PHYSICAL PRETREATMENT OF BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 243

and aqueous HCl (as example of an acid) leaches carbonates and sulfates of alkaline
earth and other elements. The use of ammonia and acids is usually not recommended
as pretreatment process for bioenergy supply due to their costs. Simple water leaching
in a controlled processing step is also associated with additional investments and oper-
ating costs mainly as a result of the after drying needed.
Herbaceous harvests are usually left exposed to the weather with occasional
rainfalls; this already diminishes in particular the alkali and chlorine species with
an associated reduction of deposition and corrosion in boilers achieved (Van Loo
and Koppejan, 2004). On the other hand, qualities of biomass obtained in these cases
can be highly varying as weather conditions vary and biomass partly degrades.

8.5 SIZE REDUCTION

Size reduction is applied to modify the particle size distribution of a biomass feedstock
in order to obtain a larger fraction of finer sizes to comply with logistics and conversion
technology demands. Particle size reduction leads to an increase of available specific
surface area and a reduction of cellulose crystallinity and degree of cellulose
polymerization (Kratky and Jirout, 2011; Zhang et al., 2007). It also generally leads
to a denser product. These effects are needed for improved heat and mass transfer
characteristics in subsequent processing steps, leading to, e.g., reduced processing
time in biomass digestion and higher yields in hydrolysis. For the very heterogeneous
biomass sources, size reduction is complex. Unlike coal and most mineral matter, only
a minor fraction breaks down under crushing forces as biomass is often fibrous and
tenacious. Most of these materials deform, stretch, or are simply compressed by
crushing forces, so that shearing, ripping, and cutting actions are needed for size
reduction (Niessen, 2010). Another complication may be that once grinded, biomass
particles tend to stick together (see Van der Burgt in Knoef, 2005).
Drawbacks of size reduction are that the amount of energy required may be high
depending on particle size targeted at. Furthermore, in herbaceous biomass, the
morphology and (hard) silica content lead to extensive wear and tear of machinery.
Also, size reduction often leads to a redistribution of particle sizes and creates
comparatively fine fractions.
The main techniques used for size reduction of biomass are chunking, chipping,
crushing, and milling/pulverization.

8.5.1 Chunking

When large biomass parts become available after harvesting, e.g., by tree cutting,
chunkers are used to downsize them to a coarse size range of 50 - 250 mm.

8.5.2 Chipping and Shredding

Chipping or shredding can be applied when the biomass or waste material is relatively
tough and possibly wet and downsizing to a relatively coarse size range of 25 - 50 mm