(See also Chips)
Producing wood chips from energy wood in short rotation faces two fundamental challenges:
- Fine or bark content in wood chips
- Water content
Exactly, the bark content in willows in short rotation is relatively high, which decreases the quality of goods. Basically, the bark content reduces in total wood chips, the thicker the trunks are in harvesting. However, since willows grow with multiple shoots and do not reach a too large diameter, the relationship is rather unfavorable here. Large heating plants deal with this material better than small burners. Poplars have more advantages in this area. The bark content of robinia wood chips should not be estimated so negatively, since robinia demonstrate a very good heating value.
Farm-fresh wood chips of poplars and willows have the water content of 50-60%, while robinia with its about 35% is advantageous here. Thus, if you harvest it in short rotation (2-4 years) with a harvesting machine, then the material must be either used immediately in (large) burning facilities or dried forcedly.
Self-heating results in the known problems of fungal infection (health hazard!) and the continuous loss of dry matter.
Medium to long rotations (4-10 years) and harvesting with forest machines have a clear advantage here: Wood is not chopped immediately during harvesting, but put into interim storage for 1 season in sunny and airy conditions and, accordingly, dried down to below 30% of water content, becoming well usable or also storable.
Here an very interesting folder from the Austrian Biomass Association: Folder_wood_chips