968

THE COLLIERY GUARDIAN.

November 8, 1918.

to a table of cosines will give the amount of the
useful output, and then the difference between the
squares of the two gives the square of the wattless
component, as explained.

Trigonometry also affords another method of finding
the wattless component. The tangent of any angle in
a right-angled triangle is equal to the perpendicular
divided by the base. In the triangle PON, the
tangent of </> = PN divided by ON ; multiplying both
sides by ON we get PN = ON multiplied by tangent

; that is to say, the wattless component can be found
when the useful output and angle </> are known. The
table already referred to also gives the value of the
tangents of different angles between 0 degs. and 90
degs. Tangent 0 deg. equals 0 (this is again the
case where the power factor is unity) ; tangent 18
degs. equals 0-324 ; of 25 degs. equals 0-466 ; of 37 degs.
is 0-753 ; of 45 degs.,-1-0; of .60 degs., 1-732; and of
66 degs., 2-246. These figures represent the relative
value of the wattless component compared to the use-
ful output. If the useful output is 100 kilowatts, with
the angle </> = 18 degs., the wattless component is 32-4
kilo-volt-amperes ; with</>= 25 degs., the wattless com-
ponent is 46-6 kilo-volt-amperes; with </> = 45 degs.,
the wattless component is equal to the useful output;
with </) = 60 degs. it is 73 per cent, above the value
of the useful output, and with </> =66 degs. the wattless
component is 2} times the value of the useful output.
Fig. 8 is a curve of (tangent values.

The foregoing figures will enable those who are
interested in the subject to get a fairly clear idea of
the influence of the lag of the current imposed by
the conditions of alternating current working. In
subsequent articles .it is proposed to show how the
various power factors of the different parts of the
distribution system are dealt with to obtain resultant
power factors, how the inefficiency due to the
operation of the power factor may be neutralised, and
how to make the calculations necessary.

UTILISING LOW-GRADE FUEL.

An expert, appointed by the French Ministry of
Munitions to report on the best methods of using low-
grade fuel, has drawn up the following directions,
which, it is suggested, should be placed in every boiler
house:—

Fuel mixtures should be as uniform and regular as
possible, and all lumps, especially of lignite, should be
broken to sizes no larger than an egg.

Good coal (low percentage of ash) must never be
mixed with dirty fuel, but a small amount of good coal
may be used to advantage after cleaning the fire.
When working with natural draught, the ashpit doors
should be kept wide open.

The fire should be fed with small quantities every five
to eight minutes, according to the kind of coal used.
Fires of small coal should be only 2 in. to 2% in. thick,
and those of broken coke or nuts not more than 4 in.
No large lumps of coke or coal should be used at all.
The grate bars should be well covered, especially at the
back, so that no air slips through. Five shovelfuls is
the average feed for a furnace 4J ft. to 6 ft. long by 40 in.
wide, commencing with two thrown on to the back end.
The stoker should be able to keep an even surface with
the shovel alone, by filling up hollows where the fire
burns most rapidly. With wet washery sludge, an
extra two (smaller) shovelfuls can be thrown on. The
formation of clinkers is prevented by waiting until the
fierce flame has subsided and the bed of fuel becomes
visible. It is advisable to keep a little water, if possible,
in the ashpits when coal that has a tendency to stick to
the grate is used.

Under normal conditions, the flue damper should be
open just sufficient to enable the fire to burn properly,
and every time the flue door is opened the damper
opening should be reduced by three-quarters. In the
case of coking coals, including certain washery sludges,
the surface crust should be broken with the rake about
three minutes after feeding, and the fire levelled
all over.

The poker may be used to open a passage for air into
the bed of fuel, but care must be taken not to mix the
clinkers with the coal. The fire will need cleaning at
intervals, depending on the quality and quantity of the
ash. With the present kinds of coal the cleaning should
take place about every three hours at the outside, the
fires being cleaned in succession, with intervals of about
20 minutes, so that the one just previously cleaned is
again in good working order. To clean the fire, the
burning fuel in one half of the grate should be thrown
on to the other half, and after waiting three to four
minutes for the remaining coke and clinker to brighten
up again, the clinkers are raked out—the operation
being repeated as regards the other side of the fire.

With forced draught the air pressure should be
from | to f in. for coals with 12 to 20 per cent, volatiles
(lignite included); from | to J in. for anthracite with
8 to 12 per cent, volatiles; J to in. for second grade
nuts with 7 to 8 per cent, volatiles; and to 11 in.
for hard anthracite with 3 to 4 per cent, volatiles; all
according to the state of the ash and the size of the
grate spaces (the above values relating to grates with
30 to 50 per cent, of air space between the bars). The
width between the bars should be small—J- in., or even
u in. in the case of small coal. A water spray is
necessary when burning certain anthracites (such as
those from the Mure mines). The damper opening
should be reduced to a minimum, the best way of
regulating it being to close down until smoke begins
to pour out of the firedoor, and then opening the
damper again a little way. Under these conditions a
flickering flame is produced; and if a handkerchief be
held in front of the open fire door only the lower edge
will be drawn slightly forward by the draught. The
proper position of the damper should then be clearly
marked, and should not be altered, except when the
fan draught is increased or the flues become choked
with dust, and then the opening should not be increased
by more than 4 to 1 in.

The method of feeding and cleaning the fire is the
same as with ordinary draught, and during the latter
operation the fan should be stopped. Moreover, in the
case of forced draught there is a greater tendency for
air to slip through the bed of the furnace, and therefore
greater attention must be bestowed on the state of
the fire.

DIFFUSION OF OXYGEN THROUGH
STORED COAL.*

By S. H. Katz.

This study of the rate of diffusion of oxygen through
coal forms part of an extended investigation of the
conditions affecting deterioration and spontaneous
combustion of stored coal that has been undertaken
by the Bureau of Mines, under the direction of Horace
C. Porter, formerly chemical engineer of the bureau.

The experiments herein described were made in the
attempt to determine the effects of the size of the coal
pieces and the proportion of voids in a coal pile on
the rate at which oxygen can be brought by diffusion
from the air around the pile to the coal within, the
purpose being to discover the bearing of diffusion on
the rate of spontaneous heating within the pile.

The fact that coal oxidises at ordinary temperatures
has been well established by many investigators.
Winmill measured the heat evolved during such oxida-
tion and found that, per unit of oxygen consumed,
it is nearly as great as that produced by combustion
at high temperatures. Combination of oxygen with
coal during storage causes a loss in heating value
of the coal. Porter and Ovitz showed that coal from
New River W.Va., exposed to the air out of doors
at Key West, Fla., for twTo years, lost 1-85 per
cent, of its calorific value. At Portsmouth, »N.H., a
comparative lot of the same coal lost 0-77 per cent,
of its calorific value in two years. Industrial and
economic conditions often necessitate the storage of
large quantities of coal, so that the loss of fuel values
throughout the country from this cause is con-
siderable.

As spontaneous combustion frequently occurs when
bituminous coal is stored in large quantities for any
length of time, there is need of more information
about the causes of spontaneous combustion, and the
present work has a bearing on this problem also.

When coal is stored in the air, the diminution of
oxygen in the air within the mass through oxidation
of the coal causes oxygen from the outside air to
enter by both convection and diffusion, thus providing
the means for further exidation. The amount of
oxidation from diffusion while air is entering by
convection will vary with the magnitude of the con-
vection. Hence the amount of oxidation that may
be attributed to either convection or diffusion will
differ greatly with conditions. This paper, however,
considers only the effects of diffusion.

Description of Apparatus.

The apparatus used in the investigation consists of
a Kipp generator for the hydrogen used to remove
the oxygen by combustion, two 20-litre bottles for
collecting and measuring the hydrogen, and a bubbler
for showing the rate of flow of the hydrogen.

The chamber may be considered divided into three
compartments, placed one above the other. Details
of the lower compartment and of the coal compart-
ment are given in fig. 1, in which a1} a2 and a3 repre-
sent tubes opening into the interior, and a2 being
closed with rubber stoppers. Through a3 was run a
capillary tube b from the centre of the lower compart-
ment to the gas-sampling apparatus. The coal was
supported by a stiff steel wire net, d, of g in. mesh,
resting on the lugs c and c. When coal finer than
§ in. mesh was used, a fine wire gauze was laid over
the net.

The circular reservoir e was built around the coal
compartment. Near the bottom of the reservoir the
rod i, bent into a circle, was solidly fixed by means
of the rods k and j, which were placed at intervals.
The reservoir was kept two-thirds full. When the
upper compartment was in place, the water seal
separated the gas inside from the air outside. On top
of this compartment were five tubes, k1} k2,' k3} k4 and
k3) leading to the interior. Tube kr was closed by
a rubber stopper; tube k2 was joined to a stopcock;
through tube k3 passed a glass tube with cock j for
conducting the hydrogen. Inside the compartment
this tube was joined to an absorption tube I filled
with glass wool to prevent backfiring. The hydrogen
was led further to the silica nozzle m3 at the centre
of the compartment. The jet of hydrogen was ignited
by the electrically heated platinum spiral n. A glass
window allowed the progress of combustion to be
watched. The capillary tube c for taking gas samples
led from the centre of the compartment through the
tube k±. Also at kA the tube d connected the water
manometer e with the interior of the apparatus. A
solution of equal parts of glycerine and water was
used in the gas sample tubes g and the levelling
bottles h to displace the gas when procuring samples.

While the oxygen was being burned from the upper
compartment the coal was covered with a diphragm
of 1 ‘ rubberised ” cloth I (fig. 1). Diffusion was
started by removing the diaphragm through pulling
proper strings. These strings were tied at reinforced
holes, and were arranged as shown. In use the
diaphragm was placed over the coal compartment and
the strings brought down and under the circular
rod i (fig. 1), and up and over the outside rim of
the reservoir. By pulling the proper strings the
diaphragm could be stretched tightly over the coal
with its edge on all sides dipping below the surface
of the water and forming a practically gas-tight
covering. When the upper compartment was in posi-
tion, the diaphragm could be removed by pulling
other strings. Both edges of the reservoir were
finished to allow the strings and diaphragm to slide
readily. As assembled for experiment, the upper and

* United States Bureau of Mines Technical Paper 170.

the lower compartments of the apparatus contained
very nearly equal volumes.

Data and Calculations.

Samples of gas were taken periodically from the
lower and the upper compartment and analysed to
indicate the change of concentration of oxygen.
From the analytical results and the time the samples
were taken, the time required for a definite degree
of diffusion to take place was calculated.

The size of the particles, the percentage of voids,
and the depth of coal in the coal compartment were
varied. Comparison of the calculated values of time
for diffusion, which were obtained with the different
conditions in the coal, showed the effect of these
varying conditions on the rate of diffusion through
the coal.

Facts Brought Out by the Investigations.

The investigations have shown the following facts : —

(1)	That in itself the size of the coal particles has
no effect on the rate at which oxygen diffuses through
a mass cf broken coal.

(2)	That, with masses of coal having different pro-
portions of voids, the time required for a definite
amount of diffusion increases in about inverse pro-
portion to the percentage of voids.

From this information the following principles of
practical import in reducing deterioration of stored
coal may be deduced: —

Although it is best that there should be no fines
in stored coal, yet when coal of mixed sizes is to
be stored it is advantageous to handle it so that the
large and the small pieces are kept evenly mixed.

In building a coal pile the aim should be to prevent
fine and coarse coal segregating through the larger
pieces rolling down the sides of the piles.

Coal Used in Experiments.

In each experiment described in this report the
coal used was from the Pittsburg bed. Coal that had
been exposed to the air several months and had
oxidised to some extent was selected so as to reduce
the amount of oxidation during the experiments.

For some of the experiments the coal was
crushed and carefully screened to size, and the
percentage of voids was determined in a
thoroughly compacted sample. In preparing the
apparatus for experiment, the coal was put in and
well shaken so as to make it as compact as possible.
Especial care was taken to have a minimum and
uniform amount of voids, and also to have the same
proportion of voids in the apparatus and in the sample
in which voids were determined by comparing the
real volume of the coal as calculated from weight
with the volume of the coal compartment. The coal
was then made level with the top edge of the coal
compartment.

In some of the experiments the quantity of 2-mesh to
4-mesh coal required to fill the coal compartment was
taken. To this was added the calculated volume of
40-mesh to 60-mesh coal required to fill the voids in
the larger coal. The two sizes were evenly mixed and
put into the apparatus. The percentage of voids was
calculated from the data obtained in previous deter-
minations of the voids in each size of coal.

To vary the depth of the coal compartment for
certain of the experiments, a sheet-metal collar of
larger diameter and proper height was placed upon
the fixed support c (fig. 1), and the screen was placed
on this collar. A weight of coal equal to the difference
between that used to fill the deep coal compartment
and to fill the shallower coal compartment was placed
on the bottom of the lower compartment. In this
way the lower compartment was raised to a height
equalling that to which the screen supporting the
coal was raised. The gas-sampling tube b (fig. 1) was
extended so as to reach the centre of the new position
of the lower compartment.

Procedure in Experiments.

The capacity of the upper compartment, calculated
from dimensions, was 37-3 litres. This included the
volume of the annular space above the water and
below the level of the coal when the water in the
reservoir was 3J in. deep. To combine with the
oxygen in the quantity of air required to furnish
37-3 litres of nitrogen (plus inert gas), 19-8 litres of
hydrogen were required. For each experiment 19-8
litres of hydrogen measured under atmospheric
conditions were used.

Before making a combustion test the apparatus was
assembled, all openings to the outside were closed,
pieces of ice were put on the top to absorb the heat,
and the platinum glower was heated to redness. Then
the hydrogen was admitted.

The duration of each experiment was measured in
hours, the time of igniting the hydrogen being taken
as zero.

During the early part of the combustion the pres-
sure rose because of the heat, but in a few minutes
fell below that of the external air. The rubber
stopper closing the tube cq (fig. 1) was then removed
and air was allowed to flow from the outside into
the lower compartment, whence it passed through the
coal, raised a submerged edge of the diaphragm above
the water, and entered the upper compartment.
When about 14 litres of hydrogen had been used, com-
bustion became less active and the hydrogen was not
consumed as fast as it entered. The pressure rose
to atmospheric, and at this point the rubber stopper
was again replaced at ar. The flow of hydrogen was
continued till 15 to 20 minutes from the start of
combustion, when the required 19-9 litres had entered
the diffusion apparatus. The pressure had risen and
the heated glower slowly continued the combustion.
After a total period of combustion of about 1| hours
the pressure had decreased to about | in. of water.
Combustion was then stopped.

To allow the temperature of the apparatus to
approach that of the room, the ice was removed from
the top and the top wiped dry. It was allowed to
stand in this condition for about five minutes.