When aqueous ammonia, 10—15 wt%, is percolated through biomass at temperatures between 150 and 170 °C with a fluid velocity of 1 cm/min and a residence time of 14 min, lignin depolymerizes and the lignin — carbohydrate linkages break. This process is known as ammonia recycle percolation (ARP) (Iyer et al., 1996). This process is advantageous in that it does not inhibit downstream biological processes. A water wash is there­fore not necessary (Kumar et al., 2009a). Additionally, it is possible to recover and recycle the ammonia. On the downside, ARP is inefficient when used to pretreat soft­wood pulp (Mosier et al., 2005) where lignin had already been removed.

Ozonolysis Pretreatment

A pretreatment option that is appropriate for grassy biomass and some softwood is ozonolysis.

In this case, ozone is used to degrade the lignin and hemicellulose in bagasse, green hay, peanut, pine, cotton straw, wheat straw and poplar sawdust (Ben — Ghedalia and Miron, 1981; Vidal and Molinier, 1988). The process primarily acts on the lignin component and only mildly affects the hemicellulose component, while cellulose is negligibly affected. Ozonolysis is notable in that it removes lignin effectively and the reactions take place at room temperature and stan­dard pressure (Ben-Ghedalia and Miron, 1981). The most significant advantage is that following an ozone pretreatment where 60% of the lignin is removed from wheat straw, the rate of enzymatic hydrolysis increases by 500% (Ben-Ghedalia and Miron, 1981). The most notable drawback is that the process is expensive due to the large volume of ozone required (Sun and Cheng, 2002).