5.1 Generation of Woody Biomass

Woody biomass derived from forest and agricultural residues is an important source of renewable energy and chemicals. According to the Food and Agriculture Organization of the United States (FAO), the global production of wood residues between 1974 and 2016 has increased from 1.85 × 10⁴ tons to 5.69 × 10⁸ tons (Food and Agriculture Organization 2018). Most of the woody biomass has been generated in Asia (51%), America (20%), and Europe (27%) (Food and Agriculture Organization 2018), which is derived from different sources, such as woody product wastes (Figure 5.1a) and yard wastes (Figure 5.1b,c). It contains different parts of woody plants, such as the trunks, branches, and leaves.

Living woody plant cells are generated from mitosis division of the apical and lateral meristems (Evert 2006). The apical meristems are located in the tips of the stems, roots, and branches and are responsible for the primary growth, i.e. lengthening of the plants; while the lateral meristems present under the bark of roots, stem, and branches are responsible for the secondary growth, that is, thickening of the plant. The stem, or the trunk, is currently the most useful part of the woody plant. The tree trunk consists of four segments: bark, phloem, vascular cambium, and the secondary xylem (Harada and Côté Jr 1985). Bark protects the plant from mechanical and biological attacks. The thin layer of cells between the phloem and the xylem is the vascular cambium, which divides the wood trunk into xylem in the inner side and phloem in the outer. Phloem is the living tissue that stores and transports nutrients to different parts of the plants, and the xylem is produced by the multiplication of meristematic cells in the vascular cambium (Lachaud et al. 1999).

5.2 General Classification and Properties of Woods

Woody materials are most commonly classified based on the seed structure and reproduction system of the plants (Figure 5.2). Hardwood, known as angiosperm, originates from deciduous trees such as oak, maple, eucalyptus, and beech. Softwood, or gymnosperm, originates from coniferous trees, such as pine, spruce, Douglas fir, and redwood. Hardwood and softwood vary in appearance and have different functional plant cells. In softwood, approximately 90% of the wood volume comprises tracheids, which transport liquid and provide mechanical support to the tree. In hardwood, fibers provide the mechanical support while vessels transport liquid throughout the plant. The transformation and storage of nutrients is performed by parenchyma cells in both softwood and hardwood (Daniel 2009).

Wood consists of elongated cells (Fujita and Harada 2000) in the longitudinal direction of the stem, and grows radially as concentric rings. The inner part of the wood ring is heartwood and the outer parts are sapwood, of which the types and contents of basic chemicals are provided in Figure 5.2. Heartwood consists of entirely dead tissues and provides mechanical support to the trees. The dead cells in heartwood contain deposited extractives, which improve biological resistance and darken the color of the heartwood. Transportation and storage of water, mineral and nutrients take place in the sapwood, although most cells in the sapwood are dead. The living cells of sapwood provide protection against microbial attacks. When the living cells in sapwood die, the sapwood is gradually converted into heartwood with aging.

5.3 Wood Chemistry

Wood is composed of various heterogeneous chemical constituents. Cellulose, hemicellulose, and lignin are the main constituents. These building block chemicals are heterogeneously distributed throughout the wood cell walls (Figure 5.3), which vary significantly among different species. Although the cellulose contents in hardwood and softwood are almost same, the softwood contains more lignin and less hemicellulose than hardwood (Kibblewhite and Brookes 1976; Alén 2011). Low molecular weight compounds such as extractives and some inorganics are nonstructural constituents of wood.

5.3.3 Lignin

Lignin is an amorphous, hydrophobic biopolymer constructed by three monolignols (Table 5.1): p‐hydroxyphenyl (H‐lignin), syringyl (S‐lignin), and guaiacyl (G‐lignin), which is the main renewable source of aromatic chemicals (Sjostrom 2013). Lignin has many remarkable properties, like high resistance to chemical, biological, and thermal degradation. The presence of multiple functional groups in the matrix makes lignin a promising alternative for carbon‐based materials (Ho‐Yin et al. 2019). Lignin has exhibited superior functionality in UV absorbance, amphiphilicity, and anti‐oxidation (Tian et al. 2017). The application of lignin and its derivatives in adhesives and green composites have been reported recently (Espinoza‐Acosta et al. 2018; Faris et al. 2016). Owing to its low cytotoxicity and pH controllable properties, lignin has also been considered for bioimaging and drug delivery systems (Dai et al. 2017). The exact linkages between the building block components are very complex and have not yet been completely decoded (Özparpucu et al. 2017), but a few major linkages have been confirmed through investigating the dissociated monomers after digestion. β‐aryl (β‐o‐4) and α‐aryl (α‐o‐4) ether linkages are the representative chemical linkages in the plant cell wall, accounting for at least 43–50% and 6–8% (in number) in softwood, respectively (Li et al. 2015). The potential to cleave the two linkages in or outside the phenolic structures are process‐specific.

Name	Structure	Softwood	Hardwood
p‐hydroxyphenyl (H‐lignin)		<5%	0–8%
Syringyl (S‐lignin)		Trace amounts	46–75%
Guaiacyl (G‐lignin)		>95%	25–50%

Lignins are generally classified as hardwood lignin, softwood lignin, and grass lignin. Hardwood lignins are composed of syringyl and guaiacyl units, which originate from trans‐sinapyl alcohols and transconiferyl alcohol respectively. Softwood lignin, on the other hand, is mainly composed of guaicyl, which comes from trans‐coniferyl alcohol. The p‐hydroxyphenyl unit of grass lignin is derived from trans‐p‐coumaryl alcohol and two foregoing precursors. Native lignins are insoluble in water and acids, but isolated lignins show the maximum solubility in different organic solvents such as dimethyl sulfoxide, acetone, dioxane, tetrahydrofuran, and so on. Isolated lignins can be classified as milled wood lignin, kraft lignin, alkali lignin, and enzymatically liberated lignin.

5.4 Chemical Composition Analysis

Chemical composition analysis is often conducted to evaluate the true value of the waste‐derived biomass. The analytical procedure for structural and non‐structural components of wood is demonstrated in Figure 5.4.

5.5 Pretreatment

Conversion of woody biomass into useful products relies on a serious of complex processes, such as pretreatment, hydrolysis, fermentation, and separation. Pretreatment aims to alter and modify the basic structure of lignocellulosic biomass for functional application. The main purpose of pretreatment is to increase the biomass conversion yield by increasing the enzyme accessibility to the pretreated substrate. Lignocellulosic biomass is a complex matrix of the introduced building block chemicals, which provide strong recalcitrance to cellulose degradation (Himmel et al. 2007). Woody biomass contains more lignin and possesses greater recalcitrance than herbaceous biomass. To overcome the biomass recalcitrance, it is necessary to fractionate all the components at the very beginning of the bioconversion process. Different pretreatment techniques have been designed to remove the recalcitrance of lignocellulosic biomass (Park et al. 2017; Baker et al. 2017; Zhao et al. 2017; Brosse et al. 2009). Pretreatment removes lignin and hemicellulose, and increases the surface area and digestibility of cellulose. It can also prevent the degradation of pentose sugar and minimize the formation of inhibitory components for fermentation. Physical, biological, and chemical methods have been reported for the pretreatment of plant biomass, while biological pretreatment is less preferred industrially due to its low efficiency and processability (Abd‐Aziz et al. 2018; Mussatto 2016). Physical pretreatment like steam explosion is usually used for woody biomass, but the process alone has not been extensively used commercially in comparison with chemical pretreatment approaches (Zhu and Pan 2010).

The most extensively studied chemical pretreatments utilize a wide range of chemicals at elevated temperature (Wang et al. 2018; Terán Hilares et al. 2017). Acid, alkali, organosolv, peroxide, sodium hypochlorite, sodium sulfide, ozone, and ammonia have all been used in the chemical pretreatments (Leu and Zhu 2013). We classify here the major chemical pretreatment concepts into three categories (i.e., organosolv, acid, and alkaline pretreatment), of which the key mechanisms are presented in Figure 5.5.

The chemical reactions in cleaving the two representative ether linkages under different types of biorefinery processes are illustrated in Figure 5.6. The presented model lignin structures include two G‐lignins linked at the α‐o‐4 position. The monolignol at the bottom right contains a –OH group linking to its phenolic structure at the 4th position while its α‐ and β‐ positions both linked with other lignins. The monolignol at the upper left links with other lignins at the 4th and β‐ positions, but its α‐ position links only with a –OH group. The concept diagram demonstrates the major functions and limitations of applied reagents and/or catalysts to cell wall decomposition during thermochemical processes. Cleavage of the two linkages under an alkaline pretreatment process is shown in Figure 5.6a. At elevated pH, the cleavage reactions can initiate most rapidly at the –OH group at the 4th position of the phenolic structure, while it can be carried out only very slowly when the 4th position links to another lignin. During the pretreatment process, more –OH structures can be created to continue the cleavage of the linkages. Alkali can also cleave some hydrolysable linkages in lignin and polysaccharides, which result in a reduction in the degree of polymerization and crystallinity of the substrate (Kim et al. 2016). For acid pretreatment processes, bond cleavage can start randomly at either the α‐ or β‐ positions regardless of whether there is a hydroxyl group bonding with the aromatic ring at the 4th carbon. With different reagents the α‐ and β‐ linkages can be cleaved and the original functional groups can be replaced by hydrogen, ethyl, or sulfonate groups. In addition, acid‐based biorefinery processes can effectively cleave glucosidic linkages in hemicelluloses and amorphous cellulose, which further improve the accessibility of enzymes to cellulose to promote enzymatic hydrolysis (Leu and Zhu 2013).

Acid pretreatments have been successfully applied to solubilize hemicellulose and efficiently used to remove the recalcitrance of biomass. Both dilute and concentrated H₂SO₄ acids are most commonly used for pretreatment of lignocellulosic biomass (Mosier et al. 2005). Dilute acid pretreatment processes are more preferable than concentrated acid processes because concentrated acids mediate the generation of fermentation inhibitors (Taherzadeh and Karimi 2008). Acid pretreatment can be done at low temperatures (<120°C) for longer retention times or at high temperatures (>180°C) for shorter retention times. Other acids such as nitric acid, hydrochloric acid, phosphoric acid, acetic acid, and formic acid are also successfully studied in biomass pretreatment (Gámez et al. 2006; Rodrıguez‐Chong et al. 2004).

Alkali preferentially removes lignin over hemicellulose from biomass (Leu and Zhu 2013). Alkali reacts with acetyl and uronic acid groups in the hemicellulose and thus increases the accessibility of enzymes (Chang and Holtzapple 2000). The hydrolysis of ester linkages more effectively removes lignin with minimal cellulose and hemicellulose solubility (Sun and Cheng 2002). NaOH is more effective than other alkalis in reducing cellulose crystallinity and degree of polymerization, and altering the structure of lignin (Kumar and Wyman 2009).

Organosolv techniques are mainly used for the extraction of high‐quality lignin. Ethanol, methanol, propanol, butanol, butanediol, ethylene glycol, glycerol, acetic acid, acetone, and tetrahydrofuran are most commonly used in combination with inorganic acids, salts, or alkali catalysts (Zhao et al. 2009; Bajpai 2016). Organic solvents are most effective in lignin degradation than other chemical pretreatments. Lignin removal increases the accessible surface of substrate for enzyme saccharification (Agbor et al. 2011). Pretreatment temperature, reaction time, solvent concentration, and catalyst concentration greatly determine the crystallinity, fiber length, and surface area of pretreated biomass. The effects of the different catalysts such as H₂SO₄, NaOH, and MgSO₄ have been studied on ethanol organosolv of pine. The results showed that H₂SO₄ is the most effective catalyst for pine (J.Y. Park et al. 2010; N. Park et al. 2010). The combination of ethanol organosolv to ball milling increased the lignin separation and glucose yield and decreased the pretreatment severity of organosolv (Hideno et al. 2013). Ionic liquid and deep eutectic solvents are recyclable organic compounds, and have been widely exploited as solvents for pretreatment of lignocellulose before further processing to energy and chemicals (Moniruzzaman and Goto 2018; Kanbayashi and Miyafuji 2016). Presence of solvent in the substrate reduces the enzyme activity of cellulose hydrolysis (Mosier et al. 2005). Removal and recovery of solvent also reduces the operational cost of the process (Sun and Cheng 2002). Organic solvents should be carefully handled because they are inflammable and can cause explosions.

5.6 Saccharification and Fermentation

Saccharification is the conversion process to break down polysaccharides into monomeric sugars (Zhu et al. 2017). Acid hydrolysis and enzymatic hydrolysis are commonly applied procedures for saccharification of lignocellulosic biomass. Acid hydrolysis can directly depolymerize the polysaccharide chains in the lignocellulosic biomass without any pretreatment, which can effectively release more than 90% of monomer sugars from the feedstock (Lavarack et al. 2002; Frederick Jr et al. 2008). However, concentrated acid hydrolysis can further react with the dissolved substrates (i.e., carbohydrates), resulting in high concentrations of fermentation‐inhibiting products, such as organic acids, furfural, and hydroxymethylfurfural (HMF) (Badger 2002).

Enzymatic hydrolysis is currently the most widely applied/discussed approach for biomass conversion. This process accounts for approximately 27% of the total operation cost of the overall bioconversion process, and therefore needs to be optimized with the pretreatment processes for cost reduction (Bbosa et al. 2018). Saccharification efficiency can be affected by enzyme‐related (Gupta et al. 2016) and substrate‐related factors (Leu and Zhu 2013). The mechanisms of the interactions between dissociated and residual components to enzyme activities are the functions of the biomass and pretreatment process. Figure 5.7a–d shows some images of the pretreated substrate before and after enzymatic hydrolysis. The surface of woody biomass before pretreatment (Figure 5.7a) is smooth, with the inner surface well protected from hydrolysis. The pretreatment by ethanol, alkaline, and SPORL opens the plant cell wall (red arrow) and creates different levels of dissociation (Figure 5.7b–d, respectively). With increased severity of pretreatment conditions and degree of lignin removal, the damage in the cell wall become visible, together with lignin precipitation on the substrate surface (Figure 5.7d). The cell wall structure is further decomposed after hydrolysis, while enzymes may be captured under a scanning electron microscope (SEM) within 24 hours of hydrolysis (arrow mark in Figure 5.7e). The cell wall can be completely decomposed after enzymatic hydrolysis (Figure 5.7f).

Enzyme hydrolysis of biomass suffers with a low reaction rate due to the high crystallinity of cellulose, and less accessible surface area for enzyme and non‐productive binding to cellulase enzyme (Dadi et al. 2006). Enzymatic hydrolysis requires pretreatment to break the regular structure of cellulose and to increase surface area for enzyme access (Badger 2002). The yield of enzymatic hydrolysis increased from 20% to 90% after proper pretreatment of biomass (Table 5.2). The efficiency of both acid and enzyme hydrolysis depends on the crystallinity, molecular structure, degree of polymerization, surface area, swelling, lignin content, and hemicellulose content (Dadi et al. 2006). The extent of hydrolysis to be done depends on the targeted chemicals. Complete hydrolysis is necessary to produce ethanol. On the other hand, complete hydrolysis reduces the efficiency of production of butanol and lactic acid from the biomass.

Fermentation is a metabolic process that consumes sugars and produces alcohols or other chemicals depending upon the strains used in the fermentation. Fermentable sugars from the biomass can be converted into different chemicals by different enzymes. In the United States, less than 5% of total chemical consumption is obtained from the fermentation of lignocellulosic biomass. Common fermentation products in the recent literatures include ethanol, butanol, and lactic acid (Macedo and Brigham 2014; Zheng et al. 2015; Wang et al. 2015). The US Department of Energy and Department of Agriculture are committed to replace at least 30% of petroleum consumption with biofuel by 2030 (Perlack 2005). The US Department of Energy (Zhu and Zhuang 2012) reported that the United States currently uses 140 billion gallons of petroleum transportation fuel per year. To replace 30% of petroleum with bioethanol will require about 60 billion gallons of ethanol (US Congress 2005) The US alone can sustainably produce annually 1.366 billion tons of dry biomass, of which 998 million tons is from agricultural residues and 368 million tons from forest wood residues (Somerville 2006). This total biomass can theoretically produce 130 billion gallons of ethanol, which is about 60% of the current petroleum energy used in the US. Moreover, the use of bioenergy alters energy dynamics and reduces greenhouse gas emissions. China has also promised to contribute heavily to biofuel production and utilization (Nguyen et al. 2017). China is expected to replace 30% of total oil imports with cellulosic ethanol by 2050 (Zhao 2015).

5.7 New Functions of Wood Residues

Wood has been found in many ancient and historical structures like Egyptian pyramids, Chinese temples, and tombs, as construction materials. Beside the more traditional uses in construction, furniture, pulp paper, and bioethanol production, wood has been used recently for the manufacture of advanced materials like wood–plastic composites (WPCs) and cellulose nanomaterials. Some modern applications of wood materials are given in Table 5.3. These composite materials have good mechanical properties to replace traditional materials for different construction purposes. Wood extractives are widely used in the pharmaceutical industry because of their medicinal properties.

5.8 Conclusions and Future Trends

Recent development of technology has resulted in dramatic changes in consumer behavior and views of woody products and the forestry industry. While the demand for structural timber remains, the need of paper and related products has continuously decreased, lignocellulosic biomass has been seen as an emerging renewable feedstock to replace petroleum refinery products. With the depletion of fossil fuels, bioenergy and biomaterials will eventually substitute plastics, resins, additives, solvents, carbon fibers, medicine, and/or many other materials. Currently, however, technical and economic challenges still remain, preventing wide acceptance of the second‐generation biorefinery. As the feedstock of the conventional refinery is still sufficient, there is no immediate demand to develop and/or use the related products. Developing technology should focus on assisting municipal waste treatment or handling the byproducts of related industry (e.g., kraft lignin). Accumulation of experience of this application should create useful information for the future development of the biomass biorefinery, and therefore needs to be emphasized and practiced.

Acknowledgement

The authors thank the Hong Kong General Research Fund (15212317), Environment and Conservation Fund (ECF 85/2017), and Sinopec Chemical Commercial Holding Company Limited for financial support. The authors also thank the University Research Facility in Chemical and Environmental Analysis (UCEA) of The Hong Kong Polytechnic University for sample analyses.

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