Environmental Engineering Reference
In-Depth Information
e equilibrium
f fluid
f
falling-rate period related
i initial
moist moisture
p
particle
w
water
wb
wet bulb
Superscripts
sat
saturated
8.1
INTRODUCTION
Biomass and waste materials are available in widely varying physical appearances.
Often, such sources contain substantial amounts of water and are available in the form
of relatively large particles when having been harvested or obtained in other collection
processes. Just to illustrate its wide spectrum of morphologies, biomass can have a
tough, fiber-like structure or have a sticky, paste-like structure.
Themajority of energy conversion processes that are dealt with in detail in Part III of
this topic need small particles for supply to the reactors, as large particles hamper feed-
ing and fuel conversion extents due to the usually limited available residence times.
Thus, particle size reduction is usually needed. Also, biomass is usually relatively
wet, with typical moisture contents between 30 and 60 wt% (as received (ar)). In most
processes,
with the exception of digestion processes
requiring water and hydrothermal processes
this cannot be accepted
—
because a high moisture content is at
the expense of processing efficiency due to the lower effective heating value of the fuel.
Therefore, moisture needs to be removed, which is a highly endothermic process.
Often, due to usage of fertilizers and differences in rainfall and harvesting character-
istics with time, the ash content of biomass is highly varying, which may impact its
handling, in particular in thermochemical conversion processes.
In view of the aforementioned considerations, physical pretreatment of biomass is
crucial, and in practice, many of the problems encountered in the processing of
biomass can be traced back to improper, nonoptimized physical pretreatment.
Physical pretreatment serves the following purposes:
—
Storage ability improvement
Reduction of the content of harmful species, e.g., removal of hard stones and
adherent soil but also ash reduction (e.g., alkali salts)
Size reduction
Moisture content reduction
(Volumetric) energy density increase
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