Liquid hot water percolation pretreatment

Pressurized liquid hot water that is percolated or otherwise forced through a packed bed of biomass particles has been shown to result in high removal of both hemicellulose and lignin, with high recovery of hemicellulose-derived sugars (primarily in oligomeric form) and high digestibility of the resulting pretreated solids (47, 48). These processes may be difficult to commercialize due to the high volumes of liquid required to sustain a continuously-flowing percolation process, although some efforts to address the high liquid volume requirements using intermittent-flow approaches have been investigated (16). Nevertheless, percolation pretreatment techniques are useful 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.

14.5.2 Acidic pretreatments

14.5.4.1 Dilute acid batch/co-current pretreatment

Dilute acid pretreatment is probably the most thoroughly investigated biomass pretreatment technique. A variety of acidic catalysts have been investigated in numerous batch/co-current dilute acid pretreatment reactor designs on a wide range of woody, herbaceous, and agri­cultural residue feedstocks. For cost reasons, most dilute acid pretreatment studies have utilized sulfuric acid or gaseous sulfur dioxide (in steam explosion applications), although several processes that utilize nitric, phosphoric, hydrochloric, or carbonic acid have also been investigated. Dilute acid pretreatment studies find wide distribution in the published literature and have been summarized in several review articles (10-15, 49, 50).

Dilute acid batch and co-current pretreatments are generally aimed at achieving near­complete solubilization of the hemicellulose fraction of biomass while also achieving high yields of hemicellulose-derived sugars. Many processes seek to directly achieve monomeric sugar formation, although care must be taken to prevent excessive sugar degradation of monomeric sugars. If performed properly, dilute acid pretreatment can be very effective at achieving reasonable monomer sugar yields via both hemicellulose hydrolysis and enzymatic digestion of the cellulose in the resulting solids across a range of biomass feedstock types (51). However, in batch or co-current mode, there will likely be some degradation losses of hemicellulose-derived sugars and possibly a requirement for conditioning the hydrolyzate liquid fraction prior to fermentation. Dilute acid pretreatment approaches have been tested in continuous co-current pilot scale reactor systems (52, 53) and have been the subject ofin — tensive process simulation and economic analysis for potential commercial-scale operations using this pretreatment approach (54).