The Story of a Piece of Coal: What It Is, Whence It Comes, and Whither It Goes

by Edward A. Martin

The second of these portions of the flame corresponds with the "reducing" flame of the blow-pipe, since this part, if turned upon an oxide, will reduce it, i.e., abstract its oxygen from it. This part also corresponds with the jet of the Bunsen burner, when the holes are closed by which otherwise air would mingle with the gas, or with the flame from a gas-stove when the gas ignites beneath the proper igniting-jets, and which gives consequently a white or yellow flame.

The third portion, on the other hand, corresponds with the "oxidising" flame of the blow-pipe, since it gives up oxygen to bodies that are thirsting for it. This also corresponds with the ordinary blue flame of the Bunsen burner, and with the blue flame of gas-stoves where heat, and not light, is required, the blue flame in both cases being caused by the admixture of air with the gas.

Thus, in order that gas may give the best illumination, we must increase the yellow or white space of carbon particles at a white heat, and a burner that will do this, and at the same time hold the balance so that unconsumed particles of carbon shall not escape in the way of smoke, will give the most successful illuminating results. With this end in view the addition of albo-carbon to a bulb in the gas-pipe has proved very successful, and the incandescent gas-jet is constructed on exactly the same chemical principle. The invention of burners which brought about this desirable end has doubtless not been without effect in acting as a powerful obstacle to the widespread introduction of the electric light.

Without entering into details of the manufacture of gas, it will be as well just to glance at the principal parts of the apparatus used.

The gasometer, as it has erroneously been called, is a familiar object to most people, not only to sight but unfortunately also to the organs of smell. It is in reality of course only the gas-holder, in which the final product of distillation of the coal is stored, and from which the gas immediately passes into the distributing mains.

The first, and perhaps, most important portion of the apparatus used in gas-making is the series of retorts into which the coal is placed, and from which, by the application of heat, the various volatile products distil over. These retorts are huge cast-iron vessels, encased in strong brick-work, usually five in a group, and beneath which a large furnace is kept going until the process is complete. Each retort has an iron exit pipe affixed to it, through which the gases generated by the furnace are carried off. The exit pipes all empty themselves into what is known as the hydraulic main, a long horizontal cylinder, and in this the gas begins to deposit a portion of its impurities. The immediate products of distillation are, after steam and air, gas, tar, ammoniacal liquor, sulphur in various forms, and coke, the last being left behind in the retort. In the hydraulic main some of the tar and ammoniacal liquor already begin to be deposited. The gas passes on to the condenser, which consists of a number of U-shaped pipes. Here the impurities are still further condensed out, and are collected in the tar-pit whilst the gas proceeds, still further lightened of its impurities. It may be mentioned that the temperature of the gas in the condenser is reduced to about 60° F., but below this some of the most valuable of the illuminants of coal-gas would commence to be deposited in liquid form, and care has to be taken to prevent a greater lowering of temperature. A mechanical contrivance known as the exhauster is next used, by which the gas is, amongst other things, helped forward in its onward movement through the apparatus. The gas then passes to the washers or scrubbers, a series of tall towers, from which water is allowed to fall as a fine spray, and by means of which large quantities of ammonia, sulphuretted hydrogen, carbonic acid and oxide, and cyanogen compounds, are removed. In the scrubber the water used in keeping the coke, with which it is filled, damp, absorbs these compounds, and the union of the ammonia with certain of them takes place, resulting in the formation of carbonate of ammonia (smelling salts), sulphide and sulphocyanide of ammonia.

[Illustration: FIG. 35.—Filling Retorts by Machinery.]

[Illustration: FIG. 36.—CONDENSERS.]

Hitherto the purification of the gas has been brought about by mechanical means, but the gas now enters the "purifier," in which it undergoes a further cleansing, but this time by chemical means.

[Illustration: FIG. 37.]