Transportation - Engineering and Science of Biomass Feedstock Production and Provision - page 144

Fig. 6.3 Tractor trailer carrying round switchgrass bales

container with 32 round bales was investigated, and the conclusion was that the
packing time required to load the container presented the limiting factor [ 14 ]. Round
bales have higher biomass losses than rectangular bales during storage, but they are
preferred for completely open storage since they shed rain water more effectively.
A developing trend for harvesting and collecting dedicated energy crops and
agriculture residues is the single-pass harvest-chopping-baling combine. This
machine has the advantage of reducing soil contamination and biomass loss by cir-
cumventing infield windrowing, at the cost of requiring lower biomass moisture
content at harvest time. The single-pass system can also be applied to stover har-
vesting, where a combine separates grain and agricultural residue simultaneously
[ 15 ]. The development of the single-pass machine started over a decade ago, when
the Haimer company produced the Biotruck 2000, which combined a SPFH with a
drying system (elegantly using the engine's waste heat) and a pelletization unit. The
material density of the pellets ranged from 850 to 1,000 kg m −3 , and the bulk density
of the pellets, from 300 to 500 kg m −3 [ 16 ].

6.3.2 Size Reduction

As shown in Table 6.1 , comminution of biomass is imperative, since the conversion
processes cannot directly process crops as they grow in the field. Size reduction can
take place during harvesting, such as in the SPFH, which contains a chopping mech-
anism and a chute that pneumatically conveys the low-density chopped material into
a wagon. Balers typically also have a cutting mechanism, but the size of the material
strands is kept sufficiently long to allow baling using either string material in the
case of square bales or netting or other forms of wrapping in the case of round bales.
Further size reduction can take place at local depots or CSPs using wood chippers,
such as those employed for tree harvesting, hammer mills, knife mills, and tub
grinders that are often used to comminute bales for animal feed. Disk and attrition
mills produce biomass particles of more uniform shape and finer size at a cost of
higher energy consumption compared to hammer and knife mills [ 17 - 20 ]. The
vibratory ball mill was found to be more effective than the rotary ball or rod mill in
reducing cellulose crystallinity of spruce and aspen chips, generating fine particles