**4. Ammonium fiber explosion (AFEX)**

#### **4.1. Description**

In AFEX, liquid (anhydrous) ammonia at moderate-to-high temperatures (60–200°C) and pressures (6.5–45 bar) is mixed with moist biomass for about 5–30 min, followed by a sudden drop in pressure to atmospheric. Ammonia is usually fed at less than 2 kg/kg of dry biomass. AFEX leads to the removal of lignin and some hemicelluloses, in addition to the decrystallization of cellulose, partly due to the strong affinity of ammonia for such biomass components. According to Chundawat et al. [53], pretreatment causes morphological and physicochemical changes to cell walls of the material, by creating nanoscale network of interconnected tunnels within the cell wall structure through the cleaving of lignin-carbohydrate ester bonds, and the partial removal and subsequent deposition of extractives on cell wall surfaces, leading to enhanced enzymatic access to cellulose. Further, Maillard reactions between ammonia and carbonyl-based aldehydic groups give rise to several intermediate products [54].

AFEX is generally affected by the moisture content and particle size of biomass, ammonia loading and process conditions including temperature and residence time. Higher temperatures cause more ammonia to flash causing greater disruption of the fibrous structure. Both glucan and xylan conversion (at fixed temperature and ammonia loading) was found to increase with moisture content of switchgrass [55]. In another study, particle size reduction increased the conversion of cellulose and xylan during pretreatment of corn stover [56].

Its main demerit is its unsuitability for handling materials with high lignin content such as wood. Much of the hemicellulose is fractionated to oligomers making it more challenging during fermentation. High pressures are usually required due to high ammonia loadings and high vapor pressure of ammonia. Moreover, ammonia is expensive and recovery of all feed ammonia for reuse is challenging. Safety issues arising from the corrosive and toxic nature of ammonia present additional challenges in process operation at industrial level. Compared to soaking in aqueous ammonia (SAA), AFEX requires expensive reactors and

0.8 Dry distillers' grains 90% cellulose conversion to

**Biomass Results Reference**

native biomass

xylose recovery

xylan conversion

glucose

theoretical ethanol yield

42.5 g glucose and xylose/100 g

Emerging Physico-Chemical Methods for Biomass Pretreatment

http://dx.doi.org/10.5772/intechopen.79649

60–85% glucose recovery, 50–85%

[57]

49

[58]

[59]

[60]

[62]

2<sup>a</sup> Agave bagasse ~100% carbohydrate preservation;

Rice straw 93.6% glucose yield; 97%

sorghum and maize

stovers

170°C, 10 min 5 Giant weed 94.2% glucan conversion; 84.4%

150°C for 30 min 1.5 Switchgrass 98% xylose yield [61]

SAA involves treatment of biomass with aqueous ammonia (5–50%w/w) at low temperatures (25–90°C) under ambient pressure in a batch reactor. Pretreatment is undertaken for residence times ranging from about 1 h to 3 months. Pretreatment efficiency is depended on variables such as temperature, reaction time and ammonia concentration. Lignin dissolves in the aqueous solution without appreciable decrease in the carbohydrate content, and high levels of solubilization are observed with high temperatures and times. In addition, severe conditions also cause release of acetyl groups, hemicelluloses, extractives and ash into pretreatment liquor [66]. In other aqueous ammonia treatment, moderate temperatures (≥100°C) are used to achieve high delignification of biomass using pressure vessels [67]. Higher temperatures

equipment.

**Reaction conditions NH3**

102°C, 30 min, 2.24 MPa

165.1°C, 69.8 min, 14.3% NH<sup>3</sup>

70°C, 350–430 psi, 14–18 min

of CO2

a Wet-basis.

, 2.2 MPa

**Table 3.** Results of AFEX pretreated biomass.

 **loading, g/g dry mass**

40–110°C, 1.4 MPa 1 Rice and wheat straw,

**5.1. Description**

**5. Soaking in aqueous ammonia (SAA)**

are compensated using lower reaction times.

#### **4.2. Applications**

AFEX has been widely applied to various class of lignocellulosic materials. Some results obtained from AFEX pretreatment of some biomass are given in **Table 3**.

#### **4.3. Positive attributes and drawbacks**

AFEX is a dry-to-dry process since no liquid stream is produced, making it potentially less costly compared to steam explosion [63] and dilute acid methods [64]. The process is simple as it reduces requirements of post-pretreatment washing, stream separation and nutrient supplementation, and produces intermediates that are of value in developing advanced bioproducts. Reaction temperatures are moderate and energy requirements are low. Large solids (up to 5 cm) can be fractionated with good yields. Moreover, desired solid loadings are easily obtained, and high solid loadings are easier to implement due to low water demands. High glucose and xylose yields are both obtained under similar process conditions which simplify the optimization of process parameters. Moreover, except for some phenolic fragments of lignin and cell wall extractives that may form on the surface of pretreated solids, no enzymeinhibitors are produced [50]. AFEX give high sugar yields at low enzyme loadings of 1–10 FPU cellulase/g of dry biomass [1]. Klason lignin and carbohydrates are preserved and pretreated substrates possess high fermentability. Recently, process improvements bordering on ammonia loading and recovery, ammonia recycle concentration, and enzyme loadings have been developed and shown to reduce the cost of operation of AFEX-based biorefinery [65].


**Table 3.** Results of AFEX pretreated biomass.

Its main demerit is its unsuitability for handling materials with high lignin content such as wood. Much of the hemicellulose is fractionated to oligomers making it more challenging during fermentation. High pressures are usually required due to high ammonia loadings and high vapor pressure of ammonia. Moreover, ammonia is expensive and recovery of all feed ammonia for reuse is challenging. Safety issues arising from the corrosive and toxic nature of ammonia present additional challenges in process operation at industrial level. Compared to soaking in aqueous ammonia (SAA), AFEX requires expensive reactors and equipment.
