WATER GAS
695
The water-gas process is affected adversely in practice owing to the faet that considerable quantities of undecomposed steam are frequently permitted to pass through the fuel-bed, particularly in the later minutes of the gasmaking period. This introduces the question of the reversible reaction :—
CO + H2O ^CO2 + H2 + 1,500 B.Th.U. per Ib. of carbon.
This reaction plays an important part in establishing the relative concentration of carbon dioxide and carbon monoxide. The following ratio in the equilibrium of the reaction has been found by Hahn :—
g. _ Per cent, of CO X per cent, of H2O Per cent, of CO2 X per cent, of H2
K varies with the temperature as follows :—
Temp. Cent. 686 786 688 986 1,086 1,205
K . . 0-53 0-84 1-20 1-57 1-986 2-13
From this it will be clearly seen that increase in temperature favours the produc-tion of carbon monoxide. On. the other Land, excess of steam tends to promote an increase in the equilibrium concentration of carbon dioxide and hydrogen.
Catalysis in the Water-Gas Process
The synthetic production of methane from carbon monoxide or carbon dioxide and hydrogen in water gas has assumed a new importance at the present time in view of the fact that if synthesis can be carried out with success on a practical scale a means is provided for producing a comparatively high-grade straight water gas without the use of enriching agents. A method for the synthesis of methane is afiorded by the reduction of carbon monoxide by hydrogen in the presence of nickel. Thus—
CO + 3 H, = CH4 + H,O.
With an active catalyst the reaction occurs at 180°-200° C., and at 250° C. the reaction is practically complete. When carbon dioxide is substituted for carbon monoxide the reaction becomes—
CO3 + 4H2 = CH4 +2H2O.
The reaction temperature in this case is rather higher, commencing at about 230° C. and becoming rapid over 300° C. At temperatures above 250° C. in the case of carbon monoxide a secondary reaction occurs, namely, the conversion of carbon monoxide into carbon dioxide and free carbon—
2 CO^CO2 +C.
This reaction complicates the process owing to the deposition of the free carbon on the catalyst, thus rendering the latter inactive.
Ordinary straight water gas contains approximately equal quantities of hydrogen and carbon monoxide, and if passed over a nickel catalyst at about 450° C. the