04/11/2013 | Autor / Editor: Holger Lieberwirth *, Lothar Schmeier ** / Marcel Dröttboom
Whereas in Maritsa Iztok-1 the residues are distributed and dumped by trucks before being compacted by dozers the Medupi power plant as well as Enel OGK-5 will operate residue dumps with mobile spreaders linked to the overland conveyor system by extendable/shiftable conveyors (Fig. 2). This technology has been proven reliable in large open pit mines for decades. With OGK-5 the shiftable conveyor is step-wise slewing around a fix pivot point with the spreader operating in high cast and low cast (Fig. 9).
The conveyor needs to be shifted about twice per year. The shifting will need about 12 to 16 hours. Normally, the shifting is scheduled during regular maintenance shifts of the boilers. The residue coming in during that time is either temporarily stored in the ash silos or dumped onto an emergency stockpile which is fed via the permanently installed emergency conveyor. The dump is large enough to accommodate the residues for about 25 years at the present capacities.
The low ground pressure of the crawler mounted spreader allows travelling on freshly dumped material. When the dump capacity on the ground level is fully utilized the spreader may dump the material in the high cast in a pre-calculated slope, making it possible to move the shiftable belt conveyor together with the spreader onto the next higher level giving sufficient dumping space for years to come.
At the Medupi plant a crawler mounted spreader is used also to form the residue dump. The ash is supplied by a shiftable conveyor tripper car to a link conveyor connecting the boom spreader with the tripper. The tripper car runs on rail tracks mounted to the shiftable conveyor’s sleepers. The dumping operation of a spreader in high as well as in low cast is shown in Fig. 10.
There will be two of these systems operating simultaneously on either side of the extendable conveyors. Once the full capacity of the pile is exhausted the equipment will be shifted to the next dumping area. The system will be operated for about 50 years with some 6 million m³ of ash being dumped per year.
Special attention is focused on environmental issues. The whole pile will be built on HDPE-liners placed on the ground after removal of the top soil. After shifting the conveyors the newly built dump is covered by liners and, subsequently, by the earlier removed top soil. So the area of residues exposed to wind and rain is limited to a minimum.
The growing demand for electrical energy in many countries as well as the replacement of old boilers by environmentally friendly new units are the drivers for modern residue removal systems. Although the solution has to fit to the specific requirements of each project a trend towards continuously operating systems can be observed.
In particular, the combination of tube conveyors with shiftable conveyors and mobile spreaders offers reliable, economic and environmentally friendly solutions. As could be shown here, these systems can be operated even under adverse climatic conditions. n
* * Prof. Dr.-Ing. Holger Lieberwirth The author is Director of the Institute for Mineral Processing Machines, TU Bergakademie Freiberg, Germany, Tel. +49-(0)3731-39-2558, E-Mail: Holger.Lieberwirth@iam.tu-freiberg.de
* ** Lothar Schmeier The author is Sales Manager Asia at Tenova Takraf, Germany, Tel. +49-(0)341-2423-500, E-Mail: firstname.lastname@example.org
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