Compression Processing and Load Compression of Young Scots Pine and Birch Trees in Thinnings for Bioenergy

Abstract

In many boreal countries forest biomass from dense young stands has great potential to meet growing demands for bioenergy. However, there is currently no cost-effective operational system for harvesting such stands. When harvesting, it would be desirable to compress trees and leave some of the foliage and fine branches in the stand in order to reduce extraction of nutrients and improve the harvested material’s fuel properties from a reduced ash content point of view. The objectives were to evaluate experimental systems (a boom-tip mounted unit for processing bunches, and a small prototype forwarder for compressing loads) in terms of their utility for compressing and partially debranching fresh and stored bunches of Scots pine trees and compressing forwarder loads of fresh Scots pine and birch trees. Processing of fresh bunches resulted in mass losses of about 10% (dbh-class 5–8 cm; not significant) to 15% (dbh-class 12–15 cm; significant) with 35 to 50% reductions in ash contents and increases in bulk and net energy density of about 80 to 160%. In-stand storage of bunches before processing generally yielded no significant advantages except for a 10%-unit reduction in moisture content. Compression of forwarder loads resulted in 16 (pine) to 32% (birch) increases in load densities, with a significant increase for birch trees. The described techniques facilitate productivity increases of off-road and road transport of tree parts, and a substantial proportion of the ash-rich materials such as needles and fine branches are left in the stand. If both tested techniques are applied during the harvesting of young trees the benefits would be substantial.