176

THE COLLIERY GUARDIAN

July 26, 1918.

see how it could be done, yet he did not think the
problem was insoluble. He also somewhat disagreed
with Mr. Walls that the management should be merged
with labour. He thought the co-partnership idea
would have to be extended to include three partners,
viz., capital, labour, and administration, each of
which received an adequate reward. He thought there
was a very great deal in Dr. Carpenter’s view that
it was distinctly injurious to the working man—cer-
tainly until he was better educated economically than
he was to-day—to trust him with large sums in the
way of bonuses, paid quarterly, half-yearly or
annually, because undoubtedly the money did get
squandered. He was forcibly reminded, in'that con-
nection, of the statement made a short time ago by
the managing director of one of the largest coal and
iron concerns in this country, to the effect that the
three years drink bill of the workmen was equal to
the entire capital of the company. Facts like those
impressed one with the danger of entrusting workmen
with large sums of money.

LOW-TEMPERATURE CARBO-
NISATION.

In the course of his paper on this subject before the
annual meeting of the Society of Chemical Industry
at Bristol last week, Mr. Edgar. C. Evans first of all
dealt exhaustively with the history of the low-tem-
perature carbonisation processes, the work done on
the constitution of coal, and with the theory of the
coking process, and then proceeded to describe several
of the carbonisation processes in detail, giving the
following summary of the main differences between
high and low temperature carbonisation: —

(1)	The low temperature (450 degs.—550 degs. C.)
of the walls of the oven reduces enormously the rate
of transmission of heat through the charge, or, in
other words, it reduces the velocity of the zone of
fusion. For this reason, a thickness of four to five
inches is the maximum that can be treated in
stationary charges within economical limits of time.
This factor brings in its train the following results:
(a) The capital outlay is increased owing to the in-
crease in the number of units, (b) labour charges are
necessarily increased, (c) the space taken up by the
plant is increased, and (d) the maintenance cost is
increased.

(2)	The resistance of the fused zone to the passage
of gas is enormously increased as the temperature
diminishes. It is estimated by O. B. Evans (Journ.
Gas Lighting, 1913, 587) that the resistance offered
to the passage of gas at 540 degs. C. is about seven
times greater than at 700 degs. C. Owing to this
extremely high resistance, if for any reason the outer
walls become choked, the gas accumulates in the charge
to such an extent that serious gas pressures are
developed. In several cases the author has found that
when dealing with resinous coals, the gas escapes from
the central portion of the charge not in a direction at
right angles to the containing walls, but parallel to
them, so that the resulting coke appears as if it were
built up of extremely thin layers. This seems to indi-
cate that in low temperature carbonisation, the outer
layers are much more resistant to the passage of gas
than is the case in high temperature carbonisation.

(3)	The time during which the coal is in a state of
semi-fusion is considerably prolonged.

In the case of high temperature carbonisation the
time-temperature gradient is rather steep, and the
interval of time during which the coal is in a state
of fusion is comparatively short. In the case of low
temperature carbonisation, however, this period is
considerably prolonged, and owing to this prolonga-
tion of the pasty stage, with the high pressures that
are induced in the coal mass, the cell cavities in low
temperature coke are considerably larger than is the
case with high temperature coke. This produces
ultimately a considerable expansion in the coke, an
expansion often great enough in the case of some
resinous coals and with well filled retorts to choke up
the gas outlet completely. With such coals a consider-
able space must be left in the retorts to allow room
for expansion, and the economic efficiency of the pro-
cess is thus seriously affected. Further, the resulting
coke becomes porous and friable.

(4)	Any free space left at the top of the coal charge
increases the amount of air that is left in contact with
the coal. This exerts a most deleterious action when
the coal is carbonised at low temperatures and results
in the formation of a friable, powdery coke.

Externally Heated Retorts.

The various types of “Coalite” retorts are on the
whole typical examples of low temperature retorts.
Their failure was due to the lack of detailed knowledge
regarding the constitution of coal, and also to the
difficulty of adapting the system for carbonisation on
a commercial scale. The Coalite trials proved one
thing very clearly, however, and that was the necessity
of carbonising the coal in layers as thin as was com-
patible with commercial requirements. This result is
achieved in a very simple manner by the Tozer retort
of the Tarless Fuel Company. In this retort, the coal
is charged in concentric layers, so arranged that no
layer is more than 4 to 5 in. thick. It is obvious that
much larger charges can be got into the same space
than in the coalite process, the coal can be charged
much more readily, labour charges for handling are
reduced, and the heating of the retorts can be made
very uniform.

The retorts are used in conjunction with Simpson’s
process for heating coal under a vacuum of from 20
to 26 in. of mercury. The use of such a high degree
of exhaustion has certain obvious advantages. The
oxygen left in the retort after charging is reduced to
a minimum, the liquid and gaseous products would be
removed very rapidly from the retort and possibly dis-
tillation would be effected more readily. The influence
of the vacuum on the quality of the coke produced is

not clear. Porter and Taylor state (Tech. Paper 140,
U.S. Bureau of Mines) that Pittsburg bituminous coal
yielded a light, inferior, porous coke when slowly
heated at atmospheric pressure, but at a pressure of
less than 30 mm. it produced a dense coke. The reason
for this is not very clear, but apparently the use of
a vacuum produced a decrease in the tenacity of the
tar film.

On the whole, a vacuum process offers certain advan-
tages over those carried on at atmospheric pressure,
but, on the other hand, from a commercial standpoint
it has certain disadvantages which are obvious to
workers familiar to coal carbonisation on a large scale.
These can be summarised thus: (1) Increased capital
outlay; (2) increased power consumption; (3) difficulty
of avoiding leakage (this would be a difficult matter
under works conditions, especially when working on
a big scale with unskilled labour in a colliery district
liable to subsidence).

Taking the Tarless Fuel process as a whole, it is
attractive in many respects, but it has yet to prove
its capacity for satisfying commercial requirements
on a big scale.

A process that has attracted considerable attention
is that carried on by the Barnsley Smokeless Fuel
Company. This differs in many essential aspects from
customary low temperature practice, and it might
perhaps be briefly dealt with. In the first place, the
Barnsley retorts are made of fireclay instead of cast
iron, the 'usual material used in low temperature
retorts. Cast iron is certainly not an ideal material;
its disadvantages were early recognised by the gas
industry and led to its substitution by clay retorts.
In the Barnsley plant vertical retorts of rectangular
cross section are used which are somewhat wider than
those used in most low temperature processes. In
these (Eng. Pat. 108,200) four varying zones of heat
were maintained, the lowest being at a temperature
of about 450 degs. C., the next 500 degs., the next
550 degs., etc., whilst finally, in the free space at the
top of the charge, a temperature of 900 degs. to
1,200 degs. C. was maintained. In this space was sus-
pended a grid made of some suitable material (metal,
metal oxide, fireclay, or carbon) so that the gaseous
compounds of distillation were subjected over as great
an area of contact as possible to the temperature
necessary to convert the paraffinoid tars to aromatic
hydrocarbons.

The author has had no experience of this plant, but
from purely theoretical considerations the chances of
success would be small if the above temperatures were
adhered to.

Internally Heated Retorts.

In this type, the coal charge is heated by the actual
passage through it of inert gas preheated to a tem-
perature sufficiently high to carbonise the charge. On
purely theoretical grounds, this proposal is extremely
attractive. The bulk of the time occupied in present
systems of carbonisation, is taken up in heating the
innermost layers of the coal, and if these could be heated
from the outset considerable economies in time could
be effected. A considerable number of attempts have
been made to carbonise coal by passing through it a
current of inert gas heated to a temperature of from
400 degs. to 600 degs. C., and Parr and Olin (Bull. 60,
Univ, of Illinois) had some excellent results on a small
scale by this method.

As far back as 1890, Parker (Eng. Pat. 67, 1890)
proposed to pass steam, water, gas, or some other
suitable gas superheated to 500 degs. to 600 degs. C.,
with a view to making smokeless fuel, and a number
of other inventors have followed along similar lines.
In th case of bituminous (resinous) coals, the old
difficulty arises that when the coal reaches the pasty
stage, it becomes impervious to the passage of the gas,
but there seems to be no reason why the method should
not be used for shales, cannels, or for coals that are
not fusible.

The results obtained by McLaurin with a process of
this type (J.S.C.I., 1917, 620) are extremely interest-
ing. As would be expected, cannel coal proved to be
quite easy to work when carbonised by means of a
stream of hot producer gas, but it was also found that
Cadder coking coal, if screened, came out of the retort
in the same shape and same size as it was put in.
McLaurin suggests that this is due to the slow heating
to which the coal was subjected and that under those
conditions it did not intumesce. If this condition is
applicable to all coals, it opens up possibilities of an
extremely interesting character. The author, how-
ever, has not found it possible to repeat this result
with the highly resinous coals of South Wales except
under conditions in which oxygen was present in the
heating gaseous medium. The effect of oxygen when
coal is carbonised at low temperatures has already been
discussed, and the author is inclined to believe that
the small quantity of oxygen which would be present in
the hot producer gas in McLaurin’s experiments played
an important part in the slow heating. A typical
analysis of the producer gas gave 0-9 per cent, of
oxygen, so that there seems to be every reason for
believing that the coal was carbonised in an atmosphere
containing a small proportion of this gas, an idea
which is confirmed by a study of the properties of the
tars obtained.

Another proposal of this type is that given by
Lamplough (Eng. Pat. 108343, 1917), the heating
medium in this case being steam.

On the whole, internal heating seems to offer consider-
able possibilities in the treatment of shales, cannels,
and possibly certain types of coal if carefully screened,
but as McLaurin and others have found, difficulties
would arise in the carbonisation of fine coal in this
way. There is perhaps one point that would need
consideration, and that is, if a small proportion of
oxygen were systematically introduced into the heating
gas, what would be the effect upon the tars produced ?
These apparently would have a similar composition to
the old Jameson tars, which were produced under con-
ditions theoretically analogous to those existing in
McLaurin’s process.

Continuous Processes.

The third class of retort which has been proposed
for the low temperature carbonisation of coal is that
in which the coal is carried forward through the fur-
nace by means of a conveying system.

The Archimedean screw has been suggested as a
suitable carrier for quite a long time. A most interest-
ing anticipation of one of the most modern of these
systems is given in Patison’s Eng. Pat. 569, 1873, in
which coal, shale, or other fuel is carried through a
heated retort by means of a worm conveyor. For low
temperatures—with a high oil yield—the inventor pro-
posed to use a cast iron retort, but for high tempera-
tures he suggested the use of a fireclay furnace.

The well-known Del Monte system differs only
slightly from this system, but it had one innovation,
and that was in the use of a screw mounted on a
hollow shaft, which was heated internally by a row of
jets from a central gas pipe. Further, the coal was
subjected to a fractional distillation, one end of the
retort being maintained at a considerably lower tem-
perature than the other. This retort again could be
used for wood, shale, lignite, or peat, but with coking
coals, the swollen pasty mass which was produced com-
pletely prevented the working of the retort.

It seems difficult to believe that any system which
involves the stirring up of the coal in any way can
be used for bituminous coals, and for that reason, it
is doubtful whether a continuous system on the lines
of the Woodhall-Duckham or the Glover-West retorts
can be used for the low temperature carbonisation of
these coals, except perhaps under conditions already
touched upon.

On the other hand, there seems to be no reason—
apart from those connected with engineering difficul-
ties—why the coal should not be conveyed through a
furnace, by some system in which the coal itself is
not stirred.

A system of this type that has been worked with
some degree of success, is that introduced by Pringle
and Richards. In this the coal is carried by means
of an endless conveyor through a retort the tempera-
ture of which is maintained at about 500 degs. C. The
conveyor itself is fitted with compartments so as to
subdivide the coal into a number of separate small
charges, each of which is rapidly heated to the desired
temperature. The coal itself is undisturbed in its
passage through the retort, and leaves as a porous
semi-coke, which is an excellent domestic fuel. This
process as yet has only been worked on an experimental
scale; the obvious difficulties are of an engineering
character, and whether or not they can be overcome
on a large scale plant remains to be proved.

A system of this type seems to offer several advan-
tages for low temperature carbonisation; temperature
control could be readily effected, labour charges would
be low, and by suitable engineering methods the rather
heavy capital outlay and high maintenance and repair
costs which would probably be inherent in this par-
ticular system could be reduced to a limit which would
make it a commercial proposition.

Economics of Low Temperature Carbonisation.

An attempt might, however, be made to review from
a general standpoint the financial possibilities of low
temperature carbonisation, and this perhaps is all the
more necessary because of the extravagant estimates
that are periodically issued from certain sections of
the financial Press. In this connection it must not be
overlooked that there are many coals which are almost
unusable in the raw state, and in some cases even
there are whole coalfields in which the coal is of such
quality that under the economic and geographical con-
ditions prevailing in their immediate vicinity they are
commercially unworkable. Several such cases have been
brought to the author’s notice in which a well-designed
system of low temperature carbonisation would result
in the economic salvation of the coalfield. Disregard-
ing for the present these exceptional cases, there
remain two, or perhaps three, main directions in
which the adoption of low temperature carbonisation
has been strenuously advocated.

The first is in the treatment of cannel coal. From
a technical standpoint, this does not appear to pre-
sent any very serious difficulties, and the question
practically resolves itself into a commercial one. The
matter has already been fully treated recently (“A
New British Oil Industry,” Craig, Perkin, Berry and
Dunstan, Journ. Inst. Petrol Tech., April, 1918).

Taking everything into consideration, the gas
engineer is well advised in rejecting low temperature
carbonisation as being unsuitable for the purpose of
producing gas in as great a quantity and of as high a
quality as possible.

There remains the last and greatest aspect of the
question—the low temperature carbonisation of bitu-
minous small coal. The total quantity of coal raised
in the United Kingdom in 1916 was approximately
256,000,000 tons. Of this, 37,600,000 tons was car-
bonised, yielding something over 8,000,000 tons of gas
coke and 13,200,000 tons of metallurgical coke. Of
the remaining 218,400,000 tons, allowing 4| million
tons of anthracite and, say, 30 million tons of low
volatile coals, there remains nearly 190 million tons of
bituminous coal, of which over 60,000,000 tons would
be small coal.

The use of raw coal was characterised over 30 years
ago as a barbarous procedure; the statement is true
to-day, but it has gained an enormously greater force
as the result of our experience during the past four
years.

Cost of Plant.

This would depend on the type of installation that
would be found serviceable. At first sight a low tem-
perature plant working at only 500 degs. C. would be
less costly than a coke-oven plant working at
1,000 degs. If the stationary type of retort were
erected, this factor would be counterbalanced by the
greater number of units found necessary, whilst the
price of d, continuous retort would be-increased owing
to the engineering devices found necessary. The cost
of a 300-ton a day coking plant before the war would
be approximately £50,000 to £60,000, and this figure