NAPHTHALENE IN COAL GAS
461
FORMATION OF NAPHTHALENE
The greater proportion of naphthalene produced in the carbonization of coal is obtained in the tar distillates, the average total quantity yielded being approxi-mately 0-3 per cent, by weight of the coal, or from 6 to 7 1b. per ton of coal dealt with. The quantity of the product formed, however, depends very largely upon the system of carbonization and the treatment to whicli the evolved gases are sub-jected. Accordingly, while the tar from horizontal retorts may contain. 10 per cent, by weight of naphthalene, that from continuous vertical retorts frequently shows no more than 5 to 7 per cent. In general, the warm crude gas derived from horizontal retorts contains naphthalene equivalent to from 1 to 1| 1b. per ton of coal carbonized.
It must be realized that naphthalene is not formed as a direct distillation product from the coal itself, but that it is the result of secondary influences wliich affect the gas during its passage from the retort. Under the influence of heat many hydro-carbon. vapours have a tendency to form naphthalene, and it seems that this tendency is the greater the higher the proportion of carbon in the compound.
According to Lewes the hydrocarbons to a great extent leave the coal as vapours of condensible hydrocarbons ; the breaking-down of these to such simple gaseous compounds as ethylene, already formed, into acetylene and methane, and the poly-merization of the former into higher compounds then occurs. Bodies like naphthalene and anthracene have such great stability that when once formed they resist any effort to decompose them again by lieat.
Naphthalene formation may be explained by the fact that when subjected to excessive heat benzene probably polymerizes, so that the following compounds containing Condensed benzene nuclei are formed :—
Düring the formation of such compounds hydrogen is evolved in some quantity,. and probably accounts for the high percentage of this gas usually found in the resultant gas from light charges. With regard to the “ free ” carbon deposited, it is interesting to note that Colman considers that with still higher temperatures and with increase of time the above compounds yield more hydrogen, and Condensed