408 MODERN GASWORKS PRACTICE
tion of heavy hydrocarbons present. Hydrogen lias increased in volume by 50 per cent., whilst methane remains moderately steady, showing that the extra hydrogen has not been obtained at its expense. The hydrogen appears to be chiefly the product of the decomposition of the heavy hydrocarbons and of the gasification of products which. would otherwise yield tar.
Cobb and Hollings also conducted experiments on the decomposition of the various hydrocarbons in the presence of hydrogen and methane, and, therefore, under conditions approximating to tliosc prevailmg in a- retort. A mixture was taken consisting of equal parts of hydrogen and methane to which were added small percentages of the hydrocarbons to be tested. Temperatures of 800° C. and 1,100° G. were selected for the study of decomposition, and only experiments in which the gases were passed through a tube packed with coke have so far been recorded. With regard to methane it was found that at 800° C. only about 2 per cent, was decomposed in orte minute, whereas at 1,100° C. degradation was very rapid. In the latter case, however, there was an increase in volume of about 44 per cent.; but a considerable loss of calorific value was noticed owing to the carbon of the decomposed methane being deposited in the solid state. The results of the two experiments are shown below.
Methane
At 800° C. At 1,100° C.
ri , n (Methane ■Gas before ) TT , (Hydrogen r, p , , (Methane Gas aiterwards s TT . (Hydrogen Percentage of methane decomposed .... 50-9 per cent. 49-1 „ 49-5 „ 50-5 2-8 50-9 per cent. 49-1 „ 6-6 93-4 87
8o far as ethane is concerned it was found that the chief products of its decom-position are ethylene and methane. At 800° C. the following results were noted after heating for forty-six seconds :—
Ethane, at 800° C.
Composition of gas. Before. After.
Ethane 4-2 per cent. 0-9 per eent.
Methane 47-5 48-9
Hydrogen 48-3 48-9
Ethylene nü 1-3
Acetylene nil Traee
In this case the percentage of ethane decomposed amounts to 79, but at a temperature of 1,100° C. the figure rose to 88 per cent.