[34] They are frequently rendered unfit for use in three months, and at some works in two months, owing not less to the irregularity of the temperature at which they are worked, than to the carelessness of the workmen.
Only one-third part of the top and bottom plates of the rotary retort being exposed to the action of heat, are alone liable to deterioration. It is only necessary therefore that these parts of the vessel be renewed, while the other parts remain uninjured for years. The new top and bottom plates being rivetted to the old and undecayed part, without deranging the rest, the retort is rendered as good as new.
Gain in the quantity of gas.—A large increase in the quantity of gas obtained, is a natural consequence of the mode in which the decomposition of coal is effected by means of the horizontal rotary retort.
Every body knows that coal, when decomposed slowly, affords a larger quantity of tar and ammoniacal liquor, but a less quantity of gas than when decomposed rapidly.
In the former case, the formation of the proximate products which coal is capable of furnishing is effected properly; the bituminous part of the coal is developed under the most favourable circumstances.
But when coal, after being previously deprived of moisture, is very suddenly heated to a high temperature, in thin strata, and small portions at a time, so that the vaporous products instead of becoming condensed, are made to come into contact with a substance (which in this case is the roof of the retort,) kept constantly at a temperature rather higher than that at which gold, silver, and copper melts, (32°, Wedgwood, or 5237°, Fahrenheit,) a very different arrangement of principles takes place.
The greatest portion of tar which the coal is capable of furnishing, instead of being produced in a liquid form, becomes then decomposed into carburetted hydrogen, and olifiant gas. That portion of tar which escapes decomposition, is condensed in the perpendicular pipe H, fig. 2, [plate II.], or H, fig. 2, [plate VII.], and falls back again into the retort, where it is also decomposed when the coal upon which it falls comes under the process of decomposition.
Hence the quantity of tar obtained by means of horizontal rotary retorts, is very small; it seldom exceeds the proportion mentioned [page 123], when the retort is worked to the greatest advantage. This quantity is considerably diminished, when Newcastle coal, broken into pieces of the size of split pease is decomposed in strata, not exceeding two inches in thickness. The quantity of tar afforded by a chaldron of coal then amounts to thirty pounds, whilst at the same time the quality of the gas is improved; because coal tar furnishes olifiant gas, which the coal alone, when distilled by means of cylindrical or other shaped cast-iron retorts of the usual form, cannot produce, or at least but in a small quantity. One gallon of coal tar yields 15 cubic feet of olifiant gas, which greatly increases the illuminating power of the carburetted hydrogen.
From what has been so far stated, it will be understood why one chaldron of Newcastle coal, when decomposed by the new process, may readily be made to produce from 15,000 to 18,000 cubic feet of gas and upwards, whereas the same quantity of coal, if decomposed by the old method, yields only upon an average 10,000 cubic feet of gas.[35]
[35] The experiments exhibiting the maximum quantity of gas obtainable from coal, see [page 44], were made with the horizontal rotary retorts at the Royal Mint. Similar results have also been obtained at the Westminster Gas-Works.