Soaking Aqueous Ammonia (SAA)
In the soaking aqueous ammonia (SAA) pretreatment method biomass is soaked in a 15-30% dilute aqueous ammonia solution at moderate temperatures (25-60°C) in a batch reactor from a few hours to several days. This is a relatively mild method compared to AFEX and ARP pretreatments, and utilizing lower temperatures and less extreme pHs reduce the associated chemical and energy costs and may reduce the formation of carbohydrate degradation products. The method has been tested on different types of biomass including switchgrass [97-99], corn stover [100, 101], wheat straw [102], oat straw [103], rice straw [104], barley hull [105], rapeseed straw [106], soybean fiber [107], oil palm fronds [108], and miscan — thus giganteus [109]. The research group of Ko et al. have optimized aqueous-ammonia soaking pretreatment conditions for ethanol production from rice straw using simultaneous saccharification and fermentation processes [104]. The optimal pretreatment conditions were identified as soaking in a solution with concentration of 21% ammonia (w/w) at 69°C for 10 h. The ethanol yield of 83.1% of theoretical maximum could be achieved under optimized pretreatment conditions [104]. For corn stover, another set of optimized conditions for soaking aqueous ammonia pretreatment is known, which is 1:6-10 solid-to-liquid ratio (dry biomass (g): weight of 15 wt.% aqueous ammonia), 60-80°C, and 8-24 h [110], resulting in 85% glucan digestibility and 78% xylan digestibility with 15 FPU/ g-glucan enzyme loading. These conditions resulted in a 77% theoretical ethanol yield based on the total carbohydrates in untreated corn stover [110]. Even though the effectiveness of soaking aqueous ammonia (SAA) pretreatment technology in improving enzyme hydrolysis and subsequent ethanol fermentation is well demonstrated, the SAA method has some major problems. The most striking ones are the use of large quantities of water and high energy demand in recycling the solvent.