Ammonia Fiber Explosion/Ammonia Recycle Percolation)

The ammonia fiber/freeze explosion (AFEX) process is a physicochemical process in which the biomass is subjected to liquid anhydrous ammonia under high pressures and moderate temperatures and then is rapidly depressurized. The AFEX resembles very much the steam-explosion pretreatment technology. However, compared to the steam-explosion process the temperatures (60—100°C) are much more moderate, meaning less energy input and overall energy costs associated with the AFEX process. Major variables in the process are the operation temperature, ammonia concentration and reaction time. The temperature will influence the degree of disruption to the biomass struc­ture, as it will affect the rapidness of the ammonia vaporization within the reactor during depressuriza­tion. Typical ammonia loading for many feedstocks are around 1 kg ammonia per kilogram dry biomass. The residence time can be altered from minutes to half an hour duration depending on the degree of satu­ration needed for the selected biomass (Chundawat et al., 2007). The biomass is saturated for a period of time with the ammonia in a pressurized reactor before being released to atmospheric temperature resulting in a rapid expansion of the ammonia gas causing swelling of the biomass feedstock. This creates hydrolysis of the hemicellulose fraction, a disruption in the lignin- carbohydrate linkages, ammonolysis of glucuronic cross-linked bonds, and partial decrystallization of the cellulose structure, all leading to a higher accessible surface area for enzymatic attack (Chundawat et al., 2007). An important prerequisite to make the process economic is a very efficient recovery of the ammonia gas. Under typical AFEX conditions this pretreatment does not remove lignin or any other substances from the biomass; however, the lignin-carbohydrate com­plexes are cleaved, and the lignin is deposited on the surfaces of the material possibly causing blockage of cellulases to cellulose (Kumar et al., 2009; da Costa Sousa et al., 2009). An overview of the advantages and disadvantages is listed in Table 17.5. Ammonia recycle percolation (ARP) has often been paired with the AFEX pretreatment process, but it can have some different characteristics. In the ARP process, aqueous ammonia of concentration between 5 and 15% (wt%) is sent through a packed bed reactor containing the biomass feedstock at moderately high temperatures (140—210 °C) and longer reaction times compared to the AFEX process, increasing the energy costs (Bro — deur). The advantage of the ARP process over AFEX is its ability to remove the majority of the lignin (75—85%) as well as solubilize more than half of the hemicellulose (50—60%) while keeping the cellulose in its polymeric form. This results in short-chained cellu — losic material containing a high amount of glucan with a high degree (>86%) of enzymatic digestibility and a limited amount of inhibitors. Up to now mostly herbaceous biomass has been treated with this process. Many of the primary concerns with the AFEX process (high energy costs and liquid loadings, along with many disadvantages associated with the AFEX process)

need to be addressed before an economical process can be envisioned (Brodeur et al., 2011).