PHYSICAL PRETREATMENT OF BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 245

a horizontal or somewhat inclined rotating vessel is filled with balls up to a charge of
approximately 30%. These balls grind the material by friction and impaction by their
tumbling (Nag, 2008).
The ball-milling technique is also used to characterize the grindability of coals by
the so-called Hardgrove grindability index (HGI). The HGI is a standardized test that
has been developed empirically and is commonly applied to various coal classes to
describe their uniformity and ease of pulverization. The traditional test involves grind-
ing a sample mass of coal (50 g with an initial size between 0.6 and 1.18 mm) in a
special milling apparatus, which crushes the coal using steel spheres at a specified
load and number of revolutions (60) (British Standards Institution [BSI], 1995).
The mass of the resulting pulverized coal fraction that passes through a 75 m sieve

determines the HGI. Thus, a higher HGI indicates a sample that is more easily grinded.
The method first uses a calibration based on four known coals. Bridgeman et al.
(2010) modified this traditional standardized test for (torrefied) biomass by using a
fixed volume (50 cm 3 ) instead of a fixed mass (i.e., 50 g) and calibrated their results
using coals of known HGI. The results of this modification are promising with respect
to further standardization for heterogeneous biomass.
Table 8.1 gives an overview of the different techniques that are used for
milling of biomass. Based on its high size reduction ratio and adequate control
of the particle size range with comparatively good cubic particle shape, the
hammer mill is widely used (Mani et al., 2004). Knife mills have shown success-
ful functioning for shredding forages for different crops and machine conditions
(Ige and Finner, 1976). Sometimes, disk mills are used to produce very fine
particles, but input feed is needed from either knife mills or hammer mills
(Hoque et al., 2007).
Generally, the energy requirements increase with increasing moisture content and
decreasing final particle size distribution and also with increasing rotational speed of
most equipment. Spliethoff (2010) showed that the energy consumption increases
strongly with decreasing particle size (see Figure 8.1). The moisture content also
has a significant impact on energy consumption as material with higher moisture con-
tent is tougher; for milling of straw with a moisture content of 30 wt% and a sieve size
of 2 mm, the energy demand was more than 8% of its heating value (Spliethoff, 2010).
The energy needed for size reduction by milling can generically be expressed as
(Ghorbani et al., 2010)

μ

E = C ð 2
1

dL
L n

ð Eq : 8 : 1 Þ

kg −1 ), C a constant, dL the differ-
ential size (dimensionless), and L the screen opening size (mm). Different models
have been proposed in the literature. Bond (1961) assumed a value of 3/2 for n,
and Rittinger assumed that the process is basically shearing and thus that the energy
requirement is proportional to new surface created, so that n = 2. Finally, Kick
assumed the energy requirement to be a function of the common dimension of the
material only, and thus n = 1 (Henderson and Perry, 1976).

with E being the specific energy consumption (kJ