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Twig 25 – 35%
Leaf 25 – 35%
Bark 5 – 10%
The proportions of the different components vary depending on environmental factors, tree size and
spatial configuration, with a range of lesser factors also impacting on component proportions at time
of harvest.
As a tree develops, leaf canopy size will develop to near maximum size within the first few years and
then the growth of wood will continue while canopy size remains relatively constant and so declines
as a proportion of the total biomass. Young trees therefore have higher leaf to wood ratios, with this
ratio reducing as the tree ages. Tree spatial configuration also impacts on the leaf to wood ratio with
the highest leaf area ratio recorded in single or double row configurations rather than multiple row
configurations. This is discussed in greater detail in section 1.2.1. Both the relative proportions of leaf
and stem, and total component yield will impact on harvest strategies depending on the final product
being targeted.
Each of the whole tree components (wood, twigs and bark and leaf) has different chemical
compositions (leaf and twig material is significantly higher in alkali metals than stem wood), and
physical characteristics and consequently different potential commercial values. More significantly
there are a number of different potential products which can be derived from the different tree
components.
Whilst load densities in the order of 350-400 kg/m 3 have been recorded (Bartle, J, Pers Com, 2011) a
varying mix of the components in a woodchip blend (leaf, twigs, bark and woodchip), along with
component size, can potentially impact on transport density and subsequently transport cost.
Potentially also, moisture content of leaf components may also impact on the compliance of smaller
components and subsequently dry packing density of the product. This will be amplified with respect
to fresh weight density. The inherent variability in product density will potentially therefore impact
on transport cost and subsequently on the distance the product can economically be taken for
processing.
Leaf and twigs in the ex-harvester product present other issues. Whilst hardwood chip has good
storage characteristics, the leaf material is significantly more prone to degradation, and this will
impact on potential value. If oil extraction is to be undertaken this must occur within days of harvest.
Depending on the intended uses of the product being harvested and the transport distances involved,
separation of the product into components at or soon after the harvesting process could offer
significant advantage. Nominally, strategies which could be appropriate include:
• Separate the product on the harvester, with the components of lower industrial value being
rejected and deposited back into the field. This is similar to grain harvesting where all material
other than grain is rejected or sugarcane, where the aim is to separate the trash and return it to
the field.
• Separate the products on the harvester, but have parallel transport systems to forward the
material to different processing nodes. Figure 4.3 illustrates a system being trialed at a Brazilian
sugar mill, where the trash separated by the harvester extractor system is transported to the mill
in a separate transport system instead of being left in the field. Figure 4.4 shows an option
which is available for tree choppers and forage harvesters to remove a proportion of the leaf
and lighter material, however conceptually this could evolve into strategies to simultaneously
take two product streams from the field.
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