Modern Gasworks Practice

358 MODERN GASWORKS PRACTICE THE CONSTITUTION OF COAL Coal, so far as its Constitution is concerned, is still largely a mystery, and new light is constantly being shed on the final mode of existence of tJic degradation products of the original vegetation. Recent investigation has mainly consisted of attempts to find out by extraction with. suitable solvents what coal consists of. In this way it has been established that the majority of coals contain constituents, one of which. is soluble in pyridene, and which is of a resinous character, while the other is insoluble. The insoluble portion is of a humus nature, and appears to be a degradation product of the cellulose contained in the original vegetation. The resinous portion, on the other hånd, accounts for the coking and swelling properties of coal, and is, therefore, of main interest to the gas engineer. Of the earlier investigators, Lewes1 suggested that the coal substance consisted of four fundamental bases which he denoted as (a) carbon residuum, (b) humus bodies, (c) resin bodies, and (d) hydrocarbons. He stated that in general the various types of coal owe their varying characteristics to the proportions in. which. these substances are present, and each of the constituents has its own distinguishable products of decomposition when subjected to distillation. For instance, the ability to form coke and to yield a high proportion of gaseous products depends upon whether the hydrocarbons and resin bodies are in excess (in which case the coal is of the gas-making type), or whether the carbon residuum or humus bodies are present in the greater proportion, when a non-caking coal of an anthracite nature results. In faet, as the anthracite stage is approached the constituerits are composed largely of the carbon residuum basis, the remainder consisting chiefly of resin compounds. Lewes 2 says tliat for a coal to have the property of caking or “ coking,” the resin bodies and hydrocarbons have togetlier to be present in the proportion of at least 50 per cent. In the ordinary type of gas coal he says tliat the four constituents are present in approximately the following proportions :— Carbon residuum ......... 30 per cent. Humus bodies ......... 20 „ Resin bodies .......... 25 „ Hydrocarbons .......... 25 „ With an anthracite coal, on the other hånd; the distribution is given as foliows:— Carbon residuum ......... 72 per cent. Resin bodies .......... 28 „ In this case it will be seen that the high calorific power of the anthracite coals is cliiefly dependent upon the entire elimination of the lowheating-power constituents. For instance, the humus bodies (which, on lieating. account largely for water vapour, carbonic acid, and carbonic oxide, having a comparatively low calorific power) give way to resins and carbon. It is the humus which accounts largely for the low heating valne of the lignites. The approximate calorific powers of the four fundamental substances may be taken as follows :— 1 The Garbonization of Goal, by V. B. Lewes. 2 Loc. tit.