Tree-processing Using a Chain System in Finland

by E Alakangas, VTT Energy and K Rieppo, Metsäteho Oy, Finland

Introduction

Researchers in Finland are assessing a transportable tree chipping unit based on chain delimbing and debarking. More than 100 units of this type are in use in North America. In Finland, one unit is in operation which, since 1991, has been used mainly for handling pine from first thinnings.
 In chain processing, the wood cut in haulage lengths is delimbed and debarked with one machine, prior to chipping. The whole process can be carried out at the roadside, storage or consumption site, or at the mill as the unit can be mounted on a truck. Small wood can be handled by the unit separately.

How it Works

The trees are fed to the unit by a loading crane. They are delimbed and debarked by steel chains, then moved on to a three-knife disk chipper. Chips are blown out via an outlet pipe either onto the ground or into a truck.
 A by-product of the process is limb and bark waste which, after being homogenised by a crusher, is suitable for combustion (eg in heating plants). This useable waste amounts to about one third of the biomass of the whole tree.

Siting

In Finland, researchers have structured the best siting for the unit. The results are complex as the economics depend on many variables, such as the working time, raw material, transportation distances and the amount of merchantable wood available at the site.

Chip Quality

Ensuring the quality of the chips is very important; the bark content should be less than 1% of dry mass. The Finnish experience is that the quality of chips produced using the chain system depends on the characteristics of the wood, on technical factors and even weather conditions. Finnish operators have found that if the wood is very small, dried or frozen, the limit for bark content is exceeded, normally by up to 1%. The most difficult wood to process in this way seems to be frozen, unlimbed spruce, for which satisfactory results have not been achieved.
 Development work is improving the debarking results and two projects are in the testing stage.

Applications

The chain delimbing-debarking method could supply at least part of the raw material for pulp mills in Finland. If timber is procured using the whole-tree system, the top part of the tree, down to 5 cm diameter, will be used for energy production.
 The next generation of units could replace large debarking drums. Bolts of small diameter could be multi-tree handled.

Advantages

The advantages of chain processing include the flexible handling of small wood, which reduces raw material loss. The investment cost of the unit is much lower (possibly by up to 50%) than that of a large debarking drum.
 Both unlimbed and delimbed trees can be handled by the system, allowing flexibility in wood procurement. More fuel wood can be harvested together with merchantable wood. By using several chain units in place of a single large debarking drum, the effect of maintenance stoppage on production would be greatly reduced.
 This study of the applicability of chain processing to Finnish conditions was carried out by Metsäteho Oy, the Finnish Forest Research Institute, Pertti Szepaniak Oy and Enso Oy.

For more information contact the CADDET Finnish National Team in Helsinki.

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