Modern Gasworks Practice

418 MODERN GASWORKS PRACTICE The coke yielded contained 2-56 per cent, of volatile matter and 88-53 per cent, of carbon. The nitrogen, it will be noticed, is at a minimum (3-6 per cent.) at the eighth hour, but is five times this amount at the twenty-seventh. LIGHT AND HEAVY CHARGES Little more than a decade ago it was the recognized custom to employ light charges in the retort, and to work these ofE in short periods of carbonization. Six hours was looked upon as a convenient period, so that the retort could be filled four times during the twenty-four hours. When such systems are adopted the coal lies in a tliin layer upon the floor of the retort, thereby leaving a large crown space un-occupied. By prolonging the period to either eight or twelve hours the labour entailed is curtailed by one-third or a half, and many further advantages accrue. With. the six-hour charge the weight of coal distilled in a “ through ” retort of average cross-section was usually 6 cwts.; and it may be taken as a Tough, rule that the mimlber of cwts. shall be equivalent to the period of carbonization in hours. Thus, a 6-hour charge consists of 6 cwts. an.8-hour „ „ „ 8 „ and a 12-hour ,, „ „ 12 „ In each case it will be seen that the weight of coal employed in twenty-four hours is approximately the same; but whereas in the six-hour charge it has to be håndled on four distinet occasions, with the twelve-hour charge only two handlings are necessary. The primary objection to the six-hour charge is, however, the excessive degradation and decomposition which. takes place in the greater free space above the coal. Owing to the tendency of the gas to linger in the retort, and to its slow passage therethrough in contact with incandescent surfaces of carbon, the destructive influences are afforded exceptional facilities for working their evil. It must be remembered, moreover, that although decomposition by heat occurs in what may seem a minute space of time, in the ordinary way the thermal degradation of the hydrocarbons takes place very slowly in comparison with the rate at which many other chemical reactions occur. Thus an increased time of exposure teils heavily in this direction. As the bulk of gases travel along the free space to the outlet pipe, portions of them will be in contact with the sides of the retort, etc., where their temperature quickly rises to that of the surfaces, and it is here probably that the most intense degradation occurs. The gases towards the middle of the bulk, how-ever, absorb the radiant heat traversing the space, and as the more complex Compounds have a greater absorptive capacity for radiant heat, than the simpler bodies, the tendency is for the more stable gases, such as methane, to remain unharmed whilst the complex vapours are split up. It is for this reason that the condensible vapours and gaseous hydrocarbons are of a distinctly beuzenoid character with. light charges. Furth.ermore, the excessive degradation gives rise to the deposition of free carbon, which, in turn, causes stopped pipes; and naphthalene makes its appearance in undesirable quantities. The naphthalene formation may be explained by the faet that when subjected to excessive heat benzene probably polymerizes. so that compounds containing Condensed benzene nuclei are formed.