PHYSICAL PRETREATMENT OF BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 238

8.2.3 Grass

Grasses are usually harvested by cutting with mowers. In larger-scale agricultural
practice, mowers are driven by tractors. The mown grass is left on the field for drying.
Regular tedding ensures faster drying by exposing the grass to airflow and solar
irradiation more evenly. Subsequently, by raking, lanes of dried grass are formed,
which can then be optimally harvested as hay using a baling machine. Alternatively,
grass can be ensiled, which needs less drying, but still the moisture content must be
reduced as the optimal moisture content for ensiling grass is between 30 and 50 wt%.
Too wet biomass leads to significant degradation during the silage process, associated
also with smell issues. Another problem is that when pockets of biomass are stored
that are too wet, self-heating and autoignition may occur, forming a serious hazard
potential and leading to possibly severe economic loss.
Taller grass species (e.g., Miscanthus giganteus sinensis ) need a forage chopper.
This machine does not only cut off the stems but also chops the material in one
continuous operation.

8.2.4 Aquatic biomass

A relatively novel development in the field of the production of biomass-to-energy
carriers is the utilization of grown wet biomass plant material, particularly algae.
Algae can be divided into two subclasses: microalgae and macroalgae (also termed
“ seaweeds ” ).

8.2.4.1 Microalgae Microalgae can be cultivated in open or closed systems.
Concerning open systems, one can discriminate both large open unmixed ponds and
open mixed ponds. An unmixed pond is relatively simple and offers the advantage
that no supplementary input of energy is required. This type of cultivation system
can be used, e.g., for wastewater treatment. Mixed ponds are characterized by a high
mixing degree that ensures exposure of the algae inventory to light, nutrients, and gases
(in particular CO 2 ), causing higher growth rates of up to a factor of 10 higher than in
unmixed ponds. Most commonly used is the raceway pond , which is a shallow circuit
with a depth between 20 and 35 cm, ensuring adequate sunlight exposure while the
inventory is stirred using paddle wheels (Benemann, 2008). A raceway pond is usually
lined with plastic or cement, though this adds to its cost. Some of the main advantages
of this concept are relatively low investment costs, simple operation, and easy
scale-up (to the order of magnitude of hectares). Drawbacks compared to a closed
system are the low volumetric productivity, little control of culture conditions,
vulnerability to contamination, and high evaporative losses (Ugwu et al., 2008).
Currently, raceway ponds are used for commercial generation of Spirulina , Dunaliella
salina , Chlorella vulgaris , and Haematococcus pluvialis (Benemann, 2008).
Closed systems are called photobioreactors, and they exist as tubular, flat-panel,
and vertical column reactors. The most pronounced difference with open systems
is the separation from open outside air so that gas exchange can be better controlled
and contamination prevented. Different algae require rather tailor-made solutions