GAS-MAKING AND OTHER COALS
359
Resin bodies Hydrocarbons . Carbon residuum Humus bodies
16,400
15,900
14,600
13,300
B.Th.U.
per Ib.
From the above it will be seen that the maximum calorific power is yielded when .the presen.ce of the humus bodies can no longer be traced, as is the case with the anthracites and many of the steam coals.
Lewes points out that the composition of the humus and resin bodies is as follows :—
Humus
Resins
Carbon.
63
79
Oxygen. Hydrogen.
32 ' 6
11 10
He shows that the whole of the oxygen, present in the coal is due to these two bases. Hydrogen, in addition, is present in the hydrocarbon substance, but the unsaturated hydrocarbons in the gas are chiefly derived from the resinic compounds, whilst the hydrocarbon base is responsible for the paraffin group. In the past, authorities on carbonization have emphasized the importance of the “ unoxidized hydrogen ” content of the coal, and by deducting the quantity of combined hydrogen from the total have arrived at a figure for hydrogen which they cönsider of importance owing to the supposition thai this is the only available hydrogen for the yield of free hydrogen and hydrocarbons in the gas. Thus a coal which on ultimate analysis shows 5 per cent, of hydrogen, and 8-8 per cent, of oxygen would. be rated on an
“ unoxidized hydrogen ” content of
per cent.
It must. be
pointed out, however, that only a portion of the original oxygen in the coal combines to form water in this männer, a fur ther material amount being found in combination in the condensible vapours forming the tar, and as oxides of carbon. Primarily, the proportion, of oxygen gives a useful indication of the exteirt to which. humus bodies are present in the coal, whilst the direct subtraction of the hydrogen per-centage from the oxygen, percentage enables an opinion, to be formed as to calorific power. For instance, Constam and Kolbe have given the followiug figures :—
Coal. Oxygen. Hydrogen. Difference. Net Calorific Power.
Per cent. Per cent. B.Th.U. per Ib.
Nottingham, bright . 10-20 5-30 4-90 13,912
Ditto hard. 9-24 4-97 4-27 14,072
Durham I 5-66 5-48 0-18 15,392
Durham II 3-70 5-46 1-76 15,766
These figures indicate that with coals of the bituminous type the calorific power is inversely proportional to the excess of oxygen over the hydrogen, and therefore bears a ratio to the quantity of humus bodies present. At the same