Characterization of biomass using instruments – Measurement of forest and crop residues - Biomass as Energy Source: Resources, Systems and Applications - page 57

during a contract period. In the case of large shipments, such as 5000 m.t. vessel, the size of
the shipment often motivates specific sampling. In other cases, determination of ash content or
fractional distribution might be done on a regular basis such as with a weekly or monthly interval.
3.3 THE FUEL CHAIN AND ITS IMPACT ON THE MOISTURE CONTENT
3.3.1 The fuel chain
There are several routes for forest biomass to the end user. Forest residue is either transported
directly to the end user or stored at a terminal. Sawmills and pulp factories deliver a part of their
by-products to the energy producers. However, pulp factories use part of the residue for their own
energy productions while saw mills deliver saw dust for the production of pellets rather than use
by CHPs.
Wood for burning, and felling residue such as tops and branches, are often stored for some time
in the forest. Often, the material is left to dry and its extraction coordinated with other activities in
the forest. Chipping or crushing might take place on site in the forest, at a terminal or at the power
plant (Fig. 3.1). Due to economies of scale, it is desirable to perform this task in large quantities.
However, it is often made in the forest if the receiving plant has no chipping equipment or when
transport costs make it too expensive to send the forest residue to a terminal for processing. In
Sweden, processed residue is usually collected in metal containers in the forest. This way, the
material can be stored on several forest sites and be collected by a vehicle typically being able to
carry three containers.
Part of the material is transported straight to the energy producer while other fuel might be
stored at a terminal in order to build up a supply before the cold season, i.e. the winter. The
fuel is often transported in side-tipping trucks (Fig. 3.2). The volume is somewhat larger than
for container trucks since it is desirable to distribute overhead on the largest possible volume.
Side-tipping trucks are also common for transportation of residue from pulp factories and saw
mills. However, since the containers are fixed to the truck, this type of vehicle is not suitable for
collection of material in the forest.
3.3.2 Sources of variation in moisture content
3.3.2.1 The forest
While the moisture content in the respective parts of a tree or a fresh log is relatively stable,
the measured moisture content in the resulting biomass tends to vary depending on storage and
transportation.
In the forest, the moisture content of the forest residue is affected by the season of the year, the
storage time and by the choice of storage area. The traditional pattern is that the moisture content
of the delivered forest residue is higher during the winter than in the summer. In some cases, forest
residue is left on the forest floor in order to dry during the late spring and summer. However,
several factors affect the moisture content of a certain shipment. Examples of actions that affect
moisture content include if the material is put in a high and dry place, or in a low place where
rainwater collects. There will also be a difference in moisture content depending on whether the
forest residue has been protected by a covering material or not. Further, when biomass is extracted
in the winter, rain and snow might settle in the open metal containers when they are stored in the
forest. Another example from the Scandinavian winter is that roads often have to be opened from
snow in order to access the biomass. To facilitate transport, the forest residue is stored adjacent to
the roads. Therefore, snow walls are pushed into the piles that will be partially filled with snow.
This snow will then be transported along with the biomass to the energy producer.
3.3.2.2 Terminal storage
When forest residue is stored at a terminal in order to build up supplies, material is gathered in
large piles. These piles tend to be filled with biomass from several origins and with different
humidities. Further, the moisture content will change during storage.