Demag cranes: The travelling crane

For resource-saving generation of energy from biomass, process cranes optimally adapted to the individual case are used.

The degree of automation is based on cargo-handling volumes and the requirements of the operator. But as green cranes, the installations always work quietly and in an energy-saving and highly efficient manner.

“Recycling before disposal”: more and more companies and local authorities are implementing this principle of putting sustainable economic activity into practice and using recyclable materials to generate energy for their own use.

In this way, they are not only acting in a responsible manner but also saving costs over the mid- and long term, as they are making themselves more independent from rises in energy prices.

The number of companies, local authorities and energy suppliers who use renewable energy sources is even greater.

What this has to do with cranes perhaps only becomes apparent at a second glance. Materials such as waste paper, straw or wood chippings are used as energy carriers; they must be fed into the installation for energy generation or further processing.

The use of process cranes individually designed for each case of application is expedient for this purpose.

This applies with respect to the degree of automation and likewise to the grab, and integration of the crane control system into the data flow of the company or the power plants.

Some more up-to-date examples from the practical engineering experience of Demag cranes illustrate the challenges presented here.

Paper manufacturer uses waste materials

In addition to wood chippings, the paper manufacturer SCA also uses solid-liquid waste materials from the production process for energy generation in its Sundsvall plant in Sweden.

For continuous feeding of the two furnace lines, a Demag double-girder overhead travelling crane was installed. This crane works fully automatically and undertakes several tasks at the same time: it piles up wood chippings in the storage area and spreads the supplied sludge over them.

A homogeneous, energy-rich fuel is thus created which is fed via a hopper for incineration. The crane is designed to operate perfectly, even at temperatures as low as minus 30 °C; eight motors for long travel and four motors for cross travel ensure exact positioning, even in strong winds.

The crane does not automatically switch off until wind force 10 is reached. The very low noise level during operation is also worth a mention: Since the crane is working in the vicinity of a residential area, special noise abatement measures have been implemented with great success.

Profile check by scan: not lifting higher than necessary

For fully automated process cranes, the Company has now implemented innovative technologies for reliable 24/7 operation: a scanner installed on the crane bridge records the material height in the storage area each time a crane travels over it.

From this data, the crane software creates a height profile and steers the crane exactly over the material in the bunker, so that the grab does not have to be raised higher than necessary.

This saves time and thus increases the handling rates. It also reduces the energy consumption of the crane installation, which in technical terms is, therefore, also well suited to its “green” environment, i.e. the generation of energy from biomass.

A further reduction in energy consumption can be effected by use of mains power regeneration. Here in the braking and lowering processes, redundant energy is fed back into the power supply system.

Correctly-sorted handling of wood chippings

Wood chippings are not only an interesting alternative to fossil fuels for private households but for power station operators too. In Hamburg-Lohbrügge and in Ludwigsburg, two identical wood-fired co-generation power stations, which were planned by the consulting engineers Ingenieurbüro Schuler (IBS) from Bietigheim-Bissingen, provide power for around 8,000 households.

Originally, thought was given as to whether wheel loaders should be used to manage the storage area and load the furnaces.

However, both noise abatement and the desire to work with as low a level of emissions as possible spoke for the construction of bunkers with process cranes.

This solution also offers the advantage that it can be implemented saving much more space. Since material transport takes place at the third level, no paths need to be kept clear and large quantities of biomass can be stored in the smallest space.

At each of the locations, one process crane distributes the material delivered by road truck – approx. 32,000 t/a – and stores it according to quality grade in one of three predefined storage zone.

When loading the day silo, the crane takes the fuel from the three zones in accordance with the specifications of the warehouse management computer in order to achieve a homogeneous heat output.

The crane control system thus ensures consistently high efficiency of the power station, which works in a CO₂-neutral fashion in accordance with the ORC principle (Organic Rankine Cycle).

In addition to the superordinate warehouse management computer and the crane control system, the automation concept of the crane also includes integrated crane scales, measuring sensors to monitor the filling level in the silos and inclusion of the crane control system in the process control technology of the CHP power plants.

It was decided to opt for a Demag process crane because it works in a highly energy-saving fashion.

Configurable strategies for optimised crane use

The handling of bulk goods in continuous processes places high demands on crane technology. Here, in addition to the specific design of the grab software, the control technology also plays a crucial role.

For assured maintenance of the incineration processes, system-appropriate design of the grab strategies is mandatory.

These strategies, which can be selected and adjusted via control screens, allow for the size and shape of the transfer points as well as subsequent further transport of the material to the incinerators.

The crane feed option to the sliding floor sections, as used for example in the power stations already implemented by IBS, requires materials to be picked up from the various storage zones and to be their continuously delivered in accordance with the operating cycle times of the sliding floor sections.

A continuous flow of material for incineration must also be ensured when the hoppers are fed. To this end, the grab arms open at finely adjustable intervals - depending on the condition of the hopper, in order to unload the material in a controlled fashion.

Depending on the size of the hopper, various transfer points are also approached. The optimisation of grab strategies serves the aim of preventing the formation of bridges in the hoppers.

Bales of straw: biomass as unit loads

For the above-mentioned examples, biomass is available as bulk material. For an English power plant operator, Demag designed and built a crane installation for handling bales of straw as bulk material.

The bales are subsequently further processed into pellets. A spreader with six travelling grabs that can pick up up to twelve bales of straw at the same time serves as a load handling attachment.

As an additional feature, a moisture measuring sensor was integrated in the grab. Sensors check the moisture content of the straw and only supply to the production process bales that do not exceed a defined maximum content of residual moisture.

This partly automated crane thus rightly bears the name process crane: it is completely integrated into the overall process of production, which generates approx. 100,000 tonnes of straw pellets per year and thus reduces the annual CO₂ output of the neighbouring coal-fired power station by approx. 120,000 tonnes.

Solutions on a standardised basis

The high demand for Demag cranes for handling biomass was the reason why the Company decided to develop a standardised solution that can be adapted to match individual operating conditions with low requirements in terms of engineering.

These mini bulk solutions, which can be operated manually or automatically, are already used for storage area management in biomass installations – among others in the woodchip installation of a British media company.

Conclusion: a new range of tasks for cranes

The trend in energy technology is clearly towards decentralised energy generation and the use of biomass is of particular significance.

A growing range of tasks for process cranes is the result. These cranes make it possible to achieve a clearly structured material flow with a low space requirement.

They also work provide for energy-efficient, emission-free and low-noise operation – green material flow technology for a resource-friendly application.

Grabbing the advantage

The correct “grab strategy” also contributes to efficient crane operation. In the case of the automated cranes delivered for the wood-fired cogeneration power stations in Hamburg and Ludwigsburg, a multi-jaw grab with a capacity of 8 m³ is used. With its high closing force, this grab can also pick up compacted material.

After penetrating the wood chippings, the jaws do not initially close completely, so that the first time the grab is pulled, a small part of the material can drop out.

Only after a few seconds does the grab close completely. This prevents material being discarded while in transit.

Handling rates are increased and energy efficiency, too: only material that actually reaches its destination is transported.

When the material is discharged, the crab moves in such a way that the material is distributed evenly on the sliding sections.