There are two possible outcomes for the chemistry behind the high pH treatment: 1) one is essentially a degradation reaction that liberates lignin fragments and leads to lignin dissolution, and 2) the other is condensation reactions that increase the molecular size of lignin fragments and result in lignin precipitation. As you can see, lignin is a complicated molecule, with a variety of linkages, so reactions are complicated due to lignin complexity. Addition of oxidizing agents greatly improves delignification.
There are multiple processes that have been developed for this type of treatment. Figure 5.21 shows the lime pretreatment process flow diagram. The pretreatment can be done under various conditions, such as oxidative and non-oxidative conditions, short term high temperature (100-200°C, 1-6 h), and long term low temperature (25-65°C, 1-8 weeks). Figure 5.22 shows the soaking in aqueous ammonia (SAA) process flow diagram.
One of the more developed high pH processes is the ammonia fiber expansion (AFEX) process. Lignocellulosic biomass is soaked in liquid ammonia (causing swelling) followed by rapid release of pressure (causing expansion). Anhydrous liquid ammonia is used, and key parameters include temperature, residence time, ammonia concentration, and moisture content of the biomass. During this process, there is virtually no compositional change, but lignin is relocated, cellulose is decrystallized, and hemicellulose is depolymerized. This method increases the size and number of micropores in the cell wall to allow for greater accessibility of chemicals for the following stages of processing. A process schematic is shown in Figure 5.23.