A complex project to convert coal conveyors was made more challenging by the need to carry out the work while the plant was fully operational, write Peter Müller and Erwin Last
Arched belt conveyor as coal feeder
These units are fueled by lignite from the open pit mines at Garzweiler and Hambach. In August 2012, after an investment of €2.6 billion ($2.95 billion), RWE Power put two additional power plant block units of 1100 MW each and improved systems technology into operation.
So with its production of more than 4200 MW, the power plant covers more than 10 per cent of the installed output by RWE Power AG-owned plants.
However, units F and G revealed a need for improvement. The coal feeders to the coal pulverizers did not work satisfactorily and caused high costs for downtime and maintenance. During an inspection demanded by the customer, Aumund engineers diagnosed that the pan conveyors installed as coal feeders would fail completely in the near future due to existing deficiencies and foreseeable subsequent damage.
Such a breakdown would disrupt power production in block units F and G, because the lignite is discharged from the coal bunkers onto the pan conveyors operating as coal feeders. Two individual pan conveyors transport the lignite to the coal pulverizers, where the coal is ground to dust. There are four pulverizers from where the ground lignite dust is blown into each boiler. So, in total, the two boilers of block units F and G are supplied with fuel by eight pulverizers and 16 pan conveyors.
The coal feeders were designed as chain pan conveyors with two chains each. Due to this design there was an increased wear on the chain links, causing a differential lengthening of the two chains.
The pans of the conveyors got twisted and rose from their assemblage. The protruding pans acted like smoothing planes and abraded the material being conveyed. Besides, since the pans did not overlap, the material was trickling in between them and causing wedged pans.
Portions of the material trickled into the casing and onto the feedback conveyor. The wedged pans caused the material to bulge over the sideboards of the coal feeder and to trickle into the casing and onto the feedback conveyor as well. The spillage conveyor was actually designed to feed the spilt material back into the material transport only. The excessive material feeding caused heavy wear. Occasionally the spillage or cleaning conveyor transported more material than the main pan conveyor. In that way, continuous heavy wear on all moving parts of the machines evolved.
Installation of new pan conveyors
Due to the tight constructive situation and the high costs for new machines, Aumund Fördertechnik won the contract for conversion of the conveyors. The company was tasked to optimize the existing pan conveyors. The order included design, construction and supervision of installation as well as regular reviews and maintenance after conversion.
First the existing conveyors had to be retrofitted by Aumund’s conversion specialists during a rotational plant downtime while the furnace kept working. Thus a general overhaul of the heavily worn conveyor during an unscheduled boiler downtime, a medium-term necessity under the circumstances, could be avoided.
Conversion during operation
The most demanding phase of the project was at its very beginning. After taking the measurements of the installation and an analysis of the problems, a pre-design was drawn even before an offer was written. The pre-design became part of the offer.
After receiving the order, the detailed design was drawn and the project planning set up. Already at this point, the planning of the parts transport to the machines to be converted and the installation sequence was decisive for the execution of the project. Only certain routes of transportation were usable, since a number of routes were blocked by maintenance and repairs of other parts of the power block unit. This especially had to be taken in consideration for delivery of the new parts to the coal feeders just in time.
Optimizing the conveyor
Structurally, the casings of the original machines were preserved as fa as possible, and reinforced. On the outside sections of the machines, transmissions, engine and bearings were re-utilized.
On the inside, however, all components including the drive shafts were replaced by Aumund machine parts of the BPB 250 line. The spillage conveyor was redesigned and its tensioning and drive axes were exchanged. Thus the drive shafts of the spillage and main conveyors were positioned on the same side of the coal feeder in the end – an advantage for accessibility and maintenance of these decisive components.
One of the few changes to the casing was the installation of a maintenance opening for better access to the drive shafts. As a consequence of the change to the drive shafts, the tensioning stations of both conveyors had to be adapted to the new design.
The feedback conveyor was stabilized by conversion from two-strand to three-strand design. Originally it was built from two chains with a flat steel bar in between for pushing the feedback material along. Due to the increased material feeding because of the leaking main conveyor, the chains lengthened here as well, and the flat steel bars started bending. An additional third chain strand now acts as a stabilizer in a cross direction.
Longitudinal stabilization was achieved by increasing the preload through installation of additional axes. Due to their own weight, the chains were sagging between the individual axes. However, between the axes the chain is self-tensioning, so the distance between the axes was too great. It was decreased by installation of additional axes.
Also, one of RWE Power AG’s specifications was to build the coal feeder pressure tight up to one bar. This specification was not met by the original machine. Aumund stress analysts calculated the installation anew, and based on these calculations, pressure tightness according to specification was achieved in the course of the conversion.
For the conversion, RWE focused on pan conveyors with an average performance of 150 tonnes/hour. During peak times, the conveyors can transport 200 tonnes/hour. One pan conveyor transports sufficient lignite to produce about 1 MW. While arranging the new machines within the existing casings, some constructive tricks became necessary to achieve the conveying performance needed: Aumund converted the machines with welded pan conveyors, which had to be adapted very individually for the connection to the hopper.
Special construction had to be realized
The pans of these pan conveyors overlap in the same way as a long brick laying structure. This prevents the material from trickling between the pans and blocking them, or from trickling trough between them.
While equipping the pan conveyors with new chains, the designing engineers selected Aumund’s AU6052 chain. With a significantly higher chain safety than is normally chosen for comparable uses, notably higher durabilities can be achieved.
Limited space challenge
The Aumund conveyors with a standard width of 200 to 300 mm had to be built into the extremely cramped space. Simultaneously with an adaption and a complete exchange of the former material feed, the conveyors were equipped with a new surface. Due to the limited space available between material feed, toe board and rollers on one side and the outer edge of the casing – pressure tight up to one bar – on the other side, a special construction had to be realized. The conveying speed was given.
Optimizing the conveying performance was only possible through changes to the cross section of the conveying elements. The cross section results from the width of the conveyor and the height of the toe board. The height of the toe board was increased, resulting in the need to lower the feedback conveyor. In addition, a smaller sprocket wheel was used. Literally every millimetre of the available casing’s interior was used. In some instances the usual minimum distance between conveyor and casing wall was undershot. Because of the negative pressure loading of the coal pulverizers, special attention was given to a better sealing of the entire casing to avoid air leaks.
The material loads underneath the bunker chutes were brought under control by installing a baffle beam. This beam deflects the shearing forces caused by material being fed onto the running pan conveyor into the structural steel work. The use of new carriers between the chain strands, the lowering of the tensioning axis and the installation of a new sprocket wheel made the conversion perfect.
To complete installation and put the equipment into operation within the planned timeframe, Aumund developed an installation schedule specifying in detail each step and the manpower needed for each step. All work was completed on time. The supervision of the installation was undertaken by up to four Aumund supervisors on site.
In two shifts, they supervised the expert execution of the conversion by the client’s installation company. At times, the installation company worked on different parts of the machine simultaneously. In these situations at each site, a supervisor was present.
Besides supervising the installation, it is the duty of the supervisor to be the contact person for the foreman of the installation company and for the client’s project leader. Certainly, the special challenge of this project was the co-ordination within the fixed time frame.
Every millimetre of available space was used
After 12 months of operation, the converted coal feeders were inspected under Aumund’s Preventive Maintenance Service (PREMAS) without tracing any new unusual wear. That proves the machines to be ready for long-term use at present and in future.
With precise time targets and cramped space conditions, the coal feeders were optimized so that they met expectations for the first time. The conversion by the Aumund specialists proved to be much more economical for the client than buying a new machine.
Through professional and detailed project planning it was possible to stay within schedule and budget, despite the very complex task at hand.
Peter Müller is Senior Sales Manager and Erwin Last is Manager Field Service at Aumund Fördertechnik GmbH, Rheinberg, Germany.