Download (4Mb) - USQ ePrints - University of Southern Queensland

The quality of cut may be less important in trees cultivated for biomass where the supply chain
objective is a material suitable for burning when compared to other cultivated agricultural crops.
Nevertheless, the quality of cut may impact on the coppice or regrowth that grows from dormant buds
under the bark of the cut stumps.
The mallee harvester principle is to use a heavy disc saw to cut the base of the mallee stems and in
many instances this will also skim the top off the lignotuber from which the stems sprout (Figure
2.6). Sawing is a process that involves many small cuts rather than a single blow by a knife as in the
case of a canecane base cutter.
The quality of the cut by the saw is a factor for the regeneration of the crop, as a clean cut without
shattering the stump will reduce the opportunity for disease, and in young mallees a clean cut will
minimise the risk of uprooting the lignotuber. The mallees are also relatively strong, and at some
point the forces of saw impact may also become a matter of harvester durability, especially after
several harvests when the lignotuber and root system have become quite large.
Figure 2.6 The top of a mallee lignotuber removed by the harvester’s saw (above) and the condition
of the site with cleanly cut stumps after passage of the harvester (right)
It is anticipated that harvest speed will be restricted to less than about 4 km/hr to control the size of
each saw-tooth’s cut, maintain an acceptable level of impact force, and to allow the harvester to be
directed onto the mallee stump without excessive lateral forces on the harvester’s head or the mallees.
Processing the harvested mallees through the chipper is a complex process of passing very sharp
knives, working against a fixed anvil, through strong wood at specific angles to the fibres of the
wood. The process of chip formation involves both cutting of the fibres, and equally importantly, the
splitting of chips behind the knife which allows the knife to continue its passage through the wood.
Knife sharpness is critical and when knives lose their edge, even before they appear blunt to the
naked eye, there will be a significant increase in the specific energy of chipping and reduced the
capacity of the chipper to feed itself. Forestry chippers typically have a knife change about once a
shift. The angle of grind on the knives is also a critical issue, so knives are normally removed and
reground with specialised workshop machinery. The angle of knife impact upon the wood also needs
to be within a narrow range to ensure chip quality is maintained and specific energy is minimised.
44