January 8, 1915.

THE COLLIERY GUARDIAN.

75

temperatures are so related to each other that when
plotted on the diagram the points representing the pro-
portions in which they occur fall nearly upon a straight
line within a certain range of temperature. In other
words, the composition of the products is determined bj
a definite law that may be expressed by an equation.
The percentage composition is not constant, but for any
given percentage of one constituent the percentage of
either of the others is fixed according to a definite
relationship that varies with different coals. In general,
however, as the temperature rises, the ratio of
CO2 + CO to H2O increases, and there is a more or less
constant relationship between CO2 and CO, as shown by
the tendency of the line indicating compositions to
approach an apex of the triangle.

Nature of the Oxidising Action.

There is no indication, therefore, that a uniform com
plex is produced from different coals by oxidation, but
it is probable^that in each coal several different

PRODUCTS FORMED PER GRAM OF DPV COAL

o

100

o

tn

MILLIGRAMS

2

50

75

0

50

150

200

2 50

£2
n. .1

I 00

TEMPERATURE,°C.



300

JI Fig. 10.—Amounts of Water, CO2 and CO
I Produced in the Oxidation of Coal
I	No. 650 (Pittsburg).

' complexes are formed, the same ones occurring possibly
in all coals, but in different proportions. In one coal
the complex may break up more rapidly than in another
and may produce a larger proportion of water or of
CO2. In general, however, oxidation at temperatures
*below 200 degs. Cent, tends both to fix oxygen and to
form products in which water predominates, whereas
above that temperature there is little or no fixation of
oxygen, and CO2 begins to predominate in the products.
An easily-ignited coal begins to produce CO2 in large
proportion at lower temperatures than a coal that is
more difficult to ignite. Curves plotted in fig. 12 show
the net increase in weight in coals No. 43 (Wyoming),
No. 48 (Illinois), and No. 650 (Pittsburg) by fixation
of oxygen.

Inasmuch as there are probably only traces of free
carbon and probably no free hydrogen in coal, there
seems to be, when coal burns, no possibility of the
direct formation of CO2, CO, or H2O from the elements,
unless it may be assumed that a preliminary distillation
of volatile matter, that leaves a residue of free carbon
in the form of coke, takes place. Such an assumption
is hardly justified, in view of the facts that coal does
not completely set free its volatile matter below 900 degs.

«---COzjPER cent

Fig. 11.—Composition of Oxidation Products
from Coals No. 43 (Wyoming), No. 48 (Ttlinois)
and No. 650 (Pittsburg).

Coal 48 (Illinois) \ ,
■ Coal 43 (Wyoming) \

Coal 650 (Pittsburgh)
AZ_________________\Z______

//	d380/

^3Z0*
350’	/

O

\ %

--A50 **

««

#Z6O\

to 1,000 degs. Cent., that considerable time is ordinarily
required to heat the coal to that temperature, and that
during this time under the usual conditions of furnace
combustion the coal is surrounded by oxygen and is
undergoing oxidation in the manner described above.

The combustion of some hydrocarbons has been
shown by Bone * * * § to involve an intermediate formation
of an oxygenated molecule which decomposes into the
final products CO2 and H2O. Rhead and Wheeler f at
the coaldust experiment station of the British Home
Office, who have studied the mode of combustion of

# W. A. Bone, and others, “ The Slow Oxidation o^
Methane at Low Temperatures,” Jour. Chem. Soc. (L ondon),
vol. 81, pt. 1, 1902, p. 536, vol. 83, pt. 2, 1903, p. 1074; “ The
Slow Combustion of Ethane,” vol. 85, pt. 1, 1904, p. 693 ;
“ The Combustion of Ethylene,” vol. 85, pt. 2, 1904, p. 1637 ;
“The Combustion of Acetylene,” vol. 87, pt. 2, 1905,
p. 1232; “ The Explosive Combustion of Hydrocarbons,”
vol. 89, pt. 1, 1906, p. 660.

t T. F. E. Rhead and R. V. Wheeler, “ The Mode of
Combustion of Carbon.” Jour. Chem. Soc. (London), vol. 103,
1913, pp. 461,1210.

carbon, advance the hypothesis, which is supported by
experimental evidence, that “in the normal burning
of carbon the carbon dioxide and carbon monoxide
found as the apparently primary products of combustion
arise from the decomposition at the temperature of
combustion of a complex, the formation of which is
the first result of the encounters between oxygen and
carbon molecules.” Their work was done with charcoal.
They consider the complex formed to be physical-
chemical, no definite molecular formula being assigned
to it, and a high temperature or reduced pressure being
required for its decomposition.

Inflammability of Coal Dust.

Investigators in England * and in France f have
advanced the theory that, assuming like conditions as
to fineness of division, percentage of inert material, and
amount of previous exposure, the variation in degree of
inflammability of different coaldusts suspended in air is
due largely to ease of decomposition by heat, or, in
other words, to the relative amount and character of
the volatile products distilled from the coal by moderate
heating.

The following considerations call attention to another
factor that probably influences the inflammability of
coal more than does the amount or character of the
volatile matter itself. Comparative inflammability tests
at the Bureau of Mines laboratories J of the four kinds
of coal used in the investigation of oxidation herein
described have shown that coal No. 43 (Wyoming), in
spite of its large water content is much more highly
inflammable than any of the others, and that the three
other coals follow it in order of decreasing inflammability,
as follows :

No. 48 (Illinois), No. 395 or 650 (Pittsburg), and
No. 9614 (W. Ya.).

Studies § of the volatile matter produced from these
same coals have shown that the volatile gases from coal
No. 43 (Wyoming) and from No. 48 (Illinois)—the more
inflammable of the four coals—contain i elatively large
proportions of water of^decomposition and CO2, which

tn

-10

eo

40

60

50

70

20

30

IO

50

43(Wyo.)

(Wyoming), No. 48 (Illinois) and No. 650
(Pittsburg) by Oxygen Fixation.

IOO 150	200	250	300	360

TEMPERATURE ?0G.

Fig. 12—Increase of Weight in Coals No. 43

probably tend to retard the ignition of inflammable
gases. No experimental evidence has been obtained to
show that the volatile matter produced by the usual
methods of coal dust ignition is distilled from coal
during the momentary period of heating prevailing in
the ignition of coal dust, or that, if any such is distilled,
the amount is sufficient to become a factor in ignition.
Even if such preliminary distillation occurs, it does not
seem likely, in view of the nature of the early volatile
matter from the more inflammable coals, that the
inflammability of the coal is determined thereby.

Relative rapidity of oxidation can, on the other hand,
reasonably be regarded as an index to the degree of
inflammability of a combustible material, since ignition
is so largely a process of oxidation. Oxidation is an
exothermic process, producing heat even in its initial
stages, and the rate of oxidation rapidly increases with
rise of temperature; therefore oxidation is self-pro-
moting, and may increase its own intensity with
explosive rapidity under favourable conditions. Dis-
tillation of coal, on the other hand, is, in its early
stages, endothermic, and is therefore not self-promoting,
but tends to withdraw heat from surrounding sources.

Although frequent irregularities occur, the relative
inflammability of coal dusts suspended in air is found
to vary approximately as the percentage of volatile
matter in the coal. This fact does not, however,
establish any causative influence of one variable on the
other.

In the coals thus far tested the comparative rate of
oxidisability also varies approximately as the percentage

* Second Report, Explosions in Mines Committee, British
Home Office, 1912.

f Report of First Series Laboratory Investigations,
French Collieries Commission, May 1912.

J J. C. W. Frazer, E. J. Hoffman, and L. A. Scholl,, jr.,
“ A Laboratory Study of the Inflammability of Coal Dust.”
Bull. 50, Bureau of Mines, 1913.

§ H. C. Porter and F. K. Ovitz, “The Volatile Matter of
Coal.” Bull. 1, Bureau of Mines, 1910, 56 pp.

of volatile matter, therefore the rate of oxidisability
may perhaps be a truer index of inflammability than
the percentage of volatile matter.

A simple test for comparative oxidisability of com-
bustible materials is desirable, for oxidisability may
prove to be an index of the relative “ ignition points ”
of the materials or their degrees of inflammability when
suspended as dusts.

Other Conclusions.

The comparative tendency of different coals to
deteriorate in storage or to develop heat spontaneously
conforms to their relative oxidisability.

The effect of weathering or of preliminary moderate
heating on the coking quality of coal is explained as an
effect of oxidation, whereby the fusible organic con-
stituents of the coal are decomposed or altered. The
alteration does not occur in a non-oxidising atmosphere.

It is known that weathering of coal causes an increase
of “ combined ” water—that is, of water that is not in
the normal free state, and that has at any given
temperature a vapour pressure lower than the normal.
This water remains in the coal after “ air-drying.” Its
increase by weathering is due to oxidation, and to the
formation thereby of a complex easily decomposed by
heat so as to yield water.

The official methods of analysis for coal, recommended
by a committee of the American Chemical Society,
prescribe a preliminary air-drying procedure to bring
the sample into moisture equilibrium with the air,
and to minimise alteration during exposure. For
ceitain kinds of high-moisture coals this air-drying pro-
cess requires from two to four days at 30 to 35 degrees
Cent. It will be seen from the experiments to determine
the oxidation of coal previously described that an err or-
is thus introduced into the analysis through the
formation of an oxygenated complex that decomposes
on drying at 105 degs. Cent., and yields an amount of
water in excess of the true original content of the coal.
The official method for determining moisture prescribes
drying in air at 105 degs. With some coals errors due
to oxidation also enter into this process through an
increase of weight from oxygen absorption, and, when
water is determined by direct weighing, from the pro-
duction of an excessive amount of water through the
breaking down of the oxygenated complex.

In a sealed area of a coal mine, or in a place where
there is no ventilation, the oxygen content of the air is
rapidly reduced by absorption in the coal. If the coal
becomes warmed in gob piles or elsewhere by spon-
taneous heating, appreciable quantities of carbon
monoxide may be formed by oxidation. When a fire
area in a mine has been sealed in order to extinguish
the fire by reducing the air supply, the analyses of gas
samples from behind stoppings give indications of the
progress of the fire. As in the oxidation of coal
considerable carbon monoxide is formed at temperatures
as low as 200 to 300 degrees Cent., its presence in mine-
fire gases may not always indicate active combustion,
but it does indicate a temperature high enough to
re-ignite the coal when an adequate air supply is
admitted.

LETTERS TO THE EDITORS.

The Editors are not responsible either for the statements
made, or the opinions expressed by correspondents.

All communications must be authenticated by the name and
address of the sender, whether for publication or not.
No notice can be taken of anonymous communications.

As replies to questions are only given by way of published
answers to correspondents, and not by letter, stamped
addressed envelopes are not required to be sent.

GERMAN PATENT SAFETY CATCHES.

Sirs,—During the past 50 years it has been fashion-
able in Germany to use safety catches on pit cages. In
Britain, in discussions, it was often said : “We ought
to protect cages here as they do in Germany whilst
inventors were invited to study and imitate the safety
catches in Germany.

Now that it is a simple matter to confiscate German
patents, cage owners have a chance of making these
things for safety of British cages.	S. M.

January 4, 1915.

Coal Contracts in War Time.—At the meeting of the
Lambeth Board of Guardians on Wednesday, the Contracts
Committee reported : “ We have had submitted to us letters
from Messrs. Sargeant, Longstaff and Company, dated
November 27 and 30, acknowledging the Guardians’ notice
of acceptance of their tender for the supply of coal during
the four months ending March 31, 1915, stating that since
tendering they have been faced with considerable labour
troubles, owing to difficulty in replacing men who are joining
the Army, and in obtaining extra labour to cope with the
winter trade; also as regards seaborne coal, greater restric-
tions are being imposed upon shipping in the North Sea,
which, coupled with the fact of the entire dislocation of the
railway transport arrangements, may make it necessary for
them to ask the Guardians to assist them in defraying the
extra expenses which they will have to meet, but assuring
the Guardians that they may be relied upon to do everything
possible under most unusual and difficult circumstances.”
No action was taken on the presentation of this report, but
a report of the Schools Committee stated that in consequence
of the inability of the contractors to deliver East .Hetton
Wallsend coals owing to labour and transport difficulties, the
superintendent of the Norwood Schools had accepted York-
shire Silkstone coals at 3s. per ton in excess of the price
specified in the contract for the delivery of this coal to the
North Lambeth institution, the difference representing the
additional cost of cartage to the Norwood institution, as
provided in the contract. The Guardians approved the
action of the superintendent.