1022

THE COLLIERY GUARDIAN.

November 15, 1918.

common to all ovens in the early period of coking
would be reduced, and during the later period, when
only one ascension pipe would be required, the others
could be cleaned without interruption to the working
of the oven. An investigation by the Otto Company
in America showed that 50 per cent, of the total gas
was given off in the first 40 per cent, of the coking
time. With 30 hours coke, 50 per cent, of the gas
would leave the oven in the first 12 hours, and 50
per cent, in the last 18 hours. Assuming coal con-
taining 15 per cent, total moisture, and that the
moisture was all driven off in the first 12 hours, the
volume of gas and steam leaving the oven during that
period at, say, 750 degs. Cent, was twice the volume
of gas leaving the oven at, say, 850 degs. Cent,
during the remaining period. In other words the
average volume per hour passing through the ascen-
sion pipes for the first 12 hours was three times
greater than in the last 18 hours; hence the reason
for the increased pressures when an oven was newly
charged. It would be of interest to remark that the
average heat transfer per hour through the oven
walls in the first period was about 6| times that of
the second period.

Mr. C. H. Merivale (Leeds) said they had talked
a great deal about moisture in coal, and they were
all agreed that the moisture was very often too high.
He would like to ask how low one might safely take
it without losing the cloud of moisture which appa-
rently conserved the nitrogen as it was given off from
the oven. Some of the older colliery companies were
considering the question of coking all the coal coming
from the collieries, and the point as to how low they
dare go would become important. Dare they go as
low, say, as 5 or 6? Another point that occurred
to him was the enormous value of the gas which
many of them were not making the most of. He saw
no mention in the paper of selling gas for illuminating
purposes or of dual mains. He was told that in some
parts of the country people were putting up dual
mains and taking off the richer gas and selling it,
say, for town lighting. He himself was selling gas
to-day for town lighting, and he was putting up dual
mains to try and improve the quality. He thought
this might lead to their being able to use higher
suctions, as mentioned by Mr. Foxwell, and therefore
possibly increasing the yield of ammonia. Owing to
the fact that they had the dual main, they could
afford the high suction on the oven, and the low grade
gas could either be burnt under the ovens or enriched
and disposed of in other wTays.

Mr. R. Holiday (Ackton Hall) said many plants
gave figures of weights, but what he found was the
difficulty of getting at the true weights, and this, of
course, affected the accuracy or the value of the
figures very much. Mr. Colquhoun suggested a con-
tinuous band, but the trouble was that as they had
a certain percentage of moisture in the coal, the coal
very often adhered to the belt, and although they
might have balanced their machines perfectly well,
after a time the belt became coated with fine coal,
which they weighed over and over again, thus spoiling
the accuracy. On the other hand, he had tried letting
this coal remain on, and balancing the belt while it
was dirty, but then the coal broke off in cakes, and
thus the weight was thrown out again. The continuous
weighing machine would be very good if there was
some way of keeping the readings continuously accu-
rate without having somebody cleaning the machine
all the time.

The President, in proposing a vote of thanks to
Mr. Colquhoun for his interesting and valuable paper,
said he felt that papers of that kind at the present
time were of great value, because in the past they
had perhaps been too much dependent, as far as coking
was concerned, on foreign experts, more particularly
Germans, and an interchange of ideas between English
engineers would certainly lead to their being more
independent of outsiders.

The vote was seconded by Mr. Neal, and carried.

Mr. Colquhoun, in reply, said he did not think
the question of how low one might go with advantage
in the moisture content of coal had ever been raised
before. The greatest trouble was to get low enough,
and the only thing that prevented a low moisture was
that the coal could not be compressed. In America
they used practically no washed coking coal. It
was all dry, and therefore their moisture was
extremely low, and they made an excellent coke. As
far as he understood Mr. Merivale’s question, he
thought the American practice was a sufficient answer.
The question of dual mains, and the use of surplus
gas for other purposes than those he had referred
to, was a very large one. He had written some para-
graphs on it, but afterwards omitted them in order
to keep the paper within bounds. With regard to
the weighing of coal on bands, that had not been
taken up as it might be. Whatever error there
might be in the weighing, it must be constant, and
if, for instance, they had wet coal sticking on to the
band, it was constant to all intents and purposes.
There would, of course, be momentary errors arising
out of flakes dropping off the band, but, taken year
in and year out, he would consider that the band
had at least a constant error, which was a great deal
nearer accuracy than the rough-and-ready method
which they at present used. Moisture, of course, had
to be allowed for. The varying moisture of the coal
that was being delivered on the band must be deter-
mined, and could be ascertained by taking a pro-
portion of the coal off the belt once an hour and
mixing and quartering that sample in order to arrive
at an average moisture for the day. With regard to
the suggestions made in the written contributions to
the discussion, he proposed to consider them more
fully and to reply at a future meeting.

Mr. W. Holehouse, managing director of the Davenport
Engineering Company Limited, and patentee of the Brad-
ford Cooling Towers, has severed his connection with the
company, and is commencing in business at 21, Morley-
street, Bradford, on his own account to design and manu-
facture special cooling plant for large turbines.

ECONOMIC CONSIDERATIONS IN COKE
OVEN PRACTICE.*

By W. Colquhoun.

Compared with the beehive coke oven, the by-
product oven produces an equal quality of coke at a
lower cost, uses less coal per ton of coke, and widens
the range of coals available for coking. It therefore
has a greater steadying influence on fluctuations in
the cost of coke, develops more fully the potential
value of the coal, and conserves national resources.
The reason why so much coke is still made in beehive
ovens is not the large capital cost, for the value of
the coal saved will itself pay the interest and depre-
ciation charges (since with a coal yielding 60 per cent,
of coke in beehive ovens and 70 per cent, in by-product
ovens, 23 tons more coal would be required to make
100 tons of coke in the beehive oven; that is to say,
to give the same output as a battery of 40 by-product
ovens, beehive ovens would require 16,000 tons more
coal per annum), but because they may be started with
little preparation and cost when the market is favour-
able, and stopped in less favourable times, and at a
particular colliery the output of coking slack may
fall short of the quantity that will most advan-
tageously carry a by-product plant through bad times.
But if a combination of interests can be brought
about, the output of slack from two or more neigh-
bouring collieries may be amalgamated for coking in
by-product ovens, with advantage to each.

Location of Ovens.

When there exists no special reason for linking up
the ovens to the colliery, such a combination may
advantageously join the blast-furnace owner and
assemble the coals at the iron works. The colliery
irregularities of supply are then largely eliminated,
and the ovens may be worked intensively and without
those variations which rob the operation of its
economies. The coals may be mixed at the ironworks
to give a more constant average content of volatile
matter and produce a more uniform quality of coke,
although one or more of the constituent coals be of
only indifferent coking quality.

Mixing Coals.

Wherever the ovens may be located, the mixing of
the coal ought to receive more attention than it does
at present. It is due largely to the use of inferior
coking slacks without adequate mixing that some of
the by-product coke produced to-day merits all the
prejudice which attended its earliest introduction.
The blast-furnace manager wants coke uniform in
quality, and at least constant in its imperfections.
An intimate and uniform mixture of the slacks for
by-product ovens is not easily made and maintained
over long periods. When derived from two or more
seams, the slacks are not sufficiently mixed by tipping
alternate wagons into the raw coal hopper and by
the turning over it receives in the washery and
drainage bunkers. Better results are obtained by an
arrangement consisting of several under rail hoppers
fitted with revolving feed tables drawing, regulated
quantities from the several hoppers on to a common
belt supplying the washery. The wagons are tipped
by electric crabs, which span the four sidings.

Moisture.

It is generally agreed that washed slack should be
drained for 48 hours before crushing. The moisture
may then be 10 to 12 per cent., or even more, accord-
ing to the fineness of the slack. In coal containing
4 per cent, of combined water and 10 per cent, of
added water, 14 tons of water are charged into the
ovens for each 86 tons of coal, and, while a limited
quantity of moisture is advantageous to the forma-
tion of ammonia and necessary to the process of com-
pressing the charge, it all has to be evaporated to
steam in the oven—an appliance singularly unfitted for
the purpose.

To show the work that has to be done by the con-
densing apparatus, it may be mentioned that in gas
at about 400 degs. Cent, (when the cooling com-
mences) from coal with 14 per cent, total water, the
water alone is accountable for the absorption of 3*2
times the heat units contained in the dry gas itself;
and in coal with 18 per cent, total water, for the
absorption of more than 4-4 times the heat units.
On entering the condensers at about 100 degs. Cent,
these differences are respectively 10-5 and 14-1 times.
Realising this, every effort should be made to limit
the amount of moisture left in the washed slack.
None of the drying apparatus used in certain indus-
tries is practicable in this case, nor capable of dealing
with the large quantities involved, but by insistence
on 48 hours’ drainage and the introduction of a pro-
portion of nuts or large coal the moisture can be kept
under control. The extra profit on the direct sale of
such nuts or large coal is apparently lost, but it may
be regained several times over in the reduced cost and
larger output of coke.

Service Bunkers.

For a battery of ovens charged with compressed
cakes, the bunkei’ for the washed and crushed coal
is one in which the coal is drawn by a revolving feed
table, in quantities regulated, first by the height of
the cone of coal on the table, and secondly by the
depth of cut of the scraper arm. It is delivered on
to a band conveyor under the table, carried to the
stamper runway, and distributed into the compression
box by an automatic reversing tripper, not in inter-
mittent layers, but continuously. With this arrange-
ment the time taken to compress an 8 ton cake is from
12 to 18 minutes (varying with the fineness of crushing
that the coal has received), whilst a 12 ton charge
takes 24 minutes. Only a youth is necessary to attend
to the operation of the table and conveyor.

Weighing.

The coal usually passes through the screens directly
into the washery, and the weight of washed slack
charged into the ovens is ascertained only from the

* From a paper read before the Midland Institute of
Mining, Civil and Mechanical Engineers on November 9.

coke and breeze made, based on the laboratory yield.
It would be of great advantage if the washed slack
could be weighed, and where a band conveyor is in
use, such as already described, it is quite practicable
to instal one of the continuous type of weighing
machines.

Compressing.

The stamping of charges was first introduced in
Germany for coal of semi-coking character that would
not otherwise make good coke, but the operation now
has a wider application, and serves to improve the
quality of many British cokes. Its adoption in this
country was retarded by the troubles due to un-
reliable and cheap machinery imported from Germany,
and by the slow and unequal heating of some of the
earlier ovens. Careful tests made in Semet-Solvay
ovens on coals containing from 26 to 28 per cent, of
volatile matter show that the weight of the charge
is increased approximately 30 per cent., which is
reduced by the longer coking time to a net gain in
production of about 13 per cent. If it is established
by trial that the quality of a coke is improved by
compression, that operation is preferable to top-
charging in many ways. The top of the charge has
the same density as the bottom, and the coke is
therefore . more uniform; the proportion of breeze
is reduced; the charge is a constant quantity and
a measure of the capacity of the plant; the capacity
is maintained at a maximum independently of the
skill or care of workmen; the oven being completely
filled, the amount of free space on top of the charge,
and consequently the degradation of by-products, is
minimised; the gas off-take may be in the centre of
the oven length, and the path of the gases shortened;
finally, except in the smaller plants, the labour cost
per ton of production is definitely below that of the
best system of top charging.

Size of Oven.

The number of charging and discharging operations
per shift is usually limited by the capacity of the
workmen, or by trade union rules, and therefore the
operation of the battery becomes more economical with
every increase in the weight of the charge. In recog-
nition of this, the 12 ton oven (11’65 tons of dry coal)
is an accomplished fact in this country, having been
built at Hoyland Silkstone Colliery and Thorncliffe
Ironworks. For a given output there are fewer inter-
ruptions for recharging, and the number of ovens
opened to the air is reduced, as well as the propor-
tion of soft coke from the top and ends.

In American practice the standard oven now holds
from 15 to 16 tons. The coal cannot be stamped, the
height of the stamped cake being too great, and, being
unwashed, it cannot in any event be compressed; to
add water merely for that purpose would be indefen-
sible. The higher thermal conductivity of the
standard silica construction, the higher working
temperature practised, and the relative dryness of
the coal together enable American cokeries to coke
in an average time of 50 minutes per inch of width.
The resulting large output of coke yields a quicker
return on the capital outlay, but, as may be expected,
at the expense of part of the by-products, which are
reduced by 20 per cent, for tar and benzol and 10 per
cent, for sulphate.

Oven Taper.

The retarding influence on output due to the taper
in an oven is not sufficiently realised. The wide end
prolongs the coking time, and this cannot well be
corrected by a proportionate supply of heat, because
it is at this end that the flue walls are narrowest.
Coking is completed at the narrow end some three
or four hours before the wide end, and the tempera-
ture at the wide end never reaches that of the narrow
end, all of which tends to unequal coking in the
batch.

The relief afforded in pushing out a batch with two
inches of taper in a length of 33 feet is only that
due to 1116 inch of taper per foot of movement, and
is really objectionable in respect to stamped charges,
unless the cake has a similar taper and is charged
from the hearth side.

Horizontal Flues.

The temperature of combustion of coke oven gas
is so high that the bricks will fuse unless an excess
of air is used, the amount of which will vary with
the temperature to which the air has been pre-heated.
In the Semet-Solvay oven the excess air is admitted
only into the top flue, where it is heated to the
temperature of the products of combustion, and
furnishes the air required in the second and subsequent
flues. In these flues there is no danger of fusion,
owing to the presence of burnt products from the
flues above; combustion in the whole system can
therefore be so regulated that the theoretical quan-
tity of air is only slightly exceeded in order to ensure
complete combustion. Such regulation involves atten-
tion, since the combustion of the gas is affected
directly by the increasing temperature of the flue
walls as the coking of a charge proceeds.

In vertical flues the excess air necessary for safety
is introduced at every gas admission, and is not used
up in succeeding flues, but finally passes up the stack
at a temperature which may represent a considerable
loss in heat units. No other condition seems possible
in vertical flue ovens; the air and gas cannot be regu-
lated from hour to hour, and therefore after the
first setting they are not regulated at all. In prac-
tice, certain types of vertical flues show unequal
heating of the walls when worked as waste heat ovens,
but the same ovens built with regenerators, with the
air pre-heated to 1,000 degs. Cent., become at once
more lively, and coke in shorter time. This improved
working may probably be attributed to the flooding
of the flues with larger volumes of gas and air corre-
sponding to the higher temperature. In the case of
coal with 28 per cent, of volatile matter and 12 per
cent, of moisture, the average speed of coking of the
Semet-Solvay oven up to, say, 20 in. wide is 90
minutes per inch of width.

Surplus Energy.

It is customary to assume that the whole of the
surplus gas from regenerative ovens—say, 5,500 cu. ft.