Page 217 - Advances in bioenergy (2016)

PRODUCTION OF REFORMABLE BIOFUELS

Biomass can be used directly for the production of hydrogen via gasification processes.
However, these processes are hampered with difficulties and inconveniences, especially when
the objective is related to distributed power production, due to the high cost of transportation
of raw biomass. For such applications, the most appropriate approach is to produce
transportable biofuels, or biomass derivatives, which concentrate a large fraction of the energy
of raw biomass and posses reduced mass and volume. The most convenient biofuels for
reformation purposes are bioethanol, bio-oil, and biogas, whereas glycerol is produced as by-

product of diesel production processes.
Bioethanol is a first generation biofuel that is already in the market for many years. The well-
1,2
known conventional fermentation process is applied for its production. Sugars and starches
are the most convenient raw materials. However, their use creates limitations and controversy
3,4
as these materials are also used for food. Furthermore, only part of the plant energy is
1
converted to biofuel, whereas fossil energy is often used in cultivations. Thus, in many cases,
there is only modest energy gain and GHG emission benefits. Furthermore, the production
process is considered expensive due to high feedstock cost (fertilizers, harvesting,

pretreatment) and energy-intensive distillation and dehydration steps.
For the reasons stated above, second generation bioethanol processes have been proposed
aiming at: (1) exploitation of nonedible biomass, (2) increase of energy efficiency, reducing

fossil fuel consumption and GHG emissions, and (3) cultivation of energy crops suitable to
local climate. Nonedible biomass is composed of cellulose, hemicellulose, and lignin.
Cellulose is a crystalline glucose polymer, strong, and resistant to acid or enzymatic
1,2
hydrolysis. Hemicellulose is a sugar polymer of five different sugars, having a random,
amorphous structure and being easily hydrolyzed to its monomer sugars by dilute acid or base.
Cellulose and hemicellulose make up 60–90 wt% of terrestrial biomass, they are abundant in
1,2
plants and wood, and represent a large percentage of domestic and industrial wastes. Lignin
composes 10–25% of biomass and consists of highly branched, substituted, mononuclear
1
aromatic polymer. Lignin, cellulose, and hemicellulose are known as lignocellulose and, due
to their abundance, are the biomass materials that can guarantee the security of raw materials
supply. However, decomposing these highly complex compounds presents technical
difficulties.

Production of second generation bioethanol, via the biochemical conversion of lignocellulose,
is difficult because of the refractory nature of biomass and the different nature of sugar
monomers, which cannot be fermented by the same microorganism. Processing includes
pretreatment, hydrolysis, fermentation, and product separation/distillation. Although
pretreatment increases production cost, it is essential to enhance hydrolysis yields, from
2
typically <20 to 90% after pretreatment. Hydrolysis methods convert cellulose into monomers
and can be classified in two categories, acid and enzymatic hydrolysis. Fermentation that
follows can be performed under the same conditions as sugar and starch fermentation. The
major disadvantages of bioethanol production with the known enzymatic technology is the