Addition of sulfur dioxide (SO2) at elevated temperatures has been described as an alternative approach to enhance the recovery of both cellulose and hemicellulose fractions from SB [14, 29]. Carrasco et al. [39] studied SO2 explosion mediated pretreatment of SB at temperatures 180-205 °C, with residence times of 5-10 min using SO2 as a catalyst. Pretreatment at 190 °C for 5 min generated pentose yield 57 %. The pretreated SB showed 87 % hydrolysis at 2 % substrate concentration after enzymatic hydrolysis.

16.3.2.4 Ammonia Fiber Expansion (AFEX)

Ammonia fiber expansion (AFEX) is an attractive method of pretreatment for SB/SL because of several advantages such as economic, fast, and highly efficient. However, handling of ammonia solution for the pretreatment at large scale is a problem due to the environmental concerns [14, 15]. Ammonia acts strongly toward lignin removal with the minimum degradation of hemicelluloses and ameliorating the accessibility of cellulase enzyme action on cellulose and remaining hemicellulose [8, 20]. Am­monia acts on C-O-C linkage in lignin including ether and ester bonds in cellulignin complex [20]. Ammonia after pretreatment can be recycled for further applications. It is important to emphasize that the formation of sugar degradation products are minimized during ammonia pretreatment of SB/SL [8, 40]. This process can im­prove the enzymatic hydrolysis of lignocellulosic material depending on use and the optimal conditions used in the process [41].

In this method, SB/SL is exposed to liquid aqueous ammonia (1-2 kg of am — monia/kg of dry biomass) under moderate or high temperature (40-140 °C) and pressure (250-300 psi) for a period of time (<30 min). During AFEX, no significant inhibitory by-products are produced and ammonia can be recycled after pretreatment

[15] . Maximum polysaccharide conversion of AFEX pretreated SB and SL by enzy­matic hydrolysis using cellulases was almost 85 % while the use of hemicellulases promoted the xylan conversion to 95-98 % levels [8]. Xylanase supplementation also promoted the cellulose conversion into glucose [8, 42].

Ammonia recycle percolation (ARP), modified method of ammonia-mediated pre­treatment uses aqueous ammonia (10-15 %) at elevated temperatures (150-170 °C) followed by its recovery and separation of biomass. Similar to steam explosion, this method also promotes the high degree of depolymerization of lignin and cleavage of lignin-carbohydrate linkages, increasing the accessibility of cellulolytic enzymes to carbohydrate skeleton of SB/SL [15,43]. Ammonia pretreatment in any form (aque­ous ammonia pretreatment, soaking in aqueous ammonia, ammonia freeze explosion, ammonia recycling percolation) is generally preferred to low lignin containing lig — nocellulosic substrates like corn stover than SB/SL [20, 29]. In fact, more research needs to be done for ammonia pretreatment optimization of SB/SL.