Chapter XXI: Act 1874: ); and in valuing land whether for rates or taxes the value of (8)

Coal, the raw material from which the gas is produced by a process of
destructive distillation, varies very widely in composition (see
COAL), and it is only the class of coals rich in hydrogen, known as
bituminous coal, that can with advantage be utilized in gas
manufacture. Coals of this character are obtained in England from the
Newcastle and Durham field, South Yorkshire, Derbyshire and Barnsley
districts, and an idea of their ultimate composition may be derived
from the following table:--

+--------------------------+-------+------+--------+------+-------+------+------+
| |Carbon.|Hydro-|Sulphur.|Nitro-|Oxygen.| Ash. |Moist-|
| | | gen. | | gen. | | | ure. |
+--------------------------+-------+------+--------+------+-------+------+------+
| Newcastle gas coal | 82.16 | 4.83 | 1.00 | 1.23 | 6.82 | 3.20 | 0.76 |
| Durham gas coal | 84.34 | 5.30 | 0.73 | 1.73 | 4.29 | 2.42 | 1.14 |
| South Yorkshire silkstone| 80.46 | 5.09 | 1.66 | 1.67 | 6.79 | 3.30 | 1.03 |
| Derbyshire silkstone | 76.96 | 5.04 | 2.39 | 1.77 | 6.92 | 3.28 | 3.64 |
| Barnsley gas coal | 75.64 | 4.94 | 2.84 | 1.65 | 7.25 | 4.28 | 3.40 |
+--------------------------+-------+------+--------+------+-------+------+------+

Our knowledge of the composition of coal is limited to the total
amount of carbon, hydrogen, nitrogen, oxygen and foreign materials
which it contains; and at present we know practically but little of
the way in which these bodies are combined. This being so, the
ordinary analysis of a coal affords but little indication of its value
for gas-making purposes, which can only be really satisfactorily
arrived at by extended use on a practical scale. Bituminous coal,
however, may be looked upon as containing carbon and also simple
hydrocarbons, such as some of the higher members of the paraffin
series, and likewise organic bodies containing carbon, hydrogen,
nitrogen, oxygen and sulphur.

Destructive distillation of coal.

On submitting a complex substance of this character to destructive
distillation, it will be found that the yield and quality of the
products will vary very considerably with the temperature existing in
the retorts, with the size of the charge of coal used, with its
distribution in the retort, with the length of time the distillation
has been going on, and with an infinity of other factors of a more or
less complex nature. If bituminous coal is distilled at a low
temperature, the tar is found to contain considerable quantities of
light paraffin oils; and there is no doubt that paraffin hydrocarbons
are present in the original coal. These paraffins, under the influence
of heat, split up into simpler members of the same series and into
olefines; and if we imagine the action in its simplest form, we should
have the gases, as they were evolved, consisting of (say) ethane and
ethylene. These have now to pass down the heated retort on their way
to the ascension pipe, and the contact with the heated sides of the
retort, and the baking from the radiant heat in the retort, set up an
infinity of changes. Ethane, when heated to this degree, splits up
into ethylene and hydrogen, whilst ethylene decomposes to methane and
acetylene, and the acetylene at once polymerizes to benzene,
styrolene, retene, &c. A portion also condenses, and at the same time
loses some hydrogen, becoming naphthalene; and the compounds so formed
by interactions amongst themselves build up the remainder of the
hydrocarbons present in the coal tar, whilst the organic substances
containing oxygen in the coal break down, and cause the formation of
the phenols in the tar.

There is very little doubt that the general course of the
decompositions follows these lines; but any such simple explanation of
the actions taking place is rendered impossible by the fact that,
instead of the breaking-down of the hydrocarbons being completed in
the coal, and only secondary reactions taking place in the retort, in
practice the hydrocarbons to a great extent leave the coal as the
vapours of condensible hydrocarbons, and the breaking down of these to
such simple gaseous compounds as ethylene is proceeding in the retort
at the same time as the breaking up of the ethylene already formed
into acetylene and methane, and the polymerization of the former into
higher compounds. Starting with a solid hydrocarbon of definite
composition, it would be theoretically possible to decompose it
entirely into carbon, hydrogen, ethylene and methane, and, by rapidly
removing these from the heating zone before any secondary actions took
place, to prevent formation of tar. But any such ideal is hopeless in
practice, as the coal is not a definite compound, and it is impossible
to subject it to a fixed temperature.

Effect of temperature in the retort.

If the retorts are at a temperature of 1000 deg. C. when the charge of
coal is put in, the temperature of the distillation will vary from
about 800 deg. C. close to the walls, to about 400 deg. C. in the
centre of the coal; and in the same way, in the space above the coal,
the products which come in contact with the sides of the retort are
heated to 1000 deg. C., whilst the gas near the coal is probably
heated to only 600 deg. C. Moreover, the gases and vapours in the
retort are subjected to a period of heating which varies widely with
the distance from the mouth of the retort of the coal that is
undergoing carbonization. The gas developed by the coal near the mouth
of the retort is quickly washed out into the ascension pipe by the
push of the gas behind, and the period for which it has been exposed
to the radiant heat from the walls of the retort is practically nil;
whilst the gas evolved in the portion of the retort farthest from the
mouthpiece has only its own rate of evolution to drive it forward, and
has to traverse the longest run possible in the retort, exposed during
the whole of that period to radiant heat and to contact with the
highly heated surface of the retort itself. Hence we find that the tar
is formed of two distinct sets of products, the first due to
incomplete decomposition and the second to secondary reactions due to
the products of the decomposition being kept too long in the zone of
heat.

Of the first class, the light paraffin oils and pitch may be taken as
examples; whilst benzene, naphthalene and retort carbon represent the
second. The formation of the second class of bodies is a great loss to
the gas manufacturer, as, with the exception of the trace of benzene
carried with the gas as vapour, these products are not only useless in
the gas, but one of them, naphthalene, is a serious trouble, because
any trace carried forward by the gas condenses with sudden changes of
temperature, and causes obstructions in the service pipes, whilst
their presence in the tar means the loss of a very large proportion of
the illuminating constituents of the gas. Moreover, these secondary
products cannot be successfully reduced, by further heating, to
simpler hydrocarbons of any high illuminating value, and such bodies
as naphthalene and anthracene have so great a stability that, when
once formed, they resist any efforts again to decompose them by heat,
short of the temperature which breaks them up into methane, carbon and
hydrogen.

The ammonia is derived from the nitrogen present in the coal combining
with hydrogen during destructive distillation, the nitrogen becoming
distributed amongst all three classes of products. The following table
will give an approximate idea of the proportions which go to each:--

Per cent.

Nitrogen as ammonia 14.50
" as cyanogen 1.56
" free in gas and combined in tar 35.26
" remaining in coke 48.68
------
100.00

The effect produced by alteration in the temperature of the retort
upon the composition of both gas and tar is very marked. As the
temperature is raised, the yield of gas from a given weight of coal
increases; but with the increase of volume there is a marked decrease
in the illuminating value of the gas evolved. Lewis T. Wright found,
in a series of experiments, that, when four portions of the same coal
were distilled at temperatures ranging from a dull red heat to the
highest temperature attainable in an iron retort, he obtained the
following results as to yield and illuminating power:--

+-----------------+-------------+------------+---------+
| | Cubic ft. of|Illuminating| Total |
| Temperature. | Gas per ton.| Power, | Candles |
| | | Candles. | per ton.|
+-----------------+-------------+------------+---------+
| 1. Dull red | 8,250 | 20.5 | 33.950 |
| 2. Hotter | 9,693 | 17.8 | 34.510 |
| 3. " | 10,821 | 16.7 | 36.140 |
| 4. Bright orange| 12,006 | 15.6 | 37.460 |
+-----------------+-------------+------------+---------+

_Composition of the Gas._

+-----------------+-----------+-----------+-----------+
| | 1. | 2. | 4. |
| | Per cent. | Per cent. | Per cent. |
+-----------------+-----------+-----------+-----------+
| Hydrogen | 38.09 | 43.77 | 48.02 |
| Marsh gas | 42.72 | 34.50 | 30.70 |
| Olefines | 7.55 | 5.83 | 4.51 |
| Carbon monoxide | 8.72 | 12.50 | 13.96 |
| Nitrogen | 2.92 | 3.40 | 2.81 |
| +-----------+-----------+-----------+
| | 100.00 | 100.00 | 100.00 |
+-----------------+-----------+-----------+-----------+

The gas analysis of No. 3 was lost, but the illuminating power shows
that it was intermediate in composition between Nos. 2 and 4. From
this it will be seen that, with the increase of temperature, the
hydrocarbons--the olefines and marsh gas series--gradually break up,
depositing carbon in the crown of the retort, and liberating hydrogen,
the percentage of which steadily increases with the rise of
temperature.

The tar formed is affected to an even greater extent than the gas by
alterations in the temperature at which the destructive distillation
takes place. The lower the temperature, the smaller will be the volume
of gas produced, and the lighter the specific gravity of the tar,
whilst with increase of temperature, the volume of gas rapidly rises,
and so does the specific gravity of the tar. Working with a caking
coal Wright obtained the following results:--

+--------------+------------------+
| Yield of Gas | Specific Gravity |
| per ton, | of Tar. |
| Cub. ft. | |
+--------------+------------------+
| 6,600 | 1.086 |
| 7,200 | 1.120 |
| 8,900 | 1.140 |
| 10,162 | 1.154 |
| 11,700 | 1.206 |
+--------------+------------------+

Analysis of the tar showed that the increase of the specific gravity
was due to the increase in the quantity of pitch, which rose from
28.89 to 64.08% in the residuals; whilst the ammonia, naphtha and
light oils steadily fell in quantity, the creosote and anthracene oils
doing the same, but to a smaller extent. Naphthalene also begins to
show in quantity in the tar as soon as the yield of gas reaches 10,000
cub. ft. per ton of coal carbonized.

In spite of these variations, however, the products in their main
characteristics will remain the same. They may be divided into--(a)
Solids, such as the coke and retort carbon; (b) liquids, consisting of
the tar and ammoniacal liquor; and (c) gases, consisting of the
unpurified coal gas. The proportions in which the products are
approximately obtained from a ton of gas coal have been given as
follows:--

10,000 cub. ft. of gas = 380 lb. = 17.0 per cent.
10 gallons of tar = 115 " = 5.1 "
Gas liquor[1] = 177 " = 7.9 "
Coke = 1568 " = 70.0 "
---- -----
2240 100.0

Solid products.

The chief solid residue, coke, is not absolutely pure carbon, as it
contains the mineral non-volatile constituents which remain behind as
ash when the original coal is burnt, and which, to a great extent,
existed in the sap that filled the cells of the plant from which the
coal was formed. The retort carbon formed as a dense deposit on the
crown of the retort by the action of the high temperature on the
hydrocarbons is, however, carbon in a very pure form, and, on account
of its density, is largely used for electrical purposes.

Liquid products.

The liquid products of the destructive distillation of coal are tar
and ammoniacal liquor. Tar derived from ordinary bituminous coal is a
black, somewhat viscid liquid, varying in specific gravity from 1.1 to
1.2. The ultimate composition of tar made in the London Gas Works is
approximately as follows:--

Carbon 77.53
Hydrogen 6.33
Nitrogen 1.03
Sulphur 0.61
Oxygen 14.50
------
100.00

These elements in tar are built up into an enormous number of
compounds (see COAL TAR), and its value as a by-product may be
gathered from the fact that on fractional distillation it yields--(1)
benzene and its homologues, from which aniline, the source of most of
the coal-tar colours, can be derived; (2) carbolic acid, from which
picric acid, used as a dye, a powerful explosive, and to give the
bitter flavour to some kinds of beer, is made, also many most valuable
disinfectants; (3) naphthalene, used for disinfecting, and also as the
"Albo-carbon" employed in an enriching burner for gas; (4) pitch,
extensively used in path-making, from which such bodies as anthracene
and saccharin can be extracted.

The second liquid product of the destructive distillation of coal is
the ammoniacal or gas liquor, which consists of water containing
ammonia salts in solution, partly condensed from the hot gas, and
partly added to wash the gas in the scrubbers. It contains, as its
principal constituents, ammonia, partly combined with carbonic acid
and sulphuretted hydrogen to form compounds which are decomposed on
boiling, with evolution of ammonia gas, and partly combined with
stronger acids to form compounds which require to be acted upon by a
strong alkali before the ammonia contained in them can be liberated.
The ammonia in the first class of compounds is technically spoken of
as "free"; that present in the latter as "fixed." The following
analysis by L.T. Wright will give an idea of the relative quantities
in which these compounds exist in the liquor:--

Grammes per litre.

/ Ammonium sulphide 3.03
Free < Ammonium carbonate 39.16
\ Ammonium chloride 14.23
/ Ammonium thiocyanate 1.80
Fixed < Ammonium sulphate 0.19
| Ammonium thiosulphate 2.80
\ Ammonium ferrocyanide 0.41

From a scientific point of view, the term "free" is absolutely
incorrect, and in using it the fact must be clearly borne in mind that
in this case it merely stands for ammonia, which can be liberated on
simply boiling the liquor.

Gaseous products.

The gas which is obtained by the destructive distillation of coal, and
which we employ as our chief illuminant, is not a definite compound,
but a mechanical mixture of several gases, some of which are reduced
to the lowest limit, in order to develop as fully as possible the
light-giving properties of the most important constituents of the gas.
The following analysis gives a fair idea of the composition of an
average sample of gas made from coal, purified but without
enrichment:--

Hydrogen 52.22
Unsaturated hydrocarbons 3.47
Saturated hydrocarbons 34.76
Carbon monoxide 4.23
Carbon dioxide 0.60
Nitrogen 4.23
Oxygen 0.49
------
100.00

These constituents may be divided into--(a) light-yielding
hydrocarbons, (b) combustible diluents and (c) impurities. The
hydrocarbons, upon which the luminosity of the flame entirely depends,
are divided in the analysis into two groups, saturated and
unsaturated, according to their behaviour with a solution of bromine
in potassium bromide, which has the power of absorbing those termed
"unsaturated," but does not affect in diffused daylight the gaseous
members of the "saturated" series of hydrocarbons. They may be
separated in a similar way by concentrated sulphuric acid, which has
the same absorbent effect on the one class, and not on the other. The
chief unsaturated hydrocarbons present in coal gas are: ethylene,
C2H4, butylene, C4H8, acetylene, C2H2, benzene, C6H6, and naphthalene,
C10H8, and the saturated hydrocarbons consist chiefly of methane, CH4,
and ethane, C2H6.

The light-giving power of coal gas is undoubtedly entirely due to the
hydrocarbons. The idea held up to about 1890 was that the illuminating
value depended upon the amount of ethylene present. This, however, is
manifestly incorrect, as, if it were true, 4% of ethylene mixed with
96% of a combustible diluent such as hydrogen should give 16- to
17-candle gas, whereas a mixture of 10% of ethylene and 90% of
hydrogen is devoid of luminosity. In 1876 M.P.E. Berthelot came to
the conclusion that the illuminating value of the Paris coal gas was
almost entirely due to benzene vapour. But here again another mistaken
idea arose, owing to a faulty method of estimating the benzene, and
there is no doubt that methane is one of the most important of the
hydrocarbons present, when the gas is burnt in such a way as to evolve
from it the proper illuminating power, whilst the benzene vapour,
small as the quantity is, comes next in importance and the ethylene
last. It is the combined action of the hydrocarbons which gives the
effect, not any one of them acting alone.

The series of operations connected with the manufacture and
distribution of coal gas embraces the processes of distillation,
condensation, exhaustion, wet purification by washing and scrubbing,
dry purification, measuring, storing and distribution to the mains
whence the consumer's supply is drawn.

Site of gas works.

The choice of a site for a gas works is necessarily governed by local
circumstances; but it is a necessity that there should be a ready
means of transport available, and for this reason the works should be
built upon the banks of a navigable river or canal, and should have a
convenient railway siding. By this means coal may be delivered direct
to the store or retort-house, and in the same way residual products
may be removed. The fact that considerable area is required and that
the works do not improve the neighbourhood are important conditions,
and although economy of space should be considered, arrangements
should be such as to allow of extension. In the case of a works whose
daily make of gas exceeds four to five million cub. ft., it is usual
to divide the works into units, there being an efficiency limit to the
size of apparatus employed. Under these conditions the gas is dealt
with in separate streams, which mix when the holder is reached. From
the accompanying ground plan of a works (fig. 4) it will be possible
to gain an idea of the order in which the operations in gas
manufacture are carried out and the arrangement of the plant.

Retorts.

The retorts in which the coal is carbonized are almost universally
made of fire-clay, and in all but small country works the old
single-ended retort, which was about 9 ft. in length, has given way to
a more economical construction known as doubles, double-ended, or
"through" retorts. These are from 18 to 22 ft. long, and as it is
found inconvenient to produce this length in one piece, they are
manufactured in three sections, the jointing together of which demands
great care. The two outer pieces are swelled at one end to take an
iron mouthpiece. The cross sections generally employed for retorts are
known as "D-shaped," "oval" and "round" (fig. 5). The "D" form is
mostly adopted owing to its power of retaining its shape after long
exposure to heat, and the large amount of heating surface it presents
at its base. The life of this retort is about thirty working months. A
cast iron mouthpiece and lid is bolted to the exterior end of each
retort, the mouthpiece carrying a socket end to receive the ascension
pipe, through which the gas passes on leaving the retort. The retorts
are heated externally and are set in an arch, the construction
depending upon the number of retorts, which varies from three to
twelve. The arch and its retorts is termed a bed or setting, and a row
of beds constitutes a bench. It is usual to have a separate furnace
for each setting, the retorts resting upon walls built transversely in
the furnace.

The heating of the retorts is carried out either by the "direct
firing" or by the "regenerative" system, the latter affording marked
advantages over the former method, which is now becoming extinct. In
the regenerative system of firing, a mixture of carbon monoxide and
nitrogen is produced by passing air through incandescent gas coke in a
generator placed below the bench of retorts, and the heating value of
the gases so produced is increased in most cases by the admixture of a
small proportion of steam with the primary air supply, the steam being
decomposed by contact with the red-hot coke in the generator into
water gas, a mixture of carbon monoxide and hydrogen (see FUEL:
_Gaseous_). The gases so formed vary in proportion with the
temperature of the generator and the amount of steam, but generally
contain 32 to 38% of combustible gas, the remainder being the residual
nitrogen of the air and carbon dioxide. These gases enter the
combustion chamber around the retorts at a high temperature, and are
there supplied with sufficient air to complete their combustion, this
secondary air supply being heated by the hot products of combustion on
their way to the exit flue. This method of firing results in the
saving of about one-third the weight of coke used in the old form of
furnace per ton of coal carbonized, and enables higher temperatures to
be obtained, the heat being also more equally distributed.

There are a great number of methods of applying the regenerative
principle which vary only in detail. Fig. 6 gives an idea of the
general arrangement. The furnace A is built of fire-brick, coke is
charged at the top through the iron door B, and near the bottom are
placed fire bars C, upon which the fuel lies. The primary air
necessary for the partial combustion of the coke to "producer" gas
enters between these bars. The gases are conducted from the furnace to
the combustion chamber E through the nostrils D D, and the secondary
air is admitted at the inlet F a little above, this air having been
already heated by traversing the setting. Complete combustion takes
place at this point with the production of intense heat, the gases on
rising are baffled in order to circulate them in every direction round
the retorts, and upon arriving at the top of the setting they are
conducted down a hollow chamber communicating with the main flue and
shaft. The amount of draft which is necessary to carry out the
circulation of the gases and to draw in the adequate amount of air is
regulated by dampers placed in the main flue. By analysis of the
"producer" and "spent" gases this amount can be readily gauged.

Retorts are set in either the horizontal, inclined or vertical
position, and the advantages of the one over the other is a question
upon which almost every gas engineer has his own views.

Charging and drawing.

The introduction of labour-saving appliances into gas works has
rendered the difficult work of charging and discharging horizontal
retorts comparatively simple. Formerly it was the practice to carry
out such operations entirely by hand, men charging the retorts either
by means of shovel or hand-scoop, and the coke produced being
withdrawn with hand rakes. Now, however, only the smaller gas works
adhere to this system, and this work is done by machinery driven by
either compressed air, hydraulic or electric power. In the first two
cases a scoop, filled with coal from an overhead hopper carried by the
travelling machine, is made to enter the retort and is turned over;
the operation is then repeated, but this time the scoop is turned over
in the opposite direction, the coal thus assuming such a position that
as much of its under surface as possible is exposed to the heated side
of the retort. With "through" retorts charging machines feed the
retorts at both ends, the scoop, which has a capacity of about 1-1/2
cwt., entering and discharging its contents twice at each end, so that
the total charge is about 6 cwt., which is allowed from four to six
hours to distil off according to the quality of the gas required. The
machines charge simultaneously at each end, so that the lids of the
retorts may be shut immediately the coal enters. The charging machines
travel on lines in front of the retort bench, and the power is
transmitted by connexions made with flexible hose. A device of more
recent introduction is an electrically-driven charging machine, in
which the centrifugal force created by a fly-wheel revolving at high
speed is applied to drive coal into the retort. If the velocity is
sufficiently high the coal may be carried the whole length of a 20-ft.
retort, the coal following banking up until an even layer is formed
throughout the length of the retort.

For the purpose of discharging the coke from the retort either
compressed air or hydraulic machinery is employed, a rake being made
to enter the retort and withdraw the coke on returning. With this
method it is necessary that the rake should enter and discharge
several times before the retort is clear, and thus the use of a
telescopic ram worked by hydraulic power, which pushes the coke before
it and discharges it at the other end, is an advantage. As much as
one-third on each ton of coal carbonized is saved by the use of
machinery in the retort-house. Taking into account the original cost
of such machines, and the unavoidable wear and tear upon the retorts
brought about by using labour-saving appliances, and the fact that the
coke-dust is very detrimental to the machinery, it is clear that the
suggestion of setting the retorts at an incline in order to facilitate
the work presented great inducements to the gas manager. The object
aimed at in thus setting retorts is to allow gravity to play the part
of charging and discharging the coal and coke, the retorts being
inclined at an angle to suit the slip of the class of coal used; this
angle is between 28 deg. and 34 deg. The coal, previously elevated to
hoppers, is dropped into the feeding chambers, which are so arranged
that they can travel from end to end of the retort-house and feed the
coal into the retorts. When the retort is to be charged, an iron stop
or barrier is placed in the lower mouthpiece, and the door closed. The
shoot is placed in the upper mouthpiece, and the stop or door, which
retains the coal in the chamber, is released; the coal is then
discharged into the retort, and rushing down the incline, is arrested
by the barrier, and banks up, forming a continuous backing to the coal
following. By experience with the class of coal used and the
adjustment of the stops in the shoot, the charge can be run into the
retort to form an even layer of any desired depth. For the withdrawal
of the residual coke at the end of the carbonization, the lower
mouthpiece door is opened, the barrier removed and the coke in the
lower part of the retort is "tickled" or gently stirred with an iron
rod to overcome a slight adhesion to the retort; the entire mass then
readily discharges itself. Guides are placed in front of the retort to
direct its course to the coke hoppers or conveyer below, and to
prevent scattering of the hot material. This system shows a greater
economy in the cost of carbonizing the coal, but the large outlay and
the wear and tear of the mechanical appliances involved have so far
prevented its very general adoption.

The vertical retort was one of the first forms experimented with by
Murdoch, but owing to the difficulty of withdrawing the coke, the low
illuminating power of the gas made in it, and the damage to the retort
itself, due to the swelling of the charge during distillation, it was
quickly abandoned. About the beginning of the 20th century, however,
the experiments of Messrs Settle and Padfield at Exeter, Messrs
Woodall and Duckham at Bournemouth, and Dr Bueb in Germany showed such
encouraging results that the idea of the vertical retort again came to
the front, and several systems were proposed and tried. The cause of
the failure of Murdoch's original vertical retort was undoubtedly that
it was completely filled with coal during charging, with the result
that the gas liberated from the lower portions of the retort had to
pass through a deep bed of red-hot coke, which, by over-baking the
gas, destroyed the illuminating hydrocarbons. There is no doubt that
the question of rapidly removing the gas, as soon as it is properly
formed, from the influence of the highly-heated walls of the retort
and residual coke, is one of the most important in gas manufacture.

In the case of horizontal retorts the space between the top of the
coal and the retort is of necessity considerable in order to permit
the introduction of the scoop and rake; the gas has therefore a free
channel to travel along, but has too much contact with the highly
heated surface of the retort before it leaves the mouthpiece. In the
case of inclined retorts this disadvantage is somewhat reduced, but
with vertical retorts the ideal conditions can be more nearly
approached. The heating as well as the illuminating value of the gas
per unit volume is lowered by over-baking, and Dr Bueb gives the
following figures as to the heating value of gas obtained from the
same coal but by different methods of carbonization:--

Vertical Retorts, 604 British thermal units per cub. ft.
Inclined " 584 " " "
Horizontal " 570 " " "

Of the existing forms of vertical retort it remains a matter to be
decided whether the coal should be charged in bulk to the retort or
whether it should be introduced in small quantities at regular and
short intervals; by this latter means (the characteristic feature of
the Settle-Padfield process) a continuous layer of coal is in process
of carbonization on the top, whilst the gas escapes without contact
with the mass of red-hot coke, a considerable increase in volume and
value in the gas and a much denser coke being the result.

Hydraulic main.

From the retort the gas passes by the ascension pipe to the hydraulic
main (fig. 7). This is a long reservoir placed in a horizontal
position and supported by columns upon the top of the retort stack,
and through it is maintained a slow but constant flow of water, the
level of which is kept uniform. The ascension pipe dips about 2 in.
into the liquid, and so makes a seal that allows of any retort being
charged singly without the risk of the gas produced from the other
retorts in the bench escaping through the open retort. Coal gas,
being a mixture of gases and vapours of liquids having very varying
boiling points, must necessarily undergo physical changes when the
temperature is lowered. Vapours of liquids of high boiling point will
be condensed more quickly than those having lower boiling points, but
condensation of each vapour will take place in a definite ratio with
the decrease of temperature, the rate being dependent upon the boiling
point of the liquid from which it is formed. The result is that from
the time the gaseous mixture leaves the retort it begins to deposit
condensation products owing to the decrease in temperature.
Condensation takes place in the ascension pipe, in the arch piece
leading to the hydraulic main, and to a still greater extent in the
hydraulic main itself where the gas has to pass through water.

Ascension pipes give trouble unless they are frequently cleared by an
instrument called an "auger," whilst the arch pipe is fitted with hand
holes through which it may be easily cleared in case of stoppage. The
most soluble of the constituents of crude coal gas is ammonia, 780
volumes of which are soluble in one volume of water at normal
temperature and pressure, and the water in the hydraulic main absorbs
a considerable quantity of this compound from the gas and helps to
form the ammoniacal liquor, whilst, although the liquor is well
agitated by the gas bubbling through it, a partial separation of tar
from liquor is effected by gravitation. The liquor is run off at a
constant rate from the hydraulic main to the store tank, and the gas
passes from the top of the hydraulic main to the foul main.

Condensation.

The gas as it leaves the hydraulic main is still at a temperature of
from 130 deg. to 150 deg. F., and should now be reduced as nearly as
possible to the temperature of the surrounding atmosphere. The
operation of efficient condensing is not by any means as simple as
might be supposed. The tar and liquor when condensed have a dissolving
action on various valuable light-giving constituents of the gas, which
in the ordinary way would not be deposited by the lowering of
temperature, and for this reason the heavy tar, and especially that
produced in the hydraulic main, should come in contact with the gas as
little as possible, and condensation should take place slowly.

The main difficulty which the condenser ought to overcome and upon
which its efficiency should depend is the removal of naphthalene: this
compound, which is present in the gas, condenses on cooling to a solid
which crystallizes out in the form of white flakes, and the trouble
caused by pipe stoppages in the works as well as in the district
supplied is very considerable. The higher the heat of carbonization
the more naphthalene appears to be produced, and gas managers of
to-day find the removal of naphthalene from the gas a difficult
problem to solve. It was for some time debated as to whether
naphthalene added materially to the illuminating value of the gas, and
whether an endeavour should be made to carry it to the point of
combustion; but it is now acknowledged that it is a troublesome
impurity, and that the sooner it is extracted the better. Gas leaves
the retorts saturated with naphthalene, and its capacity for holding
that impurity seems to be augmented by the presence of water vapour.
The condenser, by effecting the condensation of water vapour, also
brings about the deposition of solid naphthalene, apart from that
which naturally condenses owing to reduction of temperature.

Condensers are either air-cooled or water-cooled, or both. In the
former case the gas traverses pipes exposed to the atmosphere and so
placed that the resulting products of condensation may be collected at
the lowest point. Water is a more efficient cooling medium than air,
owing to its high specific heat, and the degree of cooling may be more
easily regulated by its use. In water-cooled condensers it is usual to
arrange that the water passes through a large number of small pipes
contained in a larger one through which the gas flows, and as it
constantly happened that condenser pipes became choked by naphthalene,
the so-called reversible condenser, in which the stream of gas may be
altered from time to time and the walls of the pipes cleaned by
pumping tar over them, is a decided advance.

The solubility of naphthalene by various oils has led some engineers
to put in naphthalene washers, in which gas is brought into contact
with a heavy tar oil or certain fractions distilled from it, the
latter being previously mixed with some volatile hydrocarbon to
replace in the gas those illuminating vapours which the oil dissolves
out; and by fractional distillation of the washing oil the naphthalene
and volatile hydrocarbons are afterwards recovered.

Exhauster.

The exhauster is practically a rotary gas pump which serves the
purpose of drawing the gas from the hydraulic main through the
condensers, and then forcing it through the purifying vessels to the
holder. Moreover, by putting the retorts under a slight vacuum, the
amount of gas produced is increased by about 12%, and is of better
quality, owing to its leaving the heated retort more quickly. A
horizontal compound steam-engine is usually employed to drive the
exhauster.

At this point in the manufacturing process the gas has already
undergone some important changes in its composition, but there yet
remain impurities which must be removed, these being ammonia,
sulphuretted hydrogen, carbon disulphide and carbon dioxide. Ammonia
is of considerable marketable value, and even in places where the
local Gas Act does not prescribe that it shall be removed, it is
extracted. Sulphuretted hydrogen is a noxious impurity, and its
complete removal from the gas is usually imposed by parliament. As
nearly as possible all the carbon dioxide is extracted, but most gas
companies are now exempt from having to purify the gas from sulphur
compounds other than sulphuretted hydrogen. Cyanogen compounds also
are present in the gas, and in large works, where the total quantity
is sufficient, their extraction is effected for the production of
either prussiate or cyanide of soda.

Atkinson Butterfield gives the composition of the gas at this point to
be about

per cent. by vol.

Hydrogen from 42 to 53
Methane " 32 " 39
Carbon monoxide " 3 " 10
Hydrocarbons--
Gases " 2.5 " 4.5
Light condensable
vapours " 0.5 " 1.2
Carbon dioxide " 1.1 " 1.8
Nitrogen " 1.0 " 5.0
Sulphuretted hydrogen " 1.0 " 2.0
Ammonia " 0.5 " 0.95
Cyanogen " 0.05 " 0.12
Carbon disulphide " 0.02 " 0.035
Naphthalene " 0.005 " 0.015

Washers.

It happens that ammonia, being a strong base, will effect the
extraction of a certain proportion of such compounds as sulphuretted
hydrogen, carbon dioxide and hydrocyanic acid, and the gas is now
washed with water and ammoniacal liquor. The process is termed washing
or scrubbing, and is carried out in various forms of apparatus, the
efficiency of which is dependent upon the amount of contact the
apparatus allows between the finely divided gas and water in a unit
area and the facility with which it may be cleared out. The "Livesey"
washer, a well-known type, is a rectangular cast iron vessel. The gas
enters in the centre, and to make its escape again it has to pass into
long wrought iron inverted troughs through perforations one-twentieth
of an inch in diameter. A constant flow of liquor is regulated through
the washer, and the gas, in order to pass through the perforations,
drives the liquor up into the troughs. The liquor foams up owing to
agitation by the finely divided streams of gas, and is brought into
close contact with it. Two or three of these washers are connected in
series according to the quantity of gas to be dealt with.

Scrubbers.

The final washing for ammonia is effected in an apparatus termed a
"scrubber," which is a cylindrical tower packed with boards 1/4 in.
thick by 11 in. broad, placed on end and close together; water is
caused to flow down over the surface of these boards, the object being
to break up the gas as much as possible and bring it into close
contact with the water. In this wet purifying apparatus the gas is
almost wholly freed from ammonia and from part of the sulphuretted
hydrogen, whilst carbon dioxide and carbon disulphide are also
partially extracted.

Purifiers.

The final purification is carried out in rectangular vessels, known as
"dry purifiers" (fig. 8). Internally, each purifier is filled with
ranges of wooden trays or sieves A, made in the form of grids (fig.
9), and covered with the purifying material B to a depth of about 6
in., the number of tiers and size of purifier boxes being proportional
to the quantity of gas to be purified. The gas enters at the bottom by
the pipe C, the inlet being protected from any falling material by the
cover D; it forces its way upwards through all the trays until,
reaching the lid or cover E, it descends by the exit tube F, which
leads to the next purifier. The edges of the lid dip into an external
water seal or lute G, whereby the gas is prevented from escaping.

When the gas had to be purified from carbon disulphide as well as from
sulphuretted hydrogen, slaked lime was employed for the removal of
carbon dioxide and the greater quantity of the sulphur compounds,
whilst a catch box or purifier of oxide of iron served to remove the
last traces of sulphuretted hydrogen. Not fewer than four lime
purifiers were employed, and as the one which was first in the series
became exhausted, i.e. began to show signs of allowing carbon dioxide
to pass through it unabsorbed, it was filled with fresh slaked lime
and made the last of the series, the one which was second becoming
first, and this procedure went on continuously. This operation was
necessitated by the fact that carbon dioxide has the power of breaking
up the sulphur compounds formed by the lime, so that until all carbon
dioxide is absorbed with the formation of calcium carbonate, the
withdrawal of sulphuretted hydrogen cannot proceed, whilst since it is
calcium sulphide formed by the absorption of sulphuretted hydrogen by
the slaked lime that absorbs the vapour of carbon disulphide,
purification from the latter can only be accomplished after the
necessary calcium sulphide has been formed. The foul gas leaving the
scrubbers contains, as a general average, 30 grains of sulphuretted
hydrogen, 40 grains of carbon disulphide and 200 grains of carbon
dioxide per 100 cub. ft. On entering the first purifier, which
contains calcium thiocarbonate and other combinations of calcium and
sulphur in small quantity, the sulphuretted hydrogen and disulphide
vapour have practically no action upon the material, but the carbon
dioxide immediately attacks the calcium thiocarbonate, forming calcium
carbonate with the production of carbon disulphide vapour, which is
carried over with the gas into the second box. In the connexion
between the first and the second box the gas is found to contain 500
grains of sulphuretted hydrogen and 80 grains of carbon disulphide per
100 cub. ft., but no trace of carbon dioxide. In the second box the
formation of calcium thiocarbonate takes place by the action of carbon
disulphide upon the calcium sulphide with the liberation of
sulphuretted hydrogen, which is carried over to the third purifier.
The gas in the connecting pipe between the second and third purifier
will be found to contain 400 grains of sulphuretted hydrogen and 20
grains of carbon disulphide. The contents of the third box, being
mostly composed of slaked lime, take up sulphuretted hydrogen forming
calcium sulphide, and practically remove the remaining impurities, the
outlet gas showing 20 grains of sulphuretted hydrogen and 8 grains of
carbon disulphide per 100 cub. ft., whilst the catch box of oxide of
iron then removes all traces of sulphuretted hydrogen. It will be
noticed that in the earlier stages the quantity of sulphur impurities
is actually increased between the purifiers--in fact, the greater
amount of sulphiding procures the ready removal of the carbon
disulphide,--but it is the carbon dioxide in the gas that is the
disturbing element, inasmuch as it decomposes the combinations of
sulphur and calcium; consequently it is a paramount object in this
system to prevent this latter impurity finding its way through the
first box of the series. The finding of any traces of carbon dioxide
in the gas between the first two boxes is generally the signal for a
new clean purifier being put into action, and the first one shut off,
emptied and recharged with fresh lime, the impregnated material being
sometimes sold for dressing certain soils.

The action of oxide of iron, which has now partly replaced the lime
purification, depends on its power of combining with sulphuretted
hydrogen to form sulphide of iron. Such is the affinity of the oxide
for this impurity that it may contain from 50 to 60% by weight of free
sulphur after revivification and still remain active. Upon removing
the material from the vessel and exposing it to the atmosphere the
sulphide of iron undergoes a revivifying process, the oxygen of the
air displacing the sulphur from the sulphide as free sulphur, and with
moisture converting the iron into hydrated oxide of iron. This
revivification can be carried on a number of times until the material
when dry contains about 50% of free sulphur and even occasionally 60%
and over; it is then sold to manufacturers of sulphuric acid to be
used in the sulphur kilns instead of pyrites (see SULPHURIC ACID).

Apart from the by-products coke, coke-breeze, tar and retort carbon,
which are sold direct, gas companies are now in many cases preparing
from their spent purifying material pure chemical products which are
in great demand. The most important of these is sulphate of ammonia,
which is used for agricultural purposes as a manure, and is obtained
by passing ammonia into sulphuric acid and crystallizing out the
ammonium sulphate produced. To do this, saturated ammoniacal liquor is
decomposed by lime in the presence of steam, and the freed ammonia is
passed into strong sulphuric acid, the saturated solution of ammonium
sulphate being carefully crystallized. The market value of the salt
varies, but an average figure is L12 per ton, whilst the average yield
is about 24 lb. of salt per ton of coal carbonized. In large works the
sulphuric acid is usually manufactured on the spot from the spent
oxide, so that the sulphuretted hydrogen, which in the gas is
considered an undesirable impurity, plays a valuable part in the
manufacture of an important by-product.

Cyanogen compounds are extracted either direct from the gas, from the
spent oxide or from ammoniacal liquor, and some large gas works now
produce sodium cyanide, this being one of the latest developments in
the gas chemical industry.

Gasholder.

The purified gas now passes to a gasholder (sometimes known as a
gasometer), which may be either single lift, i.e. a simple bell
inverted in a tank of water, or may be constructed on the telescopic
principle, in which case much ground space is saved, as a holder of
much greater capacity can be contained in the same-sized tank. The
tank for the gasholder is usually made by excavating a circular
reservoir somewhat larger in diameter than the proposed holder. A
banking is allowed to remain in the centre, as shown in fig. 10, which
is known as the "dumpling," this arrangement not only saving work and
water, but acting as a support for the king post of a trussed holder
when the holder is empty. The tank must be water-tight, and the
precaution necessary to be taken in order to ensure this is dependent
upon the nature of the soil; it is usual, however, for the tanks to be
lined with concrete. Where the conditions of soil are very bad, steel
tanks are built above ground, but the cost of these is much greater.
The holder is made of sheet iron riveted together, the thickness
depending upon the size of the holder. The telescopic form consists of
two or more lifts which slide in one another, and may be described as
a single lift holder encircled by other cylinders of slightly larger
diameter, but of about the same length. Fig. 10 shows the general
construction. Gas on entering at A causes the top lift to rise; the
bottom of this lift being turned up all round to form a cup, whilst
the top of the next lift is turned down to form a so-called grip, the
two interlock (see fig. 11), forming what is known as the hydraulic
cup. Under these conditions the cup will necessarily be filled with
water, and a seal will be formed, preventing the escape of gas. A
guide framing is built round the holder, and guide rollers are fixed
at various intervals round the grips of each lift, whilst at the
bottom of the cup guide rollers are also fixed (fig. 11). In the year
1892 the largest existing gasholder was built at the East Greenwich
works of the South Metropolitan Gas Company; it has six lifts, its
diameter is 293 ft., and when filled with gas stands 180 ft. high. The
capacity for gas is 12 million cub. ft.

Governor.

The governor consists usually of a bell floating in a cast iron tank
partially filled with water, and is in fact a small gasholder, from
the centre of which is suspended a conical valve controlling the gas
inlet and closing it as the bell fills. Any deviation in pressure will
cause the floating bell to be lifted or lowered, and the size of the
inlet will be decreased or increased, thus regulating the flow.

Enrichment.

The fact that coal gas of an illuminating power of from 14 to 16
candles can be made from the ordinary gas coal at a fairly low rate,
while every candle power added to the gas increases the cost in an
enormous and rapidly growing ratio, has, from the earliest days of
the gas industry, caused the attention of inventors to be turned to
the enrichment of coal gas. Formerly cannel coal was used for
producing a very rich gas which could be mixed with the ordinary gas,
thereby enriching it, but as the supply became limited and the price
prohibitive, other methods were from time to time advocated to replace
its use in the enrichment of illuminating gas. These may be classified
as follows:--

1. Enriching the gas by vapours and permanent gases obtained by
decomposing the tar formed at the same time as the gas.

2. Mixing with the coal gas oil gas, obtained by decomposing crude
oils by heat.

3. The carburetting of low-power gas by impregnating it with the
vapours of volatile hydrocarbons.

4. Mixing the coal gas with water gas, which has been highly
carburetted by passing it with the vapours of various hydrocarbons
through superheaters in order to give permanency to the hydrocarbon
gases.

Enrichment by tar.

Very many attempts have been made to utilize tar for the production
and enrichment of gas, and to do this two methods may be adopted:--

(a) Condensing the tar in the ordinary way, and afterwards using the
whole or portions of it for cracking into a permanent gas.

(b) Cracking the tar vapours before condensation by passing the gas
and vapours through superheaters.

If the first method be adopted, the trouble which presents itself is
that the tar contains a high percentage of pitch, which tends rapidly
to choke and clog up all the pipes. A partly successful attempt to
make use of certain portions of the liquid products of distillation of
coal before condensation by the second method was the Dinsmore
process, in which the coal gas and vapours which, if allowed to cool,
would form tar, were made to pass through a heated chamber, and a
certain proportion of otherwise condensible hydrocarbons was thus
converted into permanent gases. Even with a poor class of coal it was
claimed that 9800 cub. ft. of 20- to 21-candle gas could be made by
this process, whereas by the ordinary process 9000 cub. ft. of
15-candle gas would have been produced. This process, although
strongly advocated by the gas engineer who experimented with it, was
never a commercial success. The final solution of the question of
enrichment of gas by hydrocarbons derived from tar may be arrived at
by a process which prevents the formation of part of the tar during
the carbonization of the coal, or by the process devised by C.B. Tully
and now in use at Truro, in which tar is injected into the
incandescent fuel in a water-gas generator and enriches the water gas
with methane and other hydrocarbons, the resulting pitch and carbon
being filtered off by the column of coke through which the gas passes.

Enrichment by oil gas.

The earliest attempts at enrichment by oil gas consisted in spraying
oil upon the red hot mass in the retort during carbonization; but
experience soon showed that this was not an economical method of
working, and that it was far better to decompose the liquid
hydrocarbon in the presence of the diluents which are to mingle with
it and act as its carrier, since, if this were done, a higher
temperature could be employed and more of the heavier portions of the
oil converted into gas, without at the same time breaking down the
gaseous hydrocarbons too much. In carburetting poor coal gas with
hydrocarbons from mineral oil it must be borne in mind that, as coal
is undergoing distillation, a rich gas is given off in the earlier
stages, but towards the end of the operation the gas is very poor in
illuminants, the methane disappearing with the other hydrocarbons, and
the increase in hydrogen being very marked. Lewis T. Wright employed a
coal requiring six hours for its distillation, and took samples of the
gas at different periods of the time. On analysis these yielded the
following results:--

_Time after beginning Distillation._

+------------------------+----------+------------+------------+------------+
| | 10 | 1 hour | 3 hours | 5 hours |
| | minutes. | 30 minutes.| 25 minutes.| 35 minutes.|
+------------------------+----------+------------+------------+------------+
| Sulphuretted hydrogen | 1.30 | 1.42 | 0.49 | 0.11 |
| Carbon dioxide | 2.21 | 2.09 | 1.49 | 1.50 |
| Hydrogen | 20.10 | 38.33 | 52.68 | 67.12 |
| Carbon monoxide | 6.19 | 5.66 | 6.21 | 6.12 |
| Saturated hydrocarbons | 57.38 | 44.03 | 33.54 | 22.58 |
| Unsaturated " | 10.62 | 5.98 | 3.04 | 1.79 |
| Nitrogen | 2.20 | 2.47 | 2.55 | 0.78 |
+------------------------+----------+------------+------------+------------+