754

THE COLLIERY GUARDIAN.

April 20, 1916.

Morin therefore concluded that, in these experiments,
the old workings were not the only cause, or even the
most important one, in the variation of the amount of
methane found. In order to test this question further,
he made some precise determinations of the amount of
methane in old workings at Lievin. In a chamber in
the most gaseous part of the mine, a tube 16 ft. long
was fastened along the roof, and the chamber was then
filled with gob. The end of the tube that projected into
the gangway was plugged. After three months, samples
drawn from the pipe contained as much as 0-46 per cent,
methane. A similar experiment in another chamber
gave 0-75 per cent. Meanwhile the amount of methane
in the ventilating currents in returns near by varied
from 0-3 to 0-6 per cent. In old workings, in another
part of the mine, where the ventilation was mostly cut
off, some of the air from the gob was found to contain
2 to 2-7 per cent, of methane. These tests indicate that
although some gas is given oft by old workings, even if
closely filled, it does not increase to any important extent
the volume in the returns. No air rich in gas could be
obtained from any part of the old workings at Lievin,
even though sampling pipes were placed close to the
roof and in the highest places.

At one stage of Morin’s observations the ventilation of
the mine was changed, with the result that when the
volume of air passing through one of the main returns
was diminished, the proportion of methane became only
0-4 per cent. Samples of this air taken at intervals,
therefore, did not show any concordance with changes
of atmospheric pressure, although the air traversed a
considerable area of old workings. In some working
faces, considerable gas came from enclosing strata, but
the amount was variable and a much greater volume
came from the coal in the face and from piles of broken
coal.

In order to determine whether there was any relation
between the barometric pressure and the amount of
methane given off by fresh coal surfaces, Morin made a
series of tests of the return air from an individual
chamber. Only negative results, however, were
obtained.

Effect of Settling of Roof and Floor.

The negative results of above test, confirming the
supposition that the high internal pressure of the gas
in the solid coal would not be affected by barometric
change, and the lack of evidence that any great amount
of gas came out of the gob and old workings, led Morin
to conclude that the variations in the rate of escape of
the gas were due to other causes. He suggested, there-
fore, that the barometric changes must mainly affect the
volume of gas coming from floor and roof, whether this
gas is contained in the rocks themselves or comes from
fissures connected with adjoining coal beds. He pointed
out that as the coal is removed the roof settles and the
floor tends to rise, causing fractures, from which gas from
adjoining virgin coal may enter, the workings. The pres-
sure affecting the flow of gas through such fractures is
independent of the original pressure in the relatively
impermeable coal, and may be so light as to be closely
balanced by atmospheric pressure. Morin believed that
this source of gas is the most susceptible to atmospheric
changes, and is responsible for the great variations in
volume of methane observed.

Relative Volumes from Different Workings.

In order to ascertain the relative amounts of methane
given off by workings of various kinds and extents
under changing atmospheric pressures, Morin collected
samples of return air from various splits of return No. 1
at Lievin before and after a barometric fall from 30-72
to 29-12 in. The results showed that volume of gas in
old workings is not the sole factor to consider. In all
the workings but those in one return, the adjoining beds
had not been mined. Old workings in beds of coal
equally gaseous and worked to the same extent gave off
greatly different amounts of gas from the same volume
of voids. The volume of gas in the airway from
workings No. 1 in the Auguste, bed, and from workings
in adjoining beds, did not vary during the great fall of
pressure. On the other hand, the methane from work-
ings in the Frederic bed, which are undermined in about
the same area by workings in the Du Souich bed, 50 ft.
below, was more than double that from the Du Souich
workings; also the volume of gas from the latter work-
ings varied less with the change of atmospheric pressure,
indicating an escape of gas upward from the Du Souich
bed to the Frederic bed workings.

The volume of gas in an airway draining a small area
of the Auguste workings and a rise 800 ft. long, did not
vary; neither did the volume of gas in the air from the
Arago bed, in which a gangway 1,300 ft. long was in
progress, and over which there were many blowers. In
this case the emission of gas from coal in place and from
blowers was not affected by a great change of atmo-
spheric pressure.

The tests show that the virgin coal is only slightly
permeable, and therefore the gas pressure in the rock
strata affected by the mining is independent of the
pressure in the virgin coal, so that, as the gas is drained
by advance of the workings, the pressure becomes feeble.
From the facts observed the following conclusions are
drawn :—(1) The pressure and voluine of gas in the
rock strata (near the working face) is low; (2) the coal
beds are less gaseous near the working faces than in
more remote parts, and in certain cases the drainage
of the beds is complete and the last of the gas escapes
under feeble pressure; (3) the coal beds far from work-
ings remain gaseous; (4) the strata affected by coal
workings, even though distant, contain little gas, and
. in consequence after a settling of roof or rise of floor
the gas pressure in the zone of fracturing remains very
. feeble.

Morin gave the following resume of the experiments
in the Du Souich bed :—The quantity of gas liberated

does not depend alone on the amount of change of baro-
metric pressure, but also on the absolute pressure, and
therefore becomes proportional to the time between the
two extremes of pressure limiting the fluctuation. When
a period of constant atmospheric pressure followed a
fluctuation the proportion of gas remained at the figure
that it had reached as a result of the fluctuation. A low
or a high pressure lasting for several hours induced a
maximum or a minimum proportion of gas lasting for
the same length of time. A given pressure was not
always attended by the same proportion of gas, because
of complications caused by the effects on the atmosphere
in old workings. After an increase to a given atmo-
spheric pressure the percentage of gas was smaller than
after a diminution to the same pressure. He suggested
that probably this could be explained by the supposition
that during a period of high pressure the gas accumu-
lates in old workings, whence during periods of low
pressure the gas flows out into the returns.

Finally, Morin concluded that variations in gas out-
flow, induced by fluctuations in atmospheric pressure,
are due not only to old workings, but also to gas from
adjoining strata (roof and floor), and from fissures
through these strata which admit gas from adjoining
coal seams.

Observations in England and the United States.

It has been observed in an English mine that when the
mine is hot—that is, has a relatively high temperature—
the presence of methane is less evident, but when an
increased volume of cold air has been introduced the
proportion of gas increases rapidly. This has been
noted many times. Therefore, the variations of
temperature must have had a close relation to the
amount of gas given off in that mine.

For several years at the Auchincloss mine, near
Nanticoke, Pennsylvania, a comparison was made of
barometer readings and daily reports of fire bosses as
to the amount of gas in the headings, and it was found
that there was no relation between them. The mine is
relatively new, without large areas of old workings.

Tests with Pennsylvania Coal.

In tests to determine the amounts of gas given off
under diminished pressure, Chamberlin obtained some
results that throw light on the effect of barometric
changes. The relative amounts of gas given off by
bituminous and anthracite coals under long-continued
low pressure were compared with those obtained at
ordinary air pressure. Two samples were taken, one of
bituminous coal from the Mansfield mine, Carnegie,
Pennsylvania, and one of anthracite coal from Nanticoke,
Pennsylvania. Each sample was divided into two parts,
one of which was placed in a vacuum bottle from which
the air was pumped; the other was sealed in a bottle
at ordinary air pressure. All the bottles were sub-
merged in water. After seven days the anthracite
samples were tested, and in both bottles the methane
evolved was 1-07 times the volume of the coal. At the
end of 10 days the bituminous coal kept in a vacuum had
given up 0-25 of its volume of methane, whereas the
sample under ordinary air pressure had given off only
0'14 times its volume. As the amount of methane
evolved in the air-filled bottles was greater than the
volume of oxygen absorbed from the air by the coal, the
final pressure was slightly greater than the barometric
pressure toward the close of the test.

In these experiments the diminished pressure appeared
not to have increased the emanation of gas from the
anthracite. Chamberlin decided that barometric varia-
tions that do not exceed 1| in. cannot have much effect
on gas escaping from the interstices of the coal, and
that time was the principal factor in the escape of gas.
Blowers and feeders escaping under low pressure from
fissures are naturally much more sensitive and might
be affected by barometric changes.

Earth Movements and Methane Emanation.

Many suggestions have been made as to the effects of
earthquakes, sun spots, and other natural phenomena on
the emanation of gas in coal mines, but most of the
evidence presented is not convincing. Several papers
on the subject give comparative statistics of explosions,
overlooking the fact that many of them are caused by
coal dust, and due to some mishap in the mine that
would produce the same result at any time. The sug-
gestion that variations in atmospheric pressure cause
an up-and-down movement of the earth’s crust, which
changes the conditions of gas emission, may have some
bearing. Darwin estimated that very hi^h barometric
pressure—good weather conditions—may bend down the
earth’s crust 3 or 4 .in., and conversely diminished
pressure may be followed by expansion. The latter
might cause the opening of crevices and let out gas,
especially where mining has set up uneven stresses in
the strata. Gas being under great pressure in the strata
its emission might be influenced by a cause of this kind.

Milne reviewed the effect of the earth pulsations due
to meteorological influences, and cited experiments of
Chesneau, <at Anzin, who for many months noted rela-
tive movements of a tromometer and amount of methane
given off. The relation was well marked in some cases,
but in general it kept pace with barometric changes.

From a consideration of the evidence presented, it
would appear that earth movement cannot affect the
emanation of methane unless possibly to precipitate an
outburst that is nearly ready to take place; ordinarily
a blast in a near-by chamber would cause much more
local movement.

Duration of Methane Emanation.

Many of the facts stated in the foregoing show that
coal loses the greater part of its gas content while it is
being mined and broken; and, in the same way, much
of the gas in the coal exposed in faces, ribs, and pillars
drains off rapidly. In the mine, however, this drainage
of gas is superficial, for at greater or less depth the solid

coal contains its ordinary volume. This gas is disposed
to flow to the exposed surface, but the rate of move-
ment must depend on the pressure and volume of the
gas, and the porosity and solidity of the coal. In
general, in gaseous places it is found that the gas emana-
tion is rapid when the workings are progressing rapidly;
but after awhile, as the district is opened, the volume of
gas greatly diminishes. It is a frequent practice in
gaseous mines to extend gangways rapidly into gaseous
places to effect a general ventilation, and then to leave
the place for awhile until the gas diminishes so that
mining ceases to be dangerous. Slight cracking of roof
and floor facilitate gas escape, and in some cases gives
rise to feeders. The diminution of methane emanation
is shown by the fact that in many mines after Sunday
cessation, or any other shut down, the amount of gas
diminishes. There are many examples of gaseous beds
that have yielded little gas in areas where the adjoining
beds have been worked out, the gas having drained off
through cracks and porous strata.

Gas Drainage in Mining.

The project of tapping off gas in gaseous districts is
one that many coal engineers have considered, and occa-
sionally have carried into practice. In many mines it
has been found desirable to extend gangways rapidly
through a gaseous district, and then, after ventilation
has been established, to let the gas draw off for weeks
or even years before general mining is begun. Working
out an overlying bed, in order to drain off gas from the
lower one, is often tried, but the efficacy of drainage
depends on the permeability of the intervening strata.
The extension of bore holes horizontally in the coal bed,
or from the surface above, is a promising expedient, but
in most cases it does not draw much gas from solid coal,
because the surface exposed is too small, and gas travels
through the coal too slowly to affect a wide area. This
has been proved by tests of pressure and volume from
boreholes; after a hole has been opened a long time,
and then closed, the pressure is soon the same as at first.
Many tests have been made of drainage holes in
European mines, but the results have been entirely
unsatisfactory. Some experiments by Lange, in the
Rochebelle mines (France), to drain off carbon dioxide
by boreholes were unsuccessful. Old workings, how-
ever, have been drained by tubes sunk from the sur-
face, sometimes to great advantage. The author
observed an instance of this in the Otto Colliery, near
Pottsville, Pennsylvania, where tubes tapped the top of
the workings in inclined beds, and a very large volume
of methane was brought up, by escaping air from a
small ventilation split. The proportion of gas was so
high that the mixture could be lighted, and would burn
for hours.

Methane Content and Character of Coal.

Investigations having shown that the amount of
methane in coal varies greatly in different coal beds in
the same series and in different parts of the same bed, it
is concluded that the original methane content is closely
related to the constituents of the original deposit, pos-
sibly modified somewhat by conditions under which the
coal was formed. In making comparisons, however,
the original amount of methane should be taken into
consideration, because many parts of the coal bed that
may have been very gaseous when they were formed
have lost more or less of the gas by drainage to the
surface. Such drainage through overlying strata takes
place, partly by percolation, and in part by crevices, due
to fracture. It is also a fact that the gas emitted in
outbursts, blowers, and even in notably gaseous work-
ings, is not always an indication of the total amount of
methane in the coal that may be given off in time. No
relation has yet been found between the proportion of
methane and the texture of the coal or of the com-
ponents as indicated by chemical analyses, for coals high
in volatile constituents are not the highest in methane.
Undoubtedly, the original vegetable material now repre-
sented by coal varied greatly in character in different
places and at different times, even in the same basin;
and the rate of decomposition and of burial by later
deposits, and the nature of these deposits, varied greatly,
and these variations had much to do with the develop-
ment of gases. It is the author’s belief that" a relation
will be found between methane content, or at least the
original methane content, and the organic constituents
of the coal; and it is along this line that future detailed
study of the problem should be directed.

Error in Methane Determinations.

In most records of the proportion of methane in mine
air, the figures are given to the second place of decimals,
and even with greater apparent precision. In an
investigation of the probable accuracy, of ordinary
methane determination, G. A. Burrell has found that
the probable error is nearly 0-03 per cent. Accordingly,
in two analyses reported as 0*17 and 0'19 per cent., for
example, the amount of methane may be the same, or
the first may be 0-19 per cent, and the second 0-16 per
cent. This probable error invalidates all arguments
based on such small differences as 0*03 per cent. "When
this probable error is calculated into cubic feet per
minute the error increases, for air volume determina-
tions are necessarily somewhat inaccurate. Usually the
area of cross section is not precisely determinable or
determined. Most anemometers have an error much
greater than 1 per cent.; and, moreover, the actual
velocity of the air current varies in different parts of
the cross section. Probably the ordinary determination
of volume of air per minute has an error of at least
5 per. cent. That is, a current reported as 10,000 cu. ft.
a minute may be anywhere between 10,250 and 9,750
cu. ft. Therefore, in such a volume of air analysing
1'0 per cent, of methane the true amount of that gas
is between 102J and 97J cu. ft. per minute. This is not
allowing for probable error of analysis, by which the true
proportion may range from 1*01 to 0*98 per cent., so that