Engineering Properties of Biomass - Engineering and Science of Biomass Feedstock Production and Provision - page 28

Fig. 2.3 Hausner ratio of biomass grinds at different particle sizes ( filled diamond : switchgrass;
filled square : corn stover; filled triangle : wheat straw)
packing structure to a more compacted form. Normally, a low Hausner ratio indi-
cates that the initial bulk density of the packed particles after loading is already quite
close to the tapped density. Those packed particles required fewer number of tapping
to rearrange the particle packing structure in order to reach the final tapped density.
The particles with poor flowability will limit the tendency of the packed particles to
rearrange their pack structure to achieve a closer packing caused by tapping. For
example, the particles with a rougher surface may increase the internal friction
between the particles and thereby limit the particles to move to fill up the bulk pores.
When the biomass particles were further fractionated into individual particle
fractions and their individual bulk density was measured [ 19 ], the bulk density of
the switchgrass and wheat straw stem particles increased with decreasing particle
length and the bulk density of the switchgrass and wheat straw stems increased by
10-50 % due to tapping. Switchgrass achieved lower Hausner ratio than corn stover
and wheat straw for different particle sizes between 0.25 and 0.71 mm (Fig. 2.3 ). It
was observed that the switchgrass particles flowed into the container relatively
faster than wheat straw and corn stover, and hence, the initial bulk porosity was the
lowest. Upon tapping, switchgrass particles had less space to fill up the interspace
voids and the bulk density increased slightly to reach the final tapped density.
The flowability of the biomass grinds with the same particle sizes was different
by the results of the angle of repose [ 19 ]. This difference in flowability was attrib-
uted to different particle shapes and the forces between the particles. The moisture
content of wheat straw, switchgrass, and corn stover after grinding was roughly
between 5.1 and 8.7 % in this study. This suggests the capillary force acting on
those biomass particles is within a close range, and moisture content did not show
much effect on the cohesiveness of the grinds. Nzokou et al. (2005) report that lig-
nin and extractive content on the wood surface affects the wettability and the Van
der Waal's force between the particles [ 54 ]. It may further help to explain the effect
of lignin and extractives content on different biomass flowability.