**2. Challenges**

#### **2.1 Moisture content**

Biomass materials with high moisture contents is not a suitable feedstock for conventional thermochemical conversion technologies such as gasification and pyrolysis. High moisture can reduce the effectiveness of conversion processes. Moisture in raw biomass materials is also undesired because fuel produced from these materials can contain moisture. The fuels, which have high moisture contents, cannot burn easily. Some part of energy in the fuel are consumed for vaporization of water, which is present in the fuel. In order to maximize the heating value of the fuel produced from these materials the moisture content of biomass should be less than 20%. Drying the materials before being used in the conversion process is not preferable because of high cost. On the other hand, some biomass conversion processes use biomass with high moisture contents. For instance, hydrothermal conversion processes, which use supercritical and subcritical water as reaction medium, and biological processes such as alcohol production from carbohydrates by biomass hydrolysis and fermentation can be applied to the biomass with high moisture content without the need for drying. In these processes, moisture in the biomass plays an important role in the conversion, either as a major reactant, or as a reaction environment.

High moisture content in biomass causes biological degradation, mold formation and losses in the organic contents (e.g., carbohydrates) during storage [1], that could reduce the yield of the fuel produced from these materials. Storing biomass at <10% can extend the conservation time of the materials and reduce major losses (sugars) in the biomass during the storage period [2]. The drawbacks of high moisture contents can be mostly solved by compressing the biomass material for more uniform properties that process is called densification. Increasing bulk density of biomass materials by densification reduces transportation costs and storage volume. However, this process increases the price of the end product.

#### **2.2 Density**

The bulk density of lignocellulosic biomass materials is generally low (80–150 kg/m3 for grass biomass and 160–220 kg/m3 for woody biomass). This creates difficulties to handle such large quantities of feedstocks and increases their transportation and storage costs [3]. The bulk density of biomass should be between 190 and 240 kg/m3 for efficient transport in various sizes of trucks with 25 ton loads [4].

The size, shape, moisture content, particle density, and surface characteristics are the factors affecting the bulk density of a material. The challenge for low density and different size and shapes of biomass can be overcome by densification process (**Figure 1**). In this process, biomass materials are mechanically compressed to increase their density and convert them to uniform shapes and sizes (briquetting, pelletizing, or cubing) [5, 6].

The density of biomass material can be increased ten-fold depending to biomass type, moisture content, processing conditions, etc. The costs of handling, transportation, and storage of resulted densified materials can be considerably reduced. Because of uniform sizes and shapes, the materials can be easily handled with standard machines or equipment [6].

**5**

**Figure 2.**

*Structure of lignocellulosic biomass.*

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

**2.3 Complexity and diversity**

*Increasing density of biomass.*

**Figure 1.**

Lignocellulosic biomass materials is mainly composed of three components which are lignin, cellulose, and hemicellulose (**Figure 2**). These polymers are organized in complex non-uniform three-dimensional structures and each one has different polymerization degrees. Polymerization degree and/or structures of these biopolymers can vary among biomass species. Cellulose is a linear structure composed of β(1–4) linked glucose subunits. Cellulose molecules determine the cell wall *Challenges of Biomass Utilization for Biofuels DOI: http://dx.doi.org/10.5772/intechopen.83752*

*Biomass for Bioenergy - Recent Trends and Future Challenges*

biofuels in detail.

**2. Challenges**

**2.1 Moisture content**

reaction environment.

**2.2 Density**

(80–150 kg/m3

pelletizing, or cubing) [5, 6].

standard machines or equipment [6].

240 kg/m3

Following sections will focus on the challenges for conversion of biomass to

Biomass materials with high moisture contents is not a suitable feedstock for conventional thermochemical conversion technologies such as gasification and pyrolysis. High moisture can reduce the effectiveness of conversion processes. Moisture in raw biomass materials is also undesired because fuel produced from these materials can contain moisture. The fuels, which have high moisture contents, cannot burn easily. Some part of energy in the fuel are consumed for vaporization of water, which is present in the fuel. In order to maximize the heating value of the fuel produced from these materials the moisture content of biomass should be less than 20%. Drying the materials before being used in the conversion process is not preferable because of high cost. On the other hand, some biomass conversion processes use biomass with high moisture contents. For instance, hydrothermal conversion processes, which use supercritical and subcritical water as reaction medium, and biological processes such as alcohol production from carbohydrates by biomass hydrolysis and fermentation can be applied to the biomass with high moisture content without the need for drying. In these processes, moisture in the biomass plays an important role in the conversion, either as a major reactant, or as a

High moisture content in biomass causes biological degradation, mold formation and losses in the organic contents (e.g., carbohydrates) during storage [1], that could reduce the yield of the fuel produced from these materials. Storing biomass at <10% can extend the conservation time of the materials and reduce major losses (sugars) in the biomass during the storage period [2]. The drawbacks of high moisture contents can be mostly solved by compressing the biomass material for more uniform properties that process is called densification. Increasing bulk density of biomass materials by densification reduces transportation costs and storage volume.

However, this process increases the price of the end product.

for grass biomass and 160–220 kg/m3

The bulk density of lignocellulosic biomass materials is generally low

difficulties to handle such large quantities of feedstocks and increases their transportation and storage costs [3]. The bulk density of biomass should be between 190 and

 for efficient transport in various sizes of trucks with 25 ton loads [4]. The size, shape, moisture content, particle density, and surface characteristics are the factors affecting the bulk density of a material. The challenge for low density and different size and shapes of biomass can be overcome by densification process (**Figure 1**). In this process, biomass materials are mechanically compressed to increase their density and convert them to uniform shapes and sizes (briquetting,

The density of biomass material can be increased ten-fold depending to biomass type, moisture content, processing conditions, etc. The costs of handling, transportation, and storage of resulted densified materials can be considerably reduced. Because of uniform sizes and shapes, the materials can be easily handled with

for woody biomass). This creates

**4**

## **2.3 Complexity and diversity**

Lignocellulosic biomass materials is mainly composed of three components which are lignin, cellulose, and hemicellulose (**Figure 2**). These polymers are organized in complex non-uniform three-dimensional structures and each one has different polymerization degrees. Polymerization degree and/or structures of these biopolymers can vary among biomass species. Cellulose is a linear structure composed of β(1–4) linked glucose subunits. Cellulose molecules determine the cell wall

**Figure 2.** *Structure of lignocellulosic biomass.*

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 alcohol, and sinapyl alcohol, forms lignin structure.

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

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.
