Woody biomasses such as loblolly pine (Pinus taeda) could potentially be an excellent feedstock for the production of biofuels due to their widespread availability in the United States (Smith et al. 2002). However, woody biomass suffers from difficulty in pretreatment to obtain concentrated sugar solutions at sufficient yields (Stenberg et al. 1998; Soderstrom et al. 2003; Zhu et al. 2009). Higher lignin content (Soderstrom et al. 2003) and cellulose crystallinity (Bansal et al. 2010) in woody biomasses have been recognized as two major barriers in accessing the sugars after pretreatment. Various studies on different types of pretreatment also showed that the lignin undergoes changes during pretreatment including depolymerization and re-condensation and comes out from the process in more condensed form than native lignin (Funaoka et al. 1990; Trajano et al. 2013). The lignin condensation requires that this lignin be used only for the combined heat and power application (Sannigrahi et al. 2009).
The biomass to biofuel conversion process involves pretreatment and enzymatic hydrolysis to fractionate the polysaccharides and then recover the monomeric sugars from the complex woody biomass, which then can be further converted into fuels and chemicals via biochemical or catalytic pathways (Petrus and Noordermeer 2006; Alonso et al. 2010). A major obstacle for efficient biofuel production from woody biomass is the pretreatment of biomass, which plays the most crucial role by impacting all the further downstream processes (Kumar et al. 2009). Pretreatment is the first and very expensive step in the overall biomass to biofuel conversion process (Wyman et al. 2005). To overcome the challenges associated with the pretreatment of woody biomass, it is critical to understand the composition and the chemical characteristics of woody biomass.
Lignocellulosic biomass primarily consists of cellulose, hemicellulose, and lignin. The cellulose is a homogenous, linear polysaccharide with repeating cellobiose units connected by p (1 ^ 4) linkages (Pandey 2009). The degree of polymerization varies based on the type of biomass and imparts high tensile strength (Sjostrom 1999). The inherent bonding ability of cellulose is derived from inter- and intramolecular © The Author(s) 2017
V. Rana, D. Rana, Renewable Biofuels, SpringerBriefs in Applied Sciences and Technology, DOI 10.1007/978-3-319-47379-6_1
hydrogen bonding (Sjostrom 1999). Cellulose microfibrils consist of crystalline and amorphous regions which offer different resistance to attack by acids or alkali (Sjostrom 1999). Hemicellulose on the other hand is an amorphous heteropolymer of hexoses (glucose, mannose, and galactose) and pentoses (xylose and arabinose) and has a lower degree of polymerization (Sjostrom 1999). The predominant hemicellulose is O-acetyl-galactoglucomannan which is a mixture of glucose and mannan on the backbone (Sjostrom 1999). Lignin has completely different properties and structure than cellulose and hemicellulose (Sjostrom 1999). Lignin is the only renewable source of aromatics in the world (Holladay et al. 2007). Lignin is composed of three major phenolic components, namely, p-coumaryl alcohol or para-hydroxyphenyl (H), coniferyl alcohol or guaiacyl (G), and sinapyl alcohol or syringyl (S) (Ralph et al. 2004; Buranov and Mazza 2008).
The chemical composition of loblolly pine consists of 35.97 % cellulose, 19.73 % hemicellulose, and 30.65 % lignin. The recalcitrance of woody biomass is mainly derived from the higher crystallinity of the cellulose and higher lignin content (Soderstrom et al. 2003). Cellulose present in the woody biomass contains higher amounts of crystalline regions than amorphous regions as compared to nonwoody biomass (Sannigrahi et al. 2010). Crystalline regions of cellulose are considered to be more difficult to degrade than noncrystalline regions due to strong intermolecular hydrogen bonding between the cellulose chains (Sannigrahi et al. 2010). Lignin which is an aromatic polymer provides the highest recalcitrance to the biomass owing to its three-dimensional, highly heterogeneous polymeric, and least understood structures (Capanema et al. 2004).