November 10, 1916.

__________________________________________________________________	___________________________________

904

THE COLLIERY GUARDIAN.

the relation of moisture to vapour pressure, will be
alluded to in considering the thermal effect of wetting
dry coal.

An experiment with infusorial earth which is com-
posed largely of silica in a very fine state of division,
showed that infusiorial earth has an effect similar to
that of coal in lowering the normal vapour pressure
of the water which it absorbs. The lowering effect
appears to be somewhat less marked than that of the
Illinois type of coal, but greater than that caused by
the Pittsburg type. (See fig. 1.)

Thermal Effect of the Wetting of Coal.

Heat is produced by wetting dry coal or a partly-
dried coal containing less than its normal percentage of
inherent water. The relative quantity of heat
generated depends on the kind of coal and its relative
deficiency in inherent water as referred to its maximum
normal content. In other words, the thermal effect of
wetting varies directly as some function of the relative
vapour pressure deficiency in the coal.

Method of Determination.

Experiments were made to determine the thermal
effect of wetting dry and partly-dried coal. The method
consisted in mixing the dried or partly-dried sample of
coal with relatively the smallest amount of water com-
patible with efficient stirring and accurate reading of
the temperature. The calorimeter (fig. 2) consisted of
a nickel cup c, of about 70 c.c. capacity, resting on a
cork e, near the bottom of a Dewar vacuum jacketed
tube d, 36 cm. long and 8 cm. in diameter. This tube
was in turn surrounded by a large glass vessel for pro-
tection from air currents, and both of the larger vessels
were provided with fibre-board covers /. The stirrer a
was a small metal vane attached to a glass rod rotated
by a motor from the disc h. The thermometer g was
a calibrated Beckman graduated in divisions of 0*01 deg.
each.

The coal sample was reduced to 60-mesh size, and
dried as desired in a vacuum desiccator over sulphuric
acid. Just before the experiment the sample was
divided into two parts, one for determination of mois-
ture and the other for the test. The latter part was
introduced into a small bulb b, of thin glass, and the
bulb quickly evacuated to a pressure of about 20 mm.
of mercury, and sealed. The bulb was then submerged
in the water of the calorimeter vessel by means of a
wire spring between the walls of the crucible, and after
a number of temperature readings, to establish the
rate of change by radiation, the bulb was broken to wet
the coal. The procedure was found to be necessary in
order to wet the coal thoroughly. Temperature read-
ings were continued until a regular rate of cooling was
established, and the radiation correction was then
applied in the usual manner.

The moisture content of the coal as tested was deter-
mined by heating a weighed sample for one hour at
llOdegs. Cent, in a glass tube surrounded by an elec-
tric heater, the tube being connected to a vessel con-
taining phosphorus pentoxide and evacuated by a
mercury pump to about 1 mm. The coal was contained
in a glass boat which, for weighing, was quickly trans-
ferred from the heater to a glass stoppered weighing
tube.

Results of Tests.

The results of calorimetric determinations of the heat
of wetting for different coals and for various percentages
of water content are shown in fig. 3. Sub-bituminous
coal from Wyoming that had been dried, produced by
complete wetting 19-2 calories of heat per grm. of
dry coal; brown lignite from North Dakota generated
25’5 calories; bituminous coal from Franklin County,
Illinois, evolved 6-8 calories; and bituminous coal from
the Pittsburg bed in Pennsylvania produced 1 calorie
per grm. On the basis of the known specific heats
of these coals, if the values of the saturated coals are
taken, and it is assumed that no heat is lost to the
containing vessel, or to excess water, or by radiation,
the heat developed would raise the temperature of the
different coals 43, 64, 20, and 4degs. Cent., respec-
tively. The results of the tests are shown summarised
in Table III.

Table Ilf.

Heat developed by complete sa( uration with water,
Per-	calories per gramme of dry coal.

centage of /---------------------*----------------------

water in coal as	Sub-bitu minous	- Bitu- minou	Ritu- s minous	Anthra- cite	Lignite (Lab.  No. 14).	Peat (Lab. No. 27),
tested.	(Lab.	(Lab.	(Lab.	(Lab.		
	No. 43).	No. 48),	. No. 650).	No. 663).		
o-o	... 19’81 .	6’56	... 1 03 .	.. 0’71	. 26’03 .	18’58
0’6	... 19’51	5’67	... 0’74 .	—	—	—
0’7	... 19’39 .	—	—	—	.’ 26’02 .*	16’95
1’1	—_	—	... 0’32 ’.	—	—	—
1’2	—-	4’84	—	—	—	—
2'2	—	—	—	—	' 21’47 .	—
2*5	—	2’80	—	—	—	12 94
2’7	14’50 ’.	—	—	—	—	—
2’9	—	—	—	0’20	—	—
4'5	11'08 ‘	1'91	—	—	—	—
5'6	—	—	—	—	: 13-16:	—
7'1	—	—	—	—	—	6’01
7'4	7’42 :	—	—	—	—	—
7’6	—R	0'42	—	—	—	-
9’5	—	0'10	—	—	; 9’60 ;	—
11'0		~	—	—	—	—	4*40
14'4	—	—	—	—	; 4’56 :	—
17'1	1'84	—	—	—	—	—
17'7	—	—	—	—	—	0’56
21’3	0'36 ’	Discussion of Results.			’ 0’54 ’.	—

The results in Table III. on Wyoming coal (Lab.
No. 43) and the curves c and d m figure 3 show that
uniformly lower results are obtained from a coal sample
prepared by hydrating than from a sample prepared by
dehydrating. The water content in two samples may
be shown by analysis to be the same, but the one pre-
pared by partly dehydrating fresh coal seemingly con-
tains more material capable of producing heat on being
moistened than does the sample made by adding water

to a thoroughly dry coal. The reason for this variation
is not entirely clear. It may be that the minute inner
pores, after complete dehydration, do not fill as readily
with water as do the larger pores emptied by partial
dehydration. It is possible, also, that the dry coal
substance, when re-moistened, holds parts of the water in
such manner that the water has practically no vapour
pressure and is not determinable by analysis. This
would account for the fact that such a sample of coal,
although showing by analysis a smaller water content,
may give the same effect on wetting as a sample
prepared from fresh coal by dehydrating.

A similar variation was found between two samples
of the same coal in the vapour-pressure determinations.
Although the explanation given for variation in heat
effect may also in a small measure apply to the variation
in vapour pressure, it is likely,»as before mentioned, that
one reason for the discrepancy in the vapour pressures is
the extremely slow rate at which vapour-pressure equili-
brium is established by the method used, and to the fact
that probably not enough time was allowed for this in
the experiments.

A sample of infusorial earth dried over sulphuric acid
produced on being thoroughly wetted about 2 calories
per grm. of dry earth.

Parks has shown that freshly precipitated silica, after
being highly heated in a vacuum, developed 11’4 calories
per grm. on being wetted. Rodewald and Kattein, in a
study of the specific heat of wheat starch as affected by
its degree of hydration, showed that this material after
being thoroughly dried produced on being wetted 28-8
calories per grm.

Exothermic Action of Toluene.

Liquids other than water produce heat when mixed
with finely divided solids. Pure dry toluene when mixed
with dry powdered coal produced from 0-5 to 5 calories
of heat per grm. of dry coal, depending on the kind of
coal and the method used in preparing it. With coal
from the Pittsburg bed and from New River, West

3000

o

TS

1500

1000

5-(
<D
P-

a wbutstarch

b Lignite

C Wyoming (by dehydrating) ;

d Wyoming (by hydrating .

e Peat

f Illinois

g Anthracite

A Pittsburgh	|

tn

a

&

c3
O

Water in coal as tested, per cent.

Fig. 3.—Heat evolved

by Coals when wetted.

Virginia, toluene showed more heat-producing effect
than water. With Illinois and Wyoming coals the
reverse was true. The exothermic action was slow, and
had a very indefinite end point. With infusorial earth
toluene produced 3*8 calories per grm.

Effect of Fineness of Coal.

It was found that with Wyoming sub-bituminous coal
no difference in the thermal effect was caused by varying
the fineness of division of the coal from “ 20 to 40
mesh’’ to “ under 200 mesh.”

The bearing of these facts on the probable explanation
of the condition of the water in coal is considered later.

Relative Specific Heats of Dried and Undried Coal.

The specific heat of the water in coal, calculated
according to Kopp’s law, from the determined specific
heats of dried and undried coal, is considerably less
than the normal specific heat of free liquid water.
Determinations of these values have been reported pre-
viously by Porter and Taylor.*

Unfortunately, the specific-heat determinations made
with dry coal were subject to a considerable error by the
method used, namely, that of mixing the heated coal
with a liquid of knowm specific heat. In determinations
with dry coal the use of water was, of course, avoided,
owing to its known thermal effect, and dry toluene was
substituted, as its calorific action on the coal appeared
to be small and very slow. The error was, however,
appreciable, and only an approximate correction could
be made for it, because of the slow and variable rate of
heat development. The values given in Table IV.,
which shows the specific heats of dried and undried
coals, and of certain other substances for comparison,
are to be regarded as having an accuracy of not greater
than 3 per cent., and they serve merely as an (indication
of the fact that the inherent water of coal has a mole-
cular heat lower than the normal.

Probable State of the Inherent Water in Coal.

Several hypotheses as to the state of the water in coal
may be considered. The observed phenomena of the
lowering of the vapour pressure and specific heat of the
water in the coal, and the development of heat by
wetting dried coal, are in conformity one with another,
and are to be explained as phenomena exhibited by
colloidal material and not as being caused by chemical
combination of the water and coal.

The Colloidal Theory.

The conception of a “ hydrogel ” or “ colloidal gel ”
may be applied to coal or to some part of the coal sub-
stance as it is to many organic substances when
coagulated from a solution or suspension in water.

* Porter, H. 0., and Taylor, G-. B. “ The Specific Heat
of Coal.” Jour. Ind. Eng. Chem., vol. 5, 1913, p. 289.

Table IV.

Water
content.

Kind of sample.

Bituminous coal (Lab. No. 48)
from Illinois:—

Undried ........................

Dry .................................

Sub-bituminous coal (Lab.

No. 575) from Wyoming:—

Air-dried.................

Dry ....................

CuS04, 5H2O...............

CuS04 (anhydrous)__________

BaCl2, 2H2O...............

BaCl2 (anhydrous) .....•___

MnS04, 5H2O .............

MnSO4(anhydrous)..........

......

Wheat starch .............

.....

Do., air-dried.............

Do.......................

Do. ....................

Specific Specific
heat of heat of
sub- water
stance, content.

P.c.

8’4	... *0'334  ... *0’287	... 0’85
11’0	... *0 350	... 0’84
—	... *0’289	—
36’1	... 0’269	0’48
—	... 0’151	—
14’7	... 0’151	0’51
—	... U’090	—
37’4	... 0’407	// 0’78
—	..	0’181	—
22’6	... 0’3663	058
15’6	... 0’33< 6	0’46
8’6	... 0’3083	... 0’36
—	... 0’3063	—

* Results obtained by Porter and Taylor (loc. cit.) at 28 to
63 degs. Cent, corrected fox errors due to generation of heat
by wetting.

___________________________________________________

There is some ground for supposing that part of the
substance of coal may, at one stage in its formation, have
been deposited as a colloidal gel from suspension in
water. Whether this supposition be true or not, it is
commonly agreed that coal has been formed by the
degradation of vegetal matter that is in part, at least,
colloidal in character. It is, therefore, likely that apart
of the coal substance, relatively larger in the cretaceous
coals, may retain some of the properties of a colloidal
gel. These colloidal properties are manifested in coal
by the retention of water in the peculiar state which the
writers have termed “inherent water.’’ An explana-
tion of the exact state of the water would involve a dis-
cussion of the theories of colloid chemistry or physics.
It will suffice here to say that a peculiar physical condi-
tion, possibly a densification, is brought about in the

interfacial layer between the solid and liquid phases,
tending to lower the normal vapour pressure of the
liquid, to increase its latent heat of vaporisation, and to
liberate a heat of “ condensation ’’ or a heat of forma-
tion when the liquid enters into this condition.

This conception does not imply a molecular aggregate
of definite proportions, and therein it differs from the
conception of a hydrate or a chemical compound.

Theories involving the conception of “ solid solution ”
of a liquid in a solid and of adsorption or densification
of the liquid on the walls of microscopic capillary open-
ings or pores have been proposed and studies have been
made of hydration phenomena in connection with starch,
gelatin, wood fibre, and various inorganic colloids in the
light of these theories.

Relation of Theory to Observed Results.

The facts observed in the study of water in coal are,
in the main, readily explained by the colloidal hypo-
thesis. As previously brought out, the lowering of the
aqueous vapour pressure observed in coal is normal to the
colloidal condition of water, and the heat generated by
wetting the coal is in all probability the transformed
energy that results -when the liquid assumes this
colloidal state of densification or association.

The fact that the size of the coal, or its fineness of
division, makes little difference in the “ heat of
wetting,” is probably to be explained by assuming that
each particle of the dried coal acts as a sponge, or a
tissue permeated with microscopic pores. Water is
drawn into the pores of the larger pieces "with nearly the
same facility as it is into those of very small particles.

Liquids other than water are capable of forming this
molecular association with coal and with other solids.
Saturation with toluene, for example, produces as much
heat in finely divided silica, or the older coals, such as
those of bhe Appalachian region, as wetting with water
does. Adding toluene does not, however, produce as
much heat as does wetting in coals of more recent origin,
such as the cretaceous coals of the Western States, which
probably contain more colloidal material.

This fact suggests that the heat may be produced by
two processes : (1) the mere penetration of the liquid
into minute capillaries, and (2) the formation of a
colloidal condition, possibly more readily entered into by
water than by other liquids.

The fact that the specific heat of water is lower in the
state it assumes in the coal substance suggests a different
molecular condition, probably an association of the
molecules.

Summary.

Part of the water in coal is held more tenaciously than
free water by forces due to colloidal conditions, which,
in a practical way, are manifested in the following
features : (1) The impossibility of drying coal completely