Modern Gasworks Practice

HIGH TEMPERATURE CARBONIZATION OF COAL 415 heat unit. The standard of comparison to-day is, therefore,. the total number of gaseous heat units obtained per ton of coal. The figure must necessanly be elastic owing to the variation in the quality of gas coals now utilized. For a good dass Durham coal, however, it should be possible to obtain 6,750,000 B.Th.U. per ton, although. it may be meationed that in some instances a yield of 7,000,000 units is now being reached under the best conditions. These figures refer, of course, to Straight coal gas production, and are not applicable to those cases in which the coke in the retort is being partially gasified by the admission of steam. In all cases of comparison, however, it is important to remember that, in calculating the cost of production of a gaseous heat unit or a “ therm,” the net cost of coal (after deducting the revenue from residuals) should be taken, for the most economic process is not necessarily that which yields a maximum of gaseous units with, probably, a minimum of coke and ammonia. The question as to the increase of volume of permanent gas obtained by over-cracking in the free space is a vexed one, and some authorities point out that th.e yield is swelled to only a meagre extent in this way. If we consider any of the degradation reactions, however, it is possible to show by means of Avogadro’s law that some increase of volume does take place. For instance, if the reaction CH4 = C + 2 H2 is taken as an example, it is seen that one molecule of methane gives rise to two molecules of hydrogen. In otter words, a cubic foot of methane yields on degradation double its volume of hydrogen; that is, an increase of 100 per cent, in permanent gas. Similarly, etliylene (C 2H4) may decompose into acetylene (C2H2) and hydrogen (H2). Thus one molecule has yielded two molecules of gas; again an. increase in volume of 100 per cent. The extent of such, decomposition, however, is relatively small in comparison with. the total volume of gas present, and Lewes has shown that in the long run intentional over-cracking in the retort will probably account for only an additional 400 or 500 cubic feet. Lewes’ experiment on the efiect of degradation of the primary products is of extreme interest. First, he obtained a semi-primary gas having an analysis approximately as follows:— 27'5 per cent. 48-0 10-1 3-0 7-3 2-5 1-6 52-3 per cent. 34-0 2-5 8-8 1-0 1-4 Hydrogen _______________________..... Sa tura ted hydrocarbons—1, Methane „ „ 2, Higher members Unsaturated hydrocarbons Carbon monoxide ___________________.... Carbon dioxide ___________________.... Nitrogen____________________________...... This gas was then passed through a tube containing broken porcelain, and heated to 1,100° C., when an increase of volume of 11 per cent, resulted, the com-position then being as follows :— Hydrogen Methane Unsaturated hydrocarbons Carbon monoxide Carbon dioxide Nitrogen....