Coal gas

“ PART VIII. Purifying Apparatus, or Lime Machine. Coal gas, even as obtained from the best species of coal, must be rendered pure before it is fit for the purpose of illumination. The gas in its crude state always contains a portion of sulphuretted hydrogen and carbonic acid; and when burnt, although its illuminating power is greater in an impure than in a pure state, it produces an oppressive and suffocating odour, which is speedily perceptible in confined places. The gaseous product evolved during its combustion, blackens paint and tarnishes metallic bodies ”

“ Half a cubic foot of coal gas, obtained in the ordinary way of manufacturing coal gas, from Newcastle coal, is equal in illuminating power and duration of time, to the light produced by a tallow candle six in the pound, burning for one hour, and as such a tallow candle lasts five hours, therefore fifteen cubic feet of coal gas, are equal in value with regard to illuminating power to one pound of candles. ”

“ From experiments made on this subject, I am justified in stating that one half of the quantity of quick-lime that is required for the purification of coal gas in the ordinary way, is sufficient, if the column of the liquid opposed to the gas is raised to twenty inches high, nor is the evolution of the gas in any degree retarded under such a pressure. ”

“ Every body who has made the trial on a large scale, is convinced as already stated, that the best form of the retort for manufacturing coal gas where the process is conducted on the plan of decomposing coal in masses or layers of from four to eight inches in thickness, is a cylinder six and a half feet long, and one foot in diameter, and accordingly retorts of this shape and dimensions are now used in all the best regulated gas establishments in the metropolis. ”

“ It is partly the impure coal gas evolved after the fresh charge of coal which originates the smokes, when not properly supplied with air; but it is a very mixed question. When a fresh charge of coal is put upon the fire, a great quantity of evaporable matter, which would be called impure coal gas according to the language of the question, is produced; and as that matter travels on in the heated place, if there be a sufficient supply of air, both the hydrogen and the carbon are entirely burnt. ”

“ The most primitive method of making coal gas on a small scale is to fill a tin—which must have folded, not soldered, joints—with small coal, punch a hole in the bottom, and place it lid downwards in the fire. ”

“ In America the improvement is more rapid and more general.
For luminiferous purposes we have seen that there is no comparison between the consumption of crude coal and that of coal-gas. Allowing the mean proportion of 10,000 cubic feet of gas to the ton of coal which we have before taken, the consumption of an ordinary gas-burner, whether an argand or a fish-tail, is about five cubic feet per hour, giving a light of from twelve to sixteen candles, according to the richness of the gas. ”

“ There is perhaps no subject in the art of manufacturing coal gas, on which practical men are less agreed, than they are on the temperature most economically to be employed for the production of coal gas in the large way. It must be sufficiently evident, that cast-iron retorts, when worked at a low temperature, will last longer, than when exposed to higher degrees of heat. ”

“ When coal gas was first employed during the early part of this century as an illuminating agent, the low pitch of the old fashioned rooms, and the excess of impurities in the gas, rendered it imperative that the products of combustion of the sulphur-laden gas should be conducted from the apartment, and for this purpose arrangements of tubes with funnel shaped openings were suspended over the burners. ”

“ Coal-gas is a mixture of various compounds of the two elementary bodies, hydrogen and carbon; and when the gas burns, these substances are respectively uniting with the oxygen of the air, producing water and carbonic acid gas. Now, when coal-gas is burnt in the ordinary manner as a source of light, the supply of oxygen is too small to admit of the complete combustion of all its constituents ”

“ The crude coal gas contains moreover sulphuretted hydrogen, whose combustion for light would exhale an offensive sulphureous odour, that ought to be got rid of as much as possible. Carbonic acid and carbonic oxide gases, generated at first from the decomposition of the steam by the ignited coal, enfeeble the illuminating power of the gas, and should be removed. The disengagement of gas in the retorts is never uniform, but varies with the degree of heat to which they are exposed ”

“ Coal gas, as it leaves the retort, has many impurities. It is accompanied with about 3 its weight of coal tar, 1/2 its weight of H2O vapor, 1/50 NH3, 1/20 CO2, 1/20 to 1/50 H2S, 1/300 to 1/600 S in other forms. The tar is mostly taken out at the hydraulic main, which also withdraws some H2O with other impurities in solution. The condensers remove the rest of the tar, and the H2O, except what is necessary to saturate the gas. At the main, the condensers, and the washers, NH3 is abstracted, CO2 and H2S are much reduced, and the other S compounds are diminished. ”

“ 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. ”

“ But in many cases the greater expense of the coal gas is more than compensated by its easy distribution, the facility and cleanliness of its application, the general freedom from the mechanical loss, unavoidable in the case of coal fires, the prevention of black smoke and so forth. The following table shows the average composition of coal gas by volume and weight, together with the heat developed by its single constituents, the latter being expressed in kilogram-calories per cub. ”

“ In these circumstances, the coal gas, or other gas, supposed to be generated in the slightly heated mass beneath, cannot escape destruction in passing up through the bright open flame of the oven. As the coking of the coal advances most slowly and regularly from the top of the heap to the bottom, only one layer is affected at a time, and in succession downwards, while the surface is always covered with a stratum of redhot cinders, ready to consume every particle of carburetted or sulphuretted hydrogen gases which may escape from below. ”

“ The carburetted hidrogen, or coal-gas, may be freed from the non-inflammable gases, and afterwards forced in streams out of small appertures, which, when lighted, may serve as the flame of a candle to illuminate a room or any other place. It is thus, that from pit-coal a native production of this country, we may procure a pure, lasting, and copious light ”

“ But meanwhile many valuable experiments as to its production and adaptation were going on. The most productive source of illuminating gas was found to be bituminous coal. Though gas could be produced by distillation from other substances, such as shale, lignite, petroleum, water, turf, resins, oils, and fats, none could compete in quality, quantity, and economy with what is known as ordinary coal gas, at least, not until the time came, quite late in the century, when it was found that non-luminous gases, such as water gas, could be rendered luminous by impregnating them with hydrocarbon vapor. ”

“ From what has been stated in the preceding pages it becomes obvious, that a substance yielding an artificial light may be obtained from common coal in immense quantities. The attempt to derive advantage from so valuable a discovery is surely no idle speculation. Let us therefore now consider to what objects of public and private utility this mode of procuring [100] light may be applied with effect. It is obvious that coal-gas may be preserved in a reservoir for any length of time and that it may be conveyed by means of tubes to any distance flowing equably and regularly like water. ”

“ The fact that coal gas of an illuminating power of from 14 to 16 candles can be made from the ordinary gas coal at a fairly low rate, while every candle power added to the gas increases the cost in an enormous and rapidly growing ratio, has, from the earliest days of the gas industry, caused the attention of inventors to be turned to the enrichment of coal gas. ”

“ And if we give credit to the assertion of those workmen, whose business it is to manufacture a given quantity of gas by means of a certain quantity of coal delivered to them, it would appear that coal which affords gas abounding in sulphuretted hydrogen, is the kind of coal most easily to be decomposed. This, as far as it regards the decomposition of Scotch Splent or cannel coal, is certainly true. No species of coal affords gas at a lower temperature, and of none is the gaseous product more loaded with sulphuretted hydrogen gas. ”

“ One pound of coal, on a large scale, affords only 31/2 cubic feet of gas. A gas jet, which consumes half a cubic foot per hour, gives a steady light equal to that of a candle of the above-mentioned size.
The cannel coal, known in Scotland by the name of parrot coal, is very inflammable, takes fire immediately, and produces a brilliant flame. It is used by the poor as a substitute for candles. This coal, we have seen, furnishes an abundance of carburetted hydrogen gas. ”

“ As far as the gas-making properties of the first three are concerned, the relative proportions of carbon and volatile products are much the same. Everybody knows a piece of cannel coal when it is seen, how it appears almost to have been once in a molten condition, and how it breaks with a conchoidal fracture, as opposed to the cleavage of bituminous coal into thin layers; and, most apparent and most noticeable of all, how it does not soil the hands after the manner of ordinary coal. ”

“ The art of purifying coal gas, has at length however, been carried to such a perfection, that every possibility of a disagreeable odour arising from its combustion has been wholly removed, in all cases where attention is paid to the perfect combustion of the gas, by keeping the flame of the same of a proper magnitude. ”

“ The temperature necessary for the decomposition of different kinds of coal, varies. Some species of coal are more readily decomposed, and require a less portion of fuel than others; they yield up their maximum quantity of gas, in an almost equal stream from beginning to end, and no extraordinary increase of temperature is required to terminate the distillatory process. Other kinds of coal require a different treatment ”

“ Royal Society, V. XLI. so long ago as the year 1739, is recorded a paper, exhibiting an account of some experiments made by Dr. James Clayton, from which it appears that the inflammable nature of coal-gas was then already known. Dr. Clayton having distilled Newcastle coal, obtained, as products of the process, an aqueous fluid, a black oil, and an inflammable gas, which he caught in bladders, and by pricking these he was enabled to inflame the gas at pleasure. ”

“ It must be obvious, that in proportion as the mass of coal in the retort becomes carbonized or converted into coke, the exterior surface becomes a gradually increasing obstacle to the action of the heat upon the interior or central part of the coal remaining to be decomposed. The heat required on that account must be more intense, and kept up to no purpose, and the extrication of gas becomes slower and slower, as the operation proceeds. ”

“ When we consider the before mentioned varieties of coal in an economical point of view, as fuel to be used in the gas-light process, for heating the retorts, it appears from a series of experiments that have been made under my direction, that the second class of coal comprehending those varieties which contain a larger quantity of carbon than bitumen (p. 45,) afford the most economical fuel, they act less on the grate bars, and fire bricks of the furnace than those varieties which take fire easily and burn briskly with a strong blaze. ”

“ The gas from the gas-works is the hydrogen of the coal separated from the carbon. When we heat it with a match to set it to uniting with oxygen, we have nothing but a flame. You know the coal is heated in air-tight retorts; it is heated hot enough to burn, but it can't burn because there is no oxygen for it to unite with; but the heat causes the hydrogen to separate from the carbon, and then it finds its way out through the opening in the retort into the pipes, and when it reaches the air at the end of a pipe, you can heat it a little with a match, and it will begin to unite with the oxygen. ”

“ Were coal gas exclusively employed in such establishments, the fixed position which can be given to the burners, and the absence of all danger from sparks must give a degree of security to those places from fire, far beyond what they at present possess, even when superintended with the greatest possible caution and fidelity. ”

“ Mineral coal is fossilized vegetable matter; anthracite has had most of the volatile hydro-carbons removed by distillation in the earth; bituminous and cannel coals retain them. These latter coals are distilled, and furnish us illuminating gas.
Experiment 129.—Put into a t.t. 20 g. of cannel coal in fine pieces. Heat, and collect the gas over H2O. Test its combustibility. Notice any impurities, such as tar, adhering to the sides of the t.t., or of the receiver after combustion. Try to ignite a piece of cannel coal by holding it in a Bunsen flame. ”