Modern Gasworks Practice

HIGH TEMPERATURE CARBONIZATION OF COAL 401 THE DEGRADATION OF THE HYDROCARBONS The secondary reactions which take place during the carbonization of coal have been the subject of an unusual amount of research. The variable and complicated conditions under which these changes occur, however, render the path of the investi-gator extremely difficult. When a comparison of results is made, therefore, it is not surprising to find the expression of a number of contradictory opinions, although with regard to many of the more important facts a certain amount of uniformity is displayed. Primarily, it is recognized that when subjected to certain severe influ-ences many of the less stable products evolved from coal are unable to withstand their tendency to decompose, with the result that they are split up into more stable compounds of a similar nature. This degradation or reconstruction chiefly applies to the carbon and hydrogen compounds, although—as previously shown—substances (such as ammonia and carbon dioxide) composed of other elements are by no means immune. It must be recognized that in order to obtain the maximum yield of gas from a definite bulk of coal it is not sufficient to submit the solid mass alone to a very high temperature, but the primary volatile products must afterwards come under the influence of further heat, so that some portion of the condensible vapours will be converted into permanent gas. Moreover, whereas tliere is no limit of temperature beyond which it might be inadvisable to heat the solid residue, the same does not apply to the primary products, as too great exposure to intense heat has detrimental results in their case. In the gas retort this secondary reaction is occa-sioned by the passage of the products through the heated coke mass and during their travel to the exit of the retort in contact with the heated walls of the vessel and by the action of radiant heat in the free space. Accordingly, the extent to which the primary products undergo decomposition depends partly upon the limits to which the retort is heated, but in greater degree upon the manner in which the volatile products leave the retort and upon the time spent in contact with the hot surfaces. Degradation, then, is mainly influenced by the following conditions :— (a) The temperature to which the products are submitted. (b) The nature of the solid surfaces to which they are exposed. (c) The lengtJi. of contact time with these surfaces. (<?) The atmosphere prevailing within the retort. When the decomposition is such as to be beyond the economical limit, “ over-cracking ” of the gases and vapours is said to occur. It will be seen that heat may be encountered by the products in two distinct ways—namely, by contact with. the heated coke and retort walls and by direct radiation from the heated surfaces. Some years ago Young pointed out that the action of these two forms of heat is distinct; that whereas contact with the hot surfaces appears to act with more or less equal efiect on all substances, the radiant heat tends to have a selective action, and most readily attacks those molecules which absorb heat the more easily. In the light of recent research, however, it would appear that this conclusion is by no means established; the most severe efiects seem to occur during the passage of the pro- D D