Modern Gasworks Practice

412 MODERN GASWORKS PRACTICE (rf ) The employment in the hydraulic main of a vacuum slightly greater than the intensity of seal on the dip-pipe. (e) The use of heavy charges of coal in the retort. (f) Steaming the coal charge, thus producing water gas in situ. (g) The employment of prolonged charges in preference to those of short duration as used some few years ago. (h) Over-exhausting. Nothing has been said with regard to the quality of coal made use of, this having some considerable influence on the final results. It is, however, a factor over which the average gas engineer has little control, it usually being necessary (on the grounds of. economy) to take the coal from the nearest colliery. It will be seen that the increased volume is composed chiefly of— (a) Hydrogen, from the coke residue, from degradation, and from the gasified tarry vapours. (&) Methane, from the gasified tarry vapours and from degradation of heavy hydrocarbons, also to some extent from synthetic sources at the higher temperatures. A portion of this methane is eventually decomposed, so that in the final gas it shows some deficiency compared with lower yields of gas. (c) Nitrogen, from in-travel of the furnace gases, from the residual coke, which. ■chiefly evolves this gas in the final stages, and from the atmosphere. (<Z ) Carbon monoxide, from the formation of water gas (particularly if “ steaming ” is practised), and from the reduction at high temperatures of carbon dioxide. (e) Carbon dioxide, from the in-travel of furnace gases. Any proportion of carbon dioxide above the normal may be äccounted for by ■excessive vacuum on the retorts, followed by the introduction of air through defective lids and mouthpiece joints. It seems probable, therefore, that a portion of the carbon monoxide and hydrogen must in this way be burnt to carbon dioxide and water direct in the retort. Consequently, if inert constituents are to be kept within reasonable limits, all joints should receive particular attention. It is an. excellent plan to put a whole bench periodically under sliglit pressure (by closing back tem-porarily the retort hou.se governor) wlien. defective joints immediately show themselves. (f) Oxygen, free from the atmosphere. E.g. from imperfectly fitting doors or bad joints on that portion, of the apparatus which. is under a vacuum. Also intro-duced at the purifiers for purposes of revivification. As regards item (6) it is interesting to note the increase in gas volume which would result from the complete thermal degradation of the hydrocarbons and other gases containing hydrogen. The following table 1 shows that, by completely decom-posing 100 cubic leet of normal coal gas, the final volume obtained would be swelled to 133 cubic feet:— 1 R. Lessing, Gas Jcxlv, p. 493.