*3.2.1 Hydrolysis*

Chemical hydrolysis treatments are classified into acid hydrolysis and alkaline hydrolysis.

In alkaline hydrolysis, effective lignin removal and low inhibitor formation have been observed, although the reaction times are relatively long and the cost of the alkaline catalyst is high; however, it does not degrade sugars. The most commonly used reagents in alkaline hydrolysis are NaOH, NH3, CaO, and Ca(OH)2, and unlike acid hydrolysis, the temperatures are lower, in the range of 50–90°C. The use of an alkali causes the degradation of the ester and side chains, altering the structure of the lignin. This causes a loss of cellulose crystallinity and partial solvation of hemicellulose [16].

It has been reported that acid hydrolysis of biomass removes hemicellulose and partially lignin at high reaction rates; the limitation of acid hydrolysis is the corrosion of the reactor material, as well as a high formation of sugar degradation inhibitors. In acid hydrolysis, dilute or concentrated acid is used, the most used being H2SO4; the biomass is subjected to temperatures in the range of 100–160°C [17].

#### *3.2.2 Solvent extraction*

In recent years, research has been conducted on the generation of third-generation biofuels, also called advanced biofuels due to the raw materials and technological processes used for their production. The raw material for third-generation fuels are microalgae, which promise a high production of biodiesel per unit area due to their high lipid content, which surpasses all biodiesel sources currently used. Microalgae are cultivated in photobioreactors, which only need a liquid culture medium, some nutrients, and sunlight to stimulate the growth of the microalgae biomass. This makes it feasible to use land that is not suitable for the cultivation of human and animal food products for the assembly of photobioreactors. Studies for the extraction of oil from algae for subsequent transformation into biodiesel, either chemically or enzymatically, have been the subject of numerous investigations in numerous countries [18].

The study of the extraction process of lipids from microalgae begins with the knowledge of the composition of the cell wall of the algal biomass to be extracted, to select the solvents that allow high extraction efficiency and the lowest cost of the process. A wide variety of organic solvents have been used in the extraction of algal oil, the most popular being hexane and ethanol, with the extraction of more than 98% of the fatty acids present in the algal biomass [19]. Since ethanol is a polar solvent, its selectivity towards lipids is relatively low compared to other solvents, so that other components of the microalgae such as sugars, pigments, or amino acids (primary and secondary metabolites) may appear in extractions with ethanol.

#### *3.2.3 Supercritical fluids*

Supercritical fluids (SCFs) are good solvents due to their ability to dissolve substances in a similar way to organic solvents, and because their viscosity and diffusion coefficient are close to those of gases, thus facilitating the transport properties of these fluids. Moreover, since the surface tension of FSCs is equal to zero, these fluids are particularly suitable for the extraction of substances contained in solid matrices such as lignocellulosic biomass to obtain cellulose, hemicellulose, and lignin [18, 20]. A fluid is called supercritical when it is forced to remain at conditions of pressure and temperature higher than its critical pressures and temperatures, under these conditions, the fluid has characteristics of both a gas and a liquid, which gives it some special properties such as low viscosity and high relative diffusivity, which allows them to easily penetrate the solids and provide a faster extraction.

All supercritical solid extraction processes consist of two stages: extraction and separation of the solvent from the extract produced. In extraction, supercritical CO2 flows through the solid and dissolves the extractable components. The solvent loaded with the extract is evacuated from the extractor and fed to the separator, where the pressure is reduced so that the solute is not soluble and precipitates. Another advantage is the use of FSCs is the possibility of changing their solvating power by

*Valorization of Biomass as a Raw Material to Obtain Products of Industrial Interest DOI: http://dx.doi.org/10.5772/intechopen.104108*

variations of the pressure and/or temperature of the fluid, thus allowing fractional extraction of the solutes, and complete recovery of the solvent by simple pressure adjustments [20, 21].

Of all the supercritical fluids that have been studied, carbon dioxide (CO2) is the most widely used due to its low critical temperature (TC = 31°C) and pressure (PC = 74 bar), non-toxicity, availability, and low cost. CO2 is a "green" solvent that is found in the atmosphere, in food and beverages, and of which no minimum content needs to be fixed in extracts, so it can be safely used [21]. In fact, it is considered a GRAS solvent. The supercritical fluid method emerged as an alternative to the traditional use of large quantities of toxic solvents for extractions, being this type of processes the most promising, besides these techniques are characterized by having short extraction times and high selectivities [20].

#### **3.3 Physical processes**

The physical processes most used in the transformation of biomass into value-added products are presented.

#### *3.3.1 Mechanical crushing*

The reduction of wood to a size compatible with the subsequent process is the first step in the pretreatment of biomass. The reduction of lignocellulosic materials through a combination of chipping and/or grinding can be applied to reduce cellulose crystallinity, increase mass transfer due to a larger contact area and increase the efficiency of the subsequent process, whether chemical, thermochemical, or biological. The size of the materials is usually 10–30 mm after chipping and 0.2–2 mm after milling [22].

#### *3.3.2 Mechanical extraction*

Mechanical extraction is usually performed through an expeller press also called screw or extruder press. This press is a continuous mechanical extractor, where the oil is extracted from the raw material in a single step, with high pressure. Mechanical extraction has been used as a tool for the extraction of microalgae components and includes several kinds of mechanical devices such as cell homogenizers, ball mills, pressing systems [10], concluding that the highest percentage of oil extraction was obtained when using a ball mill with 1 mm crystal spheres for one minute. Mechanical extraction methods have the disadvantage of difficulty in recovering the extracted oil, so these kinds of methods are used in combination with chemical solvent methods.

#### *3.3.3 Biomass briquetting*

Biomass briquettes are a biofuel, made mostly from dried and compressed green waste and other organic materials (rice and groundnut hulls, bagasse, municipal solid waste, and agricultural residues), which can be used in boilers to generate steam or electricity from it, also used in ovens for cooking and heating. Briquettes are burned together with coal to generate heat through combustion, generating low total net greenhouse gas emissions compared to fossil fuels. The dimensions of briquettes are diameter > 5 cm and length between 50 and 80 cm [23].

#### **3.4 Biological processes**

Biological processes use biological agents (microorganisms, algae, or enzymes) to convert biomass into value-added products such as electricity, heat, bioproducts, and fuels. Biological processes can be divided into biocatalysis (enzymes are used as biocatalysts), fermentation, and anaerobic digestion.

#### *3.4.1 Enzymatic process*

Enzymatic processes are present in several areas of biotechnology, such as pharmaceuticals, food, energy, detergents, textiles, as well as in the environment, mainly in water and waste treatment processes and in the formation of biofuel, specifically biodiesel. Biodiesel can be generated from the triglycerides of tallow, vegetable oil, or microalgae oil by transesterification [24].

It is important to highlight that in the processes of bioethanol and biogas formation and other products of interest by microbial means from biomass, there is a critical step, which is the release of fermentable sugars from the polysaccharides of the biomass to be converted with high yields into high value-added products. Therefore, the most recent research in the field of bioresources has focused on the development of certain biomass pretreatments, such as delignification and enzymatic hydrolysis of cellulose, in which a low production of inhibitory compounds and high release of fermentable sugars are achieved so that they can be efficiently transformed into valueadded products via microbial means and with a low environmental impact [25].

#### *3.4.2 Anaerobic digestion of biomass*

In this process, organic matter (lignocellulosic biomass, municipal waste, livestock, and agricultural industry waste) is degraded to form biogas by the action of anaerobic bacteria at temperatures of approximately 30°C. Anaerobic digestion is the cheapest, most stable, and well-established technique that recovers a greater amount of energy from the source; the process consists of three fundamental stages: hydrolysis-acidogenesis, homoacetogenesis-acetogenesis, and methanogenesis [26]. The first stage involves acid-forming bacteria that use carbohydrates as raw material, the second stage involves acetic acid-forming bacteria that can be inhibited by H2, and the third stage involves acetophilic and hydrogenophilic bacteria that use acetic acid, carbon monoxide, and hydrogen to generate the product of digestion, which is biogas.

Biogas is a mixture of methane (CH4), carbon dioxide (CO2), small amounts of hydrogen (H2), hydrogen sulfide (SH2), and nitrogen (N2). Biogas can be used as an important energy source in the combustion process carried out in engines, turbines, or boilers operated in the industry. In addition, the degraded biomass that remains as a residue of the biogas production process is an excellent fertilizer for agricultural crops [27].

#### *3.4.3 Biomass fermentation*

Fermentation is an anaerobic process where the substrate is transformed into organic products through the action of microorganisms. The types of fermentation that exist, according to the microorganism present in the process are alcoholic, malic, lactic, acetic, propionic, and butyric.


*Valorization of Biomass as a Raw Material to Obtain Products of Industrial Interest DOI: http://dx.doi.org/10.5772/intechopen.104108*

> **Table 1.**

*Summary of the various products derived from biomass.*

The substrate is mainly fermentable sugars, obtained from starch, cellulose, fruits, vegetables, and in general from lignocellulosic biomass. **Table 1** shows some of the products that have been obtained by fermentation using biomass [11, 16, 28, 29].

The transformation of biomass into chemical products and biofuels is increasing worldwide. **Table 1** shows some products obtained by fermentation from lignocellulosic biomass. Biomass is a neutral and economical resource, however, to transform biomass into value-added products, a pretreatment of delignification and saccharification is necessary to release fermentable sugars.

The environmental impact of using biomass is the reduction of CO2 emissions due to the substitution of fossil fuels and the valorization of certain wastes as raw materials. The use of indigenous biomass helps to convert potentially problematic waste for the future into available resources. In addition, this action would reduce forest fires and contribute to the positive management of ecosystems and to the mitigation of climate change.

The social impact of the use of biomass is to stimulate the economy of the region through the employment of groups linked to rural areas and to stop the depopulation of rural areas and the economic savings that the use of waste allows. Finally, the economic value of biomass utilization requires the mobilization of a series of human and capital resources and an intense relationship with suppliers, as biomass has to be supplied to industries. This benefits the primary sector (agriculture, forestry, and livestock), as well as the secondary sector (agri-food, forestry, chemical, pharmaceutical, food, materials, etc.).
