Transportation - Engineering and Science of Biomass Feedstock Production and Provision - page 145

and improving their digestibility [ 21 , 22 ]. Polynomial relationships between bulk
density and particle size of switchgrass, corn stover, wheat, and barley straw grind-
ings were found [ 22 ] as

3 2

r b =+++ ax bx cx d (6.1)

Here, ρ b is the dry bulk density in kg m −3 , x is the geometric mean diameter of
particles in mm, and a , b , c , and d are regression coefficients. The relationship
between the dry bulk density and particle size of wheat straw and switchgrass (for
particles larger than 8 mm) can be described by a power law equation [ 10 ] in the form

- b

r b = ax (6.2)

where ρ b is the dry bulk density in kg m −3 , x is the nominal particle size in mm, and
a and b are regression coefficients. The functions as shown are specific to crop spe-
cies, initial biomass properties, milling machine type, and machine parameters.
Size reduction can also increase the bulk density of the biomass and, therefore,
can be regarded as a form of densification. The bulk density of chopped biomass
(greater than 25.4 mm) before finer size reduction is typically less than 80-100 kg m −3 .
For Miscanthus, it has been shown that the bulk density of the ground biomass
through a screen with an aperture size of 4 mm can reach values of 150 kg m −3 ,
which are well in the range of typical field-produced bales [ 20 ]. By grinding bio-
mass through a 1-mm screen and therefore performing further size reduction, a
density of 250 kg m −3 , equal to that of bales produced by high-pressure balers, can
be reached [ 20 ]. Although size reduction is an operation that requires ample energy,
it could also be used as an alternative to chemical pretreatment [ 23 , 24 ].

6.3.2.1 Energy Requirement of Size Reduction

Since size reduction is a key operation within the biomass provision chain, it is imper-
ative to assess the energy consumption of the machinery. This can be achieved by
monitoring the net input power that the machine requires for comminution and inte-
grating this power over time. In general, the energy requirement of comminution is a
function of the cutting mechanism (knife, hammer, ball), motor speed, feed rate, mate-
rial feeding mechanism, strength of the milled material, and degree of size reduction
[ 20 , 25 - 31 ]. The specific energy consumption of biomass comminution is given as

= (

)

Ef rsccxmcp , , ,, , (6.3)

Here, E is the specific energy consumption, r is motor speed of the milling machine,
sc is the milling or chopping machine scale, c is the material composition, fiber angle
and/or structure, x is the ratio of initial and output particle sizes, mc is the moisture
content in % w/w, and p is the applied axial pressure [ 10 , 20 , 22 , 32 ]. Independent of
the machine scale, a power (or exponential) law was found appropriate to describe the
relationship between energy consumption and resulting particle sizes.