2
Coal Deposits
and Properties
Coal Deposits
Several terms are used by the U.S. Geological Survey to quantify coal deposits. "Total resources" are surmised to exist based on a broad interpretation of
geological knowledge and theory. 1-4 Coals in thin as well as thick seams that occur in unmapped and unexplored areas to depths of 6,000 feet are included in
total resources.
"Identified resources" are those specific bodies of coal whose location, rank,
quality, and quantity are known from geologic evidence supported by engineering measurements. Included are beds of bituminous coal and anthracite 14 inches
or more thick and beds of subbituminous coal and lignite 30 inches or more thick
that occur at depths to 6,000 feet and whose existence and quantity have been
delineated within specified degrees of geologic assurance as measured, indicated, or inferred. Also included in this category are thinner and/or deeper beds
that presently are being mined or for which there is evidence that they are commercially minable. Identified resources are about one-half of the total resources.
The "demonstrated reserve base" (DRB) is a more restrictive classification
and represents 100% of coal in place (measured and indicated) as of a certain
date (see Table 2-1). DRB includes beds of bituminous coal and anthracite 28
inches or more thick and beds of subbituminous coal 60 inches or more thick that
occur at depths to 1,000 feet, beds of lignite 60 inches or more thick that can be
surface-mined, and thinner and/or deeper beds that presently are being mined or
for which there is evidence that they could be mined commercially at this time.
13
10.7
-- -- -- - 127.7
- -
-
366.1
17.5
4.4
697.5
10.7
17.5
21,141.9
39,984.9
6,501.6
13,098.2
6,077.2
12,693.5
67,705.0
4,541.5
10,621.1
2,198.6
4,482.0
1,644.0
994.8
411.0
289.0
14.0
25.5
96.4
25.7 67,705.0
424.3
3.73994.8
4.4
424.3
.7
8.1
1.1
255.3
366.1
997.7
997.7
826.2
127.7
826.2
Total-Bituminous
Subbituminous
125.5
13,098.2
Anthracite
474,556.2
241,874.9
42,892.9
65,532.0
182,443.0
7,345.4
70,014.0
2,198.6
10,621.1
1,644.0
6,155.5
16,214.7
9,971.2
9,106.3
2,965.7
5,444.0
19,034.719,034.7
6,077.2
1,580.2
120,468.8
6,759.8
1,842.6
12,693.5
3,412.4
103,317.8
15,765.7
1,316.8
24,117.3
,696.6
2.3
1,083.0
5,676.8
6,500.5
1,385.4
3,537.9
21,141.9
23,332.1
7,095.7
30,427.9
Coal
Processing
and
Pollution
Control
39,984.9
Lignite
14
ve Base*(million
of Coalshort
in thetons)1
United
States
on
January 1, 1979, by Rank
9,971.2
Table 2-1
* Includes those parts of the measured and indicated resource categories as defined by the EIA and
represents 100 percent of the coal in place.
t Excludes coal-bearing states in which either the resources are not currently economically recoverable or the publicly available resource data do not provide the detail required for DRB delineation.
** Data may not add to totals shown due to rounding.
tt Data shown not completely
reconciled with DRB assessment
by state.
Coal Deposits and Properties
-Principal
deposits
in Pennsylvania;
and Virginia. Anthracite
small
- meta-anthracite
Figure 2-1. Coal fields of the United States.
Alaska.)
deposits
in Arkansas,
Colorado.
deposits in Massachusetts
New
15
Me:o:ico.
and Rhode Island
(Coal rank not distinguished
in
The United States has major reserves of all ranks of coal: lignite, subbituminous, bituminous, and anthracite; their geographical occurrence is shown in Figure 2-1. Table 2-1 is a summary of demonstrated U. S. coal reserves classified by
state and by grade.l The demonstrated reserve base (DRB) given in Table 2-1 is
actually less than 10% of the total resources of coal that are believed to exist in
the United States. The DRB does not include deep coals that could be recovered
by evolving technologies, such as in situ gasification; this could expand the DRB
significantly. 5
It is interesting to compare the U. S. total resources of coal with those deposits
in other countries. Table 2-2 gives total resources as well as projected future production levels for major coal-producing countries in the world.3,4 Note that the
reserve figure for the United States differs from that given in Table 2-1, due to
the time of the estimate and different sources of data. It is clear that China, the
USSR, and the United States all have dominant positions relative to coal resources, both inferred and recoverable. Data on properties of coals in different
parts of the world have been presented by Singer. 6
Anthracite coal is the highest rank of coal, having the highest carbon content.
Anthracite reserves are approximately 7 billion tons (all reference to tonnage is
in "short" tons as opposed to metric tons) in the United States and are largely
concentrated in the state of Pennsylvania. Bituminous coal, with reserves of 242
16
Coal Processing
and Pollution Control
Table 2-2
Coal Deposits and Production
Reserves
49,560
183,866
27,533
61,356
1977-2000
108,902
4,672
15,419
113
1,982
730,102
22,026
4,625
,434
Resources
47,357
13,686
65,639
37,906
121,035
4,295
3,6345,352,435
19,823
3,3042,830,403
209,252
36,123
11,839,029
1,583,753
660,795
355,767
252,384
3,414
7,379
271,807
153,910
89,228
79,295
*
for the World in Million Tons of Coal*
Production
Cumulative
Notes: I. Data is reported in million short tons (2000 Ib = I ton).
2. Data from World Coal Study and World Energy Conference.3
billion tons, constitutes the largest contributor to total U. S. coal reserves. Relatively high-grade bituminous deposits are found in the Appalachian regions of
Kentucky, Ohio, Pennsylvania, and West Virginia. Significant bituminous coal
deposits also occur in Colorado, and in the midwestern states of Indiana, Illinois,
and Missouri. Bituminous deposits of lesser magnitude are located in the states
of Alaska, Kansas, New Mexico, Utah, and Wyoming.
Subbituminous coal deposits are of a lower rank than bituminous, but of a
higher heating value than lignite. Significant quantities of subbituminous coal
are found in the Rocky Mountain regions of Colorado, Montana, New Mexico,
Utah, and Wyoming, usually in very thick seams, ranging over 100 feet in some
cases. Total estimated U.S. subbituminous deposits are approximately 182 billion tons.
Lignite is a low-rank coal. The two major deposits of lignite in the United
States are in the Northern Great Plains and Gulf Coast regions, with limited deposits elsewhere. North Dakota and Montana contain large deposits of lignite.
Other deposits occur in Texas and to a lesser extent in Arkansas, Louisiana, and
Mississippi. Total tonnage is about 43 billion tons.
The U.S. coal deposits can be classified by rank of coal, by method of mining,
by heating value, and by sulfur content, as shown in Figure 2-2. More than twothirds of the total demonstrated coal reserves in the United States must be extracted by underground mining, because they occur at depths greater than 300
feet below the surface. 1.2 Most of the underground coal reserves are in the eastern and midwestern United States, while a major portion of the coal reserves that
Coal Deposits and Properties
17
2%
9%
By Rank
By Method of Mining
3%
By Sulfur Content
Figure 2-2. Classification
By Geographical
Distribution
of coal reserves.
could be extracted by surface mining occur in the Great Plains or Rocky Mountain regions of the western United States.
Figure 2-2 also indicates that more than half of the U.S. coal reserves are bituminous. Geographical distribution of coal deposits (Figure 2-2) indicates that
approximately 17% of the total U.S. reserves are found in the Appalachian region and 25% in the interior region. Approximately 50% of the nation's total
demonstrated coal reserves are found in the western region, which indicates the
increasing importance of western coal reserves.
The five most important factors affecting the selection of coal reserves for production are
I. The transportation cost.
2. The heating value of the coal.
18
Coal Processing
and Pollution Control
3. The depth of the coal deposits beneath the surface and nature of overburden.
4. Seam thickness and continuity.
5. The sulfur content of the fuel.
The transportation cost is largely a matter of the distance and the mode selected;
these issues are discussed in Chapter 4. The heating value determines energy
content and is essentially a function of the rank of coal.
The depth of coal deposits beneath the surface and nature of the overburden
determine the mining method employed. Usually underground mining is used -with overburdens greater than 250 to 300 feet. Surface mining is used for shallower deposits. The seam thickness and continuity influence the specific mining
practices as well as the resource recovery attainable for a given deposit. More
details on extraction methods and their application to U.S. coal reserves are
given in Chapter 3.
Sulfur content is important in determining air pollution potential and ultimate
environmental acceptability. It determines which desulfurization techniques can
be employed in order to use coal in an environmentally acceptable way. In the
early 1970s certain coals were classified as "low sulfur" coals, meaning that
they could be burned in a boiler without flue gas desulfurization. However, the
SOl standards have been tightened during the past ten years so that all coals now
processed must be subjected to sulfur removal. The higher sulfur content coals
tend to be found in certain Appalachian states such as West Virginia, Ohio, and
western Kentucky plus the interior states of Illinois, Indiana and Missouri. Lowsulfur coal reserves tend to occur in some areas of the eastern United States (eastern Kentucky and southern West Virginia), but mainly in the western United
States. The sulfur content of most U.S. coals ranges between 1% and 3%, giving
an average of approximately 1.8% by weight.
Coal Ownership
Coal ownership is an important aspect of the utilization of coal reserves. A
detailed estimate of coal ownership by type of company in the United States has
been made by Schmidt. 4 This reference indicates the distribution of coal reserves
with respect to ownership: public lands, oil (energy) companies, coal companies,
and railroads.
Total defined coal reserves by owner in the United States are approximately
300 billion tons as shown in Table 2-3. Of this total, approximately 187 billion
tons are owned by the federal government, primarily in the western United
States. Most of the federal coal ownership is in the states of Utah, Wyoming,
Montana, Colorado, New Mexico, and Arizona. Significant federal action will
be necessary in order to develop these coal deposits in the future.
Coal Deposits and Properties
19
Table 2-3
Ownership
of Coal Deposits by Industry in the United States4
Industry
Classification
Oil Companies
Railroads
Coal Companies
Mining Companies
Electric Utilities
Steel Companies
Other Groups
Total
Federal Government
Number of
Companies
Coal Reserve
106 Tons
20
46,289
24,133
16,970
15,190
7,540
5,680
5,360
10
138
9
16
7
42
242
121,162
106 Tons
Company
2.314
2,413
122
1,688
471
811
128
501
Percent of
Total
38.2
19.9
14.0
12.5
6.2
4.7
4.5
100.0
186,855
The two largest industrial holders of coal reserves in the United States are not
coal companies, but oil companies and railroads. A significant portion of the
nation's proven coal reserves are owned by 20 different oil companies, for a total
of 46 billion tons of coal. The average holding per oil company in this industrial
segment is 2.3 billion tons per company.4 Consolidation Coal Co., a wholly
owned subsidiary of Continental Oil Co., is the nation's largest single holder of
coal, with reserves of 13.3 billion tons. Exxon is the second largest owner of
coal from among the oil company group. Peabody Coal Company is the largest
of the coal companies with a total reserve base of 9.0 billion tons. Burlington
Northern Railroad owns almost half of the railroad-owned coal reserves with
11.4 billion tons, or 47.2 % of the total. An additional 41 % of the railroad coal is
owned by the Rocky Mountain Energy Company, a wholly owned subsidiary of
the Union Pacific Railroad Corporation, with a total coal holding of 10.0 billion
tons. These two large western railroads control more than 24.4 billion tons of
coal, or nearly 90% of the total coal holdings owned by railroads.
Coal Properties
Coal has its origin in fresh water swamps, analogous to the modern-day peat
bog. Peat is a porous brown mass of organic matter (e.g., remains of roots,
leaves, twigs, and trunks) in which the plant material is clearly recognizable.7
Peat accumulation requires conditions which have a low level of bacterial activity, slowing the rate of decomposition, namely
I. Abundant flora.
2. Excess moisture or high water table.
3. Low temperature (70° -120°F).
-----------------.--------- -.--===============~----------------_._---_._-----
20
Coal Processing
and Pollution Control
4. Low concentrations of carbon, phosphorus, and nitrogen with partially oxidizing conditions.
5. Very high or very low pH (optimum pH for metabolic processes is 6.5 to
7.5).
If the rate of decomposition of organic material is too rapid, peat will not accumulate. Peat accumulation usually occurs during periods of active tectonism or
mountain building, leading to rapid burial of peat. The geologic periods exhibiting these characteristics-the
Upper Carboniferous, Permian, Upper Cretaceous, and Tertiary-account
for 94% of coal formed. Eastern U.S. coals were
formed during the Carboniferous period (300 million years ago), while most reserves of western coals have their origin in the Tertiary period (60 million years
ago), with the rest arising from the Cretaceous period (160 million years ago).
The coalification process begins with the biochemical decay of dead organic
material, which produces peat. This first stage of coalification, called diagenesis, involves attack by aerobic as well as anaerobic microorganisms of the plant
material (cellulose, hemi-cellulose, lignin, fats, waxes, resins, and proteins).
Microbial activity continues until the decaying mass becomes too acidic to support fungi and bacteria (growth ceases at pH of 2 to 3). Sulfur appears in coal due
to the presence of sulfate ion; sulfate ion is reduced to sulfide by bacteria and
combined with ferrous iron to form pyrites.
Metamorphism is the second stage of coalification. Pressure and temperature
along with time are the major factors controlling this step. Metamorphism is associated with a decrease in water content, loss of oxygen, and an enrichment of
carbon, which also increases the calorific value of coal. Burial by succeeding
layers of sediments subjects the peat to increased pressure, with heat provided by
the natural geothermal gradient. A series of chemical reactions occurs during
metamorphism, mainly stripping and condensation, where the oxygen and hydrogen contents are decreased due to heat addition. The condensation reactions
eliminate functional groups such as -OH, -COOH, -OCH3,
and -CH3 from
the coal matrix.
Peat will form lignite when exposed to slightly greater temperatures and pressures than found at the surface of the earth. If lignite, which is a very soft coallike material, is exposed to a greater depth of burial or a higher temperature, it
may metamorphose into bituminous, or soft coal, and eventually into anthracite,
or hard coal, and graphite. "Rank" represents the degree of progressive alteration that the organic matter has undergone from lignite to anthracite.
Coal Petrography
Coals can be classified according to the origin and nature of the plant material.
NeaveF has called coal a "fruitcake" because of its diverse ingredients. Microscopic (petrographic) study of the visible features of coal is the basis of coal pe-
Coal Deposits and Properties
21
Table 2-4
Coal Macerals and Maceral Groups9
Maceral Group
Maceral
Composed of or Derived from
Vitrinite
Collinite
Tellinite
Humic gels
Wood, bark, and cortical tissue
Liptinite/Exinite
Sporinite
Cutinite
Resinite
Spores
Leaf cuticles
Resin bodies and waxes
Alginite
Micrinite
Macrinite
Algal remains
Unspecified detrital matter, < 10. fLm
Similar, but 10-100 fLm grains
Semifusinite
Fusinite
Sclerotinite
Fungal sclerotia and mycelia
Inertinite
"Carbonized"
woody tissue
trography;7.8 polished coal specimens are examined mainly in reflected or transmitted light. Petrographic components are called macerals, which are to coal
what minerals are to rocks. Macerals are divided into three groups: vitrinite, liptinite (exinite), and inertinite. These macerals are identified under a microscope
by using incident light comprised of a plane-polarized beam that is monochromatized. Based on the refractive index and absorptive index at different locations
on the specimen, a property called reflectance can be computed which determines the rank of the coal.7
Vitrinite is the commonly occurring maceral, arising from gels, wood, and
bark. Vitrinite appears translucent by transmitted light and gray by reflected
light. In reflected light, liptinite is dark gray and inertinite is white. Table 2-4
summarizes the coal maceral groups and their origins.9 The amounts of each
group are found by heating the coal with an inert gas. Inertinite includes the inert
infusible diluents in coal, while the first two components are subjected to chemical change. Liptinite is composed of bitumen, exinite, and resinite. A typical
vitrinite fraction for U.S. coals is 70% to 80%, while inertinite is on the order of
10% to 15%. The exinite fraction is usually very low.
Chemical Structure of Coal
Structure at the atomic level of coal is difficult to investigate because coal is
insoluble in common solvents and is generally non-crystalline and nearly
opaque. These properties make analysis of structure via either spectroscopic or
wet chemical methods very difficult. Coal has no repetitive structure, but rather
is a mixture of macromolecules. Each seam and each sample is different in its
composition, structure, and properties. The macromolecular entities in coal are
probably composed of sub-units, some of which may themselves be released
22
Coal Processing
and Pollution Control
more or less intact upon mild dissolution or by other chemical treatments which
only affect the weakest bonds. Several investigators have attempted to characterize coal in terms of a statistical average, including
1. Molecular size distribution.
2. Sizes and types of sub-units (including such features as their aromaticity
and chemical functionalities).
3. Degree and types of crosslinking between sub-units.
Coal is presently believed to be a highly cross-linked amorphous copolymer with
weak links (usually aliphatic) between stable cluster units (average molecular
weight of a cluster, composed of two to four fused rings, ranges from about 200
to 500). However, the actual structure varies with rank.7.10.11
Most of the hydrogen contained in coal is aliphatic, with relatively few methyl
groups. The degree of carbon aromaticity ranges from 60% (lignite) to 80% (bituminous), depending on coal rank. The molecular weight of coal probably
ranges in the low thousands. Oxygen is mainly contained in a phenolic (-OH)
form, but also with some ethers, carboxyl, and carbonyl groups. Sulfur functionality is similar to oxygen, as thiols (-SH), ethers, or thiophenes. Nitrogen
appears mainly as pyrrole and pyridines. Several molecular models have been
proposed, notably the Given and Wiser models. 10,11The Wiser model for the coal
molecule is depicted in Figure 2-3, Organic structural aspects of coal are very
important in understanding coal liquefaction and pyrolysis behavior (see Chapters 6 and 8); however, coal structure is less important in coal gasification and
combustion because of the severe chemical transformations involved in these latter two processes.
Ultimate and Proximate Analyses
It is customary to report the components of coal using two different analyses:
"ultimate" and "proximate." The ultimate analysis from a dried sample is defined as the chemical determination of carbon, hydrogen, sulfur, nitrogen, and
ash found via complete combustion of the coal; oxygen, the remaining component, is normally estimated by difference (although other methods to avoid indirect measurement are used on an experimental basis). Table 2-5 gives representative values for the ultimate analysis for a range of coal ranks,9 In addition, an
elemental analysis of the ash can also be performed, and the heating value for
coal is obtained using a bomb calorimeter (ASTM D 2015), Usually the higher
heating value of coal (determined by assuming water in the combustion products
is in the liquid state) is reported6,12 in the U.S. (Europe normally uses the lower
heating value,)
Coal Deposits and Properties
23
H
I
o
H
H
H
H-C-H
-++
-.+
H-C-H
H-C-H
'-
S
1
H
H
Figure 2-3. Wiser molecular model for coal.
Table 2-5
Variation of Chemical Composition
II
3
21
24
50
5
63Oxygen
71
74
35
Carbon
44
94
84
100
59
Hydrogen
* Values are representative
with Coal Rank9
Mass Percent"
averages. dry basis.
t Not a coal.
The ultimate analysis can be used to predict an approximate heating value of
the coal using the rule of Dulong and Petit, 12 which is
HHV
=
14,600C+62,OOO
(H-O/8)+4,050
S
-------------~_._._._._._._-----------_._---------------------------._------.--~--.-._-_.
-,-
__
24
Coal Processing
and Pollution Control
where C, H, 0, S express the weight fraction of one pound of the major atomic
components of coal. 12 In this equation oxygen is assumed to be combined with
hydrogen as water, which is only an approximation. Other correlations found to
be more accurate than the above equation for certain types of coals have been
reviewed by Ergun 13 and Ringen et al. 14 Errors in measuring heating values can
arise from the lack of a representative sample.
The proximate analysis is defined as the determination of moisture, volatile
matter, and ash, with fixed carbon determined by difference. The proximate
analysis provides information to identify coal rank; while there have been several
criteria proposed to determine rank,6 the accepted ASTM classification is based
on the proximate analysis and the calorific values on a mineral-matter-free basis.
Table 2-6 summarizes the ASTM D 388 classification of coals by rank and the
calorific value ranges. The use of Table 2-6 does not, however, provide a satisfactory method of categorizing the physical and chemical behavior of a range of
coals; NeaveF has argued for a different classification based on petrography.
Table 2-6 shows that anthracite has a high percentage of fixed carbon and little
volatile matter. It is thus hard and very brittle, dense, and shiny black. Bituminous coals form the largest group of coals in Table 2-6. They derive their name
from the fact that when heated, they often are reduced to a cohesive, binding,
sticky mass. Their higher volatile content makes bituminous coals more attractive for combustion. A distinct granular structure exists for bituminous coals,
which are grayish-black in color, and some of these coals are soft and crumble
easily.
Subbituminous coals are brownish black or black, and generally have a high
moisture content (15% to 30%). When these coals are dried, they tend to break
apart. Lignites, the lowest coal rank, are usually brown and of a laminar structure due to woody fibers. Lignite is usually high in volatile matter and moisture
content and disintegrates rapidly after losing moisture to air. Unconsolidated lignite is also known as "brown coaL" Brown coals are generally found close to the
surface with more than 40% moisture.
The proximate analysis serves as a simple means for evaluating coal characteristics. When the coal is heated, water vapor is first released at the boiling temperature of water; this weight loss is called the moisture content. The second loss
occurs when coals are heated in a covered crucible, which prevents the oxidation
of the carbon residue. This weight loss, called volatile matter, is measured as the
coal is heated in 7 minutes to about 1750°F (950°C). If the remaining residue is
subsequently combusted, the residue left after the combustion is called ash, and
the weight loss on combustion is the fixed carbon.
The proximate analysis of a coal is not a difficult operation, and is described in
ASTM D 2013 and D 3175. Several commercial instruments automatically perform the proximate analysis. The following paragraphs discuss several aspects of
the proximate analysis parameters.
Calorific
Fixed Carbon
Class
I. Anthracitic
I. Subbituminous
II. Bituminous
III. Subbituminous
IV. Lignite
(Dry, MineralMatter-Free
(Moist,
Mineral-MatterFree Basis)
Basis)
Equal or
Greater
Than
-
coal
Limits, Btullb
Group
-781492988669283122
22
31
14
I. Meta-anthracite
2.
3. Subbituminous
Subbituminous
coal
coal
B
2.
5.
High
CBCbituminous
I. Lignite
Lignitevolatile
A
Value
Limits, %
-
Agglomerating
Character
Equal or
Less
Than
Greater
Than
Less
Than
98
92
86
78
69
Nonagglomerating
coal
A coal
13,000
10,500
8,300
6,300
9,500
11,500
6,300
00
C11
:J
14,000
ro
w
~.
"0
-0
roO
N
0
Q.
"0
!!!.
10,500
8,300
9,500
13,000
14,000
11,500
Commonly
Nonagglomerating
Agglomerating
agglomerating
0
(3IJ)
3-en
26
Coal Processing
and Pollution Control
Moisture. Water in coal is held in cracks, crevices, and large pores; however,
studies of water absorption isotherms and absorption swelling experiments revealed that part of the water did not seem to be behaving as free liquid water, but
rather as chemisorbed and/or hydrated water. There is evidence that the rate of
heating, the final temperature of heating, the period of heating at the final temperature, the composition of the surrounding atmosphere, the absolute pressure,
and the particle size, all affect the amount of moisture driven away from a coal
specimen. 13
The ASTM D 2013 procedure calculates three moisture values. First, if the
coal is too wet to crush and sieve without losing moisture, it must be air dried (at
20°-30°F above room temperature), which results in the value of "air-dry moisture." The air-dried coal is then crushed and further dried in a forced-air-circulation-oven at 220°-230°F (l05°-11O°C) to constant weight. The weight lost in
this step is referred to as the "residual moisture."
The "total moisture" is the sum of the two weight losses. In the ASTM Standard Specifications for Classification of Coals by Rank (ASTM D 338), high
volatile bituminous and lower rank coals are classified according to their calorific values on the "moist" basis. "Moist" refers to coal containing its natural
moisture but not including visible water on the surface of the coal. As a means of
estimating the bed moisture of coal, an equilibration method is used which is
believed to restore the dried or wetted coal to essentially its bed moisture condition. The method consists of wetting the coal with water, draining off excess
water, and equilibrating the sample at 86°F (30°C) in a vacuum desiccator containing a saturated solution of potassium sulfate that maintains a relative humidity of 96%. Equilibration requires 48-72 hours, depending on the rank of the
.coal. The equilibrated sample can then be air dried to obtain its air-dry moisture
content and further dried to calculate its residual and total moisture.
Volatile matter. Volatile matter consists of hydrocarbons and other gases that
are obtained by distillation and thermal decomposition of coal. The main constituents of volatile matter include hydrogen, oxygen, carbon monoxide, methane,
water, and hydrocarbons, which may liquefy or solidify at room temperature.
The composition of volatile matter varies greatly for different ranks of coal. As
discussed above, volatile matter is used to establish the rank of coals. Because of
the arbitrary nature of this test, care must be exercised in comparing the results of
volatile analyses based on ASTM D 2013 with those obtained from tests run in
other countries using different standards.
Fixed carbon. The fixed carbon is the combustible residue left after driving off
the volatile matter. It is not completely carbon, and it usually contains small
amounts of hydrogen, oxygen, nitrogen, and sulfur. The fixed carbon in a proximate analysis is calculated by difference using the percentages of moisture, volatile matter, and ash.
Ash. Ash is the noncombustible residue after complete combustion of the coal.
The weight of ash is usually slightly less than that of the mineral matter originally present before burning. During the burning process, various chemical and
Coal Deposits and Properties
27
physical changes in the ash take place. Because the conditions of oxidation determine the number and extent of such changes, variability can be expected in separate determinations of ash content even for the same coal sample unless standardized procedures are closely followed, particularly for coals with relatively large
amounts of carbonates or pyrite. The term "inherent" or "fixed" ash content is
used to designate that portion of the ash content that is structurally part of the
coal after the coal is pulverized and mechanically cleaned prior to use.
Recently there have been many improvements in instrumental analysis which
have been successfully applied to coal. Much of this laboratory technology has
arisen out of the need to assess potential pollutant emissions from coal utilization. Appendix A gives a review of these modern developments in analytical
chemistry. Below we discuss in more detail those constituents in coal which influence pollutant emissions when coal is utilized. Table 2-7 lists ASTM tests
which are employed for analyzing various physical and chemical properties of
coal.
Coal Mineral Matter/Ash
Coal is primarily an organic substance with varying amounts of inorganic material. This is due to clay, carbonate, or other mineral matter which was carried
into the peat swamps during the initial stages of coalification. The mineral matter
is usually external to the organic material and not chemically combined with carbon or hydrogen, although in some low-rank coals alkali and alkali-earth elements (e.g., Ca, Mg, Na, Ba, Sr, and B) are often combined with carbon or
hydrogen. After combustion of coal, the inorganic material is found mainly to be
associated with the ash, which can comprise from 3% to 40% of the original coal
volume. The major minerals and species which occur in coal mineral matter include pyrite, quartz, kaolinite, illite, siderite, calcite, and sphalerite. 13
Due to the difficulty of quantitative analysis of the ash minerals, ash analysis
is usually determined by chemical analysis for most elements in the residue produced by combustion of a coal sample at 1350°F. The oxidation is performed at a
slow rate and low enough temperature to prevent fusion of the ash. This means
that "ash" and' 'mineral matter" are not the same material, physically or chemically (although they are related). An alternative to high temperature ashing
is a low temperature procedure (400°F) which uses an oxygen plasma. The oxygen breaks down the organic matter without major decomposition of the inorganic mineral species (see Appendix A). Tremendous variability in ash composition is exhibited by U. S. coals, as seen in Table 2-8. 12
Table 2-8 also shows other properties related to ash fusibility, both for reducing and oxidizing atmospheres. ASTM D 1857 discusses the procedures for determining the reference temperatures mentioned in Table 2-8. This test is an observation of the temperatures at which triangular pyramids or cones prepared
from coal ash pass through certain defined stages of fusion and flow when heated
N
ex>
oo
e!..
\J
(3
()
Table 2-7
ASTM Standards for Testing Coal
*D1756
*D2361
*D 291
*D 440
*D 547
*D 1857
*D1412
*D2014
Cubic Foot Weight of Crushed Bituminous
Drop Shatter Test for Coal
Dustiness, Index of, of Coal and Coke
Fusibility of Coal Ash
Equilibrium Moisture of Coal at 96 to
97% Relative Humidity and 30°C
Expansion or Contraction of Coal by the
Sole-Heated Oven
*D 720
D 409
Free-Swelling Index of Coal
Grindability of Coal by the HardgroveMachine Method
*D2015
Gross Calorific Value of Solid Fuel by the
Adiabatic Bomb Calorimeter
DI812
D2639
Carbon Dioxide in Coal
Chlorine in Coal
Plastic Properties of Coal by the
Gieseler Plastometer
Coal
D 197
*D 271
D 492
*D2234
*D2013
*D 410
D 311
D 310
*D 431
*D1757
*D2492
*D 441
* Approved as American National Standard by the American National Standards Institute.
CD
(J)
(J)
:S.
(Q
Plastic Properties of Coal by the
Automatic Gieseler Plastometer
n>
:J
D.
Sampling and Fineness Test of Powdered Coal
Sampling and Analysis, Laboratory,
of Coal and Coke
Sampling Coals Classified
Ash Content
According to
Sampling Mechanical, of Coal
Samples, Coal, Preparing of Analysis
Screen, Analysis of Coal
Sieve Analysis of Crushed Bituminous
Size of Anthracite
Size of Coal, Designating
its Screen Analysis
Sulfur in Coal Ash
Sulfur, Forms of, in Coal
Tumbler Test for Coal
from
\J
Q.
~
o·
:J
oo
::t
Q.
Coal
on
--0
---
047.27
u;
Utah
Texas
Illinois
Ohio
2900
25.026.0
4.04.0
0.20.5
0.60.7
0.51.0
]48.0
1.2
6.6
7.0
1.5
]1.06.6
0.2
20.0
4.25
0.81
22,250
62,290
20.]]
29.28
1.60
3.53
10.87
12.8
1.25
37.64
22.81
0.36
0.78
4.17
17.36
47.52
1.02
20.13
5.75
1.77
]7.87
4.0
.6
41.8
0.1
1.1
117.6
1.97
0.28
0.85
1.00
22.96
3.30
CD
]4.10
.5
30.0
60.0
0.5
4.0
0.6
+
1.6
1.5
]2.3
0.7
97.5
80.80
95.74
95.20
84.3
6.4
98.44
98.8
No.6
No.92,220
Pocahontas
No.3
02.5
]3.6
.6
01.30
2,180High Volatile
bituminous
2,220
2,240
2,450
2,450
2,210
2,140
2,225
2,180
2,150
2,250
2,370
2,320
2,240
2,175
2,160
2,130
2,060
2,120
2,190
2,265
1,990
2,030
2,000
2,300
1,975
2,070
roo
2,030
2,420
2,460
2,300
2,540
2,610
2,290
West
Virginia
2,385
2,430
2,]90
West
Virginia
Pittsburgh
Wyoming
Lignite
Bituminous
en
0..
"0
~
~.
"U
0~ SiOz
Temperatures
........
of Some U.S. Coals and Lignite
················
Low Volatile
.
12
2,620 Table 2-8 Sub2,480
2,670
2,450
2,605
~
"0
I»
CD
N
0
30
Coal Processing
and Pollution Control
at a specific rate. As with other coal analytical methods, this method is empirical
and several different methods are employed worldwide.6
Four stages of fusion temperature are reported in ASTM D 1857:
1. Initial, or the first rounding of the cone.
2. Softening, when the height has diminished until it is equal to the width at
the base.
3. Hemispherical, when the height of the lump equals one-half the width of
the base.
4. Fluid, when the mass is no higher than 1/16 inch.
The ash fusion temperatures can be used as an approximate measure to indicate
the magnitude of coal-ash-deposit buildup in combustion or gasification. Generally high fusion temperatures yield low slagging potential in dry ash removal systems, while low fusion temperatures are required for wet-bottom (slagging) systems.
The application of ash fusion data to predict fouling behavior in combustion or
gasification facilities is not completely quantitative. Often correlations involving
the ash composition are used to explain coal ash properties; e.g., the base to acid
ratio:
Fez03
+ CaO + MgO + NazO + KzO
SiOz + Ab03 + TiOz
Those components in the numerator are the basic oxides, which tend to lower the
fusion temperature. The acid oxides in the denominator tend to raise the fusion
temperature. Other correlating groups employed to evaluate coal-ash behavior,
especially as pertains to deposition on both furnace walls and convection surfaces, include6
1. Iron/calcium ratio
2. Silica/alumina ratio
3. Iron/dolomite ratio
4. Dolomite percentage
5. Ferric iron percentage
More details on ash fouling in coal combustion are presented in Chapter 9.
Trace metals present in coal are associated either with the mineral matter or
the organic fraction. 15,16 Typical mineral associations include the following:
Trace Element
Mineral
As, Be, Cu, Sb
B, Cd, Zn, Hg
B, Cd, Mn, Se, Mo, V
B, Cr, Mn, Cd, Mo, Se, W, Zn
B, Cu, F, Hg, Sn
Pyrite
Sphalerite
Calcite
Quartz
Clays
Coal Deposits and Properties
31
Those trace metals shown to have high organic affinities include selenium, strontium, arsenic, cadmium, boron, and antimony. 15These elements tend to be concentrated in coals, whereas trace metals generally associated with mineral matter
tend to occur in concentrations not significantly different from their average
crustal abundance. Table 2-9 gives some representative data for trace metal occurrence in U.S. coals;15 because of the wide variations in trace metal composition that may occur in U.S. coals, only the order of magnitude values are important in this table. Analytical procedures for trace element and mineral analysis
are discussed in Appendix A.
Coal Sulfur
The total amount of sulfur in coal varies from 0.2 to 10 wt% but in most samples is in the range of I to 4 wt%. Two forms of sulfur occur in coal: organic
sulfur, which is bound to the hydrocarbon structure of the coal and inorganic
sulfur, which is the remainder. The typical weight fraction of these two general
classes of sulfur varies from 40% to 80% inorganic sulfur. 2
Inorganic sulfur appears mainly in two forms: as pyritic sulfur (i.e., sulfur
combined with iron as pyrite or marcasite, crystalline forms of FeS2) and as sulfatic sulfur in the form of iron, calcium, and barium sulfates. Exposure of coal to
air can increase the amount of sulfatic sulfur via oxidation of the pyritic sulfur to
sulfates. Variations in coal sulfur content are usually due to the mineral matter,
which may vary widely in the same coal seam. The sulfatic sulfur is usually water-soluble and easily removed by washing (see Chapter 5).
Organic sulfur is usually distributed in a uniform manner in the coal matrix
including such functional groups as sulfidic (R-S-R'), disulfidic (R-S-S-R'),
mercaptanic (R-S-H), and thiophenic (ringed sulfur compounds). No organic sulfur compounds can be isolated from coal without changing its organostructure,
which is why organic sulfur is the most difficult form of sulfur to remove from
coal. Analytical procedures for sulfur analysis are discussed in ASTM D 2492 as
well as in Appendix A.
Other Physical Properties
Physical properties in coal vary systematically (although not always monotonically) with coal rank, so that many of them can be predicted from the carbon
or volatile content of coal. Properties such as porosity, density, and surface area,
pass through a more or less well-defined maximum or minimum, usually between ~85% and 89% carbon, and thereby reflect the rank-dependent surfaceto-volume ratio of coal, or the transition from bituminous to anthracitic
coals. 17,18Some of the important properties are discussed in the following paragraphs.
en
c.
0N
S'
(")
I!)
0
0Q.~ elements
Antimony
(Sb)
~
0
-0- -
Q.
:3
""U0.10
2.3
(Q
0.05
""U <0.6
!:!!.
2.0
0.32
4.9
2.6
1.0
0.36
50
0.18
1.6
0.006
0.08
0.7
.0
630.7
300
0.70
0.7
2.0
0.75
231.2
1.7
.8
2.7
2.6
0.88
0.04
0.24
0.16
<0.75.5
1.4
0.007
<0.8
3.2
1.6
0.12
1.19
0.18
0.31
0.68
1.70
0.54
0.78
1.0
0.33
100
70
75
50
0.06
0.10
0.21
0.04
0.06
0.20
0.01
0.21
0.01
0.52
0.015
2.0
0.005
10.02
0.01
2.2
1.1
0.05
0.12
0.05
0.08
1.4
15
25
5.0
150
300
500
100
2.0
2.0
bituminous
Bituminous
o'
Average
Average
Lignite
(ppm)
-
0.00
60.002
0.8
0.48
2.7
0.02
2.0
0.35
0.07
10
0.05
0.24
0.44
0.9
0.15
100
1.5
Anthracite
-
Worldwide
Table
2-9
(,)
Estimated
en
(1)
pUKe.)
~
-u0- 0.46
0()
en
'0
5.5
2.7
1.0
0.18
0.012
II
22
6.1
1.3
0.2
1110
.0
1.6
30.45
0.60
15
1.5
220
5715
323
207
0.71
74
70.4
0.17
16
19
15
20
37
1000
63
0.12
14
25
10
25
90.30
0.21
0.25
0.13
0.7
10
1.20
1.6
44.7
720
77
0.20
15
37
0.34
IS
1.l0
2.2
12.3
11.5
7.7
1.3
0.16
0.6
1.4
0.11
0.06
0.3
2065
0.08
19
0.05
0.07
10
3.2
bituminous
Bituminous
en Average
Table 2-9 Continued
Average
Lignite
:J
(D'
'0
0c,,)
~.
c,,)
n>
~
-
-
255
50
0.40
0.61
0.13
0.47
0.3
20
27
61
10
0.15
72
33
Anthracite
-
Worldwide
0.09
7.5
0.75
0.13
CD
<3
Estimated
mated
a.
0~
en
(1)
.".
::3
(Q
()
()
-u
-
-4.6
a0.1
5.3
0.50
0.17
2.3
0.07
6.1
2.2
1.7
0 S'
1010
10
20
I3100
3950
20
19
0.0
53
30
0.42
0.1
0.07
2.5
1.9
0.20
1.6
0.27
0.26
0.12
2.7
0.3
\.3
1.2
lOa
I2.7
4.1
330
100
0.5
25
lOa
15
\.3
0.5
5.3
300
500
50.07
52.2
31.6
1bituminous
.5
0.1
2025
2.90
1.6
\.3
30
2.5
13.3
1.9
0.50
Bituminous
0.98
Average
Lignite
~ ~Average
::3
::3
c)"
Q.
en
-I -
5.4
lOa
20
50
16
53.5
1.5 0.1
10
Anthracite
tI>
-
Worldwide
Co)
0
Coal Deposits and Properties
35
Plastic properties. Plastic and agglutinating properties of coal are important
in evaluating coking and combustion suitability. Certain coals (mainly bituminous) pass through a transient plastic state when the coal is heated. The coal successively softens, swells, and finally resolidifies. These coals are said to be caking (non-swelling coals are non-caking). There is no sharp dividing line between
these two types of coals and there are wide ranges in plastic behavior. The freeswelling test, as outlined in the ASTM procedure (D 720, entails heating of 1
gram of - 60-mesh coal in a silica crucible to 820°C (± 2°C) in 21/2 minutes,
and determining the free-swelling index (FSI) by comparing the resultant "button" with a series of standard profiles. A noncoherent residue is assigned an index of 0, and indices of 2 or 3 usually imply that the coal is only marginally (or
weakly) caking. An alternative method, which is used in some European countries and assesses the mechanical strength rather than the distension of a coke
button, is the Roga test. 6
The ASTM has developed another semiquantitative procedure for determining
the relative plastic behavior of coal when heated under prescribed conditions in
the absence of air. ASTM D-1812 entitled, "Standard Method of Test for Plastic
Properties of Coal by the Gieseler Plastometer," determines
1.
2.
3.
4.
Initial softening temperature
Maximum fluid temperature
Solidification temperature
Maximum fluidity
A thorough discussion of the plastic properties of coal has been presented by
Habermehl, et al. 19 and Berkowitz. 18
Porosity. Porosity in coal is composed of micropores «20A), mesopores (20A
to 200A), and macropores (>200A). The micropores contribute mainly to internal
surface area, while the macropores allow access to the inner pore spaces. Typical
porosity is 25% to 30% for low-rank coals, 5% to 10% for anthracite, but only
1% to 2% for some bituminous coals, with the minimum porosity at about 91 %
carbon (dry, ash-free, or daf). 17 Porosity is measured by mercury penetration under pressure.
Surface area. The surface area of coal is measured by adsorption of either carbon dioxide or nitrogen. However, the measured area depends upon the specific
adsorbate used, due to polarity and molecular size. Surface areas range from
100-200 m2/g (low rank), 50-100 m2/g (bituminous), and> 100 m2/g (anthracite). Note that the minimum surface area occurs with bituminous coal and not
anthracite, at a daf carbon content of about 85% to 90%.
Density. The bulk density of coal depends upon the degree of packing in the
coal matrix. Usually this property is measured by helium or mercury displacement. Generally the density decreases as the coal rank is decreased, mainly due
to the increased moisture fraction. High-rank coals can have densities over 1.5 g/
cm3, with 1.3 g/cm3 typical for lower rank coals.
36
Coal Processing
and Pollution Control
Hardness. The hardness or grindability of coal affects the relative ease of pulverization, as determined by ASTM D 409. More details on this test are given in
Chapter 5. There are also several tests used to determine friability. 13
Reactivity. Coal reactivity may be defined as the rate that it combines with oxygen at temperatures above the ignition point; reactivity and rate of combustion
are essentially synonymous. Reactivity generally increases as the coal rank decreases. Differences in reactivity from one coal rank to another can usually be
explained by the changing levels in organically-bound oxygen with rank. Organically-bound oxygen is an inherent part of the coal structure, exclusive of water
and mineral matter. As rank decreases from low-volatile bituminous to lignite, "organic oxygen increases by a factor of 5 to 6. When the fuel is heated, a portion
of this oxygen becomes available for the oxidation process.
Coal Sampling
There are several procedures for sampling coal for various analytical tests.
(See References 6, 12, 20, and 21.) Two sampling procedures are recognized in
the present ASTM standard (D 492). They are the commercial-sampling procedure and the special-purpose sampling procedure. The "commercial-sampling
procedure" applies to the average commercial sampling of coal. This procedure
is designed to measure the average ash content of a large number of samples
within ± 10% to a 95% probability.
The "special-purpose-sampling
procedure" applies to the sampling of coal
when special accuracy is required. This procedure should be used to supply samples for the classification of coals and the establishment of design or performance parameters. The special-purpose sample can be one of two sizes, either 4
times that of the commercial sample, giving an accuracy of ± 5% of the ash content of the coal samples, or 9 times that of the commercial sample, giving an
accuracy of ±3.33% of the ash content of the coal sampled.20
References
I. Demonstrated Reserve Base of Coal in the United States on January 1,
1979, U.S. Department of Energy, DOE/EIA-0280 (79), Washington,
D.C., (May 1981).
2. Averitt, P., "Coal Resources," Chemistry of Coal Utilization, M. A. Elliott, (Ed.), Wiley, New York, (1981), Ch. 2.
3. Wilson, C. L., Coal-Bridge
to the Future: Report of the World Coal
Study, Ballinger Publishing Company, Cambridge, Massachusetts,
(1980).
4. Schmidt, R. A., Coal in America: An Encyclopedia of Reserves, Production and Use, McGraw-Hill Book Company, New York, (1979).
Coal Deposits and Properties
37
5. Booz, Allen, and Hamilton, "Underground Coal Gasification Program,"
ERDA 77-51/4, Contract Ex-76-C-01-2343, (March, 1977).
6. Singer, J. G., (Ed.), Combustion-Fossil Power Systems, Combustion Engineering, Windsor, Connecticut, (1981).
7. Neavel, R. c., "Origin, Petrography, and Classification of Coal,"
Chemistry of Coal Utilization, M. A. Elliott, (Ed.), Wiley, New York,
(1981), Ch. 3.
8. Bouska, v., Geochemistry of Coal, Vol. 1, Coal Science and Technology
Series, Elsevier Scientific Publishing Company, Amsterdam, The Netherlands, (1981).
9. Probstein, R. F. and R. E. Hicks, Synthetic Fuels, McGraw-Hill, New
York, (1982).
10. Whitehurst, D. D., "A Primer on the Chemistry and Constitution of
Coal," Organic Chemistry of Coal, ACS Symposium Series No. 71,
American Chemical Society, Washington, D.C., (1978), Ch. l.
11. Larsen, J. w., (Ed.), Organic Chemistry of Coal, ACS Symposium Series No. 71, American Chemical Society, Washington, D.C., (1978).
12. Steam: Its Generation and Use, 37th Edition, Babcock and Wilcox Company, Barberton, Ohio, (1972).
13. Ergun, S., "Coal Classification and Characteristics," Coal Conversion
Technology, C. Y. Wen and E. S. Lee, (Eds.), Addison Wesley, Reading,
Massachusetts, (1979), Ch. 1.
14. Ringen, S., J. Lanum, and F. P. Miknis, "Calculating Heating Values
from Elemental Compositions of Fossil Fuels," Fuel, Vol. 58, No. 69,
(1979).
15. National Research Council, "Trace Element Geochemistry of Coal Resource Development Related to Environmental Quality and Health," National Academy Press, Washington, D.C., (1980).
16. Torrey, S., (Ed.), Trace Contaminants from Coal, Noyes Data Corporation, Park Ridge, New Jersey, (1978).
17. Sharkey, A. G., and J. T. McCartney, "Physical Properties of Coal and
its Products," Chemistry of Coal Utilization, M. A. Elliott, (Ed.), Wiley,
New York, (1981), Ch. 4.
18. Berkowitz, N., Introduction to Coal Technology, Academic Press, New
York, (1979).
19. Habermehl, D., F. Orywal, and H. Beyer, "Plastic Properties of Coal,"
Chemistry of Coal Utilization, M. A. Elliott, (Ed.), Wiley, New York,
(1981), Ch. 6.
20. Gould, G., and J. Visman, "Coal Sampling and Analysis," Coal Handbook, R. A. Meyers, (Ed.), Marcel Dekker, New York, (1981), Ch. 2.
21. Swanson, V. E., and C. Huffman, Jr., "Guidelines for Sample Collecting and Analytical Methods Used in the U.S. Geological Survey for Determining Chemical Composition of Coal," U.S. Geol. Survey Circular
735, (1976).