Recycling of balsa wood from rotor blades for the production of insulation materials

Rotor blades of wind turbines have a service life of 10 - 20 years (Figure 1). After this, they are usually incinerated at a cement plant. A renewed material utilization of the components does not take place.

With a length of 40 m and a mass of approx. 26 tonnes, a rotor blade consists of approx. 24 t glass fiber-reinforced plastic (GRP), 1.3 t balsa wood and 0.5 t metal. With an average balsa wood density of 150 kg/m³, such a rotor contains around 8.5 m³ of valuable balsa wood, which is available for recycling processes.

Subsequent to the first use in the rotor blade, the outstanding material properties of the balsa wood should then be used in a second application in insulation material, preferably through cascade utilization.

In an innovative SME project at the Fraunhofer Institute for Wood Research WKI in collaboration with the Nuremberg Tech (TH Nürnberg) and seven small and medium-sized partner companies, suitable recycling and reutilization procedures for the accruing large volumes of balsa wood are to be defined.

The project partners are focusing on a holistic consideration of the recycling chain for the balsa wood. They thereby undertake rotor-blade dismantling in the field, mechanical material processing and the homogenous separation at the recycling depot as well as the development of recovery procedures – which form the main focus of the project.

Dismantling and shredding of rotor blades

The renewed utilization of the balsa wood requires efficient dismantling and the targeted separation of the sandwich material. Within the framework of the project, water-jet cutting is being examined as the procedure which will minimize both wear on the cutting material and dust generation.

Separation of the balsa wood from the glass fiber-reinforced plastic (GRP)

The partners at the TH Nürnberg carried out initial comminution tests on rotor blades, during which the parameters of comminution type, mass flow, particle size distribution and energy consumption were examined more closely. The first successful experiments on the separation of balsa wood and GRP were achieved with the aid of density differences (Figure 2). Over the further course of the project, the researchers validated these results industrially and compared them with other procedures, such as the zigzag sifter. They subsequently subjected the recovered balsa wood to various sieve analyses in order to determine the particle size.

Production of insulation materials from balsa wood

For the production of wood-fiber insulation materials, the recycled balsa wood was optimally defibrated in a refiner by means of thermomechanical pulping. Utilizing both conventional and newer binding agents, the scientists produced differing wood-fiber insulation materials with densities of 20 to 250 kg/m³. Via the pulping of the balsa wood in an atmospheric refiner, a wood-fiber/water mixture can be extracted which, through a foaming and subsequent drying process, can be applied as a pressure-resistant wood foam without synthetic binding agents.

Outlook

In further planned work, the researchers intend to use the balsa wood for the production of lightweight “wood polymer composites” (WPC) in order to reduce the usually very high densities of WPC (approx. 1 100 kg/m³) by half.

An ecological balancing of the reprocessing steps and the manufacturing procedures for the various insulation materials should ultimately lead to optimal recycling processes.