April 27, 1917.

THE COLLIERY GUARDIAN.

807

turbance of the beds in their places, and the libera-
tion of gas, he observes that in the Donetz basin there
are many interesting examples of the influence of
thrust movements on the increased liberation of gas.

On the limits of the explosiveness of methane, the
author made an important series of experiments with
gas from the blowers of the Sofia shaft, 12 mixtures in
all, adding that the experiments showed, with a con-
tent of methane in the air of 6-62 to 6-00 per cent.,
the explosion takes place under the pressure of
332-6 mm. of mercury. When the content of methane
in the mixture fell to 5-63 per cent, at the beginning
of pressure of 410 mm. mercury, the explosion was so
weak that the combustion was not complete. The
flame from the platinum spring did not extend (as
usual) to the whole jar, and only reached downwards;
and after the explosion there were found to be 2-35
per cent, of unconsumed gas along with 3-50 per cent,
of carbonic acid. With a 6 per cent, content of gas,
with the same pressure, but with the methane at 0-4
per cent, more, the combustion was complete. This,
and many other experiments, showed, as far as they
could be relied on, that one may conclude that the
variation in the pressure on this and that side from
the atmospheric, about one-half an atmosphere, does
not exert much influence on the lower limit of the
explosiveness of methane. These variations are of
much more importance in the complete combustion of
methane at the moment of the explosion. In this case,
pressure has much more influence than the concentra-
tion of the gassy mixture.

Reviewing all the data provided by himself and
others on mine equipment, he says one must have in
view that at first, after ignition, when the extension
of the gases in the underground workings may go on
unhindered, the pressure cannot be particularly high.
But later, when the wave of the explosion meeting con-
ditions favourable to its continual increase extends
over a great part of the workings, the pressure may
reach a very high tension. The dimensions of mine
damage with gas explosions leads one to suppose that
the pressure of gas with explosions may attain to
several tens of atmospheres.

Electricity as a gas igniter does not occupy much
of the work under review. It is probably of limited
interest to the Donetz coal people, owing to the
limited application of electricity in the local mines.
But in view of the contemplated great extension of
the use of electricity in the Donetz coal basin, it will
no doubt command greater attention there soon.

The question of the distribution of methane in mine
air occupies a considerable part of the author’s
treatise. He says that the question of the distribu-
tion of methane in various parts of one and the same
vertical section, besides its theoretical interest, is of
great practical importance, connecting the ease of
ignition of mine gas with an accidental accumulation
of it in the upper parts of the working, which makes
it important to investigate it experimentally. He
took 53 samples of the air from the shafts of two
mines. One of these had improved ventilation; in the
other the ventilation equipment was more modest.
The samples were taken in glass graduated vessels with
two ground cocks. The analyses were made with the
Brockman apparatus. The limit point for methane
may be taken as 0-02 per cent., so that the disagree-
ment of the control analyses in most of the original
cases was less than 0*02 per cent., or equal to it.

Eor the distribution of methane in the air current
at places away from the goaves, four samples were
taken in the ventilation galleries, along which moved
the outgoing current of an incline; and in each case
except one when the position where the sample was
taken was 15 sazhens from the goaf, and consequently
the process of diffusion could not have been completed,
there was a uniform distribution of CH4 in various parts
of one and the same cutting. Some irregularity was
observed in the last case (the exception above referred
to), but it was too slight to call it an exception.

A further series of tests in cases of still air in places
fairly distant from the chief sources of methane, that
is, a goaf, showed that no notable difference in CH4
could be seen in the upper parts of the workings com-
pared with the lower. The time that passed before
the samples were taken does not let us think that the
pillars of coal could be important sources of gas.
Therefore, the regular distribution of methane can be
easily explained by the finished process of diffusion
in the cases now considered.

As to the points near the goaf, the -results of
numerous tests lead the author to conclude as follows:
“All the facts quoted can be easily explained by accept-
ing the proposition that the process of diffusion,
having as its result the regular distribution of methane
through the whole section of the working, requires a
certain time for its completion. The nearer the place
where the samples are taken to the source of gas for-
mation (goaf, pillars, smalls), and the stronger the
last, the longer time will be required for the process of
diffusion to complete itself. Thanks to this circum-
stance, in places adjoining the goaves, or very imper-
fectly ventilated, we should observe an enrichment in
methane of the upper parts of the workings; while in
more distant galleries from the fresh goaves there
should be a regular distribution of this gas in various
parts of one and the same cross section. An increase
in the speed of the air current hastens the mixture of
the gases.

“In the case of ventilation of the goaf with the aid of
screens, an irregular distribution of methane is
observed also in the horizontal section of the working.
The coal of the goaf adjoining the screen usually con-
tains a larger quantity of CH4 than does coal placed
away from it. This fact occurs with uniform exact-
ness, as in the case of the movement of air from behind
the screens, just as in the case when the current passes

beyond the screens. The explanation of this occur-
rence should probably be found in the greater speed of
parts of the air in places adjoining the screens, because
of the reduction of the section of the current produced
by these. The central part of the goaf in the cases
referred to is always the least gassy.”

POWDERED FUEL IN CEMENT WORKS.

In the report of the Department of Public Works,
New South Wales, for the year ending June 30, 1915,
Mr. O. Le M. Knight deals with tbe question of the
fuel used in the Portland cement manufacturing plants
of the United States and Canada, where rotary kilns
are employed, such fuel used being necessarily of a
gaseous nature to permit efficient injection and
burning, and therefore restricted to pulverised coal, oil,
natural gas and producer gas. As a matter of fact,
over 75 per cent, of the cement produced in the United
States and Canada is burned by pulverised coal.

Oil is perhaps the most convenient fuel to use, but in
most localities its cost is prohibitive. It is now used
only in those parts of the United States where it is
abundant, and where coal is expensive. The common
method of introducing it into the kilns is through three
or more nozzles or atomisers at about 101b. oil pressure,
and 12 oz. air pressure. The consumption per barrel of
cement burned is about 12 or 13 gallons of oil.

Natural gas is used at a few plants in districts where
it is plentiful, particularly Kansas. It was previously
used in some of the Eastern plants, but increase in cost
caused its rejection in favour of pulverised coal. The
gas is generally introduced through some special type
of burner, in which it is thoroughly mixed with air, the
gas pressure being usually from 3 to 4 oz.

Producer gas is not now used in any cement plant in
tbe United States. It - has been sufficiently used to
demonstrate its suitability for burning cement clinker,
but has always been found uneconomical. Only 70 to
80 per cent, of the coal burned in the producer gets to

MLveRlZEQ COAL BIN,

Coal Injector For Rotary Cement Kiln.

the kiln as gas, whereas all the coal is burned when
injected in pulverised form. Also the cost of gasifying
is considerably more than the cost of drying and
grinding the coal, the producers requiring a good deal
of attendance and showing very high labour costs.
Furthermore, the quality of the pulverised coal does not
show as great a variation as does producer gas, and
therefore permits a better regulation of the flame. It
is possible that in districts where lignite or a very poor
quality of coal is the only fuel obtainable the producer
might be used satisfactorily.

Coal.—Methods of burning powdered coal in the
rotary kiln have been so perfected that it has replaced
all other fuels, save in those localities where some
natural advantage is greatly in favour of gas or oil.
The general underlying principle which has been found
essential to the successful burning of pulverised coal, is
that to completely burn the fixed carbons the com-
bustion must be completed while the coal is in
suspension in the air. This condition necessitates a
careful preparation of the coal as regards fineness, the
usual practice being to grind to 95 per cent, past
100 mesh and 80 to 85 per cent, past 200 mesh.

The fineness plays such an important part in the
combustion of the coal that to some extent poor coal
may be successfully used if additional care be taken
with the grinding. It is found necessary, in order to
make this fine grinding possible, and to ensure perfect
combustion in the kiln, that the coal be dried so that it
does not contain over J per cent, moisture. Considerably
less power is required to pulverise the coal wlmn it has
been properly dried. The cost of drying, grinding, and
conveying the coal in the United States varies between
Is. 3d. and 3s. per ton of 2,0001b., or about Jd. to 2d.
per barrel of cement burned. Drying is accomplished
in a rotary dryer, of which there are various types, by
far the most extensively used being the Ruggles-Coles
double drum-dryer.

The types of mills most commonly used for coal
pulverising are the tube mill, preceded by some prelimi-

nary grinder; the Fuller Lehigh mill, or similar type of
mill; and the aero pulveriser, the latter also taking the
place of the fan for injection. Of the three types, the
last shows a much larger horse-power per ton than either
of the others, The Fuller Lehigh mill is quite an
efficient grinder of coal, and it and the tube mill are the
most largely used.

Great cai e must be taken in the coal mill to prevent
sufficient air from entering the pulverised coal to
suspend it until it is actually at the point of delivery to
the kiln. Pulverised coal burns slowly when in a state
of repose, bur. it is highly explosive when in suspension
in air, and a clean, well-ventilated mill is essential.
The coal should be ground only as required, and in up-
to-date plants, from the time it enters the mill up to
the time of its delivery to the kiln, it is handled in
closed conveyors and elevators, from whr h air is care-
fully excluded. Anything in the nature of air separa-
tion or conveying is, of course, impossible, and an
attempt to handle the pulverised coal in this way in the
early Edison plant caused a disastrous explosion.

The coal is usually injected into the kiln under a
pressure of 3 to 4 oz. of air, the blast only supplying
about one-fourth of the air required for combustion.
The remainder is drawn into the kiln through the base
of the firing hood, having passed over the hot clinker in
the cooler, and enters the kiln at about 300 degs. Fahr.

Injection.—The principle of the pulverised coal-burner
is the feeding of the coal into a blast of air issuing at a
high velocity from a nozzle, and its conveyance by this
air into the kiln, where it is ignited by heat radiated
from the kiln walls. The difficulties encountered in
this process are the obtaining of a perfectly uniform
feed of the coal dust, and an even distribution and
suspension of it in the blast of air. These troubles may
result in incomplete combustion, or an unnecessary
amount of excess air and back flashes.

In the latest types of coal feeder the distribution is
greatly improved by causing the coal to pass over a
perforated plate, or through a small screen after leaving
the screw conveying it from the bin, instead of falling

straight from the end of the screw into the air jet, the
length of this screen and size of perforations being so
adjusted that the coal dust just reaches the end of the
screen for tbe maximum capacity of the feeder. Even
this construction does not entirely prevent the tendency
of the injection to take the form of puffs, and in some
recent feeders the single screw has been replaced by
two screws feeding on to two air jets, which discharge
into a common blast pipe. By this means a much more
even feed of the coal is obtained than was previously
possible with the single screw. A coal feeder combining
these features is shown in the illustration.

In some types of blast arrangement a pipe leading
from the pit under the discharge end of the kiln conveys
hob air to an enlarged section of the blast pipe imme-
diately in front of the firing hood, where it is drawn in
by the blast. It is questionable whether much benefit
is derived from this construction, though the plant
using it claims to effect a slight economy by its means.

The Coal Fields of North-Western France.— Judging
from the tone of a recent discussion in the Prussian Diet,
the Germans attach great importance to the rich beds of
coal and iron in French Lorraine. Of these, the basins of
Briey and of Longwy, in the Department of the Meurthe-
et-Moselle, are the most important. The former are
situated about 30 miles east of Verdun, near Metz, whilst
those of Longwy, further north,' are near the Belgian fron-
tier. According to the Economiste du Littoral, published
at Nice, the output of the 17 principal coal mines in the
Briey district during the two years preceding the war was
12,532,240 metric tons in 1912, and 14,823,740 metric tons
in 1913, showing an increase of 2,291,500. metric tons in
1913, as compared with that of the previous year. The
total quantity of coal raised in this district and sent to
Germany, and lost to France, cannot be less than 15,000,000
metric tons at the present time. It is also probable that
the output of iron ore, which previous to the war was very
important, has not fallen off, and that the quantity sent
to Germany at the present time far exceeds the production
of the pre-war years.