Cellulose is the P-1,4-polyacetal of cellobiose (4-O-P-D-glucopyranosyl-D-Glu). Cellulose is more commonly considered as a polymer of Glu because cellobiose consists of two molecules of Glu. The chemical formula of cellulose is (C6H10O5)„ and the structure of one chain of the polymer is presented in Fig. 8.2. Many prop­erties of cellulose depend on its degree of polymerization (DP), that is, the number of Glu units that make up one polymer molecule. The DP of cellulose varies from

5,0 in native wood to approximately 1,000 in bleached wood pulp [15]. Each d — anhydroglucopyranose unit possesses hydroxyl groups at C2, C3, and C6 positions, capable of undergoing the typical reactions known for primary and secondary al­cohols. The molecular structure imparts cellulose with its characteristic properties: hydrophylicity, chirality, degradability, and broad chemical variability initiated by the high donor reactivity of hydroxyl groups.

The nature of the bonding between the Glu molecules (|5-1,4glycosidic) allows the polymer to be arranged in long linear chains. The latter arrangement of the molecule, together with the fact that the hydroxyl groups are at C2, C3 and C6 positions, allows for the formation of intra — and inter-molecular hydrogen bonds between the molecules of cellulose [16]. The coalescence of several polymer chains leads to the formation of microflbrils, which in turn are united to form fibers.

The hydrogen bonds in the linear cellulose chains promote aggregation into a crystalline structure and give cellulose a multitude of partially crystalline fiber struc­tures and morphologies [17]. The average degree of crystallinity of native cellulose ranges 50-70 % [18, 19]. The ultrastructure of native cellulose (cellulose I) has been discovered to possess unexpected complexity in the form of two crystal phases: Ia and Ip [20]. The relative amounts of Ia and Ip have been found to vary between samples from different origins. The Ia-rich specimens have been found in the cell wall of some algae and in bacterial cellulose, whereas Ip-rich specimens have been found in cotton, wood, and ramie fibers [21,22]. Native cellulose also contains para — crystalline and amorphous portion. Para-crystalline cellulose is loosely described as chain segments having more order and less mobility than amorphous chains segments but less-ordered and more mobile than chains within crystals [23, 24]. The presence of crystalline cellulose, with regions of less order, and the size of the elementary fibrils work together to produce interesting combination of contrary properties such as stiffness and rigidity on one hand and flexibility on the other hand [25].

Crystalline cellulose has a very limited accessibility to water and chemicals. Chemical attack can, therefore, be expected to occur primarily on amorphous cellu­lose and crystalline surface. Cellulose is a relatively hygroscopic material absorbing 8-14 % water under normal atmospheric conditions (20 °C, 60 % relative humid­ity) [26]. Nevertheless, it is insoluble in water, where it swells. Cellulose is also insoluble in dilute acid solutions at low temperature. The solubility of the polymer is strongly related to the degree of hydrolysis achieved. As a result, factors that affect the hydrolysis rate of cellulose also affect its solubility that takes place. In alkaline solutions extensive swelling of cellulose takes place as well as dissolution of the low molecular weight fractions of the polymer (DP < 200) [27].

Cellobiose dehydrogenase (CDH; EC 1.1.99.18; cellobiose: [acceptor]

1- oxidoreductase) is an extracellular flavocytochrome secreted by several wood-degrading fungi (white-rot and brown-rot fungi) under cellulolytic culture conditions. It oxidizes soluble cellodextrins, mannodextrins, and lactose efficiently to their corresponding lactones by a ping-pong mechanism using a wide spectrum of electron acceptors including quinones, phenoxyradicals, Fe3+, Cu2+, and tri-iodide ion [116]. CDH activity was first discovered by Ulla Westermark and Karl-Erik Eriksson as a cellobiose-dependent reduction of quinones in the two white-rot fungi T. versicolor and P chrysosporium. This enzyme has been isolated from the white-rot fungi P. chrysosporium, T. versicolor, P. cinnabarinus, Schizophyllum commune; the brown-rot fungus Coneophora puteana; and the soft-rot fungi Humicola insolens and Myceliophtore thermophila (Sporotrichum thermophile) [117]. Interestingly, no CDH activity has been reported so far from cultures of C. subvermispora, even though it is a selective delignifler [118].

Recently, it was found that CDH has shown to participate in the ligninolytic metabolism of white-rot fungi in the presence of H2O2 [118]. Henriksson et al. [119] have summarized the findings of various researchers regarding the CDH activity in ligninolysis that it reduce Fe3+ to Fe2+ and cellobiose or cello-oligosaccharides to H2O2. In the presence H2O2, the reaction favors the formation of Fenton’s reagent that trigger the production of hydroxyl radicals. This hydroxyl radical is highly reactive and known to attack lignin and cellulose. Further, Henriksson et al. [117] have discussed the following hypothesis/theory about the CDH activity:

• CDH supports the lignin degradation by reducing the aromatic radicals, which is produced from lignin oxidation reaction by LiP and Lac. Enzymatic reaction is a reversible reaction; therefore lignin degraders may favor the polymerization of the radicals in vitro condition. CDH may inhibit polymerization by reducing the radicals created by LiP and Lac.

• CDH supports MnP.

• CDH reduces toxic quinones to phenols that can be used as redox mediators by lignolytic enzymes.

• CDH reduces compound II of lignolytic peroxidases and thus, complete the catalytic cycle in the absence of peroxidase substrate.

• CDH degrades and modifies cellulose, hemicelluloses, and lignin by generating hydroxyl radicals in a Fenton type reaction.

All the above theory/hypothesis is not yet proved practically and still unclear con­cepts. Although, the hypothesis is unclear, the last point about generation of hydroxyl radicals gives plausible explanations for many of the characteristic properties of CDH and it may be the most attractive suggestion for the function of CDH [117]. Further, Dumonceauxa et al. [120] suggested that CDH is not important in lignin degradation, at least for T. versicolor delignifying and concluded that it is possible that some other enzyme masked the effect of the lack of CDH by performing reductive reactions. Hence, the CDH-deficient mutant can still degrade or modify the lignin in a similar manner as the wild type but does not degrade cellulose [121].

An operational large-scale steam pre-treatment platform similar to the Cambi pro­cess is available with the integrated biomass utilisation system (IBUS) developed by Inbicon A/S (a subsidiary company of Dong Energy, Denmark), using straw as the principal process input. The Inbicon IBUS process is mainly focused on the production of ethanol from lignocellulosic biomass. Besides this main product con­sideration, to improve the process economics, other residuals from the process are reused in different process applications. The IBUS process has been tested since 2003 in a pilot plant and was realised in 2009 with the establishment of a demonstration plant in Kalundborg, Denmark. Although the entire IBUS technology incorporates other processing steps to meet the desired product goals, with the emphasis on steam pre-treatment, only the steps involving the auto-hydrolysis process will be highlighted in this chapter.

Prior to the steam pre-treatment step of the IBUS process a mechanical crushing step is initially applied to facilitate a reduction of the biomass particle size of the biomass. The particles are then treated hydrothermally with steam under pressure. In the thermal hydrolysis, the biomass is continuously mixed with water to a dry matter

Fig. 3.3 The overall process flow in the IBUS process [40] content of 30-40 % and then treated at 180-200 °C for 5-15 min. Results of the op­timisation of the IBUS process has shown that a 100 % of the cellulose and «70 % of the hemicellulosic content of the biomass can be recovered in practice [40]. Fol­lowing the steam pre-treatment, the easier to degrade polysaccharide content of the post treated lignocellulosic biomass is then subjected to enzymatic fermentation pro­cesses, which coupled with thermal processes, are used for the production of ethanol, other chemicals and energy. With the IBUS process capable of handling high dry matter contents, higher concentrations of sugar can thus be available as a feedstock for the subsequent fermentation step, resulting in higher ethanol yields [40]. This also leads to a potential reduction in the handling and upgrading costs, since min­imal water is used in processing and with the output stream containing less water, hence higher ethanol concentrations. An overview of the IBUS process can be seen in Fig. 3.3.

Preheating the biomass prior to densification helps to improve a better quality pellet. It influences pellet quality more than the effect of die specifications. This helps to reduce the time to bring the material to reach the die temperature around 80-140 °C. When the biomass reaches the temperature above the glass transition temperature of lignin, it can deform and rearrange easily to form a densely packed structure [9, 10]. Preheating biomass could significantly increase the throughput of the pelletizing machine and reduce the energy requirement per kilogram of pellets formed [62]. In some cases, steam conditioning of biomass is applied to preheat the materials. Saturated steam is used because of its higher heat transfer capacity (indicated by heat transfer coefficient) compared to air. It facilitates a faster heat transfer to biomass.

Fermentation is a very versatile process technology for producing value added prod­ucts such as microbial biopolymers and since fermentation parameters have a high impact upon the viability and economics of the bioprocess, their optimization holds great importance for process development. Especially, microbial polysaccharide pro­duction is greatly influenced by fermentation conditions such as pH, temperature, oxygen concentration and agitation as well as by the composition of the culture medium [7, 10, 16]. Moreover, besides the fermentation conditions, the chemical structure, monomer composition, and physicochemical and rheological properties of the final product also change with the type of strain. This in turn allows the in­dustrial production of polysaccharides with desired specifications via controlling the fermentation conditions, choosing feasible feedstocks, and using high-level producer strains.

EPS synthesis is a tightly regulated carbon and energy-intensive process resulting in a wide range of nutritional and environmental requirements of the EPS producer strains. Consequently, dependency of the production on microbial growth, nutrient availability, and fermentation conditions are subject to significant controversy in literature and hence generalizations should be avoided [7].

Fermentations for EPS production are batch, fed-batch or continuous processes depending on the microbial system used. In most cases, optimum values of tem­perature and pH for biomass formation and EPS production differ considerably so that typical fermentations start with the growth phase followed by the production phase. Moreover, considerable changes in the rheological properties occur during the course of fermentation due to EPS production. This results in a highly viscous and non-Newtonian broth which in turn may not only cause serious problems of mixing, heat transfer, and oxygen supply but also give rise to instabilities in the quality of the end product. Whereas this is a common technical difficulty in commercial xanthan and pullulan production processes [30], it is not encountered in levan production due to the exceptionally low intrinsic viscosity of the polymer [33] as well as in micro­bial processes utilizing thermophilic microorganisms where production is realized at high temperatures [7]. Whether the production is small laboratory scale or large at industrial scale, the fermentation media are almost always designed to have high carbon to nitrogen ratio where nitrogen serves as the growth limiting nutrient [34]. Under conditions employed for industrial production of microbial polysaccharides, the same principle of high carbon to nitrogen ratios is used, but the substrates utilized are the cheapest available form.

As mentioned in Sect. 4.2, vascular plant is mainly including lignin-rich vascular tissue and cellulose-rich parenchyma tissue. Lignin is hypothesized as recalcitrance

for enzyme hydrolysis process. After steam explosion vascular tissue is obviously longer than parenchyma tissue. According to the morpha characteristics of steam ex­ploded materials, mechanical carding equipment is designed (Application Number, 201110233853.6). With steam explosion integrated mechanical carding technology, it is expected to fractionate stalk into vascular tissue fraction and parenchyma tis­sue fraction. It is composed of feed inlet, central axis, variable frequency motor, connecting conveyer, short fraction sliding door, fan, short fraction outlet, long frac­tion outlet. Materials pretreated with steam explosion would be fractionated in this equipment without drying procedure or adding a large amount of water. Steam ex­ploded materials could be loosened firstly, and then fractionated. So crashing process is avoided. Rotating speed and carding time could be regulated to fractionate dif­ferent materials into vascular tissue fraction and parenchyma tissue fraction. This equipment is simple and easy to maintain.

In the operating process (Fig. 4.1), steam exploded feed is put into inlet and is loosened by manipulator. In this process, parenchyma tissue and the minute fiber cell are separated from vascular tissue and flown out by air flow through short fraction outlet. At the same time, vascular tissue and a small amount of epidermis tissue are flown out through long fraction outlet. Therefore, parenchyma tissue is effectively separated from vascular tissue.

In designing mechanical carding equipment, two parameters could be considered: manipulator density and manipulator length.

In terms of manipulator density, the higher the density is, the shorter the vascular tissue fraction is. Therefore, the density of manipulator is decided by raw material property and the final aim of fractionation.

In terms of manipulator length, the longer the manipulator and axis are, the higher the throughput is, and then, the parameter about other parts could be decided by the length of the manipulator and axis including variable frequency motor, connecting conveyer, inlet, and outlet.

Prophase research revealed that, if corn stalk is steam exploded and mechanical carded as a whole, different organs demonstrate different fractionation results be­cause of heterogeneity in structure and component [9]. Therefore, different organs are steam exploded and fractionated, respectively. As a result, vascular tissue fraction and parenchyma tissue fraction are effectively fractionated when rind, leaf, and pith are steam exploded at 1.5 MPa for 7 min, 5 min, and 2 min, respectively.

If rice straw is taken as an example, with steam explosion integrated mechanical carding fractionation, cellulose content in parenchyma tissue fraction and vascular tissue fraction are 57.8 % and 46.2 %, respectively. The separation degree is 1.25 (separation degree = cellulose percentage in parenchyma tissue fraction/cellulose percentage in vascular tissue fraction). However, the separation degree is just 1.08 when rice straw is fractionated with the steam explosion integrated air flow method [10, 35]. Therefore, steam explosion integrated mechanical carding is an effective technique to enrich cellulose by fractionating raw material into vascular tissue frac­tion and parenchyma tissue fraction. As a result, the homogeneity of each fraction is improved in structure and component. Compared with the fiber carding method by water flow applied in paper making, mechanical carding could fractionate vascular tissue and parenchyma tissue by manipulator instead of water that would lead to pollution.

Therefore, steam explosion integrated mechanical carding is an effective tech­nique for corn stalk tissue fractionation and is clean for industrialization.

The demand for biofuels are expected to increase in the near future, and while the search for an efficient and low-cost production process continues, the global outlook is positive for the use of MW irradiation for the pretreatment of lignocellulosic biomass, sludge or biodiesel feedstock.

To overcome the limitations for scaling up MW-assisted technology for pretreat­ment, development of a continuous process offers numerous advantages, but still poses several challenges that require detailed investigation especially when working with high temperature and high pressure.

While the use of MW irradiation offers great benefits with regards to rapid and efficient pretreatment approach, safety is a big factor to consider in designing a large scale production plant.

6.2 Summary

Reviews of the recent advances of the application of MW for pretreatment of lig- nocellulosic biomass had proven the technique to be an important step for efficient and effective biomass-to-biofuel conversion. It was also shown to be more effective than the ultrasonic and chemo-mechanical pretreatments of sludge. The synergis­tic effect of combining MW and alkali could enhance the fermentable sugar yield for bioethanol production and sludge solubilization for methane production. The technique was also shown promising for the pretreatment of feedstock for biodiesel production, including efficient oil extraction and rapid treatment of FFAs.

MW-based pretreatment method offers several great advantages to the synthesis of biofuels, however, some technological challenges still remain. To meet up with the demands of the foreseen shift to the use of renewable bioenergy, the future should also look at the development of a continuous pretreatment process involving solid wastes while taking serious consideration of the safety in designing a large scale production plant.

References

Wood-rotting basidiomycete fungi are usually divided into white-rot and brown — rot fungi. As mentioned earlier, several white-rot fungi are involved in lignin biodegradation such as P. chrysosporium, C. subvermispora, Phlebia subserialis, Echinodontium taxodii, etc. [31, 32, 44, 77]. Majorly, white-rot fungi grow well on hard woods such as birch and aspen. On the other hand, certain species Heter — obasidion annosum, Phellinus pini, and P. radiata grow well on soft woods such as pine and spruce [32]. However, the feasibility of biological pretreatment is still in its infancy because of the extremely long treatment time as well as the difficulty in selectively degrading lignin [5, 78, 79].

The growth of fungi on lignocellulosic biomass results in a loss of dry matter. During the fungal growth, all the main components (cellulose, hemicelluloses, and lignin) are consumed in part by the fungus for its growth and metabolic activities. The loss and the selective degradation of lignin is greatly depends upon the strain which is taking the course of degradation. For example, Flammulina velutipes, Fomes marginatus, and Laetiporus sulfurous decompose wheat straw very slowly or poorly, hence, these white-rot fungi are unsuitable for biological delignification. Some other fungi Ganoderma applanatum, Poria sp., and Trametes gibbosa grow well on wheat straw, but they degrade the hemicellulose and cellulose; therefore, these strains are also not found suitable for biodelignification. Although it is very difficult to remove lignin alone from the lignocellulose, some unique fungal species such as Stropharia rugosoannulata, Hapalopilus rutilans, P ostreatus, C. subvermispora, Lentinula edodes, and Pleurotus eryngii have high affinity with lignin; and they are able to consume lignin faster than non-lignin content of biomass. Therefore, these strains are good delignifier and can be used efficiently in biological pretreatment of lignocellulose [80, 81].

White-rot fungi are more commonly found on angiosperm than on gymnosperm wood species in nature [82]. Generally, syringyl (S) units of lignin are more se­lectively degraded whereas guaiacyl (G) units are more resistant to degradation. The transmission electron microscopy revealed that C. subvermispora and Pleurotus eryngii partially removed the middle lamella while P radiata apparently removed the lignin from secondary cell walls, when these fungi were grown on straw [83]. In fibers, the middle lamella contains a high concentration of G lignin, while, secondary walls contain a high proportion of S lignin. Various environmental conditions like cultivation time, pH, nutrient ingredients (nitrogen source), and oxygen level have been optimized by many researchers in order to achieve the maximum degradation of lignin [84]. Lignin degradation by white-rot fungi occurs through the action of lignin degrading enzymes such as peroxidases (LiP and MnP) and phenol oxidase (Lac) [5, 78, 85]. These enzymes are regulated by carbon and nitrogen sources [16]. Almost all white-rot fungi produce Lac and MnP, but only some of them produce LiP [32].

White-rot fungi degrade lignin in biomass with two different mode of degrada­tion, named as selective and non-selective degradation. In non-selective degradation, all three components (lignin, cellulose, and hemicellulose) were almost degraded equally, whereas in selective decay mostly hemicellulose and lignin were degraded [32]. Some white-rot fungi species remove lignin without loss of cellulose from LCCs and cause white-mottled or white-pocket type of rot and those species referred as selective delignifier, for example, Phellinus nigrolimitatus [32, 86]. More than 1,500 species of white-rot fungi are able to decompose lignin with little consump­tion of cellulose [87]. There are also some fungi that are able to degrade the same wood with both types of attack selective and non-selective [49]. Good examples of such fungi are G. applanatum and H. annosum. The selective deligniflers have a piv­otal role in biopulping, biobleaching and bio-fuel industries. However, the ratio of lignin-hemicellulose-cellulose decayed by a selected fungus can differ enormously; and even different strains of the same species, for example, P chrysosporium and C. subvermispora, may act in another way on the same kind of wood. C. subver — mispora was found to be one of the most lignin removers from woody materials, but grew poorly on rice straw [88]. Furthermore, the comparative studies of C. sub­vermispora and P. chrysosporium revealed that C. subvermispora genetic inventory and expression patterns exhibit increased oxidoreductase potential and less cellu­lolytic capability relative to P chrysosporium [89]. Some examples of white-rot fungi, which posses selective degradations, are Pycnoporus cinnabarinus, P. ostrea — tus, P eryngii, P radiata, Phlebia tremellosus, P subserialis, P pini, and Dichomitus squalens [32, 86, 90]. The selective deligniflers have a potential role in pretreatment of various lignocelluloses in order to attain the considerable amount of feed-stock for the biofuel production.

Some species remove lignin more readily than carbohydrates [86]. Many white — rot fungi colonize cell lumina and cause cell wall erosion. Eroded zones form as decay progresses and large voids fllled with mycelium. This type of rot is referred to as non-selective or simultaneous rot [86]. T. (syn. Coriolus, Polyporus) versicolor and Fomes fomentarius are typical simultaneous-rot fungus [32, 61]. Therefore, the use of non-selective fungi is greatly limited by its non-selective degradation of plant cell walls and it may be used in biological pretreatment to some extent.

Saqib Sohail Toor, Lasse Rosendahl, Jessica Hoffmann, Jens Bo Holm-Nielsen and Ehiaze Augustine Ehimen

Abstract With the ever rising demand for more energy and the limited availability of depleted world resources, many are beginning to look for alternatives to fossil fuels. Liquid biofuel, in particular, is of key interest to decrease our dependency on fuels produced from imported petroleum. Biomass pre-treatment remains one of the most pressing challenges in terms of cost-effective production of biofuels. The digestibility of lignocellulosic biomass is limited by different factors such as the lignin content, the crystallinity of cellulose and the available cellulose accessibility to hydrolytic enzymes. A number of different pre-treatment methods are known to enhance the digestibility of lignocellulosic biomass by affecting these limiting factors. Some of them are: milling, thermal pre-treatment with steam or hot water, acid pre-treatment, and alkaline pre-treatment. This chapter will focus on one of the more promising technologies; thermal pre-treatment with steam.

The Norwegian company Cambi developed a process for treatment of sludge from waste water treatment plants, and the idea was based on the experience that cooking sludge under pressure at temperature from 150 °C to 180 °C improved the digestibility and at the same time increased the dewaterability of the sludge. If Cambi’s process is to be used for treatment of biomass, it will have to compete with other processes on market. The strongest competitor at present is the integrated biomass utilisation system process of DONG Inbicon which is used for pre-treatment of straw. Both processes are being described and discussed in this chapter.

Keywords Biomass ■ Steam pre-treatment ■ Lignocellulose ■ Hydrolysis

3.1 Introduction

With the ever rising demand for more energy and the limited availability of depleted world resources, it is crucial to look for alternatives to fossil fuels. Liquid biofuel, in particular, is of key interest to decrease our dependency on fuels produced from imported petroleum. Hereby lignocellulosic biomass is an important bioresource for

L. Rosendahl (H) ■ S. S. Toor ■ J. Hoffmann ■ J. B. Holm-Nielsen ■ E. A. Ehimen Department of Energy Technology, Aalborg University, Pontoppidanstrrnde 101, DK-9220, Aalborg 0, Denmark e-mail: lar@et. aau. dk

Z. Fang (ed.), Pretreatment Techniques for Biofuels and Biorefineries,

Green Energy and Technology,

DOI 10.1007/978-3-642-32735-3_3, © Springer-Verlag Berlin Heidelberg 2013

producing second generation biofuels. Through either thermochemical or biochem­ical conversion processes lignocellulosic biomass can be converted into energy or energy carrier. Biochemical conversion processes deliver biofuel, that is, bioethanol or biogas through fermentation of the biomass. It has been seen that pre-treatment of lignocellulosic biomass prior to conversion enhances the product yield and is therefore of great importance also for the economical viability of the process. Lig — nocellulose forms the cell wall of plant material and has a complex structure, but its main constituents are cellulose, hemicellulose and lignin, with 40-50 %, 25-30 % and 15-20 %, respectively [1].

Cellulose fibers are embedded in a matrix of other structural biopolymers, primar­ily hemicellulose and lignin. Cellulose is a linear polymer of glucose. Hemicellulose, a branched heteropolymer consisting of various 5-6 carbon sugars and lignin, is com­posed of three major phenolic components [2]. Cellulose, hemicellulose and lignin form structures so-called macrofibril which are responsible for the structural stability of the plant cell wall. Those macrofibrils are sub-divided into microfibrili, where cel­lulose is packed inside lignin and hemicellulose as can be seen in Fig. 3.1. Hydrogen bonds between different layers of the cellulose lead to polymerisation. Crystalline cellulose has a high degree of polymerisation and contributes to the resistance to biodegradation due to low cellulolytic enzyme accessibility, whereby high acces­sibility amorphous cellulose could have a lower degree of polymerisation and will be more susceptible to enzymes [1]. For a most effective biomass to ethanol con­version process all major lignocellulosic components should be utilised; therefore, pre-treatment to increase the amenability for hydrolytic enzymes is of great impor­tance. Different kind of pre-treatment technologies exist. They can be separated into physical pre-treatment methods, chemical pre-treatment methods, biological meth­ods and physico-chemical pre-treatment methods. During physical pre-treatment the improved hydrolysis results from decreased crystallinity and improved mass transfer characteristics from reduction in particle size. Through chemical pre-treatment, for example, by adding acids, alkali, pH-controlled hot water or ionic liquids, lignin and hemicellulose were partially removed and the degree of polymerisation of cellulose is lowered [3]. Physico-chemical processes, through, for example, steam pre-treatment or explosion, combine both physical and chemical methods and lead to hemicellulose degradation and lignin transformation due to high temperatures, thus increasing the potential of cellulose hydrolysis [1]. During steam pre-treatment and steam explosion which is part of physico-chemical method, the biomass is treated with high pressure steam. Steam pre-treatment and explosion which is being discussed in this chapter leads to a low by-product generation, has a high sugar yield, is applicable to different feedstock, has low investment costs and is already being used commercially [1].

Hammer mills can be used for a wide range of feedstocks. The mechanism of size reduction in hammer mills is impact. There are fixed or swing hammers mounted on the rotor. Hammers may be blunt or sharp. Sharp hammers allow a combination of both mechanisms of shear and impact for size reduction. The ground particles have various shapes and a wider range of size distributions in comparison to chippers. Maximum particle size can also be controlled by installing screens with different sizes.