Environmental Engineering Reference
In-Depth Information
8.10 SEQUENCING THE PRETREATMENT STEPS
In the previous sections, we have discussed quite a broad spectrum of physical
pretreatment techniques. Not all of these are needed in all cases; this depends on
the quality characteristics of the biomass as well as on the foreseen downstream
energy conversion processes that are described in the subsequent chapters including
their pretreatment requirements. The final question then is what should be the
sequence of the pretreatment steps.
Generally, some kindof storage followsupon theharvesting tocreate a buffer as direct
downstream processing is usually not possible. However, storage poses restrictions on
the moisture content in view of bacterial/fungal activity, so that drying to ~20
25 wt%
moisture content is usually applied before storing the biomass. Usually, drying is
preceded by a size reduction step to enhance the drying process. Sieving takes place after
size reduction (when needed in view of the downstream processing). Leaching with
water, sometimes used to improve the combustion characteristics of biomass, is in gen-
eral followed by a drying step for thermochemical conversion. Compacting follows after
a sufficient drying step has been applied to realize a stable product (bale/briquette/pellet).
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CHAPTER SUMMARY AND STUDY GUIDE
This chapter provides an overview of physical biomass pretreatment techniques. It is
important to appreciate the great heterogeneity of biomass and its subsequent
processing technologies both giving rise to the need of such treatments. Various
methods of harvesting are described, followed by storage and its impact on product
qualities. Washing is needed for biomass with a high alkali and chorine content that is
used as a feedstock for thermochemical conversion technologies. Size reduction is
treated, which often precedes a drying step. Mechanical drying and thermal drying
and its fundamentals are discussed next. Finally, different compaction techniques
are dealt with, leading to a stable solid fuel product enabling more cost-effective
transportation and other logistic handling.
KEY CONCEPTS
Necessity of physical pretreatment
Harvesting biomass and (local) transportation
Storage and its implications for biomass quality
Washing to ensure ash reduction
Size reduction (characterization and implication for downstream processing)
Moisture reduction via mechanical and thermal technologies
Compaction for production of stable intermediate solid products
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