PHYSICAL PRETREATMENT OF BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 247

450

Straw, cutting mill, 10% moisture
Straw, cutting mill, 20% moisture
Straw, cutting mill, 30% moisture
Misc, cutting mill, 10% moisture
Straw, hammer mill, 10% moisture
Wood, hammer mill, 10% moisture

400

350

300

250

200

150

100

50

0

0 2 4 6 8

Sieve diameter (mm)

FIGURE 8.1 Influence of sieving size on biomass-specific milling energy. (Source:
Reproduced with permission from Spliethoff (2010). © Springer.)

8.6 PARTICLE SIZE CHARACTERIZATION

Particle size distributions can be determined using standard sieve series. A sample of
ground biomass with known mass is loaded on top of a stack of such sieves; these are
closely held together by a fixation system and positioned on a vigorously vibrating
bottom plate. After shaking for several minutes, the stack is dismantled and the accu-
mulated mass is determined for each tray. Table 8.2 gives an example of particle size
distributions of pellets crushed using a two-roll mill (De Jong, 2005). The highest
value indicated in the d p range per sieve is the sieve size. Sometimes, the sieve sizes
are expressed in the Tyler number or mesh size.
Now, the question is how to determine an accurate average particle size based on
such results. One approach might be just to calculate the ordinary weight average
particle size, as is shown in Equation (8.5). In this equation, each d p,i value is deter-
mined as the average value of the indicated size range per sieve tray (d p average in
Table 8.2). The value for the top sieve is determined by taking the previous sieve
range, here no. 17, and consider the same size range for the final one:

d p = X

n

w i d p , i ð Eq : 8 : 5 Þ

i =1

Such a formulation is not very practical in view of its application for which area
(transfer phenomena, surface reactions) and/or volume (pneumatic particle transport,
fluidization) characteristics are of importance. Therefore, other formulations have