Characterization of biomass using instruments – Measurement of forest and crop residues - Biomass as Energy Source: Resources, Systems and Applications - page 59

For this reason, manual sampling will yield different measures of moisture content depending on
where the sample is taken in a shipment.
3.3.2.3 Transport
Transportation is unlikely to cause major changes in the moisture content. However, a slight
change in the moisture profile might occur so that the bulk of a shipment will have slightly
different moisture content than material close to the upper surface.
An analysis of data from Bestwood customers suggests that this phenomenon is due to factors
that may be divided into three groups. First, when forest residue is stored in an open container
it is subject to weather that will affect the moisture content near the surface. When rain falls,
precipitation will make the surface area more wet then the bulk. The contrary occurs during a
warmand sunny day. This effect, whichmight take place during storage as well as during transport,
is seasonal. Further, if an open container is transported during warm weather, the speed wind is
likely to dry the surface area. Finally, transport vibrations are supposed to make smaller particles
propagate to the lower part of the container. With the smaller fraction removed from the surface,
the upper region of the container is more likely to contain material of a slightly larger size and
somewhat lower moisture content. A similar effect is anticipated to occur in the winter when
transport vibrations cause snow particles to move towards the bottom of the containers.
As a rule of thumb, one might assume that the surface region is somewhat drier than the bulk.
The magnitude and direction of this bias depends on the season and weather. When sampling is
done close to the upper surface, this bias will affect the results of the moisture determination and
the price settled for a shipment.
3.3.2.4 Site storage and fuel handling
Storage at the CHP is subject to similar effects as in the terminal although bacterial activity tends
to be less of a problem since the average storage time tends to be far lower due to limitations in
storage capacity.
While the moisture content itself is unlikely to change during storage, several actions may be
taken in order to stabilize the moisture content of the fuel that is fed to the boiler. First, it is
common to mix different types of forest residue in order to compensate for anticipated difference
in humidity. Then, the fuel storage silo is used for further homogenization due to additional
mixing. Finally, using screws that extract fuel from an intersection of different layers of material,
the variation might be decreased further.
3.4 MOISTURE MEASUREMENT
3.4.1 Gravimetric moisture measurement
3.4.1.1 The gravimetric method
Moisture measurement traditionally uses manual sampling and gravimetric determination.
Moisture determination is often performed according to the Swedish standard SS 187 113 (1998)
but the actual procedures are carried out in a multitude of ways. The samples are combined into
a single sample, where after a smaller subsample is chosen and weighed in a tray (Fig. 3.3). The
tray is dried in an oven for 20-24 hours. After weighing the dried sample, and knowing the weight
of the empty tray, the moisture content is calculated.
An indicative precision of the gravimetric method is approx. 2%-units of moisture when two
trays from the same sample are compared. The performance depends on the heterogeneity of the
material, since the precision is likely to deteriorate if the sample is less homogeneous.
The oven method is not flawless. Volatile organic compounds with high heating value may be
evaporated during drying. This flaw can be compensated for by using default calorific values
that refer to the respective types of biomass. Further, the degree of evaporation differs somewhat
depending on the position of a certain tray in the oven and on the overall level of moisture in the