Modern Gasworks Practice

HIGH TEMPERATURE CARBONIZATION OF COAL 433 expected—was only 440° C. These results all refer to a charge 6 inches in. depth. Considerable variation was found when the thickness of the coal layer was in-creased to 12 inches. In this case the temperature of the core at the commence-mentwas no more than 94° C., but as carbonization proceeded a gradual increase was noticeable, until after eight hours tliere was only a slight difference between the base and the core of the charge. At this period the hottest portion of the charge was the siirface, which had risen to abont 1,065° C. The following table, compiled from figures given by G. P. Lewis, gives a useful insight into the conditions of temperature obtaining during an eight-hour charge:— Time after commencement of charge. i 1 2 Hours. 5 6 7 8 3 4 Degrees Fahrenheit. Light charge (6 inches thick) Bottom Centre . 1,290 1,110 1,360 1,250 1,460 1,700 1,650 1,850 1,900 1,870 1,830 1,870 1,830 ^Top . . 930 1,100 1,360 1,600 1,740 1,760 1,850 Heavy charge (12 inches thick) f Bottom 1,110 1,170 1,290 1,380 1,440 1,470 1,560 1,690 1,890 Centre . 200 210 300 410, 500 610 1,080 1,470 1,870 Top 1,380 1,470 1,560 1,590 1,620 1,690 1,780 1,870 1,920 With regard to vertical retorts of the continuous type, at the point of combustion the temperature is about 1,300° C., and at the base of the retort it is approximately 1,100° C. It must not be lost sight of that in the vertical retort progressive heating may be employed, whereas in horizontal systems this is not practicable. By such heating, the prevailing temperatures of the various portions of the retort may be varied and the charge subjected to difiering intensities of heating as it travels down-wards. Thus we get the two types of heating, namely, top and bottom, in the first of which the upper portions of the retort are subjected to the most severe temperatures, the lower portions being more highly heated in otter systems. The conditions prevailing in vertical retorts are illustrated in. Figs. 261 and 265. Whilst in con-tinuous systems the relative positions of uncarbonized coal, the viscid layer surronnd-ing it, and the coke zone remain practically constant, this is not the case with. inter-mittent systems, in which the pasty envelope gradually closes in towards the centre of the charge. With continuous charging, even if top heating be adopted, the upper portions of the mass will still be relatively cool, so, that the primary gases coming away at low temperatures are subjected to the minimum of degradation. With intermittent working, however, it is essential to heat the base of the retort more highly, for in this way the lower portions of coal are carbonized in advance of the upper, and thus a freer passage through the central core is assured. The coke, F F