Modern Gasworks Practice

Ox CO MODERN GASWORKS PRACTICE but at the same time it must be borne in mind that the single-stage plant cannot be operated without some expenditure of fuel. The significant point is that an attempt is made (not, perhaps, wholly successful) to effect both operations, i.e., carbonization and subsequent gasification, with. the same quantity of heat as is commonly employed to gasify coke only in the ordinary water-gas plant. That is to say, the sensible heat and combustibles obtained in the blow gases are utilized for carbonization of the coal; accordingly, as compared with the two-stage process, the combined plant effects, theoretically, the saving of the fuel used in the retort beuch producers. It is beginning to be realized, liowever, that to effect thorough carbonization of the coal before it enteis the water-gas zone it is necessary to employ around the coal zone a portion of th.e “ blue gas produced in addition to the heat derived from the blow gases. When it comes to a question of by-products, i.e. tar and ammonia, the complete gasification of coal in the combined plant does not compare favourably with the two-stage process. Certainly the single-stage process yields both of these products, but experience has shown that in order that the moie valuable constituents of the mixed gas may be transformed into permanent vapours it is necessary to fix th.e gas by further heat treatment, and as a result of this procedure both ammonia and tar are partly d&stroyed. Again, with the single-stage plant of ordinary construction it is usually necessary to confine the process to special coals the physical condition of which is such, that carbonization and travel from coal-gas to water-gas zone may proceed without difficulty. Success, for instance, could scarcely be looked for with coal containing a high proportion of fines, and probably one of the most favoured varieties of gas coals, namely, Durham coals, would have to be screened before use. CÖMPOSITION OF COMPLETE GAS The analysis of complete gas which has been given 011 page 757 must be looked upon as a model or theoretical composition, the attainment of which. in practice can scarcely be looked for with the process in its present stage of development. General experience has shown that, unless steps are taken to preclude it, an appre-ciable proportion of the unsaturated hydrocarbons will be deposited in the liquid state when the gas is cooled to atmospheric temperature. It would appear, in faet, that the process demands a cJioice between two evils. That is to say, the gas may be produced in such a way that portion of the hydrocarbon vapours it contains are condensible, in which case a fairly high tar yield will be obtained. On the other hånd, the gas may be submitted to a fixing process, in which case it will retain a greater proportion of hydrocarbons. as permanent gas, but the effect of fixation is to clestroy in part the tar and ammonia. Theoretically, it should be possible to retain the incombustibles in complete gas with in a proportion of 10 per cent, by volume, but in practice this is rarely accomplished, primarily owing to the faet that a portion of the blow products invariably manage to travel forward with. the gases issuing from the retort proper. This is a difficulty which will, no doubt, be solved by time and experience, and