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Chemical Constitution of Coal

Little was known until recently about the chemical constitution of coal. Considerable light, however, has been thrown upon this difficult problem by the work of Wheeler and his collaborators, to which reference will shortly be made. First of all it may be said that the number and complexity of the products of the destructive distillation of coal in the manufacture of coal;gas throw little light upon the constitution of coal itself, because these products may have been formed or altered in the process of distillation, and the proportion between them certainly varies with varying conditions of treatment. That coal cannot properly be described as an allotropic form of carbon may be judged from the fact that it is not certain that every kind of coal contains free carbon at all. Probably lignite contains no free carbon, but anthracite undoubtedly contains it. Caustic soda, potash, and ammonia extract humic acid from peat and lignite, but not from true coal. When lignite is fused with caustic soda pyrocatechin is obtained. True coals do not yield this compound.

The investigations of Wheeler and others were commenced with a view to elucidate the causes and conditions of the explosions of mixtures of coal-gas and air in mines.

It has been shown by these observers that in the careful destructive distillation of coal different volatile products are obtained at different temperatures. Ethane, propane, butane, and higher members of the series of paraffin hydrocarbons form a large percentage of the gases evolved below 450° C. The evolution of these gases ceases, however, above 700° C., and hydrogen and oxides of carbon take their place, so that the composition of the evolved gases shows a marked change at 750°-800° C., a "critical period" lying between these temperatures. It is inferred from these phenomena that " coal contains two types of compounds of different degrees of ease of decomposition - the one, the more unstable, yielding the paraffin hydrocarbons and no hydrogen; the other, decomposed with greater difficulty, yielding hydrogen alone (or, possibly, hydrogen and the oxides of carbon) as its gaseous decomposition product."

Another mode of attack was to treat the coal with various solvents. This had been done before, such solvents as petroleum-ether, benzene, and alcohol having been used. Pyridine, moreover, had been found to be a valuable solvent, which now yielded important results. Pyridine extracts from coal that portion of it which gives rise to the paraffin hydrocarbons on destructive distillation, together with some of the hydrogen-yielding portion. Re-extraction of the pyridine extract with chloroform or benzene gives the paraffin-yielding fraction fairly pure. This product is of a resinous nature, whilst the portion insoluble in chloroform is a degradation-product of cellulose, i.e. a humus substance. The "humus substances," whether left in the coal after extraction with pyridine or as a residue after chloroform or benzene re-extraction, were found to yield on distillation mainly phenols, whilst the "resinous substances" yielded no phenols, but olefines and naphthenes, as well as paraffins.

These products are the remains respectively of the resinous constituents and woody fibre of a "monster vegetation which flourished long before the earth was inhabited by man."

The distillation of bituminous coal in a vacuum at temperatures not exceeding 430° C. yielded further information with regard to the resinous substances, and further extraction of the pyridine-chloroform extract with pentane yielded crystals of paraffin wax of melting-point 55°-59° C., which formed about 0.1 per cent, of the original coal. These results have led to the formulation of a theory with regard to the compounds in coal itself which yield volatile hydrocarbons by distillation at low temperature.

Liquid or gaseous paraffins cannot have been produced by the thermal decomposition of free solid paraffins, which are not present in sufficient quantity, and neither can it be thought that they are formed by pyrogenic synthesis under the conditions of the experiments. It is believed that they are present in the coal substance combined somewhat loosely with a less volatile residue from which they are liberated by moderate heat.

The distillation of "free" paraffins from "bound" molecules is thus represented:

RH-CNH2N+1 → R + CNH2N+2
RH-CNH2N+1 → R + CnH2n+2 + Cn1H2n1.

A similar explanation is applied to the breaking down of ethylenic and naphthalenic molecules.

From a thermal study of the process of carbonisation of coal and related substances Hollings and Cobb arrive at the following conclusions:

  1. Below 400° C. cellulose shows a strongly exothermic reaction beginning at 345° C., and probably due to loss of hydroxyl groups, and consequent molecular condensation. Coal itself does not show this reaction.
  2. 400°-600° C. is the temperature interval in which the characteristic differences in different types of coal are exhibited. Oils, unsaturated hydrocarbons, higher paraffins, and oxygenated compounds are evolved, the thermal phenomena varying with the quality of the coal.
  3. 600°-800° C. is the temperature interval in which methane is evolved, and all coals are exothermic. If the carbonisation process is on the whole exothermic, the thermal reaction at this stage is the predominant cause.
  4. Above 800° C. hydrogen is the chief product, and the change is thermally neutral, or slightly endothermic.

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