Environmental Engineering Reference
In-Depth Information
Pretreatment
Hydrolysis
Fermentation
Biomass
Ethanol
Fig. 4 General pathway for conversion of cellulosic feedstocks to bioethanol
For developing feasible technology for bioethanol production from lignocellu-
losic biomass, following challenges have been identi
ed by various researchers
worldwide:
1. Development of an ef
cient pretreatment method to open up the
bers, so as to
overcome the recalcitrance of biomass.
2. Obtaining high titers of cellulase to hydrolyze cellulose into its monomers
(could be by adopting cheaper fermentation technology, employing cheaper/
crude substrate for enzyme production, employing bioengineered organism,
etc.).
3. Detoxi
cation of inhibitors produced during pretreatment which are otherwise
toxic for the yeast during ethanol fermentation.
first two steps are critical for the production of ethanol from biomass. Thus,
development of an ef
The
fibers and obtaining
high titers of cellulase to hydrolyze cellulose into its monomers are among major
challenges which adds cost to this technology. Although extensive studies have
been carried out to meet the future challenges of bioenergy generation, there is no
self-suf
cient pretreatment method to open up the
cient process or technology is available till date.
The process for bioethanol production from feedstocks consists of three key
steps. Firstly, the biomass is pretreated to break up the macromolecular structure of
the cells in order to make the cellulose more accessible for cellulolytic enzymes.
The second stage is enzymatic hydrolysis/sacchari
cation, where the cellulose is
broken down into simple sugar monomers. In the third stage, these sugars are
fermented by microorganisms to produce ethanol. Figure
4
shows general technical
pathway for bioethanol production.
6 Current Cellulosic Ethanol Technology
6.1 Pretreatment
Pretreatment aims to decrease crystallinity of cellulose, increase biomass surface
area, remove hemicellulose, and break lignin seal. Pretreatment makes cellulose
more accessible to enzymes so that conversion of carbohydrate polymers into
fermentable sugars can be achieved more rapidly and with more yields. Intensive
research is going on globally to search for a robust and universal pretreatment
method, but till date, all efforts have gone in vain as different biomass varies in its
composition and the differences exist based on geographical region as well as
seasonal difference is also signi
cant.
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