PHYSICAL PRETREATMENT OF BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 257

2 1

ii i

3

Moisture content exceeding the equilibrium value

FIGURE 8.3 Schematic of the drying rate of some granular materials; 1-2 and i-ii represent a
constant drying rate regime; 2-3 is the “first falling-rate” drying regime.

So,

=t f )

K A ln w c −

1 w e 1

K A ln

f c
f

t f = ð Eq : 8 : 20 Þ

w − w e

The total drying time is t c +t f , and as the drying rate in the constant-rate period
is equal to the rate of drying at the start of the falling-rate period, R c =K f c , and
therefore,

=

t=t c +t f = w i −

ð w c Þ 1

K A ln

f c
f

1
K A

ð f i − f c Þ

+ln f c
f

+ ð Eq : 8 : 21 Þ

K A f c f c

Associated with thermal drying are emissions of VOCs; these consist of, e.g., terpenes,
alcohols, or organic acids. In case of an integrated combustion process, these gases can
be incinerated to prevent VOC emissions. Their formation depends on biomass
properties, resin contents, storing time, and operational aspects of the drying process:
temperature, drying medium, required final moisture content, residence time, and
particle size distribution of the dried biomass (Svoboda et al., 2009).

8.9 COMPACTION TECHNOLOGIES

Several of the disadvantages of raw biomass are a result of its low bulk density and the
resulting low volumetric energy density. Therefore, in order to reduce transportation
and storage and handling costs and to significantly improve its fuel characteristics,