Pilot 6: Improving storage properties of slash

This pilot was implemented in Austria and Switzerland.

Slash, also referred to as logging residues, is a byproduct of wood harvesting consisting of branches, needles and tree tops. This material is of relatively poor quality and is used for heat and/or power generation in medium to large scale biomass heating and CHP plants in the form of wood chips. Most of the slash usually occurs in the warmer seasons of the year when the demand of heating/CHP plants is typically low. Therefore, different treatments of slash have been tested to improve storage properties and increase the possible storage period. Additionally, storage tests over longer periods have been carried out to compare and evaluate the effects different pre-treatment measures have on slash and how they influence its behavior over longer storage periods.

Objectives

The main objective of the pilot application is to find a way to increase the possible storage duration of slash by improving its storage properties. An economically feasible storage is currently hindered by high mass losses caused by microbial decomposition processes. Further, the high amount of fine material has a negative effect on ventilation and drying of the material and leads to higher corrosion, emissions and ash formation during the incineration process.

If it would be possible to store slash without high energy and mass losses in a cost effective way, this would broaden the resource base for biomass heating and power plants and could help to reduce the current competition between energetic and material use of biomass in Styria.

Pre-treatment & storage variants

Different pre-treatment measures of slash followed by storage tests of the treated samples have been carried out in order to capture, measure and compare the effects of the different treatments on the storage properties and the mass and energy loss of slash. Treatment measures include chipping, covering slash with fleece during storage, screening to remove fine particles and green material as well as technical drying with and without screening.

Storage testes have taken place from July to October 2014 (~4 months) in two different locations in Styria, Austria. The storage period corresponds with the time and season of the year when storage of slash is necessary, since the material occurs but the demand is low. The material has been stored in piles of approx. 250 m³.

Storage variants:

  • Chipped and untreated slash (reference sample)
  • Storage of unchipped slash
  • Roofed storage
  • Open air storage
  • Covered storage

Results & expected impact

Storage tests have been finished in beginning of November, samples have been taken before and after storage and were analysed by research partner Holzforschung Austria (HFA) which determined different physical and chemical parameters. First results show that screening of slash before storage is an effective method to increase the quality of the material.

The advantages derive from:

  • Reduction of water content through decreasing of fine content, which binds high amount of water
  • Reduction of ash content through screening out of soil, sand and stones
  • Increase of heating value through the two factors described above
  • Reduction of nitrogen content (contained in needles)
  • The possibility to return nutrients to forest stands

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On the other hand, the tests also showed that the quality increase achieved through screening can easily be annihilated through wrong storage. The negative effect of a weather-exposed storage can be seen in the figure below. The rain-laden summer 2014 caused an increase of moisture content between 3 and 7% in different open-air stored samples. Technical drying on the other hand leads to an acceleration that would take place anyway. The covering of the material with fleece does not reduce the moisture content during storage but seems to preserve the initial state. This might be a promising option to maintain the quality of screened slash during storage if no roofed storage area is available.

A promising possibility would be the combined chipping and screening of slash directly on the forest road: this way, the needle material which contains the majority of nutrients would remain in the forest while the necessary transport volume would be reduced.

If the quality of slash can be raised through pre-treatment and storage, demand and therefore market value is likely to increase as well, allowing forest owners, harvesting companies and biomass traders for more cost effective harvesting, processing and transport of harvesting residues. Therefore, every actor in the regional biomass supply chain will benefit from improved storage characteristics of slash and the created added value resulting from the increased quality of the material.