Biomass Feedstock Storage for Quantity and Quality Preservation - Engineering and Science of Biomass Feedstock Production and Provision - page 169

where F = friction force of the particle against the surface and N = normal force from
the surface in reaction to the object weight.
The angle of repose is related to material density, surface area, and coefficient of
friction. It is the angle formed by the sides of the biomass pile relative to the ground
surface.
Chaoui et al. [ 12 ] measured the angle of friction by placing two superimposed
molded slabs of milled Miscanthus on a hinged platform. The platform had a ledge
that mobilized the lower slab only. The platform was pulled by a cord as the ana-
lyzer's automated moving part traveled upward. The operation was manually
stopped when the top slab first slid off the lower slab. The radius of the arc formed
by the moving lever, and the distance traveled by the tip of the cord were used to
calculate the angle of friction. The angle of friction was significantly affected by
solids content; the drier the plant, the smaller the angle of friction. Solids content
affects the angle of friction as follows:

Angle of internal friction (Degrees) = 55.87 − 0.25*Solids (%)

Storage conditions, plant composition, and time in storage did not affect the
angle of friction. A lower angle of friction implies more pressure on walls of con-
tainers holding milled Miscanthus. It also determines the inclination angles of plat-
forms along which Miscanthus should slide.

7.3.4 Density

Bulk density, also reported as biomass wet density, is determined by the amount of
pore space and biomass particle density. It is a very important property of the bio-
mass since the large volume of feedstock often creates storage and transportation
challenges as highlighted in the introduction section. Bulk density determines the
weight capacity of the structure where biomass is stored before transport and the
cost of transport as well. Chapter 2 has a detailed section on bulk density.

7.3.5 Chemical Composition: pH, Lignin, and Cellulose

In the unusual case where biomass is stored wet, a pH different than that for a neu-
tral case (pH = 7) would influence the rate of microbial oxidation in anaerobic con-
ditions. Extreme pH levels can denature proteins, including the enzymes, which
hydrolyze substrates into the simple sugars consumed by microbes for energy. The
pH level can also influence the solubility of metals used as cofactors by these
enzymes. Extreme pH level can also inhibit the hydrolysis of cellulose and fermen-
tation of sugars into alcohol. If biomass became too acidic during storage, or was
treated with strong bases, pretreatment would be necessary to adjust the material pH
before conversion.