1. Introduction

Lignocellulosic biomass feedstock is one potential source of renewable energy and considered as a non-food material (second-generation feedstock) [1]. Agricultural and forest residues as well as industrial and municipal solid wastes are made up of lignocellulosic components [2]. They are environmentally friendly with a carbon-neutral footprint when converted to renewable

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

energy, compared to fossil energy sources such as crude oil, coal, and natural gas [3]. Lignocellulose biomass consists of cellulose, hemicellulose, and lignin. Both cellulose and hemicellulose are polysaccharides with cellulose being the main molecule utilized for ethanol production. Unlike cellulose, which comprises long unbranched fibrils entirely made up of glucose, hemicellulose is a branched polymer, and its polymer chains are shorter than those of cellulose which are described as water soluble because some sugar units are linked to the acetyl groups [4–6]. Lignin acts as a glue between hemicellulose and cellulose and still has some energy value, which can be converted to a variety of value-added products [7, 8].

The production of bioethanol from lignocellulosic biomass (crop residues and waste crops) has been estimated to be 422–491 billion liters per year, which is 16 times higher than global bioethanol production [9]. Bioethanol blend with gasoline (E5, E10, and E85) indicates greenhouse gas (GHG) emission advantage since bioethanol is less carbon-rich than gasoline [10, 11]. The lignocellulosic bioethanol process can be categorized into four steps: pretreatment, saccharification, fermentation, and product (ethanol) recovery [12, 13]. Pretreatment facilitates the breakdown of cell walls and internal tissues of the lignocellulosic biomass through physical, chemical, and biochemical conversion processes. This process involves the disruption and disintegration of recalcitrant structures to open channels for enzymatic reactions in the substrate [14–16]. According to the U.S. Department of Energy [17], the biomass process dramatically reduces dependence on crude oil, supports the use of diverse, domestic, and sustainable energy resources, provides a basis for bioindustry development in accelerating economic growth, and represents an effective strategy for reducing carbon emissions from energy production and consumption.

of each pretreatment technique depends on the type of biomass, composition, and resulting byproducts [23, 24]. In addition, most of the pretreatment techniques suffer relatively low sugar yields, severe reaction conditions, high processing costs and capital investment, and investment risk [25]. Research efforts are continuing to address these challenges. For instance, there is growing interest in microwave heating as a pretreatment alternative to support secondgeneration lignocellulosic biorefineries. According to Aguilar-Reynosa et al. [18], microwave heating process has attracted a series of experimental techniques because it satisfies green chemistry, reduction of time of processing by 10 times compared to other heating techniques, fast heat transfer, and essentially an alternative method to conventional heating [26–28].

Pretreatment of Crop Residues by Application of Microwave Heating and Alkaline Solution for Biofuel…

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49

This review is intended to identify the various microwave-assisted alkali pretreatment effects on the different lignocellulosic agricultural residues. Emphasis is also placed on the pretreatment process effects of the lignocellulosic biomass and its sugar yield/recovery from

Microwave (MW) irradiation refers to electromagnetic waves that consist of electric and magnetic fields. The waves are formed within a frequency band of 300 MHz and 300 GHz [29, 30]. The operational frequency of a domestic microwave oven is 2450 MHz and its heating mechanism with a material depends on shape, size, dielectric constant, and the nature of the microwave equipment. The heating mechanism in MW is aligned with dipolar polarization, conduction, and interfacial polarization. The alignment of polar molecules in an electromagnetic wave with rapid oscillation caused by microwave irradiation forces the polar molecules to align in the radiation field. Dipolar polarization is responsible for the continuous alignment of the polar molecules inside the material which generates the heat [30, 31]. Motasemi and

enzymatic saccharification.

2. Microwave heating effects

Figure 1. Effect of pretreatment on biomass [19].

Recently, many research works have described pretreatment as the most expensive stage in bioethanol production considering challenges faced during the conversion process [18]. Pretreatment accelerates lignocellulosic solubilization, thereby improving enzymatic reactions in the material [12, 15, 16]. Figure 1 shows a schematic of the effect of pretreatment on lignocellulosic biomass. An effective pretreatment technique is needed to liberate the cellulose from lignin, reduce cellulose crystallinity, and increase cellulose porosity [11]. Various pretreatment methods have been developed according to different research studies [3], but the choice of pretreatment technique for a raw material/feedstock is influenced by many factors. These include sugar recovery yield, low moisture content effectiveness, lignin recovery, required particle size, and low energy demand [20].

Pretreatment methods include microwave (MW)-assisted, dilute acid, alkali, steam explosion, ammonia fiber explosion (AFEX), lime, organic solvent, ionic liquids, and biological. A combination of these methods has also been studied, and some studies are still ongoing [3, 20]. Kumar et al. [21] and Merino-Perez et al. [22] presented advantages of pretreatment on lignocellulosic biomass such as (1) improved substrates sugar formation, (2) avoid degradation of carbohydrate, (3) avoid the generation of toxic compounds that can inhibit hydrolysis and fermentation processes, (4) avoid the decomposition of cellulose and hemicellulose, (5) reduction in the number and quantity of chemical reagents used, and (6) cost-effectiveness. Many research reports have compared various pretreatment methods of lignocellulosic biomass, indicating advantages and disadvantages of each pretreatment method. However, the choice

Pretreatment of Crop Residues by Application of Microwave Heating and Alkaline Solution for Biofuel… http://dx.doi.org/10.5772/intechopen.79103 49

Figure 1. Effect of pretreatment on biomass [19].

energy, compared to fossil energy sources such as crude oil, coal, and natural gas [3]. Lignocellulose biomass consists of cellulose, hemicellulose, and lignin. Both cellulose and hemicellulose are polysaccharides with cellulose being the main molecule utilized for ethanol production. Unlike cellulose, which comprises long unbranched fibrils entirely made up of glucose, hemicellulose is a branched polymer, and its polymer chains are shorter than those of cellulose which are described as water soluble because some sugar units are linked to the acetyl groups [4–6]. Lignin acts as a glue between hemicellulose and cellulose and still has some energy

The production of bioethanol from lignocellulosic biomass (crop residues and waste crops) has been estimated to be 422–491 billion liters per year, which is 16 times higher than global bioethanol production [9]. Bioethanol blend with gasoline (E5, E10, and E85) indicates greenhouse gas (GHG) emission advantage since bioethanol is less carbon-rich than gasoline [10, 11]. The lignocellulosic bioethanol process can be categorized into four steps: pretreatment, saccharification, fermentation, and product (ethanol) recovery [12, 13]. Pretreatment facilitates the breakdown of cell walls and internal tissues of the lignocellulosic biomass through physical, chemical, and biochemical conversion processes. This process involves the disruption and disintegration of recalcitrant structures to open channels for enzymatic reactions in the substrate [14–16]. According to the U.S. Department of Energy [17], the biomass process dramatically reduces dependence on crude oil, supports the use of diverse, domestic, and sustainable energy resources, provides a basis for bioindustry development in accelerating economic growth, and represents an effective strategy for reducing carbon emissions from energy pro-

Recently, many research works have described pretreatment as the most expensive stage in bioethanol production considering challenges faced during the conversion process [18]. Pretreatment accelerates lignocellulosic solubilization, thereby improving enzymatic reactions in the material [12, 15, 16]. Figure 1 shows a schematic of the effect of pretreatment on lignocellulosic biomass. An effective pretreatment technique is needed to liberate the cellulose from lignin, reduce cellulose crystallinity, and increase cellulose porosity [11]. Various pretreatment methods have been developed according to different research studies [3], but the choice of pretreatment technique for a raw material/feedstock is influenced by many factors. These include sugar recovery yield, low moisture content effectiveness, lignin recovery, required

Pretreatment methods include microwave (MW)-assisted, dilute acid, alkali, steam explosion, ammonia fiber explosion (AFEX), lime, organic solvent, ionic liquids, and biological. A combination of these methods has also been studied, and some studies are still ongoing [3, 20]. Kumar et al. [21] and Merino-Perez et al. [22] presented advantages of pretreatment on lignocellulosic biomass such as (1) improved substrates sugar formation, (2) avoid degradation of carbohydrate, (3) avoid the generation of toxic compounds that can inhibit hydrolysis and fermentation processes, (4) avoid the decomposition of cellulose and hemicellulose, (5) reduction in the number and quantity of chemical reagents used, and (6) cost-effectiveness. Many research reports have compared various pretreatment methods of lignocellulosic biomass, indicating advantages and disadvantages of each pretreatment method. However, the choice

value, which can be converted to a variety of value-added products [7, 8].

duction and consumption.

48 Renewable Resources and Biorefineries

particle size, and low energy demand [20].

of each pretreatment technique depends on the type of biomass, composition, and resulting byproducts [23, 24]. In addition, most of the pretreatment techniques suffer relatively low sugar yields, severe reaction conditions, high processing costs and capital investment, and investment risk [25]. Research efforts are continuing to address these challenges. For instance, there is growing interest in microwave heating as a pretreatment alternative to support secondgeneration lignocellulosic biorefineries. According to Aguilar-Reynosa et al. [18], microwave heating process has attracted a series of experimental techniques because it satisfies green chemistry, reduction of time of processing by 10 times compared to other heating techniques, fast heat transfer, and essentially an alternative method to conventional heating [26–28].

This review is intended to identify the various microwave-assisted alkali pretreatment effects on the different lignocellulosic agricultural residues. Emphasis is also placed on the pretreatment process effects of the lignocellulosic biomass and its sugar yield/recovery from enzymatic saccharification.
