APPROXIMATE COMPOSITION AND CALORIFIC VALUE
OF GENERAL GRADES OF COAL ON BASIS OF COMBUSTIBLE
+-------------------+----------------------------+--------------+
| Kind of Coal | Per Cent of Combustible | B. t. u. |
| +------------+---------------+ Per Pound of |
| |Fixed Carbon|Volatile Matter| Combustible |
+-------------------+------------+---------------+--------------+
|Anthracite |97.0 to 92.5| 3.0 to 7.5 |14600 to 14800|
|Semi-anthracite |92.5 to 87.5| 7.5 to 12.5 |14700 to 15500|
|Semi-bituminous |87.5 to 75.0| 12.5 to 25.0 |15500 to 16000|
|Bituminous--Eastern|75.0 to 60.0| 25.0 to 40.0 |14800 to 15300|
|Bituminous--Western|65.0 to 50.0| 35.0 to 50.0 |13500 to 14800|
|Lignite | Under 50 | Over 50 |11000 to 13500|
+-------------------+------------+---------------+--------------+
Anthracite--The name anthracite, or hard coal, is applied to those dry
coals containing from 3 to 7 per cent volatile matter and which do not
swell when burned. True anthracite is hard, compact, lustrous and
sometimes iridescent, and is characterized by few joints and clefts. Its
specific gravity varies from 1.4 to 1.8. In burning, it kindles slowly
and with difficulty, is hard to keep alight, and burns with a short,
almost colorless flame, without smoke.
Semi-anthracite coal has less density, hardness and luster than true
anthracite, and can be distinguished from it by the fact that when newly
fractured it will soot the hands. Its specific gravity is ordinarily
about 1.4. It kindles quite readily and burns more freely than the true
anthracites.
Semi-bituminous coal is softer than anthracite, contains more volatile
hydrocarbons, kindles more easily and burns more rapidly. It is
ordinarily free burning, has a high calorific value and is of the
highest order for steam generating purposes.
Bituminous coals are still softer than those described and contain still
more volatile hydrocarbons. The difference between the semi-bituminous
and the bituminous coals is an important one, economically. The former
have an average heating value per pound of combustible about 6 per cent
higher than the latter, and they burn with much less smoke in ordinary
furnaces. The distinctive characteristic of the bituminous coals is the
emission of yellow flame and smoke when burning. In color they range
from pitch black to dark brown, having a resinous luster in the most
compact specimens, and a silky luster in such specimens as show traces
of vegetable fiber. The specific gravity is ordinarily about 1.3.
Bituminous coals are either of the caking or non-caking class. The
former, when heated, fuse and swell in size; the latter burn freely, do
not fuse, and are commonly known as free burning coals. Caking coals are
rich in volatile hydrocarbons and are valuable in gas manufacture.
Bituminous coals absorb moisture from the atmosphere. The surface
moisture can be removed by ordinary drying, but a portion of the water
can be removed only by heating the coal to a temperature of about 250
degrees Fahrenheit.
Cannel coal is a variety of bituminous coal, rich in hydrogen and
hydrocarbons, and is exceedingly valuable as a gas coal. It has a dull
resinous luster and burns with a bright flame without fusing. Cannel
coal is seldom used for steam coal, though it is sometimes mixed with
semi-bituminous coal where an increased economy at high rates of
combustion is desired. The composition of cannel coal is approximately
as follows: fixed carbon, 26 to 55 per cent; volatile matter, 42 to 64
per cent; earthy matter, 2 to 14 per cent. Its specific gravity is
approximately 1.24.
Lignite is organic matter in the earlier stages of its conversion into
coal, and includes all varieties which are intermediate between peat and
coal of the older formation. Its specific gravity is low, being 1.2 to
1.23, and when freshly mined it may contain as high as 50 per cent of
moisture. Its appearance varies from a light brown, showing a distinctly
woody structure, in the poorer varieties, to a black, with a pitchy
luster resembling hard coal, in the best varieties. It is non-caking and
burns with a bright but slightly smoky flame with moderate heat. It is
easily broken, will not stand much handling in transportation, and if
exposed to the weather will rapidly disintegrate, which will increase
the difficulty of burning it.
Its composition varies over wide limits. The ash may run as low as one
per cent and as high as 50 per cent. Its high content of moisture and
the large quantity of air necessary for its combustion cause large stack
losses. It is distinctly a low-grade fuel and is used almost entirely in
the districts where mined, due to its cheapness.
Peat is organic matter in the first stages of its conversion into coal
and is found in bogs and similar places. Its moisture content when cut
is extremely high, averaging 75 or 80 per cent. It is unsuitable for
fuel until dried and even then will contain as much as 30 per cent
moisture. Its ash content when dry varies from 3 to 12 per cent. In this
country, though large deposits of peat have been found, it has not as
yet been found practicable to utilize it for steam generating purposes
in competition with coal. In some European countries, however, the peat
industry is common.
Distribution--The anthracite coals are, with some unimportant
exceptions, confined to five small fields in Eastern Pennsylvania, as
shown in the following list. These fields are given in the order of
their hardness.
Lehigh or Eastern Middle Field
Green Mountain District
Black Creek District
Hazelton District
Beaver Meadow District
Panther Creek District[33]
Mahanoy or Western Field[34]
East Mahanoy District
West Mahanoy District
Wyoming or Northern Field
Carbondale District
Scranton District
Pittston District
Wilkesbarre District
Plymouth District
Schuylkill or Southern Field
East Schuylkill District
West Schuylkill District
Louberry District
Lykens Valley or Southwestern Field
Lykens Valley District
Shamokin District[35]
Anthracite is also found in Pulaski and Wythe Counties, Virginia; along
the border of Little Walker Mountain, and in Gunnison County, Colorado.
The areas in Virginia are limited, however, while in Colorado the
quality varies greatly in neighboring beds and even in the same bed. An
anthracite bed in New Mexico was described in 1870 by Dr. R. W. Raymond,
formerly United States Mining Commissioner.
Semi-anthracite coals are found in a few small areas in the western part
of the anthracite field. The largest of these beds is the Bernice in
Sullivan County, Pennsylvania. Mr. William Kent, in his "Steam Boiler
Economy", describes this as follows: "The Bernice semi-anthracite coal
basin lies between Beech Creek on the north and Loyalsock Creek on the
south. It is six miles long, east and west, and hardly a third of a mile
across. An 8-foot vein of coal lies in a bed of 12 feet of coal and
slate. The coal of this bed is the dividing line between anthracite and
semi-anthracite, and is similar to the coal of the Lykens Valley
District. Mine analyses give a range as follows: moisture, 0.65 to 1.97;
volatile matter, 3.56 to 9.40; fixed carbon, 82.52 to 89.39; ash, 3.27
to 9.34; sulphur, 0.24 to 1.04."
Semi-bituminous coals are found on the eastern edge of the great
Appalachian Field. Starting with Tioga and Bradford Counties of northern
Pennsylvania, the bed runs southwest through Lycoming, Clearfield,
Centre, Huntingdon, Cambria, Somerset and Fulton Counties, Pennsylvania;
Allegheny County, Maryland; Buchannan, Dickinson, Lee, Russell, Scott,
Tazewell and Wise Counties, Virginia; Mercer, McDowell, Fayette, Raleigh
and Mineral Counties, West Virginia; and ending in northeastern
Tennessee, where a small amount of semi-bituminous is mined.
The largest of the bituminous fields is the Appalachian. Beginning near
the northern boundary of Pennsylvania, in the western portion of the
State, it extends southwestward through West Virginia, touching Maryland
and Virginia on their western borders, passing through southeastern
Ohio, eastern Kentucky and central Tennessee, and ending in western
Alabama, 900 miles from its northern extremity.
The next bituminous coal producing region to the west is the Northern
Field, in north central Michigan. Still further to the west, and second
in importance to the Appalachian Field, is the Eastern Interior Field.
This covers, with the exception of the upper northern portion, nearly
the entire State of Illinois, southwest Indiana and the western portion
of Kentucky.
The Western Field extends through central and southern Iowa, western
Missouri, southwestern Kansas, eastern Oklahoma and the west central
portion of Arkansas. The Southwestern Field is confined entirely to the
north central portion of Texas, in which State there are also two small
isolated fields along the Rio Grande River.
The remaining bituminous fields are scattered through what may be termed
the Rocky Mountain Region, extending from Montana to New Orleans. A
partial list of these fields and their location follows:
Judith Basin Central Montana
Bull Mountain Field Central Montana
Yellowstone Region Southwestern Montana
Big Horn Basin Region Southern Montana
Big Horn Basin Region Northern Wyoming
Black Hills Region Northeastern Wyoming
Hanna Field Southern Wyoming
Green River Region Southwestern Wyoming
Yampa Field Northwestern Colorado
North Park Field Northern Colorado
Denver Region North Central Colorado
Uinta Region Western Colorado
Uinta Region Eastern Utah
Southwestern Region Southwestern Utah
Raton Mountain Region Southern Colorado
Raton Mountain Region Northern New Mexico
San Juan River Region Northwestern New Mexico
Capitan Field Southern New Mexico
Along the Pacific Coast a few small fields are scattered in western
California, southwestern Oregon, western and northwestern Washington.
Most of the coals in the above fields are on the border line between
bituminous and lignite. They are really a low grade of bituminous coal
and are known as sub-bituminous or black lignites.
Lignites--These resemble the brown coals of Europe and are found in the
western states, Wyoming, New Mexico, Arizona, Utah, Montana, North
Dakota, Nevada, California, Oregon and Washington. Many of the fields
given as those containing bituminous coals in the western states also
contain true lignite. Lignite is also found in the eastern part of Texas
and in Oklahoma.
Alaska Coals--Coal has been found in Alaska and undoubtedly is of great
value, though the extent and character of the fields have probably been
exaggerated. Great quantities of lignite are known to exist, and in
quality the coal ranges in character from lignite to anthracite. There
are at present, however, only two fields of high-grade coals known,
these being the Bering River Field, near Controllers Bay, and the
Matanuska Field, at the head of Cooks Inlet. Both of these fields are
known to contain both anthracite and high-grade bituminous coals, though
as yet they cannot be said to have been opened up.
Weathering of Coal--The storage of coal has become within the last few
years to a certain extent a necessity due to market conditions, danger
of labor difficulties at the mines and in the railroads, and the
crowding of transportation facilities. The first cause is probably the
most important, and this is particularly true of anthracite coals where
a sliding scale of prices is used according to the season of the year.
While market conditions serve as one of the principal reasons for coal
storage, most power plants and manufacturing plants feel compelled to
protect their coal supply from the danger of strikes, car shortages and
the like, and it is customary for large power plants, railroads and coal
companies themselves, to store bituminous coal. Naval coaling stations
are also an example of what is done along these lines.
Anthracite is the nearest approach to the ideal coal for storing. It is
not subject to spontaneous ignition, and for this reason is unlimited in
the amount that may be stored in one pile. With bituminous coals,
however, the case is different. Most bituminous coals will ignite if
placed in large enough piles and all suffer more or less from
disintegration. Coal producers only store such coals as are least liable
to ignite, and which will stand rehandling for shipment.
The changes which take place in stored coal are of two kinds: 1st, the
oxidization of the inorganic matter such as pyrites; and 2nd, the direct
oxidization of the organic matter of the actual coal.
The first change will result in an increased volume of the coal, and
sometimes in an increased weight, and a marked disintegration. The
changes due to direct oxidization of the coal substances usually cannot
be detected by the eye, but as they involve the oxidization of the
carbon and available hydrogen and the absorption of the oxygen by
unsaturated hydrocarbons, they are the chief cause of the weathering
losses in heat value. Numerous experiments have led to the conclusion
that this is also the cause for spontaneous combustion.
Experiments to show loss in calorific heat values due to weathering
indicate that such loss may be as high as 10 per cent when the coal is
stored in the air, and 8.75 per cent when stored under water. It would
appear that the higher the volatile content of the coal, the greater
will be the loss in calorific value and the more subject to spontaneous
ignition.
Some experiments made by Messrs. S. W. Parr and W. F. Wheeler, published
in 1909 by the Experiment Station of the University of Illinois,
indicate that coals of the nature found in Illinois and neighboring
states are not affected seriously during storage from the standpoint of
weight and heating value, the latter loss averaging about 3½ per cent
for the first year of storage. They found that the losses due to
disintegration and to spontaneous ignition were of greater importance.
Their conclusions agree with those deduced from the other experiments,
viz., that the storing of a larger size coal than that which is to be
used, will overcome to a certain extent the objection to disintegration,
and that the larger sizes, besides being advantageous in respect to
disintegration, are less liable to spontaneous ignition. Storage under
water will, of course, entirely prevent any fire loss and, to a great
extent, will stop disintegration and reduce the calorific losses to a
minimum.
To minimize the danger of spontaneous ignition in storing coal, the
piles should be thoroughly ventilated.
Pulverized Fuels--Considerable experimental work has been done with
pulverized coal, utilizing either coal dust or pulverizing such coal as
is too small to be burned in other ways. If satisfactorily fed to the
furnace, it would appear to have several advantages. The dust burned in
suspension would be more completely consumed than is the case with the
solid coals, the production of smoke would be minimized, and the process
would admit of an adjustment of the air supply to a point very close to
the amount theoretically required. This is due to the fact that in
burning there is an intimate mixture of the air and fuel. The principal
objections have been in the inability to introduce the pulverized fuel
into the furnace uniformly, the difficulty of reducing the fuel to the
same degree of fineness, liability of explosion in the furnace due to
improper mixture with the air, and the decreased capacity and efficiency
resulting from the difficulty of keeping tube surfaces clean.
Pressed Fuels--In this class are those composed of the dust of some
suitable combustible, pressed and cemented together by a substance
possessing binding and in most cases inflammable properties. Such fuels,
known as briquettes, are extensively used in foreign countries and
consist of carbon or soft coal, too small to be burned in the ordinary
way, mixed usually with pitch or coal tar. Much experimenting has been
done in this country in briquetting fuels, the government having taken
an active interest in the question, but as yet this class of fuel has
not come into common use as the cost and difficulty of manufacture and
handling have made it impossible to place it in the market at a price to
successfully compete with coal.
Coke is a porous product consisting almost entirely of carbon remaining
after certain manufacturing processes have distilled off the hydrocarbon
gases of the fuel used. It is produced, first, from gas coal distilled
in gas retorts; second, from gas or ordinary bituminous coals burned in
special furnaces called coke ovens; and third, from petroleum by
carrying the distillation of the residuum to a red heat.
Coke is a smokeless fuel. It readily absorbs moisture from the
atmosphere and if not kept under cover its moisture content may be as
much as 20 per cent of its own weight.
Gas-house coke is generally softer and more porous than oven coke,
ignites more readily, and requires less draft for its combustion.
[Illustration: 16,000 Horse-power Installation of Babcock & Wilcox
Boilers and Superheaters at the Brunot's Island Plant of the Duquesne
Light Co., Pittsburgh, Pa.]