Modern Gasworks Practice

THE COMPLETE GASIFICATION OF COAL 759 the solution probably lies in the employment of some form of automatic compensating arrangement which would control the pressure during the two periods of run and blow. As an illustration of what may be expected in the way of incombustibles the author might quote from a recent report which he has upon the working of a plant of the kind. In. this case the carbon-dioxide content of the gas was found to be 16'8 per cent, by volume. The question of clinker formation is one of extreme importance, and means should be provided for ensuring that no hard masses of slag are used, but that the mineral constituents of the coal are brought down in friable form. It will usually be found, with plants as at present coastructed, that when the producer is cleaned the material removed will contain a large proportion of a partly burnt coke which, if .picked over, may be employed under the steam boilers. This waste fuel has been known to amount to as nnich as 3 cwt. per ton of coal dealt with. THERMAL EFFICIENCY OF GAS PRODUCTION The ordinary method of comparing the thermal efficiencies of gas production with different methods of treating coal is to an extent misleading in that the gas generated is made to bear the whole of the thermal losses which occur. It will be appreciated that by-products cannot be obtained without the expenditure of heat, accordingly they should bear a reasonable proportion of the heat which. is consumed and lost in the process. In order to deal with the matter in this way, liowever, assumptions would have to be made at the outset, and the faet that they would necessarily be of a hypothetical nature would largely vitiate the value of the results obtained. The only practical method of arriving at comparative figures, therefore, is to assume that all thermal losses are debited to the gas accoimt. The thermal efficiency of a gasification process is merely the percentage ratio of the potential heat in the fuel employed before gasification and the useful energy which is recovered atter treatment of the fuel. A ton of coal contains approximately 30 million B.Th.U. in the form of potential heat energy, while a ton of coke, in the normal undried state, contains on an average (2,240 Ib, X 12,000 B.Th.U. per Ib.) 27 million B.Th.U. When coal is the fuel employed the by-products enter into the reckoning, with coke (as there are no auxiliary products) the calculation is more straightforward. The following examples may be taken as illustrative of the results obtained when a good quality (say, Durliam) coal is carbonized under conditions approaching the best found in practice. Efficiencies have been calculated upon the following basis :— Efficiency of whole process = Useful heat recoveréd. Total heat supplied X100 Total heat in gas Efficiency of gas production=--—------—2----- X100 Total heat m gas + Heat lost and used