Environmental Engineering Reference
In-Depth Information
used, but these are also food sources, and as a result, competition between food and
fuel markets occurs. One can argue that this competition should be avoided at all
times and technologically there are many alternatives to bypass the food/feed
discussion. For example, the use of low-quality oils that cannot be used in food
applications can offer an alternative, and new catalytic processes can be developed
to exploit these less expensive oil streams.
This is a general point in all use of biomass. A drawback of using only the easily
processed sugar and triglyceride fractions of a plant is that these fractions form only
a part of it. Accordingly, the net energy yield that can be achieved using these
fractions is poor, and only specific crops can be used. To improve the energy yield
of fuels from biomass, lignocellulosic feedstocks must be considered despite their
complexity. This will enable the use of all parts of the current feedstocks (such as
waste from palm trees and molasses from sugarcane). In addition, trees, switch-
grasses, and other low-ranked biomass on poor lands can be considered as feed-
stocks. For a detailed discussion of the selection of lignocellulosic streams and
pretreatment thereof, we refer to the recent literature reviews (Zinoviev et al., 2010).
As a variation to this traditional transesterification of oils,
Neste
has developed a
process to fully reduce oil to a linear alkane component, including the catalytic
hydrogenation of the glycerol, which normally ends up as by-product, to propane
(tinyurl.com/l5jdxxq).
18.3 CONVERSION OF SUGARS TO HYDROCARBON FUELS
In a worldwide context, the most desirable source of biomass stems from lignocellu-
lose (Dornburg et al., 2010). Lignocellulose can be acquired from biomass waste, as
well as from woody or grass samples, and holds the promise of sustainable use. The
so-called second-generation biofuels derived from lignocellulosic biomass (see
Box 18.2) can thus still use a sugar moiety as molecular basis for a biofuel.
In order to convert sugars into biofuels, two types of organic transformations need
to be performed: (1) a substantial reduction of the oxygen content needs to be realized,
and (2) C
C bonds need to be created between biomass-derived intermediates to
increase the molecular weight for use as biodiesel substitutes (see Table 18.1,
C12
-
C18 range). When looking at a sugar molecule, two main ways can be consid-
ered to decrease the O/C ratio and thus increase its energy content, namely, the release
of CO
2
and the release of H
2
O.
As can be seen from Figure 18.1, the production of two molecules of ethanol from
glucose (C
6
H
12
O
6
) is accompanied by the formation of two CO
2
molecules. Elimina-
tion of two CO
2
molecules and one H
2
O molecule from C
6
H
12
O
6
gives C
4
H
10
O. This
molecular formula corresponds to 1-butanol, the main product of anaerobic fermen-
tation of sugars by
Clostridia
bacteria. Diethyl ether, which is usually formed by
dehydration of two ethanol molecules, is another compound having the same molec-
ular formula. Based on their structure, both these compounds are suitable candidates
for biofuels and have been considered as serious alternatives to bioethanol.
-
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