14.5.5.1 Sodium hydroxide pretreatment

Alkali pretreatment processes generally do not hydrolyze hemicellulose as extensively as acidic pretreatments, but can be effective at removing lignin, which can lead to an increase in the enzymatic digestibility of alkali pretreated solids. Several studies on alkali pretreatment using sodium hydroxide have been reported and reviewed (11, 12, 14). This pretreatment approach causes swelling of fibers, leading to an increase in internal surface area, reduction in the degree of polymerization, a decrease in crystallinity, separation of the structural linkages between lignin and carbohydrates, and disruption of lignin structure (11). The effectiveness of sodium hydroxide pretreatment has been correlated to feedstock lignin content, with high lignin feedstocks, especially softwoods, showing poor performance using this approach (14). Dilute sodium hydroxide pretreatment has been shown to be quite effective on low lignin (10-18% lignin content) straw feedstocks (58), but the cost effectiveness of this pretreatment approach has not been thoroughly evaluated.

14.5.5.2 Ammonia pretreatment

In addition to the rapid expansion AFEX pretreatment process, which utilizes ammonia to achieve both chemical and physical changes to biomass, there are a number of additional ammonia pretreatment processes. The simplest ammonia pretreatment process involves a relatively low-temperature soaking (ambient temperature up to 90°C) using aqueous ammonia (various strengths up to 29 wt% NH4OH) at solids loadings of 10-50% and res­idence times from a few hours to up to 1 day (14, 59, 60). In these processes, up to 80% delignification has been reported on feedstocks such as wheat straw and corn stover, with much lower extents of hemicellulose solubilization. However, good enzymatic digestibility of the remaining cellulose and some of the remaining hemicellulose can be achieved us­ing commercial cellulase preparations (60). There is little evidence that this pretreatment approach would be effective on woody biomass types, especially softwoods. As with other alkaline pretreatment approaches, the augmentation of cellulase enzymes with hemicellu — lase and other accessory enzyme activities could improve the enzymatic saccharification of ammonia-pretreated biomass.

A percolation-type ammonia pretreatment process, known as ammonia-recycled percola­tion (ARP), has also been investigated (10,14,19,61). The ARP process passes dilute aqueous ammonia (<15% NH3) through a packed bed of biomass at temperatures of 150-170°C. Because of the flow-through nature and ammonia-based chemistry, ARP pretreatment of corn stover can achieve very high lignin removal (above 80% delignification) and moderate hemicellulose solubilization (above 50% xylan solubilization at residence times of 20 min or more), although hemicellulosic sugars are generally recovered in oligomeric form and would require additional processing to liberate monomeric sugars. Enzymatic digestibility of ARP — pretreated corn stover is also high (about 90% conversion of residual cellulose to glucose and about 70% conversion of residual xylan to xylose) using a commercial cellulase preparation (19). However, economic analysis has revealed that the high liquid volumes and subsequent dilute process streams does not allow the ARP process to economically compete with other pretreatments, even when efficient ammonia recovery and recycle is assumed (21). In this respect, ARP is similar to liquid hot water and dilute acid percolation pretreatments in that such processes may not be economically competitive, but they are of value in research appli­cations to generate pretreated solids with a wide range of hemicellulose and lignin removal extents for enzymatic hydrolysis and related compositional and ultrastructure studies.