Technology for Wood and Natural Fiber-Based Materials

Research Project

Recycling of Waste Wood

The collective term "waste wood" is used to denote all woods and wood-based products whose end of life as a product has been reached and which therefore fall under the definition of waste. Sources of raw materials which have, until now, only been insufficiently utilized should now be developed - for example, the wood categories A III and A IV, which are classified under the Waste Wood Ordnance as contaminated. 

© Fraunhofer WKI | Peter Meinlschmidt
Waste wood and residual wood.
© Fraunhofer WKI | Manuela Lingnau
NIR sorting facility at the WKI.
© Fraunhofer WKI | Manuela Lingnau
A look at the inside of the NIR sorting facility during operation.

The German "Altholzverordnung" (Waste Wood Ordinance) provides for a division into two sub-groups, referred to as industrial residual wood and waste wood. The former comprises leftovers - such as, for example, sawdust - from wood processing, wood machining or the wood-based materials industry which are only subject to the regulation if they cannot be used within the company or marketed as a by-product and must therefore be disposed of.

The latter were formerly products and can be roughly divided in their origins into packaging materials, demolition waste wood, wood from the construction industry, wood from urban areas and wood from industrial or commercial activities. This is of particular importance because as a rule, the Waste Wood Ordnance provides for an allocation of the resulting waste wood determined by the place in which it occurs and its intended use. The amount of waste wood in Germany was, according to data from the Federal Statistical Office, approximately 11 million tonnes in 2010. Since about 10% was declared as hazardous waste or A IV timber, it can be assumed that around 10 million tonnes of materially-usable waste wood resulted in the categories A I to A III.

Currently, the material usage of waste wood in Germany takes place primarily in the wood-based materials industry for the production of particle boards and fiber boards. Whilst the proportion of recycling wood in Germany had remained at around 20% for a number of years, it rose in 2010/2011 to 33%. In the comparable countries of Great Britain and Italy, the proportion reached 55% and 89% respectively.


The goal

Sources of raw materials which have, until now, only been insufficiently utilized should now be developed - for example, the wood categories A III and A IV, which are classified under the Waste Wood Ordnance as contaminated. These wood wastes additionally contain plastics such as PVC, wood preservatives and paints which contain heavy metals. Even in this contaminated lignocellulosic material there is a not inconsiderable amount of recyclable timber which needs to be recovered through appropriate separation and sorting processes.

In order to comply with legal limits during usage, complex chemical analyzes such as GC-MS, ICP-OES and IC have been necessary up until now.

The goal is the development of a rapid detection method, with the help of which unambiguous sorting criteria can be developed which allow the determination of whether a waste wood is still materially or only thermally recyclable. Near-infrared (NIR) spectroscopy has the potential to replace a part of the elaborate laboratory analyzes.

The technology

General data on the sorting facility at the Fraunhofer WKI

  • Manufacturer: RTT-Steinert
  • Multiplexer spectroscope with InGaAs detector
  • Measurement of the diffuse reflectivity in the wavelength range 1400-1900 nm
  • 40 fiber-optic sensors
  • Illumination with 4 x 300 W halogen spotlights

Particle size

  • Minimum: 15 mm
  • Maximum: 300 mm

Conveyor belt

  • 600 mm wide
  • Speed: max. 2 m/s

Discharge system

  • Nozzle bar with 40 solenoid valves spaced at intervals of 15 mm
  • Nozzles operated with compressed air
  • from the integral compressor
  • or externally
  • Operating pressure: 5.5 bar
  • Air consumption: approx. 600 Nl/min
  • Adjustable expulsion retardation and expulsion duration
  • 2-way sorting



The results

With NIR spectroscopy, completely different types of plastics can be differentiated from one another. This means that it is also possible to differentiate between various types of WPC granulates (wood plastic composite) which have been produced using different types of plastics (PE, PP, PVC, PLA). A clean separation and recycling of this thermoplastic material is thereby possible. For this purpose, complex mathematical pre-processing steps (centering, spectral and spatial filtering, removal of spectrally-noisy border areas and, for data reduction, a principal component analysis (PCA) are required.

When suitable main components are chosen, all plastics can be clearly distinguished from each other and therefore cleanly separated.

Furthermore, differentiation between pure waste wood or uncoated particle board and particle board coated with melamine or PVC is simple to realize. A differentiation between types of wood as well as various organic wood preservatives seems to be possible.