Biofuels from Biomass - Novel Combustion Concepts for Sustainable Energy Development - page 38

Each pretreatment method has its own advantages and disadvantages. Though
several other pretreatment methods exist such as aqueous ammonia, ionic liquid,
and microwave, but all are con

ned to the research laboratories and none have been
found suitable for industrial application other than dilute acid pretreatment. Once
the biomass is pretreated, it is subjected to enzymatic hydrolysis.

6.2 Hydrolysis

The major bottleneck in the production of cellulosic ethanol through biological
route is requirement of high dosage of cellulase. This step accounts for maximum
part of the cost to this technology. Enzyme cost always remained a major hurdle to
industrial second-generation ethanol production. However, enzyme costs have
fallen 80 % over the last decade (Ethanol RFA 2013, http://ethanolrfa.3cdn.net/
d9d44cd750f32071c6_h2m6vaik3.pdf ) so more and more biofuel companies are
partnering with enzyme producers such as Novozymes and Genecor as purchasing
enzymes become competitive with producing enzymes on site from fungi such as
Trichoderma reesei (Xu et al. 2009 ).
Cellulases are currently the third largest industrial enzyme worldwide, by dollar
volume, because of their use in cotton processing, paper recycling, as detergent
enzymes, in juice extraction, and as animal feed additives. However, cellulases will
become the largest volume industrial enzyme, if ethanol from lignocellulosic bio-
mass through enzymatic route becomes a major transportation fuel.
Cellulases are a complex enzyme mainly consisting of three enzymes, endo-
cellulase, exo-cellulase and

-glucosidase, which work synergistically to hydrolyze
cellulose completely into monomers, i.e., glucose (Singhania et al. 2010 ). Apart
from these, several helper enzymes would also be present as xylanase and ligninase.
The search for potential sources of cellulolytic enzymes is still continuing in the
interest of successful bioconversion of lignocellulosic biomass. There are several
microorganism especially

β

filamentous fungi that produce cellulase with all the three
enzyme at different proportion needed for the complete hydrolysis of cellulose. T.
reesei is one among the most potent cellulase producers studied in detail. Currently,
high-ef filamentous fungi mainly

Aspergillus niger and T. reesei. Penicillium sp have come up as an alternative to
Trichoderma as it also produces cellulase at high titers having all the other com-
ponents required for synergistic action toward biomass hydrolysis (Gusakov 2013 ).
On-site enzyme production utilizing low-value substrate could be a step toward
developing an economically feasible process. Once fermentable sugars are avail-
able, it can be easily fermented into ethanol. Separate hydrolysis and fermentation
(SHF) as well as simultaneous sacchari

cient cellulases are produced commercially using

cation and fermentation (SSF) are highly
adopted technologies for bioethanol production from biomass.