**6. Irradiation-chemical pretreatment**

#### **6.1. Description**

In irradiation-chemical pretreatment, the biomass is typically soaked in a solvent (water, acid, or alkali) before undergoing irradiation via microwaves, gamma radiation, proton and electron beam, or radio frequency. In some cases, irradiation is performed before the chemical or other pretreatment, with advantages that include solubilization of lignin and hemicellulose, minimization of cellulose degradation, use of lower doses of chemical and less severe conditions. Further, undertaking irradiation before milling of biomass can reduce energy consumption (from size reduction) significantly [93].

Dielectric heating of biomass causes more energy absorption by the more polar part which creates a hot spot, resulting in generation of high internal steam pressure that induces an explosive effect, disrupting the biomass structure [94, 95]. The disruption is underpinned by radiolytic reactions that cause release of free radicals, triggering cross-linking and chain scission [96]. Cross-linking reactions are believed to happen within the cellulose structure and as such when they predominate over chain scission reactions, sugar yields are not affected.

In general, pretreatment results in degradation of hemicellulose and lignin, and the alteration of cellulose structure. There is an increase in the specific surface area and a reduction in the degree of polymerization [97], as well as a change in the crystallinity of cellulose to amorphous pattern [98]. In general, higher radiation intensities and lower biomass moisture content lead to higher rates of increase in final temperatures; however longer radiation time causes higher average final temperature and lower rate of temperature increase [99]. Increases in irradiation strength have been found to affect hemicellulose more than lignin or cellulose [96, 100].

## **6.2. Applications**

Microwave-assisted pretreatment has been applied to various materials. In a comparative study of the efficacy of mild sulfuric acid (5% v/v) application in combination with various heating modes—hot plate (100°C, 30 min), autoclave (121°C, 30 min), and microwave (200°C, 700 W, 15 min) on the biodegradability of garden biomass, microwave heat treatment was found to produce 53.95% cellulose recovery, leading to reducing sugar yield of 46.97%, which was about 10% higher than the other two modes [101]. Application of microwaves on alkali pretreated wheat straw [102] and coconut husk fiber [103] was found to produce higher ethanol concentration and yield than substrates that did not receive any radiation. In another study, yields of 25.3, 21.2, and 46.5 g/100 g biomass, respectively, was obtained during radio frequency-assisted NaOH pretreatment (27.12 MHz, 0.20–0.25 g NaOH/g biomass; 90°C) of switchgrass at solids content of 20% [94]. In an investigation to ascertain the effects of microwave chemical pretreatment on sweet sorghum bagasse (12% moisture, 1–2 mm), lime was found to enhance lignin removal, with sugar yields reaching 23.2 g/100 g biomass (38% of theoretical yield) for lime concentration of 0.1 g/10 ml of water. Microwave has also been used in conjunction with eutectic solvent, with enhanced lignin and hemicellulose removal and improved cellulose digestibility [104].

Under electron beam application, Karthika et al. [105] obtained 79% sugar yield from the saccharification (30 FPU/g-biomass, 144 h) of a hybrid grass exposed to 250 kGy of radiation, while Bak et al. [106] realized 52.1% from rice straw when it was exposed to 80 kGy and saccharified using 60 FPU/g-glucan for 132 h. Prior removal of hemicellulose using dilute acid and alkaline before irradiation exposes cellulase to enzymatic action during hydrolysis, and culminates in higher sugar yields [107]. Electron beam has also been applied together with other physico-chemical methods such as SE with good results [108]. The main challenge regarding the use of electron beam pertains to its low energy and as such some interest are focusing on proton beam.

#### **6.3. Positive attributes and drawbacks**

The mode of heating is uniform, energy efficient and offers rapid processing of biomass. Pretreatment is performed at low temperatures and at shorter period. It has the potential to be used for effective isolation of hemicelluloses. Irradiation generates no/low levels of inhibitors and by carefully controlling the chemical pretreatment, inhibitor levels are reduced.

Irradiation-chemical methods do not come without disadvantages. Microwave-assisted pretreatment comes with the risk of causing extensive degradation of hemicelluloses and contamination of dissolved lignin at severe conditions, releasing toxic compounds that inhibit enzymatic hydrolysis. Hu and team [94] argue that practical issues with scaling-up is more of a challenge in microwave than in radio frequency which can be used on large quantities of biomass, and at relatively high solids loading (20–50%) with uniform temperature profile when combined with chemical methods.

## **7. Conclusion**

There are few disadvantages associated with SAA pretreatment. Since pretreated solids contain high fractions of hemicellulose, a high demand for C5 conversion enzymes is needed to produce xylose and other pentose monomers [18]. Post-treatment washing usually result in

In irradiation-chemical pretreatment, the biomass is typically soaked in a solvent (water, acid, or alkali) before undergoing irradiation via microwaves, gamma radiation, proton and electron beam, or radio frequency. In some cases, irradiation is performed before the chemical or other pretreatment, with advantages that include solubilization of lignin and hemicellulose, minimization of cellulose degradation, use of lower doses of chemical and less severe conditions. Further, undertaking irradiation before milling of biomass can reduce energy consump-

Dielectric heating of biomass causes more energy absorption by the more polar part which creates a hot spot, resulting in generation of high internal steam pressure that induces an explosive effect, disrupting the biomass structure [94, 95]. The disruption is underpinned by radiolytic reactions that cause release of free radicals, triggering cross-linking and chain scission [96]. Cross-linking reactions are believed to happen within the cellulose structure and as such when they predominate over chain scission reactions, sugar yields are not

In general, pretreatment results in degradation of hemicellulose and lignin, and the alteration of cellulose structure. There is an increase in the specific surface area and a reduction in the degree of polymerization [97], as well as a change in the crystallinity of cellulose to amorphous pattern [98]. In general, higher radiation intensities and lower biomass moisture content lead to higher rates of increase in final temperatures; however longer radiation time causes higher average final temperature and lower rate of temperature increase [99]. Increases in irradiation strength have been found to affect hemicellulose more than lignin or cellulose

Microwave-assisted pretreatment has been applied to various materials. In a comparative study of the efficacy of mild sulfuric acid (5% v/v) application in combination with various heating modes—hot plate (100°C, 30 min), autoclave (121°C, 30 min), and microwave (200°C, 700 W, 15 min) on the biodegradability of garden biomass, microwave heat treatment was found to produce 53.95% cellulose recovery, leading to reducing sugar yield of 46.97%, which was about 10% higher than the other two modes [101]. Application of microwaves on alkali pretreated wheat straw [102] and coconut husk fiber [103] was found to produce

carbohydrate losses.

52 Fuel Ethanol Production from Sugarcane

**6.1. Description**

affected.

[96, 100].

**6.2. Applications**

**6. Irradiation-chemical pretreatment**

tion (from size reduction) significantly [93].

Among the three main stages of cellulosic ethanol production, namely, pretreatment, hydrolysis and fermentation, pretreatment presents the most practical and economic challenges in the attempt to produce ethanol at industrial-scale due its influence on both upstream and downstream processes. Thus, emerging and promising pretreatment methods that rely on physico-chemical fractionation of biomass are discussed, with prominence given to process description, advantages, drawbacks, and innovations employed to counteract inherent technical, economic and environmental challenges. The methods reviewed include liquid hot-water (LHW), steam explosion, ammonium fiber explosion (AFEX), soaking in aqueous ammonia (SAA), and irradiation-based pretreatment. Size reduction operations have been well integrated with other chemical and physico-chemical methods at the pilot and demonstration levels though energy consumption remains the rain challenge and as such research is shifting in favor of relatively low-energy methods such as wet disc milling as well as post-pretreatment size reduction. Irradiation-based methods have also shown promise at the industrial-level as demonstrated by burgeoning research interest around the world. With regards to physico-chemical methods, steam explosion and LHW-based methods have already been developed for industrial application.

[5] Xiao L-P, Shi Z-J, Xu F, Sun R-C. Hydrothermal treatment and enzymatic hydrolysis of *Tamarix ramosissima*: Evaluation of the process as a conversion method in a biorefinery

Emerging Physico-Chemical Methods for Biomass Pretreatment

http://dx.doi.org/10.5772/intechopen.79649

55

[6] Overend RP, Chornet E. Fractionation of lignocellulosics by steam–aqueous pretreatments. Philosophical Transactions of the Royal Society of London. Series A. 1987;

[7] Qing Q, Wyman CE. Supplementation with xylanase and b-xylosidase to reduce xylooligomer and xylan inhibition of enzymatic hydrolysis of cellulose and pretreated corn

[8] Yu Q, Liu J, Zhuang X, Yuan Z, Wang W, Qi W, Wang Q, et al. Liquid hot water pretreatment of energy grasses and its influence of physico-chemical changes on enzymatic

[9] Archambault-Leger V, Shao X, Lynd LR. Integrated analysis of hydrothermal flow-

[10] Ma Y, Ji W, Zhu X, Tian L, Wan X. Effect of extremely low AlCl3 on hydrolysis of cellulose in high temperature liquid water. Biomass and Bioenergy. 2012;**39**:106-111

[11] Matsushita Y, Yamauchi K, Takabe K, et al. Enzymatic saccharification of Eucalyptus bark using hydrothermal pre-treatment with carbon dioxide. Bioresource Technology.

[12] Liu J, Li R, Shuai L, You J, Zhao Y, Chen L, et al. Comparison of liquid hot water (LHW) and high boiling alcohol/water (HBAW) pretreatments for improving enzymatic saccharification of cellulose in bamboo. Industrial Crops and Products. 2017a;**107**:139-148

[13] Weiqi W, Shubin W, Liguo L. Combination of liquid hot water pretreatment and wet disk milling to improve the efficiency of the enzymatic hydrolysis of eucalyptus. Technology.

[14] Imman S, Arnthong J, Burapatana V, Champreda V, Laosiripojana N. Influence of alkaline catalyst addition on compressed liquid hot water pretreatment of rice straw.

[15] Larsen J, Haven MØ, Thirup L. Inbicon makes lignocellulosic ethanol a commercial real-

[16] Thomsen MJ, Thygesen A, Thomsen AB. Hydrothermal treatment of wheat straw at pilot plant scale using a three-step reactor system aiming at high hemicellulose recovery, high cellulose digestibility and low lignin hydrolysis. Bioresource Technology.

[17] Petersen MØ, Larsen J, Thomsen MH. Optimization of hydrothermal pretreatment of wheat straw for production of bioethanol at low water consumption without addition of

concept. Bioresource Technology. 2013;**135**:73-81

stover. Biotechnology for Biofuels. 2011;**4**:18

Chemical Engineering Journal. 2015;**278**:85-91

chemicals. Biomass and Bioenergy. 2009;**33**:834-840

ity. Biomass and Bioenergy. 2012;**46**:36-45

digestibility. Bioresource Technology. 2016;**199**:265-270

through pretreatment. Biotechnology for Biofuels. 2012;**5**:49

**321**:523-536

2010;**101**:4936-4939

2013;**128**:725-730

2008;**99**:4221-4228
