Environmental Engineering Reference
In-Depth Information
crucial importance when their mass fluxes are adequately considered. The
feasibility of AD technology will also depend upon environmental
legislation providing adequate valuation of the avoidance of negative
externalities from by-products of the biofuel industry such as water
pollution and GHG emissions. The latter, obviously, also concerns the
appropriate application of AD itself.
7.5
Conclusion and future trends
The analysis proposed and exemplified in this chapter highlights how the
contribution of AD to biomass chains can be quantified using useful/
recoverable energy as an indicator. Beyond the presented estimations, when
analyzing the use of residues for AD, theoretical studies need to consider
reality, including issues such as competition for biomass resources and
limitations of reuse related to the toxicity of by-products and health
concerns. In the first case, for example, palm kernel cake and oil palm leaves
are rich in nutrients and have been proven to be feeds of high quality. The
same is valid for leaves of the cassava plant. Other by-products (e.g. oil palm
fibers and sugarcane bagasse) can be combusted, providing significant
energy savings in industrial processes that tend to be intensive in use of
thermal energy. This is already being implemented by many industries. On
the other hand, other residues such as sugarcane trash, vinasse and palm
fruit bunches remain interesting energy sources as their current management
generally causes environmental problems.
Beyond the examples of the AD added value to biofuel chains, many
other industrial processes can benefit from AD given its potential to treat
organic resources of different quality. Table 7.4 lists some by-products that
are already being valorized using AD with the concomitant energy benefits.
The values given are just indicative as the nature of the biomass material can
change considerably according to the circumstances of its production.
The contribution of AD to biomass chains has already been realized and
documented in different parts of the world. For example, van Haandel
(2005) showed how, by digesting the vinasse and bagasse resulting from the
production of ethanol from sugarcane in Brazil, 31.5GJ can be produced in
addition to the 5000 liters ethanol produced from the original total 65-
75 ton wet sugarcane. In this way, AD could generate 23% energy of the
whole chain. The added value of AD to a grass biorefinery concept has been
demonstrated in Switzerland (Baier and Delavy 2005). In this case AD, adds
value to the biomass chain by generating 1.8GJ ton
1
grass in addition to
the 0.4 ton ton
1
fibers, and the 0.12 ton ton
1
proteins originally produced
from the initial biomass. Future trends towards a biobased economy will
strengthen the use of biomass resources for multiple purposes and issues of
efficiency in resource use and correct allocation based on environmental and
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