PHYSICAL PRETREATMENT OF BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 237

For commercial pulp and paper logging application, also debarking (see, e.g.,
Hatton, 1987; Nurmi and Lehtimäki, 2011) takes place, which means that the outside
part of the wood, the bark, is removed by machines.
When trees are harvested younger and more frequently compared to “ conven-

tional and this also holds for woody energy crop plantations (e.g., the

fast-growing plant species Miscanthus sinensis )

” forestry —

one speaks of short rotation forestry
(SRF). Here, machines are applied that fell, bunch, and process single stems or
“

—

coppice ” regrowth. A detailed overview of techniques is given by Sims (2002).

8.2.2 Agricultural Crops: Cereals' Straw

For the harvest of rice, wheat, or other cereals with a resulting by-product straw
(or stover in case of corn), in most modern farming systems, a so-called combine
is used. This machine combines cutting and threshing (removal of grains from the rest
of the crop including stalks). A combine is a self-propelled harvesting machine that
cuts the whole grain plant and separates the grain kernels from the remaining straw
material. This straw is left in windrows spread throughout the harvested field. It is
ensured that part of the straw (or stover) is left in the field for soil conditioning; also,
to prevent taking in too much soil, not the whole plant is cut. After natural drying, the
straw is usually picked up by a mechanical baling machine (baler), which compresses
the straw in packs or bales. Some combines are equipped with cutting mechanisms
for the straw enabling a speedup of straw degradation in the field. In areas where straw
is rather collected for other uses, these cutters are generally not applied.
The main method for collecting the straw in the field is baling. Baling comprises
collecting, exerting pressure on the loose biomass, and tying it together with a cord;
these bales can be wrapped in plastic in order to prevent rewetting. Baling is a
common method used to improve the characteristics of agricultural residues for
transport, storage, and further handling (Maciejewska et al., 2006). The size of the
bales as well as their density depends on the machine used, and the final density
depends on the type of biomass feedstock and the machine. Transportation of bales
in general is substantially cheaper than transportation of raw biomass, and in some
cases, the gain is up to 50% (Van Loo and Koppejan, 2004). The bales are produced
in various shapes, densities, dimensions, and weights. Bales can be square or round
and their final size depends on the machine used. The density of the bales, for a given
machine, depends on the type of biomass.
A Canadian study conducted by the Composites Innovation Centre showed that
the harvesting and baling cost of wheat straw, when done by farmers themselves,
for very large bales of 544 kg with a density of 160 kg m −3 , is approximately

t −1 with collection included), while for custom round bales, of 408 kg,
the harvesting and baling cost is 9.5

t −1 5

€

(7

€

t −1 with collection included).
Because harvesting and baling is done by farmers, labor costs have not been
taken into account. Generally, the bigger the bale, the lower the baling cost and
the cost of subsequent transportation of the bale (Composites Innovation Centre
[CIC], 2008).

t −1

€

(12.8

€