Modern Gasworks Practice

696 MODERN GASWORKS PRACTICE carbon monoxide disappears, being converted into methane and carbon dioxide, a certain amount of free carbon being deposited on the catalyst. Wh en the catalyst becomes choked with. carbon it is maintained at a temperature of about 450° C. and a current of steam is blown through it with the formation of carbon dioxide and hydrogen. The former gas in turn interacts to form methane, so that the final mixture consists of hydrogen, methane and CO 2. The last named may be removed, when a gas of moderately high calorific power is obtained. The two stages of the process may be combined by employing simultaneously suitable proportions of water gas and superheated steam. In practice, more particularly so far as gasworks are concerned, the production in this way of a gas containing methane presents considerable difficulty owing to the faet (as seen from the above equations) that theoretically three volumes of hydrogen are required for one volume of carbon monoxide, while on a working scale the necessary proportion is found to be very much. greater. With straight water gas as prochiced in the ordinary männer the proportions of the two gases are nearly equal. Moreover, water gas in the crude state contains a considerable quantity of sulphuretted hydrogen and some sulphur compounds. All sulphur bodies quickly poison the catalyst, and even if gas purified in the ordinary männer from sulphuretted hydrogen were treated, there would still remain the problem of the other sulphur compounds. The author has experimented with the process on a semi-technical scale, but concluded that from the gasworks point of view it was impracticable for many reasons. Primarily the question of relative concentration of CO and hydrogen rendered the evolution of a simple process almost impossible, while it was found extremely diflicult to obviate entirely the formation of nickel carhonyl. Again, from the point of view of gas production, it must be borne in mind that four volumes (at the lowest) of carbon monoxide and hydrogen yield only one volume of methane. Accordingly, on a volume basis the output of a water-gas plant would be very seriously reduced ; although this consideration is not of so great importance now that the therm basis of selling gas has been introduced. It will be appreciated that, although. the volume output of a given plant would suffer, the thermal output would not be affected to so great an extent. Sabatier 1 endeavoured to overcome the difficulty of adjusting volume differences by making a low temperature water gas having a high, content of carbon dioxide and a low proportion, of carbon monoxide. In this way the necessary excess of hydrogen was obtained, the carbon dioxide being removed before the mixture was passed over the nickel catalyst. Before reaching the catalyst vessel the gas was passed over heated copper for the removal of impurities. The objection to this metliod lies in the faet that large quant.ities of generator fuel in the form of carbon dioxide must necessarily be wasted. As an alternative to this Elworthy 2 proposed to add hydrogen to the water gas so as to adjust the relative volume requir.ements, but such a inethod is as yet rendered prohibitive owing to the cost of producing hydrogen. 1 French Patent 400656/1908. 2 B.P. 12461/1902 and 14333/1904.