**5. Mechanical pretreatments**

Mechanical pretreatments of biomass aim primarily to increase the surface area by reducing the feedstock particle size, combined with defibrilization or reduction in the crystallinity de‐ gree. This approach facilitates the accessibility of enzymes to the substrate, increasing sac‐ charification rates and yields. The most studied biomass mechanical pretreatment for biomass is the milling process, mainly the ball-milling and disk-milling pretreatments. An‐ other mechanical treatment to be considered is extrusion, even though this process involves additional thermal and/or chemical pretreatments.

#### **5.1. Milling**

Different types of milling processes can be used to improve the enzymatic hydrolysis of lignocellulosic materials [61]. The main objective of milling pretreatment is to reduce parti‐ cle size in order to increase the biomass-specific surface during biomass fibrillation and to reduce cellulose fiber organization, which is measured by a decrease in crystallinity. These effects can be produced by a combination of chipping, for final particle sizes of 10–30 mm, or grinding or milling, for final particle sizes of 0.2–2 mm [11]. It is important to emphasize that macroscopic particle size reduction does not lead to significant improvements of bio‐ mass enzymatic saccharification, which is solely achieved by using a milling process that al‐ ters the biomass structure at a nanoscopic level. Ball milling and wet disk milling (WDM) are the most common biomass milling pretreatments.

#### *5.1.1. Ball milling*

**4.1. Advantages and disadvantages of steam-explosion pretreatment**

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

incomplete disruption of the lignin–carbohydrate matrix [11].

additional thermal and/or chemical pretreatments.

**5. Mechanical pretreatments**

in the process [60].

**5.1. Milling**

The main advantages of steam explosion relate to the possibility of using coarse particles, thus avoiding a biomass-size conditioning step, the non-requirement for exogenous acid addition (except for softwoods, which have a low acetyl group content in the hemicellulo‐ sic portion), a high recovery of sugars, and the feasibility for industrial implementation. Moreover, the soluble stream rich in carbohydrates derived from hemicellulose in the form of oligomers and monomers may be easily removed and used as feedstock for the production of higher added-value products such as enzymes and xylitol [58]. Other attrac‐ tive features include less hazardous process chemicals and conditions, the potential for significantly lower environmental impact, and lower capital investment [59]. The fact that the steam-explosion process does not require previous grinding of the raw biomass is an important feature, considering that the energy required to reduce the particle size before the pretreatment (pre-grinding) can represent up to one-third of the total energy required

The main drawbacks related to steam-explosion pretreatment are the enzyme and yeast in‐ hibitors generated during the pretreatment, which include furfural and hydroxymethyl fur‐ fural; the formation of weak acids, mostly acetic, formic, and levulinic acids, the two latter acids being derived from furfural's and hydroxymethyl furfural's further degradation; and the wide range of phenolic compounds produced due to lignin breakdown. Several detoxifi‐ cation methods have been developed in order to reduce the inhibitory effect, which repre‐ sent additional costs in the overall process. Other limitations of this method include the

Mechanical pretreatments of biomass aim primarily to increase the surface area by reducing the feedstock particle size, combined with defibrilization or reduction in the crystallinity de‐ gree. This approach facilitates the accessibility of enzymes to the substrate, increasing sac‐ charification rates and yields. The most studied biomass mechanical pretreatment for biomass is the milling process, mainly the ball-milling and disk-milling pretreatments. An‐ other mechanical treatment to be considered is extrusion, even though this process involves

Different types of milling processes can be used to improve the enzymatic hydrolysis of lignocellulosic materials [61]. The main objective of milling pretreatment is to reduce parti‐ cle size in order to increase the biomass-specific surface during biomass fibrillation and to reduce cellulose fiber organization, which is measured by a decrease in crystallinity. These effects can be produced by a combination of chipping, for final particle sizes of 10–30 mm, or grinding or milling, for final particle sizes of 0.2–2 mm [11]. It is important to emphasize that macroscopic particle size reduction does not lead to significant improvements of bio‐ The ball milling process uses mechanical shear stress and impaction to produce pow‐ dered material [62]. This process can be done in the wet or dry state. A combination of chemicals, such as acids, bases and organic solvents, can also be applied depending on the main treatment purpose. In general, the process uses a rotary drum and balls of dif‐ ferent sizes made from different materials (tungsten, ceramic or stainless steel). The ef‐ fect of ball milling on the biomass particle size, structure and crystalline degree depends on the rotation speed, operation time and ball size. Ball milling treatment can be consid‐ ered a kind of ultra-fine grinding and fibrous materials can present between 10-20 µm in terms of particle size [63]. Highly crystalline cellulose has a strong interchain hydrogenbonding network that confers a high resistance to enzymatic hydrolysis, whereas amor‐ phous cellulose is readily digestible [64].

For biomass pretreatment after the material is fed into the rotary drum equipped with balls, drum rotation around a horizontal axis causes a reduction on the material particle size [65]. The ball milling process can drastically alter the complex heterogeneous network structure of wood cell walls, and with a long pretreatment time, cellulose crystallinity can be sig‐ nificantly reduced, which increases the ratio of amorphous cellulose, thus improving the sac‐ charification yields [65]. However, it was found that nanofibrillation of woody biomass by ball milling in the wet state can improve the saccharification yield without a significant de‐ crease in cellulose crystallinity [65]. They found the crystallinity index on the ball-milled bi‐ omass to be ca. 41% in comparison to 68% for the raw material. Thus, the decrease in particle size to a powder-like material and the increase of surface area seemed to be the main factors that promoted the hydrolysis of treated wood.

Results obtained with the pretreatment of sugarcane biomass have shown that ball mill‐ ing treatment of bagasse for 60 min and sugarcane straw for 90 min results in glucose and xylose yields of 78.7% and 72.1% for bagasse and 77.6% and 56.8% for straw, re‐ spectively. In both cases, the enhancement in cellulose digestibility was related to the reduction of cellulose crystallinity to nearly an amorphous level [66]. In another study, the 20 min pretreatment of *Eucalyptus* using a planetary ball milling process was insuffi‐ cient for improving enzymatic digestibility, even though the crystallinity index de‐ creased from 59.7% to 7.6%. When a prolonged milling time of 120 min was used, the enzymatic digestibility of both glucan and xylan increased, while the degree of crystal‐ linity of the material was almost the same as that milled for 20 min. Additionally, the digestibility of glucan and xylan and their total yield were 76.7%, 63.9% and 74%, re‐ spectively, even at a substrate concentration of 20% and an enzyme dosage of 4 FPU/g of substrate, indicating that ball milling is extremely efficient to enhance biomass reac‐ tivity to enzymes [39].

#### *5.1.2. Advantages and disadvantages of ball milling*

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

*5.1.4. Advantages and disadvantages of WDM*

**5.2. Extrusion process**

the possibility of using milling for biomass pretreatment [69].

neous combination of thermomechanical and chemical pretreatment.

sure can be significantly higher than other zones.

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

Sugarcane and Woody Biomass Pretreatments for Ethanol Production

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

61

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‐

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‐

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

#### *5.1.3. Wet disk milling (WDM)*

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].

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

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 than that for 10 cycles of WDM [69].
