the retorts in their passage, and finally enter the regenerators at
a temperature of about 1,900 F.
The retorts are heated either by a furnace immediately under
them on the open-grate system, or by a producer into which only a
small portion of the oxygen of the air is allowed to enter. In the
latter system, the oxygen, combining with the carbon, forms,
as already explained above, carbon monoxide, and a further
portion of air, previously warmed by the waste gases as they
leave the setting, is arranged to enter and mix with the carbon
monoxide and form carbon dioxide immediately under the lower
retorts ; and thus the greatest amount of heat is obtained where
it is required, and not, as in the open-grate system, in the furnace.
Dr. Siemens was the first to recommend this arrangement, which
18
PRACTICAL GAS-FITTING.
is the same in principle as the furnaces in use for smelting
iron.
The benches of retorts are usually made double that is, back
to back, and frequently the retorts are continuous for a length of
from 18ft, to 22ft., having a mouthpiece at each end. The
retorts are then known as " through," in distinction to the
Fig. 3. Sectional Elevation of Eight Single Retorts.
" singles," which have only one mouthpiece, their back ends being
closed. With the exception of this community of retorts, the
settings are distinct, and have separate flues and furnaces. An
iron mouthpiece, bolted on to the clay mouthpiece of each retort,
projects out about 16 in. from the front of the setting, and serves
to support the lid that shuts off the retort from the atmosphere
during the operation of drawing and charging. There are two
MANUFACTURE OF COAL GAS.
19
descriptions of lids employed in gasworks : one is of plate-iron,
and is made tight by means of a luting composed of spent lime
and clay, the lid being screwed against the face of the mouth-
piece. But, in modern works, self-sealing lids are employed ;
these have the rim faced and planed, the mouthpiece being
similarly treated ; the lid swings round on a swivel hinge, and is
Fig. 4. Longitudinal Section of Eight Single Retorts.
made tight against the mouthpiece by means of an eccentric lever
The mouthpiece is provided with an opening into which fits an
upright pipe, known as the ascension pipe, which is connected to
the arch or saddle pipe, and the latter in turn to the dip pipe
which conducts ths gn from the retort to the hydraulic main,
where it is prevented from returning to the retort.
20
PRACTICAL GAS-FITTING.
Figs. 5, 6, and 7 represent a setting of three retorts heated
on a modern system by means of a regenerative furnace ; the
.system has been found to give good results in practical working.
The producer is placed inside the oven under the front of the
retorts, and is 2 ft. square inside by 3 ft. 6 in. deep. The waste-
gas flues forming the regenerator are four in number, and 9 in.
square ; they occupy the rest of the oven space, and extend 3 ft.
outside the back of the oven, where they are carried to the
chimney by a cross flue. As shown in Fig. 5, the regenerator
is so arranged as to form three sets of flues, the top and bottom
sets being for the waste heat passing from the oven to the
chimney, and the middle set for the heated air. The air for the
hot-air flues is admitted at the back of the regenerator, and is
carried forward by two 4-in. fireclay pipes to nearly the front end
Fig. 5. Longitudinal Section through Oven.
of the two centre air-flues. It goes back outside these pipes, but
still in the two centre flues, and returns to the front in the two
outside air-flues, entering the top of the producer by three ports
at each side. The combustible gases from the producer combine
with the heated air, and pass up the front portion of the retorts,
over a middle wall, down the back portion, and then enter the
upper set of flues, in which they travel forward and descend to
the lower set of flues, and thence to the main flue leading to
the chimney. The primary air is admitted at the door at which
the producer is cleaned out. Fig. 5 represents a longitudinal
section through the oven ; Fig. 6 is a horizontal section on the
line G H, Fig. 5 ; Fig. 7, a vertical section on the line J K,
Fig. 6. A denotes the air- flues, D D the floor line F flue, and w
waste heat.
It is necessary to test frequently the producer and flue gases
MANUFACTURE OF COAL GAS.
21
in order to ensure that the correct amount of primary and
secondary air is being admitted to the setting, otherwise' there
will be a considerable waste of fuel. In the case of the producer
gases, an excess of primary air implies the production of carbonic
acid instead of carbonic oxide, and in the case of ilue gases an
excess of secondary air implies a cooling of the setting, while a
deficiency denotes a waste of fuel due to carbonic oxide escaping
Fig. 6. Horizontal Section through Oven.
unconsumed. The theoretical composition of producer gas is 347
per cent, carbonic oxide, and 65*3 per cent, nitrogen, but this
result is never attained in practice, as hydrogen is always present,
due to the decomposition of the steam given off from the water
Fig-. 7. Vertical Section through Oven.
in the ash-pans, while the reduction of carbon dioxide OCX (to
carbon monoxide CO) in the producer is never complete, with
the result that some C0 2 is always present in producer gas, but
it should never be allowed to exceed 6 per cent. The flue gases
should consist of COa and nitrogen, with from 1 to 2 per cent,
of oxygen. On no account should any CO escape.
22
PRACTICAL GAS-FITTING.
High heats produce more gas of a poorer quality, and a
smaller quantity of tar of a high specific gravity, and, as the heat
is lowered, a smaller quantity of richer gas and a larger quantity
of lighter tar are obtained. High heats appear to break up the
combinations of hydrogen and carbon, the latter being deposited
on the sides of the retort in the form of scurf. When an organic
substance, such as bituminous coal, is distilled at a comparatively
low temperature, the carbon passes off accompanied by but little
hydrogen, liquid compounds of carbon and hydrogen being
formed in great abundance, and as a consequence plenty of tar
but little gas is obtained, the gas, however, being of a high
illuminating power. On gradually increasing the temper-
ature, however, the liquid hydrocarbon decreases, while
the gaseous products increase that is, there is more gas
and less tar, the yield of gas increasing as the tempera-
ture increases, but the quality at the same time decreases.
The effects of temperature on the yield and quality of gas
have been very carefully investigated by Mr. L. T. Wright, F.C.S.,
and in a series of experiments conducted by him the following
results were obtained from four portions of the same coal when
distilled at temperatures which varied from a dull red heat to the
highest temperature obtainable in an iron retort :
Temperature.
Cubic feet of gas
j'er ton.
Illuminating
power
(candles).
Total candles
l*er ton.
I. Dull red
8,250
20-5
33,950
2. Hotter
9,693
IT'S
34,510
3. ... 10,821
167
36,140
4. Bright orange... 12,006
15'6
37,460
The temperature of distillation greatly affects the tar pro-
duced both as regards quantity and quality, more especially
the latter. The quantity of tar obtainable from coal decreases
as the distillation temperature increases. When ordinary gas
coal is distilled at a temperature of about 800 F. the tar is
very thin, consists chiefly of hydrocarbon of the paraffin and
olefiant series, and contains but a very small proportion of
free carbon ; but if the temperature of distillation be raised, say
MANUFACTURE OF COAL GAS. 23
to about 1,700 F., the tar then becomes thick, and contains
much free carbon, while hydrocarbons of the benzine series take
the place of the paraffin and olefiant hydrocarbons. With regard
to the effect of temperature on the production of ammonia, it
would appear that at very low distillation temperatures the yield
of ammonia is low, and that a medium temperature produces the
greatest amount of the residual, very high temperatures reducing
the yield, as shown by Mr. L. T. Wright in the following way :
Make per ton Yield of NH-^ Percentage by weight
(cubic feet). at per ton. of coal, as N Hy.
11,620 7-411 0-331
10,162 7-894 0-352
9,431 7-504 0-335
7,512 6-391 0-285
(Nils = Ammonia.)
The impurities, carbonic acid, sulphuretted hydrogen, and
carbon bisulphide, are considerably increased in quality at high
temperatures, and the production of cyanogen, which must
now be looked upon as an important residual, is also very con-
siderably augmented when high heats are employed.
The operation of carbonising may now be described. It is con-
ducted somewhat as follows : The charge of coal is placed in the
retorts either with shovels or with scoops, according to the size
of the works. In the London district the scoop is employed
exclusively. The scoops generally used are semicircular in cross
section ; they are made of the length of a "single " retort (shown
on pp. 15 and 16), and hold from 1 cwt. to H cwt. of coal. They are
furnished with a T-shaped handle, and are raised to the level of
the different tiers of retorts by a piece of round bar iron known
as a saddle, which is shaped in the centre to the curve of the
scoop. The scoop is laid lengthways on the charging floor and
filled with coal ; the handle is then raised by one man, while the
saddle is placed under the opposite end by two other men, who,
one on each side, raise it to the mouth of the retort, when the
saddle is removed and the scoop is propelled in and turned over
by the man at the handle, who is known as the scoop driver. It is
then withdrawn, refilled with coal, and again placed in the retort
and overturned, but this time on the opposite side of the retort,
so that the coal may lie in a thin even layer.
A retort usually holds 3 cwt. of coal, and is charged in from
40 to 50 seconds. The coal, having been deposited as described,
is next backed off the iron mouthpiece by the backing rake, and
24 PRACTICAL GAS-FITTING.
the iron door immediately closed, when the operation of gas-
making commences, and proceeds on an average for about six
hours, at the expiration of which time the coal will have given
off all the gas it is capable of yielding and have been converted
into coke. Ordinary bituminous coal is generally allowed to
remain in the retorts for six hours, and cannel coal for four
hours. As no air is allowed to get to the coal, the products
obtained are of the same weight as the coal. The hydrogen con-
tained in the coal is driven off by the action of the heat, and
passes off, combined with carbon, in various forms. Oxygen as
aqueous vapour, nitrogen as ammonia, sulphur as sulphuretted
hydrogen and also in the free state, C0 2 (carbon dioxide),
CS 2 (bisulphide of carbon), and nitrogen are also given off.
Sometimes slaked lime is mixed with the coal in the proportion
of r> cwt. to the ton, for the purpose of acting on the impurities,
but it is very trying to the eyes of the stokers, and also spoils the
appearance of the coke
The next operation is to draw the charge in the following
manner : The lever on the iron retort lid is first gently loosened,
while at the same time a piece of red-hot coke or ignited tarred
yarn is placed close to the edge of the lid in order to ignite the
gas remaining in the retort the instant the lid is opened. The
gas then burns away quietly ; whereas if the precaution mentioned
is not taken, and the lid is at once fully opened, cold air rushes in
and forms an explosive mixture with the gas, which shakes the
retort and is very detrimental to the setting generally. By means
of a long iron rake, the coke remaining in the retort is now with-
drawn either into iron barrows in which it is wheeled outside
the retort house to the coke heap, or, in the case of a stage- floor
retort house, it falls through an opening between the front of the
setting and the stage, into the coke-hole below, where it is
quenched with water. The retort is again ready for charging,
but before this is done it is necessary to see that the ascension
pipe at the junction with the iron mouthpiece is quite clear by
inserting a bent auger into the ascension pipe. With " through "
retorts, the operations described are performed simultaneously
by separate gangs of three men at each side of the retort
setting.
25
CHAPTER II.
COAL GAS FROM RETORT TO GAS-HOLDER.
THE course of the gas from the retort to the outlet of the
governor may be thus described : Leaving the retort, the gas
passes, by way of the iron mouthpiece, up the ascension pipe, along
the arch or saddle pipe, down the dip pipe into the hydraulic
main, bubbling through the liquid contained therein, with the
result that it is prevented from returning to the retort. The
crude gas as it leaves the retorts carries with it certain substances
which it is necessary to remove as soon as practicable ; these
consist of tarry matters (impure hydrocarbons), carbon dioxide,
sulphuretted hydrogen, ammonia, etc., as explained in the
previous chapter (p. 24). Now the first operation to which the
gas is subjected is that of cooling, which commences immediately
it leaves the retort, and in the act of cooling the vapours of
various hydrocarbons and water vapour condense into the liquid
form, hence the origin of the name condenser. The liquefied
hydrocarbon vapours constitute the well-known gas tar, and the
condensed water vapour, combining with the ammonia, carbonic
acid, sulphuretted hydrogen, etc., present in the crude gas con-
stitutes what is known as virgin ammoniacal liquor.
Fig. 8, p. 26, shows a hydraulic main with connections, A being
the ascension pipe leading from the mouthpiece, B the arch saddle-
pipe, c the dip pipe, and D the hydraulic main, which is supported
on a wrought-iron girder spanning the retort setting, and is pro-
vided with a weir valve E for regulating the level of liquor. The
object of the hydraulic main is to prevent the gas from passing
back to the retort when the doors are opened for drawing and
charging, whilst at the same time the gas can freely escape during
the time gas-making is proceeding ; in other words, the hydraulic
main, in conjunction with the dip pipe, forms a self-acting
hydraulic seal. This result is obtained by filling the main to a
certain level with water and then causing the dip pipe c to dip
a short distance (say a couple of inches) into the liquid. The
gas, on coming from the retort, has to force its way, therefore,
26 PRACTICAL GAS-FITTING.
through this light seal, but, having once passed the seal, it is
prevented from returning by reason of the large amount of liquid
contained in the hydraulic main, which in practical working is
made of such a width in proportion to the area of the dip pipes
and their distances apart as to provide sufficient liquid for
sealing them against the maximum back pressure that can reach
them.
Fig. 8. Hydraulic Main and Connections.
In Fig. 1, p. 15, the rising pipe is bent over until it looks
downward, and enters a trough- shaped pipe of much larger
dimensions, termed a hydraulic main, E ; this contains water,
which is kept at a level rather above the bottom of the entering
or dip pipe. This arrangement is made so that when the door of
the mouthpiece is opened the gas may not find its way back
clown the rising pipe. The pipe leading from the retort setting,
COAL GAS FROM RETORT TO GAS-HOLDER. 27
shown on Fig. 1, p. 15, and finishing at x, would be joined to
the pipe shown on Fig. 10, p. 29, at the point marked x. As
soon as the gas enters the hydraulic main a considerable amount
of cooling takes place, thus causing a quantity of tarry matters
and other condensable substances to be deposited, these finding
their way by gravitation to the tar well.
A section of the hydraulic main and dip pipe drawn to a
larger scale is shown in Fig. 9, A being the dip pipe, B the
interior of the hydraulic main, c the take-off pipe, and D the weir
valve. The apparatus is constructed of wrought-iron or mild
Fig. 9. Section of Hydraulic Main and Dip Pipe.
steel plate f\ in. thick, attached by 3-in. by 3-in. by f-in. angle
irons. The cover is of cast-iron, as are also the dip pipe and
take-off, the former being 5 in. in diameter. The apparatus is
22 in. wide ; the object to be kept in view when designing a main
being, as explained on the previous page, to seal the dip pipes
ag.iinst back pressure.
In modern gasworks the hydraulic main is divided into
separate lengths, corresponding to either one or two settings of
retorts, each section being furnished with a separate valve, and
connected by a take-off pipe to a gas main on the top of the retort
bench, running behind and parallel to the hydraulic main, the
take-off pipe which conveys the gas from the hydraulic main to
the gas main being provided with a weir valve for regulating the
level of the liquid in the main, so as to give the requisite amount
of seal as shown in Fig. 9. The dip pipe usually dips for a
28 PRACTICAL GAS-FITTING.
distance of about 2 in. in the liquid in the hydraulic main, so
that after the gas has once forced this small amount of seal, it is
prevented from returning down either its own or other ascension
pipes when the mouthpieces are opened during the period when
the retorts are drawn and charged. Leaving the hydraulic main
at a temperature of about 150 F., the gas next enters the con-
densing plant, where its temperature is reduced to about 60 F.,
and the remainder of the tarry bodies are eliminated, together
with considerable quantities of weak ammoniacal liquor.
The chief impurities in crude coal gas, for which qualita-
tive tests are required namely, ammonia (NH 3 ), sulphuretted
hydrogen (SH 2 ), and carbon dioxide, commonly known as car-
bonic acid (CO 2 ) are detected by the following methods :
The presence of ammonia is shown by its action on a moistened
turmeric paper or a reddened litmus paper. Turmeric papers are
prepared by soaking strips of filter or blotting paper in an
alcoholic solution of turmeric, made by digesting powdered
turmeric root in the liquid. They are allowed to dry, are cut
into strips, and are then ready for use, but must be kept in a
dark place. Turmeric papers thus prepared are of a full yellow
colour, which in the presence of ammonia changes to a brownish
tint, and sometimes to a deep crimson colour, according to the
quantity of ammonia present. The papers should be moistened
before use. A more sensitive test for ammonia is that of the
reddened litmus paper (glazed), which turns blue under the
action of ammonia.
Sulphuretted hydrogen is detected by causing a current of gas
to play on a piece of white paper previously dipped in a solution
of acetate of lead or nitrate of silver. The presence of sul-
phuretted hydrogen is shown by the paper changing to a
brownish black colour, due to the formation of lead or silver
sulphide, according to the reagent employed. Carbonic acid is
detected by causing a current of gas to bubble into lime or baryta
water, a white precipitate of calcium or barium carbonate being
formed if CO 2 is present in the gas. Lime water is the solution
most commonly employed, and is prepared for use by placing
about 4 oz. of caustic lime in a quart bottle, filling up with
distilled water to dissolve the lime, and shaking the bottle
occasionally to assist the operation. When the water has taken
up as. much of the lime as it is capable of dissolving, the excess
of lime is allowed to settle, and the clear liquid is transferred
GOAL GAS FROM RETORT TO GAS-HOLDER. 29
to another bottle, which must always be kept tightly corked
to prevent the access of C0 2 from the atmosphere. In' order to
make a test, about 1 oz. of the lime water is placed in a test
tube or small bottle, and the
gas bubbled slowly through it
by means of a glass tube hav-
ing a very fine opening, this
latter tube being connected to
the gas supply by a piece of
indiarubber tubing. In prac-
tice, if a precipitate does not
form within the space of three
minutes, it is assumed that the
gas is free from CO 2 . If sul-
phuretted hydrogen is also
present in the. gas being tested,
it will be necessary to interpose
a small oxide purifier, so as to
prevent any sulphuretted hy-
drogen entering the lime water,
since the presence of the latter
gas would vitiate the test.
The temperature of the gas
in the hydraulic main is usually
about 120 to 130 F., and this
must be reduced to atmos-
pheric temperature by means
of condensers (Fig. 10). These
are of many kinds, but may be divided into two classes-
namely, air condensers and water condensers.
30 PRACTICAL GAS-FITTING.
The gas is thus brought into a suitable condition for the
after process of purification. The tarry bodies and other sub-
stances thus eliminated would, if not removed at an early
stage in the manufacture, clog up the purifying apparatus.
Condensation commences immediately the gas leaves the retorts,
with the result that water vapour and the vapours of various
hydro-carbons condense into liquids, producing what is commonly
known as tar, and a weak, impure solution of ammonia,
known as virgin ammoniacal liquor, due to the absorption of
ammonia, sulphuretted hydrogen, carbonic acid, and hydrocyanic
acid, by the liquefied aqueous vapour. These liquids collect in
the hydraulic main, where a very considerable amount of cooling
takes place, since, by the time the gas reaches the outlet of the
hydraulic main, it will have deposited from one-third to one-half
of its condensable constituents, and have been reduced in temper-
ature from, say, 2000 F. in the retort to from 150 F. to 110 F.
at the outlet of the hydraulic main. The remaining portion of
the work of condensation is next effected in the condenser proper,
the apparatus being usually placed between the retort house and
the exhauster. In some works the gas passes through a length of
main running round the retort house, and known as the foul main,
before it enters the condensers. The air condensers embrace the
vertical, the annular, the horizontal, and the battery, while the
principal water condensers are those of Morris and Cutler, and
Livesey. The action of these different forms of air condensers is
practically the same, the principle on which they act being that
they transmit the heat from the gas passing through them to
the external air in contact with their outer surface, with the
result that the gas and vapours are cooled, and the condensable
vapours deposited.
As a familiar instance of the process of air condensation, take
the simplest form of the apparatus, namely, the ordinary vertical
condenser, which consists of a series of vertical pipes attached to
a cast-iron chest or receiver at the bottom, and connected in pairs
by semicircular bends at the top. The cast-iron chest is provided
with a series of mid-feathers, which dip to a certain depth in
liquid, forming a seal, so that the gas is caused to pass up one
pipe and down the next right along the series ; the condensed
products are deposited in the chest, whence they now by a sealed
overflow to the tar well. The battery condenser possesses one or
two features that render it a more efficient apparatus than the
COAL GAS FROM RETORT TO GAS-HOLDER. 31
other types of air condensers namely, that the gas is subjected to
more friction than is possible with the small amount of " skin "
contact which it undergoes in the ordinary form of apparatus ;
and as a certain proportion of the tar exists in the form of little
vesicles or bubbles, it is necessary, before all the tar can be elimi-
nated from the gas, that they should be broken up by some
means such as the battery condenser affords. The apparatus
consists of an oblong vessel of from 1 ft. to 2 ft. wide, 12 ft. to
18 ft. high, and of varying length. It is divided in the inside by
a series of mid-feathers placed at distances apart equal to the
width of the apparatus ; the mid-feathers extend to within a few
inches of the top and bottom of the chest alternately, the
gas passing from the inlet up and down each division to the
outlet. In order to increase its condensing power, a series of
small tubes of about 2 in. in diameter pass from side to side of the
vessel. These tubes are open to the atmosphere, so that the air is
capable of circulating freely through them ; this helps to cool the