COAL
Coal is of prehistoric origin, formed from accumulation of vegetable
matter, supposed to be the remains of immense forests. In past ages the
deposits underwent destructive distillation from great heat and was
subjected to pressure, sufficient to compress it into varying degrees
of hardness. Coal is composed of carbon, hydrogen and oxygen, with
small quantities of nitrogen and varying amounts of sulphur and ash.
The coals from different geological formations vary in quality from the
hard dry anthracites to the soft wet lignites, with the intermediate
bituminous coals; all of which furnish fuels that when burned will
produce amounts of heat, depending on their composition, the quantity
of moisture contained and the conditions of their combustion.
Carbon, of which coal is principally composed, exists in different
combinations, depending on the condition of its formation. Part of the
carbon is combined with hydrogen to form hydrocarbon that may be driven
off when heated, and which forms the volatile portion of the coal. The
remainder of the carbon appears in the form of coke--when the volatile
matter is driven off--and is said to be fixed. The fixed carbon and
volatile constituents together make up the combustible.
Other ingredients of coal that require attention are the moisture, and
the incombustible matter that forms ash. Moisture varies in quantity
from as low as 0.75 per cent. in hard coal to 50 per cent. in lignite.
The amounts of ash in different coals vary from 3 to 30 per cent. of
the weights of the fuel.
The heating value of coals differs in amount by reason of the variable
quantities of fixed and combined carbons, moisture and ash. Different
coals are compared in value by the number of B.t.u. per pound of dry
coal that can possibly be developed when burned, and with these factors
are given the percentages of moisture and ash.
There are no distinct demarkations between different grades of coal.
The classifications are made because of their chief characteristics
and they commonly are graded as anthracites, semi-anthracites,
semi-bituminous and bituminous coals. These classes comprehend the
most common commercial coals of the United States. Aside from those
named are forms of coal that are occasionally found, such as graphitic
anthracite, cannel coal, etc., and the various lignites.
The value of coal as a heat-producing agent is represented by the
B.t.u. it is capable of turning to useful account. The price of coal
should be based on the amount of heat it is capable of generating
when burned. In considering the value of coal for any particular
purpose, thought must be taken as to its characteristic properties,
for coals that produce excellent results for one purpose may be very
unsatisfactory in others. Soft coal containing a large percentage of
volatile matter usually produces a great amount of smoke and unless
carefully fired this will condense and form accumulations of soot that
are objectionable. For reasons of this kind bituminous coals are often
sold at a lower price than their rated heating value might indicate.
=Anthracite or hard coal= possesses bright lustrous surfaces when newly
fractured, that when handled do not soil the hands. It contains a high
percentage of carbon, a small amount of volatile matter and little
moisture. It is greatly in demand as a domestic fuel because it burns
slowly with an intense heat, practically without flame and produces
no smoke. It invariably commands a higher price than soft coal, but
in heating value is not superior to the better grades of soft coal.
In furnaces for house heating the use of soft coal often gives better
satisfaction than hard coal.
The grades of hard coal found in the market will vary with the demand
in any locality but those recognized by the trade are:
Egg Coal will pass through 2¾-inch mesh screen.
Stove Coal will pass through 2-inch mesh screen.
Chestnut Coal will pass through 1⅜-inch mesh screen.
Pea Coal will pass through ¾-inch mesh screen.
No. 1 Buckwheat Coal will pass through ½-inch mesh screen.
No. 2 Buckwheat Coal will pass through ¼-inch mesh screen.
No. 3 Buckwheat Coal will pass through ⅛-inch mesh screen.
Hard coal of stove and chestnut sizes are those most commonly used
for domestic heating, because they are well suited for furnaces and
heating stoves. Of the two sizes chestnut coal is most largely used
and on account of the greater demand, the price for this size is
usually somewhat in advance of the others; at the same time the smaller
sizes--pea and buckwheat coals--are less in price for the same grade of
coal. Under conditions that will permit their use the latter coals are
an economical form of fuel.
=Bituminous or soft coal= represents the chief fuel of commerce. The
market prices of these coals are determined largely by reason of
their reputation as desirable fuel. The variations in price depend
on the physical qualities, rather than on the amount of heat evolved
in combustion. The compositions of coals vary markedly in different
localities and often in the same locality several grades are produced.
It sometimes happens that from different parts of a mine the coal will
differ very much in heat value.
Bituminous coals are roughly classified as coking and free-burning. The
former is valuable for gas manufacture and for production of coke. The
coking coals fuse on being heated, allowing the volatile portion to
escape; and when the gas has been all distilled, the residue is coke.
When used for gas making, the volatile portion forms the illuminating
gas. When burned in a furnace, the gases from soft coal burn with a
yellow flame and usually with considerable smoke. The classification
of bituminous coals differ somewhat in the East from that of the West.
Eastern bituminous coals are commonly graded:
A. Run-of-mine coal = unscreened coal as taken from the mine.
B. Lump coal = that which passes over a bar screen with 1-1/4-inch
openings.
C. Nut coal = that which passes through a bar screen with
1-1/4-inch openings and over one with 3/4-inch openings.
D. Slack = all that which passes through a 3/4-inch bar screen.
Western bituminous coal:
E. Run-of-mine coal = the unscreened coal as taken from the mine.
F. Lump coal--divides as 6-inch, 3-inch and 1-1/4-inch according
to the diameter of the mesh through which the pieces pass the
screens.
G. Nut coal--varying from 1-1/4-inch size to 3/4-inch in diameter.
H. Screening = all coal which passes a 1-1/4-inch screen including
the dust.
Heat derived from coal--or any other fuel--in the process of combustion
is due to oxidation. Combustion or burning is caused by rapid
oxidation. When oxygen combines with carbon in sufficient quantities,
carbon dioxide is formed and at the same time heat is liberated. In
burning fuel, if the carbon is completely oxidized and changed into
carbon dioxide, the greatest amount of heat is produced. The required
oxygen is furnished by the air, which through the dampers of the
furnace regulates the rate of combustion.
=Oxidation of Hydrocarbons.=--In the oxidation of hydrocarbons, as
that of burning coal gas, the combination of the elements forms carbon
dioxide and water. The presence of the water, formed in combustion,
is often shown in the formation of moisture on the bottom of a cold
vessel when placed over a gas flame. The same effect is observed in a
newly lighted kerosene lamp, when the film of moisture forms inside the
cold lamp chimney. As soon as the surfaces become heated the moisture
is evaporated. Occasionally, the accumulation of moisture in chimneys,
from this cause, is sufficient during extremely cold weather to form
ice in the part of the chimney exposed to the outside air. Chimneys
have been known to become so stopped by accumulation of ice from this
cause as to materially interfere with the draft.
The fixed carbon of the coal, when oxidized, has a constant heating
value of 14,000 B.t.u. per pound. The volatile hydrocarbons develop
amounts of heat when burned, depending on their composition, and differ
in coals from different localities. The heat obtained from the volatile
part of coal depends on its chemical composition and differs very
materially; it may be as high as 21,000 B.t.u. per pound, or as low as
12,000 B.t.u. per pound.
A high percentage of volatile matter usually indicates a fuel that
will produce a large volume of smoke, which--unless the combustion is
complete in the furnace--will deposit soot as soon as it is condensed,
either in the chimney or in the outside air. The ash has no heating
value, and the contained moisture has a negative heating effect,
because considerable heat is required to evaporate and raise it to the
temperature of the gases of the furnace. In burning fuel the moisture
uses up the heat of combustion in proportion as it appears in the coal.
The moisture is bought as coal but requires heat to get rid of it; so
the percentage of water in coal should be considered very carefully.
It is customary in comparing the heating values of coals, to state
the proportionate parts of fixed carbon, volatile matter, moisture
and ash as well as the B.t.u. per pound of dry coal. The heat value
in B.t.u. per pound of fuel is usually obtained by burning samples in
a calorimeter from which the heat per pound is calculated. The heat
value of fuels used in power plants are often determined by careful
tests of the amount of power derived for each pound of fuel burned in
the furnace. This is done by weighing the fuel burned and measuring
the water evaporated. The ashes are weighed and this weight together
with the weight of moisture present is subtracted from that of the coal
to determine the amount of combustible of the fuel. The final results
are expressed by the number of pounds of water evaporated per pound of
combustible and also the weight to water evaporated per pound of coal
burned.
=Semi-bituminous coal= represents a class between the hard and soft
grades. It contains less carbon and more volatile matter than hard
coal. It burns with a short flame with very little smoke and is
valuable as a furnace fuel. The Pocahontas coal of West Virginia is an
example of this class. Semi-bituminous coal is often called smokeless
coal, because in burning it produces relatively little smoke. It will
be noted in the table of heat values on page 192 that coal of this
variety has high heat-producing properties. It is a very friable coal
and for that reason is apt to contain considerable dust. As a furnace
fuel it produces--when carefully fired--very satisfactory results.
=Graphitic Anthracite.=--This is a type of coal found in Rhode Island
and Massachusetts which resembles both graphite and anthracite coal. It
is gray in color, very hard and burns with extreme difficulty.
=Cannel Coal.=--This is a variety of bituminous coal, rich in
hydrocarbons. It burns with a bright flame without fusing and is often
used for open fires.
=Lignite.=--This is a type of fuel that in point of geological
formation represents the condition between true coal and peat. Lignite
occurs in immense deposits throughout the middle portion of the
western half of the United States, where beds 20 feet in depth are not
uncommon. It varies in color from black to brown and in many localities
is known as brown coal.
When newly mined, lignite contains a large percentage of water,
sometimes as high as 50 per cent. On account of this large moisture
content it has a relatively low calorific value, but when dry the
amount of heat evolved per pound compares very favorably with soft coal.
=Peat.=--As a fuel, peat has been used very little in the United States
on account of the abundance of the better grades of fuel, but in many
parts of the country it is used locally to a considerable extent. In
peat bogs from which the fuel is taken, the peat is formed from grasses
and sedges which in time produce a carbonaceous mass that becomes
sufficiently dense to be taken out in sections, with a long narrow
spade. The peat is then built into piles where after drying it is ready
to be burned.
=Wood.=--On account of its relative scarcity and correspondingly high
price, wood is no longer a commercial fuel of any consequence. The low
heating value of wood as compared with coal makes it a prohibitive fuel
except in forest localities. Wood is commonly sold by the cord and no
attention is given by dealers to its value in heat-producing capacity.
The desirability of wood as a fuel is chiefly that of reputation. It
is usually considered that hickory is the ideal fire wood, dry maple a
close second and that oak is next in desirability as fuel; following
which are ash, elm, beech, etc., depending on the density of the wood.
The price of wood per cord depends on the nearness and abundance of
supply.
The actual heating values of different woods as determined by Gottlieb
show that per pound of dry wood there is little difference in heat
value between different kinds of hard woods, and that pine gives per
pound the highest value of all. The table given below was taken from
“Steam” published by the Babcock-Wilcox Co.
Per cent. of B.t.u. per
Kinds of wood ash pound
Oak 0.37 8,316
Ash 0.57 8,480
Elm 0.50 8,510
Beech 0.57 8,591
Birch 0.29 8,586
Fir 0.28 9,063
Pine 0.37 9,153
Poplar 1.86 7,834
Willow 3.37 7,926
In considering this table it must be kept in mind that the values are
for dry wood per pound.
As given in Kent’s “Engineer’s Pocket Book” the weights of different
fuel woods per cord (thoroughly air-dried) are about as follows:
1 cord hickory or
hard maple 4,500 pounds equal to 1,800 pounds coal
1 cord white oak 3,850 pounds equal to 1,540 pounds coal
1 cord beech, red
and black oak 3,250 pounds equal to 1,300 pounds coal
1 cord poplar,
chestnut and elm 2,350 pounds equal to 940 pounds coal
1 cord average pine 2,000 pounds equal to 800 pounds coal
The above values in pounds of coal may be taken to represent average
bituminous coals. As given by Suplee’s “Mechanical Engineers’ Reference
Book,” eight samples of coals representing bituminous coals from mines
east of the Mississippi River give an average heating value of 13,755
B.t.u. per pound.
=Charcoal.=--This is made from wood by driving off the volatile
constituents; the residual carbon, which forms the charcoal is a fuel
that burns without smoke or flame. Charcoal is made by piling wood in
a heap, which is covered with earth. In the bottom of the heap a fire
generates the necessary heat for distilling off the volatile matter.
Charcoal holds to wood the same relation that coke bears to coal.
=Coke.=--This is the residue from the distillation of coal. It
comprises from 60 to 70 per cent. of the original coal and contains
most of the carbon and all of the ash of the coal. Coke is gray in
color and has a slightly metallic luster; it is porous, brittle and in
handling gives out something of a metallic ring. It is often sold for
fuel as a byproduct by gas factories. In heating value gas-coke gives
about 14,000 B.t.u. per pound when dry and as a consequence is rated
as an excellent fuel. Clean coke burns without flame and is capable of
producing an intense heat. On account of its porous nature it occupies
a relatively large volume per ton. It is most successfully burned in
stoves and furnaces with large fire-boxes.
=Gas-coke=, which is the residue from the gas retorts, is somewhat
inferior in heating value to coke made in ovens but it is an excellent
fuel where furnaces are adapted to its use. Gas-coke is often stored,
by piling it in heaps, in the open and on account of its porous nature
it absorbs considerable moisture. Where exposed to the weather the
amount of contained moisture depends on the amount of rain or snow the
coke has absorbed. This amount is easily determined by weighing a fair
sample and driving off the moisture in an oven. The sample should be
weighed several times until the weight remains constant.
=Briquettes.=--Briquetted coal and other fuels are produced by
compressing coal dust or other powdered fuel, mixed with coal tars or
other bituminous binder in sufficient quantity to cause the adhesion of
the particles when pressed into form under great pressure. Owing to the
relative cheapness of fuel, briquettes have been used but very little
in the United States. With the advance in the price of coal of the past
few years, they have found a place on the market and have become a
common form of fuel.
The heat value of briquettes will depend on the kind and quality of
material that enters into their composition. Quite generally, they
produce heat equal to the average grade of soft coal. In the Northwest
briquettes made of West Virginia semi-bituminous coal sell at the same
price as run-of-mine coal of the same quality. Their use has proven
satisfactory as a furnace fuel and they will very likely be sold in
increasing quantities.
=Comparative Value of Coal to Other Fuels.=--Until a comparatively
recent time, coal has been sold by weight and reputation alone; but
conditions are rapidly approaching, which will require it to be sold
according to its composition and heating value. Among manufacturers and
others using large quantities of fuel, the practice of contracting for
coal by specification is becoming increasingly common. The determining
factors are the amounts of moisture, ash, sulphur, carbon, and volatile
matter the coal contains, as well as the size of the pieces and freedom
from dust. In a few of the most progressive cities, coal dealers are
required to supply coal for schools and other municipal uses, which
has been subject to the approval of the City Engineer. The time is
not far distant when dealers will be required to submit samples of
all fuel, for sale to the public, to the examination of the municipal
authorities.
The following table of the heating values of various fuels is taken
from Benson’s “Industrial Chemistry.”
BRITISH THERMAL UNITS FOR ONE CENT FROM DIFFERENT FUELS
Acetylene, from carbide at 10 cents per pound 600
Denatured alcohol, at 40 cents per gallon 2,000
Air gas (from gasoline, 80°Bé at 25 cents per gallon) 3,000
Water gas, at $1 per 1000 cubic feet 3,000
Coal gas, at $1 per 1000 cubic feet 6,500
Gasoline, at 20 cents per gallon 7,500
Kerosene, at 15 cents per gallon 11,000
Natural gas, at 50 cents per 1000 cubic feet 18,000
Charcoal, at 10 cents per bushel (15 pounds) 20,000
Petroleum at 5 cents per gallon 30,000
Producer gas, from anthracite, $7 per ton 30,000
Producer gas, from coke, $5 per ton 36,000
Anthracite, at $7 per ton 46,000
Producer gas, from soft coal, at $3 per ton 50,000
Coke, at $5 per ton 54,000
Mond producer gas from soft coal, at $3 per ton 65,000
Soft coal, at $3 per ton 80,000
=Price of Coal.=--The value of coal as a fuel will depend on the
amount of heat it is capable of producing when burned; its price
should therefore be determined by the heating value per pound of fuel
as purchased. Secondary determining factors in price are those of
convenience of handling and the difficulty in burning the fuel such
as the size and uniformity of the pieces, the formation of clinkers,
smoke and accumulation of soot. Soft coals, containing a large amount
of volatile matter, usually produce much soot and smoke and as a
consequence sell for a lower price than coals that produce little smoke.
The selection of fuels will depend on the type of heating plant in
use, whether by stoves or by furnaces. If by stoves, whether it is
possible to use soft coal as a fuel. The automatically fed stove, of
the base-burner type, are usually designed for the use of hard coal and
in such stoves the use of soft coal would not be possible. Other stoves
and furnaces are usually capable of burning soft coal with varying
degrees of satisfaction, depending on the design and surrounding
conditions.
The following prices, from the local market, show the usual ratings of
various fuels in common use. These prices vary with the locality and
somewhat with the season. It is usually possible to purchase coal at
some reduction in price during the summer months when the demand for
coal is light.
Hard coal--stove size $10.25 per ton
Hard coal--nut size 10.50 per ton
Semi-bituminous--run-of-the-mine 9.00 per ton
Pennsylvania bituminous--run-of-the-mine 7.50 per ton
Soft coal--Ohio--run-of-the-mine 7.50 per ton
Soft coal--Illinois--bituminous--run-of-the-mine 7.50 per ton
Soft coal--Iowa--bituminous--run-of-the-mine 7.50 per ton
Briquettes-mixture semi-bituminous coal dust 9.00 per ton
Wood (oak), sawed, stove length and split 8.50 per cord
The price of coal is determined in many localities by the distance from
the sources of supply and the means of transportation. The fact that
coals from all of the principal mining areas from Pennsylvania, west to
Iowa, are sold at points in the Northwest for the same price, is due in
greatest measure to transportation rates on the Great Lakes. The prices
of Eastern coals at Duluth are such that in competition with Western
coals they are sold at the same price as is shown by the table.
It is usually impossible for the average householder, or even the
dealer, to determine definitely the exact locality from which his
fuel is mined. Even when such information is obtainable, the quality
is still in doubt, unless analysis is obtainable by sample. The data
given in the following tables is such as will furnish a fair knowledge
of the relative heating values of coals from the principal mining
areas of the United States. The data was obtained from a considerable
number of authorities but chiefly from the reports of the United States
Geological Survey. The different items are not intended to be exact,
they merely represent reliable average conditions.
The varying conditions of available heat and percentage of moisture
given in the following table are such as to be of little use to those
unaccustomed to problems of this kind, unless a systematic method of
comparison is made of the different fuels.
APPROXIMATE COMPOSITION AND CALORIFIC VALUE OF TYPICAL AMERICAN COALS
--------+----------+--------+--------+--------+------+---------+------
| | 3 | | | | 7 |
| 2 |Number, | | 5 | 6 | B.t.u. |
1 | Kind of | of |Moisture|Volatile|Fixed |pound per| 8
Locality| coal |samples | | matter |carbon| dry coal| Ash
| |examined| | | | |
--------+----------+--------+--------+--------+------+---------+------
Pa. |Anthracite| 12 | 5.05 | 5.52 | 82.54| 12,682 |11.53
Md. |Semi- | 5 | 2.39 | 17.73 | 75.44| 14,530 | 7.40
|bituminous| | | | | |
Pa. |Semi- | 15 | 3.60 | 19.26 | 74.46| 14,211 | 8.32
|bituminous| | | | | |
W. Va. |Semi- | 12 | 2.50 | 19.00 | 75.70| 14,758 | 5.24
|bituminous| | | | | |
Ala. |Bituminous| 6 | 3.55 | 29.99 | 59.24| 13,522 |10.73
Ark. |Bituminous| 2 | 1.42 | 16.58 | 73.37| 14,205 |10.05
Colo. |Bituminous| 6 | 9.89 | 37.34 | 52.53| 12,325 |10.32
Ill. |Bituminous| 22 | 10.31 | 36.73 | 50.52| 11,504 |12.73
Ia. |Bituminous| 8 | 7.72 | 39.15 | 50.54| 12,656 |10.33
Kan. |Bituminous| 3 | 4.25 | 32.20 | 51.17| 12,031 |13.75
Ky. |Bituminous| 9 | 5.99 | 34.58 | 56.56| 13,341 | 8.86
Mo. |Bituminous| 9 | 11.52 | 37.85 | 48.11| 12,398 |14.04
Ohio. |Bituminous| 14 | 5.65 | 38.51 | 50.59| 12,839 |10.65
Okla. |Bituminous| 3 | 5.72 | 34.83 | 52.76| 12,648 |12.41
N.M. |Bituminous| 1 | 12.17 | 36.31 | 51.17| 12,126 |12.52
Pa. |Bituminous| 15 | 2.44 | 33.41 | 58.31| 13,732 | 8.40
Tenn. |Bituminous| 4 | 2.53 | 36.58 | 58.21| 14,098 | 5.47
Tex. |Bituminous| 3 | 3.84 | 35.05 | 48.99| 12,302 |15.96
Va. |Bituminous| 5 | 2.71 | 31.32 | 62.47| 14,025 | 6.92
W. Va. |Bituminous| 10 | 2.61 | 33.92 | 58.80| 14,094 | 7.27
Colo. |Lignite | 6 | 19.75 | 45.21 | 45.85| 10,799 | 8.93
N. D. |Lignite | 5 | 35.93 | 44.33 | 43.21| 10,420 |12.45
Tex. |Lignite | 6 | 30.86 | 44.06 | 39.21| 10,297 |16.76
Wyo. |Lignite | 4 | 14.71 | 48.47 | 44.49| 11,608 | 7.035
--------+----------+--------+--------+--------+------+---------+------
The following table was prepared from the date of that preceding
combined with the prices of various coals to be obtained in the local
market. The table is intended to present a method of comparing the
values of fuels from different coal areas. The consumer is interested
to know the amount of heat purchased in the form of fuel. The table
shows in the column headed “Heat per $1,” the number of B.t.u.
purchased for $1 in coal; the number of available B.t.u. in the
different kinds of coal may be taken as a relative comparison of their
values as fuel.
The gas-coke in the table is that sold by the local gas company. The
amount of moisture in this case is relatively high because of the fact
that the coke is stored in a yard exposed to the weather, where it
absorbs all precipitated moisture. A less amount of moisture would give
a higher value for the same fuel.
------------+-----+------+----------+---------+-----------+-----------
| | | | |B.t.u. per |
|Price| Per | |B.t.u. to|100 ÷ cost |
| per |cent.,|B.t.u. per|evaporate| per 100 | Heat
Kind of coal| ton |water |100 pounds| water | pounds | per $1
------------+-----+------+----------+---------+-----------+-----------
1,336,565
Bituminous $7.50 2.44 1,340,000 - 3,439 = --------- = 3,564,000
Pennsylvania $0.375 B.t.u.
----------------------------------------------------------------------
Semi- 1,415,685
bituminous $9.00 3.06 1,420,000 - 4,315 = --------- = 3,145,000
West Virginia $0.45 B.t.u.
----------------------------------------------------------------------
1,101,012
Gas-coke $7.00 10.00 1,117,900 - 16,888 = --------- = 3,145,000
$0.35 B.t.u.
----------------------------------------------------------------------
North 607,282
Dakota $4.50 35.90 668,000 - 50,728 = ------- = 2,703,000
lignite $0.225 B.t.u.
----------------------------------------------------------------------
1,017,602
Bituminous $7.50 10.31 1,032,000 - 14,398 = --------- = 2,980,000
Illinois $0.375 B.t.u.
----------------------------------------------------------------------
994,529
Bituminous $7.50 13.10 1,012,000 - 18,471 = ------- = 2,652,000
Iowa $0.375 B.t.u.
----------------------------------------------------------------------
1,225,905
Hard coal $10.50 3.05 1,230,000 - 4,195 = --------- = 2,335,000
Pennsylvania $0.525 B.t.u.
----------------------------------------------------------------------
Semi-bituminous coal commands considerable favor as a house-heating
fuel, because of the fact that it burns with much less smoke than
bituminous coal. In available heat it is considerably above the Western
bituminous coal and it sells at a price $1.50 higher per ton. The
reason for the difference in price is not so much on account of its
heating value, as because of relatively small amount of smoke produced
in combustion. Other coals capable of producing more heat are sold at
less price because of smoke and soot produced in burning.
Hard coal at $10.50 is the most expensive coal of all. The ratio of
available heat units per $1 for hard coal, as compared with the best
soft coal, is as 23 is to 35. This means that at the stated prices
those who burn hard coal pay the additional price, because of the
physical properties it possesses.
In constructing the above table, 100 pounds of coal was taken as a unit
of comparison. The price per ton is that given in the table of local
prices. The per cent. of moisture and the B.t.u. per pound of fuel was
taken from table on page 192.
In explaining the method by which the different items were obtained, it
will be necessary to discuss briefly the condition of combustion and
the heat losses that take place when fuel is burned.
The moisture in the fuel is the undesirable part, because it requires
a large amount of heat to dispose of it. It is looked upon as so much
water, that must be raised in temperature from that in which it is
taken from the coal bin to the temperature and condition of vapor in
which it passes into the chimney. When the fuel enters the furnace
the water is heated to the boiling point. In changing temperature
it absorbs 1 B.t.u. for each pound of water, through each degree of
change. Suppose that, as in the case of Pennsylvania bituminous coal
which contains 2.44 pounds of water to each 100 pounds of coal, the
coal entering the furnace was at 50°F. To raise its temperature to the
boiling point (212°F.) required a change of 162°. The 2.44 pounds of
water raised this amount
162 × 2.44 = 395.28 B.t.u.
To change the 2.44 pounds of water, into steam at the atmospheric
pressure requires 969.7 B.t.u. (heat of vaporization), practically 970
B.t.u. per pound of water. The heat required to vaporize 2.44 pounds of
water is
2.44 × 970 = 2366.80 B.t.u.
The vapor is now raised in temperature, to that of the furnace, which
we may assume is 1200°F. The furnace being at atmospheric pressure the
vapor merely expands in volume as a gas. The specific heat of steam at
atmospheric pressure is 0.464; that is, 1 pound of steam requires only
0.464 B.t.u. to raise it a degree, and 2.44 pounds of water will absorb
0.464 × 2.44 × 1200 = 1356.00 B.t.u.
Of this last amount of heat, approximately 50 per cent. is recovered as
the gases pass through the furnace. The total loss of heat due to the
evaporation of the water is
Raising temperature from normal to 212° 395 B.t.u.
Evaporation 2,366 B.t.u.
Changing temperature of vapor, less 50 per cent. 678 B.t.u.
------------
Total heat loss 3,439 B.t.u.
In the 100 pounds of coal under consideration, there is 100 pounds,
less 2.44 pounds of water, or 97.56 of dry coal, each pound of which
contains 13,732 B.t.u. as given by the table on page 193. This gives
97.56 × 12,682 = 1,339,753 = practically 1,340,000 B.t.u.
From this quantity is subtracted the loss of heat, 3439.
1,340,000 - 3439 = 1,336,561 B.t.u.
This represents the total available heat in 100 pounds of coal. If this
quantity is now divided by the cost of 100 pounds of coal at $7.25 per
ton, the result, 3,564,000 B.t.u., will be the available heat bought
for $1 as given in column 7 of the table.