By J.W. THOMAS, F.C.S., F.I.C.
A simple experiment, capable of yielding results which shall be at least
comparative, has long been sought after by large consumers of coal and
artificial fuel abroad in order to ascertain the relative calorific
power possessed by each description, as it is well known that the
proportion of mineral matter and the chemical composition of coal differ
widely. The determination of the ash in coal is not a highly scientific
operation; hence it is not surprising that foreign merchants should
have become alive to the importance of estimating its quantity. While,
however, the nature and quantity of the ash can be determined without
much difficulty, the determination of the chemical composition of
coal entails considerable labor and skill; hence a method giving the
calorific power of any fuel in an exact and reliable manner by a simple
experiment is a great desideratum. This will become more obvious when
one takes into consideration the many qualities and variable characters
of the coals yielded by the South Wales and North of England coal
fields. Bituminous coals--giving some 65 per cent, of coke--are
preferred for some manufacturing purposes and in some markets.
Bituminous steam coals, yielding 75 per cent, of coke, are highly prized
in others. Semi-bituminous steam coals, yielding 80 to 83 per cent, of
coke, are most highly valued, and find the readiest sale abroad; and
anthracite steam coal (dry coals), giving from 85 to 88 per cent, of
coke (using the term "coke" as equivalent to the non-volatile portion of
the coal) is also exported in considerable quantity. Now the estimation
of the ash of any of these varieties of coal would afford no evidence
as to the class to which that coal belongs, and there is no simple test
that will give the calorific power of a coal, and at the same time
indicate the degree of bituminous or anthracitic character which it
possesses.
In order to obtain such information it is necessary that the percentage
of coke be determined together with the sulphur, ash, and water, and
these form data which at once show the nature of a fuel and give some
indication of its value. To ascertain the quantity of the sulphur, ash,
and water with accuracy involves more skill and aptitude than can
be bestowed by the non-professional public; the consequence is that
experiments entailing less time and precision, like those devised by
Berthier and Thompson, have been tried more or less extensively.
In France and Italy, Berthier's method--slightly modified in some
instances--has been long used. It is as follows:
70 grammes of oxide of lead (litharge) and 10 grammes of oxychloride of
lead are employed to afford oxygen for the combustion of 1 gramme of
fuel in a crucible. From the weight of the button of lead, and taking
8,080 units as the equivalent of carbon, the total heat-units of the
fuel is calculated. This experiment is very imperfect and erroneous upon
scientific grounds, since the hydrogen of the fuel is scarcely taken
into account at all. In the first place, hydrogen consumes only one
quarter as much oxygen as carbon, and, furthermore, two-ninths only of
the heating power of hydrogen is used as the multiplying number,
viz., 8,080, while the value of hydrogen is 34,462. In other words,
one-eighteenth only of the available hydrogen present in the fuel is
shown in the result obtained. Apart from this my experience of the
working of Berthier's method has been by no means satisfactory. There
is considerable difficulty in obtaining pure litharge, and it is almost
impossible to procure a crucible which does not exert a reducing action
upon the lead oxide. Some twelve months ago I went out to Italy to test
a large number of cargoes of coal with Thompson's calorimeter, and since
then this apparatus has superseded Berthier's process, and is likely to
come into more general use. Like Berthier's method, Thompson's apparatus
is not without its disadvantages, and the purpose of this paper is to
set these forth, as well as to suggest a uniform method of working by
means of which the great and irreconcilable differences in the results
obtained by some chemists might be overcome. It has already been
observed that a coal rich in hydrogen shows a low heating power by
Berthier's method, and it will become evident on further reflection that
the higher the percentage of carbon the greater will be the indicated
calorific power. In fact a good sample of anthracite will give higher
results than any other class of coal by Berthier's process. With
Thompson's calorimeter the reverse is the case, as the whole of the
heating power of the hydrogen is taken into account. In short, with
careful working, the more bituminous a coal is the more certain is it
that its full heating power shall be exerted and recorded, so far as the
apparatus is capable of indicating it; for when the result obtained is
multiplied by the equivalent of the latent heat of steam the product is
always below the theoretical heat units calculated from the chemical
composition of the coal by the acid of Favre and Silbermann's figures
for carbon and hydrogen. On the other hand, when the heating power of
coal low in hydrogen is determined by Thompson's calorimeter, much
difficulty is experienced in burning the carbon completely; hence a low
result is obtained. From a large number of experiments I have found that
when a coal does not yield more than 86 per cent, of coke, it gives its
full comparative heating power, but it is very questionable if equal
results will be worked out if the coke exceeds the above amount although
I have met with coals giving 87 per cent. of coke which were perfectly
manageable, though in other cases the coal did not burn completely. It
will be noted that the non-volatile residue of anthracite is never as
low as 86 per cent., and this, together with the very dry steam coals
and bastard anthracite (found over a not inextensive tract of the South
Wales Coal field), form a series of coals, alike difficult to burn in
Thompson's calorimeter. Considerable experience has shown that in no
single instance was the true comparative heating power of anthracite
or bastard anthracite indicated. With a view to accelerate the perfect
combustion of these coals, sugar, starch, bitumen, and bituminous
coals--substances rich in hydrogen--were employed, mixed in varying
proportions with the anthracitic coal, but without the anticipated
effect. Coke was also treated in a like manner. Without enlarging
further upon these futile trials--all carefully and repeatedly
verified--the results of my experiments and experience show that for
coals of an anthracitic character, yielding more than 87 per cent. of
coke, or for coke itself, Thompson's calorimeter is not suited as an
indicator of their comparative calorific power, for the simple reason
that some of the carbon is so graphitic in its nature that it will not
burn perfectly when mixed with nitrate and chlorate of potash. A sample
of very pure anthracite used in the experiments referred to, gave 90.4
per cent. of non-volatile residue, and only 0.84 per cent. of ash. This
coal was not difficult to experiment with, as combustion started with
comparative ease and proceeded quite rapidly enough, but in every
instance a portion of the carbon was unconsumed, and consequently
instead of about 13° of rise in temperature only 10° were recorded.
Since the calorific power of a coal is determined by the number of
degrees Fahrenheit which a given quantity of water is raised in
temperature by a known weight of fuel, it follows that every care should
be taken that the experiment be performed under similar atmospheric
conditions. The oscillation of barometric pressure does not appear to
affect the working, but the temperature of the room in which the
work was done, and especially that of the water, are most important
considerations. It has been observed by some who have used this
apparatus--and I have frequently noticed it myself--that the lower the
temperature of the water is under which the fuel is burnt the higher is
the result found. This has been explained on the assumption that the
colder the water used, the greater is the difference between the
temperature of the room and that of the water; hence it would be
expedient that in all cases when such experiments are made the same
difference of temperature between the air in the room and the water
employed should always exist. For example, if the temperature of the
room were 70°, and the water at 60°, then the same coal would give a
like result with the water at 40° and the room at 50°. This has been
regarded as the more evident, because the gases passing through
the water escape under favorable conditions of working at the same
temperature as the water, and are perfectly deprived of any heat in
excess of that possessed by the water. Under these circumstances it
would seem only reasonable that this assumption should be correct. It
was, however, found after a large number of experiments upon the same
sample of coal that this was not the case. 30 grammes of coal which
raises the temperature of the water 13.4°, when the water at starting
was 60° and the room at 70°, gives 13.7° rise of temperature with the
water at 40° and the room at 50°. Conversely, when the water is at 70°
and the room at 80°, a lower result is obtained. The explanation appears
to be this: The gas which escapes from the water was not in existence in
the gaseous form previous to the experiment, and the heat communicated
to the gas being a definite quantity it follows that the more the gas
is cooled the greater the proportion of chemical energy in the shape of
heat will be utilized and recorded as calorific power.
In order, therefore, to make the experiment more simple and workable
at all temperatures, a sample of coal was selected, which should be
perfectly manageable and readily consumed. Appended is an analysis of
the coal employed (from Ebbw Vale, Monmouthshire):
Composition per cent.
Carbon...............................88.33
Hydrogen............................. 5.08
Oxygen............................... 3.28
Nitrogen............................. 0.55
Sulphur.............................. 0.70
Ash.................................. 1.26
Water (moisture)..................... 0.80
-----
100.00
In the following experiments the standard temperature of the water was
taken as 60° F., and as the coal gave 13.4° of rise of temperature, 67°
F. was selected as the standard room temperature. The reason for this
room temperature is obvious, for, whatever heating effect the higher
temperature of the room may have upon the water in the cylinder during
the time occupied by the first half of the experiment, would be
compensated for by the loss sustained during the second half of the
experiment, when the temperature of the water exceeded that of the room.
The mean of numerous trials gave 13.4° F. rise of temperature, equal to
14.74 lb. of water per lb. of coal. When the water was at 50° and
the room at 57°, the mean of several experiments gave 13.5° rise of
temperature. When the water was 40° at starting and the room at 47°,
13.65° was the average rise of temperature. Trials were made at
intermediate temperatures, and the results always showed that higher
figures were recorded when the water was coldest. With a view of getting
uniformity in the results it was thought well to make experiments, in
order to find out what temperature the room should be at, so that this
coal might give the same result with the water at 50°, 40°, or at
intermediate temperatures. Without going much into detail, it was found
that when the temperature of the room was at 40° and that of the water
40°, and the experiment was rapidly and carefully performed, 13.4° rise
of temperature was given; but this result could be obtained without
special effort when the room was 42° and the water 40° at starting. It
is evident that the cooling effect of the air in the room upon the water
cylinder is very appreciable when the water has reached 13° above that
of the room. When the water was at 50° and the room at 55°, the coal
gave 13.4° rise with ease and certainty, and it would not be out of
place to remark here that with those coals which burn well in Thompson's
calorimeter, the results of several trials are remarkably uniform when
properly performed. With the water at 70° and the room at 80°, a like
result was worked out. Experiments at intermediate temperatures were
also carried out (see table in sequel). It is true that the whole
difference of temperature we are dealing with in making these
corrections is only 0.25, but 0.2 in the result, when multiplied by 537
to bring it into calories, as is done by the authorities in Italy, makes
more than 100 heat units--a serious difference when 5d. per ton fine is
attached to every 100 calories lower than the number guaranteed.
Taking the latent heat of steam as 537° C., and multiplying this number
by 14.74, the evaporative power of the coal used in these experiments,
its equivalent in calories is 7,915. From the analysis of this coal,
disregarding the nitrogen and deducting an equivalent of hydrogen
for the oxygen present, the _total heat units_ given by Favre and
Silbermann's figures for carbon (8,080) and hydrogen (34,462) will
be 8,746. It will be seen, therefore, that the calorific power, as
determined by Thompson's apparatus, gives a much lower result when
multiplied by 537 than the heat units calculated from the chemical
composition of the coal. When I used Thompson's apparatus in the
chemical laboratory at Turin to determine the evaporative power of
various cargoes of South Wales coal, it was agreed by mutual consent
that the temperature of the water at starting should be 39° F. (the
temperature at which the _heat unit_ was determined). The temperature
of the room was about 60°, but this varied, as the weather was somewhat
severe and changeable. Under these conditions, with the water at 39° and
room 60°, the coal which gives 14.74 lb. of water per lb. of coal,
will give as high as 15.88 lb. of water per lb. of coal. This result
multiplied by 537=8,496 calories, approaching much more nearly to the
theoretic value. This method of working is still practiced abroad, but
experience has shown that very widely differing results follow when
working in this manner, especially if the temperature of the room is
changeable, as it naturally is where ash determinations and other
chemical work is proceeding simultaneously. The time the experiment
lasts, taking the reading on a quickly rising thermometer and other
considerations, render the experiments anything but trustworthy when
0.2 of a degree makes a difference of more than 100 calories. In the
instructions supplied with Thompson's calorimeter nothing is said as to
the temperature of the room in which the experiment is performed, but
simply that the water shall be at 60° F. If, with the water at 60°, a
room were at 50°, as it often is in winter, a good coal would give 14
lb. of water per lb. of coal as the evaporative power; but if in summer,
the room were at 75° and the water at 60°, the same coal would give 15
lb. of water per lb. of coal. If further evidence were needed of the
effect of temperature consideration of the experiments already referred
to will show how necessary it is that some general rule shall be
adopted. Considerable stress is laid (in the instructions) upon the
quantity of oxygen mixture used being determined by rough experiments.
This I have found leads to erroneous conclusions unless a number of
experiments are tried in the calorimeter, as it often happens that the
quantity which appears to be best adapted is not that which yields a
trustworthy result. There are many samples of South Wales coal, 30
grains of which will require 10 parts of oxygen mixture in order to burn
completely, but since a little oxygen is lost in drying and grinding,
and few samples of chlorate are free from chloride, it is not safe to
use less than 11 parts of oxygen mixture, but this amount is sufficient
in _all_ cases, and never need be exceeded. I have made numerous
experiments with various coals (anthracite, steam, semi-bituminous, and
bituminous, including a specimen of the ten yard coal of Derbyshire),
and find that with 11 parts of chlorate and nitrate of potash, they are
all perfectly manageable and yield the best results. It is quite clear
that the excess of chlorate is decomposed in all instances, and the
latent heat of the oxygen evolved, but those coals which are best to
experiment with did not yield results that differed when the quantity of
oxygen mixture was reduced to nearly the limit required for combustion
of the coal. Under these circumstances, therefore, the constant use
of 11 parts of oxygen mixture--a suitable quantity for all coals
exported--would enable operators to obtain similar figures, and make the
test uniform in different hands.
The following is a brief outline of the method of procedure recommended:
Sample the coal until an average portion passes through a sieve having
64 meshes to the square inch. Take about 300 grains (20 grammes) of this
and run through a brass wire gauze having 4,600 meshes to the square
inch, taking care that the whole sample selected is thus treated. One
part of nitrate of potash and 3 parts of chlorate of potash (dry) are
separately ground in a mortar, and repeatedly sifted through another
wire gauze sieve, having 1,000 meshes to the square inch, in order that
the oxygen mixture shall _not_ be ground to an impalpable powder, as
this is very undesirable. It absorbs moisture rapidly, and interferes
with the regularity of the combustion when very fine. 330 grains of the
powder are weighed out (after drying), and intimately incorporated
with 30 grains of coal--better with a spatula than by rubbing in a
mortar--and then introduced into a copper cylinder (3½ inches long by ¾
inch wide, made from a copper tube), and pressed down in small portions
by a test-tube with such firmness as is required by the nature of the
coal, not tapped on the bottom, since the rougher portions of the oxygen
mixture rise to the surface. As the temperature of a room is almost
invariably much higher than the water supply, a little hot water is
added to that placed in the glass cylinder, until the difference of
temperature between the water and the room is about the mark indicated
in the following table: