Transportation - Engineering and Science of Biomass Feedstock Production and Provision - page 146

Experiments have shown that the energy consumption of size reduction is high:
For instance, to grind air-dried (8 % moisture) Miscanthus through a screen with an
aperture size of 1 mm, 5 PIHV was required. Willow ground through the same
screen required up to 12 PIHV. By extrapolating these results, a particle size repre-
senting 100 PIHV, at which the energy required for comminution is equal to the
inherent heating value of the material, was 80 μm for Miscanthus and 50 μm for
switchgrass.
The biomass moisture content also has a significant impact on the energy require-
ment for comminution: The energy requirement for comminution of Miscanthus
and switchgrass with a moisture content of 15 % was roughly 1.5 times higher than
that of the same crops with a moisture content of 8 % (air-dried) [ 21 ].

6.3.3 Biomass Compression

Table 6.2 shows the energy densities of Miscanthus giganteus , switch grass, sugar
cane bagasse, corn, and coal. When Miscanthus is baled, its energy density ranges
from 2,223 to 2,565 MJ m −3 . Corn in bulk form on the other hand has an energy
density of 11,002 MJ m −3 , over four times higher. Anthracite coal in comparison has
an energy density of 30,005 MJ m −3 , which is 11.7 times higher than that of baled
Miscanthus. It is clear that mechanical compression of biomass is essential to opti-
mize the transportation efficiency, since at low material bulk densities the transpor-
tation medium reaches its volume limit far before its weight limit [ 33 ].
On-road flatbed and box trailer vehicles in the United States are limited to carry-
ing materials with a density of 231 kg m −3 ; thus, the achievable infield density of
bales is well matched to on-road vehicles. This is, however, not the case for long-
distance rail transport. Typical “gondola-type” railcars for coal are designed such
that they reach their weight and volume limits simultaneously [ 34 ]. If biomass could
be transformed into particulates with a bulk density equal to that of coal (850 kg m −3 ),
the existing coal transportation infrastructure could be expanded upon to accom-
modate the huge feedstock transportation task in the future.
Mechanical compression of biomass is a poorly understood process since the
biomass' mechanical properties in general and rheological properties in particular
are rather elusive. The compression process can be divided into three distinct phases:
(1) removal of air, (2) compression of biomass under material reorganization, and
(3) compression of material in a settled matrix. In the first process, little pressure is
needed, since merely the material porosity is reduced. In the second process, during
which particulates move and fill the pores, exponential or power law functions seem
to adequately describe the relationship between the applied force and biomass vol-
ume [ 4 ]. The third phase, where the biomass essentially behaves like a solid and
Hooke's law may apply, is only reached at extremely high pressures. During a high-
pressure experiment using Miscanthus as a test medium, this behavior was observed
at applied pressures of more than 350 MPa [ 35 ].