Pretreatment strategies have generally been categorized into biological, phys­ical and chemical processes, or a combination of these approaches.

Biological pretreatments typically utilize wood degrading fungi (soft, brown and white rot) to modify the chemical composition of the lignocel — lulosic feedstock. Generally, soft and brown rot fungi primarily degrade the hemicellulose while imparting minor modifications to lignin. White-rot fungi can more actively attack the lignin component [12]. Although there has been a fair amount of work done in this area, the primary application has been as a biopulping option for the pulp and paper industry rather than as a pretreat­ment for bioenergy applications. In addition to the requirements for careful control of growth conditions and for large amounts of space to perform bio­logical treatments, a major disadvantage of biological/fungal treatments is the typical residence time of 10-14 days. For these reasons, biological pretreat­ments are considered to be less attractive commercially.

Physical pretreatments involve the breakdown of the biomass feedstock into smaller particles that are more amenable to subsequent enzymatic hydrolysis. Physical treatments such as hammer — and ball-milling [13-16] have been shown to improve hydrolysis yields by disrupting cellulose crys­tallinity and by increasing the specific surface area of the feedstock, rendering them more accessible to attack by cellulases. One of the advantages of physi­cal pretreatment is that it is relatively insensitive to the physical and chemical characteristics of the biomass employed. However, the physical pretreatment processes are energetically demanding and do not result in lignin removal. Lignin has been shown to restrict access and inhibit cellulases [17,18]. Fur­thermore, physical pretreatments have yet to be shown to be economically viable at a commercial scale.

Most of the chemical pretreatments that have been assessed to date (typ­ically acid and alkali based) have had the primary goal of enhancing enzyme accessibility to the cellulose by solubilizing the hemicellulose and lignin, and to a lesser degree decreasing the DP and crystallinity of the cellulosic component. Pretreatments that reduce cellulose crystallinity include mild swelling agents such as NaOH, hydrazine and anhydrous ammonia, and ex­treme swelling agents such as sulfuric acid, hydrochloric acids, cupram, cuen, and cadoxen [19]. Treatments that reduce the lignin content of the substrate include organosolv pulping with various solvents including ethanol, glycerol and ethylene glycol.

Typically, all chemical pulping processes in commercial use today involve the removal of lignin to produce pulp for various paper products. Although these processes could be considered as potential pretreatment methods, they are optimized to maintain the fiber/strength integrity of the pulp, not to in­crease accessibility to the cellulose. The relatively high value of pulp (at the time of writing, approximately US$730 per tonne of northern bleached soft­wood Kraft pulp in Europe according to the PIX Pulp Benchmark Index) can justify the high capital and operating costs of chemical pulping, while lower-value biofuels must seek cheaper pretreatment alternatives. Despite these apparent drawbacks, various groups have looked at modified pulping processes as potential pretreatment methods, most likely since these pulp­ing processes produce readily hydrolyzable substrates. For example, in a Kraft pulping process NaOH and Na2S are combined in an aqueous liquor to cook wood chips under elevated pressures, followed by a pressure-release defi — bration step. The resulting Kraft pulps have been shown to be receptive to subsequent hydrolysis by cellulases [16], most likely because of the combina­tion of chemical dissolution of lignin and a decrease in average particle size that occurs during physical defibration.

Pretreatments that combine both chemical and physical processes are re­ferred to as physiochemical processes. These pretreatment methods have received the most attention in recent years and are the major focus of this review. In particular, steam pretreatment has received significant attention for its suitability in generating easily hydrolyzable substrates from lignocellu — losic biomass. However, several aspects that affect the viability of the overall process will be discussed in more detail later in this review, including the handling and preparation of the feedstock prior to the pretreatment step, the need to minimize processing costs, and the need to maximize the value of co­products derived from the hemicellulose and lignin streams. For example, if a pretreatment method has a requirement for very fine, uniform feedstock with a particle size of less than 10 mm, this will have a significant impact on the overall economic viability of the overall process because of the en­ergy requirements to produce this fine material [20,21]. Similarly, although acid-based pretreatment processes have been shown to be effective on a range of lignocellulosic substrates, downstream costs including the need for alka­line neutralization chemicals such as CaOH2 [22], must be considered. At the same time alkaline-based pretreatment methods such as lime, ammonia freeze explosion (AFEX), and ammonia recycle percolation (ARP) processes can effectively reduce the lignin content of agricultural crops such as wheat straw and corn stover, but have a much more difficult time processing recal­citrant substrates such as softwoods.

To summarize this general introduction, it is unlikely that one pretreat­ment process will be declared a “winner” as each method has its inherent advantages/disadvantages. However, as discussed in more detail below, steam pretreatment is one method that is effective on a range of lignocellulosic sub­strates and, through companies such as Masonite, has been shown to work effectively at a commercial scale.

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