210 Introduction to Renewable Biomaterials
6.7.7 Ammonia Pretreatment
Ammonia pretreatment is used usually for delignification of biomass. It is performed
through two methods, namely ammonia recycle percolation (ARP) and ammonia fiber
expansion (AFEX).
6.7.8 Ammonia Recycle Percolation (ARP)
ARP is a process based on ammonia. Aqueous solution of ammonia (5–15% w/v) is
passed through a reactor packed with biomass at elevated temperatures (100–180∘C),
and then the ammonia in the effluent is separated and recycled. This method leads to
biomass swelling and delignification. When incorporated into a biomass, saccharifica-
tion and enzymatic hydrolysis increase. ARP technology almost completely fractionates
biomass into the three major constituents. During the ARP process, all xylane dissolves
in ammonia while more than 92% of cellulose remains in the biomass.
6.7.9 Ammonia Fiber Expansion (AFEX)
The AFEX process treats lignocellulosic materials with liquid ammonia under pressure
and then rapidly releases the pressure, which leads to the following: (i) cellulose is
decrystallized, (ii) hemicelluloses are prehydrolyzed, (iii) lignin in the pretreated
material is altered, and (iv) the fiber structure is disrupted and accessible surface area
for enzymatic hydrolysis is increased.
AFEX can achieve higher than 90% conversion of cellulose and hemicellulosic
materials including wheat straw, rice straw, corn fiber, sugarcane, switchgrass, and so
on (Sun and Cheng, 2002; Teymouriet al., 2005; Alizadehet al., 2005).
6.7.10 Defects of AFEX Process
- The cost of ammonia.
It is an expensive method, and for the commercialization of this process, ammonia
must be recycled.
- High temperature leads to xylose degradation.
AFEX method was developed by Dillet al. in Michigan State University for pretreat-
ment of switchgrass and corn cob. It is more effective for the pretreatment of grassy
plants such as switchgrass than woody plant with higher lignin content.
- Cellulase enzyme cannot function in the presence of ammonia; thus it should be
extracted from biomass before enzymatic hydrolysis.
- Pollution of air due to release of ammonia is undesirable.
6.7.11 Enzymatic Pretreatment
Enzymatic pretreatment is used to break down celluloses and hemicelluloses to
fermentable sugars like xylose and glucose. Enzymatic pretreatment is a biocompatible
procedure and involves using carbohydrate enzymes (cellulase and hemicellulase) to
hydrolyze lignocellulosic materials to fermentable sugars (Keshwani and Cheng, 2010;
Hamelincket al., 2005; Laureano-Perezet al., 2005). Enzymatic hydrolysis usually
is conducted by cellulase enzyme. Cellulase enzyme is produced by some species
of bacteria and fungi. Cellulase enzyme derived from fungi is the best enzyme in
commercial scale (Keshwani and Cheng, 2010).
