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The average composition of the gas supplied to London is, on the authority of the late Dr Letheby, thus stated:

Hydrogen

Light carburetted hydrogen.
Condensible hydrocarbons...
Carbonic oxide.................................
Carbonic acid..

Aqueous vapour.

Oxygen

Nitrogen

Ordinary Gas,
12 Candles.

Cannel Gas,
20 Candles.

27.7

46.0

39.5

50.0

3.8

13.0

7.5

6.8

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Site and Arrangement of Works.-The choice of a site for a gas establishment is necessarily conditioned by local circumstances; but the facts that a considerable area is required, and that, at best, the works do not improve the amenity of any neighbourhood, are important considerations. A central position with respect to the area to be supplied is certainly desirable, but in the circumstances it is seldom to be obtained. Of even greater consequence for a large work is ready access to a railway or other means of transport; and most of the great establishments are now connected by sidings with lines of railway, whereby coals, &c., are delivered direct from the waggons to the store or retort

Cannel gas is now, however, supplied only to the Houses house, and in the same way the coke and residual products

of Parliament and to certain of the Government offices.

MANUFACTURE OF COAL-GAS.

The series of operations connected with the preparation and distribution of coal-gas embrace the processes of distillation, condensation, exhaustion, scrubbing or washing, purification, measuring, storing, and distribution by the governor to the mains, whence the consumers' supply is drawn. In connexion with consumption, pressure of the gas, measurement of the amount consumed, and the burners and other arrangements for lighting are the most important considerations.

are removed. Where the arrangement is practicable, it is also desirable that the works should be erected at the lowest level of the area to be supplied, since coal-gas, being specifically lighter than atmospheric air, acquires a certain amount of pressure as it rises in pipes, which pressure facilitates its distribution, and it is much easier to control than to beget pressure. In the planning of works, regard must be given to economy of space and to labour-saving arrangements, so that the cost of manual labour may be minimized, and operations proceed in an orderly, methodical, and easily-controlled manner. The accompanying ground plan of gas-works (fig. 1) has been kindly furnished by Mr James Hislop of Glasgow, a gas engineer of known skill

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A, a line of rails leading into the works; B, waggon weighing machine; C, the coal store; D, retort house; E, chimney; F, coke yard; G, condensers; H, scrubbers; I, tar tank; J, exhauster house; K, pump room; L, store room (water tank is placed above K L); M, purifier house; N, lime store; O, meter house: P, gus-holders; Q, governor house; R, photometer room: S, board room; T, office; U. weighing machine; V, superintendent's house; W, joiner's shop; X, smith's shop; Y, engine boller.

and experience; and while it shows arrangements of the most approved character, it will also enable the reader to recognize the position of the various erections and apparatus as they follow each other, and as they will now be described. Retorts.-Retorts for destructive distillation of coal are formed of cast iron, clay, brick, or wrought iron. Various shapes have been adopted in the construction of these vessels; nor have their forms been more varied than the modes in which they have been disposed in the furnaces. In many instances they have been constructed of a cylindrical shape varying in length and diameter. Those first employed were of iron, with the axis vertical, but experience soon showed that this position was extremely inconvenient, on account of the difficulty which it occasioned in removing the coke.

The retorts were therefore next placed in a horizontal position, as being not only more favourable to the most economical distribution of the heat, but better adapted to

the introduction of the coal and the subsequent removal of the coke. At first the heat was applied directly to the lower part of the retort; but it was soon observed that the high temperature to which it was necessary to expose it, for the perfect decomposition of the coal, proved destructive to the lower side, and rendered it useless long before the upper part had sustained much injury. The next improvement was, accordingly, to interpose an arch of brickwork between it and the furnace, and to compensate for the diminished intensity of the heat by a more equally diffused distribution of it over the surface of the retort. This was effected by causing the flue of the furnace to return towards the mouth of the retort, and again conducting it in an opposite direction, till the heated air finally escaped into the chimney. This arrangement was continued so long as iron retorts were in use, but on the general adoption of clay retorts the furnaces were constructed to allow the fire to play freely around them.

a

The cylindrical form of retort a (fig. 2) was long in favour on account of its great durability, but it is not so well fitted for rapid decomposition of the coal as the elliptical b, or

Fig. 2.

the flat-bottomed or D-shaped retorts d, which are now principally in use. Retorts are also made of a rectangular section with the corners rounded and the roof arched. Elliptical retorts are varied into what are called ear-shaped or kidney-shaped c, and it is not unusual to set retorts of different forms in the same bench, for the convenience of filling up the haunches of the arch which encloses them. The length of single retorts varies from 6 to 9 feet, but they are now in some cases made 19 feet in length and 12 inches in internal diameter, these being charged from both ends.

Every retort is furnished with a separate mouthpiece, usually of cast iron, with a socket b (fig. 3) for receiving the stand-pipe or ascension-pipe, and there is a movable lid attached to

mouth,

the

together

with an ear-box cast on each side of the retort for receiving the ears which support the lid. Fig. 3 shows. a form of mouthpiece attached to the retort a, and also the method of

b

Fig. 3.

screwing the lid to the mouthpiece. That part of the lid which comes in contact with the edge of the mouthpiece has applied to it a lute of lime mortar and fire clay, and when the lid is screwed up, a portion of this lute oozes out round the edges and forms a gas-tight joint.

Except for small works, where the manufacture is intermittent, and where, consequently, the retort heat has to be got up frequently, iron retorts are now little used. Clay retorts, which at present are in most general use, wear out quickly; they very frequently crack so seriously on the first application of heat that they must be removed from the bench before being used at all, and in scarcely any case are they in action perfectly free from cracks. Numerous attempts have been made to introduce retorts built of brick; but the difficulty of making and keeping the joints airtight has proved a serious obstacle to their use. In the

carbonize 500 tons cannel coal, or 2000 tons per oven of four, without any repairs whatever. Decayed bricks may be removed from these retorts and new ones inserted, and when thoroughly repaired they are again equal to new. Thus the durability of each retort is so great that they are calculated to cost about th of a penny per 1000 cubic feet of gas generated, as against 1d. in the case of moulded retorts, and 7d. with iron retorts, for the same production of gas. In the Hislop retort the arched bricks are made plain, without groove or rebate joints-being thus stronger, more readily put together, and also cheaper. Carbon does not collect so rapidly on brick retorts as on those of clay, the bricks being harder pressed and better burned. On first lighting brick retorts, a charge of coke, breeze, and tar mixed makes them perfectly gas-tight.

Retort Setting.-A furnace or bed of retorts is composed of a group or setting, heated by a separate fire. The furnace is lined with the most refractory fire-bricks, and while the whole brickwork is made of such strength and solidity as ensures the safety of the retorts, the internal construction is so planned that the heat has the utmost possible amount of direct play on the retorts. The number of retorts to one furnace varies from 1 to 15, from 4 to 7 being the number most commonly adopted; and these are variously arranged to bring them all as close to the furnace. heat as practicable. In some retort-houses the furnaces are built in two stages or stories, from the upper of which the retorts are charged and drawn, while at the lower level the glowing coke is removed and quenched. The whole range of furnaces constitutes the retort bench, having a common flue which leads to the chimney shaft by which the products of combustion are carried away. The gas-coal for charging the retorts is broken into fragments about 1 b in weight or thereby. Figs. 5 (elevation) and 6 (section) illustrate the

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brick retort made of Glenboig Star fire-clay, according to the plan of Mr James Hislop, it is claimed that the difficulty is surmounted, and that both the retort and its setting present great advantage and economy. These brick retorts (fig. 4) are Q-shaped, 9 feet long and with diameters of 22 and 13 inches, set four in an oven to one unarched furnace, as in fig. 7. Each retort will, it is affirmed,

FIG. 5.-Elevation of Hislop's Gas Retort Furnace. retort setting and arrangement of furnace and flues adopted by Mr Hislop for his brick retorts, in which, by the use of centre blocks, as seen in the open front illustration (fig. 7), the necessity for internal arching is avoided.

Retort furnaces are commonly fired or heated with a portion of the coke which forms one of the bye-products of

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FIG. 7.-Retort Setting in Hislop's Furnace. tions of a retort setting on this plan, and a description of the various arrangements connected with the regenerators and the controlling of the air and gas currents, will be found in the article FURNACE, vol. ix. pp. 846, 847.

Ordinarily the work of charging and drawing the retorts is accomplished by manual labour, by means simply of shovels for charging, and long iron rakes for drawing the spent charge. In the larger works it is usual to charge the retorts with a scoop semi-cylindrical in form, made a little shorter than the retort, and of such a diameter that it can

with ease be pushed in and overturned within the retort. The scoop deposits the coal neatly over the sole of the retort, and of course the lid is much more quickly replaced than can be done with shovel charging. Numerous attempts have been made to introduce purely mechanical means of feeding retorts, hitherto with indifferent success,such devices as a travelling endless sole and a rotating sole having been tried without good effect. A charging machine and a drawing machine, worked by hydraulic power, have been introduced by Mr Foulis, the engineer of the Glasgow Corporation Gas Works, but after prolonged trial both in Glasgow and in Manchester, these have not yet proved satisfactory in action. In West's patent the charging is effected by the introduction of a small waggon within the retort, which distributes the charge evenly and uniformly. Neither has it, however, met general acceptance.

The retorts are kept at a bright red heat, and for coal with a high percentage of volatile matter a higher temperature is requisite than is needed for coal less rich in gas. As the retorts in one setting are necessarily subject to somewhat different amounts of heat, the charges in those nearest the furnace fire, and consequently most highly heated, must be drawn more frequently than the others, as otherwise the quality of the gas would be deteriorated, and a large proportion of sulphur compounds would be given off from the overburnt coke.

In drawing a charge the lid is first slightly opened and the escaping gas lighted, to prevent an explosion or "rap" that would otherwise ensue. The gas is prevented from escaping outward by the ascension pipe dipping into the hydraulic main as afterwards explained; but in some cases special valves are fitted on the ascension pipe to prevent a back rushing of the gas. A carbonaceous deposit forms on the sides of the retorts, which requires to be periodically removed by "scurfing" with chisels, or burning it off with free admission of air or steam.

The Hydraulic Main.-From the retorts the gas, after its production, ascends by means of pipes called ascensionpipes B (figs. 5 and 6) into what is termed the condensing or hydraulic main HH, which is a large pipe or long reservoir placed in a horizontal position, and supported by columns in front of the brick-work which contains the retorts A. This part of a gas apparatus is intended to serve a twofold purpose:-first, to condense the tar and some ammoniacal liquor, and secondly, to allow each of the retorts to be charged singly without permitting the gas produced from the others, at the time that operation is going on, to make its escape. To accomplish these objects one end of the hydraulic main is closed by a flunge; and the other, where it is connected with the pipes for conducting the gas towards the tar vessel and purifying apparatus, has, crossing it in the inside, a partition occupying the lower half of the area of the section, by which the condensing vessel is always kept half full of liquid matter. The stand-pipes are connected by a flange with a dip-pipe C, arising from the upper side of the condensing main HH, and as the lower end of it dips about 2 inches below the level of the liquid matter, it is evident that no gas can return and escape when the mouthpiece on the retort is removed, until it has forced the liquid matter over the bend, a result which is easily prevented by making it of a suitable length. The tar which is deposited in the hydraulic main overflows at the partition, and is carried by a pipe to the tar well.

Condensation. The gas as it passes on from the hydraulic main is still of a temperature from 130° to 140° Fahr., and consequently carries with it heavy hydrocarbons, which, as its temperature falls, would be deposited. It is therefore a first consideration in ordinary working to have these condensable vapours at once separated, and the object of the condenser is to cool the gas down to a temperature

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pipes connected in pairs at the top by semicircular pipes e, e, and terminating at the bottom in a trough X Y containing water, and divided by means of partitions in such a way that, as the gas enters the trough from one pipe, it passes up the next pipe and down into the next partition, and so on to the end of the condenser. The cooling power of this air condenser, as it is called, is sometimes assisted by allowing cold water to trickle over the outer surface of the pipes. Annulur tubes for condensing are also used, in which the gas is exposed to a much greater cooling surface, and in some large works the condensers are cooled by a current of water. In passing through the pipes the gas is considerably reduced in temperature, and the tar and ammoniacal liquor condense, the tar subsiding to the bottom of the troughs, and the ammoniacal liquor floating on the surface. In course of time the water in the trough is entirely displaced by these two gaseous products, and as they accumulate they pass off into the tar-tank, from which either liquor can be removed by means of a pump adapted to the purpose. The New York Gas Lighting Company employ a multitubular condenser, consisting of two sets of eight boxes, each containing 100 tubes 3 inches diameter by 15 feet long. Through each set of tubes, up one and down another, the gas travels, cooled by an external stream of water, while it traverses the 240 feet of piping in the condenser.

The practice of condensation and separation of tarry matter by rapid cooling is condemned by Mr Bowditch and many eminent authorities, on the ground that thereby a proportion of light hydrocarbons are thrown down with the heavier deposit, which on another method of treatment would form part of the permanent gas and materially enrich its quality. A system of treating gas has accordingly been introduced by Messrs Aitken & Young, in which the gas, kept at a high temperature, is carried from the retorts into an apparatus termed an analyser, which consists of an enclosed series of trays and chambers arranged in vertical series, in principle like a Coffey still, the lower portion of which is artificially heated. In action the analyser separates the heavier carbonaceous part of the tarry matter in the lower part or chambers, and as the gas gradually ascends from one tray or tier to another, it is at once cool. ing and depositing increasingly lighter fluids, while it is meeting and being subjected to the purifying action of the light hydrocarbons already deposited. Thus on entering the analyser it meets, at a high temperature, heavy tar deposits, and it passes out of the apparatus cooled down to nearly atmospheric temperature after being in contact with the lightest fluid hydrocarbons.

Exhaustion.-To the subsequent progress of the gas considerable obstructions are interposed in connexion with its further purification and storing in the gas-holders, and the result of which would be that, were it not artificially propelled, there would be a pressure in the retort equal to the amount of the resistance the gas meets with in its onward progress. The relief of this back pressure not only improves the quality of the gas, but also increases its amount by about 10 per cent. Among the numerous methods of exhaustion which have been proposed since the operation was first introduced in 1839, there are several rotary exhausters, having more or less of a fan action, and recently an apparatus on the principle of a Giffard's injector has been introduced, chiefly in Continental works. A most efficient form is found in the piston exhauster, a kind of pumping engine with slide valves, which exhausts the gas in both the upward and the downward strokes of its piston. The action of the exhauster is controlled by a governor, which back passes a proportion of the gas when the apparatus is working too fast for the rate of production in the retorts; and "pass by" valves are arranged to carry the gas onward without passing through the exhauster should it cease to work from accident or any other cause.

Purification. The operations embraced under this head have for their object the removal from the gas of ammonia, sulphuretted hydrogen, and carbonic acid as the main impurities, with smaller proportions of other sulphuric and of cyanogen compounds.

The agencies adopted are partly mechanical and partly chemical, the separation of the ammonia being first effected in the "scrubber," from which the gas passes on to complete its purification in the "purifiers." In early times the purifying was performed in a single operation by the use of milk of lime in the wet purifier, a form of apparatus still in use where wet purifying is permissible.

The Wet Purifier.-This apparatus was supplied with a cream of lime and water, but, although it was a most efficient purifying agent, the ammonia now of so much value was lost by its use, and the "blue billy," as the saturated liquid holding the impurities was termed, created an intolerable nuisance, and could be in no harmless way got rid of. Except in small works, wet purifying is not now practised.

The Scrubber.-The object sought in an ordinary scrubber is to cause a large amount of gas to come in contact with the smallest possible quantity of water, so as at once to dissolve out ammoniacal gases, which are exceedingly

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and down the other, and from the top a constant small stream of weak ammoniacal liquor trickles down. Such a scrubber, it is stated, is subject to clogging by deposits of tar, and equally efficient work is done without that drawback by an apparatus in which perforated iron plates occupy the place of the coke, and in the Livesey scrubber layers of thiu deal boards are employed. These boards are set in tiers perpendicularly, slightly crossing each other, with about of an inch between each tier. Anderson's washer is a form of scrubber recently introduced, in which the interior is occupied with a series of rotating whalebone brushes, which dip into troughs of ammoniacal liquor, and in their revolution meet and agitate the gas in its passage upwards through the tower or column. The scrubber shown in section and plan in figs. 9 and 10 is a form introduced by Mr James Hislop. It contains 10 tiers of trays of cast iron, perforated with 7-inch holes at a distance of 2 inches from centre to centre. The gas passes upwards through these, meeting in its course a shower of ammoniacal liquor pumped up and distributed by the rose arrangement shown in fig. 9. The bottom part of the scrubber, to the height of the first course of plates, is filled with liquor, which is repumped till it reaches the strength desired for the manufacturer of ammonia sulphate.

The Purifiers.-The ordinary lime purifier, by which sulphuretted hydrogen and carbonic acid are abstracted from the gas, consists of a large rectangular vessel seen in section in fig. 11. Internally it is occupied with ranges

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E

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FIG. 11. Section of Lime Purifier.

of wooden trays or sieves A, made in the form of grids of -inch wood, with about half an inch between the bars. These are covered with slightly moistened slaked lime B to the depth of about 6 inches, and from three to six tiers of such sieves are ranged in each purifier. The gas enters at the bottom by a tube C, the mouth or inlet being protected from lime falling into it by a cover D, and it forces its way upward through all the trays till, reaching the lid or cover E, it descends by an internal pocket F to the exit tube G, which leads to the next purifier. The edges of the lid dip into an external water seal or lute H whereby the gas is prevented from escaping. The purifiers are generally arranged in sets of four, three being in use, through which the gas passes in succession while the fourth is being renewed; and to control the course of the gas current among the purifiers, the following ingenious arrangement of centre valves and pipes was devised by Mr Malam (fig. 12).

It has a cover fitting within it in such a way as to communicate with the pipe a and either of the four inlet pipes, and also to communicate between one of the outlet pipes and the pipe h, which carries off the purified gas. The inlet pipes, b, d, f, admit the gas from the central case to the bottom of the purifiers; and the outlet pipes, c, e, g, return the gas from the purifiers back to the case, after it has passed up through the layers of lime, and descended at the back of a partition plate in each purifier to the outlet pipes at the bottom. a is the main inlet pipe for conveying the gas from the scrubber or the condenser, and h is the main outlet pipe for conveying the gas to the gasholder. The central cylinder contains water to the depth of 10 inches, and the ten pipes rise up through the bottom to the height of 12 inches, so that the mouth of each is

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