Environmental Engineering Reference
In-Depth Information
Woody biomass is typically processed at the roadside landing area into woodchips, which can then
travel considerable distances because of their higher energy density and mass density.
7.4.2 c
hipping
Woody biomass materials have a high mass and energy density compared with similarly processed
herbaceous material. Although the high density of woodchips improves storage and transport, it also
leads to higher costs to grind the woody materials. Size reduction is one of the major uses of energy
in wood harvesting operations (Naimi et al. 2006). Wood chipping is achieved by mechanical force
applied to the biomass, commonly using a set of knives, hammers, or an auger. Knives are mounted
on the surface of a flywheel that is powered by an engine.
Woody biomass chipping, combined with a traditional lumber harvest, can turn treetops and other
nonuniform wood into uniform chips, which can be sold for profit. Chips have ready markets and
sufficient density and other desirable flow characteristics that make it inexpensive to transport and handle.
7.4.3 f
orESt
r
ESiduE
B
undling
After delimbing, limbs can be bundled using a mechanical bundler to make trunk-sized bundles
for more efficient storing or hauling. Some feller/bunchers are designed for thinning small-
diameter wood and have an integral bundler system. The bundler can also be used for loading
trucks that line-haul woody biomass. Bundling may be considered as a form of densification of
forest residues, much like baling is for agricultural crops. Further size reduction and/or chipping
operations can be conducted at a satellite preprocessing facility, a biorefinery, or an industrial
bioenergy power plant.
7.5 short-rotatIon Woody BIomass harvest
Short-rotation woody biomass crops are trees, commonly willow and poplar, grown specifically for
use as biomass. In a process called “coppicing”, these fast-growing trees are harvested every 3-5
years, being cut off at the base while leaving the root system intact, which allows the trees to regrow
for future cuttings. Because they are grown in even-age plantations, these trees are more uniform
in size and shape than forest residues. This uniformity allows the material to be chipped, bundled,
or baled, with yields from 10-20 Mg DM/ha per year (Savoie et al. 2006). Bundled woody biomass
can be baled wet (40-50% moisture) and will still air-dry, unlike herbaceous biomass that must be
baled dry or ensiled if wet. These woody crops can dry slowly in bales with minimal degradation
because of their relatively slow moisture transfer through the bark and evenly distributed porosity
in the bales, which allows rapid surface drying while the interior of the wood is still moist. Short-
rotation woody crops can also be harvested using a forage harvester (called a wood chopper) with
a modified head to produce woodchips. As with forage harvesting and silage chopping, trucks are
needed to follow the wood chopper to collect chips. Chips are often stored in piles for weeks or
months, during which time there may be some biomass degradation and dry matter loss, depending
on the tree species, local climate, ventilation, soil contact, and nutrient content.
7.6 densIFIcatIon
Densification and preprocessing refer to the handling steps that occur at the farm or a regional
satellite preprocessing facility (Carolan et al. 2007) to minimize storage and transport costs.
Preprocessing usually indicates size reduction whereas densification indicates an increase in
bulk density. Densification is currently used in almost all farm-level hay harvesting procedures
during baling or ensiling. Densification can be used in any herbaceous biomass handling system to
minimize handling and storage costs.
