Page 57 - Advances in Eco-Fuels for a Sustainable Environment

34 Advances in Eco-Fuels for a Sustainable Environment

Sugar-based ethanol from cane, beets, and sweet sorghum is popular, due to the
feedstocks’ high yield of sugar per acre and low conversion costs, with the only main
disadvantage lying in their natural seasonal availability. Sugarcane must in fact be
processed within 24–72h after harvesting. Sugar is extracted by crushing the stalks
to release the juice. Lime (calcium hydroxide) is added to precipitate fibers and
sludge, then the liquor is filtered, evaporated to concentrate and crystallized sugar,
then centrifuged for its removal. Noncrystallized sugar and salts are concentrated
in a syrup-like liquid called “blackstrap molasses” that is used as a raw material
for conversion into ethanol [36]. Fermentation for 6–10h by yeast Saccharomyces
cerevisiae at a temperature of 33–35°C and cell density of 8%–17% is carried out.
At approximately 10% (v/v) concentration, ethanol fermentation is interrupted and
the broth is sent to a distillation and rectification phase, which product is the
azeotrophic solution of 95% (v/v) ethanol. Further concentration to absolute ethanol
(high grade or anhydrous) is achieved by molecular sieves or distillation using ben-
zene or cyclohexane.
Sugar beets represent a major sugar crop in Europe and North America, and are
used for biofuel production in France and Italy. They generate good yields
(50–100t/ha), growing in temperate climates with lower rainfall than is needed for
sugarcane. On average, the ethanol yield is 95L/t of sugar beets; however, its produc-
tion from this feedstock requires greater chemical and energy inputs, and thus is more
expensive than the one produced from sugarcane. Sweet sorghum varieties, on the
other hand, are few and not widely grown, although they might have significant
sucrose content (2000L syrup/hectare). Like sugarcane, sweet sorghum stores sugar
in the main stalk, and is recovered by pressing stalks with rollers. Yields on average
are 75L ethanol per ton of stalks. Farmers could still use sorghum grains because eth-
anol is produced from the sugary stalk juice.
Alcoholic fermentation from sucrose feedstock for bioethanol production is a well-
known and mastered process. The main challenges of bioethanol production from
sucrose feedstock are: (a) the introduction of new feedstocks, (b) fuel production from
bagasse (second-generation fuel), (c) selection of more efficient yeast strains,
(d) contamination in the fermentation process, and (e) reduction of vinasse volume.
Challenges in using sweet sorghum as feedstock include breeding new sorghum vari-
eties, planting techniques, harvesting time (especially during rainy seasons in tropical
countries), and reduction of compounds affecting the industrial processes such as
aconitic acid, phenolic compounds, and starch [37].
Starch-based production, the most utilized for ethanol production in North America
and Europe, depends on long chains of glucose units containing 1000 monomeric units
or more per amylose structure, and 1000–6000 units or more monomers per amylo-
pectin structure stored in grains. It is first necessary to break down these chains to
obtain glucose syrup, which can then be converted to ethanol by yeasts. Polymers
of glucose are broken into glucose by hydrolysis catalyzed by a gluco-amylase
enzyme, yielding destrose or D-glucose. Hydrolysis is then followed by fermentation,
distillation, and dehydration to yield anhydrous ethanol [38].
Bioethanol can also be produced from lignocellulosic materials (second-generation
bioethanol). These consist of cellulose, hemi-cellulose, lignin, and solvent extractives.