DISTILLATION or THE COAL.—The coal used for the manufacture of gas contains as its elementary constituents, carbon, oxygen, hydrogen, sulphur, and nitrogen, together with a small but varying quantity of mineral matter. When large masses of such coal are subjected to destructive distillation in heated retorts, a variety of hydrocarbons are formed, some solid, some liquid, and some gaseous ; while, at the completion of the distillation, a large percentage of the original carbon is left in the retorts in the form of coke, which, in addition to carbon, contains the whole of the mineral constituents of the coal, and a portion of the sulphur. The character of the hydrocarbons produced depends principally upon the temperature to which the coal is exposed. At a low red heat, liquid and solid hydrocarbons are produced in large quantity, with comparatively little gas, which, how ever, is of high illuminating value. At a stronger heat, the amount of permanent gases formed increases, and the quantity of liquid products decreases in proportion ; while, at a full cherry-red heat, the liquid products are small in quantity and the yield of permanent gases is very large. The chemical changes which occur during the distillation are somewhat complex, especially when an elevated temperature is employed. The oxygen of the coal unites with a portion of the hydrogen to form water which is evolved in the form of vapour ; part of this water-vapour passes away unchanged, and part is decomposed by contact with carbon, carbon dioxide carbon monoxide (CO), and free hydrogen being formed. The greater part of the hydrogen of the original coal which has not entered ioto combination with oxygen, passes off, partly in union with carbon, as various solid, liquid, and gaseous hydrocarbons, and partly in the free state. The nitrogen is evolved, chiefly in union with hydrogen, as ammonia (N11,), and combined with carbon as cyanogen (ON), while the sulphur passes off mainly as sulphuretted hydrogen a minor quantity forming carbon disulphide and other compounds, whose exact nature and composition is not known.
With regard to the hydrocarbons produced from the destructive distillation of large masses of coal in closed retorts, there is little doubt that a large percentage of the compounds first generated are in great measure decomposed before the products leave the retort. It is well known that the higher the temperature to which organic substances are exposed, the simpler are the products of decomposition which are formed ; while at comparatively low temperatures, compounds of a more complex nature are produced. Thus, when a mass of coal is placed in a heated retort, the material is some time in acquiring the same temperature as that to which it is exposed. The exterior of the mass comes first under the influence of the heat, and the produced gas, if quickly removed from contact with the heated surfaces, is of high illuminating value, and rich in olefiant gas and other hydrocarbons containing a high percentage of carbon. After some time, the exterior of the
mass of coal becomes thoroughly charred, its temperature equals that of the retort, and it is completely decomposed ; the interior of the mass is, however, at this stage, still at a comparatively low temperature, and, as the heat travels further and further inwards, the hydrocarbons which are first produced, having to pass through the surrounding casing of heated carbon, become more or less decomposed in transit, a deposition of carbon taking place, and compounds containing a larger proportion of hydrogen and less of carbon being produced. At this stage of the distillation, the gaseous products contain much marsh-gas (CHO. As the whole bulk of the coal becomes more and more heated, the hydrocarbons generated in the interior of the mass have to traverse a gradually increasing layer of spent carbonaceous material at a high temperature, and their decomposition becomes more and more complete, until at the close of the distillation, the evolved gas is nearly pure hydrogen. It will, therefore, be clearly understood why the richest gas, that is to say the gas which contains the greatest proportiou of carbon, and possesses the highest illuminating value, is produced during the first period of the distillation ; also why the gas becomes poorer and poorer in light-giving constituents as the distillation progresses, until towards the completion of the process, the gaseous products are altogether destitute of illuminating power. It follows also that the method at present iu use for the production of illuminating-gas is, from a scientific point of view, unsatisfactory in the extreme, as a large proportion of those constituents which possess the greatest illuminating value are decomposed during the carbonization of the coal. Better results would no doubt be obtained by the use of large flat-bottomed retorts, in which the coal could be rapidly carbonized in thin layers ; but there are practical objections to the use of such vessels. The perfection of carbonization would be attained by the use of a mechanical process, by which the coal in a moderately fine state of division could be introduced into a heated retort, and then be rapidly carbonized, the spent material being discharged at the opposite extremity. Such a process was patented some years since by Porter and Lane, and the gas produced was superior in illuminating qualities to that made by the ordinary method ; but the scheme was not commercially successful, owing, it is said, to two causes :—Firstly, the working parts could not be maintained intact for any length of time at the high temperature which it was necessary to employ; and, secondly, the coke produced was more or less broken and friable, and consequontly of less commercial value than that made in the ordinary way.
Having explained the actions which occur during the destructive distillation of the coal, the next part of the subject to be dealt with is the character and subsequent treatment of the crude gas.