Modern Gasworks Practice

394 MODERN GASWORKS PRACTICE as is shown by the large increase in this by-product which. is obtained when im-properly dried washed coal is used. When coal is carbonized a small proportion of free oxygen is always evolved from the coal substance as distinct from that which may be drawn into the retort through, ill-fittmg doors. This oxygen has a decidedly lowering influence on both. ammonia and benzene, which. have comparatively low ignition temperatures. Sommer 1 points out that the oxidation of ammonia by air beging at 150° C., and increases in effcct with rise in temperature. At 250° 5-86 per cent, of the ammonia is oxidized, although moisture acts as a decided preserving agent. At 450° 12-89 per cent, of ammonia was shown to be lost under dry conditions and 2 per cent, in the presence of moisture. It is interesting to note, however. that sulphuretted hydrogen, which is a primary product of carbonization and which is, therefore, invariably present in the gas mixture simultaneously with ammonia, is oxidized very much more readily. It has accordingly a distinct protective action, particularly as—unlike ammonia—its tendency to oxidize is accentuated by the presence of moisture. Sommer found, in fact, that at 100° C'. 25 per cent, of sulphuretted. hydrogen was oxidized, and at 425° 95-3 per cent, disappeared; whereas at the same temperatures under dry conditions only 0-26 and 1-48 per cent, was affected. Thus the sulphuretted hydrogen by its more ready action with oxygen tends to conserve the ammonia and prevent its destruction. Thomas Gray has stated that the ammonia formed duriug the earlier stages of carbonization is evidently a primary product; but as the heat penetrates the charge water still present in the inferior is vaporized, and, along with the water vapour which results from the decomposition of the coal, passes through the highly heated and almost completely carbonized outer skin. It is then partly decomposed by the inoandescent carbon, with. the formation of carbon monoxide and carbon dioxide, while the hydrogen liberated unites with the nitrogen of the coke to form ammonia synthetically. This is brought about by subjecting the ammonia to the superior affinity which. exists between the combined nitrogen in the coke and free hydrogen at a particular temperature and in a diluting atmosphere. Thus, duriug the later stages of carbonization, when hydrogen is being freely evolved, the Tervet reaction comes into play, and continues thronghout the remainder of the period, though the ammonia which is formed towards the end of the process is, no doubt, decomposed to some extent. J. W. Porteous2 has given the opinion that asli in coal is injurious to the yield of ammonia for the reason that when it contains a certain amount of iron in an oxidized state, or even as pyrites, the iron influences the ammonia already formed, and by catalytic action, at certain temperatures (600° C. and upwards) accounts for the destruction of ammonia. THE SULPHUR OF COAL Ä portion of th.e sulph.ur present in the coal substance is of a more or less “ fixed ” nature and remains behind in the coke. This portion is usually present in the original o rH 2 Coke Oven Managers’ Assocn., November, 1919.