An Approach To Balance Nutrient Exports and Nutrient Recycling In The Future - Quantification of Element Contents In Different Wood Chip Size Fractions

In recent years, continuing bark beetle calamities led to high supply of low-quality raw wood chips. To meet the specifically demanding quality requirements of small-scale combustion systems these fuels must be processed technically. During the screening process, fines, needles and bark particles are removed, as they would otherwise have a negative effect on combustion (e.g. higher emissions and ash content). However, screening residues contain a higher proportion of nutrients and organic ingredients compared to their volume. At present, these residues are mainly composted or incinerated in large-scale heat and power plants and are there for permanently unavailable for forest sites. The Bavarian State Institute of Forestry investigates whether screening residues could be redeployed to forest sites as a natural "fertilizer" or buffer to mitigate nutrient exports due to harvesting operations. Thus far, the state of knowledge does not provide data on overall quantity or proportional nutrient content in wood chip screen residues for respective sieve sizes. Therefore, a new method for measuring the distribution of nutrients in different particle fractions was developed. 30 samples of wood chip screening residues as well as their source raw material (if available and retraceable) were collected to provide an overview of current practices of the Bavarian market. Based on EN ISO 17827-2 for particle size distribution in wood chip samples, the standardised sieve sizes were adapted to the dominant particle sizes in residues and source material. Two further sieve fractions (1 mm – 3,15 mm and < 1 mm) we installed below the standard fine fraction (< 3.15 mm). A first, statistical analysis of the proportional occurrence of particle sizes showed a prevalent inclination towards smaller fractions in the source raw material (8 mm – 16 mm at 30 %, 16 mm - 31,5 mm at 22 %). The most used sieving sizes known from a former case study were 10-12 mm, 5 mm and 18-20 mm. At present, all samples have been compartmentalised in the predefined particle sizes (according to EN ISO 17827-2) and will be milled to analysis grain size (< 63 µm) for the elemental analysis by a benchtop XRF spectrometer. Of particular interest is the fraction containing the highest amounts of essential nutrients, which are crucial for tree growth and the forest ecosystem, such as N, Mg, Mn, P, etc. This allows to determine an optimal sieve size for nutrient availability and forest growth and guidelines for future nutrient-optimised wood chip supply could be developed.

This new method could help to derive a nutrient-optimal wood chip screen size for different wood chip assortments and support wood chip suppliers in determining which screen sizes are most advantageous for nutrient recycling to buffer nutrient exports from forest ecosystems.