520

THE COLLIERY GUARDIAN.

March 17, 1916.

Rock pressure in ■ mines increases directly with the
depth, and averages about 1 lb. to the square inch for
yeach foot of depth. Therefore coal at a depth of 1,000 ft.
is under pressure of a-half ton to the square inch. The
bearing of this factor on gas in the pores of the coal has
not been fully considered, but when the coal is removed
and released from such a great compressive strain it
expands materially, and there is readjustment of stress
from the interior -to the surface of each lump of coal, as
well as of the face, of the pillars, and of the roof and
floor.

Permeability of Rocks and Coal to Gas.

The gas in the coal, especially where the strata are
uplifted and flexed, is only a remnant of the original
amount, for leakage has been in progress for thousands
of centuries. That gas will penetrate rock for long
distances has been well established, and although sheets
of fine-grained materials must greatly impede its circu-
lation, yet time is by far the most important factor.
Many cases are known of securely plugged gas wells
from which the gas has flowed laterally into other wells
or into mines. The rate at which gas escapes to the
surface in any coal field is, as a rule, very slow, and
the escape is so general that it is not apparent, except in
places along faults or joint planes where at some
localities it manifests itself most vigorously.

Escape of Gas from Strata near Wilkes-Barre.

Not long ago a large amount of coal gas was found
bubbling up through the water of the Susquehanna River
and the sloughs opposite Wilkes-Barre, Pennsylvania.
At one place a pipe forced some distance into the sand,
where bubbling occurred, supplied gas enough to light
and heat a house near Market 'Street bridge. Some
years earlier, a vigorous “ blower ” was uncovered in the
deep cut of the Nanticoke Branch of the Central Railroad
of New Jersey, a mile east of Nanticoke, Pennsylvania,
which as yet shows little diminution of force. There
are in various parts of the field many small gas outlets
.that illustrate how general is the escape of gas.

The proportion of gas that has escaped from coal beds
can not be determined, but it varies greatly, and in places
the gas is practically all gone. This is the case along
the outcrop zones, and in shallow basins where the cover
is thin, or consists of coarse-grained rocks. Probably all
coal contains some methane, but the word “ gaseous ”
is only applied when there is a sufficient emanation to
show in a safety lamp.

Escape of Air from Sealed Chamber in Maryland Mine.

An interesting observation in this connection was made
some years ago by Randolph in a mine in the Georges
Greek district, Md., working the Pittsburg bed, which
is there soft and friable "and intersected by numerous
slips. During the installation of a new pumping system
the water was allowed to flood the lower workings, and
as some of these chambers rose from the main gangway
and had no outlets, a large body of air was entrapped in
them under a water head of about 40 ft. part of the
-time. At the end’of 18 months, when the water was
pumped out, it was found that the chambers had been
.filled with water to the roof, and consequently all the
air had been forced out through the nearest outlet, which
was 200 ft. away through the coal bed. When the mine
was unwatered below the level of the chambers, some
of the air worked back again, and finally presented a
partial vacuum equal to that produced by a water column
7 ft. high.

Randolph also observed an instance of the impenetra-
bility of the “ slate ” above the coal in this mine.- A
heading passing over a dome was' partly filled with
water that imprisoned considerable air. When the
mine was pumped out it was found that the water level
had never reached the roof, although at one time the
air had supported a head of upwardxof 80 ft. of water.
In order to escape, the air had to pass through the
“ slate ” at right angles to the stratification, but could
not do so to any perceptible degree.

The slowness of movement of gas through the coal
has been demonstrated by some of the pressure tests
already described, which were so conducted as to throw
light on this condition. However, the demonstration
applied mostly to solid coal, and the time covered by
the experiments was short.. iSimoh, too, found evidence
of the passage of gas into the overlying strata at Lievin.

Gas in Strata Adjoining the Goal.

■Shales and sandstones above or below the coal bed or
interlaminated with it contain more or less gas which is
liberated as mining progresses. Morin believes that
such beds are an important source of gas in mines, and
the results of some of his tests appear to substantiate
the idea. It is found that generally the amount of gas
in a mine increases >as the mining area is extended, even
if the area of working faces, does not increase. This
increase is due in large part to increased area of coal
exposed in rib, pillar, and gob, but Morin believes that
these soon lose their gas, and the increment comes from
the floor and roof. As coal is removed, and for a while
afterwards, the overlying and underlying rocks move
more or less, and thus many fissures are formed from
which gas escapes either from the rock itself or from
adjoining coal or coaly beds. Naturally, this emission of
gas is more rapid near the working faces, when there
is more gas and fissuring is most active. Boreholes
drilled into the floor and roof of the coal at Lievin
showed little gas at a distance back from the face,
although there might be considerable of it above and
below the solid coal.

Simon tamped 20-ft. tubes in the roof of the, Frederic
coal bed (Lievin), and found a pressure of only 1-4 lb.
per square inch, though the pressure in the coal below
was from 62 lb. to 80 lb. In- the newly opened Alfred
bed the returns at working faces contained 0’4 per cent,
of methane, whereas at the extremity of the same return
the amount was nearly 0-9 per cent., and, under similar
conditions, 0*5 to 0’9 per cent, has been found in the
Arago bed. A further example was afforded by an air-

way passing above the Du Souich bed. It showed no
gas even when the ventilation was not effective, but
when two working chambers in the coal below were
extended under this airway, gas became perceptible,
which undoubtedly worked up through fractures in the
strata, due to the removal of coal.

Morin also made tests of the amount and the pressure
of gas in rocks. One test was in an overturned part of
the Alfred bed, where a 5 ft. hole in the wall gave a
pressure equal to 0-4 in. of water (about lb. to the
sq. in.). A second hole in a gangway two' years old
yielded gas containing 6-3 to 5-7 per cent, of methane,
the volume being one-eighth to one-fifth of a cu. in. a
minute. Similar tests in the Du Souich workings at
different places gave varying results. One hole 5 ft.
deep, in the wall of some workings three years old, gave
off air and gas containing 2-2 per cent of methane, but
no pressure was evident; another hole in the roof at
another point, in workings a year old, gave off gas con-
taining only 0-39 per cent, methane, but doubtless this
hole had crossed a fissure; and a third hole in the roof
gave off a small flow of gas containing 5 and 4*6 per-
cent. of methane. These tests all prove that methane
lingers in adjoining strata after the coal has been
removed. Some tests were made in undeveloped sec-
tions, one being 65 ft. above the Arago bed. A hole 6 ft.
deep yielded samples of gas containing only 0-39 and
0-065 per cent, of methane. Another hole 5 ft. deep in
an unworked region yielded air containing 0-19 per cent,
methane, whereas a third hole, 4 ft. deep, 13 ft. above
the Beaumont bed, yielded gas with a methane content
of 3 per cent. These results show that at a distance
from the coal the amount of methane in the strata is
very small, but near the coal the amount is consider-
able.

In one shaft in Prussia gas began with clay “ slate ”
at 13 ft. below ground, and extended to the top of the
Wealden sandstone, 551 ft. below. The gas outflow was
very strong at 515 ft. in dark bituminous clay “ slate.”
In sinking the shaft (No. 3) for the Woodward mine just
north of Wilkes-Barre (Pennsylvania), gas was found in
the sandstones and other rocks in large quantity, and
gave trouble all the way down. At one stage of pro-
gress the outflow was estimated at 1,000 cu. ft. a
minute. Possibly the‘gas was rising from the under-
lying coal beds, but some of the rocks were shales,
relatively impervious.

A boring in the Schaumburg district was in shales; at
a depth of 695 ft., or 280 ft. above the coal bed, gas was
encountered which threw a 2 ft. column of water for
1-J- hours, and was in vigorous action at times for 24
months.

Gas in a Sandstone Stratum in Pennsylvania.

A rock tunnel in the “ Harry E ” anthracite mine,
near Wilkes-Barre, penetrated a sandstone that gave
off considerable methane. A sample of the fresh rock
was sealed, and sent to R. T. Chamberlin for a deter-
mination of its contained gases. The sample was broken
into fragments the size of small marbles, which were
then placed in a vacuum bottle, and the air exhausted.
Of course, considerable gas was lost in this process. At
the end of 21 days the gas that had accumulated in the
vacuum bottle, during this time was pumped out and
analysed. The volume of gas at 32 degs. Fahr, and
30 in. pressure, amounted to only 0-013 per cent, of the
volume of the rock used. The composition of the gas
was as follows :—Methane, 4-2 per cent.; carbon dioxide,
1-9; air, 70-0; nitrogen (excess over air), 23-9 per cent.
It is probable that the methane in this rock came from
the coal below, and was stored in the pores of the sand-
stone. Doubtless most of it passed out of the rock
rapidly, as the excavation progressed.

Goal Oxidation as a Source of Gas.

A possible factor in the mine gas problem is the
abstraction of oxygen from the air by coal. This process
appears to progress in all kinds of coal, but at very
different rates, some coals showing slight, disposition to
absorb oxygen, whereas others absorb a large amount.

The result of oxidation is the production of carbon
dioxide, accompanied by an increase of temperature,
which accelerates the emission of the methane held in
the coal. Haldane and Meachem found that the result-
ing increase of temperature was notable. At the
Hamstead Colliery, in South Staffordshire, at the bottom
of the shaft, which is 1,880 ft. deep, the average air
temperature was about 11 degs. higher than at the sur-
face, and rose 6 degs. Fahr, for every 3,000 ft. along
the main intake, rising to 80 or 85 degs. Fahr, in the
workings. The temperature of the return air diminished
from the face to the upcast, but at a slower rate than
the increase in the intake air. In one split, the tempera-
ture of the intake air was 60 degs. Fahr.; of the rock in
the mine (due to depth), 68 degs. Fahr.; and of the
upcast air, 77 degs. Fahr. The ventilating air lost in
oxygen 3-13 times more than it gained in carbon
dioxide in this mine, but in other mines the ratio was
only about 1-6.

Haldane and Meachem concluded that the increase of
temperature was mostly due to oxidation of coal, and
that in general “ the temperature increment of the
mine air rose with the diminution of oxygen.” They
also suggested that carbon dioxide in various kinds of
mines and pits may be largely due to oxidation. A self-
recording maximum thermometer placed in a 10 ft. hole
in the gangway rib soon recorded 66 degs. Fahr., but
four years later the temperature had risen to 90 degs.
Fahr. This and other observations show the gradual
penetration of oxidation, -and the resulting rise of tem-
perature in the coal exposed.

Absorption of Oxygen by Coal Samples.

Porter and Ovitz found that oxygen is absorbed rapidly
and for a long period by fresh coal, and but little carbon
dioxide results. An excellent illustration was a test with
22 lb. of Connellsville coal. In one day after mining it
absorbed nearly 60 cu. in., or half of the oxygen in

610 cu. in. of air, and gave off little more than one-tenth
as much carbon dioxide as would have been formed if
all the oxygen absorbed had combined with carbon and
been evolved as carbon dioxide. They suggest that the
reaction is probably a direct combination, of the oxygen
with certain fixed components of the coal, as it has been,
shown by Boudouard and others that certain compounds
in coal are unsaturated with oxygen, and its absorption
produces humic acid or related substances.

• The rapidity of absorption was shown by tests with
lignite from Sheridan, Wyoming, and bituminous coal
from Benton, Illinois, which in less than 15 days had
exhausted oxygen from a volume of air equal to their
own volume, but neither coal gave off more carbon
dioxide than one-tenth of the volume of oxygen
absorbed. That this oxygen is probably taken into
chemical combination rather than mechanical absorption
is shown by a test of Benton coal. A sample of this
coal, that had been supplied with oxygen for five
months, absorbed nearly seven times as much oxygen
as it originally contained. The sample was heated to
212 degs. Fahr, for 15 minutes. The gases evolved con-
tained no excess of oxygen over that in air.

(To be continued.)

SEALING OFF MINE FIRES.*

By Joseph Cain.

The following methods have been used in overcoming
no less than 23 mine fires which came within the
author’s personal experience, and which required more
than the ordinary agencies, water and dirt, to put out.

The last of these fires (August 2, 1915) started by the
ignition of a gas feeder in the firing of shots in the
advanced workings, embracing five working places,
namely, three entries and two cross cuts, 6 ft. high, and
between 10 and 12 ft. wide (see fig. 1). All places where
shots are fired by regular shot-firers are examined within
two hours after shooting; and the shot inspector, in
making his rounds, travels with the intake current, occa-
sionally crossing to the main return of any one district
to note the conditions of the return air. On this occa-
sion, as soon as the fire was detected, a careful inspec-
tion was made, the return being tested at intervals with
safety lamps. On reaching a point where smoke,
fumes, and heat became unbearable, and it was unsafe
to go farther, it was decided to seal up the fire zone.

Checking Access of Air to the Fire.

In the meantime, quite a large portion of the intake
current was short circuited, to prevent the entire volume
of air reaching the fire. This would enable each of the

Fig 1.—Location of Fires and Seals.

Air Current before Seating’
»	>’ after

working districts to be curtained off as near to the fire
as possible, so as to prevent the smoke and fumes going
to each of these districts, and confine all the smoke to
one return. The necessary material for sealing, viz.,
lumber, nails, ducking, tools, two kinds of clay, all avail-
able safety lamps, 75 ft. of rope, and quite a quantity
of drinking water, were procured, and taken in on the
main intake side.

A watchman was placed at the fan, and another at
the first opening to the main return, so as to prevent
anyone entering. The other employees were divided as
follows : Two to get the lumber to the sealing location,
one to procure the clay, one the water, one to mix the
clay properly, two to saw the lumber to the .desired
lengths, and two to prepare the place for the sealing.
No naked lights were allowed at or near these places.

In the meantime a further inspection was made, by
going as near to the fire as possible, and testing the three
openings leading to it, in order that the work of sealing
should be done in the surest, safest, and quickest
manner possible. On the intake side a point was
reached near enough to hear the roaring, crackling

* From a paper read before the Kentucky Mining Institute.