Lignocellulosic (or second-generation) bioenergy is being intensively developed, due to its use of renewable but nonfood or feed feedstocks; valorization of agricul­tural, forestry, first-generation bioenergy, or municipal by-products or waste; and potential to significantly replace fossil feedstocks for energy or chemical indus­tries. From lignocellulosic biomass materials, cellulose and hemicellulose are converted by (hemi)cellulolytic enzymes or chemical means to fermentable sugars (mainly glucose and xylose), which are then fermented by yeast or bacteria to ethanol or other chemicals. The processes could run alone, fed by selected biomass feed­stocks, or along with the starch/sugar bioethanol or bio­diesel processes, fed by the lignocellulosic by-products from first—generation bioenergy processes.

The main by-product from lignocellulosic bioenergy processes (Figure 20.8) is lignin or lignaceous residue, whose valorizations are the focus of rigorous research efforts and may include uses for the production of phe — nolics (e. g. vanillin, vinyl guaiacol, ferulic acid), lignans, carbon fiber, or as additives for paper and pulp industry, roadbed construction, or soil augmentation (Ceylan et al., 2012; Chapters 22, 23 of this book). Other by­products from lignocellulosic bioenergy processes include stillage and pretreatment liquor, which may be inhibitory to fermentation or enzymatic hydrolysis but rich in phenolics and oligosaccharides (Klinke et al., 2002; Persson et al., 2002). Mechanically separated biomass components (upstream to pretreatment, hydro­lysis, and fermentation) may also serve as sources for
phytochemicals, such as tree barks for tannin production.

Woods, especially those not suited for conventional forestry products, are attractive feedstocks for lignocel — lulosic bioenergy. Prior to enzymatic or chemical conver­sion to fermentable sugars, woody materials might be subjected to treatments (such as the leaching processes widely used for dedicated phytochemical production, as mentioned in Section (Extraction and Isolation from Specific Plants) to yield extractives comprising pheno — lics (phenols, flavonoids, and anthocyanins), terpenoids (essential oils), nitrogen-containing phytochemicals (alkaloids) or organic acids (citric, oxalic, acetic, malic, benzoic, etc.) (Huang and Ramaswamy, 2012; Turley et al., 2006). In addition to agricultural and forestry by-products (e. g. corn stover, wheat straw, and wood residues), switchgrass and other dedicated "energy crops" may serve as viable feedstocks for not only bio­energy but also phytochemical coproducts. For instance, valued phytochemicals like antioxidants and flavonoids might be extracted from switchgrass prior to the pre­treatment of the bioenergy process (Huang and Ramaswamy, 2012; Uppugundla et al., 2009; Wang and Weller, 2006).