PHYSICAL PRETREATMENT OF BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 249

An alternative for the characterization of particle size distributions forms laser diffrac-
tion measurement; the treatment of this characterization technique is

considered to be out of the scope of this topic (the reader is referred to, e.g.,
Barth and Winefordner, 1984; Merkus, 2008; Pankewitz, 2006; Syvitski, 2007).
Results of hammer milling of different types of biomass, corn stover, wheat straw,
and switchgrass, show that the particle size distribution can be described by a
Rosin

— and others —

- Rammler distribution:

b

d p
a

e − M cu =1 − ð Eq : 8 : 10 Þ

in which M cu is the cumulative fraction of particles (wt%) with a size smaller than d p ,
d p is the particle size (assumed equal to the nominal sieve aperture size), a is the size
parameter (also called the Rosin

Rammler geometric mean diameter; this is the size
at which 63.2 wt% of the particles are smaller than d p ), and b is the distribution
parameter (also called the Rosin

-

- Rammler skewness parameter).

Question : How would you determine b for a given size distribution?

8.7 SCREENING AND CLASSIFICATION

In order to separate possible adhering contaminants from the biomass and to generate
a product stream with reasonably uniform particle size distribution, sieving and
classification are needed. The biomass streammay contain contaminants such as sand,
ferrometals (e.g., nails), nonferrometals, plastics, etc.
Iron (and other magnetic metals) can be separated by either permanent magnets or
electromagnets, which can be integrated in a belt conveyor construction. The content
of sand can be reduced by separating it from the biomass flow with, e.g., a horizontal
vibrating sieve. Sieving technologies for biomass comprise disk sieving systems
(flexible stacked sieves driven as a belt), plate sieves, and rotating (trommel) sieves.
Wind sifting can be used to remove light contaminants, such as plastics.

8.8 METHODS OF MOISTURE REDUCTION

Most applications of biomass in energy production systems require drying to a more
or lesser extent. For example, for fluidized bed combustion, a moisture content of
about 60 wt% still results in acceptably low heat losses; this is common practice in
sludge combustion to reduce the volume of this waste. In contrast, in the production
of wood pellets, the moisture content must be reduced to approximately 10

15 wt%.
This difference in the required reduction of the moisture content also determines
which technologies for dewatering can be used.
Moisture contents in widely differing types of biomass can be reduced by different
techniques. These are natural drying, mechanical drying, and thermal drying.

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