Modern Gasworks Practice

692 MODERN GASWORKS PRACTICE the gas through, the district mains. Deterioration of the kind may, however, be guarded against by operating both carburettor and superheater at tlieir correct temperatures, and by attending to the periodical cleaning and rechequering of these vessels. The condensers, too, must operate on efficient lines, so that the temperature of the gas is reduced as near as possible to that of the atmosphere betöre entering the oxide purifiers. S. Carter has pointed out that 21° C. may be looked upon as a critical temperature in the process, and that with gas leaving the purifiers at this temperature oil will be precipitated in the holders. THEORY OF MANUFACTURE The generation of water gas depends upon the action of steam upon red-hot carbon, the latter being present in the form of coke (containing about 90 per cent, of carbon). As is well known, water, or steam, cannot be decomposed by the action of heat alone ; but if subjected to heat in the presence of a reducing agent the oxygen will combine with this agent, with the evolution of hydrogen in the free state. Heated carbon possesses a greater affinity for oxygen than does the hydrogen with wliich the oxygen is combined in the form of steam. Hence the sequence of reactions which. result. The oxygen of the steam combines in the first place with the carbon to form carbon dioxide, and sortie carbon monoxide. Tlius, in the lower portions of the fuel-bed we have the following reactions:— [• («) C +2H2O = CO2 +2H2. ( (b) 0 + H2O = CO + H2. Accordiug to the generally accepted theory the carbon dioxide is subsequently “ reduced,” by combination. with another carbon atom, during its travel through the remainder of the fuel-bed :— (c) CO2 + C = 2 CO. The manner of the combustion of carbon has already been discussed from the point of view of retort-bench producers in Chapter III. The main problem requiring definite elucidation is as to whether the carbon bums first to carbon dioxide or to carbon monoxide. Rheacl and Wheeler 1 have concluded that the two oxides of carbon are formed simultaneously. They assume that in the first instance a physico-chemical complex CxOy is formed, and that this subsequently breaks up into CO and CO2. In all probability the reactions take place concurrently, so that the result amounts practically to the following :— 2C + 3O = CO+ CO2. It should be borne in mind that during the blowing period in a water-gas plant the replenishment of heat is the main requirement. Therefore, the carbon must be burnt as completel y as possible to CO2. • This is all the more important when blue water gas is manufactured, in that, as no heat is required in auxiliary vessels for carburetting, any CO escaping from the generator perforirs no useful function. 1 Trans. Chem. Soc., Cl, p. 846.