Oxidative processes for biomass pretreatment applications are often referred to as wet ox­idation processes. This approach was born out of efforts in the pulp and paper industry to develop oxygen delignification processes to reduce chlorine use in pulping. The most common approach for wet oxidation as a biomass pretreatment involves the injection of pressurized O2 into a pretreatment reactor at temperatures up to 200°C and pressures up to about 1.5 MPa (69). Another approach utilizes a percolation-type pretreatment that incor­porates wet oxidation, among other approaches, into a biomass fractionation process (70). Most work has been performed in batch reactors at low solids loading, occasionally with the use of small amounts of co-catalysts or solvents. Much of this work has included the use of alkaline buffers (usually sodium carbonate) to maintain reaction pH in the neutral to alkaline range. There have been sporadic efforts to investigate alkaline peroxides and ozone as other types of oxidizing agents (71-73).

Wet oxidation extensively delignifies biomass with production of monomeric and oligomeric phenols, followed by oxidative cleavage to a variety of carboxylic acids. When the reaction is not buffered and pH is allowed to drift naturally down, extensive forma­tion of furfurals occurs, which can also be cleaved to form carboxylic acids in the oxidative environment. Hemicellulose is typically solubilized to about 70% conversion, primarily as oligomers. The combination of extensive delignification and at least 50% hemicellulose removal can result in highly digestible pretreated solids (70).

14.5.9 Biological pretreatment

Most biological pretreatment approaches utilize certain classes of lignin-solubilizing mi­croorganisms that will produce a lignocellulosic feedstock that is more amenable to enzy­matic saccharification than native biomass. Many studies have focused on a class of microor­ganisms known as white-rot fungi, which produce lignin-degrading enzymes, lignin perox­idases, peroxide producing enzymes, polyphenol oxidases, laccases, and quinine-reducing enzymes (11, 12, 74, 75). In such biological pretreatment processes, biomass is inoculated with appropriate fungal cultures and incubated for several weeks, followed by evaluation of the enzymatic hydrolyzability of the treated biomass. While increased digestibility has been attributed to the delignification action of these microorganisms (75,76), there is often some parallel loss of cellulose and/or hemicellulose during the biological pretreatment.

Other biological pretreatment approaches have been investigated as a means ofpreparing stored biomass for subsequent mechanical, thermal or chemical pretreatment, potentially reducing the required severity and cost of the pretreatment process. This approach can effec­tively soften especially woody biomass types, reducing the mechanical power requirement for size reduction via disk refining by about 40%, although there is some associated loss of mass due to fungal action of lignin and structural carbohydrates, suggesting a potential loss of available carbohydrates to the conversion process (77).