Environmental Engineering Reference
In-Depth Information
Enzyme
production
Size reduction
SSF
Detoxification
and
neutralization
Fermentation
of cellulosic
sugars
Solid
and liquid
separation
Enzymatic
hydrolysis
Product
recovery
Pretreatment
Ethanol
Fermentation of
hemicellulosic
sugars
Residue
processing
Co-products
FIGure 3.1
Traditional cellulosic biomass conversion to ethanol on the basis of concentrated acid pretreat-
ment followed by hydrolysis and fermentation. (From U.S. DOE,
Breaking the Biological Barriers to Cellulosic
Ethanol: A Joint Research Agenda
, DOE/SC/EE-0095, U.S. Department of Energy Office of Science and
Office of Energy Efficiency and Renewable Energy, 2006, available at http://genomics.energy.gov/gallery/.)
the fermentation process. Recycling and disposing of the large quantity of sulfuric acid used in this
process also make it expensive (Lu and Mosier 2008; Yang and Wyman 2008).
Lime removes lignin under various conditions depending on the type of the lignocellulosic feed-
stock used (e.g., 100°C for 13 h for corn stover, 100°C for 2 h for switchgrass and 150°C for 6 h at
14 atm for poplar wood). When pretreating woody biomass containing a lot of lignin (e.g., poplar),
adding oxygen can improve the process (Lu and Mosier 2008; Yang and Wyman 2008).
There are two pretreatments that use ammonia: ammonia fiber expansion (AFEX) pretreatment
and ammonia recycle percolation (ARP) pretreatment. The AFEX process is widely applicable
to pretreating biomass from grasses including sugarcane bagasse. This process permits nearly a
complete conversion of cellulose and hemicelluloses to fermentable sugars at very low enzyme
loadings. Being an efficient procedure, it is also less expensive than other methods (Holtzapple et al.
1991; Delarosa et al. 1994; Reshamwalla et al. 1995; Dale et al. 1996; Moniruzzaman et al. 1997). In
the ARP process, aqueous ammonia solution, 15% of weight, is passed through the lignocellulosic
biomass in a reactor at 80-180°C and then the ammonia is separated and recycled. This process is
very efficient in separating fermentable sugars from the lignin component and also in recycling the
ammonia (Iyer et al. 1996; Lee et al. 1996; Wu and Lee 1997; Lu and Mosier 2008).
Enzymatic hydrolysis is performed by cellulase and hemicellulase enzymes, either individually
or in tandem (cellulosomes). These enzymes break down polymeric substrates into glucose. The
best understood system is from
Trichoderma reesei
, a fungus that contains several enzymes (Lu
and Mosier 2008; Nakagame et al. 2010). These enzymes include endoglucanases that “decrease the
degree of polymerization of macromolecular cellulose by attacking accessible sites and breaking
the linear cellulose chain” (Mousdale 2008), cellobiohydrolases that “attack the chain ends of the
cellulose polymers, liberating the disaccharide cellobiose” (Mousdale 2008), and β-glycosidases
that “hydrolyze soluble cellodextrins and cellobiose to glucose” (Mousdale 2008). Additionally,
