Another explosion pretreatment is the ammonia fiber explosion (AFEX) process. Instead of using liquid water under high pressure, liquid ammonia is used. AFEX is an effective and somewhat economically attractive method to increase the yields of fermentable sugars from LB (Holtzapple et al., 1991; Holtzapple et al., 1992). In this method LB is exposed to liquid ammonia, not ammonium hydroxide (i. e. no water/moisture), at moderate temperatures and elevated pressures for a longer period of time. After the appropriate residence time, the system is rapidly vented allowing the liquid to vaporize and literally explode the fibrous material. Typically, 1—2 kg of liquid ammonia is used for each kg of dry biomass. The system operates at temperatures below 100 °C, pressures above 3 MPa, and is quite tolerant of pH. Any pH under 12 appears suitable. The residence time is between 10 and 60 min. Under these conditions, the system forms few degraded sugar prod­ucts yet gives a high yield of desirable sugar products (Mosier et al., 2005).

AFEX is an attractive treatment method for a variety of herbaceous crops and grasses as it significantly im­proves the saccharification rates. It has been tested on a variety of LB including aspen chips, softwood and kenaf newspaper, alfalfa, wheat chaff, wheat straw, barley straw, rice straw, bagasse, coastal Bermuda grass, switchgrass, corn stover, and municipal solid waste. One of the benefits is that AFEX only solubilizes a trivial amount of solid material. Also, compared to acid pre­treatment and acid-catalyzed steam explosion, very little hemicellulose or lignin is removed. Lastly, the structure of the material changes such that the result is an increase in water-holding capacity and improved digestibility. Although physically modified, the chemical composi­tion of the material following AFEX pretreatment is essentially unchanged from its original condition. The benefit is illustrated as follows: over 90% hydrolysis of the cellulose and hemicellulose may be obtained after AFEX pretreatment of Bermuda grass where 5% of that is lignin. The result is similar for bagasse except 15% of the hydrolysate is lignin (Holtzapple et al., 1992). These low-lignin containing biomasses readily hydro­lyze at near theoretical yields of sugars. The resulting sugars ferment rapidly with a high yield into a variety of desired products. Since the AFEX treatment produces very few inhibitors to the downstream biological pro­cesses, a water wash is not necessary (Dale et al., 1984; Mes-Hartree et al., 1988).

Materials with a high lignin content, around 25%, have proved to be recalcitrant to AFEX. Therefore, AFEX is a less effective pretreatment method for hard­wood chips, some newspaper material, and nut shells (Teymouri et al., 2005). AFEX does not require a small particle size for it to be an effective treatment option (Larson and King, 1986) like steam explosion and hot water treatments.

The most significant cost is that associated with recy­cling the ammonia following pretreatment (Kumar et al., 2009a). Since pure ammonia is used in the process, more stringent environmental and recovery procedures are required. Thus, recycling is necessary to reduce the envi­ronmental impact and the cost of the procedure. To recover the ammonia, a superheated ammonia vapor, at temperatures upward of 200 °C, is used to vaporize and strip the residual ammonia from the pretreated biomass. The evaporated ammonia is then drawn off the system by a pressure controller for final recovery (Holtzapple et al., 1990). Using this recovery method has demonstrated that over 99% of the ammonia can be recycled successfully. Even so, the overall capital and operating costs are higher than other comparable methods.