In many respects, the AFEX process is the alkaline equivalent of sulfur dioxide-catalyzed steam explosion pretreatment (13). In the AFEX process, biomass is treated with liq­uid anhydrous ammonia at temperatures between 60 and 100°C and pressures of 250­300 psig with residence times of about 5 minutes (20). The pressure is then released, result­ing in a rapid expansion of the ammonia gas that causes swelling and physical disruption of biomass fibers and partial decrystallization of cellulose, along with some lignin solubiliza­tion and re-arrangement and some solubilization of hemicellulose primarily to oligomeric sugars (10, 14). AFEX is typically conducted at high solids loadings (about 40% solids) and high ammonia loadings (about 1.0 g NH3/g dry feedstock), although the rapid expansion and high volatility of ammonia may permit near-complete recovery and recycle of ammo­nia (20, 38). The associated complexity and costs of ammonia recovery processes may be significant and must be better understood in order to assess the commercial potential ofthe AFEX process (21).

AFEX has been shown to deacetylate and increase the digestibility of biomass (39­41), although it does require that both cellulose and hemicellulose be enzymatically hy­drolyzed due to limited hemicellulose hydrolysis during AFEX pretreatment. The AFEX pretreatment is more effective on agricultural residues and herbaceous crops, with limited effectiveness demonstrated on woody biomass and other high-lignin feedstocks (14). AFEX has largely been practiced as a bench scale technique, although a larger, continuous ver­sion of AFEX based on extrusion technology, known as FIBEX, has been developed and tested (42).