Pretreatment of Lignocellulosic Biomass
Pretreatment is the process used to liberate cellulose and hemicel — lulose from the lignin seal and its crystalline structure so as to render polysaccharides accessible for a subsequent hydrolysis step. The resistance of plant cell walls to deconstruction is known as the recalcitrance property, and pretreatment is the first step in overcoming biomass recalcitrance. The main factors that contribute to the recalcitrance of lignocellulosic biomass to hydrolysis are poor accessible surface area, protection of cellulose by lignin, the heterogeneous character of biomass particles, and cellulose sheathing as shown in the schematic representation of pretreatment in Figure 5.1. As illustrated in this figure, pretreatment improves the accessibility to cellulose and hemicellulose by liberating them from the lignin shell [1, 2].
In addition to this encapsulated arrangement, crystallinity of cellulose is also an important factor, because pure crystalline cellulose is difficult to hydrolyze without a pretreatment. Transformation between crystalline and amorphous forms of cellulose is reversible; both forms can break into glucose oligomers, however, the amorphous form degrades faster than the crystalline form, as shown in
Figure 5.1 Schematic representation of the pretreatment process. (Reprinted with permission from reference [2]; copyright 2011 Elsevier).
Figure 5.2 Schematic representation of transformations of crystalline and amorphous forms of cellulose to glucose oligomers, glucose, and to degradation products. (Adapted with permission from reference [1]; copyright 2005 Elsevier).
the kinetics schematic in Figure 5.2, with rate constant k2 >> k1. In principle, an effective pretreatment causes disruption of these barriers so that hydrolytic enzymes can penetrate and cause hydrolysis (Fig. 5.2) and also minimizes degradation of sugar to undesired degradation products shown in the last step of Figure 5.2. [3, 4].
Pretreatment of lignocellulosic biomass may produce degradation products with an inhibitory effect on the fermentation process. These undesired products are produced by the degradation of sugars as well as degradation of lignin. Pentose sugar monomers may dehydrate to the 5-carbon aldehyde furfural. Similarly, hexose sugars like glucose may degrade to 5-hydroxymethylfurfural (HMF). Furfural and HMF affect cell growth and respiration, and HMF is considered less toxic than furfural and its concentration in hydro — lyzates is usually low. A variety of compounds like aromatics acids,
phenols and aldehydes may be released from the degradation of lignin fraction. Phenolic compounds have a significant inhibitory effect and are generally more toxic than furfural and HMF. Low molecular weight phenols are the most toxic. However, at temperatures lower than 180°C lignin degradation is not so significant if no strong acid or alkaline conditions are present in the pretreatment medium. Some of the common inhibitory compounds formed during the pretreatment step are shown in Figure 5.3.
These inhibitors have toxic effects on the fermenting organisms, thus reducing the ethanol yield and productivity. The level of toxicity depends in part on fermentation variables including cell physiological conditions, dissolved oxygen concentration and pH of the medium. In many cases it is essential to remove these inhibitors before exposure to cellulose and hemicellulose hydrolyzing enzymes.
Pretreatment of biomass for cellulosic ethanol process has been the topic of a number of excellent review articles in recent years [5-11,1,12-16].
There are a number of key factors in a good pretreatment method, which include the following:
1. Produces highly digestible solids that enhance sugar yields during enzyme hydrolysis.
2. Avoids the degradation of sugars, especially the pentose derived from hemicellulose.
3. Minimizes the formation of inhibitors for subsequent fermentation steps.
4. Is cost effective by operating in reactors of moderate size and by minimizing heat and power requirements.