Demag’s client was Amagerforbraending, a waste management company owned by five municipal authorities, including the south of the capital Copenhagen.
The four furnaces of the Amagerforbraending incineration plant were in continuous operation for 29,000 hours in 2006. Each year Amagerforbraending collects and incinerates 430,000t of refuse, which is used to generate energy for the area. Amagerforbraending is also responsible for the district heating system for 500,000 inhabitants.
Together with a neighbouring power station, electricity is generated for 70,000 homes and 75,000 homes are supplied via the district heating system in winter. In the summer, when the power stations in the inner-city area are switched off, Amagerforbraending supplies district heating for all of Copenhagen and the surrounding area. Operators of waste incineration plants have been obliged to operate their facilities as combined heat and power plants in Denmark since the early 1990s.
The Amagerforbraending plant dates back to 1970, making it one of the pioneers in Denmark. The first cranes used at that time to serve the chutes were supplied by Thomas Smid, a company that was later taken over by Demag. These two cranes were replaced in 2002 to cope with the ever rising tide of refuse. The most important requirement was a higher handling capacity. Demag achieved this with a pair of double-girder process cranes, each with 16t SWL, 29.5m span, 90m/min long travel speed and 80m/min hoisting speed. Waste is handled by a powered multiple-shell hydraulic grab.
The client stipulated that during installation, the plant’s operations could not be interrupted. Therefore, to avoid opening the roof, a crane was introduced into the building through a large round window opening and installed. The second overhead travelling crane was installed after the two old cranes had been removed.
Since installation of the new cranes, incoming refuse trucks are controlled by means of a traffic light system and barriers. The crane control system communicates with the warehouse management computer and ensures that one of the six tipping stations is only released when the crane bypasses the area. This ensures that the crane travel motion is not impeded by falling refuse.
Although the crane installations were equipped for automatic operation from the outset, they were only used this way periodically. The cabin installed in the bay was occupied by two crane operators during the day shift, while one employee monitored the automatic processes during the night shift.
However, as far as the owners were concerned, this was only ever an intermediate solution. The declared aim was to transfer to full automation, 24 hours a day, seven days a week.
This would not only allow redeployment of crane operators but also increase the handling capacity. Increased handling was not necessary in order to put more refuse into the four bunkers. Their incineration rate remained identical. It was more important to blend the refuse better in advance and to achieve uniform incineration. This would keep the heating value as constant as possible. For this purpose, the cranes pick up the newly delivered refuse at the tipping position and build a heap of material. The refuse trickles down the side and blends with the other material. A better blend also requires more crane cycles.
Therefore Demag returned to the project in 2006 and installed scanners below the crane bridge. These scanners permanently track the height contours of the heaped refuse as the crane travels over it. The software uses this information to determine a constantly updated height profile of the refuse bunker. The grab no longer has to be raised to the highest possible position in order for the crane to travel over it. The grab position is adapted according to the profile of the heap, even while the crane is travelling. This saves time, increases the handling rate and therefore improves efficiency.
Since the exact fill level is known for every part of the bunker, the crane reduces speed during the last metre of the lowering motion before it picks up any further refuse. In this way, damage to the grabs has been totally eliminated. A load sway damping system also enables faster travel motions without risk to the grab.
A further advantage of the new automatic system is that both cranes can now operate independently of each other and are no longer set to work in a previously defined zone. The new control system software allows their travel motions to be performed at the same time: from picking up the refuse to serving the furnaces.
The cranes are monitored from a control room on the ground floor of the building. A start-up routine is programmed for serving incinerators to ensure that the refuse chute is fed in small steps. For this purpose, the grab, which has a capacity of up to eight cubic metres, initially picks up small portions of 250kg and increases these quantities until the optimum burning temperature has been reached.
Signals are exchanged with the control computer to specify the quantity that the crane has to pick up when it feeds the chute with next load.
Further automatic programmes for the crane systems can be created by the plant managers in the control room.
After eight months of planning, installation and commissioning of the new system was swift. The plant was shut down on a Saturday and returned to fully automatic operation the following night.
Each crane is assigned two hours a week for maintenance and cleaning.
The former crane operators have been deployed to other duties in the plant, with six in the control centre and one in the company’s store.
