Environmental Engineering Reference
In-Depth Information
• We must unlock the potential of greatly untapped resources such as waste and
algae [51]. In the EU alone, unused biowaste (e.g. agriculture and forestry resi-
dues, waste water treatment sludge, organic household waste, food processing
waste, debarking waste) amounts to a total of 2.8 billions tonnes per year [49].
In addition, biomass typically exhibits a low bulk density and a relatively high
water content (up to 90% for grass), which makes its transport in its raw state
much more expensive than the transfer of natural gas or petroleum. Reducing the
cost of collection, transportation and storage through pre-processing biomass into
a higher-density, aerobically stable, easily transportable material is therefore criti-
cal to developing a sustainable infrastructure capable of working with significant
quantities of raw material [52].
The most common approach used to increase biomass density is grinding. By
chopping bailed straw, for example, a 10-fold densification can typically be
achieved. An alternative strategy that can provide a material of even higher density
is pelletisation. Through conversion of ground straw into pellets, the density of the
material could be further increased by a factor of three [53]. This pre-treatment
also provides the added benefit of providing a much more uniform material (in
size, shape, moisture, density and energy content) which can be much more easily
handled (see Chapter 3). Pre-processing might be performed onsite but can also be
done during harvesting. An example of technology recently developed to address
the engineering challenge presented by low-bulk-density biomass such as wheat
straw is a multi-component harvester that can simultaneously and selectively
harvest wheat grain and the desired parts of wheat straw in a single pass [52].
Another issue associated with the use of (fresh) biomass is its perishable
character or susceptibility to degradation. Taking straw as an example once more,
fermentation will begin if the moisture content of baled straw is kept above 25%
for a prolonged period of time, resulting in a dramatic reduction of the quality of
the raw material. In some cases, spontaneous combustion in the stacks can even
take place [54]. This issue is particularly important given that, in contrast to fossil
resources (which are of permanent availability and are continuously pumped and
mined), the availability of biomass is seasonal. In order to ensure a continuous
year-round operation of the biorefinery, biomass may have to be stabilised (e.g.
dried) prior to (long-term) storage. For example, the Austrian green biorefinery
tackles this problem by processing not only direct-cut grasses but also silage,
which can be prepared in the growing season and stored in a silo [55, 56].
In summary, it is essential that we develop versatile and sustainable biomass
supply chains and cost-effective infrastructures for production, collection, storage
and pre-treatment of biomass. As highlighted by Nilsson and Kadam, the eco-
nomic success of large biorefineries will greatly depend upon the fundamental
logistics of a consistent and orderly flow of feedstocks [54, 57]. Localised small-
scale (and perhaps mobile) pre-treatment units will be necessary to minimise
transportation costs and supply the biorefinery with a stabilised feedstock (e.g. in
