Page 209 - Advances in Eco-Fuels for a Sustainable Environment

Anaerobic digestion of various feedstocks for second-generation biofuel production 173

and cellulose, fermentation of sugar, separation of lignin residue, and retrieving and
refining the ethanol to fulfill fuel standards. Conversion technology for ethanol has
been established on two stages, which can be referred to as the synthesis gas (or syn-
gas) stage and the sugar platform, Hemicellulose and cellulose are first transformed to
fermentable sugars in the sugar platform and then fermentable sugar, which includes
glucose, xylose, arabinose, galactose, and mannose, are converted to produce ethanol
by the fermentation process. In this syngas platform, the biomass is subjected to the
gasification or pyrolysis process by the oxygen-free environment or one-third of the
oxygen supply required for complete combustion. The syngas produced from this gas-
ification process can be fermented by specific microorganisms or catalytic conversion
to produce ethanol. In the fermentation of sugar process, only the simple carbohydrate
elements are utilized for ethanol production, whereas in the syngas platform, all three
components of the biomass (cellulose, hemicellulose, and lignin) are converted to
ethanol [78].
Enzymatic hydrolysis is a promising way for converting lignocellulose materials
into sugars, but the little enzymatic availability of the natural cellulose is a significant
difficulty for processes to convert biomass into ethanol. The lignin is partially cova-
lently connected with hemicellulose and avoids the contact of hydrolytic agents to cel-
lulose. Also, the cellulose crystalline structure itself signifies an additional
complication to the hydrolysis process. Pretreatment techniques can be broadly clas-
sified into five groups: chemical, physical, physiochemical, biological, and thermal.
Physical pretreatment processes employ the mechanical reduction of size or radioac-
tivity processes to change only the physical characteristics of biomass. The
physiochemical process utilizes hot water, steam, ammonia water, and gases such
as SO 2 and CO 2 . The chemical processes employ acids (H 2 SO 4 , HCl, organic acids,
etc) or alkalis (NaOH, Na 2 CO 3 , Ca(OH) 2 ,NH 3 , etc). The thermal pretreatment system
involves the slow pyrolysis of the biomass. Among these pretreatment systems, acid
pretreatment is the preferable method for several typical processes because of the high
yield of conversion. The main drawback of the acid pretreatment process is that it
might be highly corrosive. The steam and hot water pretreatment system requires
higher pressure and absorbs more power for this pretreatment process.
Pyrolysis is a suitable energy-efficient, less-corrosive method for pretreatment of
lignocellulosic materials. During treatment of these materials at temperatures higher
than 300°C, there is a rapid decomposition of cellulose to produce gaseous products
and char whereas less decomposition at a slower rate and fewer volatile products are
formed at lower temperatures. Mild acid hydrolysis of the residues from the pyrolysis
pretreatment has led to an 80%–85% transformation of cellulose to decreasing sugars
with more than 50% glucose [28]. The gaseous products from the pyrolysis process are
converted to ethanol by fermentation by specific microorganisms or catalytic
conversion [67].
The biomass hydrolysate fermenting consists of a continuous process where the
cellulose hydrolysis and fermentation process advances in various steps. The biomass
from the pyrolysis process is subjected to enzymatic hydrolysis. It is the important step
for bioethanol production where complex carbohydrates are transformed to pure
monomers. Enzymatic hydrolysis requires mild surrounding conditions and less