Biomass Feedstock Storage for Quantity and Quality Preservation - Engineering and Science of Biomass Feedstock Production and Provision - page 168

at the preprocessing plant, since less force and less energy will need to be exerted to
mill the material.
Kaack and Schwarz [ 10 ] measured the force exerted by a plunger before the
rupture of Miscanthus × giganteus and M. Sinensis node and internode segments.
They also measured the distance by which nodes and internodes bent before breaking,
and derived the modulus of elasticity of the point of inflection. Womac et al. [ 11 ]
measured the cutting force of switchgrass stems by using a double shear setup.
Stiffness of plant nodes and internodes is a measure of the cutting forces needed
to cut shoot segments. In Chaoui et al. [ 12 ], an LRX Plus Materials analyzer (Lloyd
Instruments Ltd., UK) was used to evaluate the stiffness of Miscanthus. Materials
testing machines are test frames used to test the tensile and compressive properties
of materials. Stiffness is the highest slope of a strain versus time graph when a blade
with a 1 mm edge is pressed into a Miscanthus sample. The height and horizontal
diameter of the sample are measured and factored by the LRX Plus Materials ana-
lyzer (Lloyd Instruments ltd., UK) program when calculating stiffness. The effect of
plant age, storage temperature, and packing density on stiffness were demonstrated
and expressed in models by Chaoui et al. [ 12 ]. Nodes were significantly more resis-
tant to cutting than internodes. Lignin, hemicellulose, % solids, and segment diam-
eter were the other factors that significantly affected stiffness. Packing density,
cellulose content, time in storage, and storage temperature did not affect stiffness.
These effects were modeled as:

• Stiffness of a node (N/mm) = 2,989.0 + 130.17 *Lignin (%) + 54.91*Hemicellulose

(%) + 2.92*Solids (%) + 1.69*Diameter (mm)

• Stiffness of an internode, N/mm = 130.17 *Lignin (%) + 54.91*Hemicellulose

(%) + 2.92*Solids (%) + 1.69*Diameter (mm)

These results show that older and drier plants are more resistant to cutting,
regardless of storage conditions. Therefore, it might be more effective to store and
process Miscanthus plants (or other cellulosic biomass) harvested before
senescence.

7.3.3 Coeficient of Friction and Angle of Repose

The coefficient of friction and angle of repose would allow designing container
walls that can withhold the pressure exerted by the biomass pile within them. These
coefficients also facilitate designing the slanted surfaces of augers, which would be
placed inside storage containers to allow the interspacing of air and biomass vol-
umes, for better drying or diffusion of gaseous additives. The coefficient of friction
is derived from the angle at which a surface is inclined when friction forces no
longer keep an object, a biomass particle, adhered to the given surface. The coeffi-
cient of friction μ is defined as

m= F
N

(7.1)