PART XV.
Gas from Coal Tar.
Although the tar which forms one of the products obtained from the decomposition of pit coal, in the manufacture of coal gas, has become an article of commerce, being found applicable to most of those purposes to which vegetable tar has hitherto been used, it appears from experiments made on a large scale, that instead of thus disposing of the coal tar, it is more profitable, under certain circumstances, to submit this substance to a destructive distillation, for the purpose of obtaining from it carburetted hydrogen gas, which it is capable of affording, not only in abundance, but of a superior quality.
The chief circumstances which must determine the manufacturer of coal gas in this respect, is the price at which he can sell the coke produced in his establishment. If the price of this article is high, if he finds a ready market for coke, there is every reason to believe, that the manufacturer will find it more to his advantage to dispose of the tar, and to manufacture gas from coal alone, in order to increase his store of coke. But if coke happens to be at a low price, and not disposable with advantage, the manufacturer will do well to make the coal go as far as possible in the production of gas, and under such circumstances he will keep and convert the tar into gas, thus consuming less coal and having less of the burdensome article, coke, to dispose of.
The profit however, to be gained from the sale of coke, must be both certain and considerable, to induce a preference for the former course; because the decomposition of coal tar, besides superseding a proportionate quantity of coal, is attended with several other very tempting advantages.
From experiments lately made in the metropolis on this subject, in which I have been engaged, it appears that in all large gas light establishments, where the quantity of coal tar rapidly accumulates, and must be got rid of, and in all places where the tar cannot be sold for more than four shillings the hundred weight, it will be certainly advantageous to the manufacturer to decompose the tar for the production of carburetted hydrogen gas.
The price of coal cannot effect the operation, because where coal bears a high price, the manufacturer of the tar gas, will diminish the quantity of coal which he would otherwise be called upon to employ for the production of the requisite quantity of gas. And in places where coal is cheap, the decomposition of the tar will be attended with less expence.
The carburetted hydrogen gas produced from coal tar, possesses a greater illuminating power than the gas obtained from coal.[53] It consists chiefly of supercarburetted hydrogen or olifiant gas, and a less quantity of it is of course sufficient.
[53] Vegetable tar, also affords carburetted hydrogen gas in abundance, and this no doubt might be employed to great advantage for the production of artificial light in places where it is cheap. 212 pounds of the most viscid Swedish tar, produce 1484 cubic feet of carburetted hydrogen, (or seven cubic feet to one pound of tar,) the illuminating power of this gas is equal to the gas obtained from pit coal.
The gas thus obtained, is purified likewise with far greater facility, taking only one hundred and twentieth part of the quantity of quicklime which is required for the purification of carburetted hydrogen obtained from pit coal. The apparatus for the production of carburetted hydrogen from coal tar, is moreover less bulky, less expensive, and less complicated; and it can be managed by fewer workmen. And as the combined result of these several advantages, it is obvious, that by the substitution of coal tar, the new mode of lighting by gas can be pursued on a smaller scale; which it can never be with any profit, where coal itself is immediately employed for the production of the gas.
The apparatus employed by Mr. Clegg, for the distillation of tar, is extremely simple. It consists of two hollow cast iron cylinders, twelve inches in diameter, and nine feet long, furnished with moveable lids or mouth pieces, and joined together at the extremity opposite to the mouth piece. These cylinders are fixed in a brick furnace, so that each inclines eleven degrees, one above and the other below the horizontal base of the furnace.
When the apparatus has acquired a dull red heat, the coal tar is suffered to flow into the upper cylinder, by small portions at a time.
The tar is contained in a closed vessel, situated at any convenient place above the apparatus. It has a small aperture for the admission of air. But as a sufficient small quantity of viscid tar does not flow freely in a thin stream, a larger portion than is wanted, is made to flow first into a a small box, upon the apex of a pyramid which divides the stream, so that the excess runs off by a waste pipe, whilst a due quantity only is conveyed into the retort where it is decomposed.
This apparatus[54] therefore differs only from the apparatus described in the Journal of Science and the Arts, 1816, No. II., p. 282; that the cylinders may be detached, for cleaning them out more conveniently.
[54] Now erecting at Birmingham.
The following statement exhibits the result of a series of experiments, made (1816,) at the Westminster Chartered Gas Light Establishment,[55] for the purpose of ascertaining how far, and under what circumstances the decomposition of coal tar is a measure of economy.
[55] Communicated by Mr. T. S. Peckston.
Two tar retorts worked seven hours, produced 3054 cubic feet of gas. The quantity of tar decomposed, amounted to 354 lb. therefore 8 cubic feet of gas, (omitting fractions), were obtained from 1 lb. of tar.
Two tar retorts, worked nine hours, produced 4591 cubic feet of gas. The quantity of tar decomposed, was 525 lb. Hence 1 lb. of tar yielded nearly 83⁄4 cubic feet of gas.
Fifteen cwt. 16 lb. of tar, produced 16,112 cubic feet of gas, = 91⁄2 cubic feet of gas, to 1 lb. of tar.
Five cwt. 3 quarters, 22 lb. of tar, produced 6660 cubic feet of gas, = 10 cubic feet of gas to 1 lb. of tar.
Five cwt. 17 lb. of tar, produced 5193 cubic feet of gas, = 9 cubic feet of gas to 1 lb. of tar.
One cwt. 81 lb. of tar, produced 1737 cubic feet of gas, = 9 cubic feet of gas to 1 lb. of tar.
One cwt. 30 lb. of tar, produced 13131⁄2 cubic feet of gas, = 8 cubic feet of gas to 1 lb. of tar.
Five cwt. of tar, produced 5880 cubic feet of gas, = 101⁄2 cubic feet of gas to 1 lb. of tar.
Two cwt. of tar, produced 2072 cubic feet of gas, = 91⁄2 cubic feet of gas to 1 lb. of tar.
Three cwt. 18 lb. of tar, produced 3717 cubic feet of gas, = 101⁄2 cubic feet of gas to 1 lb. of tar.
Two cwt. 6 lb. of tar, produced 22421⁄2 cubic feet of gas, = 93⁄4 cubic feet of gas to 1 lb. of tar.
From the preceding operations it becomes obvious, that 91⁄2 cubic feet of gas, were obtained in the large way from 1 lb. of tar. But this proportion appears evidently too small, our own operations assign fifteen cubic feet of gas to one pound of tar. Professor Brande, obtained eighteen cubic feet[56] from the same quantity of tar.
[56] Journal of Science and the Arts, 1816, No. II. p. 282.
Gas from Oil.
“Messrs. J. and P. Taylor[57] are the first persons who have resorted to oil as a substance from which gas for illumination could be easily and cheaply prepared; and in the construction of a convenient apparatus for the decomposition of this body, they have fully shewn its numerous advantages over coal, while they have afforded the means of producing the most pure and brilliant flame from the inferior and cheap oils, which could not be used in lamps. The apparatus for the purpose is much smaller, much simpler, and yet equally effectual, with the best coal gas apparatus. The retort is a bent cast iron tube, which is heated red by a small convenient furnace, and into which oil is allowed to drop by a very ingenious apparatus; the oil is immediately volatilized, and the vapour in traversing the tube becomes perfectly decomposed. A mixture of inflammable gases, which contains a great proportion of olifiant gas passes off; it is washed by being passed through a vessel of water (which dissolves a little sebacic acid, and which seldom requires changing), and is then conducted into the gasometer.”
[57] Copied from the Journal of Science and the Arts, Vol. VI. p. 108.
“The facility and cleanliness with which gas is prepared from oil in the above manner, may be conceived from the description of the process. A small furnace is lighted, and a sufficient quantity of the commonest oil is put into a small iron vessel, a cock is turned, and the gas after passing through water in the washing vessel, goes into the gasometer. The operation may be stopped by shutting off the oil, or, to a certain extent, hastened by letting it on more freely; the small quantity of charcoal deposited in the retort is drawn out by a small rake, and the water of the washer is very rarely changed.”
“The gas prepared from oil is very superior in quality to that from coal; it cannot possibly contain sulphuretted hydrogen, or any extraneous substance; it gives a much brighter and denser flame; and it is also more effectual, i. e. a lesser quantity will supply the burner with fuel. These peculiarities are occasioned, in the first place, by the absence of sulphur from oil, and then by the gas containing more carbon in solution. As the proportion of light given out by the flame of a gaseous compound of carbon and hydrogen, is in common circumstances in proportion to the quantity of carbon present; it is evident that the gas which contains a greater proportion of olifiant gas, or supercarburetted hydrogen than coal gas, will yield a better and brighter light on combustion.”
“It is necessary, in consequence of the abundance of charcoal in solution, to supply the gas when burning with plenty of atmospheric air, for as there is more combustible matter in a certain volume of it, than in an equal volume of coal gas, it of necessity must have more oxigen for its consumption.[58] The consequence is, that less gas must be burnt in a flame of equal size, which will still possess superior brilliancy; that less is necessary for the same purpose of illumination; and that less heat will be occasioned. From five and a half to six cubical feet of coal gas are required to supply an Argand burner for an hour; two cubical feet to two and a half of that from oil, are abundantly sufficient for the same purpose.”
[58] Dr. W. Henry’s experiments gave the following result:—100 cubic inches of carburetted hydrogen from coal, require, for burning, 220 cubic inches of oxigen, and produce 100 cubic inches of carbonic acid—100 cubic inches of carburetted hydrogen gas procured from lamp oil, require 190 cubic inches of oxigen, and produce 124 cubic inches of carbonic acid,—100 cubic inches of carburetted hydrogen obtained from wax, require 280 cubic inches of oxigen, and produce 137 cubic inches of carbonic acid.
“One important advantage gained by the circumstance, that so small a quantity of this gas is necessary for burners is, that the gasometer required may be small in proportion. The gasometer is the most bulky part of a gas apparatus, and that least capable of concentration; and where-ever it is placed, it occupies room to the exclusion of every thing else. Some very ingenious attempts have been made to diminish its size and weight, as in the double gasometer,[59] and others, but without remarkable success. Here, however, where the room required to contain the gas is directly diminished, the object is so far obtained; and when that takes place to one half, or even one third, it is of very great importance. It in a great number of cases brings the size of the apparatus within what can be allowed in private houses; and in consequence of the rapidity with which the retort can be worked, the gasometer may again be reduced to a still smaller size.”
[59] This contrivance is more expensive and complicated than any of the gas holders of which a description has been given; nor is it safe, for if the slightest leak should happen in the interior vessel of the double gas holder, an explosive mixture would be formed, and dreadful consequences might follow; this can never be the case with any of the machines now in use.—Note of the Author.
“Another advantage gained by the small quantity of gas required for a flame, is the proportionate diminution of heat arising from the lights. The quantities of heat and light produced by the combustion of inflammable gases are by no means in the same constant relation to each other; one frequently increases, whilst the other diminishes; and this is eminently the case when coal gas and oil gas are burned against each other. The quantity of heat liberated is, speaking generally, as the quantity of gas consumed, and this is greatest with the coal gas; but the quantity of light is nearly as the quantity of carbon that is well burnt in the flame, and this is greatest in the oil gas.”
“The very compact state in which the apparatus necessary for the decomposition of oil can be placed, the slight degree of attention required, its certainty of action, its cleanliness, and the numerous applications which it admits of in the use of its furnace for other convenient or economical purposes, render it not only unobjectionable, but useful in manufactories and establishments; and these favourable circumstances are accompanied, not from any inferiority in the flame or increased expense, but by an improved state of the first, and saving in the latter.”
“Messrs. Taylor have shewn great ingenuity in the construction of their whole apparatus, but the washer and gasometer deserve particular notice for their remarkable simplicity also. In the washer, two planes are fixed in a box or cistern, in a direction not quite horizontal, but inclined a little in opposite directions; the planes are traversed nearly across by slips of wood or metal, fixed in an inclined position on the under surface, and which alternately touch one side of the cistern, leaving the other open and free. These planes being immersed in water, the gas is thrown in under the lowest ridge, and by its ascending power is made to traverse backward and forward along the ridges fixed on the planes, until it escapes at the highest part of the uppermost ridge. Thus, with a pressure of five or six inches of water only, it is made to pass through a distance of fourteen or sixteen feet under the surface of the fluid, and becomes well washed.”
“The smaller gasometers are made of thin plate iron, and being placed in a frame of light iron work, look more like ornamental stoves than the bulky appendages to a gas apparatus, which they supply. The larger ones are made very light, and when in pieces very portable, by being constructed of a frame of wood work, in the edges of which are deep narrow grooves; plates of iron fit into these grooves, which being caulked in and painted over, make a light and tight apparatus. These are easily put together in any place, and may therefore be introduced into a small apartment, or other confined space, where a gasometer already made up would not enter.”
For the following additional information on this subject, I am indebted to Messrs. J. and P. Taylor.
“The economy of obtaining gas for the production of light from oil, may be judged of from the following data.”
“One gallon of common whale oil, produces about ninety cubic feet of gas.[60] An argand burner required a cubic foot and a half of gas per hour; and consequently a gallon of oil when converted into gas, will supply the same burner for sixty hours. The expence of the gas at a moderate price of oil, will be, allowing for coals, labour, &c. for producing the gas, three farthings per hour, and such a burner will give a light, equal in intensity, to two argand lamps, or ten mould candles.”
[60] Our experiments produced 105 cubic feet, from one gallon of common whale oil.—Note of the Author.
“The expence of an argand oil lamp, is usually admitted to be, about three halfpence per hour. Now supposing ten candles to be burning, four to the pound (two pound and a half,) they will cost 2s. 11d. of which one-tenth part will be consumed in each hour. The cost of the tallow light is then three pence halfpenny per hour.”
“If wax candles be employed, the expence of the light equal to an oil gas burner for one hour, by the same mode of reckoning, allowing the candle to burn ten hours, and taking the price of the wax candles, at 4s. 6d. per pound, will be about 14d.”
“The comparative account will therefore stand thus:
| PENCE. | |
|---|---|
| Cost of an Argand burner, supplied with oil gas, per hour | 03⁄4 |
| Ditto of an Argand lamp, burning spermaceti oil | 3 |
| Ditto of Tallow mould candles | 31⁄2 |
| Wax candles | 14 |
“These calculations on the cost of light from oil gas, are taken at the usual price of good whale oil, but cheaper oils will answer the purpose nearly as well, and as many of these are often to be procured, the whole expence becomes materially reduced by their use.”