Nature of Biomass Feedstock

Agricultural crops can be roughly divided according to the composition of their (main) economic products, such as sugar, starch (grains, tubers), oilseed, protein, or fiber crop and crops for specialty products (pharmaceutics, cosmetics, dyes, fragrance, and flowers). Besides the main harvested product, all crop processing systems yield more or less secondary products/by-products and residues. These products may find an application depending on demand and possibilities for economic conversion. Biomass residues can be categorized into three main groups: (1) primary biomass residues, available at the farm; (2) secondary biomass residues, released in the agro-food industry; and (3) tertiary biomass, which is remaining after use of products. The characteristics that influence availability and suitability of the waste biomass as feedstocks are the nature of biomass, moisture content, the density, and the seasonality of supply.

Lignocellulosic biomass comprising forestry, agricultural, and agro-industrial wastes are abundant, renewable, and inexpensive energy sources. Such wastes include a variety of materials, such as sawdust, poplar trees, sugarcane bagasse,

Table 1.1 The main components of different lignocellulose wastes (refs. Nigam et al. 2009; Dhillon et al. 2011b)

Lignocellulose waste

Cellulose (wt %)

Hemicellulose (wt %)

Lignin (wt %)

Sugarcane bagasse

40.0

27.0

10.0

Rice straw

36.2

19.0

9.9

Wheat straw

32.9

24.0

8.9

Barley straw

33.8

21.9

13.8

Rye straw

37.6

30.5

19.0

Oat straw

39.4

27.1

17.5

Corn cobs

33.7

31.9

6.1

Corn stalks

35.0

16.8

7.0

Cotton stalks

58.5

14.4

21.5

Soya stalks

34.5

24.8

19.8

Sunflower stalks

42.1

29.7

13.4

Apple pomace

7.2

23.5

Brewer's spent grain

13.1–25.4

28.4–29.96

11.9–27.8

Citrus waste

8.8

4.4

3.7

waste paper, brewer's spent grains, coconut coir and shell, fruit pomace and liquid sludge, switch grass, and straws, hull, stems, stalks, leaves, husks, shells, and peels from cereals like rice, wheat, corn, sorghum, and barley, among others. Lignocellulosic biomass is chemically composed of three main fractions: cellulose, hemicellulose, and lignin in varied concentrations (Table 1.1) with smaller amounts of proteins, lipids, and ash (Fig. 1.2). Cellulose is a polymer of glucose (a C6 sugar), which can be used to produce glucose monomers for fermentation to produce a variety of products, such as renewable fuels, platform chemicals, organic acids and biopolymers among others. Hemicellulose is a copolymer of different C5 and C6 sugars including xylose, mannose, and glucose, depending on the type of biomass. Lignin is a branched polymer of aromatic compounds. Both the C5 sugars and the lignin fragments can be used as feedstock for the production of various value-added products including high-value biochemicals in a biorefinery.

These polymers are closely associated with each other constituting the cellular complex of the vegetal biomass. Basically, cellulose forms a skeleton which is surrounded by hemicellulose and lignin (Fig. 1.2). The pretreatment of lignocellulosic biomass helps to disrupt the 3D network structure of lignin, cellulose, and hemicellulose, allowing high yields of fermentable sugars to be produced in subsequent enzymatic hydrolysis. Different pretreatments used for the separation of different polymers in lignocellulosic waste are given in Fig. 1.3. The pretreatments help the enzymes for easy excess for the biomass hydrolysis to simple sugars.

Currently, biomass pretreatment is still a necessary step to establish a cheap sugar platform for bioethanol and other biochemicals. An ideal pretreatment technology should target the three basic requirements: simple process, cost-effective, and high sugar recovery.

Cellulose and hemicellulose are sugar-rich fractions of interest for use in fermentation processes, since microorganisms may use the sugars for growth and

Fig. 1.2 The structure of lignocellulosic material and routes for its biotransformation to highvalue biochemicals

Fig. 1.3 Different pretreatments for the hydrolysis of lignocellulosic biomass

Table 1.2 Examples of biomass residues for different crops

Sources: UNDP (2007), Van Dam (2002, Rosillo-Calle et al. (2007)

aResidue ratio refers to ratio of dry matter weight to crop produced

production of value-added compounds, such as ethanol, food additives, organic acids, enzymes, among others. Submerged and solid-state fermentation systems have been used to produce compounds of industrial interest from lignocellulosic wastes, as an alternative for valorization of these wastes and also to solve environmental problems caused by their disposal. When submerged fermentation systems are used, a previous stage of hydrolysis for separation of the lignocellulose constituents is required.

Few common primary and secondary residues from agricultural crops are given in Table 1.2. There is significant variation in the quantities available. For instance, in some cases, residues amount to only about 10–20 % of the crop by weight, whereas in other cases, the residues might actually be greater than the original crop. As evident from the Table 1.2, grain crops tend to have the highest overall residue ratio, amounting to as much as double the crop weight. For this reason, utilization of straw from grains should be a much higher priority for utilization of this largely untapped reservoir of biomass resources.

Lignocellulose wastes are accumulated every year in large quantities, causing environmental problems. However, due to their chemical composition based on sugars and other compounds of interest, they could be utilized for the production of a number of value-added products. Therefore, besides the environmental problems caused by their accumulation in the nature, the nonuse of these materials constitutes a loss of potentially valuable sources. .

The underutilized biomass resources from different possible sources, such as primary agricultural production, agro-industries, and municipal waste, are generally available in abundant quantities at negligible costs. Agriculture-based wastes, such

Fig. 1.4 Process showing the pretreatment and enzymatic hydrolysis of lignocellulosic biomass to produce sugar syrups for production of bioethanol

as straws or seed hulls, can be harvested and collected at the farm or at central processing units. Others wastes, such as food industry wastes, are only available in dispersed/diluted forms and need collection systems to be installed at particular industries.

Earlier, agricultural residues were promoted mainly for energy (e.g., bioethanol production) use, often at low efficiency (Fig. 1.4). However, it is now more widely recognized that there are in fact other possible routes that may provide highervalue-added products or could serve as complementary products via coproduction schemes alongside energy applications. The sugar-rich syrups produced after pretreatment and enzymatic hydrolysis of lignocellulosic biomass can be used for the production of high-value products. Currently, such integrated processes are recurring theme in industrial biotechnology development (van Dam et al. 2005). For instance, microalgae/fungal/yeast cultivations involving production of various products result in waste microalgae/fungal/yeast biomass as a by-product. The fungal biomass is rich in chitin which can be extracted and transformed to its deacetylated derivative, chitosan (Dhillon et al. 2012a; Kaur and Dhillon 2013a). Similarly, microalgae biomass resulting after lipid extraction for biodiesel is also rich in carbohydrates, proteins and other products, such as pigments.

Crude glycerol (CG) is a waste by-product of biodiesel production process. For every 100 kg of biodiesel produced by transesterification of vegetable oil/animal fat/microalgae-derived lipids, 10 kg of CG is produced. CG is a carbon-rich source

and an emerging and less expensive feedstock for bioprocess technology. CG can be used for the production of a wide range of products, such as ethanol and biohydrogen. More recently, it has been evaluated for the production of high-value biochemical, such as eicosapentaenoic acid, docosahexaenoic acid, glycolipid, biosurfactant, 1,3-propanediol, and antibiotics, such as cephalosporin C (Pyle et al. 2008; Athalye et al. 2009; Liu et al. 2011; Shin et al. 2011; Ferreira et al. 2012).

 
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