2. Microwave heating effects

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 Afzal [32] and Xu [31] described three ways in which MW irradiation and materials can interact, namely (1) MW-transparent material (insulator) where microwaves pass through without losses like Teflon™ or quartz, (2) conducting material which cannot allow microwave penetration but reflected like metals, and (3) absorbing materials such as oil and water. The electromagnetic radiation in MW heating is shaped like energy propagating in a vacuum without any material in motion, can be observed as light, and used as waves and non-ionizing waves in mobile cell phones and infrared [33].

bioproducts minimizes environmental challenges [7, 22]. Microwave pretreatment method is a physico-chemical process involving thermal and non-thermal effects. The early discoveries of microwave pretreatment on lignocellulosic biomass were reported by Ooshima et al. [41] and Azuma et al. [42]. Since then, the technology has shown efficient applications in various ways [43–45]. Recently, many research studies have used MW heating as a pretreatment technique to assist in converting lignocellulosic biomass into useful bioproducts [46]. MW pretreatment combines both thermal and non-thermal effects within the aqueous environment of physical, chemical, or biological reactions [47], and its thermal heating may considerably decrease the time and efficiency of the pretreatment [22]. The pretreatment of lignocellulosic biomass using MW heating is done selectively especially at the polar parts, resulting in an increase in the disruption of the recalcitrant structures of the biomass [48]. To date, different pretreatment techniques to make lignocellulose accessible to enzymes for enhancing bioethanol conversion have been widely studied [20]. On the other hand, energy utilization in the pretreatment process raises the overall cost of producing bioethanol considerably, and this is a critical factor to consider before investing in biorefinery processes [20]. Darji et al. [49] and Aguilar-Reynosa et al. [18] reported the different studies on MW heating process describing MW heating as a better technology with energy efficiency to reduce energy consumption during pretreatment. Enzymatic saccharification is a biochemical conversion preceded by pretreatment and followed by microbial catalyst conversion [50]. This is a microbial degradation process, accomplished by using enzymes and the result is usually a decrease in sugar [12]. Converting lignocellulosic biomass to ethanol involves disintegrating the biomass cell wall structure, thereby releasing the simple sugars which are fermented by yeast to produce ethanol [51]. Maitan-Alfenas et al. [52] reported that microorganisms are essential in enzyme production for lignocellulosic biomass saccharification. The saccharification process in the ethanol conversion requires less energy and is done in mild conditions at pH of 5.2–6.2 and a temperature range of 45–50

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

http://dx.doi.org/10.5772/intechopen.79103

[53, 54]. There are three distinct major types of cellulase enzymes used in the process: (1) endoglucanases (E C 3.2.1.4) hydrolyze at random internal β-1, 4-glucosidic linkages in the cellulose chain producing oligosaccharides of different lengths and with a shorter chain appearance; (2) exoglucanases of cellobiohydrolases (E C 3.2.1.91) progress along cellulose chain ends and release major products as cellulose or glucose; and (3) β-glucosidases known as β-glucoside glucohydrolases (E C 3.2.1.21) hydrolyze cellulose to glucose, liberate cellobiose, soluble cellodextrins to glucose [12, 55]. For hemicellulases, hydrolysis of the hemicellulose fraction requires more complex group of enzymes, and endo-β-1, 4-xylanase enzyme is needed for the hydrolysis of xylana, the major polymer component in hemicelluloses [56, 57]. Cellulases and hemicellulases production involve many microorganisms such as filamentous fungi (Trichoderma spp. and Aspergillus spp. native or genetically modified). During saccharification process, one of the fungi lacks β-glucosidase activity (Trichoderma), and it is supplemented with Aspergillus spp. in enzymes blending to improve the conversion of lignocellulose to

Related research investigations have reported different activities of enzymatic saccharification process with limiting factors on the lignocellulosic biomass such as moisture, available surface area, crystallinity of cellulose, degree of polymerization, and lignin content [8, 20, 60, 61]. Biomass formed in a complex network of lignocellulose contents has indicated that most enzymes used in process can be absorbed by resultant condensed lignin to reduced yield by

simple sugars [9, 52, 58, 59].

C

51

In 1949, Spencer Percy discovered that electromagnetic frequency radiation could be used in dielectric heating via microwave for heating application in food and other process requiring the use of heat. Von Hippel in 1954 provided further elaboration based on understanding of theories on macroscopic interactions of microwave and matters explaining his theory with dissipated power, electric field intensity, and propagation constant [18]. MW heating is directly from inside the material—wave interactions, leading to heat transfer and basically has a higher energy yield in comparison with conventional oven techniques which transmit heat by conduction-convention mechanism [34]. Xu [31] stated that conventional heating is transferred from the surface toward the center of the material by conduction, convention, and radiation; however, MW heating converts electromagnetic energy into thermal energy.

Numerous research studies have reported advantages and disadvantages of MW relative to conventional heating [18]. Advantages of MW include shorter residence time, faster heat transfer, selective, instantaneous on and off operation, precise and controlled heating, rapid and efficient, and environmental friendly process [4]. Due to its efficient process, MW heating has limited disadvantages such as (1) poor distribution of MW power within the material because of non-homogeneous material, (2) non-uniform heating, and (3) low penetration of radiation in bulk materials [35–37].
