Environmental Engineering Reference
In-Depth Information
enzymatic hydrolysis. Furthermore, high pH conditions can reduce subsequent cellulase loading
requirements for enzymatic hydrolysis, provided that hemicellulase activity is present in sufficient
quantity during the enzymatic hydrolysis. Flow-through pretreatments can remove as much as 75%
of the lignin in addition to the hemicellulose, leaving behind mostly cellulose in the solids. (Wyman
et al. 2005a) A detailed discussion of pretreatment technologies follows.
8.4.2.1 low ph Pretreatments
8.4.2.1.1 Dilute Acid
Strong acids at 0.5-3 wt% and 130-200°C will hydrolyze most hemicellulose into soluble sugars
as well as disrupting and removing some lignin. Dilute acid pretreatment removes and recovers
hemicellulose sugars as monomers and increases glucose yields from cellulose to near 100%, but
requires costly construction materials, high operating pressures, and neutralization and conditioning
of the hydrolyzate before fermentation.
8.4.2.1.2 Hot Water/Steam Autolysis
Hot water under pressure without the addition of acid will hydrolyze hemicellulose in a process
similar to a dilute acid co-current pretreatment. Water under pressure has a lower pH, penetrates cell
walls, dissolves hemicellulose and some lignin, directly breaks hemiacetal linkages, and catalyzes
the breakage of ether linkages through released organic acids. This process has lower capital
and operating costs, reduces corrosion, and requires no chemical additions compared with dilute
acid, but it suffers from the disadvantage of incomplete hydrolysis of hemicellulose and requires
hemicellulases later on.
8.4.2.2 high ph Pretreatments
8.4.2.2.1 AFEX/Fiber Extrusion
AFEX is a batch process. The continuous process is called FIBEX (fiber extrusion). This is a dry-
to-dry process that produces no liquid stream but a solid and a vapor phase, and it produces no
hydrolysis products but only physical disruption of the biomass for increased enzymatic hydrolysis.
Biomass is incubated with anhydrous ammonia at elevated temperatures. Rapid depressurization
volatilizes ammonia for recovery and increases biomass surface area for enzymatic hydrolysis.
Little lignin is removed, but lignin structure is modified, thus diminishing interference with
enzymatic hydrolysis. AFEX can be performed in lower-cost vessels than those required for dilute
acid pretreatment. The hydrolyzate is compatible with fermentation without conditioning, and it
decrystallizes the cellulose, thus increasing downstream enzymatic hydrolysis. Most hemicellulose
is recovered as oligomers, which require important hemicellulases in the enzymatic hydrolysis for
efficient sugar recovery.
8.4.2.2.2 Ammonia Recycle Percolation/Soaking in Aqueous Ammonia
Ammonia recycle percolation (ARP) is a process in which aqueous ammonia is passed over biomass
at elevated temperatures and pressures. Ammonia swells biomass, depolymerizes lignin, and breaks
lignin-hemicellulose bonds. A lower-cost alternative is soaking in aqueous ammonia (SAA), which
treats biomass in a batch reactor at 25-60°C under atmospheric pressure. ARP enhances cellulose
digestion, reduces nonproductive binding of cellulase enzymes, and improves microbial activity,
largely because of the removal of lignin, but it is less effective at treating softwood pulps.
8.4.2.2.3 Lime
Typical conditions are 0.1 g lime plus 5 g water per gram biomass. Additional water has little effect
on the process. Temperatures can vary from 25 to 130°C, corresponding to treatment times ranging
from weeks to hours. For high-lignin materials, additional lignin removal can be accomplished
through the addition of oxygen or air. Because this is a low temperature pretreatment, biomass can
be treated without a pressure vessel. Approximately 33% of lignin is removed and 100% of acetyl
