664

THE COLLIERY GUARDIAN.

September 25, 1914

to the ovens through a series of distribution pipes G,
through nozzles N, into the vertical heating flues F.
Here it meets air which has been preheated to a high
degree in the regenerators R, and combustion takes
place. In this way the charge of coal is carbonised, and
all the gas not required for heating purposes is avail-
able for outside consumption.

Gas Oven “ B.”—The Koppers’ gas oven was
designed for the production of the maximum
quantity of illuminating gas, and hence none
of the gas evolved from the coal is used for
heating purposes. The ovens are heated by pro-
ducer gas generated in a central producer plant, and
usually manufactured from inferior fuel. In this type
of oven both the producer gas and the air necessary for
its combustion are preheated. For this purpose the
generators are divided into two sections. The producer
gas is preheated by passing through the regenerators,

coke oven	Gas Oven	Combjnotion Oven

AR.
AR”
GR-
GR

GR
GA

AR
Aft

N
G

-^/^-Air
Asa Gas Oven

Air Air Air 4

TV Crc “

Air Gas Air Gas

Fig. 1.—Cross Section through Coke, Gas, and Combination Ovens.

G R, and the air by passing through regenerators, A R.
The gas and air then meet in the vertical flues F, where
combustion takes place as in the coke oven.

Combination Oven “ C.”—It will be seen from the
illustration that the combination oven is provided with
divided regenerators similar to those adopted in the gas
oven. At the same time provision is made by means
of the distribution channel G, for the utilisation of coke
oven gas for heating as in the coke oven. If the oven
is to be worked as a coke oven, all the regenerators are
used to preheat the air. The coke oven gas, freed from
its by-products, enters the distributing flue G, and the
oven works in exactly the same manner as described
above for the working of the coke oven. If the oven is
to be worked as a gas oven, one pair of regenerators
are used for preheating the air, and the next pair for
preheating the producer or blastfurnace gas. The
heated gas and air meet in the vertical flues E, and
combustion takes place, the oven now working in
exactly the same manner as described above for the

working of the coke oven.

The temperature of the oven can be maintained just
as easily in one case as in the other, and the change
from a gas oven to a coke oven and vice versa can be
made almost instantaneously, and without interfering
with its working in any way.

The Carbonisation of Coal in Coke Ovens.

As long as the primary object is the production of
metallurgical coke, so must the selection of a suitable
coal for use in the coke ovens be of prime importance.
Finality in the matter has by no means been reached,
but it is possible, as a result of collected experiences,
to lay down certain general rules concerning the rela-
tionship between the composition of coal'and its coking
capacity. Perhaps the most important of these is that
as the oxygen content increases the coking power
decreases, and it is generally assumed that a coal with
more than 8 per cent, of oxygen on an ash-and-moisture-
free basis is a non-coking, or at any rate a poor coking,
coal. This view is further borne out by consideration
of the results obtained in the distillation of various
classes of coal. The oxygen in the original coal is dis-
tributed on distillation in the form of H2O (ammoniacal
liquor), CO2, and CO. The following results were
obtained by the author with different classes of coal, and
it will be seen that, speaking generally, the figures for
H2O, CO2, and CO are a good guide as to the coking
property of the coal. The tests were all performed with
dry coal under the same conditions.

Coal.	Coke. H2O.	co2.	CO
	P. cent. P. cent.	P. cent.	P. cent.
Cumbprland		. 69-8* ... 5 34 .	.. 0-91	... 60
Durham		76-5* ... 411 .	.. 1-02	... 4-6
Warwick 		62*0+ ... 12-98 .	.. 3-6	... 10*6
Monmouth 		80-0* ... 3-37 .	.. 0-51	... 32
Staffs 		. 71’0* ... 7’53 .	... 1-08	... 4-9
Yorks 		. 70-0* ... 6-66 .	.. 1-28	... 7’8
Lancashire 		, 70-0* ... 6-80 .	.. D48	... 5’2
Lan ark 		65-0t ... 8’09 .	.. 2’45	... 10’7
Fife 			. 68’0* ... 9-12 .	.. 1’99	... 7’6
Northumberland 		. 67-0+ ... 8-55 .	.. 2'46	... 7-8
Forest of Dean 		68-01	6*73 .	.. U58	... 7-5
Brown coal, Australia	. 48’Ot ... 12-78 .	... 8-22	... 23’0
Leicester 		6<’0t .. 10-52 .	... 2-74	... 12 4
Lancashire cannel		63-Of ... 6’19 .	.. 0-56	... 5’6

* Good coke, f Poor coke. £ Fair coke.

In a large number of tests carried out by the author,
the general conclusion arrived at is that if the yield of
H2O from the dry coal on distillation is more than 7 or
8 per cent., the coke produced is of an inferior char-

acter. There are a few exceptions to this generalisation,
but, on the whole, it is quite a satisfactory guide. The
majority of Scotch coals behave in a very peculiar way.
If the coal be wound up and carbonised immediately a
good coke is obtained. If, however, it be allowed to be
exposed to the air for any length of time, and, particu-
larly if it be in a finely divided and moist condition, its
coking power deteriorates very considerably. Indeed,
the author has found that a coal which produces a good
coke when treated immediately after grinding, will
scarcely cohere on being carbonised after a day or two’s
exposure, the reason probably being that the coals absorb
oxygen from the air on exposure, and this view is borne
out by the fact that the yield of liquor on distillation is
greater after exposure than before. Consequently, it is
important with such coals to take precautions against
the effect of atmospheric exposure, and to carbonise the
coal as soon as possible after it comes from the mine.

Many attempts have been made to convert non-coking
into coking coals, but as far as the author is aware these
attempts have not been commercially successful. It is
possible, by the admixture of a quantity of pitch, to
produce a passable coke from a poor coking coal, but
this method is expensive, and could hardly be carried out
on a large scale. The deleterious effect of weathering on
the coking property of coals has already been mentioned,
but the change from a coking to a non-coking coal may
be brought about by other very simple means. If a
coking coal in a fine state of division is exposed to a
temperature of about 200 degs. Cent, for, say, two hours,
it is found that the coking power has practically dis-
appeared. Coal under such conditions gradually
increases in weight up to a certain maximum—the per-
centage increase varying with the class of coal—and then
begins to decrease again. When the maximum weight
is reached the coking property is generally completely
destroyed. In the case of the best classes of coking

HOT GAS FROM MENS.

TAR EXTRACTOR g

STILL

REHEATER

TAR TANK

SEPARATING
TANK

SATURATOR CENTRIFUGAL
DRYER

EXHAUSTER

CONVEYOR ton

J SULPHATE

J

AMHDNIACAL:
UQUOR TANK. $

</- ■ .v .-T

* COOLER

Fig. 2.—Diagrammatic Sketch of Koppers’

Direct Recovery Process for Ammonia.

coal these still retain a little coking power even after
long heating. For each coal also there appears to be a
limit or critical temperature, and when heated above this
temperature the coking property is seriously impaired.
For the ordinary classes of coking coals, such as those
of South Yorkshire, this temperature is about 150 digs.
Cent., but the effect is much more pronounced at
200 degs. Cent. The size of the coal carbonised lias
also a marked effect on the quality of the coke produced,
especially with those coals which possess only moderate
coking properties. The author performed experiments
with a Scotch coal, divided into four portions, one
through a 90 mesh, one through a 60 mesh, one through
a 30, and one below 30. The samples were arranged in
order in a distillation-tube, and separated by a thin
cotton wool plug. They were then carbonised in the
usual way. The resulting coke was different in each
case, and the larger grain coal produced the best coke,

the quality of the others deteriorating as the size
decreased. With such classes compression of the charge
is necessary in order to produce a suitable coke; and
with compression it is possible to treat many such coals
in coke ovens which otherwise could not be so utilised.

Different coals behave in different ways when car-
bonised under the same conditions, and consequently
the method of treating the coal has to be adapted to the
particular class of coal to be dealt with. The average
Durham coal is charged into the ovens in a normal
state—that is to say, containing about 2-5 per cent, of
moisture, and crushed to a degree of fineness of about
f in. to 0 in. The coal is not compressed in any way,
and a good, hard dense coke is produced. The coke is of
about the same volume as that of the coal, and no diffi-
culty is experienced in pushing the coke from the ovens.
In South Wales the coal is charged into the ovens in a
similar way, but has a tendency to swell on being car-
bonised. Unless care is taken, therefore, the oven walls
might be damaged, and difficulty will also be experi-
enced in pushing the coke from the ovens. In Cumber-
land still another variation is met with. There the coals
contain about 30 per cent, volatile matter, against about
25 per cent, in Durham, and 15 to 20 per cent, in South
Wales. If the coal is charged into the ovens as in
Durham, the coke produced is soft and friable, and will
not stand much handling. In order to produce a harder
and denser coke, the coal is compressed into the form
of a cake containing about 10 per cent, of added
moisture, and so charged into the oven. The coke
obtained by treating the coal in this manner is quite
suitable for furnace work.

Preparation of the Coal for the Ovens.

Most of the coal used in Great Britain for coking,
purposes is subjected to a washing operation before
being carbonised, so as materially to reduce the ash
content of the coke by separating the shale, and also
to eliminate some of the sulphur compounds which occur
in the coal, such as pyrites. With a coal of only
moderate coking power a high ash content is undesirable
—apart from other considerations—because it helps to
nullify the caking power. With a first-class coking
coal the ash content does not have much effect, coals
containing as much as 30 to 40 per cent, of ash yielding
quite a good class of coke. Sulphur is an undesirable
constituent in metallurgical coke, especially in that used
for foundry purposes. Coal in certain districts contains
an appreciable amount of salt, and this should be
removed as far as possible, as the salt eventually pro-
duces a corrosive effect on the brickwork inside the oven,
causing it to crumble, and thereby necessitating frequent
repairs. After washing the coal is crushed so that it
ranges in size from about fin. to Oin., and it is then
deposited in a storage bunker, where the excess of
water is allowed to drain off. Two methods of charg-
ing the crushed coal into the ovens are employed,
namely—(1) from above, the coal passing through the
holes in the roof of the oven; and (2) through the
door, the coal being fed into the oven in the form of a
compressed cake. In the former case the charge is
levelled in the oven by means of a beam which travels
backwards and forwards through the oven. The method
of charging adopted depends upon the nature of the
coal. Generally speaking, a coal containing above 25
per cent, of volatile matter has to be compressed, whilst
coal with less than 25 per cent, is charged in the normal

state. These methods are adopted with all types of
ovens, as the principles of coking involved are the same
in all cases.

Recovery of the By-Products.

Until about the year 1903 the method adopted on
coke oven plants for the recovery of the by-products was
similar to that which is still in existence on gas works.
The gas, after leaving the ovens, was passed through
coolers, and its temperature reduced to about that of
the atmosphere, thereby removing the heavier tars, and
also the moisture contained in the gas. The moisture
which was deposited dissolved out of the gas a propor-
tion of the ammonia salts which it contained. The
gas was then passed through scrubbers, and was well
washed with water until the whole of the ammonia was
eliminated and obtained in the form of ammoniacal
liquor. In order to recover the ammonia from this
liquor it had to be treated in a separate apparatus,