**2. Overview of biomass composition**

energy production from biomass averages about 11% of the total energy produced; while in developing countries this can be up to 50% [2, 3]. For example, Europe generates about 3.5% of its energy from biomass, which is ca. 40 Mtoe/year; while countries like Austria, Finland and Sweden have about 13, 18 and 17% of their total energy produced from biomass resources; the United States on the other hand generates 3% of its energy from biomass [4]. In general, there is a huge potential for the exploitation of biomass as a source of energy because some countries in central and northern Europe have actually established large plants for heat and power production from biomass. However, there are two main routes by which biomass can be converted to energy and bio products. These are biochemical and thermochemical conversion routes. While the former involves breaking down biomass into gaseous and/or liquid fuels (such as biogas and bioethanol) through the use of bacteria, microorganisms and enzymes; the latter involves the use of heat to disrupt the complex chemical structure of biomass (particularly lignocellulosic biomass such as wood) into various products that includes heat, power, fuels, bio oil, biochar

It is evident from the above that the application of biomass as a source of energy continues to attract global attention even in the midst of its gross underutilization. The low quality of biomass, often defined in terms of its inherent characteristics (high moisture content, low energy density, low bulk density, irregular size and shape) has been one of the major reasons for its under exploitation [7]. In other words, biomass in its natural form is difficult to use for the purpose of energy production hence pretreatment is often required to overcome its recalcitrant nature and make the biomass amenable for conversion through either of the conversion routes previously mentioned for maximum product recovery; the pretreatment leads to physical, chemical and structural changes to the biomass plays a vital role in the commercial viability of the energy production process of biomass [7–9]. Thus, bioconversion and bio refinery interests define the type of pretreatment measures for biomass [7, 10]. However, there are different categories of pretreatment viz; physical, chemical and biological pretreatments. These three classes of pretreatment

Another critical step to the optimization of biomass conversion and bio refinery processes is related to the characterization of biomass to determine its suitability for the aforementioned conversion processes. This means that the effectiveness and impact of pretreatment on biomass can be determined through the use of a variety of high-quality analytical techniques able to provide information on quintessential biomass characteristics that can be used to maximize product recovery. Depending on the end application of biomass, some of the useful characterization techniques include *atomic force microscopy (AFM)*, *Fourier transform infrared spectroscopy (FT-IR)*, *scanning electron microscopy (SEM)*, *transmission electron microscopy (TEM)*, *X-ray diffraction (XRD)*, *solid state nuclear magnetic resonance (SSNMR)*, to name a few. An overview of these analytical tools is presented in subsequent sections. For a full understanding of the characteristics of biomass, its value and information for the design and operation of the energy conversion systems using the biomass as feedstock, it is vital to undertake biomass characterization before and after

pretreatment. Due to the complex nature of biomass, the study of the mechanisms involved in its conversion process to energy and bio products is quite challenging. The lack of rapid, high throughput and reliable tools for assessing and tracing biomass components relevant not just to energy production but also to other value added products remains a major bottleneck in studying the impact of biomass pretreatment and process parameters. This chapter therefore presents a critical review of biomass pretreatment and characterization and discusses the applications of state-of-the-art analytical techniques commonly used to understand the features

and chemicals [5, 6].

*Biotechnological Applications of Biomass*

**20**

are described in greater detail in Section 3.

The composition of biomass is largely diverse and dependent upon its origin and species. Besides plant biomass, which are commonly referred to as lignocellulosic biomass (LCB) due to their three major cross-linked polysaccharide constituents (cellulose, hemicellulose and lignin), there are other biomass materials whose primary components include *lipids*, *proteins*, *starch*, *inorganics and minerals*. These types of biomass materials are regarded as non-lignocellulosic biomass (NLCB) and include *sewage sludge*, *animal manure*, *algae*, etc. The major constituents of both the LCB and NLCB are organic in nature and determine the characteristics of the entire biomass [7]. In comparison to LCB however, NLCB pose a greater threat to the environment because of higher content of heavy metals and heteroatoms such as nitrogen (N) and phosphorus (P); [11–13]. The heavy metals can pollute water systems, accumulate in food chains and cause serious health issues [14, 15]. Even though the N and P composition of NLCB can serve as a source of nutrients for plants, excessive amounts of these elements can lead to eutrophication of a water body, a condition linked to the deterioration of water quality via excessive growth of algae and other aquatic plants, resulting in oxygen depletion of the water body, leading to the death of aquatic animals [12].

While the structural unit of NLCB is such that various atoms are arranged in an orderly manner, LCB is characterized by complex internal structure with main components that equally displays structural multiplicities. **Figure 1** shows the internal structure of lignocellulosic biomass and how its three primary components are distributed.

The internal structure of lignocellulosic biomass reveals a crystalline fibrous structure of cellulose, which forms the core of the complex structure of plant biomass. Positioned between the micro- and macrofibrils of the cellulose matrix is hemicellulose; while lignin plays a structural role that encapsulates both cellulose and hemicellulose.

The composition of biomass varies significantly depending on the source of the biomass. In addition to the three primary components of LCB (**Figure 1**), there are other minor components such as *extractives*, *proteins*, *water* and *inorganic components*


#### **Table 1.**

*The composition of the organic fractions of various lignocellulosic biomass materials (dry basis) [17–20].*

such as *silicon (Si)*, *sodium (Na)*, *potassium (K)*, *calcium (Ca)*, *magnesium (Mg)* and *aluminum (Al)*; these minor constituents do not markedly contribute to the formation of the total structure of the biomass [16]. The organic components of various LCB materials are summarized in **Table 1**.

physical pretreatment decreases cellulose crystallinity and degree of polymerization. It is a vital step prior to both the biochemical and thermochemical conversion of biomass [7, 26]. However, there is limited information about the mode of action of physical pretreatment processes, particularly with respect to how the chemical composition of biomass is modified or how its structure is affected. The application of biomass determines the type of physical pretreatment method to be applied. For instance, biochemical conversion of LCB will require size reduction through milling in order to improve enzymatic digestibility of lignocellulosic components. For thermochemical conversion applications of biomass, milling is required for densification, pelletization and even torrefaction prior to thermochemical conversion. In both conversion pathways (biochemical and thermochemical), prior size reduction is necessary in order to eliminate mass and heat transfer limitations. Chipping is also another physical pretreatment technique commonly employed when the biomass is to be used as feedstock in thermochemical conversion processes. This is because most thermochemical conversion systems are size specific hence require feedstock

*A layout showing different pretreatment methods for biomass and their corresponding energy conversion routes.*

As previously stated, densification, pelletization and torrefaction are all considered physical pretreatment methods for biomass intended as feedstock for thermochemical conversion processes [7]. These pretreatment techniques use heat to initiate changes that leads to improved biomass characteristics. A major drawback of the physical pretreatment technique however is its lack of ability to get rid of the lignin content of LCB materials, which renders the cellulose content of the material inaccessible. Other shortcomings include high energy consumption and the prohibitive cost of scale-up for commercial purposes. Studies [26, 27] have shown that the process of lignin removal from LCB materials could be one of the major reasons for the high energy demand of physical pretreatment techniques hence the overall energy efficiency of a bio refinery process may be ultimately affected by this

Chemical pretreatment of biomass involves the use of organic or inorganic compounds to bring about disruption of biomass structure through interaction with intra- and inter polymer bonds of primary organic components. Biomass, particularly LCB materials are resistant to chemical breakdown hence biomass is regarded as being recalcitrant in nature. A host of factors are responsible for the recalcitrance

size in the range 50 mm by 50 mm in diameter [7].

*Biomass Pretreatment and Characterization: A Review DOI: http://dx.doi.org/10.5772/intechopen.93607*

pretreatment method [26].

**Figure 2.**

**3.2 Chemical pretreatment**

**23**

The composition of NLCB, on the other hand, vary from material to material and contains more multifarious elements (such as those previously mentioned) that are embedded in its structural unit; in contrast to LCB, the different compositions of NLCB leads to different thermochemical conversion behaviors when these materials (NLCB) are used as feedstock in the mentioned bioenergy conversion routes [21].

#### **3. Biomass pretreatment**

Pretreatment is a necessary process step for both biochemical and thermochemical conversion of biomass and involves structural alteration aimed at overcoming the recalcitrant nature of biomass. It is required to improve biomass characteristics in order to enhance the energy utilization efficiency of the biomass [7, 22]. In pretreatment processes requiring heat, the degradation ability of LCB is controlled by its polymeric and aromatic constituents (cellulose, hemicellulose and lignin), while the heteroatoms and inorganic elemental components of NLCB act as catalysts to facilitate decomposition, leading to the formation of a product with a carbon framework and structural changes that increases the performance of the pretreated material in bioconversion processes [23–25]. The most important barriers facing current pretreatment technologies are high costs and how to obtain a pretreated product with minimal degradation of vital components. These issues are yet to be convincingly tackled by past and present research and development hence extensive studies aimed at the development of technologies that will further exploit the physical, chemical and biological pretreatment approaches are required. The pretreatment methods must be specifically tailored toward biomass origin and its application in bioconversion and bio refinery processes. **Figure 2** shows a schematic layout of the classes and types of pretreatment processes required for the two main conversion routes (biochemical and thermochemical) for biomass.

The following subsections present a further description of the main classes of pretreatment.

#### **3.1 Physical pretreatment**

Physical pretreatment of biomass is intended to reduce particle size by mechanical comminution in order to increase surface area and pore size. For LCB materials,

#### **Figure 2.**

such as *silicon (Si)*, *sodium (Na)*, *potassium (K)*, *calcium (Ca)*, *magnesium (Mg)* and *aluminum (Al)*; these minor constituents do not markedly contribute to the formation of the total structure of the biomass [16]. The organic components of various

*The composition of the organic fractions of various lignocellulosic biomass materials (dry basis) [17–20].*

**Type of lignocellulosic biomass Cellulose (%) Hemicellulose (%) Lignin (%)** Hardwood 40–55 24–40 18–25 Softwood 45–50 25–35 25–35 Grasses 25–40 35–50 10–30 Leaves 15–20 80–85 – Sugarcane bagasse 40–45 30–35 20–30 Wheat straw 33–40 20–25 15–20 Sweet sorghum bagasse 45 27 21

The composition of NLCB, on the other hand, vary from material to material and contains more multifarious elements (such as those previously mentioned) that are embedded in its structural unit; in contrast to LCB, the different compositions of NLCB leads to different thermochemical conversion behaviors when these materials (NLCB) are used as feedstock in the mentioned bioenergy conversion routes [21].

Pretreatment is a necessary process step for both biochemical and thermochemical conversion of biomass and involves structural alteration aimed at overcoming the recalcitrant nature of biomass. It is required to improve biomass characteristics in order to enhance the energy utilization efficiency of the biomass [7, 22]. In pretreatment processes requiring heat, the degradation ability of LCB is controlled by its polymeric and aromatic constituents (cellulose, hemicellulose and lignin), while the heteroatoms and inorganic elemental components of NLCB act as catalysts to facilitate decomposition, leading to the formation of a product with a carbon framework and structural changes that increases the performance of the pretreated material in bioconversion processes [23–25]. The most important barriers facing current pretreatment technologies are high costs and how to obtain a pretreated product with minimal degradation of vital components. These issues are yet to be convincingly tackled by past and present research and development hence extensive studies aimed at the development of technologies that will further exploit the physical, chemical and biological pretreatment approaches are required. The pretreatment methods must be specifically tailored toward biomass origin and its application in bioconversion and bio refinery processes. **Figure 2** shows a schematic layout of the classes and types of pretreatment processes required for the two main

conversion routes (biochemical and thermochemical) for biomass.

The following subsections present a further description of the main classes of

Physical pretreatment of biomass is intended to reduce particle size by mechanical comminution in order to increase surface area and pore size. For LCB materials,

LCB materials are summarized in **Table 1**.

*Biotechnological Applications of Biomass*

**3. Biomass pretreatment**

**Table 1.**

pretreatment.

**22**

**3.1 Physical pretreatment**

*A layout showing different pretreatment methods for biomass and their corresponding energy conversion routes.*

physical pretreatment decreases cellulose crystallinity and degree of polymerization. It is a vital step prior to both the biochemical and thermochemical conversion of biomass [7, 26]. However, there is limited information about the mode of action of physical pretreatment processes, particularly with respect to how the chemical composition of biomass is modified or how its structure is affected. The application of biomass determines the type of physical pretreatment method to be applied. For instance, biochemical conversion of LCB will require size reduction through milling in order to improve enzymatic digestibility of lignocellulosic components. For thermochemical conversion applications of biomass, milling is required for densification, pelletization and even torrefaction prior to thermochemical conversion. In both conversion pathways (biochemical and thermochemical), prior size reduction is necessary in order to eliminate mass and heat transfer limitations. Chipping is also another physical pretreatment technique commonly employed when the biomass is to be used as feedstock in thermochemical conversion processes. This is because most thermochemical conversion systems are size specific hence require feedstock size in the range 50 mm by 50 mm in diameter [7].

As previously stated, densification, pelletization and torrefaction are all considered physical pretreatment methods for biomass intended as feedstock for thermochemical conversion processes [7]. These pretreatment techniques use heat to initiate changes that leads to improved biomass characteristics. A major drawback of the physical pretreatment technique however is its lack of ability to get rid of the lignin content of LCB materials, which renders the cellulose content of the material inaccessible. Other shortcomings include high energy consumption and the prohibitive cost of scale-up for commercial purposes. Studies [26, 27] have shown that the process of lignin removal from LCB materials could be one of the major reasons for the high energy demand of physical pretreatment techniques hence the overall energy efficiency of a bio refinery process may be ultimately affected by this pretreatment method [26].

#### **3.2 Chemical pretreatment**

Chemical pretreatment of biomass involves the use of organic or inorganic compounds to bring about disruption of biomass structure through interaction with intra- and inter polymer bonds of primary organic components. Biomass, particularly LCB materials are resistant to chemical breakdown hence biomass is regarded as being recalcitrant in nature. A host of factors are responsible for the recalcitrance nature of biomass including the structural complexity and heterogeneity of biomass, the crystalline nature of its cellulose content, and the extent of lignification [7, 28]. Throughout the chemical pretreatment process, the structural recalcitrance of LCB is disrupted, resulting in the reduction of cellulose crystallinity and depolymerization as well as the degradation of cellulose and the breakdown of lignin [29, 30]. For biochemical conversion of biomass, particularly LCB, chemical pretreatment is commonly undertaken in order to isolate the respective biopolymeric constituents of the material. **Figure 3** shows the effect of chemical pretreatment on LCB.

Examples of compounds that have been used for the chemical pretreatment of biomass and which had significant effect on its structure include acids, alkali, organic solvents, and ionic liquids [32, 33].
