Biomass Feedstock Storage for Quantity and Quality Preservation - Engineering and Science of Biomass Feedstock Production and Provision - page 174

erosion increase as the bales become larger. Significant runoff from bales was found
for larger bales, but the depth of weathered layer in bales stored outdoors increased
quadratically over a 6-month storage period, reaching a depth of 7.5 cm.
The effect of type of surface that a switchgrass ( Panicum virgatum L .) bale is in
contact with, and shelter from the elements, was correlated with dry matter losses
by Sanderson et al. [ 32 ]. They modeled dry matter losses in 275 kg round bales of
switchgrass (1.39 m diameter and 1.19 m long) with less than 10 % moisture at the
point of baling as

Final dryweight in kg =- 278 01. * daysafter baling (7.2)

A more accurate way of calculating dry matter loss would be through a storage
technology-agnostic model, in which the bale and ground surface interface would
be substituted with the level of water activity (also known as water potential) and
microbial activity. Since the soil is a source of water and microbes, these factors
would be elevated in the bale region close to the ground relative to other regions in
the bale. A water diffusion factor would determine the extent of spread of microbial
oxidation of dry matter from the soil/bale interface. The total dry matter losses due
to the surface area of this interface can be calculated by running a simulation on a
model to account for the above mentioned factors. Development of such a model is
one of the important research directions of this field.

7.4.4 Freezing or Cooling

Freezing has both the effect of reducing water activity and reducing temperature.
Microbial activity ceases at 4 °C; however, reaching and maintaining this temperature
requires more energy than drying biomass to a 15 % moisture level. After biomass is
refrigerated, its temperature tends to increase under the effect of entropy and the equi-
librium with ambient temperature. This is attenuated by good insulation, but energy
is still used to keep the biomass at 4 °C. Cooling reduces but does not inhibit micro-
bial activity. Oxidation rate is halved with every 10 °C decrease in temperature [ 22 ].
Freezing and cooling can be achieved by fitting storage tanks with compressors
and cooling reagents. The energy requirements of such a compressor can be calcu-
lated based on the compressor efficiency, power, ambient temperature, biomass
heating capacity, and moisture content which influence heating capacity. Ice blocks
can also be used if the biomass is to be kept cool only for the length of transportation
time, from the harvest site to the production plant. In the Midwestern region of the
USA, switchgrass and Miscanthus would be harvested in the winter months of
November to March. One argument in favor of freezing is that the ambient tempera-
ture during these months is quite low and there is snowfall. Freeze drying, therefore,
may not be such an energy intensive operation for such a situation. Recently,
Eckhoff [ 33 ] studied this option for storing high-moisture corn so as to enable the
use of corn stover for ethanol production or as animal feed. He developed a prelimi-
nary system design using this concept, which showed promise.