Alkaline pretreatment is viewed as a viable treatment method because of its low energy requirement and low capital equipment and operational costs (Zhao et al.,

2008) . This process operates at lower temperatures and pressures than other pretreatment methods. However, at these conditions, the process is measured in hours or days vs. minutes or seconds for high-temperature, high-pressure methods (Karr and Holtzapple, 2000). Additionally, one may recover or regenerate many of the caustic salts.

Alkaline pretreatment may follow an SEP and may be followed by an enzymatic hydrolysis pretreatment (Montane et al., 1994; Pan et al., 2006). The initial reac­tions of alkaline pretreatment involve solvation and saponification. Solvation, similarly associated with
dissolution or diffusion, is where the solvent surrounds an ion, typically sodium dissolved in water. Traditional NaOH treatment requires high temperatures to be effec­tive (Zhao et al., 2008). It may be supplemented with urea to lower operational temperatures and improve dissolution (Zhao et al., 2008).

The alkaline solvent then saponifies the intermolec­ular ester bonds that cross-link xylan hemicelluloses and other components including lignin and other hemi — celluloses. Removing these cross-links increases the porosity of the lignocellulosic materials. This improves the penetrability of the material to the solvent and swelling of the biomass follows. The swollen biomass is thus more vulnerable to enzymatic and bacterial activity.

Compared with acid hydrolysis, alkaline hydrolysis generally causes less sugar degradation. That said, dissolution or solubilization of LB increases with alkali concentrations. At strong alkali concentrations, peeling of end-groups may occur. This leads to alkaline hydroly­sis and degradation of the dissolved polysaccharides. Furthermore, this may also produce unwanted byprod­ucts. However, there may be a downstream advantage in subsequent conversion treatments. It increases the inter­nal surface area, decreases the DP, decreases crystallinity and separates linkages between lignin and carbohy­drates causing an overall disruption of the lignin struc­ture (Fengel, 1984). This provides opportunity for increased enzymatic and bacterial activity in down­stream processes.

An alternative process to improve sugar content is to use aqueous potassium hydroxide, which selectively removes xylan. Keeping the temperature low, at or
below room temperature, prevents peeling (Hon and Shiraishi, 2001).

It appears that monomeric forms of hemicelluloses are easily degradable to other volatile compounds. Glu — comannans and xylans are particularly vulnerable to peeling. However, by pretreating with a 3% NaOH and 12% urea at —15 °C one can achieve a 60% glucose conversion (Zhao et al., 2008).

Calcium hydroxide, or slake lime, is yet another effective alkaline pretreatment agent. It is one of the least expensive and it is highly recyclable (Karr and Holtzapple, 2000). Using common lime kiln technology, one can recover calcium hydroxide by regenerating it from insoluble calcium carbonate. Lime pretreatment removes lignin and hemicellulose and increases the CrI.

Pretreatment with dilute NaOH decreases the lignin content within a range of 24—55% to 20% and increases the digestibility of NaOH-treated hardwood from 14% to 55%. No effect was observed for softwoods with lignin content greater than 26% (Bjerre et al., 1996). Dilute NaOH pretreatment causes swelling, which, as stated previously, has downstream benefits.

The overall effectiveness of alkaline pretreatment depends on the lignin content of the biomass. Further­more, it changes the cellulose structure such that it is less dense and more thermodynamically stable than native cellulose (Hendriks and Zeeman, 2009; Liu and Wyman, 2003).