Research and innovation
Opportunities and limits of circular economy and superior use of wood chip residues - „RestUse“
The research project „RestUse" aims to identify and evaluate new ways in which residual materials from woodchip processing (e.g. fines particles, needles) can be put to higher-value use. One main goal is investigating whether such residues are suitable for nutrient recycling in the forest ecosystem to mitigate nutrient extraction caused by wood harvesting measures.
Background
Ongoing calamities (e.g. bark beetle or drought stress) evoked a rising availability of low-cost wood. This trend, as well as the increasing importance of renewable energies, led to a growing supply of biofuels such as wood chips, which are mostly combusted in biomass heat (and power) plants.
Why residual materials from wood chip processing should be put to higher value use?
Currently, residues from wood chip processing are mostly used inferiorly for applications such as composting, which means that – in terms of quantity a large amount of nutrients stored in those sieve residue materials are permanently lost for the ecosystem.
Figure 1: Success of technical sieving of wood chips (© Kuptz, TFZ)
Drying and sieving – two steps of wood chip processing for a higher fuel quality
Zoombild vorhanden
Figure 2: Sieving as an important processing step for fuel processing; here: drum sieve (© Schreiber, LWF)
Quality requirements for wood chips depend largely on the specific needs of each combustion plant. For example, smaller heating boilers (< 100 kW thermal output) require a constant and predefined fuel quality to ensure an efficient and low-emission combustion process. To provide a certain fuel quality, two mechanical processes can be applied after chipping: drying and sieving.
Sieving removes residual material such as fine particles, overlength, and needles from the wood chips. Although these residues contain a relatively large proportion of nutrients and organic compounds, they can have a negative effect on combustion and the heating plant. Especially high shares of fines and needles lead to higher emissions and ash content as well as slagging.
Sieving removes residual material such as fine particles, overlength, and needles from the wood chips. Although these residues contain a relatively large proportion of nutrients and organic compounds, they can have a negative effect on combustion and the heating plant. Especially high shares of fines and needles lead to higher emissions and ash content as well as slagging.
Project approach
During the project "RestUse", studies are being carried out to investigate established recycling channels. Various partners have been recruited through a market analysis and were part of a practice study to determine the nutrient content in different wood chip residue assortments.
In a field study, the application of residual materials in the forest and their influence on the nutritional supply of two forest stands will be investigated by sampling needles and soil. Based on the acquired field data and the practical study, a possible "fertilization effect" and an ideal residue particle size could be deduced and recommended to relevant stake holders.
In a field study, the application of residual materials in the forest and their influence on the nutritional supply of two forest stands will be investigated by sampling needles and soil. Based on the acquired field data and the practical study, a possible "fertilization effect" and an ideal residue particle size could be deduced and recommended to relevant stake holders.
Figure 3: Overview of the work packages in the 'RestUse' project.
Work package „Market analysis“
In the beginning of the "RestUse"-project, a nationwide market analysis was carried out to obtain an overview of the status quo of processing practice for residual materials from wood chip processing, biomass processors and suppliers as well as heating plants.
Around one third of the 143 respondents stated that they screen wood chip assortments. In 68 % combine this step with technical drying. Almost half of all respondents use a barrel sieve, around a quarter use a flat screen and 17 % use a star screen. Other sieves were hardly ever used.
The interviewees were also asked about the wood assortments for their respective preparation process. There were clear differences in the assortments used for participants inside and outside of Bavaria. In Bavaria, residual forest wood chips made from coniferous wood are the most commonly used assortment for screening at 55 %. Outside Bavaria, residual forest wood chips from hardwood assortments and energy roundwood chips from coniferous wood are mainly used as raw materials for the wood chip production. This means that higher quality assortments tend to be screened outside of Bavaria.
Around one third of the 143 respondents stated that they screen wood chip assortments. In 68 % combine this step with technical drying. Almost half of all respondents use a barrel sieve, around a quarter use a flat screen and 17 % use a star screen. Other sieves were hardly ever used.
The interviewees were also asked about the wood assortments for their respective preparation process. There were clear differences in the assortments used for participants inside and outside of Bavaria. In Bavaria, residual forest wood chips made from coniferous wood are the most commonly used assortment for screening at 55 %. Outside Bavaria, residual forest wood chips from hardwood assortments and energy roundwood chips from coniferous wood are mainly used as raw materials for the wood chip production. This means that higher quality assortments tend to be screened outside of Bavaria.
The screening process usually produces three fractions:
- Main fraction
- Fine fraction/residual materials
- Overlength (is usually shredded again, depending on the treatment process)
When asked about the use of the main fraction, around 60 % stated that it would be used for energy production. Only around a quarter of respondents market their main fraction to other business branches such as material utilization (e.g. for chipboard construction, as fall protection on playgrounds).
Energy use also dominates the use and marketing of the fine fraction/residual materials at around 40 %. The second most common use of the residues is bedding in stables at 23 %, followed by material utilization at 20 % (e.g. as mulch material). A small proportion (13 %) of the fine fraction is used for composting or in other areas, such as landscaping or for charring (see Figure 4).
Energy use also dominates the use and marketing of the fine fraction/residual materials at around 40 %. The second most common use of the residues is bedding in stables at 23 %, followed by material utilization at 20 % (e.g. as mulch material). A small proportion (13 %) of the fine fraction is used for composting or in other areas, such as landscaping or for charring (see Figure 4).
Figure 4: Usage of wood chip residues
Work package „Case studies on established utilisation pathways“
Following the survey, 11 companies in Bavaria were acquired for further participation and case studies in the project. During these studies, various screening residues (N=30) of different particle sizes and wood chip assortments were gathered for element content analysation to obtain information about their particle-class related nutrient contents and composition, to investigate a optimum screen-size suitable for potential nutrient recycling.
An innovative method for analysing the screening residue was developed for this purpose. After drying the screening residues, the sample is fractionated into the following particle sizes on a horizontal sieve shaker:
- 45 mm
- 31.5 mm
- 16 mm
- 8 mm
- 3.15 mm
- 1 mm
- < 1 mm
Figure 5: Examples of respective particle sizes
All individual particle sizes were milled to fine powder grade and are currently being analysed to obtain their chemical element composition. This separate analysis of the different fractions is expected to enable the derivation of an "ideal" screen-size in terms of nutrient occurrence and availability for different wood chip assortments.
Work package „Field study“
Background
A region in Bavaria, which is characterised by a limited supply of nutrients is the Franconian Forest. It is therefore important to pay attention to the nutrient balance with forest management practices. Timber harvesting operations remove biomass and their contained nutrients from forest ecosystems (Figure 4). If these removals exceed the replenishment of nutrients through rock weathering and deposition, a continues decline of soil fertility could be at risk. A return of nutrients could compensate these deficits, which is why an adaptation of timber assorting during harvesting operations, or a return of nutrients is investigated in the "RestUse" project.
Figure 6: Schematic representation of nutrient flows in managed forests (© Weis et al. 2016)
Method
The study area in the Franconian Forest (district Kronach) is one of the very alkalis-deficient forest sites in Bavaria. For this reason, the analysis of the application methods of woodchip screening residues is particularly important in such a nutrient-insufficient area.
Two different sample areas were selected: a 75-year-old old spruce stand (see Figure 5) and a 30-year-old spruce thinned stand (see Figure 6). This allows a deduction about possible effects of the various application types, and also about the influence of age.
Two different sample areas were selected: a 75-year-old old spruce stand (see Figure 5) and a 30-year-old spruce thinned stand (see Figure 6). This allows a deduction about possible effects of the various application types, and also about the influence of age.
Figure 7 (© Riebler, Wendel)
Figure 8 (© Riebler, Wendel)
Before the woodchip residues and other variants of the field trial can be applied on-site, the initial state of the forest ecosystem must be characterised. This is done by sampling needles and the present soil horizons.
Needle sampling
In November 2021, needle samples were taken from selected sample trees. For this purpose, branches were removed from the young spruce stand using a ladder system (so-called "Diestel Leiter”) and a cutting handle (three branches from the sunlit part of the crown and three branches from the shaded part of the crown) (Figure 9). In the older spruce stand, tree climbers were used to gather branches (Figure 10). In both cases, the branches were subsequently separated on-site into 1st and 2nd needle year. The first needle year includes needles that were formed in the current (or recently completed) vegetation period, the second needle year includes needles that were formed during the previous vegetation period (at least 1 year old).
Figure 9: Needle sampling of the young spruce stand. Extraction of the sampling branches (1 & 2) and on-site separation of the needle years (3 & 4).
Figure 10: On-site needle sampling (older spruce stand).
After transportation to the LWF-laboratory, all samples were weighed in “fresh” and subsequently dried, to determine the water content of the needles. Afterwards, the needles were separated from the twigs (see Figure 11). A first analysis of the needles was the so-called "100-needle weight”, which serves as a simple vitality parameter. It is determined by the average mass [g] of 3 x 100 needles per sample. The samples were then finely ground using an ultra-centrifugal mill. The needle samples are currently being analysed for element content using a benchtop XRF-device.
A total of three sampling campaigns were carried out to obtain the needle samples:
- 1st sampling – record of the initial state: winter 2021/22
- 2nd sampling - 8 months after the application: winter 2022/23
- 3rd sampling - 20 months after the application: winter 2023/24*
* Only sampling of the young spruce stand: a majority of the old spruce stand had to be felled in early summer 2023 due to a massive bark beetle infestation
Figure 11: Preparation of needle samples in the laboratory. 1) Weighing the fresh samples. 2) Drying at 60°C. 3) Difference between fresh and dried sample. 4) Separation of the needles and twigs.
Soil sampling
Soil samples were collected in spring 2022 at the study areas of the old spruce stand. Sampling was repeated in spring 2023. The second sampling was carried out without a core drive sampler, only the humus and the upper mineral soil horizons were resampled.
Different sampling methods were chosen to map the entire soil profile:
1. humus sampling with a sampling frame (see Figure 12)
The humus layer consists of largely undecomposed, fresh litter (e.g. needles, leaves, dead mosses etc. = L/Of) and an organic layer (Oh). These two layers are carefully removed within the 20 x 20 cm cutting frame using a spatula. Living roots must be cut out with scissors and estimated for their volume.
2. sampling the mineral topsoil with a stem drill (see Figure 13)
In the second step, the stem drill is inserted into the humus sampling hole for another approx. 10 cm to sample the topmost mineral soil horizons. The different horizons are then identified and documented (transitions can be recognized by changes in colour and density in the soil).
3. sampling of the mineral topsoil to subsoil using a core drive sampler (see Figure 14)
To reach the lowest horizons of the soil, a core drive sampler was drilled into the ground with a percussion hammer (depth approx. 100 cm). The profile is lifted out, as in the 2nd sampling, and the individual horizons are identified and packed separately.
Figure 12: Humus sampling with a 20 x 20 cm frame
Figure 13: Sampling of the topsoil with a stem drill
Figure 14: soil sampling with core drive sampler
All samples were brought to the LWF lab and dried. After drying, the soil samples are first sieved to 2 mm and then ground in order to analyse the element contents.
Application of different variants
After recording the initial state, the application of the different variants started in May to June 2022. A total of four variants plus a control area were spread on each test plot in five randomized repetitions (see Figure 15):
- Woodchip screening residues with 8 mm screen diameter,
- Woodchip screening residues with 16 mm screen diameter
-> both residues were produced with a barrel sieve from a standard residual wood chip assortment - Modified screening residues (8 mm + mycorrhiza)
-> The mycorrhiza "vaccination" is intended to support the exchange of nutrients with the tree and is acclimatised for spruce trees (ectomycorrhiza) - Lime fertilizer with 30 % wood ash
-> The lime fertilizer is a commercially available natural lime product specifically for forestry usages. It is made from moistened dolomite lime mixed with 30 % wood ash - Control area, untreated
The application quantities for the wood chip residues were based on the recommended quantities for lime fertilizer (3 t/ha). Based on this, double the quantity of screening residues were dispersed (6 t/ha) to ensure a user-friendly and practical application as well as a realistic availability of the necessary quantities. All variants were evenly distributed on the study areas and the selected quantities ensured that the different variants would not cover the soil too much and thus disturb the natural decomposition of the soil flora and fauna.
The dose of mycorrhiza “vaccination” is based on the trunk diameter of the treated tree. Therefore, an average measured breast height diameter of all sample trees was used for the old and young stands to choose a correct dosage for the mycorrhiza.
Figure 15: All applied variants for the field study: 1) wood chip screening residue 8 mm, 2) wood chip screening residue 16 mm, 3) mycorrhiza, 4) lime fertilizer with wood ash
Field applicationof the variants
The required quantities of all variants were transported to the test plots in the Franconian Forest and weighed on site for each plot (Fig. 1-3). The material was homogenized in a barrel before application (Fig. 4), brought to the respective test plots (Fig. 5) and then spread by hand (Fig. 6).
Figure 16: Spreading of the variants.
Results
Currently, all collected samples are analysed directly for their element content after the final grinding step using a benchtop XRF. As this is an ongoing process, only initial evaluations from the first and second needle sampling campaign are given here. The final evaluation of all results will be available in summer 2024.
Results of the 100-needle weight of the 1st and 2nd sampling campaign (BP1 and BP2)
For the 100-needle weight, 3 x 100 needles are counted from each needle sample and an average value is calculated. This value is regarded as a simple vitality parameter which, together with the element analysis, can indicate correlations with the quantity availability of the critical plant nutrients.
Figure 17: Variance in needle weight of the 2nd sampling between the variants of the application trial
- Higher needle weight in sunlit crown (vs. shaded crown) and old spruce stand (vs. young/thinning stand)
- No significant differences between the variants
* N of BP 1 and BP 2; ** 1 sample tree was removed due to bark beetle infestation infestationskäferbefall vor BP 2 entnommen
- Decrease in all variants (incl. control area)
- Significant decrease especially in the newly formed needle year of the 2nd sampling
- Signs of stress caused by several years of drought and bark beetle infestation
Outlook
In the last year of the project, the final samples (mainly soil samples) are processed, and all ground samples are analysed for their element content. Subsequently, the results will be assessed with regard to the element distribution between the sunlit and shaded tree crown as well as the old versus the young spruce stand in the Franconian Forest. In addition, the wood chip and screening residue samples from the practical trial will provide information on the distribution of nutrients in the respective sieve fractions. All methods and results of this research project will be comprehensively documented in a project report.
Contact
If you have any further experience, inquiries or suggestions regarding the use of residual materials, please feel free to contact us at any time:
Project information
| Short Title | RestUse |
| Long Title | Opportunities and limits of circular economy and superior use of wood chip residues |
| Status | In progress |
| Project Manager | Markus Riebler |
| Deputy Project Manager | Dr. Herbert Borchert |
| Project Consultant | Katharina Wendel |
| Duration | 1st January 2021 – 31st October 2024 |
| Funding | Bavarian State Ministry of Food, Agriculture and Forestry |
| Funding Code | G2/N/20/11 |