430
MODERN GASWORKS PRACTICE
would appear to be that of maintaiuing an adequate temperature in the retorts, more especially in the lower sections where the endothermic nature of the water gas reaction is most marked. The importance of temperature on the composition of the gas yielded by the water gas reaction is well illustrated by the figures given by L. J. Willian1
Effect of Temfbbatube in Wateb Gas Reaction
Temp. ° C. Per cent, of steam decomposed. Composition of gas, per cent, by volume.
Hydrogen. Carbon monoxide. Carbon dioxide.
670 8-8 65-2 4-9 29-8
760 25-3 65-2 7-8 27-0
837 41-0 61-9 15-1 22-9
955 70-2 53-3 39-3 6-8
965 94-0 48-8 49-7 1-5
1,070 99-4 50-9 48-5 0-6
The above figures clearly indicate that as steaming is progressively increased there must essentially be a corresponding rise in the proportion of carbon dioxide in the gas unless means are found to maiatain the retort at temperatures sufficiently high to permit of a maximum decomposition.
Downward Steaming
The general practice when steaming the coal charge has been to admit the steam to the base of the retort so that a scouring action on the products evolved. from the coal is effected. A method,2 however, has been introduced by T. Settie in which the usual upward steaming from the base is replaced by a downward stream admitted near the upper end of the coal charge. In this case the steam is superheated and passes at a pressure of 30 Ib. into the gas outlet fixed to the retort. The supply is so arranged, in faet, that instead of impinging on the coke with. a high velocity, as in the more common methods, the steam approaches the charge at a slow speed and is equally diffused over the whole cross-section of the retort. Continuous downward steaming, however. is not carried on, but the supply is turned into each charge in succession, over the last three hours of a 12-hour distillation period. The retort at this period contains, of course, coke and breeze only, and the top gas outlet is closed so that the steam has to travel downward through the hot coke, the products due to it passing thence by a special connection to the base of an adjoining retort, through which they travel upwards through the coal charge and thus to the gas outlet. Each retort therefore receives during the period of distillation three hours’ steaming downwards and three hours’ treatment with blue gas upwards. The advantage of the highly heated upwards stream of blue gas is that a diluting
1 Proc. New England Assoen. Gas Eng., 1919. 2 See page 165.