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nace is provided for each setting of retorts, irrespective of whether they are single or throughs. The main flue is usually placed below the setting. It will be seen that in this form of furnace both the primary and secondary air are completely under control, so that by an analysis of the producer" and "spent" gases the necessary amount of air can be easily admitted, according to requirements. The principal advantages claimed for this system of firing
A saving of 50 per cent. in fuel over the direct-fired furnace.
Saving in labour.
Increased make of gas, owing to good and regular heats. Saving in wear and tear of materials.
For further information concerning regenerator furnaces the student is referred to Graham's Construction and Working of Regenerator Furnaces" (Spon and Co.).
The amount of fuel required for heating retorts is a very important item in the economy of a gas-works. In small works, where the retorts are not kept constantly at work, it may amount to as much as 35 or 40 per cent. of the coke
produced, which is reduced in large works to about 25 per cent., which has been still further reduced by the employment of regenerative furnaces to as little as 10 lb. per 100 lb. of coal carbonized.
One of the appurtenances of a furnace is the ash-pan, which is best constructed of wrought iron. Its office is to retain the ashes and small cinders which are continually dropping through the furnace bars; the ash-pan should always be kept partly filled with water, the steam from the latter fulfilling the important function of keeping the furnace bars from becoming excessively heated. The aqueous vapour passing up continually through the incandescent fuel, becomes decomposed with the production of carbonic oxide and hydrogen, which are heating gases. Further, the steam greatly facilitates the removal of the clinker which is formed in the furnace. Clinker, it may be explained, is produced by the combination, under the influence of heat, of the inorganic or incombustible matter of the coke (the ash of the coal, as it is generally termed). This consists principally of silica, alumina, lime, iron, etc., which fuses together to form a kind of slag. This deposit chokes up the air passages between the furnace bars, and must be periodically removed, or the necessary amount of air required could not be obtained. The clinker is removed by means of a long bar, known as the "clinkering bar," and the operation of clinkering is a very arduous operation, hence the advantage of being able to remove the clinker with as much ease as possible.
The number of beds or settings to a bench of retorts, as well as the size of the retort-house, vary with the size of the works and the make of gas. It is a general custom at the present day, in the case of large works, to build the retort-houses with a stage, that is, in such a way that there are two levels; the retorts being drawn and charged from the upper of these, while the lower level, or coke-hole,
receives the coke, which, on being withdrawn from the retorts, falls through an opening in front of the settings into the coke-hole, where it is quenched.
The coke-hole thus formed effects an economy of space, and since the introduction of regenerative furnaces such an arrangement has become a necessity. The retorts are charged in small works by means of shovels, but in large works, particularly in London and its vicinity, the instrument known as "the scoop" is invariably employed. The scoop generally holds a hundredweight of coal, and in charging by means of the same three men are required, one of whom takes the cross-handle, and the other two lift the opposite end, or "nose," by means of a bar of iron fitted to the circle of the scoop, and known as a "horse," and guide the scoop into the open retort; the scoop is then pushed in as far as possible, inverted, and quickly withdrawn, when it leaves the coal in a comparatively even layer on the bottom of the retort. This operation is repeated as often as necessary. The coal is backed off the mouthpiece by means of the "backing rake," and the mouth of the retort is then quickly closed by means of the lid described later on, the whole operation being performed in something under a minute.
A great deal depends upon the manner in which the coal is deposited in the retort, as good carbonizing results can only be obtained when the charge is placed in the retort in a thin even layer; for should the coal be charged irregularly, that is, in thick and heavy, and light and thin portions alternately, then the rich illuminating gases will be expelled from the thin portion before the regular time for drawing the charge, with the result that gases of little or no illuminating value, and which in some instances are actually prejudicial, will be driven off, and probably reduce the illuminating power of the gas obtained from the charge generally, while the thick portion will probably not be pro
perly carbonized in the usual time allowed, with the result that the maximum amount of gas which is capable of being evolved from the coal will not be obtained, consequently producing a low make per ton. Further, charges of this description tend to reduce the heats of the retorts, and produce an inferior coke.
With cannel coal, or the richer descriptions of gas coals, the duration of the charge is generally either three or four hours, but with the ordinary bituminous gas coals six hours is the usual time required for carbonization. "During the first hour or so the evolution is always greatest, the gas being rich in all its constituents, but containing also a large proportion of CO2. Each succeeding hour the yield diminishes, accompanied by a very appreciable falling off in illuminating value, until towards the close of the charge the gas becomes scarcely worth collecting."
When the coal has given up all its gas the retorts are drawn. In drawing the retorts the lid is loosened a little, and a light applied, in order to avoid the slight explosion which would otherwise occur, to the detriment of the retort. The lid is then fully opened, and the coke withdrawn by means of long rakes. The rakes employed are generally made of three-quarter round iron, and are about 12 feet long, having a handle at one end; while the other end, with which the coke is withdrawn, is provided with a flattened head. In the drawing of through retorts the rakes employed are larger, and the drawing as well as charging takes place at each end simultaneously by separate gangs of men.
Plate No. 1 shows the tools employed in the retorthouse, with their respective names, as supplied by Messrs. R. and J. Dempster, Limited, of Manchester:
Stokers' drawing rakes.
Nos. 1 and 2.
There is continually being deposited on the walls of the retorts a deposit of carbon, technically known as "scurf," and this deposit is much increased when the gas is subjected to an excess of pressure in the retort, as when an exhauster is not employed; so it becomes necessary to remove this deposit of carbon periodically. This is sometimes effected by chipping it off by means of chisel bars; but there is a risk of damaging the retort by so doing. A better plan is to loosen the retort lid, and thus allow the oxygen of the air to burn away the deposit until it becomes sufficiently thin to be manageable. Clay retorts require scurfing about once a month, and they should never be allowed to continue working for too long a period without being scurfed, as the deposited carbon becomes more difficult to remove the longer it is allowed to accumulate.