**2.4 Recalcitrance and dissolution difficulties**

Despite their potential for the production of biofuels and chemicals alternative to petroleum, the complex and rigid structures of lignocellulosic materials limit their use in such applications. Success of using lignocellulosic biomass for biofuels and other useful chemical productions depends largely upon physical and chemical properties of the biomass, on pretreatment methods and optimization of the processing conditions. The compositional changes in plant cell wall and differences in ultrastructure greatly influence the pretreatment and hydrolysis (dissolution) efficiency of the biomass. Hydrolysis is a chemical reaction that releases sugars from biomass structures. Biomass dissolution involves both physical, chemical and/


**7**

*Challenges of Biomass Utilization for Biofuels DOI: http://dx.doi.org/10.5772/intechopen.83752*

which is embedded in the lignin matrix.

are not environmentally friendly.

4-glycosidic bond which is catalyzed by H+

depends on the H3O+

and sugar alcohols, etc.).

include the following:

or thermochemical treatment processes. The crystallinity of cellulose, hydrophobicity of lignin, and embedding the cellulose in lignin-hemicellulose matrix and difficulties in cleavage of some linkages (hydrogen bonding, ether linkages between the phenyl propane units, etc.) make biomass materials resistant to hydrolysis. It is relatively easy to hydrolyze hemicellulose into simple sugars compared to cellulose because hemicellulose fraction is more accessible compared to cellulose fraction

Biomass materials must first be broken down into components with smaller molecular weights (e.g., oligo- and monosaccharides) in order to be efficiently converted into a range of products. Hydrolysates from biomass can be used for producing a wide range of value-added products, including biofuels (ethanol, hydrogen, etc.), industrially important chemicals (e.g., solvents), and food products (sugar

Significant existing challenges for hydrolysis of lignocellulosic biomass materials

• Existing hydrolysis methods are expensive and time consuming. Most of them

The major hydrolysis processes typically used for the solubilization of biomass require either use of toxic, corrosive, and hazardous chemicals (e.g., acid and alkali treatments) or longer retention times (e.g., enzymatic hydrolysis), which collectively make the process environmentally unsafe and/or expensive. Mineral acids are commonly used to dissolve hemicelluloses, whereas lignin is typically dissolved by alkaline or organosolv pretreatments [14, 15]. Recovery of the chemical catalyst is often crucial to the success of these processes [16]. On the other hand, generally harsh conditions (e.g., high temperatures and high acid concentrations) are needed to release glucose from biomass complex structures. Pyrolysis and other side reactions at higher temperatures become very important, and the amount of undesirable byproducts (tars) increases as the temperature is increased above 220°C [17]. Concentrated acid hydrolysis has been applied to breakdown lignocellulosic efficiently [18–20]. The hydrolysis of cellulose to its monomer sugar component occurs by degradation of chemical bonds in cellulose by the hydrolytic cleavage of β-1,

ions of an acid. The reaction rate

ion concentration, the reaction temperature, and the chemical

• Additional steps are required (pretreatment, neutralization, etc.)

• Released carbohydrates decompose in harsh hydrolysis conditions.

environment of the glycosidic bond and the rate is increased with the increasing acid ion concentration and temperature. The acid hydrolysis process usually employs sulfuric acid and hydrochloric acid at concentrations of 1–10% using a moderate temperature (in the range of 100–150°C) [21]. A two-step sulfuric acid hydrolysis is a widely used technique for releasing sugars from biomass [18]. Biomass is first treated with concentrated sulfuric acid at a low temperature and then hydrolyzed with diluted sulfuric acid at an elevated temperature. Concentrated acid recrystallizes cellulose to less crystallized oligosaccharides followed by less concentrated and higher reaction temperature for converting recrystallized oligosaccharides to monosaccharides. Concentrated acid hydrolysis process can provide higher conversion from polysaccharides to monosaccharides with minimum forma-

tion of reaction by-products with careful control of reaction conditions.

The use of concentrated acid for biomass hydrolysis has several more drawbacks such as energy consumption, equipment corrosion, handling of non-safe chemicals,

**Table 1.**

*Various lignocellulosic biomass materials and their chemical compositions [11, 12].*

#### *Challenges of Biomass Utilization for Biofuels DOI: http://dx.doi.org/10.5772/intechopen.83752*

*Biomass for Bioenergy - Recent Trends and Future Challenges*

alcohol, and sinapyl alcohol, forms lignin structure.

three basic precursors (HGS) [9, 10].

**2.4 Recalcitrance and dissolution difficulties**

framework. The inter- and intra- chain hydrogen bonding in the structure makes the cellulose to be crystalline and this portion of cellulose does not hydrolyze easily compared to amorphous cellulose structure [7, 8]. Hemicellulose has a random and amorphous structure, which is composed of several heteropolymers such as xylan, galactomannan, arabinoxylan, glucomannan and xyloglucan. Its polymerization degree is less than cellulose. The monomer units of hemicellulose polysaccharide include xylose, mannose, galactose, rhamnose, and arabinose units unlike only glucose in cellulose. Lignin is a complex aromatic substance of phenyl propane units. Three different phenyl propane building blocks p-coumaryl alcohol, coniferyl

Phenylpropanoid monomeric units in the lignin polymer are identified as p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units, respectively. The ratio of these units varies between plants; for example in hardwoods S and G forms dominate with minor amount of p-hydroxyphenyl (H), whereas softwood lignins contain only G units. On the other hand, lignins from grasses are composed of the

Composition of lignin, cellulose and hemicellulose in biomass materials significantly differ among biomass species (**Table 1**). For instance, some biomass materials such as hardwoods contain more cellulose in their structures, while others such as straws have more of hemicelluloses. Hemicellulose fractions of softwoods mainly have D-mannose derived structures such as galactoglucomannans, while hemicelluloses in hardwoods have D-xylose derived structures such as arabinoglucuronoxylan [13]. This diversity among biomass materials can significantly affect the conversion processes for production of biofuel or other useful products from biomass materials.

Despite their potential for the production of biofuels and chemicals alternative to petroleum, the complex and rigid structures of lignocellulosic materials limit their use in such applications. Success of using lignocellulosic biomass for biofuels and other useful chemical productions depends largely upon physical and chemical properties of the biomass, on pretreatment methods and optimization of the processing conditions. The compositional changes in plant cell wall and differences in ultrastructure greatly influence the pretreatment and hydrolysis (dissolution) efficiency of the biomass. Hydrolysis is a chemical reaction that releases sugars from biomass structures. Biomass dissolution involves both physical, chemical and/

**Biomass material Cellulose Hemicellulose Lignin** Switchgrass (grass) 33.8 28.4 16.6 Miscanthus (grass) 47.7 24.6 12.3 Poplar (hardwood) 52.1 27.4 15.9 Oak (hardwood) 40.4 35.9 24.1 Pine (softwood) 46.0 25.5 20 Spruce (softwood) 45.5 22.9 27.9 Corn stover (agricultural waste) 38.5 24.5 18.5 Rice husks (agricultural waste) 32.1 20.6 17.7 Corn bran (byproduct of milling) 20.5 65.3 1.6

*Various lignocellulosic biomass materials and their chemical compositions [11, 12].*

**6**

**Table 1.**

or thermochemical treatment processes. The crystallinity of cellulose, hydrophobicity of lignin, and embedding the cellulose in lignin-hemicellulose matrix and difficulties in cleavage of some linkages (hydrogen bonding, ether linkages between the phenyl propane units, etc.) make biomass materials resistant to hydrolysis. It is relatively easy to hydrolyze hemicellulose into simple sugars compared to cellulose because hemicellulose fraction is more accessible compared to cellulose fraction which is embedded in the lignin matrix.

Biomass materials must first be broken down into components with smaller molecular weights (e.g., oligo- and monosaccharides) in order to be efficiently converted into a range of products. Hydrolysates from biomass can be used for producing a wide range of value-added products, including biofuels (ethanol, hydrogen, etc.), industrially important chemicals (e.g., solvents), and food products (sugar and sugar alcohols, etc.).

Significant existing challenges for hydrolysis of lignocellulosic biomass materials include the following:


The major hydrolysis processes typically used for the solubilization of biomass require either use of toxic, corrosive, and hazardous chemicals (e.g., acid and alkali treatments) or longer retention times (e.g., enzymatic hydrolysis), which collectively make the process environmentally unsafe and/or expensive. Mineral acids are commonly used to dissolve hemicelluloses, whereas lignin is typically dissolved by alkaline or organosolv pretreatments [14, 15]. Recovery of the chemical catalyst is often crucial to the success of these processes [16]. On the other hand, generally harsh conditions (e.g., high temperatures and high acid concentrations) are needed to release glucose from biomass complex structures. Pyrolysis and other side reactions at higher temperatures become very important, and the amount of undesirable byproducts (tars) increases as the temperature is increased above 220°C [17].

Concentrated acid hydrolysis has been applied to breakdown lignocellulosic efficiently [18–20]. The hydrolysis of cellulose to its monomer sugar component occurs by degradation of chemical bonds in cellulose by the hydrolytic cleavage of β-1, 4-glycosidic bond which is catalyzed by H+ ions of an acid. The reaction rate depends on the H3O+ ion concentration, the reaction temperature, and the chemical environment of the glycosidic bond and the rate is increased with the increasing acid ion concentration and temperature. The acid hydrolysis process usually employs sulfuric acid and hydrochloric acid at concentrations of 1–10% using a moderate temperature (in the range of 100–150°C) [21]. A two-step sulfuric acid hydrolysis is a widely used technique for releasing sugars from biomass [18]. Biomass is first treated with concentrated sulfuric acid at a low temperature and then hydrolyzed with diluted sulfuric acid at an elevated temperature. Concentrated acid recrystallizes cellulose to less crystallized oligosaccharides followed by less concentrated and higher reaction temperature for converting recrystallized oligosaccharides to monosaccharides. Concentrated acid hydrolysis process can provide higher conversion from polysaccharides to monosaccharides with minimum formation of reaction by-products with careful control of reaction conditions.

The use of concentrated acid for biomass hydrolysis has several more drawbacks such as energy consumption, equipment corrosion, handling of non-safe chemicals, an added necessary step of acid neutralization, the formation of byproducts that create an inhibitory effect in the fermentation [22, 23] and other negative environmental impacts. Thus, the current methods have undesirable processes and do not meet the needs.

Subcritical water (99.97°C < T < 374.15°C; 217.76 atm < P) is an alternative way to hydrolyze lignocellulosic biomass in an environmentally friendly manner by only operating temperature and pressure conditions. Significant advantages of subcritical water over commonly used biomass breakdown methods—alkali, acidic, and enzymatic—are summarized in **Table 2**.

The chemical properties of water are greatly changed at high temperatures and pressures due to the reduction of hydrogen bonding, which causes changes in dissociation, solubility, diffusivity, and reactivity [24]. Subcritical water has a lower relative dielectric constant and a higher ionic product than ambient water. When the temperature of water increases from ambient temperature to 250°C, its relative dielectric constant decreases from around 80 to nearly 27, which is similar to that of acetone at ambient temperature [25, 26]. Furthermore, the ion product of subcritical water substantially increases with temperature; therefore, subcritical water can catalyze chemical reactions such as hydrolysis and degradation without the use of any additional catalyst [27, 28].

Ionic product numbers of water (Kw) at various temperatures and pressures showed that when pressure is around 35 MPa and temperature is in sub- and supercritical regions under 400°C, Kw values are always higher than 1 × 10<sup>−</sup>14. The Kw increases to its maxima (~10<sup>−</sup>11) between 200 and 300°C and does not respond to changes in pressure when in this temperature range. The molar concentrations of hydrogen ion (H<sup>+</sup> ) and hydroxide ion (OH<sup>−</sup>) in these regions are almost 30 times higher than those under room temperature. Therefore, the hydrolysis yield in these regions is expected to be high, and biomass polymers could be broken down into their smaller molecular weight components efficiently [29–31].

The presence of a weak acid in subcritical water media can also improve hydrolysis of biomass materials. The use of carbon dioxide is as a pressurizing gas caused the formation of carbonic acid that plays a catalytic role in effective solubilization of biomass [32]. Some studies indicated that the addition of small amounts of hydrogen peroxide can enhance lignin removal and modify cellulose structure toward favoring enzymatic hydrolysis [33, 34].

Complexity and diversity of the biomass materials considerable affect the solubilization efficiency of these materials. The differences in the content and composition of resulted hydrolysates can change the yield of the biofuel or target compound produced from these biomass hydrolysates. The more degraded organics containing hydrolysates can positively affect the yield of certain various value-added products; for instance, production of gaseous products by hydrothermal gasification


**Table 2.**

*Comparison of alkali, acidic and enzymatic biomass breakdown methods with subcritical water treatment.*

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*Challenges of Biomass Utilization for Biofuels DOI: http://dx.doi.org/10.5772/intechopen.83752*

nents with a suitable method.

**2.5 Other challenges**

**3. Conclusions**

effective conversion of a specific product.

processes. The maximum solubilization yield of wheat straw and kenaf biomass materials was 70–75%, which was achieved at 250°C in subcritical water medium [32, 35]. However, the hydrolysates obtained in this process had high molecular weight polysaccharides that were difficult to utilize for hydrogen production by aqueous-phase reforming [31]. For maximum usability, biomass components in hydrolysates should be further broken down into smaller molecular weight compo-

Although energy demands are continuous, biomass materials are seasonal. Some biomass feedstocks have advantages in terms of production, harvesting, storage, and transportation compared to others. Non-food biomass such as energy crops (switchgrass, miscanthus, kenaf, etc.) have advantages over food crops (corn, sugarcane, sugar beet, sweet sorghum, etc.). Perrenial energy crops such as switchgrass and miscanthus do not need to be replanted each year and they do not require special care and high maintenance to grow. On the other hand, agricultural biomass residues (corn stover, wheat straw, rice husk, crop peels, pulps, etc.) as promising low-cost feedstocks since they do not need additional land for biomass growth and the land used for agriculture belongs to these types of biomass materials. Forest biomass are also large source of materials for biofuels and other value-added products production. However, high costs of their harvesting and transportation limit their use. In addition to the advantage and disadvantage listed above, different sources of biomass feedstocks do not have same composition, uniform size and shape, etc., that considerable affect efficiency of conversion processes for a specific product. Therefore, biomass feedstocks for a bio-refinery needs to be standardized.

Biomass materials have some challenges that need to be overcome for their fully utilization for biofuel and other useful products. Availability, abundance, and requirements for growth, growth rate, etc., parameters considerably affect the feedstock selection for value-added products. Besides, the content of cellulose, hemicellulose, and lignin in biomass materials and accessibility of these fractions in the biomass structures play significant roles in biomass dissolution and biofuel production from the hydrolysate. Biomass material should be used in densified forms to overcome moisture, storage and handling problems. Biomass feedstocks delivered to a bio-refinery from different sources should be standardized for an

*Challenges of Biomass Utilization for Biofuels DOI: http://dx.doi.org/10.5772/intechopen.83752*

processes. The maximum solubilization yield of wheat straw and kenaf biomass materials was 70–75%, which was achieved at 250°C in subcritical water medium [32, 35]. However, the hydrolysates obtained in this process had high molecular weight polysaccharides that were difficult to utilize for hydrogen production by aqueous-phase reforming [31]. For maximum usability, biomass components in hydrolysates should be further broken down into smaller molecular weight components with a suitable method.
