THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CXVI.

FRIDAY, SEPTEMBER 6, 1918.

No. 3010.

Fuel Economy in a Modern Steel Works.*

Report by Mr. BENJAMIN TALBOT

In the Report of the Committee appointed by the
Board of Trade on the Iron and Steel Industry after
the War, it will be found that, in the opinion of wit-
nesses and in the Report itself, there was a general
agreement that the iron and steel works in this country
were much behind their competitors in Germany and
America in their employment of fuel, and, incidentally,
also that they were extravagant in labour per unit
of output. It is recommended by them that large
modern units should be built for the manufacture of
the chief staple products, and that some of these
units could be erected at or near the sea board in close
contiguity to the coal fields.

In the Report, from the large amount of evidence
taken it was recommended that a modern unit should
be capable of turning out as a minimum at least 300,003
tons per year of the well-known chief staple products
of steel manufacture, and that if the unit could make
500,000 tons annually, it would be still better.

The plant would consist of coke ovens with by-
product recovery, blast furnaces, steel furnaces and
rolling mills; the idea being that in well-known staple
lines of steel manufacture the finished commercial
product should be made at the same plant, so as to
use up surplus power and heat generated at the coke
ovens and blast furnaces, and so keep down cost of
production by such concentration.

On the foregoing assumed output, and on the
assumption that a modern steel works is erected in
such a locality as that indicated, it is proposed to
examine the most economical fuel consumption that
can be obtained in practice.

The works are assumed to have an output of 6,003
tons of finished steel per week, and on the assump-
tion of 80 per cfent. of finished product per ton of
ingot this will necessitate the production of 7,503
tons of ingots per week. To produce this 7,500 tons
weekly of ingots, assuming the use of 20 per cent, of
scrap in the open hearth furnace, we should require
6,000 tons of pig iron per week, and, taking a con-
sumption of 23 cwt. of coke per ton of pig iron pro-
duced, we require weekly 6,900 tons of coke, the
equivalent of which in coal, taking a yield of 70 per
cent, of coke on the coal carbonised, is 9,857 tons per
week.

Location of Coking Plant.

The coke oven plant will be most economically
situated in close proximity to the blast furnace plant,
but as it has been argued by some authorities that
the colliery itself is the more economical locality, and
not the blast furnaces, it may be well to examine the
reasons for selecting the works as the site for the
coke-making plant as against the colliery.

In the ordinary regenerative coke oven, the heat
necessary to coke the coal is obtained from the com-
bustion in the flues of the coke oven of part of the
gas produced from the distillation of the coal; this
amount of gas is usually taken at 50 per cent, of the
total gas evolved during the manufacture of the coke.
This coke oven gas is a rich gas of high calorific value,
and its replacement by gas of a low heating value,
such as blast furnace gas, when such a gas is available,
is an obvious step in the direction of economy of fuel,
when the coke oven gas thus liberated can be used
to more advantage, which is the case on a large steel
works in which the process of steel manufacture is
carried out in gas-fired open hearth furnaces.

If, now, the coal be carbonised at the colliery, where
no low-power gas is available, unless producer gas is
made from coke, it is clear that this 50 per cent,
of the total rich coke oven gas cannot be used for
steel purposes, and the remaining 50 per cent, avail-
able would have to be brought by means of pipe lines
to the steel works, if it is to be utilised there.

The use of coke oven gas for other purposes, such
as illuminating gas for towns and for domestic heating
purposes, would be foreign to the present investiga-
tion, but the proposal which has been made to utilise
this rich coke oven gas under boilers for the purposes
of raising steam to produce electric power would not
seem to be in the direction of economy of fuel if such
gas could be used in towns directly for heating and
lighting purposes. .

It will be argued that power is required at the
colliery for driving the necessary machinery, etc., but
such power will be more economically obtained from
high-tension current supplied to the colliery from a
series of coke oven and blast furnace plants, which
could be linked together for supplying such electric
power from the surplus they would have over their
own requirements.

It is clear from the above considerations that the
interest of the collieries and the steel works must be
one, otherwise it would be hardly possible to make the
necessary arrangements.

Heating Coke Ovens with Blast Furnace Gas.

This use of blast furnace gas on coke ovens is not
new, although not extensively used. The published
results from a German source are, however, quite

* Appendix II. to the Carbonisation Committee’s Report.

favourable, and it is claimed that the time of carboni-
sation per charge is lessened.

Lt is therefore suggested as an economy which could
undoubtedly be carried out in such a proposed works
that the necessary coke be manufactured in coke ovens
heated by blast furnace gas, thus setting free the
whole of the rich coke oven gas for use in the steel
furnaces and other heating processes.

A possible source of saving in connection with coke
ovens would be the utilisation of the heat in the coke
as drawn from the ovens, but a practical apparatus for
this purpose has still to be designed. So far as the
writer is aware, this problem has not been seriously
attacked, but from a large battery of ovens the power
that might be recovered in the form of low-pressure
steam from this source might be worth investigation.

With the use of blast furnace gas on the coke ovens
it would, of course, be necessary to pre-heat both the
air and gas. In practical work it will be found that
blast furnace gas and coke oven gas need not replace
each other in proportion to the heat units contained
in each, but for the purposes of the present memoir,
the gases have been calculated to replace each other
in direct proportion to their heating value.

The blast furnace plant would consist of four or five
blast, furnace stacks, each provided with the neces-
sary equipment. It is assumed that the ore mixture
used would average 42 per cent, of metallic iron, and
with such a mixture of ores a coke consumption of
23 cwt. per ton of pig iron would be required. This
figure may be criticised by some as being unduly high,
and undoubtedly if dry blast be employed it could be
appreciably lowered. If a. dry blast plant were in-
stalled, the consumption of coke per ton of pig' iron,
with ores containing 42 per cent, of iron, would
probably be 21 cwt. per ton, instead of the 23 cwt.
that is assumed to be necessary with ordinary undried
blast. With such dry blast, however, the blast furnace
gas will run from 4 to 5 per cent, lower in carbonic
oxide gas—that is, instead of having, say, 27 to 28
per cent., aads usual with undried air, this constituent
will only reach 22 to 23 per cent., and the heating
value, instead of being 100 to 105, will, with the dry
air blast, only average from 75 to 80 B.T.U. The
volume of gas produced per ton of pig iron will also
be less than with undried air, so that the apparent
gain in fuel consumption is largely minimised by the
lessened amount of gas obtained and its lower heating
value, when such gas is economically employed for
the production of power for the works. Thus where
the gas is fully utilised it is highly questionable
whether the large outlay for plant for the dry blast
installation is economically justifiable.

Utilising Waste Heat from Slag.

Two other possible sources of economy in connection
with the production of pig iron may be mentioned, the
one being the more extended use of lime in place of
limestone in the blast furnace, and the other the
recovery of the heat contained in the molten blast
furnace slag. The use of lime to replace limestone is
now in practical use in some works where they use
calcining kilns. The problem of the recovery of the
heat of the molten slag is still in the experimental
stage.

From the point of view of economy of fuel, the
blast required to supply the compressed air to the blast
furnaces should undoubtedly be obtained by the use
of large gas engines, for which the motive power would
be cleaned blast furnace gas. It is also assumed that
gas engines would be used for all power purposes,
and that steam is not employed. If gas engines of
sufficient reliability and power cannot be obtained at
reasonable prices, when compared with steam instal-
lations, then economy of fuel must be sacrificed. There
is every reason to hope that post-war conditions will
give us as efficient gas engines as are obtainable in
Germany and -the United States/and that their capital
cost will not be prohibitive, nor -theiFupkeep excessive.

With the assumed weekly output of 6,000 tons of
pig iron, the quantity of free air required may be
taken to be 125,000 cu. ft. per minute, supplied at a
pressure of 12 lb., and allowing 80 per cent, efficiency
for the gas engines with reciprocating blowers, from
actual tests the power required to supply the blast
may be taken as 8,250 I.H.P. for the whole of the
furnaces. Taking the consumption of the blast furnace
gas engines to be 8,700 B.T.U., per I.H.P.- hour, the
8,250 I.H.P. would require 71,775,000 B.T.U. per hour.

In addition to the supply of blast, the auxiliary
power required in and around the furnaces and coke
ovens may be taken as approximately 30 kw. hours
per ton of pig iron produced, or on a make of 6,000
tons per week, say, 1,080 kw. hours per hour, which
would require, if supplied through gas-driven electric
alternators, at 90 per cent, efficiency, an additional
16,086,600 B.T.U. per hour, or, say, a total for the
supply of blast auxiliary power to the blast furnaces
and coke ovens of 87,861,000 B.T.U.. per hour.

Tar and Gas Firing.

In the open hearth steel department for the pro-
duction of 7,500 tons of ingots per week we have

assumed the use of coke oven gas and tar only on
the furnaces, as large experience has been gained with
this method of working, notably in the United States.
From published information, and from direct informa-
tion supplied to the writer from friends in the States,
the figure of 8,000 cu. ft. of coke oven gas per ton
of ingot, with the simultaneous injection of 10 gallons
of tar, may be taken as average figures. The addition
of a jet of tar with the debenzolised coke oven gas
has been found most useful in practical work, as the
operators state they can govern the temperature and
the direction of the gas better when the tar is used,
and the risk of burning the roof is decreased. Probably
powdered pitch injected into the stream of coke oven
gas would perform the same results if it is desired to
take off the light oils from the tar. At present the
most successful use of coke oven gas on steel furnaces
has been with these who use the tar with it.

Apart from the large saving in the use of coal
brought about by this method of working, simplifica-
tions in the construction of the furnaces, and in the
gas tubing and reversing valves are also brought about
by its use. Longer hours of actual steel melting are
also obtained by the use of this gas in place of pro-
ducer gas, owing to the fact that the time employed
at present at the week-ends in flue cleaning can be
dispensed with, and the furnace need not be cooled
down as it is at present during flue cleaning.

The electric energy required to actuate the auxiliary
machinery required on the steel works, such as the
large travelling cranes on the charging and tapping
sides of the furnace, may be taken at 10 kw. hours
per ton of ingots, or, on the weekly make assumed,
of 7,500 tons of ingots, with a working week of, say,
535 kw. hours per hour, and, assuming this electric
energy to be generated by gas engines, it would re-
quire, in round figures, 7,171,000 B.T.U. at 100 per
cent, efficiency, or, say, 8,000,000 B.T.U. per hour
at 90 per cent.

For the output suggested, the weight of the ingot
cast would be at least 3^ tons, and these would be
rolled down in three trains of rolls, which trains, with
all the accessory and repair machinery, would be elec-
trically driven by power supplied as far as possible
by gas engines actuated by blast furnace gas.

The amount of power required in the rolling mill
will, of course, vary with the section rolled, and is
somewhat difficult to fix. A figure which may be fairly
taken is 150 kw. hours per ton of ingot, and this, on
130 hours per week, with a use factor of 80 per cent.,
would require in power 10,817 kw. hours per hour that
the mills were running. Assuming this energy to be
generated by gas engines, the heat energy required
would be 145,000,000 B.T.U. per hour.

The soaking pits and wash heating furnaces for the
ingots would be heated by any excess of coke oven gas
over that required on the steel furnaces. It is assumed
that in the soaking pits and wash heating furnaces
2,000,000 B.T.U. are required per ton of finished steel,
so that for heating up 7,500 tons of ingots (the equiva-
lent of 6,000 tofts per week of finished material), with
the heating furnaces in gas for 144 hours per week,
the hourly requirements would be, say; 84,000,003
B.T.U.

Waste Heat Boilers.

A point in fuel economy in connection with steel
works to which a considerable amount of attention is
now being directed is the utilisation of the waste heat
in open hearth furnace flues by means of waste heat
boilers, as the hot gases pass between the regenerator
chambers and the stack. The writer has no actual
details of English practice, but data from several
American firms have been published, which show that
a considerable amount of power can be developed from
this source.

It is stated that a saving equivalent to 150 to _
200 lb. of coal per ton of steel ingots has been obtained
from this waste heat. Although we should not expect
the same saving when coke oven gas is made use of
on the furnaces, yet there is no doubt that quite a
large saving may still be looked for from this source.

It may be of interest to quote the following extract
from the original paper by C. J. Bacon on this sub-
ject, read in 1915 before the American Iron and Steel
Institute: —

“ Existing boilers on large open hearth furnaces
are showing a saving which, when expressed in terms
of fuel required in coal-fired boilers, is equivalent to
at least 250 lb. of coal (11,000 B.T.U. per lb.) per ton
of ingots.” Arguing on this, Mr. Bacon points out
that if these boilers be applied to the then (1913) total
annual make of steel from open hearth furnaces in
the United States, and assuming a saving of between
150 and 200 lb. of medium coal per ton, there would
be an annual saving of at least 2,000,000 tons of coal.

Reservation of Coking Coals.

As an important point in fuel economy, the writer
would emphasise here the importance of iron works
being supplied with suitable coal for the manufacture
of coke. There are certain seams of coal in Durham
which have been exported as steam coal and which are
suited for forming a fine hard coke, low in ash and
sulphur. Those seams should be reserved exclusively
for this purpose, and on no account should the export
of this particular quality of coal be permitted, as
the export of coals capable of producing a good quality