I. _Natural Gas._--From time immemorial it has been known that in some
parts of the Caucasus and of China large quantities of gases issue from
the soil, sometimes under water, which can be lighted and burn with a
luminous flame. The "eternal fires" of Baku belong to this class. In
coal-mines frequently similar streams of gas issue from the coal; these
are called "blowers," and when they are of somewhat regular occurrence
are sometimes conducted away in pipes and used for underground lighting.
As a regular source of heating power, however, natural gas is employed
only in some parts of the United States, especially in Pennsylvania,
Kansas, Ohio and West Virginia, where it always occurs in the
neighbourhood of coal and petroleum fields. The first public mention of
it was made in 1775, but it was not till 1821 that it was turned to use
at Fredonia, N.Y. In Pennsylvania natural gas was discovered in 1859,
but at first very little use was made of it. Its industrial employment
dates only from 1874, and became of great importance about ten years
later. Nobody ever doubted that the gas found in these localities was an
accumulation of many ages and that, being tapped by thousands of
bore-holes, it must rapidly come to an end. This assumption was
strengthened by the fact that the "gas-wells," which at first gave out
the gas at a pressure of 700 or 800, sometimes even of 1400 lb. per sq.
in., gradually showed a more and more diminishing pressure and many of
them ceased to work altogether. About the year 1890 the belief was
fairly general that the stock of natural gas would soon be entirely
exhausted. Indeed, the value of the annual production of natural gas in
the United States, computed as its equivalent of coal, was then
estimated at twenty-one million dollars, in 1895 at twelve millions, in
1899 at eleven and a half millions. But the output rose again to a value
of twenty-seven millions in 1901, and to fifty million dollars in 1907.
Mostly the gas, derived from upwards of 10,000 gas-wells, is now
artificially compressed to a pressure of 300 or 400 lb. per sq. in. by
means of steam-power or gas motors, fed by the gas itself, and is
conveyed over great distances in iron pipes, from 9 or 10 to 36 in. in
diameter. In 1904 nearly 30,000 m. of pipe lines were in operation. In
1907 the quantity of natural gas consumed in the United States (nearly
half of which was in Pennsylvania) was 400,000 million cub. ft., or
nearly 3 cub. m. Canada (Ontario) also produces some natural gas,
reaching a maximum of about $746,000 in 1907.
The principal constituent of natural gas is always methane, CH4, of
which it contains from 68.4 to 94.0% by volume. Those gases which
contain less methane contain all the more hydrogen, viz. 2.9 to 29.8%.
There is also some ethylene, ethane and carbon monoxide, rarely
exceeding 2 or 3%. The quantity of incombustible gases--oxygen, carbon
dioxide, nitrogen--ranges from mere traces to about 5%. The density is
from 0.45 to 0.55. The heating power of 1000 cub. ft. of natural gas is
equal to from 80 to 120 lb., on the average 100 lb., of good coal, but
it is really worth much more than this proportion would indicate, as it
burns completely, without smoke or ashes, and without requiring any
manual labour. It is employed for all domestic and for most industrial
purposes.
The origin of natural gas is not properly understood, even now. The most
natural assumption is, of course, that its formation is connected with
that of the petroleum always found in the same neighbourhood, the latter
principally consisting of the higher-boiling aliphatic hydrocarbons of
the methane series. But whence do they both come? Some bring them into
connexion with the formation of coal, others with the decomposition of
animal remains, others with that of _diatomaceae_, &c., and even an
inorganic origin of both petroleum and natural gas has been assumed by
chemists of the rank of D.I. Mendeleeff and H. Moissan.
II. _Gases obtained as By-products._--There are two important cases in
which gaseous by-products are utilized as fuel; both are intimately
connected with the manufacture of iron, but in a very different way, and
the gases are of very different composition.
(a) _Blast-furnace Gases._--The gases issuing from the mouths of
blast-furnaces (see IRON AND STEEL) were first utilized in 1837 by Faber
du Faur, at Wasseralfingen. Their use became more extensive after 1860,
and practically universal after 1870. The volume of gas given off per
ton of iron made is about 158,000 cub. ft. Its percentage composition by
volume is:
Carbon monoxide 21.6 to 29.0, mostly about 26%
Hydrogen 1.8 " 6.3, " " 3%
Methane 0.1 " 0.8, " " 0.5%
Carbon dioxide 6 " 12, " " 9.5%
Nitrogen 51 " 60, " " 56 %
Steam 5 " 12, " " 5 %
-----
100 %
There is always a large amount of mechanically suspended flue-dust in
this gas. It is practically equal to a poor producer-gas (see below),
and is everywhere used, first for heating the blast in Cowper stoves or
similar apparatus, and secondly for raising all the steam required for
the operation of the blast-furnace, that is, for driving the
blowing-engines, hoisting the materials, &c. Where the iron ore is
roasted previously to being fed into the furnace, this can also be done
by this gas, but in some cases the waste in using it is so great that
there is not enough left for the last purpose. The calorific power of
this gas per cubic foot is from 80 to 120 B.Th.U.
Since about 1900 a great advance has been made in this field. Instead of
burning the blast-furnace gas under steam boilers and employing the
steam for producing mechanical energy, the gas is directly burned in
gas-motors on the explosion principle. Thus upwards of three times the
mechanical energy is obtained in comparison with the indirect way
through the steam boiler. After all the power required for the
operations of the blast-furnace has been supplied, there is a surplus of
from 10 to 20 h.p. for each ton of pig-iron made, which may be applied
to any other purpose.
(b) _Coke-oven Gases._--Where the coking of coal is performed in the old
beehive ovens or similar apparatus the gas issuing at the mouth of the
ovens is lost. The attempts at utilizing the gases in such cases have
not been very successful. It is quite different where coke is
manufactured in the same way as illuminating gas, viz. by the
destructive distillation of coal in closed apparatus (retorts), heated
from the outside. This industry, which is described in detail in G.
Lunge's _Coal-Tar and Ammonia_ (4th ed., 1909), originated in France,
but has spread far more in Germany, where more than half of the coke
produced is made by it; in the United Kingdom and the United States its
progress has been much slower, but there also it has long been
recognized as the only proper method. The output of coke is increased by
about 15% in comparison with the beehive ovens, as the heat required for
the process of distillation is not produced by burning part of the coal
itself (as in the beehive ovens), but by burning part of the gas. The
quality of the coke for iron-making is quite as good as that of beehive
coke, although it differs from it in appearance. Moreover, the gases can
be made to yield their ammonia, their tar, and even their benzene
vapours, the value of which products sometimes exceeds that of the coke
itself. And after all this there is still an excess of gas available for
any other purpose.
As the principle of distilling the coal is just the same, whether the
object is the manufacture of coal gas proper or of coke as the main
product, although there is much difference in the details of the
manufacture, it follows that the quality of the gas is very similar in
both cases, so far as its heating value is concerned. Of course this
heating value is less where the benzene has been extracted from
coke-oven gas, since this compound is the richest heat-producer in the
gas. This is, however, of minor importance in the present case, as there
is only about 1% benzene in these gases.
The composition of coke-oven gases, after the extraction of the ammonia
and tar, is about 53% hydrogen, 36% methane, 6% carbon monoxide, 2%
ethylene and benzene, 0.5% sulphuretted hydrogen, 1.5% carbon dioxide,
1% nitrogen.
III. _Coal Gas (Illuminating Gas)._--Although ordinary coal gas is
primarily manufactured for illuminating purposes, it is also extensively
used for cooking, frequently also for heating domestic rooms, baths,
&c., and to some extent also for industrial operations on a small scale,
where cleanliness and exact regulation of the work are of particular
importance. In chemical laboratories it is preferred to every other kind
of fuel wherever it is available. The manufacture of coal gas being
described elsewhere in this work (see GAS, S _Manufacture_), we need
here only point out that it is obtained by heating bituminous coal in
fireclay retorts and purifying the products of this destructive
distillation by cooling, washing and other operations. The residual gas,
the ordinary composition of which is given in the table below, amounts
to about 10,000 cub. ft. for a ton of coal, and represents about 21% of
its original heating value, 56.5% being left in the coke, 5.5% in the
tar and 17% being lost. As we must deduct from the coke that quantity
which is required for the heating of the retorts, and which, even when
good gas producers are employed, amounts to 12% of the weight of the
coal, or 10% of its heat value, the total loss of heat rises to 27%.
Taking, further, into account the cost of labour, the wear and tear, and
the capital interest on the plant, coal gas must always be an expensive
fuel in comparison with coal itself, and cannot be thought of as a
general substitute for the latter. But in many cases the greater expense
of the coal gas is more than compensated by its easy distribution, the
facility and cleanliness of its application, the general freedom from
the mechanical loss, unavoidable in the case of coal fires, the
prevention of black smoke and so forth. The following table shows the
average composition of coal gas by volume and weight, together with the
heat developed by its single constituents, the latter being expressed in
kilogram-calories per cub. metre (0.252 kilogram-calories = 1 British
heat unit; 1 cub. metre = 35.3 cub. ft.; therefore 0.1123 calories per
cub. metre = 1 British heat unit per cub. foot).
+----------------------+----------+----------+------------+--------------+------------+
| | | | Heat-value | Heat-value | Heat-value |
| Constituents. | Volume | Weight | per Cubic | per Quantity | per cent. |
| | per cent.| per cent.| Metre | contained in | of Total. |
| | | | Calories. | 1 Cub. Met. | |
+----------------------+----------+----------+-----------+---------------+------------+
| Hydrogen, H2 | 47 | 7.4 | 2,582 | 1213 | 22.8 |
| Methane, CH4 | 34 | 42.8 | 8,524 | 2898 | 54.5 |
| Carbon monoxide, CO | 9 | 19.9 | 3,043 | 273 | 5.1 |
| Benzene vapour, C6H6 | 1.2 | 7.4 | 33,815 | 405 | 7.7 |
| Ethylene, C2H4 | 3.8 | 8.4 | 13,960 | 530 | 9.9 |
| Carbon dioxide, CO2 | 2.5 | 8.6 | .. | .. | .. |
| Nitrogen, N2 | 2.5 | 5.5 | .. | .. | .. |
| +----------+----------+------------+--------------+------------+
| Total | 100.0 | 100.0 | .. | 5319 | 100.0 |
+----------------------+----------+----------+------------+--------------+------------+
One cubic metre of such gas weighs 568 grammes. _Rich gas_, or gas made
by the destructive distillation of certain bituminous schists, of oil,
&c., contains much more of the heavy hydrocarbons, and its heat-value is
therefore much higher than the above. The carburetted water gas, very
generally made in America, and sometimes employed in England for mixing
with coal gas, is of varying composition; its heat-value is generally
rather less than that of coal gas (see below).
IV. _Combustible Gases produced by the Partial Combustion of Coal,
&c._--These form by far the most important kind of gaseous fuel. When
coal is submitted to destructive distillation to produce the
illuminating gas described in the preceding paragraph, only a
comparatively small proportion of the heating value of the coal (say, a
sixth or at most a fifth part) is obtained in the shape of gaseous fuel,
by far the greater proportion remaining behind in the shape of coke.
An entirely different class of gaseous fuels comprises those produced by
the incomplete combustion of the total carbon contained in the raw
material, where the result is a mixture of gases which, being capable of
combining with more oxygen, can be burnt and employed for heating
purposes. Apart from some descriptions of waste gases belonging to this
class (of which the most notable are those from blast-furnaces), we must
distinguish two ways of producing such gaseous fuels entirely different
in principle, though sometimes combined in one operation. The incomplete
combustion of carbon may be brought about by means of atmospheric
oxygen, by means of water, or by a simultaneous combination of these two
actions. In the first case the chemical reaction is