Page 21 - Biofuels for a More Sustainable Future

14 Biofuels for a More Sustainable Future

(d) Biochemical processes applied to produce fuel ethanol, for example, sugar/
starch fermentation leading to biogas (methane) generation in anaerobic
conditions.
In the production process of first generation biofuels, mainly sugar fermen-
tation followed by the distillation process is applied to produce dry ethanol.
Biodiesel is produced through transesterification of oils with a chemical cat-
alyst (acid/alkali) or enzyme (Dutta et al., 2009; van Gerpen et al., 2002),
followed by a two-step distillation to remove by-products (e.g., glycerol).
In regard to the second generation biofuels production (e.g., cellulosic
ethanol, butanol), the following processes are applied: pretreatment to sep-
arate cellulose and hemicellulose from lignin, enzymatic hydrolysis (i.e., sac-
charification) to extract simple sugars, fermentation, and finally distillation.
Also biogass can be produced through the same sequence of the biochemical
processes up to distillation. Accordingly, gasification or pyrolysis (thermo-
chemical processes) is applied to convert biomass at higher temperatures
and pressures than those applied in biochemical processes. The process of
biomass gasification and direct liquefaction is commonly called “biomass-
to-liquid.” Gasification is more cost intensive than other processes; how-
ever, it produces cleaner fuel that can be directly used in engines (Larson,
2007). Through the gasification process, a variety of biofuels can be pro-
duced, such as Fischer-Tropsch liquids (FTL), dimethyl ether (DME),
and other alcohols (FitzPatrick et al., 2010; Dutta et al., 2014).
The third and fourth generation biofuels can be extracted with the same
processes, while the difference relates only to the feedstock used at the input
stage (traditionally cultivated algae for the third generation fuels, and genet-
ically modified algae biomass for the fourth generation fuels). Thus different
final industrial fuels can be extracted in different processes of the third and
fourth generation biofuels production, as follows:
(a) Biodiesel through oil extraction, transesterification, and distillation.
(b) Ethanol, biomethane, and biobuthanol through biochemical processes,
while biomethane and biobuthanol are processed in addition through
anaerobic digestion.
(c) Syngas, synthetic diesel (aviation fuel), and bioenergy through the ther-
mochemical process of gasification (Dutta et al., 2014).

4 Summary and conclusions
Biofuels production has faced many sustainability challenges over the
decades of technological, feedstock, and process developments. High costs