Modern Gasworks Practice

THE HORIZONTAL RETORT BENCH 69 of clinker curtailed. In the author’s Pressure Grille Grate the primary air is in-timately mixed with. steam prior to passing into the fuel-bed. Steam in passing up through the fuel is broken up into its elements, the oxygen combining with the carbon in the following manner—■ C +H2O =CO +h2 This reaction, being endothermic, deprives the producer of heat; but the lieat so lost has been instrumental in generating a further quantity of combustible gases (CO and H2) which yield up an amount, not far short of the loss, when burnt in the combustion chamber and cooled down in the regenerator. What has in reality happened is that heat has been appropriated from the producer at a point where excessive temperatures are to be avoided and has been transferred, with. a certain amount of loss, to the portion of the setting where maximum quantities of lieat are desired. In this way steaming has a twofold value. It will be seen, moreover, that by gasifying a portion of the carbon by means of steam, instead of wholly by air, the resulting gas contains less nitrogen, for that portion gasified by the steam consists of combustible gases alone. When steam is used, therefore, the proportion of combustibles in the producer gas is greater than is the case when air only is employed. Moreover, the rather lower temperature of the producer means a lower temperature of the issuing gas, a faet which. in the ordinary type of retort setting is more than balanced by the inereased potential heat which the gas carries ; but which is of especial importance in those instances where external producers are employed, in which case a certain amount of cooling of the gas takes place before it reaches the combustion chamber. In cases such as the latter, therefore, the greater the exchange from sensible heat into potential heat the higher will the economy of the system be. Primarily, however, steam is employed in gasworks producers as a temperature regulator, the main provision being that of curtailing clinker. The quantity demanded must necessarily depend upon the character of the fuel, the moisture (if any) it contains, and the construction of the grate. The chief consideration is that of preventing fluxing of the ash, and the greater the fusibility of the ash the higher must the steam ratio be. Excessive steam, however, as shown below, leads to impoverishment of the producer gas, and the problem with a bad fuel often resolves itself into wh.eth.er it is more economical to have an excess of steam and a producer gas of inferior quality, or whether it is pre-ferable to curtail the steam and suffer some inconvenience from clinker troubles. If the quantity of steam is excessive the fuel-bed will be inordinately cooled, which may result in the oxidation of carbon to CO2 in the following manner— C +2H2O =CO2 +2H5 This reaction, being also endothermic, is depriving the producer of lieat for the formation of an inert gas, CO2. Moreover, the presence of excessive quantities of steam is likely to inerease the tendency of some of the CO to undergo oxidation— o O + w tc o II a O to + w to