PHYSICAL PRETREATMENT OF BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 241

The drag rake is a tool based on the hand rake and functions by dragging the system
over the sea or ocean floor, thereby collecting solid-attached weeds. The rakes are then
retrieved to the boat in which the macroalgae are collected, using haulers or winchers.
After this, the whole process can be repeated.
In the cutter-blade system, a reciprocating cutter blade is lowered to a controlled
height where it cuts the algae. The algae are then picked up by a conveyor belt
and loaded onto the harvesting ship. This technique can be applied for both solid-
attached algae and free-floating algae.
The suction harvester is a system consisting of pipes equipped with a bladed
impellor that simultaneously draw up and cut seaweeds, which are then collected onto
the ship or in a bag trailing behind the ship. This also is a technique that can be applied
for both solid-attached algae and free-floating algae.
The Scoubidou system is a technique by which the seaweeds are twisted around a
rotating hook, breaking the holdfasts through traction. The seaweed is then collected
onto the ship by rotating the hook in the reverse direction. The Scoubidou system is
applied to attached macroalgae species.
A fifth mechanical system for algae harvesting is directly linked with
fishery and is performed by nets cast from and pulled by ships. This technique
is used for free-floating seaweeds, which are thus simply collected by pulling
in the nets.

8.3 STORAGE

Storing harvested material forms an intermediate step between harvesting and further
physical and chemical processing of biomass. This creates a time bridge between these
steps, which enables continuous supply. Although at first sight storing biomass does
not seem to be a pretreatment process, its conditions partly determine the quality
characteristics of the biomass for further processing. Storage process conditions
can affect the moisture content of biomass, its energy value, and its dry matter content,
and as such, it is related with physical biomass pretreatment (Maciejewska et al.,
2006). Important safety and environmental aspects of storage are the moisture content
and particle size distribution, degradation associated with loss of mass and energy
content by microbiological activity (Hunder, 2005) causing self-heating, and emission
of greenhouse gases (in particular CH 4 ,N 2 O) (Wihersaari, 2005). In general, a higher
original moisture content of the biomass causes higher losses of dry matter. Therefore,
one targets at a moisture reduction to approximately 20 wt% before storage. In order to
prevent as much as possible the loss of dry matter, airtight storage or storage of
material not yet reduced in size is recommended; the smaller the size of biomass,
the worse is its ventilation (Maciejewska et al., 2006). Wihersaari (2005) studied
the release of CH 4 and N 2 O from 6 months ' wood chip storage. This amounted to

MWh −1 fuel in case the wood chips were predried to 40 wt% moisture
content; this figure increased to 144 kg CO 2eq

58 kg CO 2eq

MWh −1 for biomass that was delivered

fresh with a 60 wt% moisture content.