PHYSICAL PRETREATMENT OF BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 244

is sufficient. This technology is also used for predownsizing of particles to enhance
further size reduction characteristics (needed particle size distribution and energy
requirement). Chipping and shredding devices are standard in use for the maintenance
of, e.g., parks and gardens and make use of fast-rotating cutting blades mounted on the
face of a flywheel. Large wood chippers are frequently equipped with grooved rollers
for (reversible) material gripping in the throat of their feed funnels (tinyurl.com/
bo9lcb). The following types exist:

￿ High-torque roller : This type of uses low-speed grinding rollers, is

driven with an electric motor, and is characterized by quiet, dust-free, and
self-feeding operation.

“ shredder ”

￿ Drum : Drum chippers are among the oldest available and have a fast-rotating

drum powered by an engine usually by means of a belt. The drum is mounted
parallel to a hopper and spins toward the output chute. Modern types can handle
biomass material with a diameter of 150 - 500 mm.

￿ Disk : Knives are mounted on a steel disk, and reversible hydraulically power

wheels draw material from the hopper into the disk area, which is mounted
perpendicularly to the fed material; the knives of the spinning disk cut the
biomass into chips, which are thrown out toward the chute. Industrial-grade
types can have disk sizes as large as 4 m; similar biomass sizes as in the drum
type can be handled, and disk-type chippers produce more uniformly shaped
and sized chips.

Shredding using rotary action has been shown to result in different specific energy
consumption values for different types of biomass and derived products; e.g., paper
is much more difficult to be shred than grass; shredding paper to ~40 mm size was
shown to require an energy of 15.2 kWh t od −1

in a second pass for further size reduction to 25 mm; for switchgrass, these figures
were 8.2 and 4.1 kWh

t od −1

in a first pass and 7.6 kWh

t od −1 , respectively (Schell and Hardwood, 1994).

8.5.3 Crushing

Materials that are comparatively hard and brittle are reduced in size by making use of
their nondeformation nature, i.e., these materials rather break than bend. Crushers that
are suitable for such materials are jaw crushers, but also roller mills and hammer mills
can be used.

8.5.4 Milling and Pulverization

Mills of many different types can be used to grind biomass into the finest particle size
classes. In the past or still in (very) traditional practices, mills for producing flour from
grains (wheat) were driven either manually (e.g., mortar and pestle), with animals
(horse mills), by wind power (windmills), or water (water mills). For the processing
of coals on an industrial scale, ball mills are usually applied. In this concept,