, mercury may contribute to

health problems through bioaccumulation in the food chain.

Mercury, a portion of which is derived from coal combustion, is deposited in lakes and rivers and can be converted

to toxic methylmercury, which then accumulates and

becomes enriched in fish and birds. Cases of mercury poisoning have been documented in people who eat contaminated fish for prolonged periods, both in the United States

and abroad. Pregnant women and subsistence fishermen are

particularly vulnerable.

A rare occurrence of chronic thallium poisoning was

reported from Guizhou Province, China (Zhou and Liu,

1985). The source of the thallium poisoning appears to be

from vegetables grown in soil developed over a mercurythallium-rich mining slag. Most symptoms, such as hair

loss, are typical of thallium poisoning; however, loss of

vision in several patients was considered to be unique.

Mineralogical analysis of coal used in the homes of the

patients with visual impairment revealed abundant mercury

minerals. Chemical analysis of a sample of coal being used

in Guizhou Province, China, indicated the mercury concentration to be 55 ppm (Finkelman, 1999), about 200 times the

average mercury concentration found in U.S. coals.

SUMMARY

Coal scientists and technologists are ideally positioned

to help minimize health problems due to coal combustion

by providing information on coal quality. Information on

the concentrations and distributions of potentially toxic elements in coal may assist people dependent on local coal

sources to avoid those areas of a coal deposit having undesirably high concentrations of toxic compounds.

Information on the modes of occurrence of potentially toxic

elements and the textural relations of the minerals and macerals in which they occur may help us to anticipate the

behavior of the potentially toxic components during coal

cleaning, combustion, weathering, and leaching.

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central and eastern United States, in Gluskoter, H.J., Rice,

D.D., and Taylor, R.B., eds., Economic geology, U.S.: Denver,

Colo., Geological Society of America, The Geology of North

America, v. P–2, p. 523–546.

Flores, R.M., and Cross, A.T., 1991, Cretaceous and Tertiary coals

of the Rocky Mountains and Great Plains regions, in

Gluskoter, H.J., Rice, D.D., and Taylor, R.B., eds., Economic

geology, U.S.: Denver, Colo., Geological Society of America,

The Geology of North America v. P–2, p. 547–571.

Flores, R.M., Ochs, A.M., Stricker, G.D., Roberts, S.B., Ellis,

M.S., Keighin, C.W., Murphy, E.C., Cavaroc, V.V., Jr.,

Johnson, R.C., and Wilde, E.M., 1999, National coal

resources assessment non-proprietary data; location, stratigraphy, and coal quality of selected Tertiary coals in the Northern

Rocky Mountains and Great Plains region: U.S. Geological

Survey Open-File Report 99–376, 1 CD-ROM.

Osmonson, L.M., Rohrbacher, T.J., Molnia, C.L., and Sullivan,

G.L., 2000, Coal recoverability in the Hilight quadrangle,

Powder River Basin, Wyoming; a prototype study in a western coal field: U.S. Geological Survey Open-File Report

00–103, 23 p.

Rohrbacher, T.J., Teeters, D.D., Sullivan, G.L., and Osmonson,

L.M., 1993, Coal resource recoverability; a methodology:

U.S. Bureau of Mines Information Circular 9368, 48 p.

Schultz, J.E., and Eakins, Wynn, 2000, Availability of coal

resources in Colorado; Somerset coal field, west-central

Colorado: Colorado Geological Survey Resource Series 38,

84 p.

Tabet, D.E., Quick, J.C., Hucka, B.P., and Hanson, J.A., 1999, The

available coal resources for nine 7.5-minute quadrangles in

the northern Wasatch Plateau coalfield, Carbon and Emery

Counties, Utah: Utah Geological Survey Circular 100, 46 p.

Treworgy, C.G., Korose, C.P., Chenoweth, C.A., and North, D.L.,

1999, Availability of the Springfield coal for mining in

Illinois: Illinois Minerals 118, 43 p.

U.S. Geological Survey, 1996, Assessing the coal resources of the

United States: U.S. Geological Survey Fact Sheet FS–157–96,

available on the web at http://energy.usgs.gov/factsheets/

nca/nca.html. (Accessed February 11, 2003).

———2000, Federally owned coal and Federal lands in the

Northern Rocky Mountains and Great Plains region: U.S.

Geological Survey Fact Sheet FS–011–00, 6 p. (also available

on the web at http://pubs.usgs.gov/fs/fs-0011-00/. Accessed

February 11, 2003).

Wood, G.H., Jr., and Bour, W.V., III, 1988, Coal map of North

America: Reston, Va., U.S. Geological Survey Special Map,

scale 1:5,000,000.

UTILIZATION

American Coal Ash Association, 1995, Fly ash facts for highway

engineers: American Coal Ash Association Report

FHWA–SA–94–081, 70 p.

Attanasi, Emil, 2000, Forces shaping future U.S. coal production

and use: U.S. Geological Survey Fact Sheet FS-158-00.

Damberger, H.H., Harvey, R.D., Ruch, R.R., and Thomas,

Josephus, Jr., 1984, Coal characterization, in Cooper, B.R.,

and Ellingson, W.A., eds., The science and technology of coal

and coal utilization: New York, Plenum Press, p. 7–45.

Elliott, M.A., ed., 1981, Chemistry of coal utilization, second supplementary volume: New York, Wiley-Interscience, 2,374 p.

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Association and Coal Exporters Association of the United

States, 114 p.

Stricker, G.A., 1991, Economic Alaskan coal deposits, in

Gluskoter, H.J., Rice, D.D., and Taylor, R.B., eds., Economic

geology, U.S.: Denver, Colo., Geological Society of America,

The Geology of North America, v. P–2, p. 591–602.

HEALTH EFFECTS

Chu, P., and Porcella, D.B., 1995, Mercury stack emissions from

U.S. electric utility power plants, in Arcella, D.B., Huckabee,

J.W., and Wheatly, Brian, eds., Mercury as a global pollutant:

Water, Air, and Soil Pollution, v. 80, no. 1–4, p. 135–144.

Swaine, D.J., and Goodarzi, F., 1995, Environmental aspects of

trace elements in coal: Dordrecht, The Netherlands, Kluwer,

312 p.

Wilson, R., Colome, S.D., Spengler, J.D., and Wilson, D.G., 1980,

Health effects of fossil fuel burning: Cambridge, Mass.,

Ballinger Publishing Co., 392 p.

39

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APPENDIX 1

FEDERAL AGENCIES WITH REGULATORY OR RESEARCH RESPONSIBILITIES FOR

COAL AND COAL QUALITY
U.S. GEOLOGICAL SURVEY CIRCULAR 1143

Schweinfurth—COAL—A COMPLEX NATURAL RESOURCE U.S. Geological Survey Circular 1143