2.3. Effect of microwave power, alkali concentration, and residence time on enzyme digestibility

MW-assisted alkali pretreatment can penetrate the biomass and vibrate the molecules. The rapid oscillation of the molecules causes continuous heat generation and disruption of lignocellulosic structure, and similar results were reported by Hamzah et al. [69] and Wang et al. [70]. Table 1 describes the effect of microwave-assisted alkali technology in enhancing enzymatic saccharification. Lignocellulosic biomass conversion to bioethanol is very challenging considering the heterogeneous nature of the feedstock used in the process [22]. MW pretreatment process leads to a high lignin removal and improvement in the biomass morphology to facilitate the reactivity of the enzyme, thereby increasing sugar yields [22, 79]. Increasing the alkali concentration during pretreatment of lignocellulosic biomass helps to increase cellulose digestibility and is more effective for lignin solubilization [3, 48]; a relatively long residence time is needed to produce high sugar yields at a lower temperature for alkali pretreatment technique [3]. Increasing NaOH solutions (2–5 wt%) with high temperature (60–140 C) and residence time (10–60 min) at a fixed MW power level of 500 W dissolves a high amount of hemicellulose in the supernatant. Xylan recovery was 73%, and solubilization of lignin was highly dependent on the MW energy input [63]. Xu [31] reported that MW irradiation is an effective heat source employed in alkali pretreatment to produce temperature needed in the delignification of biomass for enzyme reactivity. However, MW-assisted alkali Pretreatment of Crop Residues by Application of Microwave Heating and Alkaline Solution for Biofuel… http://dx.doi.org/10.5772/intechopen.79103

nonspecific linkages of the enzymes [52]. In addition, Palonen et al. [62] reported that the hemicellulose removal increases the mean pore size of the biomass, thereby increasing the chances of cellulose to get hydrolyzed. Consequently, lignin content reduces enzymatic saccharification by forming a shield and blocking substrate digestible parts from hydrolyzing [60]. Janker-Obermeier et al. [63] studied solubilization of hemicellulose and lignin from wheat straw through MW-assisted alkali treatment. The result suggested that more than 80% hemicellulose and 90% lignin could be removed from the solid wheat straw substrate without

The combination of MW-assisted pretreatment and chemical pretreatment on different biomass as reported by several research studies indicated a higher sugar recovery, and various chemicals used in this process are dilute ammonia, iron-chloride and the common ones, alkaline and acid. All these chemicals assist MW pretreatment technology in removing lignin (alkali solution) and hemicellulose (acid solution) for cellulose accessibility [47]. The combined process separates lignocellulosic biomass components by disrupting the biomass structure, reducing the crystallinity of cellulose, improving the formation of fermentable sugars, and reducing the degradation of carbohydrates [64]. At lower temperatures, the combined pretreatment of lignocellulosic biomass improves enzymatic saccharification by accelerating the pretreatment reaction [65–67]. A combination of acid (H2SO4, 2% w/v) and steam (140

30 min) is reported to have efficiently solubilized the hemicellulose, resulting in 96% yield of pentose in pretreatment and enzymatic hydrolysis of soybean hull [68]. Consequently, more research studies on MW pretreatment technique are still ongoing using different feedstocks

MW-assisted alkali pretreatment can penetrate the biomass and vibrate the molecules. The rapid oscillation of the molecules causes continuous heat generation and disruption of lignocellulosic structure, and similar results were reported by Hamzah et al. [69] and Wang et al. [70]. Table 1 describes the effect of microwave-assisted alkali technology in enhancing enzymatic saccharification. Lignocellulosic biomass conversion to bioethanol is very challenging considering the heterogeneous nature of the feedstock used in the process [22]. MW pretreatment process leads to a high lignin removal and improvement in the biomass morphology to facilitate the reactivity of the enzyme, thereby increasing sugar yields [22, 79]. Increasing the alkali concentration during pretreatment of lignocellulosic biomass helps to increase cellulose digestibility and is more effective for lignin solubilization [3, 48]; a relatively long residence time is needed to produce high sugar yields at a lower temperature for alkali pretreatment technique [3]. Increasing NaOH solutions (2–5 wt%) with high temperature

C) and residence time (10–60 min) at a fixed MW power level of 500 W dissolves a

high amount of hemicellulose in the supernatant. Xylan recovery was 73%, and solubilization of lignin was highly dependent on the MW energy input [63]. Xu [31] reported that MW irradiation is an effective heat source employed in alkali pretreatment to produce temperature needed in the delignification of biomass for enzyme reactivity. However, MW-assisted alkali

2.3. Effect of microwave power, alkali concentration, and residence time on enzyme

C,

excessive saccharide solubilizing high amount of cellulose.

and chemical combinations.

52 Renewable Resources and Biorefineries

digestibility

(60–140


53


HPAEC-PAD: high-performance anion exchange chromatography-pulsed amperometric detector; DNS: dinitrosalicylic acid; NREL: National Renewable Energy Laboratory

On the other hand, an overview of key pretreatment processes employed for the bioconversion of lignocellulosic biomass was reported by Chaturvedi and Verma [79]. The study suggested that alkali pretreatment process involving lime, ammonia, NaOH, and KOH resulted in higher yields of sugars involving lignocellulosic biomass with a low lignin content like rice hull and grasses. But concerns with environment challenges were associated with ammonia because it

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

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

55

Densification of biomass is primarily achieved by pelletizing which is the application of mechanical force to compact biomass into uniformly sized solid particles [80, 81]. Densification increases the density of biomass into a pellet product having a density of 600–1200 kg/m3 [82] for efficient transportation and low moisture for safe storage [83]. Particle size and preconditioning of biomass prior to pelletization can facilitate the binding characteristics and chemical composition of biomass, thereby improving the overall pellet quality [84]. In addition, moisture content as a factor during preheating of biomass before pelleting assists in

The pretreatment process helps to complete biomass conversion into valuable bioproducts. Therefore, the pretreatment of lignocellulosic biomass is important in enhancing enzymatic cellulosic digestibility to increase glucose yields [86]. There is only one cited paper on the effects of MW-assisted alkali pretreatment and densification on improving enzymatic saccharification of biomass conversion into ethanol. Sugar yields were reported to increase after MWassisted alkali pretreatments of canola straw and oat hull pellets. Sodium hydroxide (NaOH) and potassium hydroxide (KOH) at various concentrations were used in the study. The authors highlighted that samples selected for cellulosic substrate analysis were based on parameters that describe pellet quality such as tensile strength, dimensional stability, and pellet density [3]. Table 2 shows MW-assisted alkali canola straw and oat hull pellet data and corresponding glucose yield results. The tensile strength, dimensional stability, and pellet density showed little or no significant effect on the sugar yields on canola straw and oat hull pellets. It is evident that samples ground in a 1.6-mm hammer mill screen size had a significant effect on the cellulosic enzymatic digestibility. Table 2 shows data and results from Hoover et al. [85] and Shi et al. [87] which were compared. Hoover et al. [85] indicated that preheating AFEXpretreated biomass pellets had no effect on sugar yield while the non-preheated pellet had a

is toxic to the environment.

Figure 2. Cellulosic sugar analysis methods.

2.4. Effect of biomass pelleting on enzymatic digestibility

loosening the natural binders to produce durable pellets [85].

Table 1. Summary effect of microwave power, residence time, and alkali concentration in improving enzymatic digestibility in selected agricultural crop residues.

pretreatment technology is effective depending on the lignin content of the feedstock and can lead to a high lignin solubilization and increased sugar yields [52]. According to Chaturvedi and Verma [79], results from reducing sugar yields ranging from 40 to 60% are mostly reported from MW-assisted pretreatments. The review pointed out that no pretreatment technology offers 100% conversion of biomass into fermentable sugars. To obtain the optimal MW-assisted alkali pretreatment condition that can improve enzymatic digestibility using different biomass, various microwave power levels, residence times, and alkali solutions of various concentrations were considered. Also, feedstock properties and reaction conditions are contributing factors influencing microwave pretreatment characterization and yield of the final product.

The results from Table 1 indicate that MW-assisted alkali pretreatment can enhance the acceleration of enzymatic hydrolysis process compared to the conventional method as reported by many research studies. Sodium hydroxide (NaOH) solution identified as the most widely applied in MW pretreatment process and effective alkali compared to other alkalis. It was observed that NaOH, residence time, and substrate concentration were the main factors affecting the enzymatic saccharification efficiency. From the different MW-assisted alkali pretreatment processes, a low MW power (200–400 W) and a short exposure time (1–25 min) of feedstock reactor improved enzymatic saccharification sugar yields. However, lime was not a good alkali reagent for MW pretreatment and enzymatic saccharification of sweet sorghum bagasse, whereas sodium and ammonium hydroxides were excellent with MW pretreatment and enzymatic saccharification in high yields of sugars depending on the biomass used.

Figure 2 shows the various sugar analysis methods applied in the last decade in quantifying the sugar yields from MW-assisted alkali pretreatment and enzymatic hydrolysis process of cellulosic biomass. The most widely applied method is DNS with 67% followed by NREL protocol with 28% over the last 10 years as indicated in the published research papers. None of the research studies that have used these methods indicated the most appropriate method. Rather, results of sugar yields were based on the type of biomass used and pretreatment parameters. However, there was no analysis on the cost of using any of the sugar analysis methods reported in the study.

Figure 2. Cellulosic sugar analysis methods.

pretreatment technology is effective depending on the lignin content of the feedstock and can lead to a high lignin solubilization and increased sugar yields [52]. According to Chaturvedi and Verma [79], results from reducing sugar yields ranging from 40 to 60% are mostly reported from MW-assisted pretreatments. The review pointed out that no pretreatment technology offers 100% conversion of biomass into fermentable sugars. To obtain the optimal MW-assisted alkali pretreatment condition that can improve enzymatic digestibility using different biomass, various microwave power levels, residence times, and alkali solutions of various concentrations were considered. Also, feedstock properties and reaction conditions are contributing factors

HPAEC-PAD: high-performance anion exchange chromatography-pulsed amperometric detector; DNS: dinitrosalicylic

Table 1. Summary effect of microwave power, residence time, and alkali concentration in improving enzymatic digestibility

The results from Table 1 indicate that MW-assisted alkali pretreatment can enhance the acceleration of enzymatic hydrolysis process compared to the conventional method as reported by many research studies. Sodium hydroxide (NaOH) solution identified as the most widely applied in MW pretreatment process and effective alkali compared to other alkalis. It was observed that NaOH, residence time, and substrate concentration were the main factors affecting the enzymatic saccharification efficiency. From the different MW-assisted alkali pretreatment processes, a low MW power (200–400 W) and a short exposure time (1–25 min) of feedstock reactor improved enzymatic saccharification sugar yields. However, lime was not a good alkali reagent for MW pretreatment and enzymatic saccharification of sweet sorghum bagasse, whereas sodium and ammonium hydroxides were excellent with MW pretreatment and enzymatic saccharification in high yields of sugars depending on the biomass used.

Figure 2 shows the various sugar analysis methods applied in the last decade in quantifying the sugar yields from MW-assisted alkali pretreatment and enzymatic hydrolysis process of cellulosic biomass. The most widely applied method is DNS with 67% followed by NREL protocol with 28% over the last 10 years as indicated in the published research papers. None of the research studies that have used these methods indicated the most appropriate method. Rather, results of sugar yields were based on the type of biomass used and pretreatment parameters. However, there was no analysis on the cost of using any of the sugar analysis

methods reported in the study.

Biomass MW power (W) MW

54 Renewable Resources and Biorefineries

713 6–18

acid; NREL: National Renewable Energy Laboratory

in selected agricultural crop residues.

Canola straw and Oat hull

Catalpa sawdust time (min)

(3 min interval)

200, 400, 600 3, 6, 9 MW-

Alkali solution (%w/v)

NaOH and KOH

water, NaOH and Ca (OH)2

Enzymes Sugar

Trichoderma reesei and βglucosidase

Commercial cellulase

analysis method

Sugar yield (dry biomass)

DNS Canola straw: 110.0 mg/g Oat hull: 99.10 mg/g

DNS MW/Ca(OH)2/400 W/ 6 min: 402.73 mg/g

Reference

[3]

[78]

influencing microwave pretreatment characterization and yield of the final product.

On the other hand, an overview of key pretreatment processes employed for the bioconversion of lignocellulosic biomass was reported by Chaturvedi and Verma [79]. The study suggested that alkali pretreatment process involving lime, ammonia, NaOH, and KOH resulted in higher yields of sugars involving lignocellulosic biomass with a low lignin content like rice hull and grasses. But concerns with environment challenges were associated with ammonia because it is toxic to the environment.
