Modern Gasworks Practice

112 MODERN GASWORKS PRACTICE combustion ; while in the third, or cool, zone, heat may be extracted from the hot travelling charge by carrymg round the exterior of the retort the secondary air em-ployed for the combustion of the producer gas. This last-named feature is certainly one which will appeal to that school of fuel economists wh.o are continually pointiug to the loss of sensible heat whicli is an inherent feature of all intermittent horizontal retort systems in whicli the coke must necessarily be discharged in the incandescent state and ultimately quenched with water. Later experience with the system indicates that with contmuously operating retorts difficulties are encountered owing to the resistance which. the material offers to the piunger or propeller. Increase of pressure applied to the mass in the retort may not ensure an efficient feeding and discharging action, for the coal appears to wedge itself in the retort. However, it has been found that if the retort be so shaped that the weight of the coal is distributed fairly evenly over the sides and bottom, inst,pari of being carried practically entirely by the bottom of the retort, the plunger is enabled. to effect more efficient propulsion of the charge through the retort with a minimum of pressure, for the resistance to propulsion is more uniform over the whole of the mass, and buekling or wedging of the charge is obviated. The retorts, therefore, should be made with. tapered sides, typical sections being shown at A, B and C in Fig. 56, which also shows in detail the continuous cliarging apparatus. With the tapered retort it is said that not only is the weight of the charge distributed more evenly over the surface of the retort, but, by reason of the reducecL thickness of the coal in the base of the retort, the heat penetrates and completely carbonizes the coal very readily, and so gives it greater power to resist buckling during propulsion, and at the same time reduces the friction between the charge and the retort surface. The system has quite recently undergone considerable modification, and is now designed with a large steaming chamber (Fig. 57) at the discharge end of the retort. “STEAMING” in horizontals The success which has attended the process of steaming the charge in vertical retorts (see page 424) has led to many attempts to apply the same system to horizontal retorts. A certain amount of success has been obtained in the latter direction, but, on the whole, it may be said that, unless highly superheated steam is employed, derangements, such as the reduction of working heats or the excessive deposition of water, are likely to follow. Favourable results have been obtained by working the horizontal retort on semi-continuous lines. That is to say, one ascension pipe is stopped and steam is admitted. from that end of the retort. Instead, as is customary, of discharging the whole of the coke at one operation, only half of it is pushed out at a time (the remaining half filling that portion of the retort which is ad] acent to the steam supply), while the cleared half is re-charged with coal. After the usual period of distillation the charge (now reduced to coke) is similarly pushed forward, and its place is taken by a further supply of coal. The ascension pipe on the steam inlet end of the retort is, of course, entirely blocked off, all gas passing out through, the pipe at the further end of the retort.