T. E. (Thomas Edward) Thorpe.

A dictionary of applied chemistry online

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which coal is formed, commencing to decompose
at temperatures above 300°, and yielding
primarily hydrocarbons, and the other type
probably consisting of altered cellulose, orly
decomposing at temperatures of about 700°-800°,
then yielding hydrogen as the chief gaseous
product. Good gas-making coals contain a
higher proportion of the first class of compounds,
and it appears likely that these form the coal
constituents soluble in pyridine.

The changes which take place when coal is
heated in absence of air are exceedingly com-
pKcated in character, and result in the produc-
tion of an immense number of different substances
some gaseous, some liquid, and some solid at
the ordinary temperature. The liquid con-
stituents, wmch condense together in the form of
coal tar, are by far the most numerous, the
different compounds isolated from it already
being numbered by hundreds, and doubtless
large numbers of unknown compounds are
also present {v. Tab). Moreover, both the
nature of these substances and their relative
amounts in the products vary greatly according
to the temperature to which the coal is heated,
and the manner in which the heat is applied,
and our knowledge of the nature of the reactions
occurring during the process is stUl very far in-
deed from complete, so that it is only possible
as yet to discuss these changes in very broad
outline.

In considering these reactions it is most
convenient to deal first only with the carbon and
hydrogen, these being the most important
constituents from a gas-making point of view.
When heated above its decomposition-point in
closed vessels, coal, like, almost all organic sub-
stances, is resolved into two portions: (1) a solid,
non-volatUe portion ; and (2) a volatile portion,
partly liquid and partly gaseous at the ordinary
temperature. It is an invariable rule that the
volatile portion, as a whole, contains a higher
proportion of hydrogen and lower proportion of
carbon than the original coal, whilst the reverse
is the case with the non-volatile portion. When



the temperature employed is the lowest at
which any substantial decomposition occurs,
namely, 300°-400°, a very large proportion of the
volatile products consists of substances which
are Uquid at the ordinary temperature, and only
a small yield of gaseous products is obtained,
the latter consisting chiefly of methane, ethane,
and ethylene, with smaller amounts of their
higher gaseous homologues, and only a low
percentage of hydrogen. The liquid products
consist chiefly of hydrocarbons of the paraffin
and olefine series, with smaller amounts of
members of the acetylene series, or of their
derivatives, i.e. of substances relatively rich in
hydrogen.

The residue obtained at such a temperature
still contains a fair percentage of hydrogen, and
burns in the air with production of- flame, but
not of smoke. Where the chief product desired
is a residue of this kind stai containing hydrogen,
such as the material sold now under the name of
' Coalite,' or where, as in the Scotch shale-oil
industry, a tar containing paraffin and olefine hy-
drocarbons is the product of greatest value, such
low temperature carbonisation may be advan-
tageous. The yield of gas under these conditions
is, however, so small (although of high calorific
and illuminating power) that the employment
of such temperatures is not practicable com-
mercially as a general rule, where the gas
is the primary product required, and for
such purposes higher temperatures must be
employed.

When the closed vessel is heated more
strongly, the coal placed into it only rises in
temperature gradually, and, therefore, at first
undergoes the above low-temperature decom-
position. The further action of heat then
brings about fresh changes, both in the above
hydrogen-containing residue and in the vola-
tUe substances primarily evolved from the
coal. The effect of further heat on the
residue is to drive off more hydrogen, which,
at temperatures of 700°-800° is mostly evolved
as methane and free hydrogen, the latter being
present in larger proportion, the amount of
other gaseous hydrocarbons being very much
smaller than in the gas produced at low tempera-
ture. The residual coke becomes denser and
harder, but at this temperature stiU contains
hydrogen in appreciable quantity, and on
raising the temperature to the highest obtainable
in practice, an additional yield of gas is procured
still consisting of methane and hydrogen, but
as the temperature increases the percentage of
methane falls and that of hydrogen rises. The
higher the temperature to which the residual
coke is finally heated, the greater is therefore
the yield of combustible gas.

The volatile products first evolved from the
coal, consisting chiefly of compounds which are
liquid at the ordinary temperature, also under-
go further change when a higher carbonising
temperature is employed, if they are subjected
to the action of this heat before they can escape,
yielding a large proportion of gases which are
permanent at the ordinary temperature, the
most important of these being hydrogen, meth-
ane, ethylene, and benzene. The latter, althougl i
a liquid at the ordinary temperature, is so
volatile that most of it remains in the gas.

On the other hand, simultaneously jrith the



666



GAS, COAL.



formation of these gases, the petroleum-like
products yield many substances which contain
a higher percentage of carbon and a lower
percentage of hydrogen than is present in the
substances from which they are formed, con-
sisting of hydrocarbons of the aromatic series,
such as benzene, naphthalene, anthracene, and
their derivatives. These are liquids at the ordi-
nary temperature, or, if soUd (like najphthalene
and anthracene), are soluble in the other liquids
and condense together as a fairly fluid tar in
the subsequent cooling of the gas. By heating
the primary volatile products to a moderate
temperature, therefore, the yield of permanent
gas is largely increased, and that of the tar
diminished, and the latter now consists chiefly of
aromatic, instead of fatty, organic derivatives ;
making the tar of especial value for the coal-tar
colour industry.

If the volatile products are subjected for any
length of time to still higher temperatures before
escaping, such as a bright red or white heat,
further decompositions take place, which, on the
whole, are bynomeansfavourable to the produc-
tion of gas. The tar vapours of the aromatic
series undergo further decomposition into still
detaser hydrocarbons, and a gas consisting mainly
of hydrogen ; and many of the new hydrocarbons
formed are solid even at a red heat, and are quite
insoluble in the remaining liquid tar. These
are partly deposited on the heated retort walls
as ' carbon ' or ' scurf,' and are partly carried
away with the hot gas as finely divided particles,
and are carried down during cooling with the
tar, making this much more viscous even when
hot. ^Further,- even the gaseous compoimds
undergo further decomposition; the benzene
undergoing partial conversion into hydrogen and
denser hydbrooarbons, the ethylene yielding some
hydrogen and methane, as weU as tarry matter,
and even the more stable methane undergoes a
partial decomposition into its elements hydrogen
and carbon.

The chemical considerations therefore lead
to the conclusion that while in order to obtain
the maximum yield of gas from coal it is advisable
to subject the latter finally to as high a tempera-
ture as possible, so as to drive ofi the volatile
matter completely, the gases and vapours
produced in the distillation should only be
subjected to a considerably lower temperature
than that to which the coke is eventually heated,
as too high a temperature results in the separa-
tion of carbon, which would otherwise have
remained as gaseous hydrocarbons in the gas ;
thus depreciating both its calorific and illuminat-
ing power. On the other hand, it is equally
evident that these volatile products must not
be subjected to too low a temperature, for in
that case the hydrocarbons, &c., produced in
the early stages of distfllation would not be
sufficiently decomposed, and would be lost from
a gas-making standpoint — ^being condensed in
the tar.

The remaining constituents of the coal
oonoemed in the production of gas are oxygen,
nitrogen, and sulphur. The ash remains in the
coke, and need not bo further considered here.
Oxygen is present in considerable amount in the
coal substance ; and the coal as used also always
contains either moisture in the free state or in
such a loose state of combination that it is given



ofi, at temperatures below 100° — the amount
varying usually from 1 to 4 p.o. The oxygen in
the coal substance is probably evolved in the
early stages to a considerable extent as steam
and volatile compounds of carbon, hydrogen,
and oxygen ; and the latter, on further heating,
are largely decomposed, forming steam, carbon
monoxide, and carbon dioxide. As the tempera-
ture rises, the carbon dioxide tends to combine
more and more with the red-hot carbon present,
undergoing reduction to carbon monoxide, and
the steam tends also to act on the carbon, with
production of water gas ; and the higher the
temperature to which the gases are heated, espe-
cially while in contact with the coke, the greater
is the amount of carbon monoxide formed,
and the higher also the amount of steam con-
verted into water gas. Some of the oxygen is
found in the tar as compounds with carbon and
hydrogen, such as phenol; but the bulk of it
remains as steam, which condenses out on cool-
ing, and, roughly, about one-fourth is found in
the gas as oxides of cairbon.

The eflect on the nitrogen is similar to that
on the hydrogen. At very low temperatures
some ammonia is given ofi, and also substances
containing carbon, hydrogen, and nitrogen,
which condense in the tar ; while a large pro-
portion of the nitrogen remains in the residue.
At higher, but stiU moderate, temperatures,
much more nitrogen is given off from the coal,
and the above volatile products are largely
broken up, with the formation of ammonia and
some free nitrogen ; and at such temperatures
the maximum yield of ammonia is obtained.
At stiU higher temperatures, more nitrogen is
evolved from the coke ; but the ammonia itseU
is then largely decomposed into its elements
nitrogen and hydrogen, and also reacts with the
hot carbon, producing hydrocyanic acid ; and,
in spite of the more complete elimination of the
nitrogen from the coke, the shield of ammonia is
decreased. Only from 14 to 17 p.c. of the nitrogen
in the coal is recovered in the form of ammonia
(». Ammonia). Some of the nitrogen is also
found in the tar, chiefly in the form of nitrogenous
bases, such as pyridine.

The sulphur, too, probably comes off first as
compounds with carbon, and hydrogen ; and
these, when more strongly heated, yield sulphur-
etted hydrogen, which can be removed from the
gas without much difficulty. At still higher
temperatures, however, the decomposition of
these volatile organic sulphur compounds takes
place, with formation of larger quantities of
carbon disulphide; this compound being also
produced, but probably in much smaller amount,
in the gases last driven off from the coke, due to
the sulphur stiU remaining in the latter. High
temperatures, especially if allowed to act fuUy
on the volatile products, therefore tend to
increase considerably the amount of this im-
purity, which can only be removed with difficulty.

Both with regard to sulphur and nitrogen,
therefore, as wdl as in the case of the hydro-
carbons, it is desirable that the volatile products
themselves shall only be heated to a moderate
temperature if a maximum yield of the valuable
product (ammonia) and a minimum yield of the
deleterious impurity (carbon disulphide) is to
be obtained. In the case of the oxygen com-
pounds alone does there appear to be any



GAS, COAL.



667



advantage in subjecting these products to a high
temperature, owing to the larger proportion of
carbon monoxide and smaller proportion of carbon
dioxide produced, as well as to the larger amount
of steam converted into combustible gas. On
the whole, the advantages gained in the latter
respect are more than counterbalanced by the
deleterious action of such high temperatures so
far as the hydrocarbons and the nitrogen and
sulphur compounds are concerned.

In addition to the changes brought about by
thermal decomposition, which are the most
important, formation of other compounds also
occurs by polymerisation or synthesis from two
or more of the products of thermal decompo-
sition, such as the formation of benzene and
stUbene by condensation of acetylene, and a
large number of the tar constituents are doubt-
less formed in this manner. So far as the gaseous
products are concerned, the formation of carbon
monoxide by the action of the dioxide on carbon,
and the synthesis of water gas by the interaction
of steam and carbon, have been referred to. In
addition, there is some evidence that methane
may also be formed to some extent in the
carbonising process by synthesis from hydrogen
and carbon monoxide, in accordance with the
reversible reaction CO-fSHa-^CH^+HaO. This
reaction occurs almost quantitatively when a
mixture of the gases is passed over metallic
nickel at 250° (Sabatier and Senderens, Compt.
rend. 134, 514), and although, as the temperature
rises, the reverse reaction takes place in increas-
ing degree, the evidence available points to the
conclusion that appreciable amounts of methane
are formed in this manner under some conditions
during carbonisation.

In the carbonisation of coal in the most
usual gas-works practice, the coal, as above
described, is placed in a, long narrow retort set
either horizontally or at an angle of about 32° to
the horizontal, leaving a considerable amount
of free space above the coal through which the
evolved gas must pass to reach the exit pipe
at one or both ends of the retort. The carbonisa-
tion commences on the outside of the mass —
namely, at the bottom and sides — where it is in
contact witli the heated retort wall, and also
at the top, where it is affected by the heat
radiated through the free space from the hot
crown of the retort. The coal on the outside
first becomes pasty and agglomerates, and then
undergoes the low-temperature decomposition
already described, the resultant vapours and
rich gas escaping fairly readily into the free
space above the coal. Here they are subjected
to the further action of heat in two ways —
namely, by contact with the heated walls of the
retorts, and also by the action of the radiant
heat rays which are traversing the free space.
The net result of their combined action is to
raise the temperature of the vapours passing
through the free space, and to bring about
much change ; and the higher the temperature
to which the retort walls are heated, the greater
is the amount of further decomposition that
they undergo.

The further action of heat on the pasty mass
on the outside of the charge converts this
eventually into coke ; heat passing simultane-
ously further into the charge and converting the



layer below into the pasty condition, which is
assisted by the condensation on these lower
layers of some of the tarry matters formed by
the decomposition of the hotter layers above,
and the extent to which such condensation
occurs doubtless has much influence on the
physical properties of the resulting coke. This
sequence of changes continues until the heat has
penetrated to the centre of the mass, and effected
its complete carbonisation. After the first layer
of hot coke has been formed, the gases produced
from the interior of the charge, in order to
escape from the retort, must not only undergo
the ordeal of heat in passing through the free
space, but also pass through the hot layer of
coke on the outside of the charge ; and the
temperature and area of this coke mass con-
stantly increases as time goes on. Further, as
carbonisation proceeds, the rate at which the
gas is evolved decreases, and in consequence, its
speed through the free space becomes less, and
it is exposed to heat there for a greater length
of time. From all these causes, the quality of
the gas falls off after the first hour, and especially
when the carbonisation is approaching com-
pletion. The percentages of methane, ethylene,
benzene, sulphuretted hydrogen, and carbon
dioxide steadily diminish as time progresses, and
that of hydrogen steadily increases ; while that
of carbon monoxide varies less considerably, as
shown by the following table, giving the analysis
of the gas evolved at different stages from »
retort charged with Derbyshire coal, the tempera-
ture of carbonisation being about 950° : —





Hours after commencement




Ihour


1! hrs.


aihrs.


3! hrs.


5 hrs.


Sulphuretted






hydrogen


3-8


3-1


2-8


2-1


1-2


Carbon dioxide .


3


2-8


2-6


2-3


1-7


Ethylene and












benzene


8-6


5-2


3-6


2-4


0-0


Oxygen .


0-0


0-0


0-0


0-0


trace


Carbon monoxide


4-4


5-0


4-9


4-5


3-8


Methane .


49-7


42-0


39-4


37-5


26-3


Hydrogen


29-8


37-5


42-2


46-2


60-8


Nitrogen ( by diff.)


0-7


4-4


4-5


50


6-2



Before the introduction of gaseous firing, the
retort temperatures obtained in practice did not
usually exceed about 900°C., or 1650°F. Under
these conditions, the volatile products were
not materially overheated, and gas of high
illuminating power was produced ; while the
tar simultaneously formed was fairly fluid, and
oiJy contained moderate amounts of the
objectionable ' free carbon.' The quantity of
naphthalene formed was also not excessive, and
that of ' light oils ' simultaneously produced
was, in most cases, sufficient to wash the naph-
thalene out of the gas during condensation to
a sufficient extent to prevent its subsequent
deposition in the solid state in the mains and
services. On the other hand, under these
conditions, a relatively low yield of gas per ton
is obtained, as an appreciable quantity of
volatile matter is left in the coke.

When, with the aid of gaseous firing, higher
carbonisation temperatures were employed,



068



GAS, COAL.



matters were considerably modified, as the
volatile matter of the coal was more completely
evolved with the production of a greater volume
of gas. But, as we have seen, it follows inevit-
ably that, in a horizontal or inclined retort
having a large free space above the coal, the
volatile products must also be more strongly
hefited by an increase in the retort temperature ;
and, as a result, under these conditions, the
gases and vapours undergo a more far-reaching
decomposition than before, and a reduction in
the quality of the gas is brought about. Some
of the hydrocarbons formerly present in the gas
are now deposited in solid or liquid form, which
either remain in the ietort as carbon or are
condensed with the tar, and are lost from a
gas-making point of view. Simultaneously,
other disadvantageous changes occur, inasmuch
as the tar produced is thicker and contains
much more ' free carbon,' thereby increasing the
trouble from stopped ascension pipes and pitched
hydraulic mains, and greatly increasing the
practical difficulties of retort-house working.
Further, the action of these high temperatures
on the volatile products increases the produc-
tion of naphthalene and decreases that of light
oils, and renders the cooled gas much more
liable to deposit soUd naphthalene in the mains
and services both on the works and in the district,
causing serious trouble both to suppUer and
consumer.

In spite of these last-named drawbacks, the
employment of higher temperatures has, on the
whole, proved favourable ; the increased yield
obtained from the more complete elimination of
the volatile matter of the coal having more than
counterb^anced these disadvantages. Never-
theless, a process of carbonisation is very desir-
able, in which the coal itseU can be heated to a
high temperature without simultaneously causing
too great heating of the volatile products ; and
it is largely with a view to this end that modifi-
cations of the method of working have been
introduced during the past few years.

From what has been said, it is clear that one
of the chief causes of the overheating of the gases
and vapours is the existence of a large free space
above the coal in the retort. With the hori-
zontal retort, so long as the coke had to be
extracted by a rake, worked either by hand or
mechanically, a considerable amount of free
space was necessary in order to afford room for
the introduction of the rake-head above the
coke ; but the invention of mechanically
propelled pushers, which discharge the coke by
pushing from one end of the retort, has done
away with the necessity of the free space for
the removal of the coke, and has made it possible
to put in a much larger charge of coal without
inore'asing the difficulties of discharging the latter.
In recent years, therefore, by the introduc-
tion of such increased charges, the amount of
free space has, in many works, been largely
reduced in horizontal retorts, so that the gases
and vapours are exposed to a smaller amount
of retort surface, and, still more important,
owing to the volume of the free space being
smaller, they pass through it more quickly, and
are exposed to the action of the heat for a shorter
time. As the practical result, it is found that
the coke produced is larger, a thinner tar
containing less free carbon is produced, owing



to the lessened overheating, less carbon disul-
phide is produced, less trouble from naphthalene
ensues, and retort-house working is considerably
facilitated. Whether the total heat value of the
gas obtained from a ton of coal — i.e. the multiple
of gas per ton x calorific power — is greater than
can be obtained with the smaller charges, is a
matter on which different opinions prevail at



On the other hand, owing to the increased
thickness of the charge, a longer period must be
allowed for its carbonisation if the central core
is to be completely carbonised ; and beyond a
certain weight of charge, varying apparently
with different classes of coal, the gas production .
per retort per 24 hours falls off, or can only be
maintained by increasing the carbonisation
temperature above that employed with the
smaller charges. Such higher temperatures also
increase the heat to which the gases are exposed
in the free space ; but experience seems to
show that the effect of such increase is small
when the volume of free space is low, or, in other
words, that in bringing about decomposition of
the volatile products the time during which the
gas is exposed to heat is much more important
than the temperature of the retort walls.

Carbonisation in vertical retorts. But, while
the amount of free space can be in this manner
greatly reduced, it is scarcely practicable to
eliminate it altogether in horizontal retorts. If,
however, the retort is placed in a vertical instead
of a horizontal position, this is readily effected ;
as, in the nature of things, the coal when charged
in from the top must completely fill the retort.
Although a small amount of free space may be
left at the top, matters can be so arranged that
this is not too large and that the retort walls at
this point are not heated to any great extent.
Hence latterly carbonisation in vertical retorts
has been much investigated, and is now practised
in a large number of works.

Intermittent vertical retorts. In the Dessau
vertical retort system, patented by Bueb (£ng.
Pat. 1393, 1904), the previous practice is
adhered to, in so far that the coal is charged
into the retort from an overhead hopper all at
once, and allowed to remain there until carbonisa-
tion is complete, when the coke is discharged by
gravity and the retort refilled with fresh coal.
The retorts are either 4 or S metres in length,
having an oblong cross section with rounded
corners and are tapered, increasing in size from
top to bottom to facilitate the discharge of the
coke. Until recently 10 or 12 retorts were set
in rows of two in a setting, each setting being
heated by gas from a separate deep producer
capable of being filled much above the point at
which the producer gases are drawn off, so that
it may run for 24 hours without recharging.
Fig. 10 gives a vertical section of such a setting
and producer, showing clearly the general



Online LibraryT. E. (Thomas Edward) ThorpeA dictionary of applied chemistry → online text (page 156 of 183)