CHEMISTRY OF BIOFUELS AND BIOFUEL ADDITIVES FROM BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 554

BOX 18.2: GENERATIONS OF BIOFUELS

First-generation biofuels are biofuels directly produced from food crops, such as
the oils for use in biodiesel or the sugars for producing bioethanol for fermentation.
First-generation biofuels have been criticized because they release more CO 2 in
their production than they capture CO 2 during their cultivation. In addition, the
use of food crops as source for fuel comes with ethical and global dilemmas
(Karlsson, 2007).
Second-generation biofuels are produced from nonfood crops, such as lignocel-
lulose (wood), food crop waste, and grasses. They are more cost competitive
compared to first-generation biofuels, and life cycle assessments have indicated
that their use will result in a decrease of transport-related greenhouse gas emissions
(Davis et al., 2009; Pickett et al., 2008). The production of second-generation
biofuels must be accompanied by an integral usage of all parts of the biomass,
including recycling of biomass parts that have been turned into products and
electrical energy generation from residual parts (such as lignin).
Third-generation biofuels are based on improvements in the production of
biomass. For the production of these biofuels, advantage is taken of specially
engineered energy crops such as algae. In theory, algae can produce more energy
per unit area than conventional crops, and they can be grown on land as well as
water. Furthermore, they can be engineered to produce not only ethanol but also
diesel, gasoline, and kerosene. A major disadvantage at this moment is the cost of
production, as well as problems concerning the robustness of the agricultural
production.
Fourth-generation biofuels are aimed not only at producing sustainable energy
but also at capturing and storing CO 2 . Production of these biofuels differs from
second- and third-generation production, because at all stages of production, the
carbon dioxide must be captured, leading to a reduction in CO 2 emissions.

Elimination of one CO 2 and one H 2 O molecule can be realized via fermentation
and commonly leads to acetone and acetic acid as products (see Figure 18.3). Alter-
natively, a molecule of ethyl lactate can be produced from lactic acid and ethanol,
which are products of, respectively, anaerobic and aerobic fermentation. Elimination
of another molecule of water leads to the formation of

-valerolactone (gVL). Simply
removing three molecules of water form glucose leads to hydroxymethylfurfural
(HMF). These glucose-derived molecules are denoted as platform molecules or
building blocks. They can play a role as biofuels directly (ethanol, butanol) or can
be considered as starting materials for the production of biofuels or biobased additives
for fuels, provided cheap technology is available to perform this process.
Lignocellulose feedstocks also contain approximately 25 wt% of hemicellulose,
which contains a large percentage of pentose sugars. Xylose (e.g., from straw) can
be readily fermented to ethanol. Chemical water elimination from xylose leads to fur-
fural, another platform chemical (see Example 18.2).

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