« PreviousContinue »
A TEXT-BOOK OF GAS MANUFACTURE
T will be advisable at the outset, to give a brief general
sketch of what goes on in a gas-works, as this will greatly help the student in grasping the connection between the various apparatus which go to form a gas-making plant. The latter is shown in the frontispiece.
In the manufacture of coal-gas, as ordinarily practised, bituminous coal is placed in charges of from two to four hundredweights at a time, in fire-clay vessels termed retorts, capable of being hermetically sealed, and which have been previously heated, by means of a furnace, to a suitable gasmaking temperature. The charge is allowed to remain in the retorts for a period of from four to six hours, at the expiration of which time the coal will have given off all the gas it is capable of evolving, and have become converted into coke.
The gas as it comes from the retorts passes up through a pipe leading from the mouthpiece, known as the ascension pipe, and is conducted by this into what is known as the hydraulic main, where a portion of the tarry matters become deposited. The hydraulic main extends along the whole length of the retort benches, receiving the dip pipes of the several benches of retorts.
The hydraulic main serves a twofold purpose; it acts, in the first place, as a means of conveying away the gas from the place where it is generated; and, in the second place, it acts as an automatic contrivance for sealing the dip pipes, as they are termed, and thus preventing any escape of
gas during the charging and drawing of the retorts; this is effected by keeping the hydraulic main always about half full of liquid, and allowing the dip pipes to "dip " a short distance in the same. From the hydraulic main the gas travels to the condensers, where, by traversing a series of pipes exposed to the cooling action of the air, the gas becomes cooled, and deposits the tar and aqueous vapour previously held in suspension.
The tar and condensed aqueous vapour deposited by the action of the condensers, are run off by a suitable pipe into a receptacle known as the“ tar well,” the capacity of which depends upon the amount of coal carbonized. From the condenser the gas passes to the exhauster, the function cf the last-mentioned piece of apparatus being, to remove the gas
from the heated retorts as fast as it is produced, and, at the same time, to propel it onward with sufficient force to enable it to pass through the materials used in its purification, and from thence to the holder. It is necessary to remove the gas from the retorts as quickly as possible, for two reasons; both of which depend upon the injurious effect caused by the gas being subjected to excessive pressure in the retorts, which operates prejudicially, both by causing a partial decomposition of the richest portions of the gas, with a consequent loss of illuminating value, and also by causing leakage through the porous walls of the retort.
From the exhauster the gas enters the apparatus intended for the abstraction of the ammonia in the same, consisting usually of the scrubber or washer, or a combination of the two; the last-mentioned apparatus in this case being placed first.
The function of both the scrubber and washer, is the removal of the whole of the ammonia and some portion of the carbonic acid and sulphuretted hydrogen contained in the gas, which is effected in the case of the washer, by bubbling it through water, and in that of the scrubber, by bringing the gas into contact with a large extent of surface kept wet by water. From the scrubbers the gas passes to the vessels known as the purifiers, which are filled with lime and oxide of iron, by whose action the remaining impurities, consisting of carbonic acid and sulphuretted hydrogen, are got rid of, the gas being then in a sufficiently pure condition to be supplied to the consumer.
From the purifiers the gas passes to the station meter, where the amount of being made is registered, and from thence it enters the gas-holders, where it is stored for use.
From the holder the gas passes to the district for consumption, the necessary pressure being regulated by a piece of apparatus known as the station governor, placed at the outlet of the holder.
THE CHARACTERISTIC PROPERTIES OF THE VARIOUS KINDS
OF COAL, AND THEIR VALUE FOR GAS-MAKING PUR
THEN vegetable matter decays, in absence of air, it
undergoes a similar change as when it is heated, Water, CO,, and marsh-gas are given off, and the material remaining becomes richer in carbon; the fact that CO, and CH,, are among the products of decomposition from vegetable matter, is proved by the last-named substances being found in a highly compressed condition in the coal. measures of the present day; and it is the marsh-gas, or so-called fire-damp, which causes the colliery explosions with which we are unfortunately so familiar. It is by the decomposition of decaying vegetable matter that the various kinds of coals, lignites, and peats have been formed.
The conversion of the vast mass of vegetable matter from which coal is derived, into the latter substance, has been the work of ages, but the exact causes, chemical and physical, which have brought about this change are un. known. It is probable, however, that the first change would be the loss of moisture, and the fermentation of albuminous matter and gum. This would bring about a inolecular change; there would be a rise in temperature, but an absence of a supply of air would check too rapid carbonization.
The mass of vegetation, it must be recollected, would at this time be also subjected to great pressure. The oxygen and hydrogen being most subject to the effect of this