Description of the Process of Manufacturing Coal Gas, for the Lighting of Streets Houses, and Public Buildings / With Elevations, Sections, and Plans of the Most Improved Sorts of Apparatus Now Employed at the Gas Works in London and the Principal Provincial Towns of Great Britain; Accompanied With Comparative Estimates, Exhibiting the Most Economical Mode of Procuring This Species of Light

by Friedrich Christian Accum

In the former case, the greater part of the essential oil and tar which the coal would have afforded is decomposed, as stated already by virtue of the high temperature to which the vapourous tar is suddenly exposed in the horizontal rotary retort, which is not the case when coal is decomposed in the retorts of the old construction.

Gain in the quantity of coke.—With the cylindrical or cast-iron retorts of the old shapes, the quantity of coke obtained from a given quantity of coal is upon an average 25 per cent. increase by measure from the best kind of Newcastle and Sunderland coal, but taking into account the waste incurred in breaking out and removing the red hot coke from the retort, which requires the application of rakers and crow bars, a considerable portion of it becomes reduced to dust or breeze, and hence no more than bulk for bulk of the coal decomposed can seldom be depended upon as the ultimate saleable quantity of coke.[36]

[36] There is a vast difference with regard to the quality as well as quantity of coke obtained from different kinds of coal. Some kinds of coal produce a species of coke which is so friable that it will hardly bear being moved from place to place without crumbling into dust, others produce coke in pieces of the size of small pebbles, while a third sort affords coke of a stony hardness.

In the new mode of carbonizing coal by means of the horizontal rotary retorts, the increase of coke is 150 per cent. by measure, so that one chaldron of Newcastle coal produces two and a half chaldron of coke—this is the quantity produced upon an average. But when the retort is worked at a temperature to produce at the rate of 18,000 cubic feet of gas from the chaldron of coal, the increase of coke by measure is 175 per cent.; in that case, the layers of coal in the coal boxes must not exceed two inches in thickness, so that the volume of coke is in the ratio of the quantity of gas produced and the rapidity and elevation of temperature at which the decomposition of the coal is effected.

The coke being withdrawn from the place where it is formed by merely turning the boxes containing it, upside down, all waste is avoided.

With respect, again, to the quality of the coke, it will be observed that when the coal is rapidly carbonized in thin layers, and has full liberty to expand freely, as in the case of the horizontal rotary retort, it affords a light and porous coke, whereas in the cylindrical, paralellopipedal, semi-cylindrical, or ellipsoidal retorts, the coke being compressed, the intense heat to which it is so long and superfluously exposed, renders it extremely dense, and of a stony hardness.

The latter sort of coke is unquestionably preferable for the smelter, and all furnace operations, standing the blast of the bellows well. But the coke produced in the new mode of operating, is better suited for the great majority of domestic purposes, kindling more readily, and making a more cheerful fire. The combustion of the dense, or as it is now called, cylinder coke, can be only kept up when used in a common grate, by a strong draft of air, and it is therefore not so well suited for fuel for domestic purposes, to make a small fire; but the coke obtained by the horizontal rotary retort, readily maintains its own combustion, even when in small masses; hence it may be used without any trouble, either in the fire-place of the cottager, or of the prince, and accordingly it bears a higher price in the market.

Directions to workmen, with regard to the management of Horizontal Rotary Retorts.

The circumstance most essential to the economical application of the horizontal rotary retort, is, as has been repeatedly stated, that the coal shall be spread in thin layers in the boxes of the retort, not exceeding from two to four inches in thickness; and it may be laid down as a general rule, that the thinner the layers, and the higher the temperature, the greater will be the proportion of gas, the greater the bulk of coke, and the smaller the quantity of tar.

The coal before it is submitted to the distillatory process, should be as dry as possible, and the more it is comminuted the better. The very refuse of the coal called slack, provided it is perfectly free from foreign matter, answers best. It should also be spread in the trays, in even layers.