*5.1.4. Advantages and disadvantages of WDM*

Although WDM pretreatment presents lower energy consumption than that for ball milling, it requires large amounts of water due to low solids loading (1–5%), which is a drawback that may hinder its industrial application. WDM of rice straw has been reported to require almost the same energy (5.4 MJ/kg of biomass) as hydrothermal pretreatment, exemplifying the possibility of using milling for biomass pretreatment [69].

#### **5.2. Extrusion process**

*5.1.2. Advantages and disadvantages of ball milling*

*5.1.3. Wet disk milling (WDM)*

than that for 10 cycles of WDM [69].

Mechanical disruption of cellulose by ball milling is a candidate method for a significant in‐ crease of cellulose-accessible surface area without the loss of low-molar mass components. After ball milling treatment, without the use of additive chemicals, the treated material keeps the same chemical composition of the untreated material and there is no generation of liquid fractions, gas or inhibitors. As such, ball milling is an environmentally friendly pre‐ treatment method for lignocellulosic biomass. However, milling processes are known to be very energy intensive, depending on the material characteristics and the target particle size [36]. Taking into account the high energy requirements of milling and the continuous rise of energy prices, it is likely that this process is not economically feasible [67]. Moreover at an industrial scale, ball milling equipment requires high dimensions; nevertheless, in specific cases the balls can be replaced by bars for efficient milling depending on the amount of bio‐ mass to be used. Recently, a new milling pretreatment method for lignocellulosic biomass

WDM is a recently introduced biomass pretreatment process able to produce milled biomass with low levels of inhibitors; it is considered to be feasible for industrial implementation. This technique has been shown to increase the degree of biomass fibrillation and the nano space between the microfibrils, thus promoting the accessibility of the cellulolytic enzyme pool to cellulose [69]. The disk mill is a type of crusher that can be used to grind, cut, shear, fiberize, pulverize, granulate or blend. In general, the suspended material is fed between op‐ posing disks or plates that can be grooved, serrated or spiked. The force applied in the mate‐ rial will depend on the type of disks, the distance between the disks and its rotation speed. For biomass processing using WDM, a water suspension (1–5% of solids) of the lignocellulo‐ sic material is passed between two ceramic non-porous disks that are separated by a dis‐ tance of 20–100 µm and that have a rotational speed of around 1800 rpm. This process can be repeated according to the required number of WDM cycles; very small particle sizes with

high specific surface areas have been observed for a minimum of five cycles [66, 68].

56.1% was observed for straw, confirming that WDM is more efficient for straw.

A study on WDM pretreatment of sugarcane bagasse and straw showed that enzymatic hy‐ drolysis yields increased with the number of WDM cycles; maximum sugar yields were ob‐ tained with 20 cycles, leading to glucose and xylose yields of 49.3% and 36.7% for sugarcane bagasse and 68.0% and 44.9% for sugarcane straw, respectively [66]. Hydrolysis yield data for 10 WDM cycles showed a glucose yield for bagasse of 31.5%, while a glucose yield of

As WDM is a recent procedure for biomass pretreatment, there is limited information on the pretreatment of several different types of biomass. However, reported data for the pretreat‐ ment of rice straw showed that after 10 cycles of WDM it was possible to achieve glucose and xylose yields of 78.5% and 41.5% respectively, with an energy consumption of 5.4 MJ/kg of rice straw. The authors evaluated energy consumption using 60 min ball milling for the pretreatment of rice straw. The process used 108 MJ/kg of rice straw, a value 20-fold higher

was described by using disk milling in a wet state, as described below [68].

60 Sustainable Degradation of Lignocellulosic Biomass - Techniques, Applications and Commercialization

Screw extruders were originally designed to extrude polymers and were also developed for food and feed processing [70-72]. An extruder can provide many functions, such as cooking, forming, kneading, degassing, dehydration, expansion, homogenization, mixing, steriliza‐ tion, shaping, densification and shearing [73]. These functions can be performed in the same process, depending on the size of the extruder and the screw design. Since the 1990s, there has been an increase in the number of studies that use extrusion for biomass processing, such as for the extraction of compounds [74-76], densification [73, 77] and biomass pretreat‐ ment for enzymatic saccharification [78-87]. For lignocellulosic biomass pretreatment, extru‐ sion processing can provide a unique continuous reactor working at higher throughput and solid levels. The extrusion equipment provides temperature control and efficient pulveriza‐ tion by applying a high shearing force. This process also allows the advantageous simulta‐ neous combination of thermomechanical and chemical pretreatment.

The extruder consists of a barrel with a rotating screw (or screws) that squeezes and conveys the material continuously from the input to the output. The barrel is normally segmented, which allows temperature control (heating or cooling) along its length and feeding ports for additive injection. Different types of screw elements can be installed onto the shafts for screw configuration in accordance with the process requirements. The possibility to design the screw configuration using many combinations of elements renders the process very flex‐ ible. There are conveying, backward-conveying and kneading elements that can be threaded in different ways to provide mixing, shearing, elongation flow, and pressure build up. The use of kneading disks, which can be staggered at diverse angles (typically 30°, 45°, 60° and 90° of stagger) in forward or reverse directions, can impart a high shearing stress by forcing the material to pass through the small clearances between the disks and between the disks and the barrel surfaces [88, 89]. It is also possible to configure sealed regions where the pres‐ sure can be significantly higher than other zones.

There are several types of extruders that can be classified according to the number of shafts: single-screw, twin-screw or multiple-screw extruders. The single-screw extruder presents a unique screw rotating in a stationary barrel and is more applicable for distributive mixing without changes in physical properties of the material [89]. On the other hand, some types of twin-screw extruders can provide distributive and dispersive mixing; twin-screw extrud‐ ers are normally applied to obtain changes in physical properties of materials, such as the reduction in particle size by high shearing forces [89]. The twin-screw extruder can be classi‐ fied according to the rotation directions of the two screws: counter rotating (opposite direc‐ tions) [90] or co-rotating (same directions) [88]. The counter-rotating design is used for a very high shear force; nevertheless, it can generate excessive wear and tear. The co-rotating twin screw can be operated at high screw speeds, resulting in high outputs, while maintain‐ ing the required shear force, mixing and conveying properties. Figure 4 shows an example of screw configuration and temperature control versatility of a twin-screw extruder used for biomass pretreatment [82].

[99], big bluestem, prairie cord grass, switchgrass, Indian grass [78, 83-87]. Nevertheless, there are no reports for the use of extrusion for pretreatment of sugarcane bagasse and straw

Sugarcane and Woody Biomass Pretreatments for Ethanol Production

http://dx.doi.org/10.5772/53378

63

The use of single-screw extrusion for the pretreatment of corn stover and soybean hulls at high solid loadings (75–80%) resulted in 54–61% sugar recovery [84, 100]. The pretreatment of Douglas fir wood (coniferous, *Pseudotsuga*) was performed by a counter twin-screw ex‐ truder [79] using cellulose affinity additives (ethylene glycol, glycerol and dimethyl sulfox‐ ide) to effectively fibrillate the wood cell wall and lower the extrusion torque. However, it has been suggested that torque is more effective for fibrillation than temperature and the swelling effects of additives. The enzymatic conversion of extruded products into glucose was three to six times higher than that of untreated material [79]. Ethylene glycol was found to be the most effective additive for fibrillation, achieving a glucose yield of 62.4%. In anoth‐ er study, sawdust and wood chips were pretreated using a twin-screw extruder; this result‐ ed in the recovery of 65% of glucose upon enzymatic hydrolysis, which was over 10-fold

Extrusion was also used in combination with hydrothermal and chemical pretreatment. A single-screw extruder was reported for the pretreatment of wheat straw in conjunction with NaOH, Na2S and H2O2; nevertheless, the mass flow rates and concentration were limited to low values of 10–30 g wheat straw (dry basis)/min and 12–33% solids concentration [91]. The use of a twin-screw extruder in combination with the ammonia fiber explosion (AFEX) proc‐ ess for the pretreatment of milled dry corn stover showed that the extrusion process with ammonia compares well to batch AFEX pretreatment [92]. The pretreatment of milled (un‐ der 3 mm) Douglas fir (softwood) and *Eucalyptus* (hardwood) by LHW followed by a co-ro‐ tating twin-screw extruder was also reported [80]. The glucose yields obtained by extrusion after LHW were higher than those for the individual use of LHW for both types of biomass. Results for Douglas fir were fivefold higher, compensating for the limitations of LHW for this material as LHW is known to be less effective for softwood than for hardwood. The Douglas fir wood was also treated using a batch-type kneader with twin-screw elements [81]. The biomass was pulverized by ball milling for 20 min followed by kneading for 20 min. The maximum glucose yield was 54.20% (25.40 wt% based on initial wood weight). However, glucose yield was improved by heating the extruded biomass with water under pressure (135 °C and 0.25 MPa), revealing that only mechanical kneading with water

showed limitation for enhancing the accessibility of cellulose to enzymes.

cals (NaOH, urea and thiurea) with extrusion, with no significant improvements.

The thermomechanical extrusion of wheat bran and soybean hulls led to reduced sugars yields of 65–73% and 25–36%, respectively [102]. The combination of lower temperature and high residence time (low screw speed) or higher temperature and low residence time (high screw speed) led to higher sugar yields; these authors also tested the combination of chemi‐

The combination of twin-screw extruder and diluted acid pretreatments was recently report‐ ed for rice straw [93] and rape straw [94]. When rice straw was pretreated with 3% sulfuric acid at 120 °C a low (32.9%) glucose yield was observed. However, the use of extrusion/acid pretreatment followed by a hot water extraction step enhanced the enzymatic hydrolysis

aimed at cellulosic ethanol production.

higher than that of untreated material [101].

**Figure 4.** Example of a twin-screw configuration. The elements can be exchanged to be adapted to the type of bio‐ mass and the pretreatment conditions, such as retention time and shearing force.

Thus, the co-rotating twin-screw extruder is one of the most promising configurations for biomass processing due to its flexibility to carry out biomass deconstruction under relatively high stress, temperature and pressure. The twin-screw extruder is also easy to operate and economically suitable for large-scale production [80]. However, the use of kneading ele‐ ments and sealed regions, with reverse elements, for biomass pretreatment at high rates and high solid concentration depends on the flow properties of the biomass. The extrusion of lignocellulosic biomass is difficult due to poor flow properties inside the extrusion barrel, leading to accumulation, burning and blocking of the die during the process [82]. This mate‐ rial can be mixed with water or additives to increase the viscosity and its flow capability, reducing the operational torque and transposing the aforementioned problems. The combi‐ nation of chemical pretreatment inside the extruder, i.e., alkaline [91, 92], acid [93, 94] and ionic liquids [95], can also increase flow and reduce torque, allowing the use of extrusion as a continuous mixing reactor for biomass processing.

The extrusion process was reported for starch conversion [96] and for wood pulping [97]. A twin-screw extruder was used to fibrillate wood chips to produce individual fibers, which have higher aspect ratios than the wood flour particles usually used for wood–polymer composites [98]. Twin-screw extruders were also used as extractors of lipids [74] and essen‐ tial oils [75]. The extraction of hemicelluloses via alkaline solubilization using a twin-screw extruder for pentose production from the hardwood *Populus tremuloides* was also reported [76]. The major advantage of using twin-screw for extraction of biomass compounds is that kneading and reverse screw elements can cause severe compression of the material and al‐ low the simultaneous extraction and liquid/solid separation in a very efficient manner.

Significant improvements in sugar yield after enzymatic hydrolysis have been reported for biomass pretreatment based on extrusion; examples include corn stover [92], *Miscanthus* [99], big bluestem, prairie cord grass, switchgrass, Indian grass [78, 83-87]. Nevertheless, there are no reports for the use of extrusion for pretreatment of sugarcane bagasse and straw aimed at cellulosic ethanol production.

very high shear force; nevertheless, it can generate excessive wear and tear. The co-rotating twin screw can be operated at high screw speeds, resulting in high outputs, while maintain‐ ing the required shear force, mixing and conveying properties. Figure 4 shows an example of screw configuration and temperature control versatility of a twin-screw extruder used for

62 Sustainable Degradation of Lignocellulosic Biomass - Techniques, Applications and Commercialization

**Figure 4.** Example of a twin-screw configuration. The elements can be exchanged to be adapted to the type of bio‐

Thus, the co-rotating twin-screw extruder is one of the most promising configurations for biomass processing due to its flexibility to carry out biomass deconstruction under relatively high stress, temperature and pressure. The twin-screw extruder is also easy to operate and economically suitable for large-scale production [80]. However, the use of kneading ele‐ ments and sealed regions, with reverse elements, for biomass pretreatment at high rates and high solid concentration depends on the flow properties of the biomass. The extrusion of lignocellulosic biomass is difficult due to poor flow properties inside the extrusion barrel, leading to accumulation, burning and blocking of the die during the process [82]. This mate‐ rial can be mixed with water or additives to increase the viscosity and its flow capability, reducing the operational torque and transposing the aforementioned problems. The combi‐ nation of chemical pretreatment inside the extruder, i.e., alkaline [91, 92], acid [93, 94] and ionic liquids [95], can also increase flow and reduce torque, allowing the use of extrusion as

The extrusion process was reported for starch conversion [96] and for wood pulping [97]. A twin-screw extruder was used to fibrillate wood chips to produce individual fibers, which have higher aspect ratios than the wood flour particles usually used for wood–polymer composites [98]. Twin-screw extruders were also used as extractors of lipids [74] and essen‐ tial oils [75]. The extraction of hemicelluloses via alkaline solubilization using a twin-screw extruder for pentose production from the hardwood *Populus tremuloides* was also reported [76]. The major advantage of using twin-screw for extraction of biomass compounds is that kneading and reverse screw elements can cause severe compression of the material and al‐ low the simultaneous extraction and liquid/solid separation in a very efficient manner.

Significant improvements in sugar yield after enzymatic hydrolysis have been reported for biomass pretreatment based on extrusion; examples include corn stover [92], *Miscanthus*

mass and the pretreatment conditions, such as retention time and shearing force.

a continuous mixing reactor for biomass processing.

biomass pretreatment [82].

The use of single-screw extrusion for the pretreatment of corn stover and soybean hulls at high solid loadings (75–80%) resulted in 54–61% sugar recovery [84, 100]. The pretreatment of Douglas fir wood (coniferous, *Pseudotsuga*) was performed by a counter twin-screw ex‐ truder [79] using cellulose affinity additives (ethylene glycol, glycerol and dimethyl sulfox‐ ide) to effectively fibrillate the wood cell wall and lower the extrusion torque. However, it has been suggested that torque is more effective for fibrillation than temperature and the swelling effects of additives. The enzymatic conversion of extruded products into glucose was three to six times higher than that of untreated material [79]. Ethylene glycol was found to be the most effective additive for fibrillation, achieving a glucose yield of 62.4%. In anoth‐ er study, sawdust and wood chips were pretreated using a twin-screw extruder; this result‐ ed in the recovery of 65% of glucose upon enzymatic hydrolysis, which was over 10-fold higher than that of untreated material [101].

Extrusion was also used in combination with hydrothermal and chemical pretreatment. A single-screw extruder was reported for the pretreatment of wheat straw in conjunction with NaOH, Na2S and H2O2; nevertheless, the mass flow rates and concentration were limited to low values of 10–30 g wheat straw (dry basis)/min and 12–33% solids concentration [91]. The use of a twin-screw extruder in combination with the ammonia fiber explosion (AFEX) proc‐ ess for the pretreatment of milled dry corn stover showed that the extrusion process with ammonia compares well to batch AFEX pretreatment [92]. The pretreatment of milled (un‐ der 3 mm) Douglas fir (softwood) and *Eucalyptus* (hardwood) by LHW followed by a co-ro‐ tating twin-screw extruder was also reported [80]. The glucose yields obtained by extrusion after LHW were higher than those for the individual use of LHW for both types of biomass. Results for Douglas fir were fivefold higher, compensating for the limitations of LHW for this material as LHW is known to be less effective for softwood than for hardwood. The Douglas fir wood was also treated using a batch-type kneader with twin-screw elements [81]. The biomass was pulverized by ball milling for 20 min followed by kneading for 20 min. The maximum glucose yield was 54.20% (25.40 wt% based on initial wood weight). However, glucose yield was improved by heating the extruded biomass with water under pressure (135 °C and 0.25 MPa), revealing that only mechanical kneading with water showed limitation for enhancing the accessibility of cellulose to enzymes.

The thermomechanical extrusion of wheat bran and soybean hulls led to reduced sugars yields of 65–73% and 25–36%, respectively [102]. The combination of lower temperature and high residence time (low screw speed) or higher temperature and low residence time (high screw speed) led to higher sugar yields; these authors also tested the combination of chemi‐ cals (NaOH, urea and thiurea) with extrusion, with no significant improvements.

The combination of twin-screw extruder and diluted acid pretreatments was recently report‐ ed for rice straw [93] and rape straw [94]. When rice straw was pretreated with 3% sulfuric acid at 120 °C a low (32.9%) glucose yield was observed. However, the use of extrusion/acid pretreatment followed by a hot water extraction step enhanced the enzymatic hydrolysis yield from 32.9% to 60.9%. The hot water hemicellulose extraction step allowed the conver‐ sion of 83.7% of xylan to xylose and favored cellulose hydrolysis [93]. Rape straw extrusion pretreatment with 3.5% sulfuric acid at 165 °C obtained a glucose yield of 70.9% [94].

and blocking of the die during the process. Another disadvantage in comparison to other bi‐

Sugarcane and Woody Biomass Pretreatments for Ethanol Production

http://dx.doi.org/10.5772/53378

65

Ionic liquids (ILs) can be defined as salts that melt below 100 °C and are composed ex‐ clusively of ions. The first report of a room temperature IL dates back to 1914 [110] and did not prompt any significant interest at that time. It was in the 1980s that these chemi‐ cals have come under intense worldwide attention due to the implications for their use as solvents [111, 112]. The fact that many ILs can be liquid at room temperature and, in general, present a negligible vapor pressure has justified the attention that this group of chemicals has received. They have also been suggested as candidates to substitute for low-boiling-point solvents, such as toluene, diethyl ether and methanol. In addition, ILs are versatile materials and often called designer solvents because their physical and chemical properties can be tuned to meet a specific purpose by preparing new ILs with

ILs have become increasingly trendy over the past few years in the biomass field due to the ability of some members of this class of chemicals to dissolve a wide variety of bio‐ mass types. ILs have been reported for the pretreatment of cellulose [114] and lignocellu‐ losic materials, such as rice straw [115], sugarcane bagasse [116, 117], wheat straw [118], switchgrass [119], *Miscanthus* [120] and wood [121, 122, 123], among others. However, this concept is not new since in 1934 a patent claimed that certain organic salts were ca‐ pable of dissolving cellulose and alter its reactivity [124]; nevertheless, at that time this publication did not generate any important reaction in the scientific community. In 2002, a research group from the University of Alabama investigated new compounds, now known as ILs, based on the concept of cellulose dissolution by a molten salt described by Graenacher in 1934. As result, they found that the IL 1-methyl-3-butyl imidazolium chloride ([Bmim][Cl]) could dissolve up to a 10% solution of cellulose by stirring cellu‐ lose with the IL while heating (100 °C). When heating was performed in a microwave oven, the dissolution achieved was up to 25% (wt%) [125]. Their pioneer work has now been cited over 1000 times and is considered a breakthrough that has set the basis for a

Based on the concept of cellulose dissolution described by Swatloski and co-workers [124] and lately by another work that has shown that [Bmim][Cl] was also able to partially dis‐ solve wood [126], many research groups have described processes of biomass pretreatment with ILs; most of these studies document the complete or partial dissolution of lignocellu‐ lose under heating conditions followed by precipitation with water as an antisolvent. The aim of this procedure is to recover a pretreated part of the biomass that is highly susceptible to enzymatic attack. After IL pretreatment, the biomass native structure is altered in the re‐ covered material in such a manner that the reconstructed cellulose is essentially amorphous

omass pretreatments is the low yields of enzymatic hydrolysis achieved.

**6. Ionic liquid pretreatment**

different combinations of ions [113].

novel concept for lignocellulosic biomass pretreatment.

compared to highly crystalline untreated cellulose [127].

The high sugar recovery due to extrusion pretreatment is related to fibrillation, the increase in surface area [79-81, 86, 87] and pore size [103], which facilitate the access of enzymes to cellulose. Some authors have reported that the crystallinity, which confers resistance to en‐ zymatic hydrolysis, was not significantly reduced in extruded biomass [79, 102] and there‐ fore was not related to the increase in biomass digestibility. Moreover, an increase of 82% in the crystallinity of soybean hulls by thermomechanical pretreatment, using a twin-screw ex‐ truder was reported [82]; as there was no change in material composition, crystallization of the amorphous structure during thermomechanical extrusion was suggested. Some re‐ searchers have also noted the crystallinization of cellulose in the presence of moisture and heat, as has been observed for wood pretreated by steam explosion [104], cotton linter and wood treated in aqueous media after ball milling [105] and hemp cellulose treated by wet ball milling [106]. In accordance with the aforementioned, some researchers suggested that the opening of the cell wall structure at a microscopic scale is sufficient for enzymatic sac‐ charification, regardless of the cellulose crystallinity index [79]. Furthermore, the combina‐ tion of thermomechanical and/or chemical pretreatments can deconstruct the hemicelluloses chains and/or remove part of the hemicelluloses and lignin, facilitating biomass digestibility [82, 102, 107].

#### *5.2.1. Advantages and disadvantages of extrusion pretreatment*

The twin-screw extruder is highly efficiency for pulverization by applying high shearing forces and shows adaptability to different processes, such as chemical, high-pressure appli‐ cations and explosion pretreatments (steam or other solvents) [79-81, 92, 108, 109]. The proc‐ ess is easy to operate and the extrusion process allows the continuous pretreatment of large amounts of biomass with high throughputs, which is advantageous in comparison to batch procedures for the industrial setting. Extrusion compares well to pretreatment technologies that have as drawbacks the batch processing mode, low solids loading or the use of large amounts of water, as already mentioned. Extrusion allows temperature control and does not require washing and conditioning steps, as required with diluted acid, alkali or ionic liquid pretreatments and does not produce effluent; thus there is no effluent disposal cost, no sol‐ ids loss and no significant safety issues [86]. In comparison to other mechanical pretreat‐ ments, the extrusion process is normally less energy intensive than the milling pretreatment options. If extrusion is combined with chemical pretreatment, due to its effective ability of kneading and mixing, the process requires less chemical loadings and thus less residual ef‐ fluents are formed; the combination of extrusion with chemical pretreatment can further re‐ duce energy consumption as it is economically suitable for large-scale operation. Furthermore, extrusion does not produce fermentation inhibitors, such as furfural and hy‐ droxyl methylfurfural; nevertheless, low concentrations of acetic acid have been reported [84-87]. However, the extrusion pretreatment of lignocellulosic biomass requires the use of additives to increase the flow ability inside the barrel and avoid the accumulation, burning and blocking of the die during the process. Another disadvantage in comparison to other bi‐ omass pretreatments is the low yields of enzymatic hydrolysis achieved.
